I think my earlier post may have been overlooked, so I'm re-posting my question. I am using a Yanmar diesel with a 7 degree down angle output transmission with a PSS shaft seal setup. The flexible mounts allow the engine to torque a noticable amount as it revs (mounts have very few hrs on them). My question is, how is the alignment at the coupling not thrown off as the engine revs and twists in the mounts using a down angle transmission? Are there special considerations taken when using a down angle transmission? With a parallel output transmission, it seems like the effect of engine movement would be less significant. I am asking because I am experiencing a high rpm vibration that seems to be coming from the prop (felt at the helm). I am using an Autoprop which is much heavier than the stock fixed prop and develops more thrust at lower rpms. After extensive analysis of the entire system, everything seems to check out. I am considering installing more rigid mounts (Yanmar's seem very flexible) or better yet, using a flexible coupling (not sure there's enough room). Do you think this logic makes sense? Thanks for your input.
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This Advice Is Worth What You Are Paying..
..,but I'd stay with the stock mounts. The engine should stabilize, in gear at load and at working revs. I assume you are comfortable with the static alignment of the shaft. If that is correct, I think the engine movement isn't material. All said, I would go with a flexible coupler first. I just looked at one and if you have 3/4 inch room to move the shaft back, you should be in business. In my case, I was looking for a one inch spacer that I had to have custom made, so I cannot offer an opinion as to how effective they are. I doubt the 7 degree output angle is any issue compared to a straight one. Other than that, I'd be at a loss to try anything else before going back to the fixed prop for comparison. If it goes away, my suggestion would be to work with the prop distributor for a fix. Best of luck. I just went through one of these nightmares on an old power launch. Rick D.
..,but I'd stay with the stock mounts. The engine should stabilize, in gear at load and at working revs. I assume you are comfortable with the static alignment of the shaft. If that is correct, I think the engine movement isn't material. All said, I would go with a flexible coupler first. I just looked at one and if you have 3/4 inch room to move the shaft back, you should be in business. In my case, I was looking for a one inch spacer that I had to have custom made, so I cannot offer an opinion as to how effective they are. I doubt the 7 degree output angle is any issue compared to a straight one. Other than that, I'd be at a loss to try anything else before going back to the fixed prop for comparison. If it goes away, my suggestion would be to work with the prop distributor for a fix. Best of luck. I just went through one of these nightmares on an old power launch. Rick D.
I Did
Patrick, I fitted a flexible coupling and, as it moved the prop back about 35mm I was able to fit a Stripper as well. double benefit!
Patrick, I fitted a flexible coupling and, as it moved the prop back about 35mm I was able to fit a Stripper as well. double benefit!
Vibration
The disadvantage of a spacer is that the prop will move aft 1" increasing your "unsupported shaft length". This is the distance between the strut and the face of the prop hub. If is more than 1.5 times the diameter of the shaft (ie 1.5" for a 1" shaft) then the shaft can flex and wobble causing vibration of its own. Bronze shafts are more flexible than SS. Another source of vibration comes from the prop tips being too close to the hull. The spacing should be 10 to 15% of the prop diameter. The Autoprop pivots its blades forward when the transmission is in forward. The slope of the hull causes this distance to be the least while motoring in forward. Try this test: with the boat well tied to the dock, put the boat in gear and run the power up to cruise power (or full) and note the amount of vibration. Power down, put the transmission in reverse and power up again to see if there is less vibration. In reverse the blades have pivoted aft where they have the most clearance. If the vibration level is the same in forward as reverse then you do not have a tip clearance problem. PYI sells a drive kit that puts a CV joint into your driveline that makes alignment irrelavent.
