prop rpm


Active Member

There is a 3 to 1 reduction in the gearing to the props and as a practical matter the engine RPM in water never goes above about 3400 RPM so the props normally never run above about 1100 RPM. Somewhere I seem to remember Hugh saying something like "be sure to disengage props when out of the water since the RPM can easily exceed the maximum prop RPM. I was a bit curious about that remark though since on the road the engine RPM rarely would go much above 5000 RPM which would give you a prop RPM of about 1650, not all that much higher than in the water. One thing I've noticed though is that the prop shafts run quieter when there is a water load on the props. This is probably because those prop shaft u-joints really flex a lot and the load keeps them from chattering. I'd say generally it's a good idea to disengage the props when out of the water, especially if your driving at much above "parade" speed.

John Friese
67 White
67 Red

Canadian four amphs

Amphicar Expert
I have never worryed much about spinning props out of water, as John says they spin much slower, no I would not do 60mph down highway with them on
You can hear them engaged and makes clutch work harder,
I ususally turn them off when shifting after coming out of water but sometimes forget and Amphis behind panic tooting horns etc.Leaving turn signal on is a more important thing to toot at.
but do remember you must be totally stopped before you engage them going into water