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Old 26-09-2019, 05:45   #106
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Re: serious discussion about electric power in large boats

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Originally Posted by a64pilot View Post
This is 7.16. To me as a JIm Bob type of guy and by no means a math guy, it looks the same.

In this area I'm not Jim Bob. You either accept the math and science or you don't. The math is really simple. Interpretation takes a bit more. Remember I am a naval architect. I don't just call myself one - I have a degree specifically in naval architecture and marine engineering. I've worked on everything from aircraft carriers to landing craft, SWATH to air cushion vehicles, catamarans, trimarans, and some really odd research vessels. So I'm telling you that the concept of hull speed really has no utility in the real world.



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Originally Posted by a64pilot View Post
I think your contention is the there is no exact, specific number that is an absolute limit? I absolutely concede that, but again as a JIm Bob type who never got past College Algebra we arenít wrapped up in is it 1.3 or 1.5 or 1.57.

My point is that you should be wrapped up in that because that is why hull speed as no utility.



Let's look at some specific numbers and then back up a bit.


Your boat has a length on the waterline (LWL) of 33'. That means that for the classic factor of 1.34 your "hull speed" is 7.7 kts. If the factor is 1.2 it is 6.9 kts. For 1.5 it is 8.6 kts. That is a swing of 22%. If we gave you a stall speed for an A64 +/- 11% would that be useful? No.


"Hull speed" is a point on the speed-power curve. Your boat came in two versions: a standard full keel and a shallower full keel with a swing keel. The speed-power curves for those two boats will be different; shaped the same, the deeper boat will have the entire curve shifted left. Yet the calculated "hull speed" will be the same. Not useful. Put a bow thruster in and the curve will shift left but the calculated "hull speed" will be the same. A long overhang boat with narrow beam and deep fin keel that has a 33' waterline, say a Herreshoff design, will have the same calculated "hull speed" even though it is going to be a faster boat. Not at all useful.



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Originally Posted by a64pilot View Post
We just know that at some point we are pulling the guts out of it and expending a lot of fuel which kills range, for very little benefit.

I definitely agree. "Hull speed" has nothing to do with that and isn't helpful. Here's what IS helpful. We have a speed-power curve: boat speed through the water on one axis (usually horizontal; this is considered the independent variable) and power (really total hull resistance) on the other axis (usually vertical; this is considered the dependent variable).



So Bob Johnson generates a speed-power curve. The big boys would do model testing to build the curve. Bob may have estimated one based on experience and review of curves from similar hulls appropriately scaled. That's what I would have done in his place. He picked a boat speed under power he wanted to deliver and picked off the power for that speed. Adjustments for propeller efficiency, power train losses from cutlass bearing, line bearings if any, and transmission lead to an engine power requirement. Pull down the catalogs from the engine manufacturer(s) you prefer to work with and choose an engine based on cruising rpm. Pull maximum power from the engine specs and work backwards to thrust from the propeller, back to the speed-power curve and you find the maximum speed under power.



No "hull speed" anywhere in there.



Your IP 38 came with one of two engines, a 44 HP and a 50 HP depending on the year of manufacture (ignoring all the repowers with who knows what in there). You can look up the curve for fuel consumption. That curve may be fuel consumption against power or fuel consumption against rpm. If the latter somewhere there is a power against rpm curve. Regardless you have all you need, for a specific boat and a specific engine, to build your own curve for boat speed against fuel consumption. Now this is useful, especially with a known and confirmed fuel tank size.



No "hull speed" anywhere in there.


Now from experience we know that with some variation of engine models, cost sensitivity, willingness to operate in the dark, destination distance, availability of bailouts, etc. 1800 rpm is a sweet spot for range, cost, noise, vibration, heat, etc. Because of the shape of the speed-power curve (or just more experience) we know that if we're running late we can run up to 2200 or 2300 rpm, but 2500 or 2600 rpm won't get us in much faster but will greatly increase fuel cost and make life more unpleasant below. That isn't to say it doesn't make sense to run at 3000 rpm for a while from time to time to get engine lube oil up to temperature. This is a benefit to engine service life for high speed diesels.



