Fuel Consumption and Low Speeds

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rjw
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Fuel Consumption and Low Speeds

Post by rjw »

I'm considering building a houseboat - either a Waterlodge or, perhaps, a Huck Finn pontoon towed behind a 15' tri-hull.

If I do, I plan to travel a lot, but fuel consumption and cost are a major concern.

How will cruising at very low speeds (say, 5 knots or so) affect fuel consumption?


Thanks

Bob

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Bill Edmundson
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Re: Fuel Consumption and Low Speeds

Post by Bill Edmundson »

Bob

Very difficult to answer. Assuming your boat has a planning hull, it is most efficient just above planning speed. At 24 mph you might get 4 mpg. that's 6 gal/hr. to go 24 miles. At a little over idle speed it might only take 2 gal/hr. But, you've only gone 6 miles. To go the same 24 miles will take 8 gal.

Your mileage may very. Also, if you're going to tow I'd go to a flatter pitch prop.

Bill
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galamb
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Re: Fuel Consumption and Low Speeds

Post by galamb »

Maybe I'm missing what you are asking here.

Are you talking about the powerplant on the boat you plan to build (so cruising that at 5 mph).

(confused because) You mention towing this (new) boat behind a 15' Tri-Hull (so towing at 5 mph???)

If you are talking about cruising with your houseboat at 5 mph your consumption will be "initially" dependent on what you power it with.

A small diesel inboard may be the most fuel efficient.

If you wanted outboard power, a 4 stroke kept below 3000 rpms will, on a horse for horse basis, use significantly less fuel that a 2 stroke outboard (but after 3000 rpms give or take a bit they suck "exactly the same" regardless of what the glossy adds tell you).

Regardless of what you choose, how much fuel you burn will be related to speed (ok, stated 5 mph), the total weight you are trying to push, how much hull is in contact with the water, the shape of the hull that is in contact with the water and how slippery/smooth that surface is.
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Bill Edmundson
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Re: Fuel Consumption and Low Speeds

Post by Bill Edmundson »

After some thought, I think the most efficient thing would be a 10 hp High Thrust O.B. on the house boat and tow the Tri-Hull.

Bill
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kens
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Re: Fuel Consumption and Low Speeds

Post by kens »

The most fuel efficient is a true 'displacement' hull. These are good for approx 5kts.
A pontoon,or, tri hull, doesnt make sense to me, (perhaps I am closed minded)
Think about the TOTAL wetted bottom of a (pontoon) or tri hull and add it all up as an aggregate of all the hulls.
Could you not build a single displacement hull with the same wetted bottom as a pontoon?

Think about 3 individual hulls, each is 1/3 the size of a single whole displacement standard hull.

There is another number seldom mentioned. The displacement/length ratio. This means that a long-narrow hull is most efficient.
If it is a wide floorspace you want, then get a short wide boat. (pontoon)
If efficiency is what you want, then get something 5 times as long as it is wide.
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rjw
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Re: Fuel Consumption and Low Speeds

Post by rjw »

Thanks to all for the discussions and insights

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