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Old 09-11-2005, 13:36   #31
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We swing a 22" 3 blade max prop through a BW Velvet Drive 2.81 reduction powered by a W108C6 at 2600 RPM pushing 60k displacement on 55 foot of waterline and burn 1.75 gallons per hour in flat water at 8.2 knots. So 4.68 MPG.

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Old 09-11-2005, 20:57   #32
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Ummmm, sorry to disagree Mike, but no it won't. If the hull is below it's max hull speed, then adding the 4000lbs will not increase fuel consumption. A hull 4000lb's heavier may perform better with more HP. But that's because you need HP to move and control the movement of mass. Hence why the rule of thumb for required power is so varied. Wind and wave influence are the biggest factors in soaking up power.
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Old 09-11-2005, 22:11   #33
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HP

If you need more horsepower then you need more fuel. Horsepower costs. All efficiencies being equal. Boats slow down as you add weight, so you need more power ( horsepower ) to drive them, whether it be sails or engine. Look at all the graphs for determining power for a boat. Weight is always a factor. I will agree that this extra fuel required would be difficult to measure, but in my mind there is no way you can move 4000 pounds through the water with out incurring any energy expended. Mine was just an example to illustrate a point raised earlier. I even gave an illustration about the effect of waves on two similar but different boats.
Actually I was hoping others might have some thoughts on this.
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Old 10-11-2005, 10:59   #34
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Mike and Wheels are both correct

I just may not have been very clear. Mike, as you pointed out, increased mass will lead to the boat being deeper in the water which will increase wetted surface area. In addition, having to move that water out of the way, even though that energy is not total loss as Wheels points out, translates to "friction" in terms of power loss.

Now, again, my point is that the hull design has the most significance to the horsepower requirement than a direct one-to-one significance of mass to horsepower (and fuel consumption).

I think that Wheels before pointed out another very significant factor which is just how one might compare various horsepower engines and looking at their fuel consumption to "choose" one which might, in a particular case, give the best fuel economy.

Note, for example, that for two identical automobiles traveling at 60 mph one having an 80 hp engine will probably get less economy than the other one having a 120 hp engine as long as the larger engine winds up (at that particular speed) loafing along at close to the peak point of its efficiency curve compared to the one with the smaller engine.

In my opinion it does no good to design a cruising displacement sailboat to use an engine to get better fuel economy at 3.5 kts when one should design for the best fuel economy at, say 6 kts a more desirable speed to make safe harbor ahead of the gale. It is worth the extra money.

Unlike what the Pardeys propose, I believe that a good engine is a safety item to be cared for as much as any item necessary to survive what the ocean can throw at you.

Because the world's waterways and oceans are heavily populated with various vessels it is not safe to either you or me to be without an engine. I for one, do not want to see the Pardeys attempt to come into a populated anchorage where I am anchored because they have no engine. It was fine at the time of Columbus. Welcome to reality without denial. Heck I've been through the Panama Canal where they would not let anyone transit unless they declared that they could make 6 kts so as to not slow down the process of locking at both ends.
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Old 10-11-2005, 11:48   #35
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Not sure what the word "pardeys" means , but I presume it refers to sailboats with no engine and relying on wind only???
Anyway's, even back in the ole days, Traditional sailing vessels never sailed up onto a dock. They were towed into position by the "Tug" boat which comprised of men with oars.

Mike, I 100% agree with you on several of your points. Especially the HP to fuel point.

