New 4 Blade Propeller
Once we launched the boat we quickly found that it cavitated terribly around 3,200 RPM.  Cavitation is when the blade tips on a propeller begin to spin too fast.  Tiny, low-pressure bubbles in the water collapse against the tips with enough force to actually PULL metal out of the blade.  If this continues long enough, cavitation will destroy the blade tips.  Reducing shaft speed by increasing pitch and blade area is the usual fix for this problem.  That is what I have tried to do.

My old propeller, which came with the boat, was identified as a "Federal 13X12."  This means the propeller is 13 inches in diameter with a 12 inch pitch.  Pitch is the distance a propeller would travel through a solid - if it could.  So, a 12" pitch propeller would travel 12" in one revolution.

I switched to a "OJ 12X13."  I went down one inch in diameter but up one inch in pitch.  Plus this propeller is cupped.  Cupping the blade improves performance because the blades are able to hold better when operating in a cavitation condition.  However, as you add pitch you increase the load on an engine and adding cupping adds additional load as if the pitch were increased further.  This equals to a 150-300 drop in RPM at wide open throttle.  NO BIG DEAL, you may say......  but you need to make sure that whatever propeller you install allows you to operate the boat within the recommended wide open throttle range.  For example....if your engine has the recommended wide open throttle RPM range of 4,200 - 4,600 RPM then you must be able to reach that RPM.  If you can not, then you MUST change propellers to reach the range.  If you do not, then your engine will "lug" down.  This will severely limit the life of an engine.

With all of this in mind, I increased the pitch of my prop and added cupping - thus limiting the cavitation effect but lowering my wide open throttle RPM..  Then, I also decreased my prop diameter by one inch to bring the wide open throttle RPM back up.  On paper, the math says this should work, but then again, this is my boat and if anything will go wrong - it will.  Here are two pictures of the new prop and the boat.  More on this later after I get a few test drives completed.
Here is my old three blade prop next to my new 4 blade prop.  You can easily see the size difference of the blades.  Click on the image for a larger view.  You can see the effects of cavitation on the old prop.  On the trailing edges of the blades there are sections of a different color.  It was here that the cavitation damage was occuring.  Lets all hope the new prop cures that problem.
Here is the prop installed.  You can see some areas of the rudder that have the paint removed.  This was caused by the high RPM passage of water over it  I suspect this is normal since this propeller spins at the actual RPM of the engine.  You can also see the two new drain plugs I installed.  These help get alot of water out of the bilge whenever I have the boat lifted out of the water for storage.
Here is a shot of my boat being lifted out of the water on the fork lift at the marina.
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