View Full Version : Transom engine exhaust question

09-29-2002, 07:20 PM
" I have a 1976 Luhrs with a chrysler 318cu. engine.My question is with the thru hull exhaust, I am thinking about putting on elbows at the transom to divert the exhaust down below the waterline. Shamrock has this set up on there inboard powered boats. Would this create a back pressure problem or worse yet, cause water to siphon back into to manifolds? I am looking to quiet down the exhaust as I cruise along. Seen aftermarket silencers but don't like the price! "

09-29-2002, 08:48 PM
"Back pressure would increase only slightly, and then only when stopped or underway slowly. It could put a small strain on your starter if the engine doesn't start right away. I don't see a siphoning problem as long as you have exhaust elbows extending above the waterline. However, this setup wouldn't be any quieter when the boat is running fast enough to expose the full transom.

The main drawback I see is punching one or two fairly large holes through the hull below the waterline carrying tubes that vibrate and get hot. That alone would be enough to turn me off the idea, but then I'm kinda conservative when it comes to putting holes in my hull. ;^)

IMHO, save your pennies and go with some good marine mufflers.


09-29-2002, 11:32 PM
" My Trojan has underwater exhausts for it's big block Crusaders. One of the main things they did is to put small exhaust ports tee'd into the main exhause pipes before they go below the water line and exiting the boat above the water line. This has several advantages. The engines will idle! Where as they probably wouldn't with the back pressure of the exhaust exiting below the transom. Also it gives a convenient way to check for raw water being pumped through the system. The main exhausts are long fiberglass tubes, but they don't seem to have done anything beyond the wet exhaust to deal with heat. The tubes are large (4"), but 671 naturals were also an option for this boat so the exhaust was designed to carry a lot of volume.

It's a very quiet way to do things. Underway on the aft deck you hear engine and water noise, but no rumbling exhaust.

I would certainly have a qualified fiberglass facility do the holes through the hull and make sure that you have a fool proof system for keeping the boat water tight even if you loose a hose or a hose clamp. Big, underwater exhaust holes are going to be awfully hard to keep up with if you spring a leak. On my boat the solid fiberglass tubes extend to above the water line. "

09-29-2002, 11:36 PM
BTW: My Trojan has big water bath mufflers in addition to the underwater exhaust. So I don't know how much is from the mufflers and how much from the underwater exhaust.

Try the mufflers first!

09-30-2002, 12:28 PM
" croozertoo, exhaust holes are already there as this is a typical inboard engine set up. Location is already above the waterline (1 inch to the bottom of the outlets). The height from the manifold risers is about 12 inches, sloped down to transom. Also for Bilgeguy, I am very familar with your Trojans set up and I like it! Mine are only 3 inch outlets and I hope that diverting it down slightly would take out some thunder. Thanks "

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02-27-2010, 05:11 PM
I have a Mercruiser with a 350 and Apha II outdrive. I just removed my out drive and replaced the water pump and all the hoses including the bellows. When I put the boat back in the water there is a leak coming in the boat where the outdrive connects to the transom. Water is pooling into the rectangular box above the drive shaft and dripping down on to the shaft and into the boat. It seems to be where the shifter arm is connected. I have a manual and it doesn't show or talk about what it would take to fix this problem. Is there a seal in there and do I have to remove the outdrive again to fix this kind of leak. Does anyone know how to fix this and can you give me the steps to do it. Thank you in advance.

02-28-2010, 02:17 PM

This is an old thread. Better results posting on the mercrusier board (many more knowledgable eyes).

based on memory (I haven't tore down an AlphaII) you either messed up the gasket reinstalling the outdrive or the bellows has a tear. Try the merc board and see what they say.

02-28-2010, 02:28 PM
Thanks Mark I did repost it there and am getting some help now.
Happy Sunday!