Inquiry for information from experienced boatbuilders and boat designers

Hello,

for some time now I have been reading and reviewing posts from this and other forums,

I have some questions and would be glad if someone could provide an answer to one or more of these:

1. what kind of caulking / gasketing would be sufficient to provide hindrance to water invasion in the specific location of outrigger attachment to the ama, in case of trimarans; more specifically, if the aka, or beam, is to be attached above the ama hull, in a way that the attachment slot is protruding above the ama structure (aluminium plates which would surround the attaching beam on its sides), so in the end that the horizontal bolts and pins are removable (so, not in the sense of folding amas, but 'removable' amas) and that the beam is secured from seawater intrusion - does this mean, as I read elsewhere, that the bolt drill holes in the beam itself is sufficiently defended against seawater by layering epoxy in the drill holes itself, or does this require more steps, i.e. rubber gaskets)

1.* would it be possible to attach the beam to the ama in this way, and what would be the optimal dimension of the bolts and pins taking into consideration the next question:

2. would it be sufficient to make a (horizontal) 10cm x 17.8cm composite rectangular beam (where the 10cm vertical sides are 24mm WBP plywood panels, and between those a 13cm horizontal plane which is made up of three layers of 10mm WBP plywood [bottom plate 1cm, then XPS foam 3.6cm, then middle plate 1cm, another layer of XPS foam 3.6cm, and top plate 1cm, totalling to 10.2cm, which is 0.2 more than the sides but on the bottom and top plate each 0.1cm would be supplanted and leveled by epoxy and glass fibering ) - and this beam being 200cm from the centre of the hull to the ama, in total the whole beam dimension of the trimaran being 400cm, for a trimaran which has a main hull length of 690cm, with the amas being 450cm; with the buoyancy of the hull being 800kg, the amas each having a 150kg buoyancy, and the weight of the hull being 400kg, the weight of the amas each being 70kg.

In short, would it be overkill, or less than sufficient, to make the beams in this way, and what would be required to attach them as illustrated, to prevent any water intrusion into the inside of the ama hull via the bolts/pins and their holes through the amas, as well as how can the aluminium plates (which would protrude vertically from the planes of two bulkheads positioned accordingly to surround the beam) be secured from water intrusion, i.e. is fiberglassing sufficient.

Thank you

 

Welcome to the Forum Ivan.

Trying to read your description above, and visualising what you are talking about, made my head hurt.

Do you have any drawings or sketches that could be posted?
I think that you are not allowed to post attachments on the Forum until you have 5 posts to your name. But if you do have any drawings that you would like to post, I could post them for you if, you email them to me.

 

Have you considered lashing the akas/beams?

Take a look at traditional Polynesian sailing canoes or even Wharram Catamarans for inspiration on ways to join Akas and Amas.

Otherwise if you keep with the idea of using hardware, one way some designers get around water intrusion is by using studs that have been glassed/epoxied in place or threaded inserts that have been welded in place to accept bolts.

Also using a bushing for the bolt to pass through that is glassed/welded in would allow the bolt to pass through without water intrusion.

 

Thank you, and yes, yesterday after your reply I have made some drawings but I still need to inscribe more info and when that's done I'll upload it here. Thank you for the inquiry

 

Can you describe more precisely what do you mean by "studs glassed/epoxied in place", it seems to me that it may be just what I had in mind; perhaps you can link a picture of what you mean. Is it a structure, protruding above the ama deck, into which a beam can be set and locked with bolts? In that case, what I would like to understand more clearly is what is important to keep in mind to make sure that glassing the edges of this (I assume metallic) element is done correctly; I guess just some photos would be good.

This is something which would go well with what I had in mind.

I have seen this technique before but now that you mentioned this, I went to check out Wharram designs. And I must say I spend a good 10 minutes now on the website, it seems that this lashing technique may have some innate beneficial characterestic - it seems to me James Wharram kept choosing this as the attaching technique - why would that be if not for some great advantage for self-built boats. Wondering now, I start to think about corrosion on metallic attachment elements vs. simple rope replacement. Would definitely like to check this out further. Thanks!

