"Gene Kearns" wrote in message
...
On Sat, 18 Dec 2004 11:01:15 -0500, MikeG
wrote:


The planning speed thing was what was bothering me also. It's the main
reason I got to thinking about re propping.

The present prop is in pretty sad shape. I'm hoping that even without
changing anything a new prop of the same spec's will show an improvement
in that area.


The direction of this thread seems to have made a left turn and gotten
me a bit lost....

The *prop,* per se, has *nothing* to do with the speed at which a boat
planes. While, you may well pick up some efficiency and/or top end
with a new prop, it won't change the planing speed at all.

the prop has nothing to do with the speed at which the boat planes, but it
does affect the ability of the boat to maintain that speed before falling
off plane. For instance, his boat may actually plane at 17 mph...but his
current prop can't maintain the boat on plane at 17 mph, and it quickly
falls off plane. It sounds to me like 20 mph is simply the minimum speed at
which he's able to *maintain* plane...and he's calling that his minimum
planing speed. Changing props won't change the speed at which the boat
planes, but it very well may change the boat's ability to maintain a lower
planing speed. I'd say try the 4-blade. If he drops the pitch, the engine
will overrev...unless he also increases diameter to compensate. Of course,
he could also have a prop shop add cupping to the blades, but then it
becomes even less of a science and more of trial and error.