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Old 06 September 2014, 13:19   #1
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Why don't outboards/sterndrives have multiple gears?

I remember reading about an Arctic Blue 29 or 37 which had two selectable gears and did 86 knots with a single Ilmor as a result. Why isn't this more common, surely selectable gears would increase fuel economy by allowing a higher gearing when cruising and increase holeshot with a faster ratio?
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Old 06 September 2014, 13:25   #2
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Cost, weight, complexity...

???

Cars/trucks etc are the exception lots of stuff doesn't have variable gearing.
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Old 06 September 2014, 13:33   #3
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I remember reading about an Arctic Blue 29 or 37 which had two selectable gears and did 86 knots with a single Ilmor as a result. Why isn't this more common, surely selectable gears would increase fuel economy by allowing a higher gearing when cruising and increase holeshot with a faster ratio?
Cos it's not as simple as that! my understanding is that prop grip and cavitation (in its true sense) becomes the issue. there was a variable pitch prop around which achieved something similar not sure why it wasn't a commercial success.you'd have extra weight, maintainence and drag issues to offset against any gain.
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Old 06 September 2014, 13:48   #4
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It doesent take much variation in load to totally stall your motor you can see that when fitting a slightly larger prop
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Old 06 September 2014, 16:20   #5
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Sunseeker had a dabble at a two speed box. The problem is torque. As soon as it changed you went below the optimum power band and used to bog down. We had a go at putting a Motorbyke engine in a boat. Sad results on that
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Old 06 September 2014, 17:33   #6
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We had a go at putting a Motorbyke engine in a boat. Sad results on that

I think I can remember seeing that episode of Scrapheap Challenge..?

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Old 06 September 2014, 17:36   #7
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Would have been a better result to just hang the back tyre over the back
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Old 06 September 2014, 19:11   #8
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Where's Kitten these days anyhow?
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Old 07 September 2014, 02:30   #9
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Very common in flat bottom V-drives to use a Turbo 400 or Powerglide transmission. Ironically it is only used at low speed, and often to give a usable reverse. When drag racing they all throw it in the top gear for launching. Of course most of those boats are running highly modified BIG blocks with goobs of horsepower and torque. Nothing like American Muscle

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Old 07 September 2014, 03:56   #10
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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.
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Old 07 September 2014, 04:18   #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, 13:03   #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, 15:59   #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, 04:02   #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, 05:52   #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, 07:18   #16
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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
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, 11:08   #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, 11:19   #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, 14:15   #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, 14:46   #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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