2 part question about exhaust ...

[Scott T-Bird]

My exhaust exits the transom just above the boot stripe and it is pretty much submerged when running over 6 knots as the boat squats at higher speed. Prior to my engine change from 1GM to 2YM15, the exhaust was only submerged if pressed to full throttle and reaching about 6 to 6.2 knots. I would normally cruise along at a speed where the exhaust was just above water, otherwise, I noticed black soot in the water and (often) exhaust smell in the cockpit.

I always assumed that submerging the exhaust had a tendency to cause the exhaust to ride up the transom into the cockpit.

Now with the new engine and higher cruising speed, the exhaust is always submerged. I don't notice any difference or smell, but I wonder if there is a preference for exhaust to be submerged or above waterline at all times. I don't relish changing the current location.

The second question is a moot point now but I'll ask anyway. My exhaust elbow and hose used to be 1-3/4" but the opening thru the transom was restricted to about 1-1/4" at most. Bad boat design, I suppose ... but does this contribute to engine maintenance problems? What would be anticipated problems? (Now my exhaust is 2" throughout).

 

[Bill Roosa]

T-bird!
motoring at max speed is for folks on schedules!!!!!!
I would have thought that you would have figured that out by now!

Hey Doc, it hurts when I bend my elbow. Doc says "so just don't bend your elbow"

I'd say you are well along the route to owning a trawler. Stop investing in motor an start investing in sails. A good set of light wind sails will go a long way toward not having to motor at max speed. You also might want to consider planning your trips and weather better. Going in the right direction on the way out so you can see a quick return by sail on the way back...........

 

[caguy]

What happens when you motor sail and heel?

 

[jdhohmann]

What happens when you motor sail and heel?

Nothing happens as long as you don't heel too much in which you expose your raw water intake. then you will have serious problems, like overheating your engine. Another thing to be carfeul about is if your fuel tank suction line gets exposed because the fuel is over on one side as well, you may stall your engine and end up having to prime your injectors and bleed them, if you are running a diesel....

 

[Scott T-Bird]

Funny you should mention it ...

T-bird!
motoring at max speed is for folks on schedules!!!!!!
I would have thought that you would have figured that out by now!

Hey Doc, it hurts when I bend my elbow. Doc says "so just don't bend your elbow"

I'd say you are well along the route to owning a trawler. Stop investing in motor an start investing in sails. A good set of light wind sails will go a long way toward not having to motor at max speed. You also might want to consider planning your trips and weather better. Going in the right direction on the way out so you can see a quick return by sail on the way back...........

I happily sail in light winds and slow speeds. Relaxing on a sailboat is my middle name! But, turn on an engine, 6 knots is agonizingly slow. As James Taylor once sang in a song (Traffic Jam) "It hurts my motor to go so slow".

Also, the concept of investment in relationship to sailboats is oxymoronic, isn't it? But, you are right ... buying new sails is always a good way to spend money!


Unfortunately, I don't have to do much planning ... I can only navigate a maximum of 3 miles from my dock. No, I should have shortened my question ....

Is there any reason to be concerned about submerging the exhaust port when the boat is squatting at higher speeds?

 

[Stu Jackson]

Is there any reason to be concerned about submerging the exhaust port when the boat is squatting at higher speeds?

No. My boat's been doing it for 29 years.

There should be more than enough pressure out the muffler to overcome a tad of immersion.

 

[Scott T-Bird]

no doubt that there is more than enough pressure ...

No. My boat's been doing it for 29 years.

There should be more than enough pressure out the muffler to overcome a tad of immersion.

Still, it is a restriction, albeit water. That's why I was wondering about it in the same context as my old 1-1/4" exhaust port when the whole exhaust system in front of the port was 1-3/4".

 

[Ken13559]

Newer Hunters exhaust is underwater. No issue there.

 

[kloudie1]

My 34's port is under the stern overhang just above the waterline. It submerges when running at anything over 4 knots. I don't think that the small amount of back pressure there is a problem for the engine.. as a % of the total back pressure, that amount is smaller than in the lift muffler and the elbow. That small amount of horsepower loss could be measured with some sophisticated instruments, but for all practical purposes, it is lost in the "round off" error. On my boat, I don't like the bubbling noise and have considered moving the outlet above the waterline.. so far, (24 years) it has not bothered me enough yet.

 

[centerline]

My exhaust exits the transom just above the boot stripe and it is pretty much submerged when running over 6 knots as the boat squats at higher speed. Prior to my engine change from 1GM to 2YM15, the exhaust was only submerged if pressed to full throttle and reaching about 6 to 6.2 knots. I would normally cruise along at a speed where the exhaust was just above water, otherwise, I noticed black soot in the water and (often) exhaust smell in the cockpit.

I always assumed that submerging the exhaust had a tendency to cause the exhaust to ride up the transom into the cockpit.

Now with the new engine and higher cruising speed, the exhaust is always submerged. I don't notice any difference or smell, but I wonder if there is a preference for exhaust to be submerged or above waterline at all times. I don't relish changing the current location.

The second question is a moot point now but I'll ask anyway. My exhaust elbow and hose used to be 1-3/4" but the opening thru the transom was restricted to about 1-1/4" at most. Bad boat design, I suppose ... but does this contribute to engine maintenance problems? What would be anticipated problems? (Now my exhaust is 2" throughout).

