Long Shaft Solutions

[Willallison]

No, I'm not bragging.....
I'm looking at a design for a displacement powerboat about 65' LOA
The layout requires that the shaft(s) length will be of the order of 9m long.
I'm looking for alternatives - if there are any - to using a multitude of intermediate shaft bearings
Any suggestions..... apart from the obvious, move the engine(s) aft?

[Frosty]

Hydraulics?

[daiquiri]

I would also take into consideration a diesel-electric propulsion.

[Willallison]

I had considered and basically dismissed both - on the basis of cost, simplicity &/or efficiency....

[PAR]

A fluid coupling would be out too then I suspect. I've seen some pretty long shafts run through pillow blocks and a few with U joints.

[daiquiri]

A flexible shaft, maybe?
http://www.suhner-transmission-exper...ble_shafts.cfm

[marshmat]

A nice selection from Suhner there, daiquiri. I couldn't find anything rated for more than 215 cm.kg torque (18 ft.lb or 24 N.m) though, which seems a bit low for Will's 65-footer.

I suppose, Will, you could put a thrust bearing near the stuffing box and use a large diameter, hollow shaft to span the gap from there to the engine without the array of perfectly aligned bearings that a thinner, solid shaft would need.... what kind of shaft speed are we talking about?

[daiquiri]

Originally Posted by marshmat

A nice selection from Suhner there, daiquiri. I couldn't find anything rated for more than 215 cm.kg torque (18 ft.lb or 24 N.m) though, which seems a bit low for Will's 65-footer.

Well, this is not a case where I would expect to find an off-the-shelf solution available at first try. The link is there to illustrate the idea of flex shafts and to give an example of a firm which produces them. So Will can contact them and see if they can construct a shaft according to his specifications.
Or he might have more time than I do (it is his project) to make a google search for some other manufacturer of flex shafts for big powers.

[sparky_wap]

Ever seen a long commercial truck's driveshaft? Usually broken into 3m segmants with u-joints and slip splines between section. Why re-invent the wheel when you can investigate finding or modifing an existing design. It would handle the torque and rpm of whatever you throw at it.

A single shaft would not work at high rpm due to resonance (shaft critical) and multiple bearings would not work unless you hull is very rigid. Rough seas would flex the hull and it would only take a a few millimeters to tear up your bearing and shaft. Commercial trucks have a chassis that will flex and the u-joints and slip splines accomodate.

Joe

[mydauphin]

A boat that size. Put engines farther back and use a v-drive.
Or put engines in back use surface drives.
Or put engines in back use IPS or other Zdrive.
A very large shaft is very expensive and inefficient on space, weight, maintenance, they use thrust block and bearing along the way.

[u4ea32]

The noise of a long shaft is an issue.

The noise of the engine in the middle of the boat, under interior, is a bigger issue.

Remember that noise is energy into a volume, so the energy is dispersed by the cube of the distance from the source: Distance is your friend -- get that mechanical stuff physically removed by a maximum distance from people and eardrums.

I second the "put the engine aft, close to the stuffing box" concept.

[PAR]

I guess you didn't read the original post, where the layout of the boat requires long shafts or at least, a fairly remote engine location.

This isn't uncommon, particularly in larger vessels, which is what Will is dealing with I'm sure.

I can think of several options, none especially good compared to more convention arrangements (all of which I'm confidant you've also thought of), but in light of restraints . . . the wonderful world of convoluted concessions in compromise.