Nothing about "hull speed" in there, and no shortcuts available by trying to stuff it in.



Sails are very powerful sources of propulsion. On your boat with newish sails on a close or beam reach, well trimmed, in a fresh breeze (say 20 kts) you should be able to break 8 kts through the water pretty consistently as long as you have not loaded the poor girl down too much. The speed is just where you sit on the speed-power curve for the power generated by the sails.



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Originally Posted by a64pilot View Post
But there is absolutely a speed at which with enough power you absolutely can exceed,but it takes more and more power to go faster.

Correct, and nothing to do with hull speed.


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Originally Posted by a64pilot View Post
You see it all the time in the ICW, that boat that passes you rolling that big wake is expending quite a lot of fuel to roll that big wake, the energy to make that wake of course comes from the fuel tank.

Correct. That's the wavemaking resistance I talked about in my earlier dissertation.



Still not a wall or even a knee in the curve (called an inflection point).



You referenced cars. The amount of additional fuel necessary to run at 60 vice 55 is more than the amount of additional fuel necessary to run at 55 vice 50. It's a curve, not a point.


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Originally Posted by a64pilot View Post
Now in 7.15 the end of the chart where the curve actually decreases is Iíd bet the point where that hull begins to plane

Maybe. Depends on the source of the data. It might be scaling effects (using an algorithm outside the range of applicability). It could be submarining of the model. Lots of common recreational hulls simply won't plane. They may fly, *grin* but they won't plane.


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Originally Posted by sanibel sailor View Post
I think if range is paramount, there is a fuel consumption curve for an engine that will also help find maximum efficiency. Is graph 7.16 for planing craft as shown in diagram above?

No, the speed-power curve for planing craft looks quite different. There is a very narrow speed range over which the curve tips over and looks more like the initial section. A well done set of curves will actually have two lines: one for the boat speeding up and the other for the boat slowing down. This is the measured description of the real-world case where for a nominal recreational boat you have to reach some speed (say 16 kts) to get on plane, but you can stay on plane at lower speeds (say 13 kts) with significant fuel economy. This is all (usually) for the hull without controllable appendages (engine trim or trim tabs). With plenty of time and resources you could build a family of speed-power curves for different engine trim or trim tab settings.



Aren't you glad you asked? *grin*


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Originally Posted by sanibel sailor View Post
Also what's with the hump? Does that apply to sailboats as well?

Good question, and yes it applies to sailboats. The hump is a function of wavemaking resistance. When the wavelength of the wave generated by the motion of the boat through the water is very close to the waterline length of the boat there is a regime in which the boat speed can increase dramatically with minor increases in power. Arguably this is hull speed. The problem is that the magic constant is different for every hull form, and indeed with the addition of appendages. That's why ships almost always have bow bulbs and many have stern bulbs. The bulb is designed to generate a wave that is out of phase with that generated by the hull. The reduction in potential energy transferred to the water level ("wavemaking") directly results in power savings and therefore fuel consumption. There have been some efforts using bulbs on smaller boats without enough significance to be worth the cost. The design cost alone is more than most naval architects focused on the recreational market can justify.
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Old 26-09-2019, 05:52   #107
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Re: serious discussion about electric power in large boats

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Originally Posted by a64pilot View Post
This is 7.16. To me as a JIm Bob type of guy and by no means a math guy, it looks the same.
I think your contention is the there is no exact, specific number that is an absolute limit? I absolutely concede that, but again as a JIm Bob type who never got past College Algebra we arenít wrapped up in is it 1.3 or 1.5 or 1.57.

From a BillyBob kind of guy that did get far beyond basic algebra in college (99% of which is long forgotten) I agree with all your points.


The common formula I have always heard is 1.34 * sqrt LWL but I completely agree with Auspicious that this is not real nor perfectly accurate in practice. However for the average cruiser with a typical, true displacement boat the reality will be somewhere close to that formula. It might be 1.2 or 1.4 but regardless somewhere in that general range there is a point where one will reach the point of diminishing returns IE a large increase in power and fuel consumption to get a very slight increase in speed.





