Exhaust outlet position

[penfold1]

Another thing I've been meaning to post about for yonks. A while ago I read something about Hubert Scott Paine and the work he did on fast patrol craft before, during and after the war; one of the things he experimented with was ducting the exhaust out in front of the propellor, which was found to increase top speed, giving a sort of supercavitating effect. Presumably you would need spiffy wizzbang props to get the most benefit. Has anyone here ever tried anything like this? Or did Fairey ever try it themselves? The thought was originally prompted by the horror that was removing and replacing the exhaust hose and outlets on 'Marie Louise', which made me ponder alternative positions, e.g. on the underside of the hull at the quarter, which seems quite popular in the US and offers better silencing.

cheers,
David

[Ben]

In the Scott Paine biography it does mention that they exit the exhausts of some of the boats out through the underside of the hull, as I remember this was an attempt to make the boats quieter in operation. I'm not sure of the effect on the boats performance.

His bio is Fast boats and flying boats by Adrian Rance, its still available form the aero book company who can be found on the web.

The huntress I had was fitted with an under hull exhaust and I think at least one other is like it, certainly very quiet and it does keep smoke to a minimum too, again, I'm not sure on the benefit from a performance point of view. I wanted to replace the underhull exhaust with a transom one, I like the noise!

[Ben]

I forgot something important, with an underhull exhaust you cannot see the cooling water.

[Scott Pett]

That's where the early Alan Burnard water cooled shaft seals are useful. The cooling water leaving the seal is plumbed to the side of the hull so you can easily see it from the driving position. It acts as a good tell-tail for the main sea-water cooling circuit.

Unfortunatley the later version has the water leaving the seal via the stern tube.

This leaves you two choices: a) listen for the squealing of a dying pump impeller, or b) fit an exhaust temperature alarm.

[Paul Foulston]

Whilst Scott Paine was experimenting with putting water out the bottom to ventilate the prop the Germans were building E boats with the new development of underwater exhausts, slower (diesel powered) but better armed than our Vosper boats they used the benefit lower noise to ambush our coastal shipping.

The system does require a bleed pipe, i.e. a small additional exhaust above the water line. The purpose of this additional outlet is to bleed off some exhaust at low speed and provide water tell tale. Ben does surprise me by saying that Cascade did not have a bleed off pipe; she was copied from Huntress of Bosham, which certainly has a 2 inch bleed off pipe on the transom. Without a bleed pipe exhaust back pressure is enormous at low speed and it is a mandatory requirement in engine manufacturer’s installation guidelines.

Personally I dislike underwater exhausts, due to the rude noises they make when the vessel is stopped. The noise level is not measured in decibels; more like chicken balti’s the night before per minute.
Alternatively perhaps the noise originated from our guy’s driving the little 68ft Vosper MTB’s when caught unawares by those nasty E boats who invariably fired tracer rounds at them knowing that with full tanks the MTB’s carried over 2,500 gallons of high octane petrol.

Paul

[Ben]

There was no bleed off the exhaust, just the five inch under hull jobbie. At idle it did make the boat shake when stationary as it would do nothing....then a big fart....then nothing...then another big....

[Scott Pett]

With all this talk of farting and underwater jobbies, we'll have the EU mandating the fitment of holding tanks to underwater exhaust installations.