Outboard exhaust: thru prop or above prop

[Barracuda Bill]

Is there a difference in efficiency between the two designs?
It seems that the thru prop would be better, since there isn't the obstruction of the exhaust tube.
Any comments from the experts?

 

[Chris1956]

Re: Outboard exhaust: thru prop or above prop

That is a tough question to answer, without some real world testing. Years ago OMC made all their motors with the exhaust above and behind the prop. It seemed to work OK. The thru prop exhausts are nearly universal on most large manufacturers, since the 70s, when OMC changed over.

I know that the thru prop exhaust configuration is efficient, because the vacuum of the motor moving thru the water pulls the exhaust out. That is why thru-hull exhaust on I/Os is not more efficient (in HP) than thru prop exhaust.

Of course, high performance OB props have solid hubs, and the exhaust simply spills out of the gearcase and runs over the prop blades. This affects hole shot, but improves high speed operation. Go figure out why?

 

[Frank Acampora]

Re: Outboard exhaust: thru prop or above prop

"They" CLAIM that thru-hub exhaust is more efficient and indeed, when Force wanted to boost their horsepower a bit, they designed a "dual exhaust" lower unit that exhausted both through the prop hub and also through the exhaust snout.

It is true that the trim tab on a thru-hub exhaust lower has less drag than an exhaust snout but the real advantage of thru-hub exhaust is burying the exhaust deeper in the water for a quieter engine (and Mercury advertised as such many years ago).

Of course for Maximum power, outboard race engines and some I/O units will have special lower units that exhaust above the cavitation plate. Outboard engines set up to race with solid hub props do exhaust right over the hub, BUT it doesn't foul the prop because these are surface running props with only one blade in the water at speed. Essentially, the exhaust is going into open air.

 

[Barracuda Bill]

Re: Outboard exhaust: thru prop or above prop

I suppose that with most rigs, it would be difficult to determine any performance gains switching between the two. Personal prefference is probably the name of the game.