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Originally Posted by a64pilot View Post
But there is absolutely a speed at which with enough power you absolutely can exceed,but it takes more and more power to go faster.

You see it all the time in the ICW, that boat that passes you rolling that big wake is expending quite a lot of fuel to roll that big wake, the energy to make that wake of course comes from the fuel tank.
Itís not a wall, with enough power of course you can blow through it, but it requires quite a lot of power and fuel for very little extra speed.
Attachment 200545

Also I occasionally see a boat similar in size to mine but newer and with a more modern hull design, going faster than me but with a smaller wake than mine.
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Old 26-09-2019, 08:19   #108
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Re: serious discussion about electric power in large boats

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Originally Posted by a64pilot View Post
...what I found on all three boats was the the most economical as in greatest range was idle, and any faster cut into range, there was no sweet spot like what is shown on these charts...
I don't particularly want to enter into this debate, but just wanted to add an interesting note that seems to be contrary to your tests - my consistent experience on large fast (planing) motoryachts was generally always that the faster we went the more fuel efficient the vessel was vs boat speed.

In simplistic terms this makes complete sense of course, faster = less wetted surface = the same speed for less power (or more speed for the same power).

It was always a great justification, going faster saves money...

But these were vessels (and engines) specifically designed to go fast and operate in that part of their range - ergo, going slower than their designed optimum range was inefficient.

Of course there was (or must have been) a limit to this at some point (even just from aerodynamic resistance) but it was not always reached. One boat was most efficient at 100% wide open throttle, but what was maybe good for fuel efficiency was maybe not so good for engine life. Throttled back to 90 something percent was ok.

Idling was normally terribly inefficient. These huge engines ticking over, not burning much fuel compared to wide open of course, but also not really achieving anything either compared to this consumption.

The modern MTU high speed diesels on some of these vessels were designed to operate efficiently at a very high percentage of their maximum rpm/load (and so were the hulls that they were matched to). So this was not 2000 vs 3000rpm, but more like 85 vs 92% load. IIRC in that range they produced the most amount of power vs fuel consumed. And less than 80% load started to become notably inefficient.

And as @Auspicious mentioned earlier, a certain amount of the design process becomes backwards at certain point - what engines are available that approximately meet the criteria - and after the drivetrain package, and maybe some hull tweaks, are designed backwards to match them.

The situation gets even more interesting (and complicated) when multiple engines are involved. Perhaps this multiple engine power is required to get on the plane initially, especially when full of fuel at the beginning of the voyage, but once up and moving fast maybe you can throttle back. Or maybe the enormously increased boat speed justifies this additional engine power and consumption. I have experience with multiple variations of this situation.

So, a slight thread jack, but also additional information in general to reinforce that there is no simple answer to this question.

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Old 26-09-2019, 08:54   #109
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Re: serious discussion about electric power in large boats

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Originally Posted by a64pilot View Post
Iíve used fuel flow meters and GPS speed in calm water and graphed fuel consumption over speed for three different boats.
All three were planning boats, two were Sportfishermen, one Diesel and one gas and one CC with an outboard.
I didnít find that bump around 1.0, but my data points werenít likely as numerous as would be needed to find that bump, and I think that is actually a very small bump. Iím not saying itís not there, Iím sure it is, just my testing methodology wasnít good enough to find it.
Anyway what I found on all three boats was the the most economical as in greatest range was idle, and any faster cut into range, there was no sweet spot like what is shown on these charts with the bump, and if you look at the bottom of the bump, itís still higher than lower speeds.
Now all of this ignores wind and current which further complicates things of course.

The little CC I tied into the engines bus and picked all kinds of data from it, one of those was of course fuel consumption, the Garmin 740s, one of its features was it would compute fuel mileage continuously, so you could see the effect of trim tabs, motor trim and of course RPM on the fly.
As much as I wanted there to be, I couldnít find any speed really that got better fuel mileage than a lower speed. Now right before she broke onto plane with the bow in the air was of course a bad speed for range, but just before it and just after it were better of course.

