Engine mount position.

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chrishelen

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On my Perkins perama the front STB engine mount has always been a problem. To get the right alignment the stud adjust is at the top of its scope, I know there is only supposed to be about 10mm of thread from the mount to the engine braket, but if I pack the mount up it touches the nut on the water pump. It's been fine for the past two years but in the last week the adjustment studs sheared twice whilst running. I put a new mount on after the first episode so hopefully not that, engine alignment is spot on. Any ideas what's going on? It's a commercial boat that does lots of hours.. I'm typing this on a small phone, so sorry for any grammatical errors.
 
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chrishelen said:
On my Perkins perama the front STB engine mount has always been a problem. To get the right alignment the stud adjust is at the top of its scope, I know there is only supposed to be about 10mm of thread from the mount to the engine braket, but if I pack the mount up it touches the nut on the water pump. It's been fine for the past two years but the last week I have snapped the adjustment stud twice. I put a new mount on after the first episode so hopefully not that, engine alignment is spot on. Any ideas what's going on? It's a commercial boat that does lots of hours.. I'm typing this on a small phone, so sorry for any grammatical errors.
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Not many choices there...
Spanner too long
Stud too thin
You really should only be tightening nut above against nut below engine bit. To snap a stud then you would need to stretch it apart!
 
Dougal Tolan said:
Not many choices there...
Spanner too long
Stud too thin
You really should only be tightening nut above against nut below engine bit. To snap a stud then you would need to stretch it apart!
Click to expand...

Sorry, I didn't word that very good. The studs have snapped themselves somehow when the boat is running.
 
chrishelen said:
Sorry, I didn't word that very good. The studs have snapped themselves somehow when the boat is running.
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You say the engine mount in question has always been a problem, in what way? Has it broken before since the recent two breakages? If not, how long has it run for without breakage before these recent two breakages? Can you post pictures of the broken mount?

Www.solocoastalsailing.co.uk
 
Plum said:
You say the engine mount in question has always been a problem, in what way? Has it broken before since the recent two breakages? If not, how long has it run for without breakage before these recent two breakages? Can you post pictures of the broken mount?

Www.solocoastalsailing.co.uk
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It's always been a problem because of the proximity to a nut on the seawater pump. If I pack the mount up it touches the nut, so it has to have around 80mm of stud showing before the bottom adjustment nut. I can't post pictures on here, but the stud shears off just under the bottom adjustment nut. They are m12 studs. Lost a day's work today fitting new engine mounts. I have managed to get a 12 mm pad under the mount which still gives a lot of screw showing. It's run for two years and thousands of hours without mishap before this latest episode.
 
Plum said:
Where are you measuring? If you are measuring at the gearbox flange and you have no flexible coupling you should be within 0.1mm
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I have a flexible coupling, it's around 0.5 mm differance top to bottom and around the same side too side measured the same bolt at 12-3-6-9 o'clock position
 
chrishelen said:
It's always been a problem because of the proximity to a nut on the seawater pump. If I pack the mount up it touches the nut, so it has to have around 80mm of stud showing before the bottom adjustment nut. I can't post pictures on here, but the stud shears off just under the bottom adjustment nut. They are m12 studs. Lost a day's work today fitting new engine mounts. I have managed to get a 12 mm pad under the mount which still gives a lot of screw showing. It's run for two years and thousands of hours without mishap before this latest episode.
Click to expand...

Ok, couple of thoughts.....
M12 is rather small. What size engine? Even my 21hp diesel has M16 studs.

There is more to aligning an engine than just measuring at the gearbox flange. You can have perfect alighnment but the four mounts may not be sharing their equal load. You may have the diagonally opposite mounts taking most of the load. You need to disconnect the flange, jack up the front of the engine on its centreline and check the clearance above the two nuts under the front mounts.

Look for other factors that are contributing extra load on this mount. Is there another fault in one of the other three mounts that is resulting in this one taking more that it's fair share of load?

As a last resort you may have to relocate your waterpump!


Www.solocoastalsailing.co.uk
 
chrishelen said:
I have a flexible coupling, it's around 0.5 mm differance top to bottom and around the same side too side measured the same bolt at 12-3-6-9 o'clock position
Click to expand...

