Power required

[obmij]

I have read that the 'rule of thumb' is that for a displacement boat you need 1 hp per 500lb, to make your theoretical hull speed.

I understand hull speed is found by multiplying the square of waterline length by 1.34 (in my case 6.2 knots)

Is there any way of finding the power required to move at that speed, that is less 'rule of thumb'?

 

[macd]

I understand hull speed is found by multiplying the square of waterline length by 1.34 (in my case 6.2 knots)

Square root, not square (sadly, or a 20-footer would do over 500 knots....)

 

[alahol2]

I used the following formula...
Speed / sq rt(LWL) = 10.665 / cube root(displacement / HP) Displacement is in lbs, LWL in feet, speed in knots.

This gives the theoretical shaft horsepower so you need to add another 5-10 percent to cater for transmission losses etc.

 

[Tranona]

While that formula is a useful guide, there are more issues to consider. The boat is moved by the propeller, not the engine, so choice of the correct propeller is important. Often the diameter of the propeller that can be swung is limited so you need to consider the shaft speed for the maximum size prop you can use. So choice of gearbox ratio is important to ensure that you can use all the power the engine produces, which means sizing the prop to enable the boat to achieve maximum speed at full revs. This will then give you a comfortable cruise speed at between 60-70% maximum revs.

 

[obmij]

Thanks - the rule of thumb has my required HP at 14, the equation 13.8

Not such a bad rule of thumb!

Regarding prop size and gearbox ratio, I will have to do more research, although advice from the forum is always welcome!

Thanks again.

 

[Tranona]

What boat do you have? You can check the prop size by putting your boat data into the Propcalc programme on www.castlemarine.co.uk The most common gearbox ratios - for example on the Beta or Nanni 14 hp are either 2:1 or 2.6:1 and the latter will allow you to swing a larger diameter prop. I have a 15" diameter with a 2.6:1, whereas with a 2:1 you need a smaller prop. Which is better depends on the underwater profile, and in some boats you simply can't run a 15" prop.