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Old 21-06-2011, 09:35   #1
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Repower

Hello all.
I have just repowered my 76 cal 34 with a Yanmar YGM30F with a kansaki 3.2:1 v-drive transmission. I am not getting the speed I used to out of my old Westerbeke 30 with a walter geardrive 2:1. I am wondering if anyone can tell me what you are using for a prop, gearbox etc. if you have a similar or the same configuration boat and drive system.
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Old 21-06-2011, 09:47   #2
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Re: Repower

If you change the transmission ratio you change the power transmitted for the same engine rpm and old propeller. To correct you need to recalculate the size and pitch of the propeller to restore the thrust. There are thick books that can assist in doing the calculation but you need to consider the available "propeller free space" at the end of your propeller shaft in relation to the hull or aperture if you have a full keel with cut-out for the propeller.
- - The considerations will determine the maximum diameter propeller you can install. With each diameter propeller there are only a few "pitches" available. If you need "more" propeller than you need to get a propeller with more blades.
- - Once the calculations are done then you need to "test" the resulting recommended propeller and see if you get what you are needing. If not then you "borrow" the next size propeller and try it in real life. Normally you can find a propeller shop with a large assortment of propellers you can try if you let them know you will buy the final best one from them.
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Old 21-06-2011, 10:24   #3
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Re: Repower

Quote:
Originally Posted by MikeGavel View Post
Hello all.
I have just repowered my 76 cal 34 with a Yanmar YGM30F with a kansaki 3.2:1 v-drive transmission. I am not getting the speed I used to out of my old Westerbeke 30 with a walter geardrive 2:1. I am wondering if anyone can tell me what you are using for a prop, gearbox etc. if you have a similar or the same configuration boat and drive system.
Why did you elect to go with the 3.2:1? Both motors have somewhat similar torque / hp curves...

... the jump from 2:1 to 3.2:1 would seem to suggest something like a 30% - 33% increase in pitch... given the decrease in RPM at the prop.

The problem is that slippage is likely to increase, so you will not necessarily gain the same thrust from the increase... Maybe someone like "Never Monday" will post, but if you take this question to a competent prop shop they should be able to help you...
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Old 21-06-2011, 15:16   #4
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Re: Repower

Well.... It was a rebuilt engine and the transmission came with it. I would certainly have preferred the 2.14:1 ratio that is available which would have been very close to my old one. I was hoping the increased Rpm of the yanmar would bring the shaft rpm to about the same as the old 2:1 gear. Oops! Now I have to come up with a solution. Maybe I will have to get a different transmission as I am not sure the prop I have can be repitched enough to make a difference.
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Old 21-06-2011, 17:05   #5
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Re: Repower

Usually a gear ratio like 3.2: 1 is used with larger diameter props to transmit the torque of the engine at slower speeds, more pitch may work but I'm not sure how well. You are probably limited in the prop diameter by how close the prop baldes get to the hull (you don't want to get them too close, not sure but I think 1/2 to 1/3 the prop diameter away).
The rebuilt engine with the attached transmission may have bee a good deal, but you'll end up spending the money to get it right on other things.
Good luck in getting it sorted out,
Brian
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