Marine exhausts???

[burt2]

Im converting a carver Montego 27 from twin 4.3 V6’s to a single Diesel engine and I need to design and exhaust for a Volvo AD41a 3.6L Diesel turbo inline six, I’ve made plenty of exhausts for cars but this is my first boat. I want to put the exhaust out the starboard side as far back as I can, but I don’t know what effects engine power in regards to submersing the exhaust below or above the water line, the boat is a planning hull so the exhaust will be lifted out of the water when planning allowing me to look over the side a check if water is going through the system and when at displacement speeds it will be submerged. The water line is only 300mm (1ft) from the turbo exhaust outlet so I also want a water trap right? (water lock), I was thinking of using the factory system though the leg but it seem very restrictive and has already been blocked of by the last owner who clamed the exhaust was much to small and lost a lot of HP due to it, is this true?

Cheers

[FAST FRED]

Volvo AD41a 3.6L Diesel turbo inline six

Can you live with the tiny power of this engoine when it is rated for 24/7 use ?

FAST FRED

[Ike]

Frankly I'd go with the factory system.

[burt2]

The engine will give us a cruise speed of 20-22Kts and a top speed of 27-29Kts we are not going to water ski with it LOL but will do us fine. Going with the factory system would be the tidiest option as it very Compaq and clean how ever I already have a nice water lock system I can alter to suit they are less maintenance and perform better, the factory exhaust is very small when coming through the leg, and the rubber bellow is got to be restrictive

[longliner45]

burt2 put it above the waterline! failier above wont be as bad as failier below, dry exaust is best,,,,,,good luck longliner

[StianM]

Originally Posted by longliner45

burt2 put it above the waterline! failier above wont be as bad as failier below, dry exaust is best,,,,,,good luck longliner

We build all boats with the exhaust below water, but we do this for reducing the sound since we build miletary, coust gard and patrol boat's that are suposed to not make too mutch noice. And it also reduce the IR signature.

Uligal fishing and smuglers are more easy to catch if they don't hear you sneaking up on them.

There is no other reason for leting out the exhaust abow water.

[burt2]

Noise was my reason also for putting it below the water line, since we will be at displacements speed quite often thought it would be nice to hear your self think.

[longliner45]

sound or safty, I believe in old school designs , the sandusky boat I had ,had the exuast slightly above the waterline,so does chriscraft ,once the boat is on a plan the exaust is dispersed into the water , but hey ,,,its your boat .

[burt2]

I am not sold on putting it under water yet, is there a safty issue with the exhaust going below the water line? i can't see how, as the factory exhaust runs out the leg below water line. All i no is diesels love to be able to breath well and the factory exhaust wood make alot of back pressure to much if you ask me. but i'm no expert.

On another note I was going to use a skin fitting for the raw water pick up as well since it is blanked off on the leg along with the factory exhaust, will the raw water pump make enough vacum to pull the water up?, the pump will sit almost 2ft above the water line (i was just wondering).

[FAST FRED]

The pump fitted to most factory engines is a centrifugal that will only circulate a full water system.
With keel cooling the centrifugal pump will force the water thru the necessary pipes as it would a radiator or heat exchanger..

A raw water pump is usually fitted with IMPELLERS , usually rubber . and will push water either thru a heat exchanger or thru the engine.

It is very poor practice to use sea water as coolant inside the block as about 140F the salt begins to precipitate out of solution and plug up the blocks internal cooling passages.

FAST FRED

[bilgeboy]

Hi Burt,

You asked about any safety issue about the exhuast below water line. I supposed a cracked muffler hose could cause a devastating flood, but I think much more common would be engine damage from lack of cooling water.

Ever started your engine with the seacock closed? I'll bet you have if you work on your own boat...

I'll tell you, I am in the habit of looking for a good stream of water in the exhaust immediately after start up. When underway, if not at the helm, I'll check it a few more times. With the exhaust low, you can't see it. Even at the water line, the exhaust gasses can spray the seawater and make you think you've got a flow of water in the exhaust.

Anyway, those are my humble thoughts on the issue. I really do use this quick inspection method alot. It could save a very expensive repair.

Mike

[StianM]

A preshure switch on the preshure side off the pump should do the trick.
12v and a 12v buzzer at the control panel and you would get a inoing noise if you don't have water flow trough the engine.
Not that exspensive to make your own alarm system. Only nead some basic knowledge.

I agree with all critics off leting the exhaust out under water.
Also if your hull take in water the waterleel in the exhaust will rise and reatch the valves in the end.
The waterlock don't protect you agains that

[burt2]

yeah that sounds like a good idea, I think i can handle something like that, where can i find some info on wiring up such a useful gadget?.

Ok so there is enough people telling me to put above the water line so I think that will be the way to go, now should I go striaght out the side or out the back. the back would be more structurly safe as its 60mm of glass over ply and the sides are just 10mm GRP and i would have to put in patching.

[longliner45]

and the back looks so much cooler,,,,,,,,longliner

[StianM]

Originally Posted by burt2

the back would be more structurly safe as its 60mm of glass over ply and the sides are just 10mm GRP and i would have to put in patching.

Do what ever you think look nice.
It's not just 10mm of GRP, it's alot.
My father has a 30 feet boat reatching 42kn and has 14mm GRP hull.
One night the radar reselution was set to low and at 37 they crash into a 30mm steel bar marking a bank (50mm, but 20 off them was rust) the steel bar cut a 50cm deep cut into the bow befour bending under the boat and destroy 1 off the two stern drives. The hull did not even take in anny water since it suffer no damage under the waterline, but a scratch in the gelcoat.

Don't underestimate the force off the GRP