31 FBC Waterline Change After Repower

[twonahil]

We are in the process of repowering our 31 FBC with Cummins QSB 5.9's and we are trying to set the height of the exhausts as they exit the transom. What kind of numbers have people see in water line change after going from the old 454's to the diesel engines?

I have also seen some oval shaped exhaust tubes on a few boats here. Does anyone know where you can find these?

[Tony Meola]

I have the 270 remans. Never changed the height of the exhaust, it is about 3/4 covered with water when before it was up about midway on the exhaust. Down 100 gallons and the chine is totally out of the water. Full they are submerged. In fact, it is submerged to the top of the black boot stripe when full.

Not sure on the oval exhausts, I actually think they were custom fiberglass builds.

[Al C]

Check with Tom Oakes at Holtz 772-485-4595 he just put the oval exhaust on my boat.

[Marlin]

Check with cummins on the exhaust tube diameter , I went with 6" Fiberglas and did a side exhaust , installed Qsb330 ,kept in the same water line,sits heavy with full fuel but never leave her with full fuel unless we r going off shore the next morning,I also have weight with teak transom, covering boards ,cockpit ,cabin sole,235 gal fuel tank, 75 gal water,keep her lite until u till we get ready for a trip.

[Bruce]

Marlin is right on. On all the engines I've put in the 31, never changed the water line. When fully loaded with fuel, water and supplies for a trip, It would sit with the boot strip submerged in the stern, but normal running conditions nothing changed. Its amazing how much weight the 31 will cary and displace well.

On the exhaust pipes you can buy round tubing, slice in half and either make or buy sheet fiberglass and make your own ovals.

Most everyone runs the exhaust on the outer stringers to give you a full storage capacity or room for boxes in the deck in the shaft and rudder alley. At that point you pretty much have no real choice for up or down movement going out the transom or side.

[Yannis]

Bruce,
A friend here with a 45+ Wellcraft was doing major changes to his boat last winter.

He told me that he removed all exhaust tubing, even the mufflers, and created two "bell shaped" holes under the hull and diverted the exhausts right into those bells that are looking downwards into the sea ! In other words his exhaust holes are under the hull into the water. He said he also put two relief side tubes of some sort, one on each side, outwards from each bell that are for when he idles and the pressure is not important to overcome water pressure from below. He picked 3 miles, saved tons of space and is very happy. Have you ever heard of such a conversion?

[CaptPatrick]

Yannis,

Underwater Exhaust Systems have been around for over 20 years... There's a lot of mixed feelings about it, I for one don't like them mainly because I can't visually inspect the raw water flow by just looking at the exhaust at the transom. Secondly, I believe they're fish deterrents.

Mainly you find underwater exhausts on motoryachts, but they have been used on many other boats, including the 31 Bertram. Carl at Elite Marine in Florida has installed them on at least 3 B31s. The main two advantages are theoretically a quitter and cleaner exhaust.

Tony Athens' of Seaboard Marine has "Some thoughts on Underwater Exhaust Systems".

[Yannis]

Thanks Captain, very interesting article. Tony Athens proposes a muffler; my friend just dismissed it for some reason. You know, this thought crossed my mind as by getting rid of all tubes, mufflers etc I could make more space for water tankage; but then again, what the heck am I talking about...

[Navatech]

FWIW, on the naval vessels I served on each of the 4 engines had a muffler and dual exhaust outlets... One above and one below the water line... At 1200 RPM the exhaust would automatically be directed to the underwater outlet (one damper would open and another one would close)... All these outlets were on the side of the hull... The shape of the underwater outlets was such that when the water flowed it would create suction reducing the exhaust back pressure... Manual override was possible (if you needed to keep the noise down for example when doing a clandestine approach to a beach)...