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Old 23 August 2004, 11:20   #11
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The QM2, or any other large ship is completely unlike a rib.

A rib needs a large gob of torque low down to drive the boat up onto the plane. A large ship isn't even close to hull speed and therefore has a very simple relationship between thrust and speed.

The QE2 (since the QM2 wasn't around back then) has something like 6 medium speed diesels driving alternators. It then has a bank of electric motors connected to the prop shafts.

Typically, really really big ships don't have a gearbox at all. They have a full throttle rpm of about 60rpm, and have a variable pitch prop. To reverse, some of them have to stop the engine, engage a reverse drive on the camshaft, and fire the engine up in reverse. There is no need for a gearbox on big ships since a variable pitch prop is going to be infinitely more flexible.

Where a gearbox is useful is where more power is required to get on the plane than is necessary to stay on the plane. Often, prop design is such that the prop ventilates sufficiently as the boat climbs on the plane to get the engines into the powerband, thereby getting enough power to get on the plance. Class I boats have this, look at the funny tube at the back leading down to the prop. A number of mercury's props also have vent holes, and race props on ob's and sterndrives often have over hub exhausts

There are also some variable pitch ob props available. I believe they have varying degrees of success, but are difficult to keep reliable.

So really, the only time you need a gearbox is to get up on the plane when you have such a coarse pitch prop, with so much grip, that you cannot get the engine revs up enough to get enough power or thrust to get going. But as Jon stated, you've already kind of got an auto gearbox since prop slippage is very high at low speeds.
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Old 23 August 2004, 11:27   #12
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Don't we all have props that gives us all the maximum permsisible speed for our ribs against the recommended WOT (dive Ribs excluded) for the various makes of outboard that we use?

The only point in having a gearbox surely is to exploit acceleration and reducing plane times!

That means the a gearbox would only benifit you if you raced boats or you had heavy loads aboard, other than those two resons I can't see the point of one.

Andy

PS. just read your post Mat and agree fully
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Old 23 August 2004, 11:30   #13
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Yes.
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Old 28 August 2004, 04:37   #14
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Gearboxes

ZF do a two speed gearbox - was developed, I believe, by Fabio Buzzi for his race and fast interceptor boats fitted with Trimax surface drives.

Sunseeker fit them on their boats with Trimaxes - Superhawk 40, 50 and the XS 2000.

Have driven a Superhawk 50 with them fitted to triple Yanmar 440hps with Trimax drives.

Accelleration in first gear was much better than trying to get on the plane in second - much less smoke as well. However, switching from first to second (done with a small electric switch) produced an unhealthy bang - I don't know how long they are likely to last.

It depends on how fast you want to get on the plane or if you are worried about a bit of smoke as you do so.

Duncan
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Old 28 August 2004, 05:54   #15
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I'm not quite following this gearbox thing. I don't understand how you could keep the normal prop and gear it for acceleration. Surely you'd need a 1:1 (or thereabouts) ratio and a fine pitch prop for getting up and running then the gear would change to speed up the prop to allow the boat to run faster.
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Old 29 August 2004, 09:28   #16
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Gearboxes

What you do is size the propeller for for maximum speed in second / top gear. For some engines - turbocharged diesels this can sometimes mean that the engine bogs down when trying to get on the plane. So the gearboxes first gear is significantly lower to allow the engine to run at a higher speed enabling it to produce more boost and torque and so putting the boat on the plane much quicker. Once up and running you switch to second and you get your top speed.

The Superhawk I was talking about would do 50 knots in top and about 30-35 knots in first.

The other way to get round this phenomenom it to fit a supercharger as Volvo does.

Duncan
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Old 29 August 2004, 10:25   #17
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Gear Box's

Brian

The company I started 35 years ago build thousands of gearboxes each year and has in recent years been given the contract to oversee the development of a full "variomatic" unit for, Road, Race and marine use. I am guessing, because some people on this forum who have not heard about it will say it does not work but I can assure you it does. we have had it on the back of a 800 hp test motor with very good results. The marine version will be tested this winter. It's concept is to allow a variable stepped gear ratio to suit the vairable conditions and payloads that are seen on commercialy used fast boats. Call me if you want further information Alan P
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Old 29 August 2004, 14:05   #18
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Quote:
Originally Posted by Searider
What you do is size the propeller for for maximum speed in second / top gear. For some engines - turbocharged diesels this can sometimes mean that the engine bogs down......
Duncan
Thankyou. So it doesn't address the problem of biggish pitched props at low speed, just the low power of a turbo'd diesel before the turbo is any use. That seems a missed opportunity to me. Back to the variable pitched prop, it does both.
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Old 30 August 2004, 07:42   #19
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Quote:
Originally Posted by jwalker
Thankyou. So it doesn't address the problem of biggish pitched props at low speed, just the low power of a turbo'd diesel before the turbo is any use. That seems a missed opportunity to me. Back to the variable pitched prop, it does both.
NO!

That's exactly what it does - overcomes the problem of big pitch props driven by turbocharged diesels that have little torque at low rpm.

I don't think that variable pitch props will be the answer though - isn't there enough to go wrong / need servicing in a sterndrive as it is?

Duncan
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Old 30 August 2004, 08:04   #20
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I've used variable pitched props for many years on an outboard. Completely reliable. An excellent piece of kit. It's just a pity they're not available for a bigger range of engines. I've kept one lest I have a need for it in the future. It's the only type to use!
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