Trimaran folding system attachment

[Will001]

Thanks very much for the replies, it's good to see some pictures of other boats as i also have never had a close look at how the farrier mechs are supported. Thanks Solgato.
I will try lifting the amas on land and see if the attachments points move, hopefully this weekend.

I'll have to remove the amas at some stage to really check what's going on in this area, but if there isn't movement when lifting the amas i might take it for a few runs before i get stuck into that. (and repainting etc.)

I still have a list of other jobs to do. Just finished fairing and glassing the rudder, need to bulk up the attachment point for this also... and the bottom of the hull is a bit soft... So it could be a bit slow.

In terms of bushes for the folding mechanism, I will need a bush at every point correct?
The current design uses a bolted connection for the small top member, should this be replaced with pin and bush?

 

[SolGato]

Post some photos of the upper attaching points and the pivot points so we can better access the system.

The original Australian built Tramps use a solid axle that is machined at each end to accept bolts and uses plastic bushings. Some later versions of the boats like the OTASC and Eagle used differing methods that were probably less expensive but got the job done.

I can tell you from experience that it’s important to make sure all the fastening hardware is in good condition and that there is no play in the system, otherwise you run the risk of failure and possibly tweaking the mechanism. A little play can cause a weak link putting other fasteners in shear. Be sure to look for signs of electrolysis and make sure all dissimilar metals are insulated from one another with TefGel of a layer of plastic, etc..

I had an upper support bracket shear a bolt once while folding on trailer and it was pretty scary. Upon inspection, it looked like over time the bolt had stretched and eventually gave after years of opening and closing the system from the rear beam which is common practice. Had it happened on water with motion, it probably would have bent the system.

One way to make sure everything is in good condition is to see how the alignment is when open and closed. Is everything returning to where it is supposed to? Are the bolts lining up? Amas in mirrored position same distance to hull, etc.. I would definitely replace any suspect fasteners, insulate and protect all dissimilar metals from one another, and clean and grease everything if you end up demounting the amas to work on the lower connection points.

 

[Will001]

Well i tried lifting the amas on my shoulder and bouncing them and the system seemed ok. A little flex at the hull but not major. So i decided to tighten the bolts, stbd side was fine a little creaking. But the other side just pulled the internal backing plate into the laminate (def not solid glass).
I also found some large areas of delamination near the back of the boat which were full of water... drilled a few holes. It looks like glass to foam delam. Not sure what foam is in there, some looked like polystyrene...
Guess its going to be a new bottom for this old girl.
Got a few pics of the folding mechanism.

 

[W17 designer]

Most helpful to see these pics
Interesting "patchwork quilt" of pieces for the folding system. Probably strong enough but I really don't like the attachment to the main hull up forward. Looks like the forward arms were made too short s0 an extension piece was added. I'd be looking at making a better extension piece that mated better with the main hull (suitable stiffened) and designed to take the load in-line with the arms.
Sad if they indeed used a polystyrene foam. Hope you're wrong Poor quality for this, low compressive strength and it can absorb some water too. Can survive in low stress areas but I'd plan to replace it for critical areas. Need to cut off one skin to do that ;(
An interesting main hull shape though ... I like it! Displacement low down with near vertical sides ..something that makes good sense to me for low wave making.

 

[SolGato]

Thanks for the additional photos. From them I can see a few things I would address. First is it appears the pivot axle is made of steel. I would replace it with stainless. Also with the axle designed the way it is, I don’t see any provision for bushings. The only way to add proper bushings would be to reduce the size of the axle or open up the thru holes in the aluminum arms. If your not going to be leaving the boat out on the water and/or folding and unfolding it regularly, I would just swap the axle with stainless and thoroughly and regularly grease it or coat it with a corrosion inhibitor to keep the stainless from destroying the aluminum.

Now regarding the lower mounting of the arms. Looking back at the original photos you posted and seeing the new photos, I think I now have an understanding of what is going on. In your first photos of the inside of the hull you can see what appears to be evidence of previous mounting holes below the existing holes. I can also make out what appears to be a metal structure below which likely crosses over to the opposite size. This is what I would expect to see in order to create a strong anchoring point for the folding system. The problem is, I think originally the lower mounts were supposed to tie into this structure, but someone screwed up and the system either got damaged or was mis-designed or was adapted for use on this boat and for whatever reason, the mounting points had to be move up to allow for proper articulation. Because of that, they are not bolted thru and into the metal structure inside. In the one photo it looks like the lower bolts are just catching the metal cross structure and the uppers way above and not tied in at all. Someone then crudely fabricated a backing plate for the uppers that looks to slip over the bolts to help spread the load. Because they are not tied into the cross structure, you are not only feeling looseness or deflection in the system, but you can also clearly see it in the photo you posted looking down the hull skin. See the gap between the bracket face and hull at the top two mounting bolts? The top two are relying mostly on sandwiching the hull for anchoring while the bottom are just catching the metal cross structure inside offering better strength since I’m guessing that cross structure is glassed in, then tied into the other side with two flat bars.

Anyway, the short of it is it looks like it was originally designed well, but then modified for whatever reason and now lacks the strength and support it needs.

If everything is opening and closing and lining up as it should, the solution in my opinion is to figure out how to tie the lower mounts back into the lower glass in metal cross support. The trick part I’d coming up with a solution that can be bolted in place because welding on that glasses in cross support is not possible.

