Cap'm Kurt

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Adding these tubes as first attempt to stop the reversion issue.

Is there a certain minimum angle the slash cut should be? I cut it around 50-55 deg but havent installed it yet and can re-cut it easy now before welding it on.

Also, should the slash cut go up or down. I've heard long on top, but from experience with a friends boat, it puffed the water up off the bottom when pointed down(long on top) - it did not do that facing up - long on bottom. The water at idle just flowed down the bottom of the pipe. Maybe every motor is completely different.

SB

iTrader: (1)

If you are doing the inner's that way, I'd just do straight cut. That way the bottom is as long as possible too.

If you where keeping a water jacket around your newly installed section, than it is different.

How you are doing it now, there is no water jacket, so as you had found during your friends installation, the water is falling to the bottom at low engine speeds/water volume.

The reason a slash cut would work (longer on top) is that when water runs off the top, it would fall to the outer tube ID vs in the inner tube. But, since you don't have an enclosed water jacket around the new extension the water is falling before it get's to the end of the extension.

During your final install make sure the bottom of the extension is not touching the outer pipe/hose. If it does, it will give smooth transition for water to go back in.

Boatally Insane

iTrader: (1)

This is how Eddie Marine did mine...

SB

iTrader: (1)

My Eddies where same way. I bent the very lower section back up so it was straight. This raised bottom of inner tube off of the floor of outer (rubber connector hose) further.

SB

iTrader: (1)

Mercruiser had the following kit (no one seemed to know about) to extend the point the upper water 'discharge' came out of the stock riser.

http://www.boatfix.com/merc/bullet/89/89_32.PDF

Cap'm Kurt

ok, well, I gather that slash cut is more preferred if water is mixed with the exhaust to shield the inner tube from falling water off of the end of the inner tube, yet your EM system was flat cut. Go figure, just different opinions on reversion and PERFORMANCE of the exhaust. I figure the flare would make it easier for reversion, i guess that is why you bent the bottom straight. Maybe even a very small flare inward would help stop water from getting in.

Maybe just getting the mix point further downstream is the biggest issue. I am limited by the nice cross with the rubber Hardine Marine flapper which may help but is not stopping rev at this point, so if this setup does not stop the reversion, i will cut out the flapper cross and extend the inner tube even further thru the transom area. Have to do whatever to save the valves . But i will try slash up, slash down, and flat cut before doing that.

f_inscreenname

Just not sure how you are going to keep the whole thing wet? When water comes out of the riser it runs out of the top then the exhaust gasses blows it out the exhaust spreading the water throughout. If water comes out of the riser and then just rolls under the extended pipe I’m not sure how it will cool the entire pipe.

Cap'm Kurt

I will have to watch the rubber hoses to make sure they are ok, but I've seen this done once before and believe it or not the inners stayed pretty cool. First it must not take much water to cool, and second there is a lot of water coming out each header at mid and upper rpms.

I think these merc pumps put out roughly 25 gpm at mid range and 40ish wot. A garden hose puts out about 5-6 gpm with nothing on the end so that gives you a feel as to how much water that is. The water is probably splashing around between the inner/outer tubes and covering a little bit more than we realize at those flowrates - and you don't have to cover every square inch of the pipe to keep it cool - alum conducts heat pretty good. So I see your concerns, and hey, it made me think it out a little bit more, but I think it will be ok- just will have to check it to make sure.

thanks for the comment.

Cap'm Kurt

I suppose I could put a ring of something in between the inner alum tube and the outer 4" hose if need be, with one or two outlet holes near the back, to help build up more water in between them , similar to what headers do, but hopefully wont have to.

Cap'm Kurt

Boatally Insane / SB
Is the open ring area about 1/4 or 3/8 tall, between the inner/outer - right at the left flare at the weld - is that completely open upstream? or is there some metal in between that helps to collect water and keep more water up all around the inner tube before flowing out the rear and meeting the gas.