 

I found this video and at 0:45 it seems to show plywood panels, a cross section view. It may be exactly what I could include for the beam itself;

the other question is how to make the vertical protruding elements (a continuation of the bulkheads, to say) which would be the attaching plate to the beam. (is this what you meant by "studs which are glassed/epoxied in place"?)

 

Hi I will now upload the drawings with some information inscribed.

Please note that the attachment of the aluminium vertical plate to the bulkhead is not shown (probably it could go between two bulkhead ply sheets?).

 

These sketches are very basic so I hope they can illustrate what I had in mind.

Would such an attachment be possible?
Here are some questions I have in mind now.

1. Would there need to be space between the beam and the top of the ama?

2. How exactly to create the attaching element on the ama, insuring that it is well protected against water intrusion into the ama hull - is it sufficient to make a slot with a dimension of the width of the aluminium plate and then just glass the edges around the bottom of the protruding aluminium plate, or would this structure require some caulking/epoxy filleting, or a rubber gasket around the plate below the ama top deck, or some other method?

3. What would you say about the approximate strength expected of the beam itself, would it be sufficiently large and structurally sound?
It would be required to take on loads for the following dimensions of the trimaran: the amas are 450cm long, cca. 60cm high, with a buoyancy of cca. 100kg and a mass of cca. 70-80kg, and if the total length of the beam (from centre of hull to one ama) is 200cm; and with the hull being 22.6' long, 400-450kg hull dry weight, each ama 70-80kg, 950-1100kg displ.
Thus the total width of the tri would be 400cm.

4. Would it be possible, in this regard, to have these pins/bolts installed (as the previous reply was suggesting) with a threaded insert or a bushing, so that they can be removable?

5. This would allow the beam to be disconnected from the ama, as well as disconnecting the beam from the hull, and thus allowing for easier storage. Once again I would like to point out that even if this sequence would be lengthy, i.e. more than a few minutes required for the folding beam mechanism designs, that is still okay since the removing/disconnecting of the amas, beams and the hull would be intended for storage purposes and servicing etc.and not for mooring/docking purposes, what he folding mechanisms are intended for.

 

These are screenshots from a youtube clip where assembly of the aka to the amas takes place within a few minutes, there seem to be horizontal bolts protruding downwards from the aka and once slotted to the ama, the person in the video uses a hatch to the side of the aka to fit the nuts to the bolts. It seems that the bolts are black in color, does this mean that they are covered with some kind of rubber caulk which would prevent water intrusion through the bolt holes?

 

Another post on this forum illustrates how a beam can be connected, in a very similar manner compared to what I had in mind, from what I can discern the beam is aluminium and has attachment plates welded under the beam. The beam is then connected to aluminium plates connected to the ama bulkhead and protruding vertically from the ama hull

 

Ivan,
This is by far the best way to go.
BB

 

There are lots of different approaches to making connections.

The captured shots from the YouTube video look like it’s a Multi 23 they are assembling. If so, those boats are a modern design with a well engineered connection designed to handle the high performance loads.

In that example and others, typically there is a mechanical/geometric component where a slot locks a square beam in place so that it can’t twist, and then bolts, brackets, lashings, etc.. hold it all together.

Same kind of thing for round beams -usually a groove they sit in with hardware then holding it together.

A common design used on Beach Catamarans and some larger production Multihulls that use round aluminum profiles for the beams is a combination of a slot moulded into the float that the beam sits in and then a pair of half round straps bolt down and hold it together, with the straps fitting over studs that have been glassed into the hulls to clamp it all together. I have included a photo of my 30’ Catamaran’s connection below for reference.

