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Old 25-03-2013, 08:40   #1
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Unhappy "Natural Speed" for EP consideration

I read a book many years ago that gave a rough formula for the natural speed of a boat.

Simply put, this is the speed at which a boat can move through the water with very little effective friction. A speed at which the boat, once moving, will continue to move with almost no effort(according to the author).

The formula computed a speed of something like 2 knots or so for my Venture 25.

Now I have a Newport 28 and I want to go electric for propulsion.

If I can figure out the "natural speed", for my boat, that will be my target speed for slow, leisurely travel when the duldrums hit.

Higher speeds will be used only as needed . . . not to make the next bridge or a supper date.

I will still have to figure wattage/amps used at higher speeds and will remain cognizant of that effect on my range.

But, for those times when I am willing to travel at my "natural speed", my range will be very substantially increased . . . maybe to the point where a solar panel will actually supply more than needed wattage.

My comparison to what "natural speed", occurred with my Venture 25, having found myself with only a foresail and about 5 miles downwind(a light wind), from my dock, many years ago. My battery was an old, near-dead car battery. When I think back, I often feel that the natural speed could be best described as a speed at which the boat will generally coast, a minute or so after power is turned off.

Anyway . . . even though at times, I could actually count the revolutions, I got back to the dock. I actually had to turn off my nav lights after the lights drew too much power from the battery.

So, for me to go to electric propulsion, I've decided that when the wind dies and will no longer move me . . . that I'll be satisfied with the approximately 2 knots that will take the absoluteness of minimal power.

But now, try as I might, I cannot find the formula for "natural speed".
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Old 25-03-2013, 09:10   #2
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Re: "Natural Speed" for EP consideration

The proper term is called "hull speed". A hull moving in water generates a bow wave that acts as resistance. For non planing hulls that force increases with engine power to a point where you can't go any faster by adding more power. The prop spins faster but you don't go faster. It's the limit of how fast you can go not how fast you can go.

The formula is Hull speed in Knots = 1.34 times the square root of the length of water line

Its a generic formula that could be refined a little based on hull type but in your case it's good enough.

This really has nothing to do with using electric propulsion. You really can't set up a Newport 28 to do hull speed electrically for very long. Reaching hull speed isn't that practical. Getting in and out of marinas to waters you can sail is very practical. Covering longer distances really isn't. You would need a huge battery bank that would require many days to recharge.
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Old 25-03-2013, 09:33   #3
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Re: "Natural Speed" for EP consideration

Hull speed is more like the practical limit for a displacement boat, and is useful for comparing designs. Power requirements for a given boat are close to the boat speed cubed--it will take 8 times as much power to go 4 knots as it does to go 2 knots. Put a different way, it will take twice as much energy to go a mile at 4 knots as it does to go a mile at 2 knots.

All this assumes you are in flat water with no wind.
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Old 25-03-2013, 09:41   #4
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Re: "Natural Speed" for EP consideration

Never heard of the natural speed formula - but sounds like it is meant to be different to (max) hull speed (no point trying to overcome it with a yacht - even if you can bulldoze an extra knot(?) out of her).

Perhaps it is what (I!) would describe as the "minimal effort speed" - likely most yachts could do a couple of knots at little more than tickover (slow Marina Speed) no matter the size of the engine (the engine size is not yet important?), and perhaps some boats a touch more speed than others?........could also be pretty much the speed you could easily enough maintain with a scull? (duration of own puff being a seperate matter!).

Whilst I agree with Pblais that travelling at hull speed under battery power alone (even with solar) would not be practicable for long distances, nonetheless with a sailing boat that can be a minor consideration - depending on how you use the boat (few "accidently" find themselves needing to motor at hull speed for 50 miles - that's part of the passage plan. "Need" being different from "more convenient"!)......with regard to "refueling" (with electric!) taking a couple of days, whether that is a constraint depends on boat use (no long passage under power means batteries not empty - and even if they are, if you are not actually going anywhere else for a few days does it really matter that the batteries take a while to recharge?).

As mentioned before, I have one eye on EP as well - my thinking is that I will include a generator onboard to extend the range, but with the hope that it acts only as a comfort blanket until I get comfortable with EP (pros and cons).

My overall take on EP is that it also takes a change of mindset as well as engine fuel!.....and that it won't suit everyone (Diesel still the best compromise / covers more requirements), particularly those who use the yacht to motorsail for long distance or as a motorboat (no criticism meant by that - "your" boat, so use it as you see fit ).....but if you only ever do less than 50 miles in a couple of days under power perhaps now worth at least half a look? (especially if you have a sudden need for a new motor!).
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Old 25-03-2013, 09:54   #5
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Re: "Natural Speed" for EP consideration

"natural speed" is not any sort of official or regularly used term.

