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Walt. H.
09-04-2003, 11:25 AM
An opinion tech question:

When running a strickly above water exh, system and you have your Y-pipe capped off or blocked at the base where the Y-pipe would bolt up to.
Which would give better performance for the prop's flow of water, leaving the exh bellow boot on causing a vacuum effect at the prop exh ring as the boat travels forward. Or taking off the exh bellow to allow air to be pulled through the open hole from the missing bellow. I wanted to try this test myself but I will not be able until next season when I can get the boat finished.

I figured it might be an interesting discussion over a pitcher of beer! :p

Walt......

MOP
09-04-2003, 04:25 PM
Walt there was a post awhile back about capping the Y's, one of the much more knowledgable guys said during testing that the boats with both flowing made more speed. I am sticking my neck thinking it may have been HP500 not sure.

Greg K
09-04-2003, 05:30 PM
Makes no difference in performance that I could tell when you run thru hulls only. My exhaust bellow or tube was on my King Cobra drive from the transom assembly. I suspected I had some water leaking thru my block off plate and removed the bellows and capped it with a pumbling cap to seal it at the transom assembly. Again, it made no difference in performance. Just less water in the bilge now.

Walt. H.
09-05-2003, 02:24 AM
:confused: No guy's,
I'm talking about having the stern drive exh port open from the bellows port down thru the lower unit out to the prop.
Leaving the bellows on Vs. having the bellows off. I'm not talking about water leaking thru the transom block plate or capped "Y"-pipe.
Reason:
I have read in some performance books that running both thru hull exh and stock exh,at the same time with the guts removed from a set of capt's choice or quick & quiet will pull more exhaust out make more H/P, because the prop pulls like a turbo vacuum pump.So i'm figuring if this is true then having exh passage blocked is causing some turning resistance for the prop/engine when only running the exh straight out thru the transom above water.
So it should be better to leave the exh bellows off so that the stern drive exh passage can breath when only running transom exh! Correct?

Greg K
09-05-2003, 06:41 AM
Now I get what you're talking about...I had that on my 16. Exhaust was able to exhaust thru hull and thru prop at the same time. Make any difference...who knows?? I just wanted to have thru hulls with out a diverter plate and not have to remove y-pipe and used a splitter pipe to do that from Corsa.

MOP
09-05-2003, 05:27 PM
If you are running drive out the prop exhaust the bellows between the transom shield and the drive must be in to take full advantage of the converging ring on the prop which helps a lot to extract exhaust.

Walt. H.
09-06-2003, 12:23 AM
Hi Phil,
No i'm not running the exh thru the prop at all, its just going along for the ride, straight out the transom only!
The question is regarding if there is a positive or a negative effect on the prop thrust flow by either closing off or leaving the stern drive exh passage open.["closed will cause a vacuum, open will aerate the flow"]

I can also confuse matters more by asking to run a small hub prop or big hub with either combo. :)

Say Phil, did you receive my return e-mail?

Thanks,
Walt.

MOP
09-06-2003, 07:53 AM
Walt Mail received! I need to add a tad more to this thing, If you are running just through the transom nothing through the drive then the converging ring on the prop can and should be eliminated. Its sole purpose is to extract the exhaust, it does cause drag its drag is out weighed by the additional performance when exhaust flows through it. You guys are making me sweep out the cob webs, how many of you guys remember the air foil style with the slots the one that was plastic and snapped into a groove on the back edge of the prop an OMC outboard thing. That pulled the exhaust out very well but created to much drag. When OMC threw out the electric shift drives they went right to the flared prop, I think Merc gets the credit for the flared outlet. All this with a Banger Hang Over!

Walt. H.
09-06-2003, 11:35 AM
Ok Phil,
Good to hear cause things have been a little crazy with my mail. Sometimes for unknown reasons they bounce back undeliverable this week, an AOL thing I guess. A.O.L.stands for = "All. Outer. Luck".
The prop i'm currently using if I was using the boat is a Mich, Balistic it doesn't have any flare at the hub so I guess i'm almost there. Now all i'll do is eliminate the connector exh hose and let the inside of the prop breath when the time comes. :p
And yes I do remember the black plastic cap, it looked like a nose cone and was held on with a cotter pin, it kept the shear pin in place.

Walt.