The completion of the installation of the Kubota coincided with the annual antifoul so the first outing was
with a clean prop and bum. Now I know the old Bukh was tired but I didn't know how much until yesterday.
I suppose a big difference is the Bukh was raw water cooled from a 1/2 inch hose feed and when motoring
the exhaust sort of went 'donk donk splash - donk donk splash' but the kubota is heat exchanger cooled
from a 3/4 inch feed and puts out an amazing amount of water from the exhaust. The new engine is so much
more powerful that even at 'running in revs' of 2000 rpm the amount of water it pushes up at the stern is very
noticeable. My exhaust outlet is about 2 inches above waterline right at the rear on the Port side and I now
have a bit of a problem.
When the amount and force of the exhaust water meets the water pushed up by the prop I get a noise that
sounds like Niagara Falls on a windy day. When I first opened the throttle it sounded like all the bearings in
world had seized up all at once. Missus said it wasn't coming from inside the boat so I discovered the scource
of the racket and scratched my head. I don't suppose it's doing any harm but I can do without it, so I guess
the easiest solution is to raise the exhaust outlet up hull by 4 inches or so to clear the sea water, other than
that ....I don't know. Any suggestions ??.

Trek said..
Sounds like you've made yourself a speed boat Sam! Moving the exhaust outlet up might be necessary by "sound" of it. They do that on ski boats.

Ski boats eh Trek. That's interesting, and here's me thinking I'd had an original thought about raising the outlet.... Ha.

Add a piece of pipe and a 90deg elbow with a polished stainless steel tip on it.

My exhaust outlet is under water giving a gentle gurgling sound reminding one of the fact that the sea-cock is open and the impeller working correctly.
It has the appropriate exhaust elbow inside, and a pong box, to stop flooding, of course.
No streaks of soot on the hull either.

The underwater outlet muffles sound. I am glad, my boat has one.
My mates Cav 32 with the outlet well above water is rather unpleasantly noisy.
The other boat l was racing on (had a single cyl. Bukh motor) had underwater outlet and it was pleasant.

I hope, you can solve this unpleasant situ before you burn the boat.

It seems that I am over propped. The prop that I have on now is a 3 blade fixed 13 x 10.5, how
the old Bukh ever turned that is a mystery. Porters have just told me it should 12 x 8 and they
will alter it, no problem. Should I do this or should I leave it as it is considering I now have a
bigger engine that will cope better with a big prop.

I think the gearbox ratio is also pretty important. Just bought a new prop from Flexofold and they won't advise on the right size without it.

Put a shower head on your exhaust

If possible try a temporary extension to your exhaust, try higher and under water, that way you take the guesswork out before you spend $$$ on what may not be the best out come.

samsturdy said..

So now I'm wondering if my noisy exhaust problem is caused by the over size prop pushing too much water up
to the stern whereas the boat is designed for a smaller 12x8 prop and the outlet will be above water level.

HI Sam,
Makes perfect logical sense to me. You never know, you may just fix the root cause and not a symptom.
If it turns out like that, nice work Watson!

So I've decided to do this job in two stages ( 1 ) get the prop resized. ( 2 ) get the exhaust fixed. If resizing
the prop fixes the exhaust problem.....great.
In order to keep the cost down I've contacted diver Dave Pollock to take the prop off in the water. I'll take
it to D H Porter and they can resize it. Dave said to get it propspeeded before I give it to him to put back on.
It sounds good to me and should happen one day next week.
I'll keep you posted.

Bundeenabuoy said..

samsturdy said..
So I've decided to do this job in two stages ( 1 ) get the prop resized. ( 2 ) get the exhaust fixed. If resizing
the prop fixes the exhaust problem.....great.
In order to keep the cost down I've contacted diver Dave Pollock to take the prop off in the water. I'll take
it to D H Porter and they can resize it. Dave said to get it propspeeded before I give it to him to put back on.
It sounds good to me and should happen one day next week.
I'll keep you posted.

