Logo
FULL SITE MENU

Inboard prop questions

J

jon_a

Contributing Member
"I have a BB140A (inboard&

"I have a BB140A (inboard) with more noise and vibration than I care for. This is a 27 year old Repco(Maine lobster boat) that was hardly used and the previous owner never got the new boat bugs worked out.
I believe that part of my vibration problem is that the prop passes too close to the oak strut underneath. The wheel is a Michigan 14LH13, Dyna-Jet. The engine maxes out at 4300 rpm so it is about the right size for the engine. My question is could I use a smaller diameter prop with more pitch? is there some rule of thumb about maximum/minimum pitch for a given diameter? What does this do to the efficiency of the prop?
Also, as the attached pictures show that is a stuffing box used as a stern bearing! I also have a stuffing box in the correct place. Anyone ever seen this done before?
Any comments would be appreciated.
Thanks,
Jon <center><table border=1><tr><td>http://www.marineengine.com/discus/icons/mime_html.gif""" align=left alt=""text/html"">prop installation
[url=""][b]Prop Pictures.html[/b][/url] (3.6 k)</td></tr></table></center>"
 
"For some reason I did not man

"For some reason I did not manage to open the pix, but anyway.
I suggest You reduce the Dia by 1" and change to a 4-blade (Dyna quad) with same pitch"
 
"Mort:

ALL inboards have vi

"Mort:

ALL inboards have vibration since the shock wave coming off the top of the prop arc hits the bottom of the hull. No way around it. That said, the 15 % rule is common on the clearance issue: the space between prop and hull should be equal or greater than 15 % of the prop's diameter.

You might simply need a prop tune up.

Jeff"
 
"Jeff:
You are correct that a

"Jeff:
You are correct that all props gives schock-waves but there is normally quite a difference in vibration and schock-waves from a 3 versus 4-blade prop. By going from 3 to 4 blades the dia might be reduced by normally 1" which will encrease the distance to keel/bottom and at the same time keep the blade-area and trust. Not quite comparative, but try to put a triangel wheel on Your car versus a square - which will give the most 'vibration'?
Did You manage to open the pix?"
 
I don't know what happened

I don't know what happened to the pictures. I'm going to try to post them again.

Jon
 
"cannot upload the pictures, e

"cannot upload the pictures, each picture exceeds the 100kb allowed by this board. I'll try to e-mail them.

Jon"
 
"Got the pics. no problem with

"Got the pics. no problem with space between prop and hull, but not quite happy with angle.
Suggest You first try to disconnect the shaft coupling from gear-box and check engine align-ment with a feeler gauge. .05 mm may pass (gap), 0.10 mm should not pass (gap). If ok - try a 4-blade as earlier suggested."
 
Michigan doesn't list a 4

Michigan doesn't list a 4 blade prop under 17" dia. in it's catalog.
Who else makes inboard props?
Thanks for all the help. A four blade certainly seems worth trying.

Jon
 
"I think I am outta luck with

"I think I am outta luck with the 4 blade idea. Nobody makes a 4 blade inboard prop that small (except for a ski-boat and they don't appear to be suitable for my needs). I'll check the alignment this week, I'm taking a couple of vacation days so there should be time.
Would a 3 blade Michigan of 1" smaller diameter reduce the vibration? What would the pitch need to be to still be compatible with the 14LH13?

Jon"
 
"I would keep it at 14".

"I would keep it at 14".
The distance to the bottom is not the problem, but the profile of the stem and the funny wooden support to the rudder keeps water from flowing clear to the prop in addition to the steep angle of the prop-shaft that makes the water attack the prop at a wrong angle.
I'm not sure how much work You are prepeared to do, but the installation is a mess!
Had it been mine I would have taken out the shaft and sterntube, lifted the cutlass to leave about 2" from prop to bottom to reduce the angle, made a proper profile of the stem to make water enter the prop, changed the wooden support for the rudder to a SST tube-profile or similar.
In general improved the waterflow to the prop!
Finally I would have made a slight cupping of the blades to make them hold better on the water."
 
"Thanks Morten for the input.

"Thanks Morten for the input. When I bought the boat I didn't know how much I didn't know about inboards! The price of arrogance, I should have had it surveyed. I intend to own this boat for a while as it suits my needs pretty well so I'm going to make this right.
I'm going to have to find someone local to help me lay this out, this is doable. I have the skills, the boat is in the back yard where it is easy to work on. The journey of a thousand miles begins with a single step.....


Jon"
 
"Just picked up on another pro

"Just picked up on another problem, the engine is mounted on flexible rubber mounts but the transmission connects to the shaft with a rigid flange. My research says that this is not right, that I should also have an flexible coupling on the shaft so that when the motor moves about on the rubber mounts the movement is not transmitted to the shaft. Anyone have any wisdom to offer, should I change the coupling? Any suggestions as to what works best with the MS3 trans?

Thanx,
Jon"
 
"That is as should be. The min

"That is as should be. The minor movement will be taken up by the stuffing box and rear cutless bearing.
Flexible shaft connections may be used by those which have problems in aligning the engine properly, extreemly soft engine mounts are being used, too litle space in sterntube-shaft,..."
 
Yesterday I released the shaft

Yesterday I released the shaft flange and discovered that the shaft sits 1/8" above and 1/8" to the right of the gearbox flange! Spent the rest of the day repositioning the engine and the connection is now dead-on and I cannot fit a .002" gauge in anywhere around the flange. We"ll see what that does in the Spring when we launch.

Jon
 
"Hi,
flexible engine mounts a

"Hi,
flexible engine mounts and flexible couplings are not meant to compensate for misalignment between engine and shafting. Flexible engine mounts are installed to dampen the vibration transmitted to the hull, especially if this is of plank on frame construction. In fact marine flexible mounts are constructed in such a way as to allow up and down motion only.
Flexible couplings are meant to take up the starting tourque especially on larger engine / prop installations. In both cases the gearbox flange and the shaft flange should be perfectly aligned i.e. both flanges should be parallel and concentric, rotating on the same centre line. Careful alignment, together with a decent cutlass bearing will eliminate excessive vibrations."
 
"Hey Jon, I just wanted to pip

"Hey Jon, I just wanted to pipe in and mention that my engine/trans produces alot of vibration also. I have it running pretty well but it makes alot of things on the boat buzz or vibrate. Its funny it only occurs at certain RPMs though. I can wedge a piece of paper in a certain latch that's rattling or just speed up a little and it will go away. I did take the prop in to be polished and balanced a few months ago but it only made a small if any change in the vibration.
Good luck with your projects this winter."
 
Back
Top