This is in response to the 280DP vibrations topic, I did not want to hijack that thread as my actual topic does not apply to the "A" series diesel duoprops because they have the 4 blade front prop.
To diagnose a vibration I would want to know on "B" series props if the effort to "time" or "clock" the props during install was taken. There is some credence to this needing to be done to improve performance and reduce possibilty of vibration as well as cavitation (burning of the edges of the props). The fundamental theory is that there should not be both blades of the duoprop passing under the cav plate at the same time. It is simple to clock them during install and does no harm. Put the forward prop on and move it to where a blade is pointing straight down. Then when mounting the rear prop move it around before it hits the splines until one of those blades is pointing straight up then move it onto the splines. I learned of this and have done it each time since and have had no vibrations and no cavitation. I had experienced cavitation on the first set of props and have not seen it since I clocked the new ones during install. I am not saying all boats with VP 280DP or 290DP drives react to this regarding vibration or performance, but this is also supposed to relieve the potential for cavitation of the props (burning the edges).
I started this topic to put forth the thought that any vibrations that were fixed when installing new props (if the old ones were not damaged) or even the reverse of this, introducing a new vibration after new props are installed, could be due to this phenomenon. Perhaps the old set of props was not clocked but then during installation the new props got clocked ok due to chance and "fixed" the vibration. Conversely, the old props did not vibrate but new props were put on (for a pitch change or whatever) and a vibration is introduced because the clocking is not optimal by chance.
Regarding pitch, I have a 22 foot cuddy cabin with Volvo Penta 570 275 HP and the 290DP drive. It came with B7's that were in horrible shape. I put on new B5's and it picked up 4 mph! But again the B7's were missing a lot of material and were horribly misshapen as well. The performance with the B5's is great for what we do with the boat, it is on a small lake so speeds of 50+ are not called for. We pull tubes and skis and cruise at 40 or so. The top speed is 48 and it can get there very quickly even loaded up. This hull planes fast anyway, and with the B5 duoprops it can hold lower speed planing even better. I put the original B7 set back on recently after I had them reconditioned. The hole shot was still ok and the top speed was going to be 50+, I had to settle for 50 at lower rpm due to conditions but I could tell the engine could reach top rpm of 4800 so it was a good test for me. I have a bit of a speed problem (as in holding back) so I put the B5's back on and sold the B7's. It can cruise effortlessly from 38-40mph which is plenty, and the pulling power is perfect for what we do (full boat and pulling tubes). According to my manual the engine has an rpm limiter so I do now worry about over-revving, it hits about 4800 and never tries to go above that. At first I thought (not having learned of the rpm limiter yet) that the B5's were correct, but now I am sure it could pull B7's and hit the top rpm range easily, just good stuff to know if my needs change.
To diagnose a vibration I would want to know on "B" series props if the effort to "time" or "clock" the props during install was taken. There is some credence to this needing to be done to improve performance and reduce possibilty of vibration as well as cavitation (burning of the edges of the props). The fundamental theory is that there should not be both blades of the duoprop passing under the cav plate at the same time. It is simple to clock them during install and does no harm. Put the forward prop on and move it to where a blade is pointing straight down. Then when mounting the rear prop move it around before it hits the splines until one of those blades is pointing straight up then move it onto the splines. I learned of this and have done it each time since and have had no vibrations and no cavitation. I had experienced cavitation on the first set of props and have not seen it since I clocked the new ones during install. I am not saying all boats with VP 280DP or 290DP drives react to this regarding vibration or performance, but this is also supposed to relieve the potential for cavitation of the props (burning the edges).
I started this topic to put forth the thought that any vibrations that were fixed when installing new props (if the old ones were not damaged) or even the reverse of this, introducing a new vibration after new props are installed, could be due to this phenomenon. Perhaps the old set of props was not clocked but then during installation the new props got clocked ok due to chance and "fixed" the vibration. Conversely, the old props did not vibrate but new props were put on (for a pitch change or whatever) and a vibration is introduced because the clocking is not optimal by chance.
Regarding pitch, I have a 22 foot cuddy cabin with Volvo Penta 570 275 HP and the 290DP drive. It came with B7's that were in horrible shape. I put on new B5's and it picked up 4 mph! But again the B7's were missing a lot of material and were horribly misshapen as well. The performance with the B5's is great for what we do with the boat, it is on a small lake so speeds of 50+ are not called for. We pull tubes and skis and cruise at 40 or so. The top speed is 48 and it can get there very quickly even loaded up. This hull planes fast anyway, and with the B5 duoprops it can hold lower speed planing even better. I put the original B7 set back on recently after I had them reconditioned. The hole shot was still ok and the top speed was going to be 50+, I had to settle for 50 at lower rpm due to conditions but I could tell the engine could reach top rpm of 4800 so it was a good test for me. I have a bit of a speed problem (as in holding back) so I put the B5's back on and sold the B7's. It can cruise effortlessly from 38-40mph which is plenty, and the pulling power is perfect for what we do (full boat and pulling tubes). According to my manual the engine has an rpm limiter so I do now worry about over-revving, it hits about 4800 and never tries to go above that. At first I thought (not having learned of the rpm limiter yet) that the B5's were correct, but now I am sure it could pull B7's and hit the top rpm range easily, just good stuff to know if my needs change.
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