# Thread: Formula for boats prop & hull efficiency

1. ## Formula for boats prop & hull efficiency

Don't know if this works with outboards(it should) but i know it works well with inboards. My boat comes out at 81% efficiency.

Just note that this is a method for checking the efficiency of the hull and prop package and really has nothing to do with efficiency of the motor and gear. But it all ties together. An educated guess would be that motor efficiency x gear efficiency x hull efficiency = a somewhat overall estimate of boat efficiency.

Here is the formula for figuring out your boats efficiency:

RPMs divided by Reduction Gear Ratio X=s Pitch

X=s 60 (minutes) divided by 12 (inches per foot) divided by

5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots

Example my 31 JC 370 HP Yanmar@ 3300 RPMs 2:0 gear with a 23 X 21 propeller a WSOTY speed of 22.5 knots.

3300 (RPMs) divided by 2 = 1650 X's 21 (pitch) = 34650.00 X's 60 = 2079000.00 divided by 12 ("s per foot) = 173250.00 divided by 5280 (feet in a mile) = 32.81 (miles per hour) divided by 1.15 =28.53 knots @ 100 % efficiency

22.5 knots (Boats actual speed divided by 28.53 knots ='s .79% efficiency

Therefore the only things you need to get your boats efficiency is the follow:
speed at wot @ RPMs
reduction gear

2. Neat info.

I see you run a Yanmar, this is a article you may find interesting......

http://www.sbmar.com/articles/contin...t_perspective/

3. Wow if this is an accurate representation of hull efficiency and if it indeed works also for an outboard, I'm sitting at 100% at WOT and 96% a cruise. Best news I had all day.

4. Originally Posted by Akgramps
Neat info.

I see you run a Yanmar, this is a article you may find interesting......

http://www.sbmar.com/articles/contin...t_perspective/
No NO No yanmar for me !! I've got slow turning American Iron 300HP John Deere !!! Those yanmars are in the same league as volvo diesels, high reving timebombs to get the horsepower out of them. I have learned to stay away from that type of motor.

5. Originally Posted by akroxy
Wow if this is an accurate representation of hull efficiency and if it indeed works also for an outboard, I'm sitting at 100% at WOT and 96% a cruise. Best news I had all day.
I think it does work for outboards and with no keel thats why you get better efficiency.

6. Originally Posted by potbuilder
No NO No yanmar for me !! I've got slow turning American Iron 300HP John Deere !!! Those yanmars are in the same league as volvo diesels, high reving timebombs to get the horsepower out of them. I have learned to stay away from that type of motor.
I was wondering, thought you were more of a "real diesel" guy...Thought you were using the formula on your boat, my mistake, have you done the math on your outfit?

7. Originally Posted by Akgramps
I was wondering, thought you were more of a "real diesel" guy...Thought you were using the formula on your boat, my mistake, have you done the math on your outfit?
yes i did do it for my rig 81%.

8. a little more info to mull over, especialy on diesel vs gas. This was written by a guy that has lots of knowledge on setting up motors & gears on boats.

It works at any speed. However you must start at WOT & the boat top RPM. If your boats is rated a 2800 RPMs WOT & you are only turning 2600 RPMs at WOT. your #s are not true to the boats rated performance. Also at WOT because the wheel is spinning the fastest that is when you have the greatest chances of increased slippage.

RPMs especially for diesels is very important & you need to get the rated RPMs of that motor is you expect it to last.
On every boat that I have ever run each morning on the way out I go WOT for a minute or so & at the end of the day I also go WOT for a minute or so. I look at my tach, temp, pyrometer & flow scan. To make sure that I am achieving full RPMs and temps. If one day I do the test & I am down 200 RPMs lets say, and I need to get in then I reduce my cruise RPMs by that amount so the load stays the same.

Diesels are different that gas motors. On a diesel the governor stops the RPMs of the motor by giving the correct amount of fuel. If you are overloaded & cannot get to the rated RPMs then the governor keeps on pouring in more fuel in its attempt to get to WOT.

Also with overload or over wheel not getting the rated RPMs is one thing but even at the rated cruise RPMs the load will be greater.

A properly wheel boat is when the governor hits it mark at the same time that the load of the wheel max's out in RPMs. In fact overturning by 100 RPMs is always a good thing.

Gas motors are different you just wheel them the rated RPMs. If you are overloaded the vacuum draw of the engine does not increase enough to over fuel like the diesel. If you unload you just over rev a little bit.

Here are three shots of my Yanmar Panel. One shot was at rated RPM 3300 RPMs 21 knots & 15 GPH. When I go WOT she turned up to 3500 RPM! two hundred higher that rated along with 23 knots. The boat was light that day & therefore when I would put 6 guys full fuel ice & bait for a canyon trip it would turn 3400 RPMs. This is the way all diesel boats should be wheeled.
3rd photo 18.4 knots & 3000 RPMs the sweet spot.

Lastly every year if your propeller does not look like the last photo you are cheating yourself. Wheels are where the rubber meets the road & it must be pe

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