This is a recurring problem and the topic of a previous thread in 2002. Here's what I wrote back then, and read Dyk Luben's reply. With a bad transmission, observing engine RPM is pointless, you need to be concerned with shaft RPM. While the shaft may be turning, it won't turn with enough RPM to drive the boat even at high engine RPM. I'll wager to anyone that their Martec prop is indeed opening. Climb overboard and watch!

Brad Stokes
Senior Member posted 08-08-2002 11:19 AM
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My prop opens easily by hand and when the blades are oriented properly the lower one will hang down by its own weight. Given that data, I knew with absolute certainty that if it spins, it has to open. The condition Bob and Dyk are describing happened to me just after I reinstalled the engine after rebuilding it and I had not adjusted the linkage properly. I would have to rev the engine up around 1500-1800 and it would rumble and abruptly "kerchunk" and churn water at the stern. Initially I thought that was the prop opening, but knowing how loose the prop was I knew there was no reason for it to be closed before that. So I looked at the shaft. When put into gear, it would spin but so slowly that there wasn't enough revolution to drive the boat. I pushed on the linkage lever on the transmission and wham it fully engaged and the shaft started spinning like it should - much faster. I realized the rumbling I heard was gears trying to mesh and the kerchunk I had heard was the transmission popping into gear, not the prop opening. I adjusted the linakge so now when I shift into forward, the boat moves even at low RPMs.
Dyk, Bob, I don't know how tight a brand new or recently rebuilt prop is supposed to be, but a spinning mass generates quite a bit of force. The formula for centripetal force is F=mv^2/r with m=mass of object, v=velocity at a certain radius, and r= radius. A quick check of the numbers will tell you that even at low RPM, there is easily enough force to open your prop. Keep in mind that if it is so tight that it won't open, it shouldn't be able to close either, so when sailing an open prop should be spinning like mad and you can hear and feel that. If you can assure yourself that the prop is opening and closing, the only other variable in the equation is the RPM of the shaft. I can only speak from my own experience and what worked for me but there is no reason I can think of for a clean, properly adjusted prop to not open. It has been designed to open when it revolves. Good luck!

Dyk Luben
unregistered posted 09-13-2002 02:36 PM
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I need to follow up on my propeller situation. After further study, I found that indeed the propeller shaft was spinning too slowly. It was a problem in what I called the transmission, but what the professionals call the reversing gear. The clutch was worn out and no amount of linkage adjustment would correct it. I had a new clutch and seals installed by Mack Boring, and it works perfectly now. So if anyone assumes that their propeller is not opening correctly, don't go buy a new prop like I did, but check first that the clutch is fully engaging