Sport 13 Yamaha 40-HP

Optimizing the performance of Boston Whaler boats
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Sport 13 Yamaha 40-HP

Postby bobross » Fri Jan 08, 2021 8:58 pm

I am in the midst of a project on a 1977 Sport 13. I have a 1988 Yamaha 40-HP two-stroke-power-cycle engine. The propeller is beat up, has no markings, and I can't tell its size. The owners manual for the motor lists 11-3/4 x 12 - G, but I think that number is an exemplar.

Give me a recommendation for an aluminum, three-blade, modestly priced, standard propeller that will give middle-of-the-road performance. I do not require extra top speed or rapid acceleration from a standing start. The boat will be plenty fast as it is. I require a propeller that will allow the engine to accelerate to more than 4500-RPM and less than 5500-RPM at full throttle.

Give me the diameter and pitch for the propeller you have used with a Yamaha 40-HP engine which gave good all-around performance.

Bob Ross

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Phil T
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Re: Sport 13 Yamaha 40-HP

Postby Phil T » Fri Jan 08, 2021 9:16 pm

Bob--Engines more than 30-HP are not sold with a propeller included as there is no "standard" propeller. A propeller is selected based on the engine and the hull it is mounted on.

Boston Whaler owners with a Sport 13 and a classic Yamaha 40 two-stroke-power-cycle engine have tested many propellers. Their recommendations include:
--10.5 x 12 Yamaha Performance
--10.5 x 12 Turbo Hot Shot

Propeller dimensions listed above only apply to the specific make and model listed.

Most propeller gurus do not recommend an aluminum prop since they are less efficient, suffer more damage, and do not "protect the lower unit" any better than stainless steel propellers.
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Re: Sport 13 Yamaha 40-HP

Postby jimh » Sat Jan 09, 2021 10:00 am

bobross wrote:The owner's manual for the engine lists 11-3/4 x 12 - G, but I think that number is an exemplar.

An exemplar means "one that serves as a model or example; an ideal model." On that basis, the propeller suggested by the manufacturer in the owner's guide would be a good place to start.

The propellers suggested by Phil will also be good choices.

bobross wrote:Give me a recommendation for an aluminum, three-blade, modestly priced, standard propeller...

Aluminum and three-bladed are requirements that can be easily met. Moderate price is undefined, although aluminum three-bladed propellers for a 40-HP engine will probably all be "modestly priced."

bobross wrote:The boat will be plenty fast as it is.

What is your expected maximum boat speed that will be "plenty fast as it is"?

Boat speed is a result of the ratio of horsepower to weight. With a small boat like a SPORT 13, the total boat weight will vary considerably with the number of people aboard. Each adult of perhaps 180-lbs (or more) represents a significant increase in boat weight in a hull that only weighs perhaps 350-lbs. You should select the propeller based on the typical loading you anticipate will be used for most of the boat operation.

bobross wrote:Give me the diameter and pitch for the propeller...which gave good all-around performance.

"[G]ood all-around performance" usually means having the ability to rapidly accelerate the boat onto plane and to produce the maximum boat speed expected for the boat hull, weight, and horsepower. These characteristics are not usually designated as "middle-of-the-road" outcomes. I don't believe you can forgo rapid acceleration onto plane and achievement of boat speed appropriate for the hull, weight, and horsepower and still meet the criterion of "good all-around performance." Loss of either characteristic would rule out the description of "good all-around performance."

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Re: Sport 13 Yamaha 40-HP

Postby padrefigure » Mon Jan 11, 2021 5:37 pm

For a leaping off point, I have a 10 x 13 stainless steel propeller on my 1968 13-footer powered by a 1983 Johnson 35-HP engine. The only data I can provide is the top [boat speed] is about 27-MPH. The engine has no tachometer. The engine has recoil start.

I tried a 10 x 15 aluminum propeller. The boat did not gain any speed and seemed slower to plane. The engine speed decreased based on the pitch of the engine note.

A 12-inch pitch propeller might be a little low for a more modern 40HP, but I believe the hull changed in 1972 increasing weight, probably width, and for sure transom height.

I suspect you know how to select the correct pitch propeller, and asked your question to get an idea of the starting point.

I hope these comments help.