Exhaust Size - Replacement Engine?

[Chief Engineer]

I was peripherally involved with a Yanmar installation (was not lead mechanic) at a Boatyard....They installed a Yanmar and used a reducer to use the old two inch exhaust. I offered my opinion and was rebuffed.....by the owner of the yard....said that there would have to be substantial modifications to run 3" hose and the owner would go for it (whatever that meant).

End result? The engine overheated and smoked like a WWII destroyer laying out a smokescreen.

[s/v Jedi]

Chief,

completely agree. Next thing we'll hear is that Yanmar is a bad engine because it's exhaust diameter is bigger than that of a Ford or whatever with the same hp. Something like that is bound to come up next ;-)

cheers,
Nick.

[Boracay]

After installing my engine I found that I needed a 4" exhaust for my John Deere 4045.

Oh well. Another boat buck or so and a months work. The only tricky bit was waiting till the boat came out of the water so a larger hole could be cut in the hull.

Didn't make any difference, but I guess it's one less thing to go wrong.

[bobfnbw]

So then, let me ask this...
My perkins 4-108 has a 2" exhaust.
Would there be any benifit to increasing the exhaust size to 2.5 or 3"?
I have to replace the exhause anyway. The difference in size is a small price.
The exhaust is a long run, 35 feet, with several bends of 90 degrees.
I found out a few days ago, one of the 90's had been leaking for quite some time behind the refrig box and has wet rot all thru there... not to bad though as it needed replacing anyway...
Max rpm I could get with my hurth 2.7 reduction and the fixed prop I have was around 2400 rpm. Would increasing the exhaust size help this?
Thanks
Bob

BTW, I too believe if yanmar says 3", go for that. Better to do it right the first time.

[s/v Jedi]

You have a 35' exhaust system on a 40' boat with multiple 90 degree bends!!!??? Man, you are in trouble.

ciao!
Nick.

[bobfnbw]

Yeah nick one problem with a center cockpit boat.
Exhaust goes up in a loop in the engine room, then out thru the galley, then aft along the port side to the transom. Its a lot of hose. 35' is the estimation with a tape measure.

[Boracay]

Increasing the size of my exhaust didn't change my max. rpm. I still had to repitch.

If you have a look on the Centek web site there is a table giving suggested exhaust sizes for various powers.

It could be worthwhile to get in touch with them. They may have suggestions that could help with your rebuild.

Have you considered a side exhaust outlet?

[Reg and Kerry]

You all may be interested to know that I have taken the advice offered, and today I had a new hole cut in my hull to take 3" skin fitting. Have ordered the pipe and have fitted the water lock.
Typical boat though. Whilst removing the old 2" pipe, found more work to do. We will get there yet.
Rags

[bobfnbw]

Originally Posted by Boracay

Increasing the size of my exhaust didn't change my max. rpm. I still had to repitch.

If you have a look on the Centek web site there is a table giving suggested exhaust sizes for various powers.

It could be worthwhile to get in touch with them. They may have suggestions that could help with your rebuild.

Have you considered a side exhaust outlet?

Thanks Boracay for the link.
According to the table my Perkins 4-108 which develops ~40 hp at the rpm range we use it at needs a 2.5" exhaust. Not sure why endeavour would put a 2" in, except like most production boat builders a little savings here amounts to a lot of savings over a whole production line.
I think I will have to rethink the exhaust size. At least 2.5", which shouldn't be a problem.
Having the exhaust out the side could be done, but from what I understand, it would need to out both sides... which would mean still bringing it back to the transom area under the aft berth... so might as well go out the back.

Bob

[s/v Jedi]

The exhaust diameter is related to total length of the exhaust system and to the number of bends in it in addition to the engine manufacturers specified size. Our genset is 1.5" but after we got carbon build-up, Northern Lights calculated that we had enough length and bends to bring us very close to 2" instead. We changed to 2" and never had trouble again.

So, with 35' of exhaust hose I would ask for expert advice! I think you are better off with a short side-exhaust with a checkvalve to prevent water ingress because the checkvalve will give less back pressure than what you have now...

cheers,
Nick.

