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Old 08-11-2005, 14:00   #16
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Fuel

Just to add to this. I get about 20 miles per imperial gallon at about 6 knots. 29 foot boat LOA, 25 feet LWL, 7400 pounds with a 2QM15 Yanmar and two bladed geared folding prop.
I have not measured this accurately but I think I am close enough.
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Old 08-11-2005, 16:59   #17
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At one point, returning from Hawaii, we got the burn rate down to 7 usg / Day. Light boat, 3 cyl Yanmar, glass water at about 4kn. This works out to about 14 kpg.

But what we saved in fuel was more than made up for from the poor befouled little engine. Carboned up the exhaust solid.

Running 6-6.5kn? I'd guess maybe 5.5 kpg?

Now the old jetboat, running a 455 Olds with dual carbs could drain its tanks in about 15-20 minutes. 22 gallons, 15 minutes @ 60 = 15 miles for 22 gallons.. 'bout 0.7 mpg.

It was one heck of a ride though!

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Old 08-11-2005, 20:21   #18
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My last cruising boat was 35 ft with a Yanmar 3QM30, 3-blade, and consistently got 8 to 8.5 nautical miles per US gal. The current boat is 40 ft, with a Yanmar 4JHE, 3-blade feathering, and consistently gets 11 nautical mpg.

I don't understand that there is necessarily a correlation between fuel economy and displacement because displacement should only affect acceleration, not how easily the hull is driven. The prismatic coefficient is an indication of how easily the hull is driven.
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Old 08-11-2005, 21:02   #19
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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

In the range I'm looking at we have about 4 to 14 NMPG.

Thanks to those posts that gave pertinent data relevant to my question.

Any other sailors with 40' to 45' sailboats with fuel consumption figures?
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Old 09-11-2005, 01:03   #20
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Arrrr, I think I see the mao you are trying to draw here, and you aren't going to get the picture this way. Fuel consumption is not something that can be acuratley measured by boat size and weight. For a Deisel engine, Fuel consumption is a direct corrilation to HP. Give or take small differences in efficiency between one particular design and another particular design, A metered dose of fuel produces a set and measurable amount of power when burn't. Increasing the measured dose, increases the amount of power. I need to look up my books as my memory is a bit fuzzy, but it is so many ml's of fuel per HP. So an engine of a given HP should be burning a given amount of fuel. Let me explain this another way. You could have a 100HP engine burning say 1Gal/hr at full power. (That is just a figure plucked out of the air) A 200HP engine may require say 3Gal/hr at full power, but as it passes through the 100HP point, it will also be using ruffly 1Gal/hr.
This now show's you, that when you are a knot short of hull speed, your engine is not using all it's HP. The 100HP engine may be only using say 80HP, so fuel consumption is low. But to gain that one extra knot, your fuel consumption goes up another 20%. So there is great difficulty in anyone saying accurately that there boat does so and so speed. They may or may not be close to Hull speed and two boats identical could have very differing fuel usages just because on skipper pushes his hull at 6.5knots and the other at 7knts. It can and will make a hugemoungouse difference in fuel consumption between those two speeds, if the max hull speed is 8knts.
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Old 09-11-2005, 01:08   #21
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Sorry, I failed to proof read that last post. Mao should read as Map.

And down near the bottom, I wrote "so there is great difficulty in anyone saying accurately that there boat does so and so speed" should read,

So there is great difficulty in anyone saying their boat uses so and so much fuel.
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Old 09-11-2005, 06:07   #22
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Pursuant to Alan's point, the service manual for my Yanmar 2QM15 gives the following information about its fuel consumption: 14 HP at 3000 RPM = 2.6 liters per hour (continuous operation for 10 hours per day). Presumably the rate of fuel consumption will be proportionally (although not necessarily linearly) less at lower revs.

How fast my boat can go with the engine operating at 3000 rpm or 2500 rpm or whatever, and thus how many miles I can cover per unit of fuel, will depend on hull form, windage, sea state, propellor efficiency, etc..., while the fuel consumption should essentially be a function of the revs...

Regards,

Tim
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Old 09-11-2005, 06:21   #23
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Hope this is helpful, I just skimmed the posts.

My boat is 38 LOA, 31 LWL, 22,000lbs displacement. 3 blade fixed prop.

Engine is Perkins 4-108.

I burn approx 0.78 gph at 6.5 knots.

