PO installed a Yanmar 3GM30F in my Bristol 29. He glassed over his exhaust throughhull which appears to have been about 3" above the waterline. I need to cut in my exhaust throughhull, where should I locate? How high above water line? Center, port or starboard side of transom (exhaust comes off stbd side of engine)? Keep existing waterlift setup? Or better, simpler option? Best through hull make and material? Boat's on stands under shed, so relativly easy to work on now. Thank you.
Engine exhaust
- Thread starter GABristol29
- Start date
Really no way of guessing with at least some photos of the engine in place, the waterlift muffler location and plumbing, etc.
There is a real danger of flooding your engine if the exhaust isn't correct.
As a rule of thumb, closer to the centerline is always better for exhaust. And the higher the outlet, the better, to prevent back flooding.
There is a real danger of flooding your engine if the exhaust isn't correct.
As a rule of thumb, closer to the centerline is always better for exhaust. And the higher the outlet, the better, to prevent back flooding.
A water lift muffler is the only viable choice. The other option is a dry stack, but that will entail a lot of extra plumbing and work and you end up with a very hot exhaust pipe.
The hose from the muffler to the outlet should have a loop in it that higher than the waterline at any angle of heel. This helps prevent water from back flowing into the exhaust and engine.
As you are going through a rather extensive refit, you might want to bulk up your library. Nigel Calder's Boatowners Mechanical and Electrical Manual is a good choice.
The hose from the muffler to the outlet should have a loop in it that higher than the waterline at any angle of heel. This helps prevent water from back flowing into the exhaust and engine.
As you are going through a rather extensive refit, you might want to bulk up your library. Nigel Calder's Boatowners Mechanical and Electrical Manual is a good choice.
Dave, TomY,
Thank you. I consulted Nigel Calder's book and will proceed. I am inclined to exhaust through midline of transom, at least 8" above the estimated loaded waterline. Only potential concerns re. placing at the midline that come to mind are that my exhaust hose would bisect my stern lazarette and I wonder whether a problem may be created if I proceed with a wind vane/self steering gear.
Thanks,
Matt
Thank you. I consulted Nigel Calder's book and will proceed. I am inclined to exhaust through midline of transom, at least 8" above the estimated loaded waterline. Only potential concerns re. placing at the midline that come to mind are that my exhaust hose would bisect my stern lazarette and I wonder whether a problem may be created if I proceed with a wind vane/self steering gear.
Thanks,
Matt
I know it may sound strange, but I put a scupper on the outside covering the exhaust outlet. My exhaust is relatively close to the waterline. I have watched it and seems to keep any water out of my system.
I guess you can't take the power boater out of me.
I guess you can't take the power boater out of me.
I would consult your Yanmar installation manual.. Without you giving us pictures to view its a crap-shoot. The type of pictures to give is an exhaust type! etc. if you have an elbow feeding an exhaust reservoir then out should be at the waterline, however, you must make sure the exhaust tube (PIPE) is at the correct angle to drain properly and it also prevents water from backing up to your engine. Please see your manual.
Best of luck to ya! Capt. Rob
PS.. see page on piping for your answers..
Best of luck to ya! Capt. Rob
PS.. see page on piping for your answers..
You said: "PO installed a Yanmar 3GM30F in my Bristol 29. He glassed over his exhaust throughhull which appears to have been about 3" above the waterline."
I seem to be missing something here. He installed a diesel and glassed over the exhaust port??
I seem to be missing something here. He installed a diesel and glassed over the exhaust port??
No, I didn't miss anything.. I want him to follow the manufactures procedures for installing his output...
That's why I included the manual for him.. But thanks.
Sorry heritage thought you were talking to me..
That's why I included the manual for him.. But thanks.
Sorry heritage thought you were talking to me..
There may be a well found reason for the PO sealing the exhaust port. Off the top of my head would be the new engine required a different hose diameter than the old engine. Glassing over the port and installing a new exhaust port would be a perfectly logical thing to do.
Thanks all. PO, David, has passed, I purchased the boat from his wife. David installed the Yanmar in the 90s and begin the refit I have taken over in 05. I suspect he was raising the exhaust outlet due to having added ballast weight and anticipating extended cruising with more stores. But that's only a guess at this point. This evening I was looking for the waterlift muffler box in the boat, it has been removed, so I will need to buy another. I am also having to decide wbewhet to keep or remove the auxiliary 25 gal fuel bladder David installed, to supplement the original and existing monel tank. I do not like the idea of the bladder under the motor, though David did secure it well and added vinyl sheeting to buffer chafing. Any thoughts about whether to remove the bladder are welcomed . Thanks, Matt
If the bladder should spring a leak, you'll have unto 25 gallons of diesel in the bilge.
One other thought on running the exhaust hose, try to keep the bends and turn gentle and sweeping rather that sharp. Sharp 90* bends will affect the exhaust back pressure which affects the engine performance. Check the engine specs for size.
The big advantage to midline placement is heel angles. If the outlet is close to the waterline and in the center, it will not go underwater when heeling. The further away from the midline, it becomes necessary to raise the outlet so that it is does not go under water when heeled.
The stern will squat and the waterline will rise as the boat approaches hull speed. If the outlet is too close to the waterline water can flow into the exhaust system from this stern wave. Power boats deal with the problem by install flaps on the outlet that sort of seal the exhaust system.
I installed a new water-lift muffler and exhaust on my boat two years ago and put a high loop in the lazerette on the centerline. You can see the loop through the access hole I had to enlarge in a bulkhead on the centerline of the lazarette.
If the high loop goes to one side or the other, 50% of the time it won't be a high loop and could potentially flood the engine, especially on a hull like mine (which is very similar to your Bristol 29).
Here the loop is tucked up to the deck.
My outlet is 12" above the water line. That may seem excessive but even at the height above WL, the outlet is awash at times, under sail. A following sea could force a plug of water into the outlet. Under the right conditions, without a high loop, there is a potential for a back siphon in the exhaust line.
Just keep that in mind but I would search the web for other B29 owner groups and see what they have.
You may even find the original drawing the designers set up. Engine flooding, which can happen from both the outlet and the raw water intake, isn't rare at all.
Here is a drawing of the exhaust system on Hunter sailboats. Note the loop that rises along the side of the starboard hull as far as possible before it drops to the exhaust port. Going at hull speed under power the exhaust port is slightly under water. I've never read of any problems with this system even with the port being near the water line. The high loop seems to do the job. You might have problems getting a high exhaust loop if you route the exhaust hose under the cockpit floor in order to exit near the centerline of the stern.
Thanks Heritage. I'm situated such that I should be able to get the elevation needed in my stern lazarette.
My 1996 Hunter 280 had a Yanmar 2GM20F that exhausted under the boat, below the waterline.
My current 1988 O’Day 322 also has the 2GM20F, but the exhaust is above the waterline, on the port side transom...very close to the outside edge of the hull. Might be able to see it on the port side in the photo below.
Greg
My current 1988 O’Day 322 also has the 2GM20F, but the exhaust is above the waterline, on the port side transom...very close to the outside edge of the hull. Might be able to see it on the port side in the photo below.
Greg