Laminating a new mast support beam.

[welkman]

Hi,

I’m currently working on a project to replace the mast support beam on my Achilles 24 and have come to a bit of a halt despite trying my best to research the problem. I have removed all the deck core from the inside of the cabin roof and the mast support beam, I know how to proceed with recoring the deck, but have a few questions when it comes to replacing the beam.
I think the best way of doing this will be to laminate a new beam using epoxy and wood, I am thinking oak and sepele but I am not sure if this is a good choice. I think using widths of 100 mm and thickness of 20mm sounds about right for the intended purpose but I cannot seem to find a definitive guide. The same goes for the overall size of the beam needed to support the mast and transfer the load to the internal bulkheads, I think that a total of around 6 laminations sounds about right but I am unsure. I plan to reinforce the deck above the beam with several layers of glass and then attach the beam to the deck using epoxy to bed the beam in and ‘tabbing’ around the beam.

I would be really grateful if anyone has any ideas or opinions as to if this will be strong enough or how to proceed with this.
Cheers
James

 

[Rigger Mortice]

What was there before?
As a general rule you can assume that a well made laminated beam will be stronger than a similar sized single piece of wood.
Will 20mm thck wood bend readily enough for the shape of beam you are intending to make? My instinct would be to use thinner laminations but it depends on the finished curvature.

 

[welkman]

The original beam was a 180ish mm piece of half inch ply incapsulated in glass fibre which was about 5-6mm thick. I agree that thinner laminations would make life easier but im not sure what I can get from the local timber suppliers. The span is only about 1500mm.

Thanks for the help

 

[Tranona]

120mm finished thickness is a massive beam - far bigger than you need. Think you will find 20mm too thick if there is anything of a radius. Laminations of 6-10mm would be much better to work with, and laminated round a former before fitting. Finished thickness of 40-50mm would probably be fine. A solid beam for this type of job would be around moulded 2 1/2" old money (most wooden boats were built in old money). Oak or Sapele would be fine - no need for both if you are then going to hide it.

Don't make the job too complicated. If I were doing it I would probably use 5 * 6mm ply laminations. Much stronger and arguably easier to work than solid wood.

 

[electrosys]

A different application which may be relevant ....

Last year I needed to reinforce the transom of a fishing boat which had become distorted over many years due to the weight of having a 30HP outboard attached. After much deliberation I decided to make-up a beam stretching right across the inside of the slightly concave transom from 12 layers of 4mm Brazilian hardwood ply. Attaching and building up the beam proved uneventful, although it took longer than I'd planned, due to the epoxy curing time necessary between each layer.

Using such thin ply made the job very easy, and it's something I'd happily do again - in preference to steam-bending - wherever appearance isn't an issue. For additional strength, it would always be possible to insert Aramid fibres into the outermost laminations.

 

[Rigger Mortice]

Tranona is right. It does look as if you are over engineering it.

You are planning to use epoxy which is stronger than the original GRP then you are putting in a substantially bigger beam.

The Achilles 24 was a well built boat so you don't really need to beef it up quite as much as you are proposing. I would be tempted to just double up the thickness of the ply and continue with the epoxy as already planned.

I'm fairly sure that there is an Achilles website somewhere. If there is then existing owners may well be able to give you the benefit of their experience.

 

[welkman]

Thanks for all the responces. I have tried the achiiles forum, of which I am an active member, but have not had any luck with this issue.

So just to clarify I could buy a sheet of 6mm marine plywood, cut out several strips at the length and width required and use this to build the beam instead of solid wood ? I could then fully encapsulate the beam in epoxy and glass mat as before rather than tabbing the beam to the cabon roof. This seems like a faster and stronger solution. I had not come across the idea of laminating ply before.

 

[Tranona]

Yes. Now you have said what was there before, then 4*6mm ply laminations would be still overkill - probably 3 times as strong and stiff as one piece of 12mm. You can laminate on a former - do it all in one go if you like working up a sweat. Or you could do it in situ if there is not too much of a radius and the GRP is stiff enough to stand the bend of the first lamination. Epoxy that to the GRP and then build up laminations one at a time. Use a system of posts and wedges to keep it shaped to the deck until the epoxy cures. Saves making a former, but not so pleasant doing the job in the boat.

 

[electrosys]

If you have a choice, I'd recommend laminating in situ as doing this will guarantee getting the curve 'spot on'.

The stuff I used was 4mm with equal plies, so 2/3rds of the timber ran lengthways, with the remaining 1/3rd preventing any twist (at least, that's how I figured it).

 

[Poignard]

Contact Scotty_Tradewind, who posts on here. He has carried out such a repair on a Twister.

 

[welkman]

Wow thanks for the responces. Im going to do the lamination in the boat but before I do that I need to pop a nice bit of hardwood (epoxied in place) into the underside of the mast foot ( to fill the cavity and grind it flush. I hope to get this done over the next couple of weekends. Any types of plywood I need to avoid? Any types of plywood that would be reccomended ?

Thanks again for all your help, this has been going round and round in my head for the last week.

James