Trimaran folding system attachment

[Will001]

Hi, i just joined the forum, hoping you guys could help me out.
I recently bought a custom (home built) 21 foot trimaran based on a Mystere 6.0 catamaran (very similar to a tornado i believe). 9.5m mast, 5.2m beam. The main hull and folding mech is about 7yrs old.

It's in need of a fair bit of tlc, but one thing which concerns me is the folding mechanism attachment to the main hull. It looks like the main forward attachment points are not in great condition. The lower side is compressing the hull laminate and the top is separating from the hull with some minor cracking.

It is thru bolted to aluminium backing plates on the inside of the hull.

There are small struts bolted onto the lower backing plate to transfer load to the opposite side of the hull. The upper backing plate is unsupported..

Is this a normal way to distribute load in a folding trimaran?

I was thinking about replacing the internal struts and backing plates with a single rigid beam which both upper and lower bolts are attached to?

The hull is made from polycore, a polypropylene honeycomb, with epoxy/glass inside and out.

 

[trip the light fandango]

Yes it is strange, it is like the work was done reasonably well but missed 50% of the point of having the internal brace, very odd, probably just wanted to use it, sick of working on it.. Be careful with your stainless/ aluminium, delrin washers and marine grease/lanolin help.

 

[W17 designer]

Seems that Fandango has understood your arrangement better than I Would you have a photo looking from forward back on to the folding system with the ama out? Then we can determine how the loads are applied.
For sure, you cannot securely attach something bolted through a honeycomb without making the core solid at that point.

If this is a Farrier-style linkage, the inner upper aka will be working mostly in tension or compression and the deck can be designed to take this with some form of beam at the connection point. Under the most critical condition, the lower arm will try to pull out from the shell on the leeward (in the water) side, based on the maximum buoyancy* that the ama has, while that same lower arm on the windward side, will press into the shell based on a maximum load of the weight of the ama plus all the crew weight that you may add on that ama. While the sail load will help to support this (via the tight windward shroud), its best to calculate the strength needed by the arms and their connections from the full max. loading I've noted. Outboard of the linkage, the aka will act in bending as a cantilever .. again based on the maximum loads indicated above. The lower link will generally need a solid cross connection through the main hull, from side to side ... again with any core material replaced with something solid. Even for a small boat like this, I would add extra strength with a safety factor of around 2, with more for a larger boat.
*As noted in this article, I design using 100% of the total ama buoyancy on the forward beam with about 75% generally being ok for the rear one.
https://smalltridesign.com/Trimaran-Articles/design/crossbeam.html
Hope this helps, but a photo or sketch will help if not.
mike
www.smalltridesign.com

 

[SolGato]

I think the rigid beam mod would be wise. I’d also increase the surface area of the outer plate to the hull to help spread the load over that area which has probably been weakened.

On the Farrier boats the connection points for the folding system have a lot of shape built into them for strength. They don’t just connect to a flat surface, the hull has been molded to accommodate the mounting so as to not create a stress point.

 

[Will001]

Thanks for the replies guys.

Yes it is strange, it is like the work was done reasonably well but missed 50% of the point of having the internal brace, very odd, probably just wanted to use it, sick of working on it.. Be careful with your stainless/ aluminium, delrin washers and marine grease/lanolin help.

I am planning to add bushes and upgrade to stainless pins at some stage (currently galvanised pins).

@W17 designer i will try to get a better photo, but yes it is a farrier style folding mechanism.
I'm starting to think this is probably due to trailer loads. The amas are unsupported on the trailer, plus the boat is secured with chains to this attachment point (pulling down).

 

[SolGato]

A word of caution if you decide to add bunks to your trailer to support the amas-

Your center hull supports should do all the work and the outers only helping to keep the boat upright when loading, otherwise you risk damaging the outer hull skins if the outer bunks are supporting any of the boats weight. The folding mechanism isn’t really designed for this when folded and unless your Amas were laid up with reinforcements along the inside outer skins, you risk damaging them, especially if you have a flexible bunk board, or if you lose a tie down or main hull bunk board while towing.