The disadvantage of a spacer is that the prop will move aft 1" increasing your "unsupported shaft length". This is the distance between the strut and the face of the prop hub. If is more than 1.5 times the diameter of the shaft (ie 1.5" for a 1" shaft) then the shaft can flex and wobble causing vibration of its own. Bronze shafts are more flexible than SS. Another source of vibration comes from the prop tips being too close to the hull. The spacing should be 10 to 15% of the prop diameter. The Autoprop pivots its blades forward when the transmission is in forward. The slope of the hull causes this distance to be the least while motoring in forward. Try this test: with the boat well tied to the dock, put the boat in gear and run the power up to cruise power (or full) and note the amount of vibration. Power down, put the transmission in reverse and power up again to see if there is less vibration. In reverse the blades have pivoted aft where they have the most clearance. If the vibration level is the same in forward as reverse then you do not have a tip clearance problem. PYI sells a drive kit that puts a CV joint into your driveline that makes alignment irrelavent.
I believe it to be propeller pitch loads.
Dear Patrick, Thank you for your note. In my opinion I do not believe that the down angle shaft/transmission is an easy installation to keep quite. Once the propeller is producing thrust, you are correct in assuming that the running gear is pushed forward and out of column. What we have found is that the front of the engine dives down and the aft end raises along with the engine moving side to side. The result is the coupling, no matter how well you place it in alignment, is stressed resulting in a miss-alignment during operation. We have found that with our Max-Prop the two ways to fix the upper Rpm vibrations was to change to a three blade, which I believe you have already, and to reduce any over loading that can be taking place with the prop. In your note you state that you have an Autoprop. It is very common for them to limit the obtainable Rpm’s of the engine by producing adverse loads. When we have installed Max-Props on Yanmars with down angle shafts, we have found that if any adverse loads that may be present due to aggressive pitch we experienced vibration. PYI had a great many problems with the J-44’s because of this. It was not until we altered the pitch load did we fix the problem. These loads are often the simple introduction of to much fuel for the given Rpm, which are a result of to much pitch load. This added fuel creates added pressure, which in turn causes added stress and movement to the running gear system. A way to confirm if you have excessive loads, is to run your engine up to 100% throttle and determine were the engine loads out at. If your Yanmar obtains less then 3,600 Rpm’s you should have the pitch reduced by the factory to allow the engine to get to the upper Rpm’s. In what you describe my guess is that you will not see much more the 3,200 Rpm’s at your full load throttle. Changing mounts may have some impact to reducing the noise but I do not think that it will eliminate it. I do not believe that placing a flexible coupling in the middle will fix your problem either. Because of the down angle and the introduction of a flexible element it could actually reduce the support causing more problems. Good luck and BY NO means am I inferring any negative comments towards your current propeller. Good luck Kevin
Dear Patrick, Thank you for your note. In my opinion I do not believe that the down angle shaft/transmission is an easy installation to keep quite. Once the propeller is producing thrust, you are correct in assuming that the running gear is pushed forward and out of column. What we have found is that the front of the engine dives down and the aft end raises along with the engine moving side to side. The result is the coupling, no matter how well you place it in alignment, is stressed resulting in a miss-alignment during operation. We have found that with our Max-Prop the two ways to fix the upper Rpm vibrations was to change to a three blade, which I believe you have already, and to reduce any over loading that can be taking place with the prop. In your note you state that you have an Autoprop. It is very common for them to limit the obtainable Rpm’s of the engine by producing adverse loads. When we have installed Max-Props on Yanmars with down angle shafts, we have found that if any adverse loads that may be present due to aggressive pitch we experienced vibration. PYI had a great many problems with the J-44’s because of this. It was not until we altered the pitch load did we fix the problem. These loads are often the simple introduction of to much fuel for the given Rpm, which are a result of to much pitch load. This added fuel creates added pressure, which in turn causes added stress and movement to the running gear system. A way to confirm if you have excessive loads, is to run your engine up to 100% throttle and determine were the engine loads out at. If your Yanmar obtains less then 3,600 Rpm’s you should have the pitch reduced by the factory to allow the engine to get to the upper Rpm’s. In what you describe my guess is that you will not see much more the 3,200 Rpm’s at your full load throttle. Changing mounts may have some impact to reducing the noise but I do not think that it will eliminate it. I do not believe that placing a flexible coupling in the middle will fix your problem either. Because of the down angle and the introduction of a flexible element it could actually reduce the support causing more problems. Good luck and BY NO means am I inferring any negative comments towards your current propeller. Good luck Kevin
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