I expected that right after it got onto plane that it would actually get better fuel mileage than it did at a speed less than plane, but that wasnít the case.

So in other words if you disregard wind and current cause I think that just too complex a problem to make general statements on, the slower you go the better the fuel economy.
But who can idle all day?

The other observation I made that seems to make sense is to observe your wake, if you have much of one, your burning excess fuel, slow down until you have no real wake and youíll burn much less fuel.
Of course you may choose speed over fuel,that is of course your decision to make.

For whatever itís worth, there is absolutely a sweet spot for any airplane, speeds less than or faster than will burn more fuel per distance covered, I expected a boat to have the same, but it doesnít.
I guess a boat doesnt have to balance lift and drag ratios. It only has to deal with thrust and drag.

Specific wing sections have their best L/D speeds.
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Old 26-09-2019, 09:00   #110
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Re: serious discussion about electric power in large boats

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Originally Posted by jmh2002 View Post
I don't particularly want to enter into this debate, but just wanted to add an interesting note that seems to be contrary to your tests - my consistent experience on large fast (planing) motoryachts was generally always that the faster we went the more fuel efficient the vessel was vs boat speed.

I've read about that sometimes being the case on small sport cruisers and center consoles and so forth.

With ours, the faster we go, fuel consumption increases. It's a curve, with more RPMs = more GPH = not enough faster speed to make up for it.

FWIW.

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Old 26-09-2019, 09:40   #111
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Re: serious discussion about electric power in large boats

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Originally Posted by Auspicious View Post
In this area I'm not Jim Bob. You either accept the math and science or you don't. The math is really simple. Interpretation takes a bit more. Remember I am a naval architect. I don't just call myself one - I have a degree specifically in naval architecture and marine engineering. I've worked on everything from aircraft carriers to landing craft, SWATH to air cushion vehicles, catamarans, trimarans, and some really odd research vessels. So I'm telling you that the concept of hull speed really has no utility in the real world.



My point is that you should be wrapped up in that because that is why hull speed as no utility.



Let's look at some specific numbers and then back up a bit.


Your boat has a length on the waterline (LWL) of 33'. That means that for the classic factor of 1.34 your "hull speed" is 7.7 kts. If the factor is 1.2 it is 6.9 kts. For 1.5 it is 8.6 kts. That is a swing of 22%. If we gave you a stall speed for an A64 +/- 11% would that be useful? No.


"Hull speed" is a point on the speed-power curve. Your boat came in two versions: a standard full keel and a shallower full keel with a swing keel. The speed-power curves for those two boats will be different; shaped the same, the deeper boat will have the entire curve shifted left. Yet the calculated "hull speed" will be the same. Not useful. Put a bow thruster in and the curve will shift left but the calculated "hull speed" will be the same. A long overhang boat with narrow beam and deep fin keel that has a 33' waterline, say a Herreshoff design, will have the same calculated "hull speed" even though it is going to be a faster boat. Not at all useful.






I definitely agree. "Hull speed" has nothing to do with that and isn't helpful. Here's what IS helpful. We have a speed-power curve: boat speed through the water on one axis (usually horizontal; this is considered the independent variable) and power (really total hull resistance) on the other axis (usually vertical; this is considered the dependent variable).



So Bob Johnson generates a speed-power curve. The big boys would do model testing to build the curve. Bob may have estimated one based on experience and review of curves from similar hulls appropriately scaled. That's what I would have done in his place. He picked a boat speed under power he wanted to deliver and picked off the power for that speed. Adjustments for propeller efficiency, power train losses from cutlass bearing, line bearings if any, and transmission lead to an engine power requirement. Pull down the catalogs from the engine manufacturer(s) you prefer to work with and choose an engine based on cruising rpm. Pull maximum power from the engine specs and work backwards to thrust from the propeller, back to the speed-power curve and you find the maximum speed under power.



No "hull speed" anywhere in there.