Usually when hardware is used it runs vertical not horizontal, so with your proposal of cross bolting through the aluminum plates to sandwich your beams you will likely be more prone to movement and you won’t have any way to tighten the connection up. And slop increases, you’ll have more torquing and twisting of the connection leading to failure.

Challenges like the above is why lashing is such a great way to go. It is adjustable, easily removable, and much more forgiving -if it loosens up and stretches it can absorb the movement without failing until you are able to re-lash, and you can spread the load of the connection more easily. And of course, using it avoids the water intrusion issue.

 

Thank you SolGato for this groovy suggestion. It is clear to me now how beneficial the slots/grooves are with their locking characteristics by structure,I only regarded what attaching plates/bolts/pins to use, but with a groove I'm thinking now if the beam itself could be made (since the beam in question is plywood/XPS/GRP) into a shape to even further use the advantage of locking it in place

Excellent clarification

Great - this is very usefu information. Can you please tell me, what kind of insert are the bolts screwed in, inside the hull? In your example, is it a bushing or a threaded insert? And is it a crossbeam element they are screwed into, or?
The other thing which I am highly interested is what kind of gasket (or caulking matter) did you use for the isolation of the bolt holes?

If they are, from what I can see, screwed from the bottom upwards through the hull, are they then glassed/epoxied? In this situation, would it be neccessary to set the glass cloth on the vertical portion of the horizontal deck (I mean, along the circular edge of the bolt hole), or just around the bolt after it's already screwed through?

If you could provide me with this information it would be a great reference, both your picture of the external view which is now quite clear, and the interior structural aspect (for my further understanding of scantling / preventing water intrusion).

 

And with regards to lashing, firstly thank you for your comments,

Lashing as I understand seems now to be something to definitely include along with the slot structure - what material would be best for seawater/very sunny regions with not so cold temperatures? I guess regular marine-grade rope would be ok? For the mentioned dimensions (2-metre hull to ama beam, 22.6' x 13' tri, 400-450kg hull dry weight, each ama 70-80kg, 950-1100kg displ.), would a 16mm bi-directionally woven marine rope be sufficient if there are 4 beams?
Or is it better to do with some more specific material like Kevlar rope?
Perhaps the best are straps with that tightening mechanism, as you use on your photo, as they can be easily tightened

 

To answer your original questions:
1. Water protection is achieved by a ring of thickened epoxy between the aka material and the bolt. For compression reasons the bolt location must be solid material, so you have two options:
a.) install a block of (ply)wood between the panels, drill oversize, fill with thickened epoxy, drill final hole, or
b.) install a bushing (fiberglass pipe or drilled fiberglass block)

2. The aka is loaded in bending, it's a cantilever beam (I presume you don't have waterstays). A hollow rectangular beam will have the top and bottom flanges carry the load, the sideplates keep them at the proper orientation and resist torsion.
Your proposed arrangement is possible but needlessly heavy and complicated. The middle plate is not necessary, it doesn't carry any significant loads (neutral axis of I beam). Making the top and bottom flanges out of plywood means that half the wood is going into the wrong direction and is just dead weight. An efficient beam would have the top and bottom flanges made from solid wood, and the side plates made from plywood, ideally cut diagonally from the sheet.
XPS foam is useless from a structural point of view, so the beam would probably need some additional bulkheads installed. Filling the remaining voids with foam is a personal choice between added weight and no airspace.

It doesn't matter how exactly you attach the ama to the aka. The goal is always to make a secure connection that doesn't move. For that the first thing is that the aka doesn't collapse under the load (solid blocking and washers). Second is that there is no rotation, so you need at least two spaced connectors, and third is that there is no movement in the connection under load. Lashing works, but it's complicated and high maintenance, that's why more modern wharrams transitioned to prestreched polyester webbing and ratchets. Bolts need and additional tool (the wrench) and a form of security against vibration (additional nut, locknuts, castellated nut, etc.). For stainless bolts go with bronze nuts, that prevents galling. If you can keep track of a wrench and some spare nuts, bolting is preferable.