Scott, its recommended that there is no more than 1lb of back pressure measured as close the the engine exhaust manifold as practically possible...

When i installed my riser loop to prevent backwashing in a seaway, I did some research on this as the tubing i used was restricting my exhaust to 1.25" also... what i found was an engine up to about 40hp can use 1.5 exhaust tubing.... and due to standardization, 1-7/8" or 2" is normally used.
So my reasoning tells me that my little 25hp motor or its performance wont be damaged by slightly smaller exhaust. And it hasnt been... so far.

Some boats have a submerged exhaust and some do not, but as long as water cant backwash into the engine it doesnt really matter.

A large hull outlet necked down to fit a small exhaust tube can cause water to be "hydraulically" pushed in to the system, where as larger tubing with a smaller exit is better, providing it does not build an unacceptable backpressure.

A high riser just before the hull exit is the best way to prevent a backwash, short of installing a ball valve...

 

[Scott T-Bird]

Excellent reply!

Scott, its recommended that there is no more than 1lb of back pressure measured as close the the engine exhaust manifold as practically possible...

When i installed my riser loop to prevent backwashing in a seaway, I did some research on this as the tubing i used was restricting my exhaust to 1.25" also... what i found was an engine up to about 40hp can use 1.5 exhaust tubing.... and due to standardization, 1-7/8" or 2" is normally used.
So my reasoning tells me that my little 25hp motor or its performance wont be damaged by slightly smaller exhaust. And it hasnt been... so far.

Some boats have a submerged exhaust and some do not, but as long as water cant backwash into the engine it doesnt really matter.

A large hull outlet necked down to fit a small exhaust tube can cause water to be "hydraulically" pushed in to the system, where as larger tubing with a smaller exit is better, providing it does not build an unacceptable backpressure.

A high riser just before the hull exit is the best way to prevent a backwash, short of installing a ball valve...

Thanks for that. Yes, I have a riser at the stern and a waterlift just aft of the engine.

Kloudie & Ken, that was also good to know. I've not had the same symptoms that I used to have, now that I replaced the engine (and I'm sure those symptoms were related to the engine's performance, not the location of the exhaust).

 

[jibes138]

Thanks for that. Yes, I have a riser at the stern and a waterlift just aft of the engine. Kloudie & Ken, that was also good to know. I've not had the same symptoms that I used to have, now that I replaced the engine (and I'm sure those symptoms were related to the engine's performance, not the location of the exhaust).

water pressure is 1 atmosphere for each 33 feet of depth. So at say 1 foot under water it is 14.7 psi/33 = less than half a psi.

 

[jibes138]

Still, it is a restriction, albeit water. That's why I was wondering about it in the same context as my old 1-1/4" exhaust port when the whole exhaust system in front of the port was 1-3/4".

scott was it 1.25 ID or OD? My Universal M25 uses 1.25 pipe for the exhaust. This is fixed by the exhaust manifold flange that is threaded for a 1.25 pipe. I'd think your smaller motor should be OK at this size. The M25 is 21 HP as a point of ref. the real restriction might be the exhaust valve. Might be worth looking at that cross section area to see if it is smaller than a 1.25 diameter hole area.

 

[JamesG161]

You should not have...

I would normally cruise along at a speed where the exhaust was just above water, otherwise, I noticed black soot in the water and (often) exhaust smell in the cockpit.

You may have an exhaust leak, which is noticeable when submerged exhaust is water restricted.

Black soot = engine problems.

You need to have a expert review your exhaust riser, muffler, host, et.al.

That is minor cost compared to engine fixes.
Jim...

 

[Allan12210]

I always look back at the photo Ralph Johnstone has published of the cut away mixing elbow. That internal passage, even when clean and including the humpty hump path sure looks like a lot more resistance in flow than the differences in downstream exhaust pipe sizes discussed above. I remember with car engines a certain amount of back pressure was good to assist in exhaust scavenging. Running straight exhaust would burn the exhaust valves. So, it looks like most of the required back pressure is already designed into the mixing elbow.

 

[Nelsonhow]

Excessive Heel

Three thing to consider:

1) Heel so pronounced that the cooling water intake is exposed.
2) Heel so pronounced that the fuel tank uptake sucks air in a almost empty tank.
3) Heel so pronounced that the engine oil fails to circulate properly.

In a moderate breeze and a heel of less than 25 degrees or so I don't think there's a problem with these three things.

If heel is excessive on my H-30 it usually means I'm over canvassed and I need to reduce the sail. My Hunter likes to sail kind of flat so if there's a lot of heel I reduce the head sail and consider a reef in the main. This puts less stress on the rig and on the Captain!

 

[jibes138]

IF you are sailing along with 25 degrees of heel and doing hull speed of about 7 knots why is your engine running? If you aren't sailing and just motoring along why are you heeling 25 degrees?

 

[frankies56]

I am never am surprised when someone ask a question and and the tangent changes. to heeling 25 deg and motoring wow dah-mazing, Allen 12210 I really like the photo of that elbow cutaway that is an eye opener for me. I have I heve never even bothered looking to see if it was clogged. maybe I will get another 100 rpms, thanks for that post. oh as for what exhaust smell small openings on small eng, yea we have a had that