Your IP 38 came with one of two engines, a 44 HP and a 50 HP depending on the year of manufacture (ignoring all the repowers with who knows what in there). You can look up the curve for fuel consumption. That curve may be fuel consumption against power or fuel consumption against rpm. If the latter somewhere there is a power against rpm curve. Regardless you have all you need, for a specific boat and a specific engine, to build your own curve for boat speed against fuel consumption. Now this is useful, especially with a known and confirmed fuel tank size.



No "hull speed" anywhere in there.


Now from experience we know that with some variation of engine models, cost sensitivity, willingness to operate in the dark, destination distance, availability of bailouts, etc. 1800 rpm is a sweet spot for range, cost, noise, vibration, heat, etc. Because of the shape of the speed-power curve (or just more experience) we know that if we're running late we can run up to 2200 or 2300 rpm, but 2500 or 2600 rpm won't get us in much faster but will greatly increase fuel cost and make life more unpleasant below. That isn't to say it doesn't make sense to run at 3000 rpm for a while from time to time to get engine lube oil up to temperature. This is a benefit to engine service life for high speed diesels.



Nothing about "hull speed" in there, and no shortcuts available by trying to stuff it in.



Sails are very powerful sources of propulsion. On your boat with newish sails on a close or beam reach, well trimmed, in a fresh breeze (say 20 kts) you should be able to break 8 kts through the water pretty consistently as long as you have not loaded the poor girl down too much. The speed is just where you sit on the speed-power curve for the power generated by the sails.






Correct, and nothing to do with hull speed.





Correct. That's the wavemaking resistance I talked about in my earlier dissertation.



Still not a wall or even a knee in the curve (called an inflection point).



You referenced cars. The amount of additional fuel necessary to run at 60 vice 55 is more than the amount of additional fuel necessary to run at 55 vice 50. It's a curve, not a point.





Maybe. Depends on the source of the data. It might be scaling effects (using an algorithm outside the range of applicability). It could be submarining of the model. Lots of common recreational hulls simply won't plane. They may fly, *grin* but they won't plane.





No, the speed-power curve for planing craft looks quite different. There is a very narrow speed range over which the curve tips over and looks more like the initial section. A well done set of curves will actually have two lines: one for the boat speeding up and the other for the boat slowing down. This is the measured description of the real-world case where for a nominal recreational boat you have to reach some speed (say 16 kts) to get on plane, but you can stay on plane at lower speeds (say 13 kts) with significant fuel economy. This is all (usually) for the hull without controllable appendages (engine trim or trim tabs). With plenty of time and resources you could build a family of speed-power curves for different engine trim or trim tab settings.



Aren't you glad you asked? *grin*





Good question, and yes it applies to sailboats. The hump is a function of wavemaking resistance. When the wavelength of the wave generated by the motion of the boat through the water is very close to the waterline length of the boat there is a regime in which the boat speed can increase dramatically with minor increases in power. Arguably this is hull speed. The problem is that the magic constant is different for every hull form, and indeed with the addition of appendages. That's why ships almost always have bow bulbs and many have stern bulbs. The bulb is designed to generate a wave that is out of phase with that generated by the hull. The reduction in potential energy transferred to the water level ("wavemaking") directly results in power savings and therefore fuel consumption. There have been some efforts using bulbs on smaller boats without enough significance to be worth the cost. The design cost alone is more than most naval architects focused on the recreational market can justify.
I think you are trying to over complicate things for your entertainment or to impress others. That donít impress me much.

Anyone who knows anything about boats knows that there are many more factors that affect boat speed than just the THS.

But the point is, everything else equal, the boat with the higher THS wins.

So yes, THS is a valuable formula, just not the only consideration.
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Old 26-09-2019, 09:57   #112
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serious discussion about electric power in large boats

Iíve not seen all boats of course, but I have never seen a boat that got is best fuel mileage at or near wide open throttle.
That is one of those Iíd have to see to believe.
Now the way a turbine is so inefficient at low power outputs, that could possibly be true for a turbine, but I find it hard to believe itís true for a Diesel. Itís one reason why Turbineís never made it in OTR trucking, partial power fuel efficiency is terrible.

