Propshaft A-Frame

[Sultans_Kapitein]

Hi all, I have just had an 'interesting' email from the boat yard. I asked them to have a look at the wooden pads between the Propshaft A-Frame brackets and the hull, she has two props so that is four pads in total. Also she has been 'high and dry since 2004 so I am pretty sure the wood will have taken a turn for the worse. Any way I got the email saying;

I am no expert on boat design, but I would question why this timber is here, and why the brackets were not made longer to bolt or weld directly to the hull? Obviously timber is always going to have maintenance issues. The only favourable reason for the timber pads I can think of is to provide some degree of vibration absorption, I suspect their only purpose is to achieve the correct prop-shaft alignment. I thought you may want to consider doing away with the timber altogether, by possibly using a high density plastic in its place or adjusting the brackets themselves such that no packing is required

In their defence I must say that the boat yard acknowledge their limited abilities with steel boats, and their only experience with them is on narrow boats, most of their expertise is in fibreglass boats, both manufacture and repair.
So what does the 'team' think? The boat is a 15 Meter TSDY built in steel. I do not know but it seemed an obvious practice to provide both alignment and also anti-vibration padding but I do not know. I am sure there are plenty on here who will know the 'definitive' answer.
Thanks in advance.

Mal

Also posted on Practical Boat Owners Forum

 

[scottie]

I suspect that they are packing pieces if the A brackets are cast rather than fabricated as wood Is easier to shape to fit rather than being vibration absorber.
Casting a "plastic" pad may be an option if they know what they are doing similar to engine mountings.
How hick are they?

 

[PCUK]

Brackets would normally be made to the correct size and welded on. Wood has no place below the water-line on a steel boat. I think you should thank the yard and let them do the job properly.

 

[OAF]

totally agree with PCUK Wood has no place on a steel boat when it comes to mounting engines, gearboxes or shafts, if it has been used for packing/alignment replace it with steel!

 

[Sultans_Kapitein]

I suspect that they are packing pieces if the A brackets are cast rather than fabricated as wood Is easier to shape to fit rather than being vibration absorber.
Casting a "plastic" pad may be an option if they know what they are doing similar to engine mountings.
How hick are they?

Good question, to which I have no answer

 

[Sultans_Kapitein]

totally agree with PCUK Wood has no place on a steel boat when it comes to mounting engines, gearboxes or shafts, if it has been used for packing/alignment replace it with steel!

So you are suggesting a steel pad to replace the wood?
Or have I misunderstood and do you mean to re-fabricate the A-Frames to fit directly to the hull?

Mal

 

[OAF]

So you are suggesting a steel pad to replace the wood?
Or have I misunderstood and do you mean to re-fabricate the A-Frames to fit directly to the hull?

Mal

If the wood is there just to act as a spacer then just replace it with steel, the alignment of the bracket/bearing on the shaft has to be achieved some how, without seeing the set up it is difficult to suggest the best solution,what sort of bearing is on the prop shaft, is it a plumber block bearing

 

[Neil_Y]

Totally agree with pc. I can't see any benefits with a wooden mount on a steel boat.

To reduce vibration you need the bearing to hold the shaft inline, if the mount moves so will the shaft = noise.

Rigid aligned bearings reduce noise.

 

[hlb]

Mal, I suppose you mean the P bracket near the prop, though it may be A frame looking on your boat.

Your engine, stern gland and P bracket, need to be perfectly aligned. You dont want a piece of wood in there that can wobble about.

I'd leave it till your installing the engines, then get a man that can.

You will need engine mountings that are adjustable, but it will only be a bit.

Best way to check for alignment, is to get a collar made, that fits over the gear shaft. Then when the prop shaft is introduced, it should slide into the collar easily. When your satisfied with this, you can take the collar off and put on whatever sort of coupling you have, some are more forgiving on alignment than others.

If you dont get it right, you eventually sink.

 

[Neil_Y]

That approach is good, but you need to start by confirming the bearing carriers are aligned. This could be P bracket, intermediate V and stern tube.

Easiest way is to chock shaft and then see if you can insert bearings and spin them by hand in the carriers.

 

[hlb]

That approach is good, but you need to start by confirming the bearing carriers are aligned. This could be P bracket, intermediate V and stern tube.

Easiest way is to chock shaft and then see if you can insert bearings and spin them by hand in the carriers.

Well yes, maybe that as well. I've only done it from the presumption, that they were in the right place all ready.

How ever in Mal's case. I think it could be a fresh start.

It's not the easiest of jobs, hence thats why stern drive boats are mostly cheaper than shaft drive.

 

[Sultans_Kapitein]

Thanks for all of the input, lots to go at. As Haydn says I can go which ever way is best, but as the current setup has been in place for some time (Since the 70's as far as I know) I do not think I shall be re-fabricating the A-Frames. I think I shall get some polypropylene pads made up, they will not degrade in the water and will server the both purposes of anti-vibration and fitting without changing the alignment. Of course if some knowledgeable person comes on and states that polypropylene will eat through my hull I shall reconsider
Thanks all.
Mal

 

[hlb]

Mal, I don't think you quite understand, An engine does not have rubber bit's in it to stop vibration, it is perfectly balanced.

If you introduce a piece of wood, or whatever in there, it may well stop you feeling the vibration, but that does not mean it's not happening.

The vibrations will eventually, break bolts or shafts, probably both.

I think I can let the cat out of the bag, now my boat is sold. But I asked my mechanic for three years, to look why my port stern gland was forever leaking.

He kept doing bits and bats, but never sorted it. Eventually the prop shaft took the stern gland out, then bent like a banana and drilled a six inch hole in the back of the boat.

In short, feeling vibration is good, not feeling it is bad.

 

[RobWales]

Feck......................!