# Defining required engine power and gearbox ratio

Discussion in 'Props' started by marineengineer, Dec 12, 2009.

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### marineengineerNew Member

Is there any practical way or a formulation to calculate required horse power and defining gearbox ratio for sailing boats and power boats?

Thanks

2. ### Guest625101138Previous Member

First step is to measure, calculate or approximate the drag on the hull at the target speed. You then need to determine the largest diameter prop you can fit. There will be factors such as clearance to the hull and shaft angle that dictate this. Ideally the shaft angle is horizontal but angles as high as 15 degrees are tolerable depending on the prop.

Having the target speed, diameter and the boat drag allows you to determine the best pitch, shaft speed and gear ratio. There are various ways to do this.

Rick W

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### marineengineerNew Member

Rick , thank you so much. Is there any document that shows the calculation method for all these terms?

4. ### Guest625101138Previous Member

There might be a few text books that covers the material adequately.

If you want to start by an example rather than buying books or googling for a long while then-

To start with drag - what type of hull are you considering, length, beam and speed? There is no single method that suits all applications.

Rick W

5. ### FrostyPrevious Member

On a sail boat???

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### marineengineerNew Member

I need to collect for sail and power boat.

7. ### Guest625101138Previous Member

Frosty
Of course but I would have a folding one to reduce drag under sail.

Rick

8. ### Guest625101138Previous Member

Both displacement type or displacement and planing?

Rick W

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### JoakimSenior Member

You are most likely limited to commercially available folding propellers and marine gear/drives. Thus I would not say the largest diameter you can fit is the best. If you want the large diameter propeller to work well, you need low rpm.

E.g. I have a Yanmar 1GM10C with 2.64:1 saildrive and a 14x8 folding propeller. It likely has an efficiency of ~50% at full speed of 6.4 kn, 3480 rpm (1320 rpm at shaft).

I could get a much better efficiency:

18x18 propeller at 640 shaft rpm would give over 60%

30x40 propeller at 250 shaft rpm would give over 70 %

40x56 propeller at 150 shaft rpm would give almost 80%

But what could I do with shaft rpm of 1300 rpm? Not much better considering also cavitation limits.

10. ### Guest625101138Previous Member

Joakim
You make the point well that efficiency is strongly related to diameter. We had not settled on the gearing. If the gearing is given then the prop selection will most likely be set by the gearing rather than what can be fitted.

Rick W

11. ### FrostyPrevious Member

Big props= big hubs big hubs = big shafts.

Are you suggesting that if he is building a 40 foot sail boat and he "could" fit a 4 foot diam prop he should do so?

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### JoakimSenior Member

But most likely you are limited to what is "normally" used. And that would be shaft rpm > 1000 rpm for normal sized sailboat propulsion systems.

Is it worth making a non-standard 5:1 gear and buying a much more expensive 18" propeller to get a bit better efficiency? How important is the difference between 50% and 60% for a sailboat.

13. ### Guest625101138Previous Member

The question was about existing boats. The diameter of the prop is inevitably constrained so you go for the biggest diameter that will fit. This is a starting point. We are not modifying the hull or starting a new design. You cannot buy really efficient props for pleasure boats you have to make them so this is beyond the question posed.

Unless the hull is specifically designed for a large diameter prop there will be constraints on the diameter imposed by the hull. Some hulls with tunnels will actually fit a decent diameter prop otherwise the hull and shaft angle constrain the prop diameter to something less than you would otherwise choose if you could fit it.

On higher speed boats you get to the point where the strut or leg drag becomes a consideration. As speed goes then cavitation becomes a consideration as well.

Rick W

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