I believe Auspicious is hung up on ďhull speedĒ not being a specific number of 1.34.
However I still say there is an actual speed at which point that to exceed that speed requires greater and greater power. Iím not hung up on it being 1.34 or 1.2, 1.5 or whatever, but itís easy to demonstrate that at some speed that the increase in drag is rather large with a small increase in speed.
The graphs he has linked to show it, so I assume his point is that there is no one set magic formula that you can apply to any hull from a canoe to a pickle fork hydroplane and determine where that huge drag increase occurs.

Iím fine with that, Iím not conducting a test flight, I donít have test points to hit, rough estimations are fine with me, cause if nothing else I believe that to be a number that is somewhat variable anyway, depending on sea conditions, currents and winds, and maybe even loading so a loosey goosey sorta close speed is good enough for me.
Iím not a racer, this isnít rocket science for me.

For what itís worth I can hit 8 kts, but itís on a beam reach with as he said 20+ kts of wind and Iím overpowered to do so.
But I can hit 7.5 kts in a lot less wind, without much heel and not be overpowered. To hit 8 just seems that everything is under a lot more stress than it is at 7.5, and that is only a half kt difference. So it would seem to me that the curve gets real steep at or a little past 7.5 kts.

7.5 kts is about my comfort limit, to get beyond that sacrifices comfort, and slowing down a half a kt and gaining comfort and not pushing the boat hard and feeling overpowered, is worth it to me.

So the classic number gives me 7.7 kts. All I know is I can make 7.5 in a relaxed stance but to make 8 means Iím pulling the guts out of it, heeled over and the rigging creaking etc. so somewhere between 7.5 and 8 that power vs drag curve gets steep.
I donít think 8.5 is achievable, or at least not by me in calm water.

So I think the discussion point is what it the definition of hull speed.
I donít say itís 1.34 times the square root of waterline length, but there is definitely a speed at which drag increases drastically.
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Old 26-09-2019, 10:16   #113
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Re: serious discussion about electric power in large boats

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I've read about that sometimes being the case on small sport cruisers and center consoles and so forth.

With ours, the faster we go, fuel consumption increases. It's a curve, with more RPMs = more GPH = not enough faster speed to make up for it.

FWIW.

-Chris
Yes, in all cases, more overall fuel consumption per minute/hour, etc, of course. But I was speaking of efficiency. So yes, enough faster speed to make up for it. Less consumption per kn so therefore less consumption per mile, and less total fuel consumption for the journey .
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Old 26-09-2019, 10:38   #114
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Re: serious discussion about electric power in large boats

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So I think the discussion point is what it the definition of hull speed.
I donít say itís 1.34 times the square root of waterline length, but there is definitely a speed at which drag increases drastically.
Yes, for a classic/common hull shape that doesn't plane that certainly seems to be the case, regardless of what the numbers might say.
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Old 26-09-2019, 11:40   #115
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Re: serious discussion about electric power in large boats

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Iíve not seen all boats of course, but I have never seen a boat that got is best fuel mileage at or near wide open throttle.

I learned long ago to "never say never" but I haven't seen anything like that either.



I can say with assurance that naval architects and marine engineers size propulsion to provide margin for things like heavy weather, foul bottoms, and generally keeping the engine at the power level (around 80% of full power) that leads to the longest service life. Most cruisers, like a64pilot and me, end up running somewhat slower than that for fuel economy. For most high speed diesels that ends up being 1800 rpm. For a big sportfish with huge twins that max at 3600 rpm, you may find yourself happier at 2500 or 3000 to keep the temps up and acid down because a few extra thousand hours between tear downs is worth it. For this community 1800 rpm is a good number. Fiddle with that and tally up your fuel receipts to see if the sweet spot on your boat is a bit higher or lower.



By the way, sail with a clean bottom. I have run numbers for my delivery customers a number of times to show that between my day rate and fuel costs they save money by paying for a diver. Even on my own boat when my time is not a financial factor (most of the time) paying a diver is worth it for a cruise of any consequence.



