Re-Power 1972 SPORT 13 with Yamaha F25

Optimizing the performance of Boston Whaler boats
13Sport
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Re-Power 1972 SPORT 13 with Yamaha F25

Postby 13Sport » Thu Aug 11, 2022 8:26 pm

[This discussion about the performance that will occur after a re-powering of a boat has been moved to the PERFORMANCE forum. The PERFORMANCE forum is where all such topics of boat, engine, and propeller performance are discussed--Moderator]

Q1: will a Yamaha F25 propel a 1972 SPORT 13 fast enough to get on plane with four people aboard?

Q2: with a Yamaha F25 will a 1972 SPORT 13 have a cruising speed between 13 to 25-MPH?

If any reader has a c.1972 SPORT 13 with a Yamaha F25, please comment about the boat performance.

A goal for the re-power is to have my 13-year-old daughter to be able to use it by herself and for our family of four to be able to fish from the boat, to explore with the boat, and for the boat to get on plane. We do not care about top boat speed; we just want the boat to get on plane. We will NOT be using it to pull tubes or water-skiers.

BACKSTORY

I am refurbishing a [1972--HEY--USE FOUR DIGITS FOR YEARS] SPORT 13 boat that my father gave to me when I was age 13. The boat needs to be re-powered.

The current outboard engine is a c.1980 Johnson 35-HP that's been on the transom for years. With the Johnson 35-HP, the boat speed was greater than 30-MPH.

jimh
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby jimh » Fri Aug 12, 2022 8:59 am

13Sport wrote:Q1: will a Yamaha F25 propel a 1972 SPORT 13 fast enough to get on plane with four people aboard?

Performance is all about power-to-weight ratio. On small and light boats like a SPORT 13, the effect of adding persons aboard on the total boat weight will be significant.

Whether 25-HP can push the SPORT 13 onto plane with four people is going to depend on the weight of the four people. If the four are two trim and fit adults and two trim and fit 13-year-olds, then the chances are much better than if the four people are University of Wisconsin offensive linemen that each weigh over 300-lbs.

The Boston Whaler literature indicated the minimum power to have a lightly-loaded 13-footer reach plane will be just 9-HP. With 25-HP there is no doubt the boat will get on plane when lightly loaded

The original c.1980 35-HP engine was rated for propeller shaft horsepower. The Yamaha F25 is also rated for propeller shaft horsepower. You can estimate the speed reduction that will result from decreasing to 25-HP from 35-HP using the Crouch method for predicting the speed of moderate speed planing hull boats using their weight and horsepower. Crouch predicts speed varies with the chane in power-to-weight to the 0.5 exponent. Here is an example:

If the boat could reach "more than" 30-MPH with the 35-HP, then with 25-HP, and assuming no change in total boat weight, the new speed range will then be "more than" 25-MPH.

30-MPH ×(25/35)^0.5 = 25-MPG

jimh
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby jimh » Fri Aug 12, 2022 9:09 am

13Sport wrote:Q2: with a Yamaha F25 will a 1972 SPORT 13 have a cruising speed between 13 to 25-MPH?
The ability of an engine to produce a particular power output varies with the engine speed. Often with four-stroke-power-cycle engines the engine speed required to achieve the rated power output will be very close to the maximum engine speed allowed. The influence of this on the ability to get a heavily loaded boat onto plane is important to consider.

To keep a boat on plane at the very lowest planing speed often takes a surprising amount of power. To do this, the boat engine must be able to develop a lot of power at low engine speeds. That is generally not the forte of small-displacement, high-RPM four-stroke-power-cycle engines

To allow the 25-HP engine to accelerate to an engine speed at which it can produce 25-HP, the load on the engine created by the propeller must be managed so that the engine acceleration is not stopped before the rated power band engine speed is achieved. This can be done by reducing the propeller power curve, which is directly related to the propeller pitch.

The outcome for a small and light boat with a 25-HP four-stroke-power-cycle may be that two different propellers are required, each optimized for different boat weights. This is not a huge problem as an aluminum propeller for a 25-HP engine is not very expensive, so buying two won't be a financial burden.

If you cannot find one propeller that suits your use with the boat run only by one person and also when run with four aboard, you can just change the propeller to suit the load.

goldstem
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby goldstem » Fri Aug 12, 2022 10:20 am

I believe [a Yamaha F25] will [get a 1972 SPORT 13 to go fast enough to get on plane] with the correct propeller and correct weight di distribution of passengers.

