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Starscream wrote: ↑Tue May 14, 2024 5:12 am Unlikely to be the wiring of the tach: I just looked up the BF50 and BF90 meter wiring diagrams and they have the same 5 wires, with the same color coding. Would be odd for a pro shop to make that mistake.

We can possibly eliminate the RPM gauge as a problem if we consider the speeds you were attaining at the 4200RPM/ballasted and 4500RPM/unballasted conditions. If you were in the teens then it's unlikely to be the RPM gauge, if you were in the 20s then the RPM gauge could be the issue.

13.75" x 13" prop:
4200 RPM ballasted ==> 24 MPH (GPS)
4500 RPM unballasted ==> 28 MPH (GPS)

So this means it could be an RPM gauge issue? It's original to the boat so over 20 years old.

Do you think I should replace the RPM gauge before trying another prop?

larryd97 wrote: ↑Tue May 14, 2024 3:29 pm

Starscream wrote: ↑Tue May 14, 2024 5:12 am Unlikely to be the wiring of the tach: I just looked up the BF50 and BF90 meter wiring diagrams and they have the same 5 wires, with the same color coding. Would be odd for a pro shop to make that mistake.

We can possibly eliminate the RPM gauge as a problem if we consider the speeds you were attaining at the 4200RPM/ballasted and 4500RPM/unballasted conditions. If you were in the teens then it's unlikely to be the RPM gauge, if you were in the 20s then the RPM gauge could be the issue.

13.75" x 13" prop:
4200 RPM ballasted ==> 24 MPH (GPS)
4500 RPM unballasted ==> 28 MPH (GPS)

So this means it could be an RPM gauge issue? It's original to the boat so over 20 years old.

Do you think I should replace the RPM gauge before trying another prop?

No way a 90HP pushes an unballasted M to 28mph at just 4500 RPM. My Suzi DF90A hits 28MPH at just under 6k RPM. My money is on the RPM gauge being faulty.

Your 50hp was a 3 cylinder, the 90 is a 4 cylinder. The tach works by counting ignition pulses so every 3 pulses was a revolution on the 50 but you have 4 pulses per revolution on your new engine. Your tach is under-counting your rpms by about 25%. As far as I know the OEM tach that was used by the BF40/50 does not have an adjustment for changing the number of cylinders.

Be Free wrote: ↑Wed May 15, 2024 6:50 am Your 50hp was a 3 cylinder, the 90 is a 4 cylinder. The tach works by counting ignition pulses so every 3 pulses was a revolution on the 50 but you have 4 pulses per revolution on your new engine. Your tach is under-counting your rpms by about 25%. As far as I know the OEM tach that was used by the BF40/50 does not have an adjustment for changing the number of cylinders.

Problem solved... Larry your motor and prop are in great shape at 28MPH and probably about 6k RPM at WOT without ballast.

Enjoy, just make sure your centerboard and rudders are secured full up. Replace that centerboard line pre-emptively. With a 90HP, it's bad if the board drops at speed.

Be Free wrote: ↑Wed May 15, 2024 6:50 am Your 50hp was a 3 cylinder, the 90 is a 4 cylinder. The tach works by counting ignition pulses so every 3 pulses was a revolution on the 50 but you have 4 pulses per revolution on your new engine. Your tach is under-counting your rpms by about 25%. As far as I know the OEM tach that was used by the BF40/50 does not have an adjustment for changing the number of cylinders.

That makes sense. Thank you!

Starscream wrote: ↑Wed May 15, 2024 7:23 am
Enjoy, just make sure your centerboard and rudders are secured full up. Replace that centerboard line pre-emptively. With a 90HP, it's bad if the board drops at speed.

Yes, the centerboard line concern crossed my mind as well. The existing lines are only 2 years old, so perhaps will replace at the end of this season. I'm thinking it would be a worthwhile precaution to rig up the daggerboard safety line to be able to secure the daggerboard up when motoring at high speed. That would provide redundancy in case the primary daggerboard line were to break.

FWIW - I may try a 14 x 11 at some time in the future. If I do that I will post results. The existing prop has some minor dings, etc. which helps me justify to my wife to try a new prop... It's on the order or 1 boat buck...

larryd97 wrote: ↑Thu May 16, 2024 11:30 pm FWIW - I may try a 14 x 11 at some time in the future. If I do that I will post results. The existing prop has some minor dings, etc. which helps me justify to my wife to try a new prop... It's on the order or 1 boat buck...

I wouldn't recommend that. The tach/rpm math says you're already at 6kRPM with the prop you have, and at 28mph unballasted, your current prop seems perfect. No reason to change anything: if you need a new prop, get the same one, or send your current one to a repair shop to address the dings. A 14x11 could possibly over-rev your motor.

Starscream wrote: ↑Fri May 17, 2024 7:09 am

larryd97 wrote: ↑Thu May 16, 2024 11:30 pm FWIW - I may try a 14 x 11 at some time in the future. If I do that I will post results. The existing prop has some minor dings, etc. which helps me justify to my wife to try a new prop... It's on the order or 1 boat buck...

I wouldn't recommend that. The tach/rpm math says you're already at 6kRPM with the prop you have, and at 28mph unballasted, your current prop seems perfect. No reason to change anything: if you need a new prop, get the same one, or send your current one to a repair shop to address the dings. A 14x11 could possibly over-rev your motor.

Fair enough. If I try another prop I'll design to the heaviest use case, then rely on the operator (me) to not over-rev the motor. This motor has electronic fuel injection and the same control system implements a governor / rev-limiter so I figure that gives the operator (me) a margin of safety.

Local prop shops don't seem interest in working on aluminum props - not worth the labor cost. Maybe just to smooth this one out it might be worth it. A replacement is on the order of $130 / 1.3 boat bucks.

I have carefully straightened bent edges on my aluminum propellers in the past. I used some of those props for years; I've also had chunks break off while doing it. Be patient, go slow, and use your judgement. Sometimes it's just time to get a new prop.