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I believe Auspicious is hung up on ďhull speedĒ not being a specific number of 1.34.

I'm an engineer. I am predisposed and trained for accuracy.



The point is that there is no actual speed. There is no one speed. There is a range over which fixed marginal increases in speed require greater and greater marginal increases in fuel consumption. That range is huge compared to the speed prospects of the boat. 20% or more of the available speed of the boat.



I'm still waiting for that 50 kW micro nuclear reactor to drive an electric motor that fits in my engine room. That will be a game changer.



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For what itís worth I can hit 8 kts, but itís on a beam reach with as he said 20+ kts of wind and Iím overpowered to do so.
But I can hit 7.5 kts in a lot less wind, without much heel and not be overpowered. To hit 8 just seems that everything is under a lot more stress than it is at 7.5, and that is only a half kt difference. So it would seem to me that the curve gets real steep at or a little past 7.5 kts.

All fine. I going to infer you have the swing keel version of the IP 38. You should be able to keep the boat on her feet with the keel down in 20 kts and still make 8+ kts without too much angst. Lots of people over trim their sails which slows the boat while increasing heel. That is unpleasant for everyone.



As a rule of thumb (and yes, I've been beating up on a rule of thumb) you should be able to get boat speed above half of true wind speed (not apparent) for wind speed above about 3 kts until you get about half way through the transition region of the speed-power curve. There is no excuse for cruisers to make people unhappy, including themselves, so given good sail trim just reef. That's a whole different thread that I'm sure has been beaten to death.



I have been tactician on boats that won races by reefing and getting the boat on her feet rather than burying the rail in the water.



I don't much like creaking either. Somewhere in there determinism goes out the window and subjectivity weighs in. Yes, I try to save my customers money by going as fast as we can, but not at an untenable risk to the boat. As I tell my customers, I spend your money like it is mine. So I get your point. IPs always have to think about shrouds and their anchors (did the 38 have the glassed in chain plates of the later boats?). You can really hear the boat working when she powers up.



Regardless, I like the older two digit IPs better than the three digit ones.


Quote:
Originally Posted by jmh2002 View Post
Yes, for a classic/common hull shape that doesn't plane that certainly seems to be the case, regardless of what the numbers might say.

There simply isn't a point. 20% of speed range is not a point and doesn't approximate a point.



If you don't believe the numbers then you might want to reconsider driving across bridges (built based on numbers) or riding in elevators (built based on numbers) or using any one of a number of other products whose design and manufacture is based on numbers.
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Old 26-09-2019, 12:39   #116
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Re: serious discussion about electric power in large boats

Ok, now I understand what your saying, no there isnít one single point at which the curve stays flat until that point and then goes vertical.

Of course there is a steepening curve, with these types of things there always is, and even when you think itís a fine point, when you zoom in so to speak youíll find that itís not.

However if you had to pick a specific number, I think 1.34 is pretty close for operational planning, yes you can sometimes go past it, but maybe when passage planning, best to not plan to.
Remember most of us have knot meters that in truth may not be calibrated to under a kt anyway, depending on how dirty it and the hull is etc.
You have heard measure it with a Mic, mark it with chalk, cut it with an axe, well most knot meters are that axe. Just because something will display decimals, doesnít mean itís accurate to that level.

No, my 38 is a fixed keel boat. Yes the chainplates are glassed in. I had allied titanium make me some out of grade 5 titanium and I had those installed. I donít think SS is a good metal choice for that application, but thatís just my opinion.

The older two digit IPís have a slightly deeper keel than the three digit ones and while no IP goes to windward well, I think the older ones do better then the newer ones, maybe in part due to the slightly deeper draft.
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Old 26-09-2019, 13:55   #117
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Re: serious discussion about electric power in large boats

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So yes, enough faster speed to make up for it.

That'd be great. Just happens we never see that.

