gkelly
09-11-2004, 03:30 PM
I have a perkins 4-236 80hp engine at 2500rpm.with a borg warnet 2:10:1 ratio.what would be the correct prop be for this engine.The boat is 36 foot wooden 13ton 11 foot beam 3 foot 8 draft.The prop that is on it is 22 by 19 and it will only do 1750 rpm full trottle.
FAST FRED
09-12-2004, 05:58 AM
Measuring pitch is dificult , easiest is a prop shop.
Not pulling full RPM may be engine condition , or prop condition,
OR it could be what is sometimes called a cruse prop.
Most eng mfg prefer the engine to reach goverened full throttle , so when you pull back the propeller law curve reduces the engine output considerably , to extend engine life.
Unfortunatly some boats if operated at very LRC (long range cruise) speed of sq Rt LWL X .7 or .8 the engine will be very unloaded at that low RPM and suffer .
Wet stacking , burnishing the cylinder bores , excessive blow by all come from underloading.
SO cruising folk sometimed install a prop that CAN NOT pull max RPM.
The usual "rule of thumb" is to go to full throttle for a min. and then pull back at least 10% of the observed max rpm.
Its far safer to have a pyromometer in the exhaust and make sure your not overloading the engine.
Usually DIAmeter has far more to do with power absorbed by the prop than pitch.
The old standby Skenes Elements of Yacht Design or the new Dave Galley book will figure your prop requirements .
OR just fill out the form at the Michigan Wheel site , and they will do it for free.
FAST FRED
