OUTRAGE 18 with Yamaha F115

Optimizing the performance of Boston Whaler boats
Sebash4

OUTRAGE 18 with Yamaha F115

Postby Sebash4 » Fri Mar 30, 2018 4:50 pm

How will a Yamaha F115 perform on an OUTRAGE 18?

Backstory: I have [another] boat with a Yamaha F115. While I love the old two-stroke-power-cycle outboard engines, I've come to really like the four-stroke-power-cycle outboard engines.

[Omitted back story: in another thread the OUTRAGE 18 has just been purchased and it has a 150-HP engine--jimh]

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Phil T
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Re: OUTRAGE 18 with Yamaha F115

Postby Phil T » Fri Mar 30, 2018 5:39 pm

This could be considered a FAQ.

The F115 is an excellent motor and a good fit for a classic Outrage 18, as shown by our past member Elaelap, see http://continuouswave.com/ubb/Forum4/HTML/002632.html

Please note that engine height and prop selection are VERY important. Check back with us after the engine choice has been made but before it is installed.
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jimh
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Re: OUTRAGE 18 with Yamaha F115

Postby jimh » Fri Mar 30, 2018 5:40 pm

I don't know exactly what parameter you are asking about with the very general term "performance." If you mean what will the top speed be if you change to a 115-HP from some other engine, then you can generally assess the top speed potential of a boat with different engine horsepower by using the approximation that top speed is proportional to the square-root of the horsepower to weight ratio.

For example, if a boat had 100-HP, you increased to 135-HP, and the weight stayed the same, then the top speed would increase by a factor of (135/100)0.5. This approximation works quite reliably for moderate v-hull planing boats in the speed range under 50-MPH. In this example, if the boat went 35-MPH with 100-HP then you'd expect [35*(135/100)0.5]-MPH (41.6-MPH) from the new engine. The ratio also works when you decrease power. Add the weight to the calculations if the weight changes with different engines. You use the two total boat weights, not just the engine weights.

If you tell us the top speed of the boat with the other engine, we can offer some reasonably good estimate of the top speed with the new engine, but you have to tell use the total boat weights in each case, unless you want to assume they total boat weight is a constant to make the calculation simpler (but less accurate).

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Re: OUTRAGE 18 with Yamaha F115

Postby ConB » Fri Mar 30, 2018 5:58 pm

I would not be happy with less horse power on my Outrage 18 than my 150 when I have a full tank of gas, 4 adults and a Golden Retriever onboard.

But if you have the engine you could switch it and try it, eh.

Con
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Re: OUTRAGE 18 with Yamaha F115

Postby jimh » Fri Mar 30, 2018 6:02 pm

CON--best answer so far.

Sebash4

Re: OUTRAGE 18 with Yamaha F115

Postby Sebash4 » Fri Mar 30, 2018 7:47 pm

Thanks Jim and ConB,

Jim, that's a lot of info which will take me time to digest.

ConB, I like your thoughts on making the switch and see how it performs.

When I asked the question, I thought there would be a few members that may have had experience with different engines. When looking at history and info on the Outrage 18. I found the boat maximum power was 150-HP and the minimum was 85-HP. Not sure how the 115-HP will do but I'll soon know from trial and error.

Thanks again for the info.

Sebash4

Re: OUTRAGE 18 with Yamaha F115

Postby Sebash4 » Fri Mar 30, 2018 8:08 pm

PhilT, thanks for the link to Pale ale's post (elaelap). I miss his post as he was very knowledgeable. Good info from several members on the 115 on the Outrage 18.

Thanks again.

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Re: OUTRAGE 18 with Yamaha F115

Postby jimh » Sat Mar 31, 2018 8:47 am

Sebash4 wrote:Jim, that's a lot of info which will take me time to digest.



SEBASH--the relationship between boat speed and horsepower is really as simple as my one-sentence description. More horsepower makes a moderate speed planing hull boat go faster, but the rate of increase is only to the 0.5-exponent, and that is only if the weight is constant. Maybe if I give you another example it may help:

If the horsepower on a moderate speed planing hull boat is doubled, the boat speed does not double. The boat speed increase is in proportion to the horsepower increase, but only to the 0.5-exponent. If the horsepower ratio is 2, the speed ratio will be 20.5.

It may also aid understanding to mention that raising any number to the 0.5-exponent is also the same as "taking the square root" of that number. I use the exponential notation because it is just about impossible to write mathematical formulas in HTML text with the radical sign that often is used to denote taking a square root or other root. You just can't create simple HTML text with radical sign notations in formulas.

I hope that helps you understand the relationship between propulsion horsepower and boat speed.

I also mention weight. Boat speed is inversely related to weight. I think this is really intuitive. The more weight added to a boat, the slower the boat goes. I don't think anyone has a notion that adding weight makes the boat go faster. The effect of weight is again to the 0.5-exponent, or perhaps I should say 1/weight to the 0.5-exponent. The weight is the TOTAL BOAT WEIGHT, not just the engine weight. This is intuitive, too, as the engine has to also push the boat weight, not just its own weight. You don't calculate just the engine weight change but the total boat weight change.

The boat speed is always in proportion to the ratio of power to weight. So more power makes the boat go faster, and more weight makes the boat go slower. Because not all propulsion engines have the same weight, and because engines with more horsepower are often larger engines and weigh more than smaller engines of less horsepower, you need to consider both the change in horsepower and the change in total boat weight. Maybe another example will be helpful:

If a boat weighs 2,000-lbs with its present engine and a new engine weighs 100-lbs more than the old engine, then the new total boat weight will be 2,100-lbs. In making a speed estimate we use the weight change as (2000/2100)0.5, or 0.976, that is, the boat speed with the new, heavier engine will be 0.976-times the old speed with the lighter engine.

Now if you don't want the boat to slow down by a factor of 0.976 when you in add 100-lbs, then you have to increase the horsepower. How much more horsepower? You have to get 1/0.976 speed increase of 1.025. This means the horsepower will have to increase by a factor of 1.0252, or 1.05-times.