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Author Topic:   13-footer, Yamaha F20
tjxtreme posted 05-01-2012 10:24 AM ET (US)   Profile for tjxtreme  
The current propeller on my [1978 Boston Whaler 13-footer] is a Solas 9-1/4 x 10. Yamaha recommends pitches between 8 and 12 for this motor. I am interested in improving acceleration; with two people and coolers [the 1978 Boston Whaler 13-foot boat] can be slow to get on plane. I know [the 1978 Boston Whaler 13-foot boat with Yamaha F20] will never be a speed demon. The [the 1978 Boston Whaler 13-foot] boat with a light load currently runs at 28-MPH. Suggest a replacement propeller. Thanks
Teak Oil posted 05-01-2012 08:34 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
We need to know what boat you are running the boat on.
dgoodhue posted 05-01-2012 09:13 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
It looks like he has a [1978 Boston Whaler 13-foot boat with Yamaha F20]. I missed it the first time I read his post. [Boat speed of] 28-MPH seems like a good top speed for 20-HP and 13-feet. In general, if you want more acceleration and heavy load propeller, you need to go down in pitch.
tjxtreme posted 05-02-2012 12:01 AM ET (US)     Profile for tjxtreme    
Yes. [The boat] is a 1978 13-foot Whaler. If I go down one-inch in pitch will the difference be noticeable? Thanks
dg22 posted 05-02-2012 09:25 AM ET (US)     Profile for dg22  Send Email to dg22     
If you stick with the same brand, one-pitch down will be noticeable for acceleration, but I'd be cautious about running it wide-open for too long with a light load. You may want to get a tachometer. It sounds like you have the perfect propeller right now for a light load so you are probably running at or very close to maximum RPM.
tjxtreme posted 05-02-2012 10:54 AM ET (US)     Profile for tjxtreme    
I will look into getting a tachometer. Could you explain the risk of running a proeller pitch too small?
dgoodhue posted 05-02-2012 12:28 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
[Your Yamaha F20 engine] could exceed the maximum recommend engine speed range. Having a tachometer would at least let you know the engine speed, you could adjust the throttle accordingly.
tjxtreme posted 05-02-2012 01:02 PM ET (US)     Profile for tjxtreme    
I see, I shopped around a bit. Are the tachometers that wrap around the ignition wire reliable? They seem to have mixed reviews.
Teak Oil posted 05-02-2012 06:06 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
A small hydrofoil would let you keep most of your current speed and help considerably with the planing of the boat with a load.
dgoodhue posted 05-03-2012 06:23 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
I did some quick calculations on your set up using propellor calc in the Refence section. I assume you motor has the current 2.08:1 gear ratio and 5,000 to 6,000-RPM operating range. Estimated ten-percent prop slip and estimate your engine speed to be 5,700-RPM at 28-MPH. Obviously this is theoretical, but it does perhaps suggest that you might be able to go to a lower pitch propeller and lose little to no speed.

AGetting tachometer readings, even if only temporary, would be helpful.

dgoodhue posted 05-03-2012 06:27 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
Never mind I realized that you have a 10-pitch propeller; I had calculated a 12-pitch propeller.

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