Catamaran Conversion

Mr E the full answer would have to come from the people who make the units. Also the IOB format does limit prop selection to some extent. If the older units rated for up to 11'000 lb could be easily adopted to boats of heavy weight and comercial use why would they have to go to the trouble of developing and advertiseing the new 15,000lb capable units. These units all incorporate right angle gearing of some sorts and I suspect that The amount of HP torque and stress going through these gears has its limits and a heavy boat demands more to push it.

 

Prop diameter certainly is a limiting factor, and I'd guess the bigger diameter hub that would be needed to accomodate scaled-up gears also enters the equation, which would go hand-in-hand with bigger propellor diameters.

 

Think about the actual contact area of those bevel gears. It is at best a single wide line of contact area, the length of the gear face width at best. That i sa very small area of contact to be transmitting LOTS of HP over.

And you are doing this at both the upper and lower ends. With wear come worst tolerances, which then invites accelerated wear...a vicious circle

I'm truly surprised that some of these outdrives hold up for as long as they do considering the HP being utilized with them.

Those silent chains used to connect the engine/trans in the Olds Toronado were lasting for well over 100k miles.

http://en.wikipedia.org/wiki/Oldsmobile_Toronado
The Turbo-Hydramatic heavy-duty 3-speed automatic transmission, (or THM400, TH400) came about during development of the Toronado. Called the TH425 in FWD form, the transmission's torque converter was separated from its planetary gearset, with the torque converter driving the gearset through a 2" wide silent chain-drive called Hy-Vo, riding on two 12" sprockets. The Hy-Vo chain drive was developed by GM's Hydra-Matic Division and Morse Chain Division of Borg-Warner. The chains were made from a very strong hardened steel and required no tensioners or idler pulleys because they were pre-stretched on a special machine at the factory

 

Hey, I'm not a mechanical wizz by any means, but how do 90 degree spiral bevel gears have less contact area than normal gears ? I now expect to be jumped upon, but please enlighten me.

 

They don't necessarily have have less contact area, its just that many folks don't realize how little contact area there really is.

"Whenever two curved (usually convex) surfaces are in contact under load, the contact occurs along a line or point, or, depending on the elastic constants of the materials concerned, along a very small circular or elliptical area. As a result of such small contact areas, the shear (Hertzian) stresses which develop at and near the surface are consequently very high."
http://researchspace.csir.co.za/dspace/bitstream/10204/1415/3/fernandes%202_1997.pdf

 

Well, if the bearing areas are similar, could it be that the alignment needing to be more precise in 90 degree gears ? They certainly have to mesh precisely or destroy themselves quite quickly.

 

same as a pinion gear,,,is that what you're saying?