[GordMay]

Originally Posted by bobfnbw

... Having the exhaust out the side could be done, but from what I understand, it would need to out both sides... which would mean still bringing it back to the transom area under the aft berth... so might as well go out the back.
Bob

See also the earlier thread about transverse (North Sea) exhaust systems
➥ http://www.cruisersforum.com/forums/...som-25920.html

[bobfnbw]

Thanks gord. I have seen that pic of yours in another thread.

The problem for me is its a center cockpit boat. Currently the exhause hose goes to the waterlift, then up and around the engine space which is 5 feet high, then out the engine space, between a bulkhead between the aft head and the galley, along the top of the fridge countertop, then a 90 deg turn and down the port side to the transome where it makes a 45 bend and then out. there are 6 bends in the long run of 2" hose. 2 of them are 90 deg elbows. the rest are 45 to maybe 75 deg bends.
As Nick pointed out, that is a long run.

But then as Chief Engineer pointed out.... in his thread about

Why Do Owners Insist on Changing Things on a Boat?
Using water tanks for fuel

Re-routing/changing exhaust lines

My (late) mentor told me...never-ever deviate from an original installation unless you are ready to assume the liability.

but here is a situation where the builder did a poor job of it. As I found when I started pulling apart the bulkhead where the exhaust hose went. I has been leaking for years it seems. And yeah the back pressure must be significant.
I am going to see it it can be routed straight under the stateroom sole where the bilge hose currently go. I might be able to do that.

But there is no way to put it out both sides of the boat unless I go back to the transom area, and what would be the point ??

[ericoh88]

Originally Posted by s/v Jedi

The exhaust diameter is related to total length of the exhaust system and to the number of bends in it in addition to the engine manufacturers specified size. Our genset is 1.5" but after we got carbon build-up, Northern Lights calculated that we had enough length and bends to bring us very close to 2" instead. We changed to 2" and never had trouble again.

So, with 35' of exhaust hose I would ask for expert advice! I think you are better off with a short side-exhaust with a checkvalve to prevent water ingress because the checkvalve will give less back pressure than what you have now...

cheers,
Nick.

Nick,

May I ask how long was your run from genset to exhaust thru-hull ? I understand that you have a water separator so where did the carbon build up? Did you have to replace your gas/water separator from 1.5" to 2" too?

The reason I am asking is I have just installed a Kohler 3 cylinder 4.5EFOD which has similar block to NL673.

My existing exhaust is 1.5" and the Kohler is 2". Currently I have used a reducer after elbow from 2" to 1.5". Distance from genset to exhaust thru hull is 20' and the water lift is 5'5" from genset. Plan to install a water separator.

Thanks

Eric

[s/v Jedi]

Originally Posted by ericoh88

Nick,

May I ask how long was your run from genset to exhaust thru-hull ? I understand that you have a water separator so where did the carbon build up? Did you have to replace your gas/water separator from 1.5" to 2" too?

The reason I am asking is I have just installed a Kohler 3 cylinder 4.5EFOD which has similar block to NL673.

My existing exhaust is 1.5" and the Kohler is 2". Currently I have used a reducer after elbow from 2" to 1.5". Distance from genset to exhaust thru hull is 20' and the water lift is 5'5" from genset. Plan to install a water separator.

Thanks

Eric

Pretty sure you need to buy a 2" waterlift and 2" water separator, hust like I did. My genset has a 1.5" exhaust connection so I modified the 2" inlet on the waterlift to 1.5" using a piece of 1.5" fiberglass pipe, some fiberglass tape and epoxy. You don't even need to do that

I was at less than 20'.

[sharutika30]

Hi just a thought that jumps out at me as I had to replace my entire exhaust when I went from a Volvo MD2 to a Perkins 4-108.
You will probably need a new muffler as well. Don't use adapters for the hose to muffler connection as this means you have defeated the 3" system, if the muffler has 2" fittings it is the wrong one. Cheers and good luck Dan