13Sport
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby 13Sport » Fri Aug 12, 2022 8:04 pm

The four of us add up 590-lbs.

Foulweather Jack
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby Foulweather Jack » Mon Aug 15, 2022 6:01 pm

I’m [do not know the] hull weight of the 1972 Sport 13.

I have a 2004 130 Sport with a 2004 Yamaha F25. I bought the boat for my daughters to learn to boat. The F25 would not plane the 2004 130 SPORT boat without the addition of a anti-ventilation plate foil to provide more lift. The 2004 130 SPORT boat will not plane with four adults.

My 2004 Yamaha F25 outboard engine weighs 156-lbs; the new Yamaha F25 is weighs 126-lbs. The Yamaha promotional literature boasts that it is the lightest 25-HP outboard engineon the market, including two-stroke-power-cycle outboard engines.

The higher power-to-weight ratio [of the new F25] will be an advantage [compared to the heavier older F25], but I wouldn’t count on being able to plane [the 1972 SPORT 13 boat] with four adults on board [using the newer F25 engine].

My 2004 Yamaha F25 is nearing its end-of-life. I may re-power my 2004 SPORT 150 boat with the new Yamaha F25, with the hope that its lighter weight will allow the self-bailing hull to work properly. Currently, the [old 2005 Yamaha F25 outboard engine] is so heavy that there is always water in the [sump compartments]. I keep the boat in the water all the time to serve as a rescue boat in case the wind dies while I am windsurfing, so it would be nice to keep the inside of the hull dry.

With one adult on the 2004 130 SPORT, the 2004 Yamaha F25 can propel the boat to a top boat speed of 20-MPH, which is perfect for 13-year-old kids.

If a 40-HP engine is installed on a 2004 130 SPORT 40, the boat speed increases to almost 40-MPH. A boat speed of 40-MPH is too fast for teenage kids.

jimh
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby jimh » Tue Aug 16, 2022 9:05 am

If a boat with a moderate planing hull can only reach 20-MPH with 25-HP, and it is a modern planing hull design, we can estimate the total boat weight must be around 2,250-lbs. If the horsepower is increased to 40-HP and there is no change in weight, the boat speed would probably only increase to 25-MPH. That is a very generous estimate because there will likely be an increase in weight due to the larger engine.

jimh
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Re: Re-Power 1972 SPORT 13 with Yamaha F25

Postby jimh » Tue Aug 16, 2022 9:11 am

13Sport wrote:The current outboard engine is a c.1980 Johnson 35-HP....With the Johnson 35-HP, the boat speed was greater than 30-MPH.

The boat weight can be reasonably estimated with some accuracy from the Crouch formula for boat speed as a function of power and weight, with a known hull coefficient. The smaller Boston Whaler hulls with somewhat rounded bottoms like a SPORT 13 typically have a hull coefficient of 190.

An engine made in 1980 would likely have been given a power rating based on its crankshaft power output, not its propeller shaft power output. On that basis I would de-rate the 35-HP engine to 32-HP. The ICOMIA 28-83 method of rating outboard engine power did not come into effect until 1983.

Using the calculator I have created that works according the naval architect George Crouch's speed prediction formula, for a boat to reach just 30-MPH with a 32-HP engine, the boat weight would need to be around 1,280-lbs.

Reducing the power to 25-HP, and assuming no change in engine weight, the boat speed would decrease to 26.5-MPH.

If the boat could reach 33-MPH (that is, "over 30-MPH") with the 32-HP engine, that suggests a reduced boat weight of only 1,060-lbs. At that boat weight the 25-HP engine would give a boat speed of 29.2-MPH.

The Crouch's Calculator can be found at

https://continuouswave.com/calculators/crouchCalc.php

In this instance, the hull coefficient and hull weight do not really need to be know with precision. The general behavior of boats with moderate speed planing hulls is the boat speed increases or decreases with the power-to-weight ratio change to the 0.5 exponent. Reducing to 25-HP from 32-HP is not going to turn the boat into a complete slug. For example, the ratio (25/32)^0.5= 0.88. That means the reduction is speed by lowering power to 25-HP from 32-HP will be by a factor of 0.88 as long as the total weight did not change. If the boat went 33-MPH with 32-HP then with 25-HP it would still go 28-MPH If total boat weight is reduced, then the speed reduction will be even less.