Shipbucket

You are aware of where the CVF has its MT30s are you not? Also add to the fact that there is no mechanical connection between the actual turbines and the shafts (IFEP) and it really isn't a problem.

_________________
AU Projects: | Banbha et al. | New England: The Divided States
Blood and Fire

you are so yesterday. we do this quit often in Norway, and shaft length is no problem at all. we have car ferries with shafts that are 80 meters long! what they do is having the shaft trough several joints, that mean that even if the ship bends...

you don't need shaft, only from the electric motors. all you do is connecting the jet engine to an electric generator (small fishing boats her in Norway use hydraulics) then run a couple of massive cable trough the ship to the electric motor. for example ramform ships. they have there engine mounted in the bow, over waterline. do they have an shaft from bow to stern, no, there engine is powering electric motors that again is powering the propellers.

the you will say that electric generators can't take the rpm from an jet engine, well gears. look at the Nansen class, it is CODAG, but not the Normal CODAG you are used to. it have several gears making the ship to be able to have it engine to drive the shaft directly or trough electric motors, and all engine can drive separately.

so no long shaft is needed, but a long shaft is not a problem at all. Ebba Mearsk for example have a 120 meter long shaft, you loos some on friction due to length of shaft, but you save that in, because you will be able to design a more balanced ship with better hull.

My concern wasn't with shaft runs, but overall beam at (especially) the forward engine room.

Actually long shaft runs can be a real pain. They take up more space than you think, require expensive watertight openings in the bulkheads (which for warships have to remain watertight after whipping due to underwater explosions, or even just the ship hogging or sagging if they are long in proportion to the ship) and require similarly expensive bearings to support them. Merchantman type shaft tunnels are highly undesirable for warships as they can increase vulnerability to flooding.

Although modern shafts and bearings are in theory much less susceptible to damage effects than those used in the past (but that's been said about a lot of things), reducing their length by using electric motors or moving the machinery spaces aft is still desirable. The arrangement of the Type 22 frigates was in part to reduce the shaft lengths by keeping the machinery box aft.

Ultimately, however, design is not about optimisation - although optimisation is a key part of design - but rather something closer to satisficing - a compromise between different needs. So one solution will not be the single paradigm for all cases.

RP1

_________________
"Yes siree, the excitement never stops." Togusa, Ghost in the Shell

To what extent are shaft problems a function of their relative size and speed? Aside from stiffness/vibration issues with narrower, higher-speed shafts, it seems like sealing the smaller-diameter penetrations might be easier. A smaller shaft would also presumably be more likely to completely fail in whipping and not transmit the forces and moments down the entire length of the shaft, compared to a larger and stiffer one.

These would seem to bear (no pun intended) on the push and pull between big slow screws and more pumpjet-like installations (especially if the latter can see shorter shafts in the first place).

Also there would seem to be a skeg question here (where you can have rigid structure supporting the shaft outside the shell of the ship for some considerable length).

That's a lot of questions...

For a given propeller and thus shaft RPM, a wider diameter shaft will have a higher "linear" speed in the bearings, which is going to increase friction losses. In normal operation, the minimum diameter of the shaft is going to be limited by the torque it needs to carry, including transient loads. Since the stiffness of the shaft is most dependent on the outer layers of material (like an I-beam) reducing the diameter will make it less efficient as a load carrier.

There are various ways in which the shaft can fail under, say, whipping. It could simply snap, it could snap and bend, with the whirling shaft opening up a compartment (as in Prince of Wales), the seals (through the bulkheads and shell plating) could fail, or the bearings, thrust blocks etc could fail. These latter failures can be minor - i.e. the shaft is now overheating but a temporary cooling circuit can be rigged - or major - i.e. water is getting into the people tank, which tends to be disconcerting to those involved.

To avoid this you need special flexible bearings that can withstand the relative movement of the hull and shaft (at a high rate, i.e. due to UNDEX). Another possibility is to have a shaft which is flexible, and composites are a potential solution here, as a composite shaft is very stiff radially but far less stiff in bending. These are also lighter and damp vibrations. But that does represent a big lump of composite...

All this means you want shorter shafts however as DKB noted with respect to the Type 43 - which has one shaft running for something like 50-60% of the ships length - there was extensive investigation of this problem after 2WW and the risk was felt to be small. There is always a lot of uncertainty with this, however - in a major incident about a decade ago, a lot of survivability features for machinery turned out to not work as well as might be hoped, and that was in a far less arduous scenario than an underwater explosion. So if one can avoid relying on the performance of the flexible seals, then that would be great.

Skegs can indeed provide protection for propeller shafts, although it's only up to a point. A skeg with cheap bearings in it may well be worse than an exposed shaft with proper ones. Skegs have other impacts, though; increased bare hull resistance (more skin friction), potential for dead water in between twin skegs, but their detail design can be changed to improve the propeller inflow compared to a conventional arrangement (this is something organisations like MARIN spend a lot of their time looking at). On a lot of smaller ships like FF/DD it would be difficult to fit a practical skeg - you have to be able to get inside the thing somehow - but for larger ships they might be practical. Of course, one should remember that they would be a lot longer on a modern CV/N than on a battleship or a bulk carrier, as the former has a more slender hull, with longer after runs. That means a lot of additional surface area.

RP1

_________________
"Yes siree, the excitement never stops." Togusa, Ghost in the Shell

Right, I remember my structures classes well enough to know that you're going to have minimum diameter per unit torque and all that, but (on the fringes) this would seem like it might be relevant in terms of desired shaft RPM and such.

I'd never thought about a composite shaft, though. That's a really interesting concept. God Himself help the folks who need to figure out the vibrational characteristics.

On Norwegian ferry's where the engine is placed bellow the shaft connection to the Z-drive. they had to solve it by having multiple shaft connected to each other trough bearings. but then ferry engine have a much lover rpm, and work pressure, since at least Norwegian ferries have up to 30% engine power in spare when operating.


Composite shaft: it reminds me of this one, it show very well the difference's between a steel and a ..: https://www.youtube.com/watch?v=hjErH4_1fks


I just come over this one: here is every thing run by generators, engine in the bow compartment... and the rest is everywhere.. literally:

http://www.skipsfarts-forum.net/images/ ... /33511.jpg

Z-drives and other odd arrangements are used quite a bit for small ferries. They have additional losses in the gearboxes, but they are very compact, which increases the money-making volume of the ship. This is an example of how technical inefficiency of an individual system is acceptable if it leads to a more efficient overall ship-as-a-system.

Electric drive is increasingly popular in offshore service vessels because it again frees up additional space (conventional Platform Supply Vessels have shafts running about 80& of the ships length - right through the money-making bit!). For more complex vessels such as the one Heuhen posted it is also very useful as their operating profile involves a lot of station-keeping and variable loads, so a direct drive arrangement would be problematic.

RP1

_________________
"Yes siree, the excitement never stops." Togusa, Ghost in the Shell