STEP UP GEARBOX BY ZF OR TWIN DISC??

[nabeelah]

I do not know if ZF or Twin disc manufactures step up gear boxes that increase the rpm output.For example a marine engine that runs WOT @ 2300rpm to be coupled to jet units that require nothing minimum than 2550 rpm.i.e I need the jets to get anything in between 2550rpm to 2800rpm from the same engines that can produce only a maximum of 2300rpm.
Hopefully I do not sound ridiculous!!

[seapower]

has what you are refering to. I can't speak for ZF, but I'm sure they do also. "Induction" gear boxes are what you're looking for. Frank

[nabeelah]

Frank,are they expensive as compared to regular g.boxes??

[seapower]

Yes they are expensive, just like anything else that has "marine" attached to it. They are bulkier and are considered "highbred's" (not the usual), so I would expect the price to reflect that. My only dealing's with them are through a boat builder/friend of mine. He installed one in a sportfish that had an inboard/outboard installed. They used the gear ratio to keep the diesel from destroying the outdrive. Frank

[kapnd]

Yes they are expensive, just like anything else that has "marine" attached to it. They are bulkier and are considered "highbred's" (not the usual), so I would expect the price to reflect that. My only dealing's with them are through a boat builder/friend of mine. He installed one in a sportfish that had an inboard/outboard installed. They used the gear ratio to keep the diesel from destroying the outdrive. Frank

Nabeelah, sounds like you need a different motor, not transmission.
Many modern diesels rev plenty high for your jet drive application, and would allow the use of an off-the-shelf reduction gear, 1:1 is common.
A custom tranny will cost you plenty, would be very disappointing if it didnt give the results you are looking for.

Frank, I read Gerr's propellor book, he had an upper limit for the prop tip speed. Cant remember what that was, but would think such a setup would have a very limited range of efficiency.
Hard to believe that nobody makes a production leg that can handle diesel torque. They would sell like hotcakes!

Just an afterthought, jet drives dont need a reverse gear, just use a straight drive, no tranny?

[seapower]

There are a few (very few to my knowledge) outdrive's that are built for use with higher torque. From what I understand, part's are slow in coming if you have a break-down. It's all about market demand. I too, would like to see a robust product that'll handle the torque modern day diesel's develop. One problem with making an outdrive for diesel's is, that all diesel's are not created equal. Just look at the torque curve's of a few brand's at equal horsepower. I think you'll be amazed at the difference's you see. Not only that check out the torque verses RPM. The problem Nabeelah was trying to over come is why there are induction gear's. Some application's just need speed of propulsion, not brute strength or the added weight. Frank