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post #12 of Old 12-15-2012
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Re: overpropped vs underpropped

Thanks John for the discussion. I understand that my WOT term is not the WOT that the mfg uses. It's just the max rpm that can be achieved at a given prop pitch.

Let me try to explain in a different way. As I go up the hill in my Jeep without down shifting I will slow down with the rpm going down to zero yet when I go into a headsea/headwind that is not the case for the boat. The boat will slow down yet the engine will maintain 2500rpm. Why is that? The Jeep has no slippage in that the tires maintain firm contact with the pavement while the prop on the boat is "slipping'. In other words the pitch of the prop is set for an optimal speed. I set mine for achiveing 6 knots at 2500 rpm and can approach hull speed at "wot" of 3000 under calm conditions. I just don't want to hear the engine screaming at 4000 rpm which is the max intermittent rating and is what I can achieve under no load. While a max prop allows for setting different pitches it does not allow a variable pitch while underway. I guess we will have to agree to disagree on this and note that I'm not advocationg anybody follow me on this.

This brings up another interesting point on engine operation. In my Perkins workshop manual, it mentions replacing a zinc in the heat exchanger. There is no zinc in that heat exchanger. I found a good perkins mechanic who agreed that there is indeed no zinc since the rubber end gaskets on the end of the exchanger isolated the core from the metal of the rest of the engine. My point is that even a workshop manual can be wrong...and in some cases even I.

Best Regards
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