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Author Topic:   RPM/PROP PROBLEMS ON A 15
Cliff posted 06-01-2001 11:20 AM ET (US)   Profile for Cliff   Send Email to Cliff  
Yesterday I hooked 2 tachs up to my 50 yamaha on the back of my 15 whaler and the engine is only taching WOT at 4100-4200 rpm(27.5mph). Prop is a 10-15 aluminum 3 blade.

Yamaha is telling me to call BW and ask them what the right prop is for this setup but I would rather start here with you all.

I am certain I need to lower the pitch but by how much and should I also increase the diameter, again by how much? Should I switch from a 3 blade to a 4?

I am hoping this is the problem and it is not the engine itself.

Any help would be well received since I am tired of dealing with this.Thanks in advance for any and all advice.

jimh posted 06-01-2001 12:53 PM ET (US)     Profile for jimh  Send Email to jimh     
There is propeller performance data for exactly your situation--15-Whaler and 50 HP--in the REFERENCE article on propellers, part 2:


You need to know the gear ratio of the Yamaha 50 lower unit to construct a similar analysis.

My guess is there might be something amiss with the engine, as a 50 HP should be able to swing a 15-inch pitch prop, unless you have the boat very heavily loaded.

I moved down to a 14-inch because we are often out with two people and a lot of gear, and we get into some larger waves, so I like the lower pitched prop for those situations, but the boat ran only about 100 RPM less with the 15-inch pitch prop of the same design (Merc BLACK MAX 3-blade Aluminum).

lhg posted 06-01-2001 12:58 PM ET (US)     Profile for lhg    
Cliff - I'm with Jim and guess engine problems. On selecting propellers, never worry about diameter, as that is determined by the manufacturer. A prop is bought by pitch only.
bigz posted 06-01-2001 01:18 PM ET (US)     Profile for bigz    
Cliff, run a compression check on the cylinders your way off regardless of gear ratio. With that rpm at WOT a prop ain't the answer. Good luck
Cliff posted 06-01-2001 03:45 PM ET (US)     Profile for Cliff  Send Email to Cliff     
Thanks all for the input. Taking the boat and engine to the yamaha service folks today.
jimh posted 06-01-2001 09:09 PM ET (US)     Profile for jimh  Send Email to jimh     
First thing I'd ask them to do would be to check the calibration of the tachometer that you have been using against one they know is accurate. That could be an issue.


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