And I agree part of the distortion is probably due to using those connecting points at tie down points. You might want to come up with a removable fixture that can be placed in the slots that the beams unfold into that can be used for tie down instead.

 

[W17 designer]

Although trailers are often responsible for damage to main hull bottoms when rollers are used instead of bunk boards, I rather doubt this is the case for the amas, unless they are suspended loose and are free to jump around on bumps. The folding arms should be more than amply strong enough to carry dry ama weight in any position. But amas should be lashed to avoid shock loads from jumping around loose while on the road. Heavy duty shockcords are better than chains IMHO if attached to your folding arm brackets when on the trailer as the main hull should have its own separate lashing straps.
My first concern would be to check to see that the honeycomb was removed in way of any bolted attachment, as otherwise loads (both sailing and trailing) could have damaged that area and permit movement that should not be there.
As I pointed out in my earlier post, the forward beam takes a high proportion of the load at sea, so this might explain why the forward one is more in need of repair than the aft one. It's good to consider all the load options to figure out the real cause of the problem as then you can personally find the best solution. Good luck with this.

 

[SolGato]

As a Farrier Tramp owner I can speak from experience that asking an Ama to support more than its weight with only a narrow board resting against the outer skin of the hull, or improperly tying down the boat so as to allow it to bump or rest heavily against it, or if one of the main hull bunks gives or any of the boards are mis-adjusted so as to allow an ama bunk board to do more work than it should, will likely result in outer skin hull damage.

One of my hulls was seriously damaged due to some of these issues, and on some photos of other boats you can see impressions on the outer skins from bunk boards.

I removed my ama bunks for this reason and use the 4 lower arm supports as tie downs points running down and out to strategically placed D-rings on the trailer. The only tricky issue is when loading on an unlevel surface like a sandbar or crappy boat ramp as it sometimes takes a try or two to get the boat to sit flat on the rollers and bunks before strapping it down.

From the Tramp sailing manual, “The supports for the floats are not intended for bearing their weight but to provide lateral stability on the highway and enhance the overall ride. The weight of the floats is carried by the folding arm mechanism and the main hull”.

Just mentioning this especially because the OP’s lower folding arm supports already don’t seem to be very well anchored and one might assume the ama bunks of a folding Trimaran should be doing their share of work to support the boat when on trailer, when in-fact they should not with a Farrier design. I do not know if this is the case with the Corsair models but would guess they are no different.

 

[trip the light fandango]

That sounds like good advise from both W17 and SolGato to me, I probably jumped the gun in partially assuming what I can see . W17 's point about checking the honey comb or the state of the holes in the skin for the bolts is sensible. If they did rush getting it in the water they may not have put in matt or rovings/re enforced the drilled holes . It looks like the bottom transverse bracket is holding in place the top of the folding mechanism via vertical rods and the lower plate against the skin for the struts is unsupported internally. I've never looked close at a farrier in the flesh but this mechanism looks weak and more vulnerable folded. On the water it it is probably strong enough I'm guessing, but I think the trailer tie down points could easily be an issue as SolGato and the OP are suspicious of, sharp jolts from bumps with a lot hanging if I understand correctly. Not a bad looking boat though. PS, unrelated but I'd be really interested in W17designers opinion on camber on amas/floats at some stage, regards .

 

[SolGato]

One of the major differences about this boats folding mechanism that likely isn’t Farrier designed is how and where the system mounts to the hull. The rest of the mechanism looks very similar to a Tramp. But on the tramp hull, the large cabin with its bench seats extends wider than the rest of the hull above the water line to create a wide cockpit beam. So on a Tramp, that lower attachment bracket is actually up under the wing coming off at a right angle and not bolted flush to the side of a hull, so it acts more as a guide during extension and articulation where as on the OP boat, the lower mount may be pulling and pushing on the skin of the hull as it’s forced to support weight until the Akas are bolted.

Don’t get me wrong, your boat looks like a lot of fun and I think with a little fabrication work you’ll get it sorted. After all, I am kinda biased about Farrier’s folding system.

