Through or over exhaust prop with no trough exhaust.

[PerformanceDodge]

I have a bravo leg, I am running the exhaust straight out of the back of the boat so the exhaust won’t run though the leg and prop.

I am intending to run a cleaver prop as it is a stepped plane hull and should not require bow lift.

1, Would I be better running a though or over exhaust prop.

2, If I run a through exhaust prop would drag be created at the nut end of the prop as surly there would be negative pressure over the end as there is no exhaust pressure going trough it. (surely if you drag a blunt ended object through water the blunt end would have negative pressure on its face.)

3, If this is the case has any one tried venting the exhaust inlet on the leg to air, allowing any negative pressure to be relived by air.(obviously above the water line with a one way valve)

4, Or would I be better running an over exhaust prop and then surley the step between the leg and prop would create drag.

I am sure the simple answer is, props are a black art suck it and see. But someone out there must have some technical knowledge or experience.

Many thanks in advance for any replies.

[PerformanceDodge]

Any advice please

[Jon Fuller]

Originally Posted by freddy21

I am sure the simple answer is, props are a black art suck it and see.

I think that's the crux of it.

I can't remember the details, is this engine wastegated?

At least being a std drive system you should be able to borrow plenty of props and study the actual results, rather than supposition, and that's what makes up most opinions on propellers! Especially with an unusual setup such as yours.

Jon

[PerformanceDodge]

Hi Jon

Many thanks for your reply. The turbo is a VVR (variable vane geometry), so there is no waste gate as such, the boost is controlled by the angle of the turbine inlet vanes impinging on the radial inflow turbine of the turbo (not convinced how long this set up will work in an salt water environment, may have to swap it for a conventional waste gate turbo further down the line). The turbo is mounted in the centre of the engine V and I have fabricated a water jacketed then wet exhaust system that exits the upper transom. The inner transom shield has exhaust blanking plate at present.

In theory I would expect the the most hydrodynamic set up would be to use a trough exhaust prop to follow the contour of the lower gear box bullet then run a air vent tube to the inner transom exhaust inlet to prevent negative pressure over the exhaust exit end of the prop. May be if I fit the vent with a ball valve I can experiment with it vented and non vented, then with and over hub exhaust prop try the same again. I love fiddling and it will be interesting to find the results for future reference. But in practice it may make no difference, and as you have confirmed there is a lot of superposition and the best way forward will be trial and error. I just though some one may have tried the same or similar experiment with a similar or racing set up.

By the way what a fantastic double page photo of Swip Wipes in the boat mag from the CTC, I have never seen anything like it. Well done for your achievement and finishing such a gruelling race.

Regards David

[Renegade]

Give dave a call at steel developments, if's a mate of mine, he'll know. All the best. Paul

[Renegade]

He's a mate, sorry bad spelling.

[PerformanceDodge]

Originally Posted by Renegade

He's a mate, sorry bad spelling.

Ha, found them by Google and presume this is them

Steel Developments
Propeller Repairs & Sales

240 Merton Road,
London,
SW18 5JQ
England

TEL: 020 8874 7059
Email: mail@steeldevelopments.co.uk

Many thanks

[Hydrostream]

One thing i noticed running a over hub prop from a through hub was the loss of lift from the extra gearbox length the through hub gives.

Also the aft face of the prop is a high pressure area, wouldnt think the low pressure created would make much difference.
Not at all if you run a high enough X dim

Alan

[Renegade]

Yes thats them, but I just found out he's away for a week, but if will be able to help you.

[PerformanceDodge]

Originally Posted by Renegade

Yes thats them, but I just found out he's away for a week, but if will be able to help you.

Many thanks, no rush just forward planning at the moment.