# Can anyone tell me how to know the propulsive efficiency of an outboard?

Discussion in 'Outboards' started by corsair.p, May 31, 2007.

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### corsair.pN.A.China

How you guys forecast the speed of an outboards boat?

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### sottorfmember

Very good question and a difficult one to answer. When considering an outboard engine you need to consider the propeller efficiency as well as the added drag of the leg. The propeller efficiency is roughly the same as you would get on ant submerged propeller of similar blade area ratio, pitch diameter etc. This can be calculated using B-series data.

The difficult part is the drive leg. The important thing to note here is the bullet nose part of the drive leg is NOT A STREAMLINED BODY! This is due to the blunt downstream end where the propeller is attached. The blunt end is great for venting the exhaust through as there is no back pressure for the engine but it also adds a lot of drag as there is no pressure recovery as found with a streamlined shape.

The drag of a blunt body can be calculated from Sighard Hoerner's famous work: "Fluid Dynamic Drag" If you run through the analysis of calculating the drag of the drive leg you come to the conclusion that at high speds (50 knots +), the drag of the bullet is often higher than that of the hull! Hence be really careful how you approach the problem of predicting performance of an OBM with submerged bullet. If you include the drag of the leg in the overall propulsive efficiency of the propulsion system then the OPC = 0.45-0.55 in most cases. A lot of people use 0.5. The higher the speed the lower the OPC.

The way to get around this problem is to run cleaver propellers which allows you to lift the bullet largely out of the water and vent the trailing edge to atmosphere. Setting up the engine like this brings a big increase in speed. Then you can get OPC = 0.6+

The third way to improve OPC, adopted by Volvo Penta on their stern drives, is to have a streamlined bullet with so the drag of the whole leg is lower. They vent the exhaust through the cavitation plate.

I look forward to hearing the input of other people who have grappled with this problem.

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### pistnbrokeI try

assuming its a normal 2 :1 outboard leg then multiply the pitch of the prop in inches by 2 and take 80% of that figure for your speed in mph........ Approximate and assumes light planing hull

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### Easy RiderSenior Member

With a car we need an mixture enrichment system because the engine requires a rich mixture when cold. So does an OB but the OB never gets up to temp because of the requirements of a seawater cooling system. They must run cold and therefore require a richer mixture. So the OB must have more efficiency in some way that the inboard does not to be equal. Also OBs come w a fixed gear ratio and need to perform on a much wider range of displacement and speed that the more flexible inboard. The modern OB is much heavier than in years past and more weight to support fresh water cooling is not likely to happen nor is multiple choices for gear ratio and prop size either. If the forces were there to promote manufacturers to bring these things about it would probably have done so by now. It seems OBs shine at very high speed but I don't see OBs competing at moderate speeds like 15 knots. My 16' OB w a 40 E-tech does quite well but it has an unusually large prop and low gear ratio and the boat is light.

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### PARYacht Designer/Builder

Simply, a rough guess at top speed can be made with a few questions about the hull and the outboard. Some of these questions would be weight of the boat, hull shape employed in the design, maximum loading, engine make, model and year, estimated amount of slippage and drive train loses, etc. With these "variables" plugged in, a fairly good estimate can be made. You can refine this figure considerably, though it does require a good bit more effort, for little substantive gain (fine tuning the last 5%).

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### AlikSenior Member

I would take 0.5...0.55 as quick reference for outboard engine on average planing powerboat.

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