Hope this helps

John
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ps - I think your best comparison would be by displacement (mass) and LWL (friction).
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Old 09-11-2005, 06:22   #24
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pss - 8.33 nmpg

And engine is 47 hp
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Old 09-11-2005, 09:25   #25
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Mass

Yes as stated larger mass / displacement requires more power to push so more fuel is being used.
But that is a very general statement. If we think in terms of sails, sail area to displacement shows us that more hp from the sails is required to drive a larger boat. Two boats could be the same length but a heavier one will require more power to drive at a constant speed. Again a generalised statement.
I powered alongside a US 30 which is about the same size as my boat but a bit lighter. He used less power in flat water, and more power than me when going through waves.
I still get about 20 mpg ( big gallon ) and used about 10 gallons this year.
Michael
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Old 09-11-2005, 11:05   #26
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Mass and displacement

I still have a problem with equating the amount of horsepower and fuel consumption with the mass (displacement) of a hull. Once the mass of the hull has been accelerated to a constant speed there is NO more energy required to keep it at that speed forever!

If, however, one relates the amount of wetted surface area to the displacement (mass of water displaced) THEN I can agree. Because the shape of the hull greatly alters how the wetted surface area is presented to the water according to displacement a design factor, called prismatic coefficient, relates this to how much horsepower is required to drive a hull towards its designed "hull speed".

Now obviously the windage above the water also affects the results. So, it is the friction above the water (windage) and the friction below the water ALONE which would determine how much power is required to drive at a particular speed. Therefore, the mass or displacement is not a direct, only indirect, factor.
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Old 09-11-2005, 11:10   #27
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added note

The reason that I bring up the displacement question is because there are many boats of the same displacement which demonstrate a huge difference in fuel economy at the same driven speed. Like Wheels points out it is the other factors which will affect a particular case of just what will be the fuel economy at a particular hull speed.
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Old 09-11-2005, 11:17   #28
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Mass

More weight of the boat means more water being moved out of the way, which means more energy being used, which means more fuel being used.
Some boats may have more or less drag and or prop efficiency.
Your reasoning works for two motorcycles of the same overall size with one being heavier than the other. The heavier bike requires more hp to set it in motion and to change speed once in motion. If the windage of both were the same then the fuel consumed would be about the same at a constant speed.
But with boats the " windage " which is water being moved out of the way, is greater for the heavier boat because there is more of the heavier boat in the water.
Of course we have to assume boats of a similar design shape.
But if I put 2000 pounds of weight in my boat I will need more fuel to move it at a constant 6 knots.
At least that's the way I figure it.
Michael
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Old 09-11-2005, 11:45   #29
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No Mike, Ricks correct. This is why a person can lean on the mooring rope of a 10,000 T ship at the dock and move it toward them. A hull moving through water in a full displacement mode has two opposing forces that cancel each other out. Now Jeff would probably come up with the correct scientific terms for this.
The force on the bow as it tries to "part" the water is opposed by the same force seen on the stern as the water tries to "pinch" the stern forward. Because that same force is seen equally all round the hull and under it. This is why the boat floats. If the force was unequal between front and rear, then the boat would be in perpetual motion forward. Once the hull reaches displacement mode, the attitude of the hull is changed and continuing to change as the speed increases, due to hydraulic force of the water lifting the bow. The stern is losing the opposing water force and thus the ability to negate the force exerted on the bow. Above the displacment speed, many other forces come into play and it's why the power required to plane a hull is not simply double, but it is more like exponetial to the size of hull, weight, and speed.
Now I apolagise for the above as not being very scientific, maybe not entirely accurate and maybe difficult to translate. But hopefully it may show the readers thatthis isn't a simple case of Mass in the water.
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Old 09-11-2005, 12:49   #30
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Power

If heavier boats do not require more power to drive them as they get heavier, then why do they have bigger motors.
Lets use a simple example. A boat weighs 12000 pounds and is uses X amount of fuel at 6 knots. We add 4000 pounds of weight to the boat ( put it inside ). I am saying that this boat will now consume more fuel at the same speed in the same water.
I realise there is drag and wetted surface to consider, but heavier boats have more wetted surface.
So moving water out of the way, and increased wetted surface is going to result in more fuel being used.
My theory and I am sticking to it until the rocket scientists show me I am wrong.
I am familiar with planing hulls, power and sail, sold heeps of them, sailed many.
Michael
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