 

[SolGato]

Here’s one fix that might be feasible. Using a piece of aluminum U channel, cut angles into the sides (blue) and use the back portion as a full length backing plate that extends below the lower mounting bolts against the glassed in metal plate. The angled sides then extend down to tie into the existing bolts for the cross bars acting as gussets triangulating the structure. This provides a strong boxed in support for the lower arm.

Then on the outside of the hull, you would fabricate a new base plate with the same footprint as the new inner which extends below the existing lower bolts, drill two additional holes for bolts thru and into the original interior plate to create a third lower pair which will tie the new inner plate into the existing structure once sandwiched together. The idea is to create an extension of the existing structure using the existing holes and fastener locations.

If there’s an additional bolt below that bent plate with the threaded rod through it, you could come out even further with your side gussets and tie into that bolt as well.

 

[W17 designer]

I generally agree with SolGato on this, but I suggest that before you make new plates and holes etc, to take a look at the how the force passes up and down the lower arms. Check this photo on which I've marked up the assumed line of force. Ideally, the pad on the main hull should be in line with this, but you can now see that the bracket bolted to the hull is actually way too high, putting most of the load on just the lower two bolts.
I would suggest to redesign that bracket with a deeper plate against the hull and then through-bolt to the new plate inside. To achieve this, perhaps the 'double-channel' parts that form a sort of box, will need to end up more trapezoidal, wider at the bottom than the top. I hope that is clear enough.
The axis of the lower pivot pins should align with the axis of the similar pin at the rear arm in order that it folds smoothly.

 

[SolGato]

W17 and I on the same page about the outer plate. With the way it’s setup right now, the bracket is acting as a lever to pull those top bolts out which would explain the gap.

I understand why the bracket is two piece made of U channel. Although complicated and somewhat unnecessary, it was a way to fabricate a mount of standard material without welding that could fit the outer shape of the hull because it allows for compensation of the tapering of the hull from front to back while providing a perpendicular mounting point.p for the arm. Since this mount was fabricated this way, it makes me wonder if the system was fabricated, then fitted and drilled for installation which would explain the issues inside where it has missed the inner support. Would also explain some of the extra holes. I could see someone hanging all the parts, then drilling holes and bolting it all together only to cycle it through it’s folding and encounter binding or misalignment.

The good news is from the photo above, you can see that there is still some clearance for the two channels to close (clamshell) together which would allow for a larger mounting plate to be slipped behind extending down further to bolt thru and into the inner structure as W17 shows in his photo. You would need to either slot the holes (not advisable) or better yet move the bracket half that bolts to the hull fore or aft to drill new holes for clamshelling, then your new base plate can compensate for this if you make it larger than the existing footprint of the bracket half in both length and width.

As I said, if everything is working and articulating correctly, then your goal should be to create strength by tying it into the existing structure without altering the geometry or having to perform major fabrication involving welding, unless of course you are a skilled fabricator. Otherwise a few pieces of metal from Speedy Metals, a grinder with a cutoff wheel, preferably a drill press, some stainless fasteners and you’re in biz.

 

[W17 designer]

If what I wrote was confusing, perhaps this rough sketch will better explain what I'm suggesting.

 

[Will001]

Thanks for the great suggestions and pictures. It seems like you are both understanding the situation.
I like the idea to line up the forces. Should at least be able to get close, there is a bit of space below the current mounting bolts.
So the front and rear pivot points need to be in a line do they? Yes it does fold very smoothly atm. That means i cant extend the front strut, have to extend the bracket to keep the pivot pin/axle in the current position.

In terms of the internal components it does look like the lower support is glued in with epoxy. I'm not sure if its worth trying to chip or heat this out? So i can add a stiffer supporting strut across the hull and reinforce the glass in this area.

Yes the pins are just galvanised. I got a quote for some bushes (Iglidur T Flanged Bearing TFI-1011-08) about $3.50 each. And am hoping 5/8" 316 stainless round bar will be a close enough fit to avoid getting specially machined pins. I can get a hand reamer to open up the aluminium holes.

FYI the hull actually tapers inwards down low before flaring out again about 20cm below the bracket.

 

[RV 64]

You could also make the plate the other way around . might be easier
these one are carbone with ingledur bushes and 12 mm inox bolts not fully threaded and cut to fit

 

[W17 designer]

Glad we're all on the same page .... a rather rare state on most forums ;-)

Last things first. RV64 is right of course that the bracket could indeed extend out vertically from the hull, but I would not recommend it 'in this case'. Such a layout creates annoying water disturbance and spray which may be tolerated when the bracket is small, but in this case, the pin is a significant distance from the hull, making a large bracket that will make unacceptable resistance and spray. Seems the builder wisely choose the existing bracket design to avoid that issue.

When I first saw the photo, I was also wondering if the links could be lengthened .. anything to bring the pivot closer to the hull. But the geometry of folding systems can be tricky and theoretically at least, the arm lengths should be closely the same fore & aft, and the axis of the pivot pins designed in line. However, if there's enough play in the pins, it just might work with elongated front arms, though I certainly would not bet on it. You will need to measure everything and then draw out the geometry for both rear & forward systems. Longer arms forward will swing upwards with a larger radius and could well mess things up. But I would suggest you check it out on paper first. Good luck with this ... an interesting design.