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Old 07 September 2014, 05:18   #11
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I can see how it'd work if you could use a Constantly Variable Transmission, as a CVT can be dialled into the 'powerband' and the whole HP of the motor used.
I did have that idea. You could have the box up inside the engine cowling so as not to increase the drag from the leg.
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Old 07 September 2014, 14:03   #12
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I did have that idea. You could have the box up inside the engine cowling so as not to increase the drag from the leg.
You'd have to. I don't think there'd be much advantage to it though-it'd be heavy and you won't get it to go any faster.
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Old 07 September 2014, 16:59   #13
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There was an aftermarket modified gearbox for Volvo 290DP drives called a 'step drive' that had two fwd gears. I think PRM made it not sure if they still do them. They used an old Sunseeker as a demo boat.
ZF also do two speed drives for use with Trimax surface drives. They were used on several different Sunseeker models (twin & triple engine) & Fabio Buzzi boats probably loads of others but they are the boats I know of as they were coupled to Yanmar engines.
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Old 08 September 2014, 05:02   #14
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I've been wondering why the reverse gear is down in the gearbox (yeah I know I've sort of answered my own question there) but why not have the F-N-R bit up in the cowling under the powerhead & just a straight forward drive shaft & bevel gear in the bottom of the leg. This would do away with the shift rod/linkage & make the leg a lot slimmer. Just a rambling thought
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Old 08 September 2014, 06:52   #15
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I've been wondering why the reverse gear is down in the gearbox (yeah I know I've sort of answered my own question there) but why not have the F-N-R bit up in the cowling under the powerhead & just a straight forward drive shaft & bevel gear in the bottom of the leg. This would do away with the shift rod/linkage & make the leg a lot slimmer. Just a rambling thought
I've wondered that too, it would make a massive difference to speed and efficiency if you could make the gearbox much smaller, I know sticking a smaller gearbox on a yamaha 90 can take it from 75 to close to 80mph.
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Old 08 September 2014, 08:18   #16
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Quote:
Originally Posted by Pikey Dave View Post
I've been wondering why the reverse gear is down in the gearbox (yeah I know I've sort of answered my own question there) but why not have the F-N-R bit up in the cowling under the powerhead & just a straight forward drive shaft & bevel gear in the bottom of the leg. This would do away with the shift rod/linkage & make the leg a lot slimmer. Just a rambling thought
You would be adding another gear set & therefore increase drag & also need to lube said gear set somehow
You would still need the lower gear although only 1 but couldn't reduce the diameter only the length which wouldn't help performance same goes for shift rod it runs down the front of the shaft so just runs in dead space
As for multiple gear ratios as has already been said having 2 or more ratios would only help with slow speeds as the engine can only take a certain ratio & needs to be doing a certain amount of rpm to produce it's hp
Drop the rpm & you overload the engine
You can reduce the ratio but not increase it, in the same way you can't just fit a bigger prop
Boats don't have hills to go down unlike cars
Maybe a small benefit to reduce ratio for water sports or rough water use but probably easier to switch props for water sports
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Old 08 September 2014, 12:08   #17
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Mercruiser offered the "Black Track" 2 speed gearbox on certain sterndrive packages in the 90's, never caught on, and wasn't that popular with boatbuilders.
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Old 08 September 2014, 12:19   #18
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gotchi - this should achieve something similar - but how big is your budget? Prices, Terms & Warranty
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Old 08 September 2014, 15:15   #19
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gotchi - this should achieve something similar - but how big is your budget? Prices, Terms & Warranty
Thats pretty cool, I've always wanted to make a propeller like that, couldn't quite get my head around how to do it though. Not quite sure how they did it but it's pretty clever stuff!
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Old 09 September 2014, 15:46   #20
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Old offshore race boats used two speed gearboxes, first gear was just to get on the plane as they were quite heavy especially with fuel when taking off and the used to run the props quite high.
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