Thread: Under propped?
View Single Post
Old 03-15-2014, 06:59 AM   #6
FF's Avatar
Join Date: Oct 2007
Posts: 19,849
>In theory, a properly propped boat will run exactly at the rated RPM of the engine at full throttle.<

This is the method for plaining and fast boats.It is recommebnded by the Mfg as it assures very very low loading at normal cruise RPM.

This gets them by the warrentee period easily.

It is not fuel efficient or a good way to operate an engine , high RPM and minor load, on a displacement boat.

>I talked to another owner of an identical boat that is over propped with a 25x21 prop and he runs approx. 1600 RPM to get 8 knots, but cannot reach the 2500 RPM's.<

Sounds like this owner has installed a proper cruising prop , he should have an EGT gauge and a NEW red line on the tach.

I agree that 8K sounds high for a boat with probably a 36 ft LWL.

7K as suggested may take fewer RPM and 50% less fuel.

With a hand held GPS , simply operate the boat for about 2 min at every RPM going up in 100RPM steps , and plot the results.

Where you see a need fto add big RPM and almost no gain in speed , back off one K and enjoy the quiet and low fuel bill of LRC, Long Range Cruise.

When you locate the LRC , the goal is to prop to have the engine working at 60% to 80% of its rated power AT THAT LOWER RPM!

60% if you go out in blue water and climb 5-10 ft waves at speed. Or it blows 30K+ mos times where you cruise STIDD seats recommended.

FF is offline   Reply With Quote