MISCHIEF 15 Yamaha F70

Optimizing the performance of Boston Whaler boats
Yeneews
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Joined: Sun Jul 11, 2021 9:56 pm

MISCHIEF 15 Yamaha F70

Postby Yeneews » Sun Jul 11, 2021 10:30 pm

The Yamaha F70 on my MISCHIEF 15 accelerates to 4,500-RPM turning an aluminum propeller [of unspecified brand, model, and number of blades] marked as 14.5 x 17-pitch.

[The recommended full throttle engine speed] should be more than 5,000-RPM, and I infer the propeller pitch should be adjusted.

[I plan to replace the current aluminum propeller with a] stainless steel propeller optimized for performance.

Will the weight [of the MISCHIEF 15 affect] the optimal propellor choice?

Suggest a stainless steel propellor pitch that would work best.

—Fin S

jimh
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Re: MISCHIEF 15 Yamaha F70

Postby jimh » Mon Jul 12, 2021 9:24 am

The pitch of a propeller can only be “adjusted” a small amount, perhaps an increase or decrease in pitch of one inch.

A change in pitch of 1-inch usually only affects engine speed by about 200-RPM. To pick up 500-RPM engine speed the propeller pitch will likely need to drop 2-inches, so a new propeller is going to be needed.

Total boat weight is a very significant influence on boat speed, and weight changes on small boats will be especially influential due the relatively large percentage change from added weight to a light boat from added crew or added gear.

What is the engine mounting height as described by the number of holes raised from lowest possible mounting?

Read

https://continuouswave.com/forum/viewto ... ?f=7&t=739

before replying.

What is the weight of the MISCHIEF 15?

What is the gear ratio of the F70?

What is the maximum engine speed of the F70 at full throttle?

jimh
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Location: Michigan, Lower Peninsula
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Re: MISCHIEF 15 Yamaha F70

Postby jimh » Mon Jul 12, 2021 9:49 am

With 70-HP I would expect at least 35-MPH top speed to be obtained with a MISCHIEF 15. Assuming a 2.33 gear ratio, engine speed of 5,500-RPM, and 17-pitch, that boat speed should be possible.

I suspect the engine is now mounted too low and the engine trim is not being set properly, or the engine is not in good tune, or all three are affecting performance.

dtmackey
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Re: MISCHIEF 15 Yamaha F70

Postby dtmackey » Mon Jul 12, 2021 10:19 am

If properly set up your mischief should run about 40mph and acheive an RPM around 6000rpm. First start with the engine mouting height, while everyone will say "two holes up", you can run that motor as high as you can mount it. Less motor in the water is less drag and more speed. Many people feel that even mounted as high as you can on the transom still isn't enought and can still go highter by installing a jackplate. I can attest that the motor can be run a full 5" above the transom with no cavitation or problems and I have this setup on my 15' SuperSport.

I'm currently running a stainless 17" prop, but may bump to an 18".

Image

biggiefl
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Re: MISCHIEF 15 Yamaha F70

Postby biggiefl » Sun Mar 19, 2023 6:45 pm

I had a classic 15-footer with a center console and a 1990 Johnson 70-HP that would run a constant 46-MPH and a top GPS SOG of 48.6-MPH without a jack plate. If you are getting less than 43- or 44-MPH you are not set up correctly.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).