Let me go a little further with the other converstion. I am no expert, but I do understand a little on hydrodynamics. Maybe just enough to be dangerouse.
When the hull is in "true" displacement mode, there is very little resistance to movement throught the water. Apart from the friction of the water surface directly on the hull. Even then, the thin layer of water touching the hull acts as a "slip coat" to the surrounding water, as the thin layer remains stationary, "attached" to the hull, and the outer surface moves past it and helps to reduce friction. As the hull speed increases, the friction between this water boundry also increases.
The other aspect of hydrodynamic resistance, is the impact of motion versus mass. The hull shape, especially at the bow, has a big impact. The water is forced to change direction because of the bow and move around the hull. At a low speed, the fluid effect (easier than writing fluididity ) is such that the mass of the water can "bend" around the hull shape and fill the void that appears at the stern instantly. But as speed increases, two forces are produced, or at least, accentuated. The waters mass begins to overcome the fluid effect of the water and wants to carry on in the direction it was just forced to go on. The other effect is a hydraulic force. Water can't be compressed. So the starts to create a pressure wave. We see this energy being sent out away from the hull as the wake. Plus the resistance to movement is also seen as warter riding up the face of the bow. This takes energy and all that HP is basically being turned into that wake. Just like a person throwing a bucket full of water, the further the water is thrown and the greater the volume of water thrown, the more energy it requires. The hull is "throwing" water and it takes energy to do so.
But as to weight of the hull and HP to move it, it is all about inertia. Being able to get the mass moving and controlling it. Look a Joli's post. A bigger boat than mine, twice as much weight as mine, has a faster Hull speed than mine and yet less HP than mine and as a result, uses less Fuel than mine. But I bet my boat is more responsive that theirs. Only reason why I have such a big engine, was that the original owner wanted a Launch. I feel like I am driving a fishing boat more than a sailboat when under engine power.
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Old 10-11-2005, 11:52   #36
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xort once whispered in the wind:
If I follow the numbers correctly here's what I've got so far...

Jon D with a 47' boat gets 1.5 GPH at 7.25 K for approx 4.7 NMPG

Vasco with a 39' boat gets .75 GPH at 6.5 K for aprox 8.6 NMPG

Delmarrey with a 40' boat gets .5 GPH at 7 K for aprox 14 NMPG

Irwinsailor with a ?' boat gets 2 GPH at 7.5 K for aprox 3.75 NMPG

Rick with a 40' boat gets 11 NMPG

One problem here! Boats can not be measured in NMPG due to currents and wind facters.

e.g. I can actually motor sail (motor and sail) at around 8.5 knots in a 10 kt breese out in open water. But if I motor fully open againist the current in some areas here (Puget Sound) I may only make 2-3 kt. headway. Like wise I could make 10 kt. going with the current. And this is all using the GPS for SOG.
There have been times were I was just putting along and discovered I was going backwards (by the GPS)........................_/)
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Old 10-11-2005, 12:16   #37
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Alan just a note

This is the Pardey's (link below)
Pardey's

As for the Panama Canal, I believe they have kicked that up to 6.5 kts now. No time to research, have to go to work!
http://www.pmbc.net/
Bye......................._/)
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Old 10-11-2005, 15:16   #38
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A question asked was what other info did I have. I was surfing Catalina's website and found a 44', 25,000lb boat burning 3.7GPH at 8.5K. That is 2.29 NMPG which seems quite low for an efficient sailboat hull. I have more knowledge of trawlers with relatively bad hull forms for efficiency and they are getting 1 to 2 NMPG. That's with a single 120HP Lehman. So the Catalina number surprised me and I went back to Charlie Wings book. His numbers were from a survey of 200 people and averaged above 8 NMPG.
The survey here so far has ranged from 4 to 14 NMPG.
I do understand that wind, current and waves will have an impact on all this. Hills and rough roads don't stop the EPA from giving estimates.
In the range I'm talking, 40' to 44', there isn't that huge a difference in displacement and HP...relative to boats much bigger or much smaller or to trawlers or other power boats. So I think a range shouldn't be to difficult to ascertain. I'm not looking for an exact, to the decimal point number, just a range compared to other types of boats. Some of you are trying to make this out to be much more complex than I'm asking.

