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 Motor Boat Forum

 

[Twister_Ken]

Maybe not anti-vibration and alignment, but anti-noise transmission?

 

[prv]

Maybe not anti-vibration and alignment, but anti-noise transmission?

If that was the idea you'd have thought they'd use stiff rubber, like an engine mount.

Pete

 

[Tranona]

Are the A brackets fabricated or cast? If the latter, it could be they used standard castings and used the packing to line up the shafts properly because the legs were too short, so if bolted direct to the hull, the props would overhang too much. If fabricated, the original builder could just have got them the wrong length and used the packing rather than re-fabricate.

 

[prv]

Are the A brackets fabricated or cast? If the latter, it could be they used standard castings and used the packing to line up the shafts properly because the legs were too short, so if bolted direct to the hull, the props would overhang too much. If fabricated, the original builder could just have got them the wrong length and used the packing rather than re-fabricate.

That's what the OP's yard thinks, and it's my suspicion too.

Pete

 

[penfold]

Is the A-frame made of bronze or other copper alloy? That would explain the need for a non-metallic isolator; wood is certainly less durable and more maintenance intensive than a block of acetyl or some other stiff polymer with low water absorbtion characteristics.

 

[vyv_cox]

The yard's thinking is sound in my view. The less wood you have on a boat the less trouble, although it is good to look at. Particularly underwater I would go for something man-made. Plastic sounds OK but on a steel boat I think I would prefer welded steel blocks for maximum strength and reliability. If the material is a galvanic corrosion issue it should be easy to incorporate isolating washers.

 

[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

 

[old_salt]

Hi Mal.
Yep as others have said "Steel Wood Steel not a good idea under water on a steel boat". I would be happy to pop over and have a look if you wish.
I still owe you a nosh up anyway.
My suggestion would be to first make sure the hull has a sound doubler in the region of the "A" bracket mounts and all the holes or bolts are in good order, then have a good look the "A" bracket cutlass bearings along with the shafts for straitness and check the bearing surfaces are in good order then remount and line out the shafts in the "A" brackets temporally supported in the correct place, then measure for the required spacer or packer peace, have it machined up and try it in then recheck for true alignment to the engine, gear box or thrust block bearing coupling/spigot if you have one, adjust the packer to suite then bolt up securely and re-check the line-out. If the brackets are bronze then insulation or bonding consideration for electrolyses will be required but that is a mine field and would require expert knowledge Mc Duffs are the peeps for all that and they should be able to help.
A good little book that explains it a little is:-
"Boatbuilding with Steel" By Gilbert. C. Klingel.
Another book I have is Metal Corrosion in boats By Nigel Warren.
I will happily lend you them if you want.

 

[rogerthebodger]

On my steel yacht my P bracket is welded directly to the keel box as shown in the pic below