New Guy needs advise

[belizebill]

I live on Caye Caulker, Bill

[Andy G]

Hi Belizebill, for what its worth I have an IG 36 with twin Lehman 120's and I think 4gall/hour is about right. I used to have a 34 Clipper with single 120 Lehman and found both boats cruised at the same 8knots on 1800rpm, but the single def used less fuel.The Clipper had a bow thruster which for me was a must and it was great in all but really strong cross winds.

On balance I like the twin engine set up because of the greater control it gives(me anyway) in close quarter manouvering in strong winds.

I would not worry to much about the fuel use, IMHO it is far more important to ensure the boat and engine are in good condition, get that wrong and you can be looking at some big bills,find a good surveyor, it's money well spent.

Good luck.

[Carey]

FF wrote:

Usually the rating of the engines will show the style of cruising service the boat was designed for.

A 36 ft boat with 100 hp is going to be really cheap at the Sq RT of the lwl, say 6k for LRC.

Hull speed will double or more , the burn for an extra K or 2.

If the 36 ft boat boasts 350 hp, it was designed as an inshore weekender , the speed will be double the small engined boat 15K but the fuel burn will be 10-15 times higher.

On unusual boats , where you cant simply ask owners , a quick math will work.

Find the vessels displacement , that is how much it weighs (not the USCG "tons" which is volume).

Divide the weight in pounds by 2240 for the displacement in tons.

Slow cruise will cost 2 or 3 hp per ton, so multiply the tons by 3.

Then divide that number by 16 for a diesel or 12 for gas.

This will give a rough GPH at normal cruise.

FF


I think you missed the mark a little on fuel consumption for your hypothetical 350 hp vessel Fred. For example, my 420 hp single engine boat burns only twice as much at 15 knots, compared to 8 knots. Not 10-15 times the slow cruise. I get just over 2 mpg (assuming no tidal effect), at 8 knots, and 1 mpg at 15 knots. This is factual, not incorrect theory.

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[FF]

15Gph times at the very best 20Hp per gal is 300HP

If you propped and ran the boat to use the last 120 hp (or more likely 160hp) the burn would be up another 8-10GPH.and perhaps another 2K -4K faster.

A "rule of thumb" is designed to give a rough estimate of performance , so one can understand the concept the designer had in mind for the build.

420 hp, in a 36 FT boat was built to run between marinas , NOT CONTINENTS.

1500 miles of range would be really hard, due to fuel weight and volume.