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Old 13 April 2004, 12:38   #61
Seb
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So there seems to be either major faults or operational restrictions with all of the drives that have been mentioned although only coupled to relatively small torque engines such as mercruisers etc.

What does one do if they want a drive leg for something such as a CAT diesel which has massive torque?

If there are no options, does it mean that the only way to go is jet drive?

Cheers

SEB
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Old 13 April 2004, 13:05   #62
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Quote:
Originally Posted by Seb
What does one do if they want a drive leg for something such as a CAT diesel which has massive torque?
SEB
You could go Arnerson, something like an ASD 10 would cope, or maybe a 12. Or how about shaft drive, like a cruiser powered by similar motors would use.

The Yanks put the 440 hp Yanmars on B Max's, which are a modified Bravo 1. These are available from the Bravo Shop in Florida I believe! The other one that's a possibility is the latest Imco Bravo that uses a seperate gearbox, I don't know the torque handling figures for this drive, but they are supposed to be high!

Although both of these drives are excellent, they probably aren't that practicle for everyday use, but their worth taking a look at.
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Old 13 April 2004, 13:28   #63
Seb
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Thanks. Im not actually in the market for a CAT or a drive to match it (wish i was) but am purely asking out of interest as i rarely hear about engine installations in RIBs bar Yanmar, Volvo and Mercruiser.

Does this mean then that if one wanted a CAT for their RIB, there is no standard drive which could cope without venturing into surface drives and jets which always seem to be surrounded in huge debates?

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Old 13 April 2004, 14:13   #64
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I decided a while ago that a jet was the only sound way to go with big engines for RIB's, but have been put off somewhat by other threads! I think that it's horses for courses though, and would not be put off putting a jet into a big boat - after all many commercial and rescue boats working on oilfields worldwide are jet propelled, and they do go and play in most weather. They also have to launch into it from a ship!
In another thread (can't find it now) someone asked about fitting 2 or even 3 jets into a large RIB, one thing that may be worth considering if anyone's seriously going down that route is that crewboats and fast supply boats servicing the oil industry are generally multi engine, and a configuration that was popular in the past was 2 conjventional shafted engines, with a large unsteered jet in between to provide the bulk of propulsion.
Also regarding jets and opposed to propellors in large boats, (20 - 30m) it's generally held that jets are less efficient at lower speeds but come into their own as speed increases, and the efficiency of conventional props and jets cross over at about 25knots. I would guess that on a smaller prop the cross over would be at a higher speed.
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