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]

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]

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 ;)

[burt2]

Scary well ill try not to put into a situation like that lol but sometimes these thing just happen. I think the exhaust will look much nicer out the back, and I suppose the swim platform will also muffle it slightly.

Ok so out the back on the starboard side above the waterline it is, well that was easy now I just got to do it which is much harder than talking about it. I’m going to build it in stainless to a plastic water lock then stainless up to the turbo, ill buy a pre-made through hull exhaust flange that looks nice.

The next big project is building a hard top which I think ill build out of stainless tube with a fibre glass top and clears that drop down to the screen pretty standard but anything is better than a rag top, weather is very unpredictable here in New Zealand and putting those vinyl covers up in a squall sux.

[bilgeboy]

Hi Burt,

I'm trying to visualize your exhaust system, and I can't see what you mean by a "water lock". Water locks are good if you are trying to brew beer.

Since I am not sure what you mean, I just want to humbly offer that you might want to build a "riser" into the exhaust system, and then inject your cooling water after the rise. This will protect your turbo and possibly engine from any back flow of seawater or cooling water. Once the injection occurs, you can use reinforced muffler hose to the back of the boat. This should make a well designed system that has quality stainless components, but saves work and money by using muffler hose.

Mike

[StianM]

Once the injection occurs, you can use reinforced muffler hose to the back of the boat. This should make a well designed system that has quality stainless components, but saves work and money by using muffler hose.

Mike

It eleminate vibration in the exhaust system where stressed point will be damaged. On a steel pipe you would nead a compensator, but I have changed a lot off them during my work hours so a hose like bilgeboy sugest is the bether option.

[burt2]

thanks a good point exhaust hose would keep vibration down I didn't think of that, so you guys would just run it from the riser straight out the back without any thing in between

[Ike]

Run it out the stern but think about air flow. You don't want a large concentration of exhaust and Carbon Monoxide forming a cloud at the stern that literally gets sucked back into the boat. Take a long look at your boat and try to figure out where is the best place to put it on the stern so that airflow will take it away from the boat under most conditions, particularly when at idle or going slow downwind. Look at newer boats similar to yours and see where they put them.

[burt2]

thanks, yeah i had thought of that, the engine room air intakes are on both starboard and port sides right at the rear i was think of blanking the starboard one off and making the port vent bigger as the exhaust will come out pretty near the starboard vent

[StianM]

thanks a good point exhaust hose would keep vibration down I didn't think of that, so you guys would just run it from the riser straight out the back without any thing in between

let the steel pipe from the turbo go up as far as posible befour leting it go down. Place the coolig water outlet as low as posible befour the conection off the exhaust hose.

Run it out the stern but think about air flow. You don't want a large concentration of exhaust and Carbon Monoxide forming a cloud at the stern that literally gets sucked back into the boat.

The exhaust will be cooled and somehow cleaned by the water so it won't be mutch off a problem and just to pick a litle carbon monoxside is not a problem with diesels since they work with large excess air. A good combustion give close to zero carbon monoxside. I belive your thinking off Carbon dioxide.

Carbon Monoxide is flammable and highly toxic while Carbon dioxide is not flammable and not toxic, but displace oxygen and can suffocate living creatures and put out flames. Just wanted to sort out anny misunderstanding.

[burt2]

The turbo will sit approximately 300mm above the water line, do you think I will need to extend the factory exhaust up higher; I will be using 2x check valves one in the exhaust through hull fitting and one in the plastic muffler.

[StianM]

Depends on how paranoid you want to be.
We let the pipe bend up befour we go down on our coust gard vessels, but we don't have anny valves.

[burt2]

I know what you mean, there is no way of checking that the check valves are in good order but im sure 2x check valves and 300mm above water line and the exhaust and muffler to fill with water would be a pretty safe set up for a leisure cruiser more than the factory set could provide anyway.

[inchman]

Hello, forgive my ignorrance but why is the waste cooling water always put out the exhaust?

cheers Gary

[FAST FRED]

why is the waste cooling water always put out the exhaust?

It serves to lower the temperature of the exhaust , making it easier to not catch the boat on fire.

And it can reduce the noise from the exhaust a bit.

On larger engines only a portion of the cooling water is used to cool the exhaust , as the cooling water volume would be too much for the exhaust .

FAST FRED

[bilgeboy]

Hey Burt,

How is the project coming? This thread got dredged up from the depths, thought we could get an update.

I somehow missed the check valve thing before. I don't like check valves at all- they can't be trusted, and when they fail they put you in a world of hurt. Consider how they are discouraged from use in a bilge pump system, and you will not want them in the exhaust. I tried one once to keep my bilge a bit drier, it stuck closed after a month an burned up my bilge pump. Lessoned learned. Since that event I have read some pretty bright people warning to avoid such things.

I'm still partial to the riser idea. Lots of protection, good flow, no check valves and small money.

Mike

[ChrisF]

Nobody has mentioned an exhaust water tell-tale, so I will. Tee off a little bit of the cooling water just after it exits the manifold and run it out high on the hull, wherever you can best see it from the helm. Then you just check that instead of peering over the transom. All you need is a big enough stream to see, say 3/8" pipe. This can also serve as a siphon break if you tee off at the highest point of the cooling water loop.

[chandler]

It sounds like this all power boat stuff. How about a yawl? Could you run the exhaust up and out the mizzen mast?

[alex fletcher]

as a professional engineer here is what i would do measure the exhaust gas temperature and pressure both are vitally important to a diesel motor. to little pressure your ex valves will burn out to much will reduce power considerably to high a temp will burn valves and to little u may get pre detonation any question email me Alex also twin screw to single? have you thought of the safety ramification of one drive 2 is alway safer than one

[bilgeboy]

Hi Alex,

I like to discuss diesel engine theory, especially hashing out some of the finer points. I am not sure I fully understood your post. I would appreciate it you could explain a couple of points.

When you mention exhaust gas pressure, I was thinking that this is also called "back pressure" by alot of folks. I didn't understand why too little back pressure would be harmful to exhaust valves. I thought you always wanted to minimize back pressure as much as possible, as it is poorly tolerated by diesels, causing the effect you mentioned (loss of power).

Also, why would a low exhaust gas temp cause predetonation? As I understand it, most elevated EGT's are caused by excess fuel to air ratio, causing the unburned diesel to continue burning as it exits the manifold, thereby increasing the EGT. Well combusted diesel cools rapidly as the volume of gas expands on the piston downstroke. Predetonation should only be caused by an abnormally high cylinder temperature (engine to hot), injection timed too early so that diesel is injected way before TDC, or too much diesel is injected (from dribbling injectors or an extremely overloaded engine.

I would appreciate your thoughts,

Thanks,

Mike

[alex fletcher]

to much EGP will slow the hot gas passing over the Ex valve this allows the heat to transfer to the valves thus heating them
A diesel engine is designed to operate with a amount of EGP but never no pressure the pressure differential between CYL. compression and EGP is greater and the valve is under more stress and is more likely no to hold a seal we all Know what happens when EX Valves leaks.
low EGT cools any unspent fuel vapour in the Ex gases allowing them to condense in the exhaust structure i actually should have typed Post det sorry
contact the engine manufacturer and get the specs the the engine as thay are all different and adhere to the specs and measure the parameters to make shire thay r correct and you will have years of trouble free boating
hopes this helps
Alex