0.5mm sounds rather a lot, if it's the variation between a 'datum' nut in the various clock positions.
Also worth asking if your flexible coupling is actually designed to permit that degree of intolerance. Many can only accept a tiny amount: the "flexibility" is more about reducing the transmission of engine vibration to the hull.
 
Plum said:
Ok, couple of thoughts.....
M12 is rather small. What size engine? Even my 21hp diesel has M16 studs.

There is more to aligning an engine than just measuring at the gearbox flange. You can have perfect alighnment but the four mounts may not be sharing their equal load. You may have the diagonally opposite mounts taking most of the load. You need to disconnect the flange, jack up the front of the engine on its centreline and check the clearance above the two nuts under the front mounts.

Look for other factors that are contributing extra load on this mount. Is there another fault in one of the other three mounts that is resulting in this one taking more that it's fair share of load?

As a last resort you may have to relocate your waterpump!


Www.solocoastalsailing.co.uk
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Thanks. It's a 29hp. I have done most of what you suggest, I thought I had the engine perfectly balanced on the four mounts. I will check again. The water pump is built into the engine. Someone suggested I could have a badly cut stud which could lead to weakness, but two? Anyway thanks for helping,see what happens tomorrow.
 
When studs and bolts 'shear' at a nut the cause is fatigue. In this case the fracture will be normal to the axis of the bolt. From your description there is a lot of stud extension with a heavy vibrating mass at its end. You need to reduce this by packing the mount upwards or maybe by adding washers to shift the nut downwards. There are other options, difficult to advise without seeing, but reducing the length of unsupported bolt is the aim.
 
vyv_cox said:
You need to reduce this by packing the mount upwards or maybe by adding washers to shift the nut downwards. There are other options, difficult to advise without seeing, but reducing the length of unsupported bolt is the aim.
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I think he knows this, but doesn't think he can do it because of the position of the water pump.

It's difficult to give any advice without a picture.

Pete
 
prv said:
I think he knows this, but doesn't think he can do it because of the position of the water pump.

It's difficult to give any advice without a picture.

Pete
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Yes, the water pump is in the way. I have managed to get a 12mm pad under the mount, but still too much thread showing. My mistake guys M16 studs. Small phone fat fingers. Don't know how to post pictures on here Pete.
 
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macd said:
0.5mm sounds rather a lot, if it's the variation between a 'datum' nut in the various clock positions.
Also worth asking if your flexible coupling is actually designed to permit that degree of intolerance. Many can only accept a tiny amount: the "flexibility" is more about reducing the transmission of engine vibration to the hull.
Click to expand...
I think half a mil is acceptable, consider 12 people onboard, flexing of the hull, wave motion. That half a mil is going to change a bit.
 
So, the mount cant come to Mohamed... sorry couldnt resist... the mount cant be moved closer due to the water pump..
3 choices:
move the pump
thicker stud
make the top end coupling flexible

Not easy to tell if move the pump is a sensible option (probably not)
A thicker stud will more likely break something else.
So, a flexible coupling at the top. Something like a rubber washer sandwich?
 
Dougal Tolan said:
So, the mount cant come to Mohamed... sorry couldnt resist... the mount cant be moved closer due to the water pump..
3 choices:
move the pump
thicker stud
make the top end coupling flexible

Not easy to tell if move the pump is a sensible option (probably not)
A thicker stud will more likely break something else.
So, a flexible coupling at the top. Something like a rubber washer sandwich?
Click to expand...

Ha very funny. Can't move the pump Dougal. Tried a bigger stud but the rear mount same side braket snapped, just moved the problem. Not sure what you mean 'flexible coupling' on top
 
chrishelen said:
Thanks. It's a 29hp. I have done most of what you suggest, I thought I had the engine perfectly balanced on the four mounts. I will check again. The water pump is built into the engine. Someone suggested I could have a badly cut stud which could lead to weakness, but two? Anyway thanks for helping,see what happens tomorrow.
Click to expand...

Do try to post some pictures of the mount and how it is fitted to the engine and maybe we can suggest some fixes. Good luck.
 
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