When you think about it from a mechanical design point of view, it’s pretty incredible how easy and strong the system is, but what is more amazing is that boats like mine are now 35+ years old and have been folded and unfolded countless times over the decades, and most of them have probably never had the system serviced. How many other mechanical systems can you say have stood that kind of test of time on a boat?

When I first got my Tramp, I went through the whole system cleaned the pivot axles, rebushed the arms, greased and reassembled with new hardware, and I was wowed. I thought it folded and unfolded incredibly easy before, but after the rebuild it was pretty amazing.

The other thing I’ll say about the system is how incredibly strong it is on a Tramp. I believe he overbuilt the system during the Tramp production due to multihull fear and then during later use and development with Corsair started streamlining it. I sail my boat in some pretty crazy swell and probably push my boat harder than most, certainly not sailing in the kind of conditions the boat was originally designed and marketed toward, and I can tell you launching off/riding and sailing fast through swell in my boat is incredibly thrilling but never has it not felt like one solid member. The beams are so overbuilt you forget your sailing three hulls connected together.

 

[trip the light fandango]

Farrier is is the standard others try to reach from my reading,, an impressive longevity, this one's mechanism/beams look a little more delicate. Boy there's been some serious money and time spent on folding mechanisms. I just saw the better pictures on SA and the old holes look a little suspicious, as one of the blokes on SA said the main beams should be taking most of the load,..that advise on checking flex on the land was good.

 

[W17 designer]

Fandango PS, unrelated but I'd be really interested in W17 designer's opinion on camber on amas/floats at some stage.

Rather than invade this thread, why not write me your question via this questionnaire and perhaps elaborate your needs. (and do you mean deck camber?)
Design Questions: https://smalltridesign.com/Trimaran-Design-Questions.html
cheers

 

[W17 designer]

Fandango: "...as one of the blokes on SA said, the main beams should be taking most of the load".

I've not read the SA posts but not sure I can fully agree with this in the case of a Farrier folding system.
As I've said earlier, the lower arm (and its connection to (and across) the hull) will take major loads; in tension on the leeward side and possibly less so, in compression on the windward side (depending on how many crew jump around on the rail!). As far as the horizontal top beam is concerned .. the part outboard of the pivot will be subject to bending, while the part inboard, subject mostly to compression or tension.
At the outer pivot, where the load transfers from compression load down the lower arm, to bending of the outer beam, the loads will be highest so that becomes a critical part re beam strength and should be regularly checked for cracks around that pivot.
But I too can say that IF built as per the Farrier design, this has proven a solid, reliable system and I have a close friend with a 25 yr old F25A on which the system still works very well. Another friend with a Scarab 22 is not so happy as the geometry was not perfect. It's really not such an easy mechanism to get right, and is one reason Ian wanted to manufacture all these under his own eyes. It's clear to me that the mounting of this present system (under review) to the hull is sub-par compared to a full Farrier installation, so some rework of the mounting might be justified. I'd personally like to see the main hull stiffened longitudinally inside in way of the lower attachment (the flair on the Tramp achieves that), or a solid connection added from P to S that is anchored in someway to the hull itself, perhaps near the centerline. The deck should already serve to stiffen the attachment for the upper beam.
mike
www.smalltridesign.com

 

[SolGato]

For reference, here are some photos of my Farrier Tramp showing how much structure is molded into the hull and how the lower mounting brackets are wide and mounted to the underside of the cabin wing, not just mounted to the face of a hull skin.

Our summer mooring sailing season begins in a few weeks and Trio is waiting patiently on trailer for the last swell to pass through, and for her seasonal detailing.

 

[trip the light fandango]

Fandango PS, unrelated but I'd be really interested in W17 designer's opinion on camber on amas/floats at some stage.

Rather than invade this thread, why not write me your question via this questionnaire and perhaps elaborate your needs. (and do you mean deck camber?)
Design Questions: https://smalltridesign.com/Trimaran-Design-Questions.html
cheers

Thanks and will do, I 'm probably using the wrong terminology,I'm refering to the outer hulls being angled/ splayed outward at the waterline, increasing beam, the amas still parallel but their bows leaning inward, I'll have a go at the questions .