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Old 26-09-2019, 14:12   #118
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Re: serious discussion about electric power in large boats

This whole discussion would have gone faster over adult beverages at Davis' Pub in Annapolis or somewhere similar in Albany GA. You need cocktail napkins to draw on.



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Of course there is a steepening curve, with these types of things there always is, and even when you think itís a fine point, when you zoom in so to speak youíll find that itís not.

Yes.


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However if you had to pick a specific number, I think 1.34 is pretty close for operational planning, yes you can sometimes go past it, but maybe when passage planning, best to not plan to.

Weeellll.... *grin* There are very few boats I would use anything close to 1.34 x root (LWL) for passage planning. I generally start with 125 to 150 miles per day depending on the size of the boat. I take a look at lines if I can find them; I usually can't. Then I go looking for polar charts. Lay the polars over pilot charts, which is really tedious so I cheat and use software called Visual Passage Planner. Then I derate that a bit for my plan. I usually come in a day or so early (depending on distance). Ocean currents of course make a difference, especially big ones like the Gulf Stream.



For your boat, short-handed (I assume) I'd generally plan on 110 miles per day, an average a bit under 5 kts. Then I plan for weather delays and the boat breaking. The big deal is provisioning. I generally assume I'll miss the next stop and then add a pad on top. That means leaving Southampton headed for the Azores I provision for Bermuda with a little extra. Just in case. I always press up tanks before heading offshore. I might burn three gallons of fuel from Annapolis to Norfolk (Little Creek) but I still stop for fuel and water before heading offshore to Bahamas or the Caribbean. Too much fuel, too much water, or too much food are self correcting problems.



Quote:
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The older two digit IPís have a slightly deeper keel than the three digit ones and while no IP goes to windward well, I think the older ones do better then the newer ones, maybe in part due to the slightly deeper draft.

Bob Johnson has/had a very firm grip on his market. The shallow draft of his Full Foil (tm) keel has a big impact on sailing performance but most of his customers don't care. They like the draft and mostly motor anyway. It isn't right for me (and an IP 370 was on my short list before I bought Auspicious) but there are a lot of people it is right for.



There is no question that the two-digit boats sail better than the three-digit boats. With a clean bottom, a tuned rig, and a backstay adjuster your IP 38 could do pretty darn well.



I've met Bob Johnson a few times. Bill Bolin was the IP guy I know best. I've been to the factory at least three times, all before the sale. I think pretty highly of the company Bob built.
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Old 26-09-2019, 14:37   #119
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Re: serious discussion about electric power in large boats

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...I need cruising range so I would have to add a diesel generator to the electric system...
Just to clarify, skipmac, you mean you need cruising range under power, right?

Because there are many cruising boats with unlimited range, without any engine.
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Old 27-09-2019, 03:45   #120
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Re: serious discussion about electric power in large boats

It's all about energy and storage.

First question to be asked is what is your cruising speed and how many kW you need to sustain it in average (to overcome the resistance of the water, waves and wind).

Second question would be what is the range you need to cover without refilling. (to calculate the necessary energy capacity in storage)

Third question to be asked is, how long is the time you will tollerate a stay for re-fuelling at the dock or at an anchorage.

A 15kWh LFP battery weights 150kg or 0.1kWh per kg.
Diesel has 9.8kWh/l and one liter weights 0.84kg, so 1kg of diesel has 11.66kWh energy stored. It is 116 times more energy per kg than an efficient lithium battery.

No matter how efficient electric motors are and how inefficient combustion engines, diesel will beat electric any time at the moment regarding range per kilogramm. Also refuelling 2kg of diesel is much faster than charging 15kWh lithium, even at 3C you will need 45kW connection and 20 minutes, whie 2l get in in 2 seconds. The storage of 15kWh in LFP costs 5000$, 2kg diesel can be stored in a small jerry can for 5$.

Recuperation of 300Wh/h gives you in 24h sailing "at hull speed" about 7.2kWh, so 0.8l Diesel. To fill your diesel tank of 200l you must sail non stop the whole year in the roaring 40's.

Electric only propulsion is a nice toy, but not for serious sailing under power.
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