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Old 23 August 2004, 04:42   #1
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Gearboxes

Why donít boats have gears?



Whether petrol or diesel, inboard or out, the overwhelming number of boats, from yacht tenders to the QM 2 donít have gearboxes, like cars.
There are obviously good reason for this.
What are they?
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Old 23 August 2004, 04:56   #2
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They have got gears. Forward + reverse and reduction gearing.

I know what you mean though! I suspect that having variable pitch size on your prop would have the effect of an auto box, this would improve acceleration but not top speed.

Andy
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Old 23 August 2004, 05:07   #3
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The class 1 offshore racers have gearboxes - and I believe they are filtering into the leisure market but only in the high performance sector - maybe someone else can tell you more?
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 23 August 2004, 05:14   #4
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Gears aren't really needed for normal boating coz a propeller running in water (or air) is such a good torque converter, the gearing kinda changes with speed as the efficiency/slip gets better the faster you go.
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Old 23 August 2004, 05:49   #5
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I would have thought size and weight were the main problems especially with outboards. I assume modern units will be electronic otherwise getting the gear change wrong could cause big problems at high speed.
I've never looked but does the prop spin freely in the water when not in gear or does it stop and cause drag.

Could it also be a reliability issue, less moving parts means less to go wrong

Anyway where would you put the clutch peddle
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Old 23 August 2004, 05:50   #6
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Cost & Reliability are issues
Merc once built the Deuce High that had contra rotating props & a 2 speed transmission. I believe you can also buy a 2 speed transmission that fits in the mid section of a conventional merc v6.
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Old 23 August 2004, 06:19   #7
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This mob are in our yard. tey tested this boat by going to the Needles and coming back every day for 6 months

http://www.prmstepdrive.com/index1.htm
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Old 23 August 2004, 06:22   #8
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so are this mob.

they haven't got it right yet but are working hard on their product. They are also very well funded

http://www.yellowfin.com/technology.shtml
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Old 23 August 2004, 06:38   #9
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OK then. Lets look at some of these issues.
If the prop. slip/efficiency gets better the faster you go, why not gear-up so that it goes even faster?

Rupert
I would envisage the gear-change to be automatic/electric/computer controlled or something, that would be so slick that the prop would not have time to stall in the water.
I can well imagine cost and maintenance (not to mention the extra weight) are likely to be severely constricting factors on outboards. But what about inboards on say a larger sized rib?
And as we get bigger, what issues can there be on the QM2?
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Old 23 August 2004, 09:51   #10
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Quote:
Originally Posted by Brian
OK then. Lets look at some of these issues.
If the prop. slip/efficiency gets better the faster you go, why not gear-up so that it goes even faster?
Is there a maximum speed you can do with a prop ? or a speed where the prop will just turn and not grip the water. (Am I just confusing this with cavitation)


Quote:
Originally Posted by Brian
I would envisage the gear-change to be automatic/electric/computer controlled or something, that would be so slick that the prop would not have time to stall in the water.
I can well imagine cost and maintenance (not to mention the extra weight) are likely to be severely constricting factors on outboards. But what about inboards on say a larger sized rib?
And as we get bigger, what issues can there be on the QM2?
If you have a gear box then the prop would have to be good at both low and high rpm, wouldn't this mean the geometry/size would need to change on the blades while it's running. Is a small high speed prop better for slow speed manoeuvring or a large slow one?

I guess this adds such a complex system to the design that it is cost and reliability become the main issues even on something as big as the QM2, unless she suffers generally from to much mass and the problem is not top speed but stopping.
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