For "efficient motoring" just simply the SQRT(waterline length) is often used as a target speed (in kts). . . . .slow enough to not burn too much energy but fast enough to actually get somewhere. There is a little physics behind that, but it's really more just a rule of thumb.

I believe your Newport has a waterline of 23.5ft, so the sqrt of that would give 4.8kts.

Even slower would still be more energy efficient. But as you drop down in power level, you will somewhere get to a power level where you will not be able to make progress if there are any waves or any wind forward of the beam.
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Old 25-03-2013, 11:40   #6
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Re: "Natural Speed" for EP consideration

This "natural speed", was a term the author used and it has absolutely nothing to do with Hull Speed.

It was a relatively simple formula that was based on wetted area, type of hull, the depth, and type of keel, the length at water line and the beam at the waterline. The author had devised a rough number to replace the coefficient of friction in water.

The formulation eventually gave a speed at which the boat would move before the graph of the power versus speed would begin it's steep incline.

The "natural speed" of my Venture 25(MacGregor), was about 2 knots. Other boats in the examples were all about 2 knots with the lighter boats about 3 knots and heavier boats about 1-1/2 to 2 knots.

Anyone who's pushed boats by hand, knows that there's a speed at which, when the boat reaches it, it takes almost no effort to keep the boat moving . . . this is the speed I'm seeking.

Waves and air speed are not part of the computation, and shouldn't be, since I'm interested in a speed when there's virtually no wind and therefore little wave action.

Even British warships could be moved by men in a lifeboat rowing...
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Old 25-03-2013, 12:17   #7
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Re: "Natural Speed" for EP consideration

Perhaps you are referring to the formula for Froude Numbers?
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Old 25-03-2013, 12:28   #8
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Re: "Natural Speed" for EP consideration

The slower you go, the further you will get on one tank. Off course, you get there late, but so what.

Now I would have to see the efficiency curve for that electric motor to see if this holds true for your case. Peukert's law must be followed at the same time (your tank is the battery, unless you are driving a hybrid) - here again GO SLOW, go far.

You can get a sniff of things by looking up HR's website data for their smallest boat - they give the max millage per tank / engine rpm (=boat speed) there.

Go slow, go far, go whenever.

Cheers,
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Old 25-03-2013, 12:43   #9
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Re: "Natural Speed" for EP consideration

Quote:
Originally Posted by SURV69 View Post

The formulation eventually gave a speed at which the boat would move before the graph of the power versus speed would begin it's steep incline.
The graph is just a simple square function.

Drag=.5 p V^2 C A

Where p= fluid density
V = velocity
A = cross section area
C = Drag coeffecient

The point is that there is NOT a sharp inflection point that is going to pop out as the obvious speed. Less velocity is less drag is less power consumed. Slower is better. You simply have to pick the trade-off point between power consumed and speed on a smooth curve that suits you.
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Old 25-03-2013, 14:30   #10
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Re: "Natural Speed" for EP consideration

Aside from the shape of the graph that dictates the power required vs. the speed attained, one may consider inputs like prop efficiency (the prop is likely to be optimised for one power/speed combo only). Next comes the efficiency of the engine (some engines may be more efficient at some power point than at other). And in case of electric motor, the efficiency of the power source's rate of discharge. These are just examples, not a full list.

The ultimate 'best speed' will be a combo resulting from optimization of functions (looking for minima of each factor).

So to say, it is 'slower further', but there may be, theoretically, some cases when max range will be attained at a speed different than the minimum speed.

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Old 25-03-2013, 14:37   #11
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Re: "Natural Speed" for EP consideration

^^

Agreed. The specific propulsion system may have an optimum point.

But that does not seem to be what the OP is talking about. He seems to be talking about a drag inflection point for the vessel (way under the "wave making speed" that creates the "hull speed" inflection point).

One thing that's not been mentioned that's a bit relevant is that it takes much more power to accelerate a vessel (to say 2kts) than it takes to maintain it at 2kts. That's explains why when you push the boat by hand, once you get it moving, it seems to take so little further power.
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Old 25-03-2013, 15:06   #12
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Re: "Natural Speed" for EP consideration

Exact. And the sooner (in shorter time) one wants to achieve that target speed the more power required. Just look at F1 cars taking off.

But I am not sure if the total energy used to accelerate to speed X over time T=1 versus over time T=2, differs or not (?). Assuming no slippage and all other factors equal.

Energy can't be lost, so unless the prop has lost its grip on the water the related energy consumption amounts should equal?

In any case, here too: get to target speed sooner ('faster') will probably mean some loss. I guess accelerating in small increments makes for minimum power losses.

Not only go slow, but also 'accelerate slowly' ;-)

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