SPORT 13 25-HP

Optimizing the performance of Boston Whaler boats
david860
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SPORT 13 25-HP

Postby david860 » Sat Dec 11, 2021 8:25 am

My SPORT 13 has a Mercury 25-HP two-stroke-power-cycle engine, and performance is good.

If the SPORT 13 is re-powered with a Yamaha F25 lightweight four-stroke-power-cycle engine, how will performance change?

Jefecinco
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Re: SPORT 13 25-HP

Postby Jefecinco » Sat Dec 11, 2021 10:18 am

Our Sport 13 is powered by a Mercury 25 HP four stroke with electric tilt and trim with wheel steering. I feel the performance is more than sufficient but have never towed any water sport toys. The boat handles the weight of four well fed adults without problems. Never having operated a Sport 13 with a Mercury 25 HP two stroke engine I can not offer a comparison. I suspect there would be little difference between the performance of a 25 HP Yamaha four stroke and a Mercury 25 HP four stroke similarly equipped.
Butch

david860
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Re: SPORT 13 25-HP

Postby david860 » Sat Dec 11, 2021 2:14 pm

Butch—[your report of the performance of your boat] is encouraging [me to buy the Yamaha F25].

jimh
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Re: SPORT 13 25-HP

Postby jimh » Sun Dec 12, 2021 8:57 am

There is not much information given in the initial question to allow for a good answer. The term "performance" is undefined. The term "lightweight" is undefined.

A boat's performance in terms of top speed and acceleration depends on the ratio of power to weight. More power at the same weight improves acceleration and top speed. More weight at the same power reduces acceleration and top speed.

For two engines rated the same power, there is really no basis to think their power output will be different. What may be different is the power curve or the engine speed range at which the engine produces the rated power. Generally for outboard engines, manufacturers seldom provide an engine power curve. There is no reasonable assumption that can be made here about either engine. We just consider they are both 25-HP engines. If one weighs more, then the added weight will tend to reduce performance.

About all that can be said in terms of performance is that the Yamaha F25 will very likely get better fuel economy than the Mercury two-stroke-power-cycle engine, but most of that improvement will be seen at lower engines speeds.

The new engine will most likely need a new propeller if its best performance is to be obtained.

david860
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Re: SPORT 13 25-HP

Postby david860 » Sun Dec 12, 2021 4:17 pm

The new lightweight Yamaha 25 is advertised as a portable outboard. It shares the same power head with the 15 and 20-HP engines. This means the new 25-HP has a smaller displacement that the previous 25-HP Yamaha.

I was hoping someone in the forum has knowledge or experience with the newer 25.

I’m concerned that it may be weaker in all performance measurements such as, power to plane the boat, acceleration, and top end speed.

I have never owned a four-stroke-power-cycle outboard, and I have heard complaints about their performance compared to two-stroke-power-cycle outboards.

Jefecinco
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Re: SPORT 13 25-HP

Postby Jefecinco » Mon Dec 13, 2021 9:54 am

When I see the term "portable engine" I think of an engine with tiller steering and without power tilt of trim. I saw some that were advertised with remote mechanical controls, whatever that means. Perhaps the entire Yamaha lineup of 25 HP engines are considered portable although they weigh over 100-lbs
Butch

goldstem
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Re: SPORT 13 25-HP

Postby goldstem » Wed Dec 15, 2021 8:56 am

I have a SPORT 13 with a Mercury 25 FOURSTROKE engine which is much heavier than the Yamaha [25-HP engine under discussion in this thread].

[The SPORT 13 boat] does sometime struggle to get on plane.

[The Mercury 25 FOURSTROKE engine] does not seem to have the mid-range punch the old Johnson 30-HP two-stroke-power-cycle engine had. [The Mercury 25 FOURSTROKE] really needs to rev.

jimh
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Re: SPORT 13 25-HP

Postby jimh » Wed Dec 15, 2021 9:48 am

goldstem wrote:[The Mercury 25 FOURSTROKE] really needs to rev.
For many four-stroke-power-cycle outboard engines, the engine must be able to accelerate to the very top of its allowed engine speed range in order for the engine to produce its rated horsepower output.

jimh
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Re: SPORT 13 25-HP

Postby jimh » Wed Dec 15, 2021 9:51 am

For a good understanding of how the engine power band of an outboard engine can affect the ability of the engine to accelerate under load, see my article on the load created by a propeller at

Propeller Power Curve
https://continuouswave.com/whaler/refer ... Curve.html