Exhaust mismatch

[Thistle]

The exhaust outlet on my new engine has a 50mm OD; the waterlock glassed into the boat has a 40mm OD spigot.

The old 40mm ID tube is a fairly neat fit inside the new 50mm ID tube. Would it be satisfactory to put a short section of the smaller tube onto the spigot and fit the wider tube over the top of this with a jubilee clip round the lot?

 

[tugboat]

The engine manufacturer, when they give their exhaust outlet a certain diameter, intend that this diameter is continued right to the outlet at the hull. This is for sound engineering reasons, such as cooling water throughput and exhaust back pressure. I appreciate you don't want to wreck your boat but your idea could lead to poor engine performance, possible engine damage and potential water/fume leakage.

 

[sailorman]

look on the ASAP Supplies site they sell reducers

 

[alahol2]

Vetus do a reducer from and 2 an assortment of sizes. I've just used one to go from 50mm to 45mm. Neat and easy.

 

[tugboat]

Regarding the advice about the Vetus reducer, I refer to page 111 in the Vetus catalogue. The note alongside the picture of the reducer says "Exhaust systems may be sleeved upwards but NEVER downwards"

 

[oldharry]

You could, if it all fits and can be made gastight. If so you do not need to fit 'reducers' which will only do what you will have already done.

But (and isnt there always a 'but') you will noticeably affect the engine performance. The exhaust pipe doesnt just carry fumes away over board, but has to be able to carry a specific volume of exhaust gasses away from the engine. This obviously increases as the throttle is opened. If the exhaust pipe is restricted in any way then at high throttle settings the gasses cannot escape quickly enough, and this prevents the engine from developing full revs/power. (Learner legal 2T motorbikes have their engines 'restricted' to the legal limit by simply putting a narrow neck in the exhaust to prevent them developing full power. Clever kids remove the restrictor and can go faster!).

However learner bikes are never diesel powered, and a 20% restriction in the exhaust of a diesel could increase back pressure enough to cause problems if you run the engine at full throttle for any length of time. If you are willing to accept that you cannot use the engine at much more than 3/4s throttle - ever - then you would be Ok with a restricted exhaust. It would be simple enough to fasten a 'stop' on the throttle control to ensure it was never over-run.

Much better to start again with the correctly sized exhaust and waterlock, if possible.

 

[TheoSr]

Mismatch is the right word.
Say the tube has a wall thickness of 5 mm, then the 50 mm tube has an opening of 12.5 cm2, the 40 mm tube has only 7.0 cm2, this is just 56% of the 50mm tube.
Asking for trouble, I think.
Better do it right from the beginning.
Good luck,
Theo

 

[Stemar]

When I changed my engine from 7 (actually 5 if I was lucky) to 28HP I cobbled the exhaust in the way you suggested. My exhaust comes straight out of the transom about 4" above the waterline, and at anything much over idle, the exhaust water didn't touch the sea for a metre or more!

I upgraded, changing the transom throughhull and it was a big improvement. I don't know if I've improved the power because I still have the original prop. The new engine doesn't even know it's on the end of the shaft!

 

[fireball]

Bit of a stern powershower effect then .... could catch on!!

 

[Thistle]

Many thanks - as ever - for all the wise counsel.

I hadn't - but should have - appreciated the possible effect on performance. After Tugboat's first reply I dropped the engine manufacturer a line by e-mail and they have, pretty much by return, confirmed that, in this case, 40mm is OK. Full marks to Beta Marine for their excellent service.

(As to why it's OK, I'm not sure but rather suspect that the same exhaust is used on my Beta 10 and the next one up (13.5?) so there will be some spare capacity with the smaller engine.)

 

[bob_tyler]

After consultation with Beta Marine, my 13.5hp was fitted with a 40mm exhaust. Has been no problem.

As usual Beta were most thorough and helpful.

 

[tugboat]

I suppose it's a silly question, but if 40mm is OK why did they put a 50mm exhaust stub on the engine in the first place? /forums/images/graemlins/confused.gif

 

[Avocet]

You don't say what power the engine is. I agree you can increase backpressure by reducing the bore but unless the bore is as small as it already can be for that engine, it's unlikely to cause a problem. Chances are, inside your waterlock and / or muffler, it will go through much smaller cross-sections in any case - particularly when it's sharing them with the cooling water. My 12 hp has a 40mm exhaust and I'm pretty cetain the gas and cooling water get lost in there for days at a time! The other thing to consider is the increase in backpressure for every bend in the pipe. The manuafcturers would, I imagine, be careful not to size their pipes on the limit for acceptable flow in case the exhaust on any particular boat had an unusually tortuous route. One thing I would be careful of though, if you intend to use one pipe to sleeve another, is that when warm, they might soften and loose their grip on one another. I know they shouldn't get warm but all in takes is a plastic bag over the intake or a failed impeller.... It's also a good idea to have a swelling at the end of the "male" spout so that if anything does start to move, the jubilee clip(s) should tighten their grip as they try to slide over the swelling. For these reasons, I'd be inclined to get a "proper" reducer.