Propeller question (change pitch or diameter?)

[ioan74]

Hello to all

I've got a prop question...a quite specific one actually. Maybe someone could help.

I have a 15ft, 1200lbs dry weight runabout, including the Yamaha F60 4stroke injected which powers it. Identical to this one: http://www.boten.nl/fotos/82852-3_800x600.jpg

I recently changed the prop in this way, with the following observations:

OLD PROP:
Pitch = 13"
Diameter=11-1/8
RPM at WOT: 5500
Planing at 3200 rpm

NEW PROP
Pitch = 13" (i.e. identical)
Diameter=10-3/8 (i.e. 6/8" smaller than before)
RPM at WOT: 6000
Planing at 3400 rpm
Observed: Better acceleration; spins faster because in spite of identical pitch, there is more slip due to the smaller diameter.

I am not that happy with that new prop. I feel it spins too fast (Isn?t 6000 max RPM at WOT too much?), and that top speed is a little less.

My question:

[] Should I keep the smaller diameter of 10-3/8", but increase the pitch to 14"?
[] Or buy a new prop with the old configuration, meaning keeping pitch at 13", but D=11.5"?


The new prop clearly has more slip, so I wonder if I don?t lose in terms of MPG. This is my 1st priority, accelleration is less important.

Any input is appreciated!

Ioan
Switzerland

 

[Sea Rider]

Welcome to Iboats,

If at wot with 5500 rpm and factory delivered prop, would have been ideal going for same size prop with at least 2 less pitch to pull wot rpm to 6 K assuming that 6K is the max wot rpm factory stated parameters for a Yam 60.

What was the max speed achieved with both props at wot, was speed measured with a GPS ? Ain't a 60 HP be rather small for that size boat, which is the max HP that will take ?

Happy Boating

 

[Texasmark]

Diameter is usually a spinoff for a given "size series" of props when the pitch is derived. Diameter usually increases with low pitch where heavy loads are encountered. Usually as the pitch increases the diameter will shrink for a given "model". Pitch is what makes the difference from a performance standpoint especially on light fast hulls.