To those of you who actually came up with numbers, thank you very much.
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Old 10-11-2005, 15:50   #39
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The numbers

From an old book, so drag ( resistance ) is not as great and fuel consumed is not as great. Modern foils recapture drag better than old foils so have less total drag.
For rough estimates use .6 pounds of fuel per horsepower per hour. Fuel consumption is based on horsepower, horsepower is based on resistance, resistance is based on displacement. Resistance is also calculated at a given speed length ratio.
Resistance is 45 pounds per long ton.
Lets use my boat at 7400 pounds and 25 feet LWL at six knots.
7400 is 3.3 long tons, times resistance, X 45 = 148.5
Horsepower is calculated resistance X speed X .003.
148.5 X 6 X .003 = 2.673. Now a factor must be plugged in for gearbox and prop efficiency. The book suggests 35%.
2.673 divided by .35 = 7.6 hp
So they are saying I need 7.6 hp to go 6 knots in smooth water. Fuel consumed .6 X 7.6 = 4.56 pounds per hour which is about 1/2 an imperial gallon or about 12 miles per gallon, and I told you earlier that I get about 20 mpg.
So, my engine is more efficient and my boat does not have a smuch drag as the book suggests.
Using the same formula as above, if I add 1000 pounds to the boat ( inside ) I need about one more hp to drive it and .6 pound of more fuel per hour.
We all know we need more giddyup to pound in to waves and currents.
David Gerr " The Nature of Boats " and " Skenes Elements of Yacht Design " will have the above info, and Frank Bethwaites " High Performance Sailing " will explain recaptured drag of a foil.
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Old 10-11-2005, 16:40   #40
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something must be wrong with your numbers to get 12 by the formula and 20 by your usage estimates.

And do you have a 7.6 hp engine? If you have a bigger engine then you will consume even more fuel via the wasted inneficiency of the larger than theoretically necessary engine.
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Old 10-11-2005, 17:34   #41
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The book

I have a 2QM15 Yanmar.
The book is old. My engine and boat are more efficient than the book numbers. My fuel guage may not be accurate. It was a long time ago that I did the estimate. Could be I am not getting as much as I thought. Also can not remember which prop I had on at the time.
The book is theory to give an estimate. I prefer a practical test. I will check again sometime by filling the tank to the top, motoring for a known distance, and then refill.
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Old 10-11-2005, 21:40   #42
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planing sail boat

The 44' Catalina is probably being at or real close to hull speed to do 8.5knots. It depends on the waterline and efficiency of the hull but as speed goes up, the power/fuel to make it go faster goes up. I wouldn't be at all surprized that pushing a 25,000# boat to hull speed would result in 3.7gph consumption. Cut that back to 6 or 7 knots and the fuel consumption would probably drop drastically.

Nautical miles over the bottom per gallon will vary greatly depending on wind, current and sea state. All you can do is find the most efficient no wind, smooth water speed and the rpm that gives it. Then try and power at that rpm for most condtions. Sometimes you may have to use more rpm, sometimes less but at least hanging around that rpm will give you some idea of how much fuel you are burning over time.

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Old 11-11-2005, 06:53   #43
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After reading the mileage others are getting with their boats I must have a fuel hog. My new (2003) Westerbeke gets the mileage that is stated in the owners manual and that is 2 gallons per hour at 2500 RPM. Our boat is 56' on deck and 32 tons. Are the Westerbeke (Mazda) diesels known as heavy fuel users?
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Old 11-11-2005, 09:35   #44
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After reading the mileage others are getting with their boats I must have a fuel hog. My new (2003) Westerbeke gets the mileage that is stated in the owners manual and that is 2 gallons per hour at 2500 RPM. Our boat is 56' on deck and 32 tons. Are the Westerbeke (Mazda) diesels known as heavy fuel users?
If I run the Perkins 4.236 [85 HP] at 2200 RPM or so I am also burning close to 2 Gal per hour. But I am really pushing water and going at hull speed. I typically run in the 1400 - 1600 or so range which lowers cruise to 7.25 or so and is much more fuel efficient
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Old 12-11-2005, 06:57   #45
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Mikes numbers are close. A diesel will burn 6.7 pounds (1 gallon) per 16.75 Hp used per hour or 4/10 of one pound per Hp per hour.

Lots of things go into calculating the fuel used. S/L ratio ( how hard you push the boat), the bottom condition, the correct prop, length, weight, sea conditions, accesories,.......

Irwin Sailor, yea you probably burn 2 gallons per hour easily.

This stuff is just math.
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