prop size aluminum runabout

[tyl]

i have 17.5 sea nymph aluminum runabout with a 1978 85hp johnson manual trim i have a aluminum17 pitch prop now and have a 19 pitch ss that needs repaired. The prop shop said i can stay with 19 or they can make it a 17. I plane up good and engine runs excellent .but my top speed is 30-32 mph. Does anyone have a similar setup is this the best top speed I can get. Would a 17 pitch aluminum be the same as 17 pitch ss.No tach and speedometer is the kind with the hose.This is my first boat so I am not sure about what kind of performance to expect.

 

[Bob_VT]

Re: prop size aluminum runabout

Either is okay but running w/o a tach is not good. You really want to hit about 5500 rpm on wide open throttle.

 

[tyl]

Re: prop size aluminum runabout

I installed a tach and I run about 4600 rpm at wot with 2 people. Speed is 32mph with a 17p aluminum.

 

[steelespike]

Re: prop size aluminum runabout

Is the speed by gps? Seems a little optimistic,4600 with 10% slip makes about 29mph.
The tach may be a little lower than actual.If the speed is gps then the tach is a little low.
It does appear a 15" prop may be good.

 

[tyl]

Re: prop size aluminum runabout

not gps factory speedometer and the tach is the kind where you wrap wire around 1 plug wire.

 

[Titanium48]

Re: prop size aluminum runabout

It sounds like it would be safe to get the prop shop to repitch the SS prop to 17p. Pitot-type boat speedometers are notoriously inaccurate (mine is off by about 30%), so I would only use it for relative before and after comparisons. It is also possible that an inductive tach pickup might miss a few pulses and read a bit low, but you are almost 1000 RPM below the max with a 17p so you have a lot of room even if your tach is a bit off.