Can you get a second hand prop to see if a different size removes the noise?
You may be on to the answer and then upgrade.

Well Bunny, the way I see it, regardless of whether resizing the prop fixes the noise, the prop is still too big
for the engine, 13x10.5 is significantly larger than 12x8 and I need to get it to the right size regardless of any
other problem. Perhaps someone can tell me the consequences of using a prop that's far too big.
I'm just hoping that my thinking about too much water being pushed up is correct and the resizing will fix it.
We'll see.

UncleBob said..

samsturdy said..

Bundeenabuoy said..

samsturdy said..
So I've decided to do this job in two stages ( 1 ) get the prop resized. ( 2 ) get the exhaust fixed. If resizing
the prop fixes the exhaust problem.....great.
In order to keep the cost down I've contacted diver Dave Pollock to take the prop off in the water. I'll take
it to D H Porter and they can resize it. Dave said to get it propspeeded before I give it to him to put back on.
It sounds good to me and should happen one day next week.
I'll keep you posted.

Can you get a second hand prop to see if a different size removes the noise?
You may be on to the answer and then upgrade.

Well Bunny, the way I see it, regardless of whether resizing the prop fixes the noise, the prop is still too big
for the engine, 13x10.5 is significantly larger than 12x8 and I need to get it to the right size regardless of any
other problem. Perhaps someone can tell me the consequences of using a prop that's far too big.
I'm just hoping that my thinking about too much water being pushed up is correct and the resizing will fix it.
We'll see.

Sam, not my area of expertise but I always thought that if the engine can be run at its design full revs then the prop is about right, an oversize prop will bog down the engine and not allow it to reach it's potential. Is this the case with your's ?

Not currently Bob because I'm 'running in' I'm only allowed to go to 2000 rpm. Nonetheless the engine may be
stressed without me knowing it yet and I don't want to ruin a brand new engine.
Looking back at the time the Bukh was in and thinking about how that ran I think it's safe to say it did not rev
freely, but, with a single cylinder donk with such a long stroke it may be hard to determine especially when you
have as little experience with them as I have.

Yes MB, I've already spoken to Glen so when I take the prop down to him I'll also take all the original info regarding
boat and engine ( I still have it all including the receipt for the prop he made ) plus the info on the new engine and he can work it out from
there. He's not going to make a new prop, He says he can cut down this one and alter the pitch. That'll do me.

samsturdy said..
I'm 'running in' I'm only allowed to go to 2000 rpm.

I would be talking with Kubota about that. Diesel engines love "load".

If you nurse that engine you will surely glaze the bores. This "running in" idea/proceedure is not generally relevant today.

Engine manufacturers have figured it out and no longer make engines to the size tolerances that require a "running in" period. They deliver engines "ready for work".

You are "allowed" to run the engine at whatever load capacity you "choose". Of course running it as the manufacturer recommends is recommended for warranty purposes.

Beta is not the manufacturer of the engine. Kubota is. Read your warranty document carefully.

I just got back from having the day off yesterday ( watching my granddaughter at a dance comp. ) Regarding
engines 'under load', pushing a boat along at any revs would put an engine under load, would it not ?. I thought
glazed bores were caused by running the engine to only charge batteries and doing it at too low revs.
Cisco, I guess I'm trusting Beta ( through their handbook ) to give me the same advice as would come from Kubota.

Yes you are right Cisco. The graph shows max torque at 2600 rpm with a fuel consumption of one and a half litres per hour.
Now that's almost the revs they're telling me not to exceed for 20 hrs, so it looks like there will be no harm done sitting at
maximum torque when I get going again.
Regarding the exhaust situation, it also states that the exhaust back pressure should be kept to a minimum, so that means
no under water exhaust as that will dramatically increase back pressure. So, going along with resizing the prop to correct
size to keep the exhaust outlet above water seems the right thing to do.