Mojomo is a bit bust... any ideas?

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tcm

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Enclosed some pix of damage to Mojomo. Can anyone help advise what to do and where to go to get it fixed? Or is it not too bad and fine to continue sailing wth no fix? This may need translation.

We don't think it's too serious. You can see original manufacture was by "Marechal Mats". This damage was initiated by failure of the gennaker halyard at the mast head when sailing downwind at around 7-8 knots, twin headsails wing held out, no mainsail. The gennaker flew forward into the sea, and then acted as a sea anchor, pulling down hard on the bowsprit which in turn has pulled back of the A-frame mounting plate off its seating on the main cross tube, lifting about 5mm on either side.

The bowsprit is held in place under usual use (i.e. with the gennaker flying and hence upwards force) by two steel cables down to the front edge of either hull and one back to the top of the A-frame (mainly just to keep the pole from flopping into the water when not in use). It doesn't seem that the A-frame is welded on top of the main cross member, or has any structural re-enforcement if a forward load is applied - which is reasonable because there is rarely any forward force on the A-frame - except when this happens! I am hoping it could be simply welded on top. The A-Frame also has a heavy steel cable running over it which appears to re-enforce (or backup?) the main cross member. Or maybe it does not even need that?

Alternatievly, does it all need dismantling and welding to exact dimensions? I hope not.. We are currently 19N 50W toward Antigua then Martinique and can be contacted via email or satphone. All advice welcome! overview -look at gap where a-frame meets crossmember detail- a screwdriver can just fit in the gap. The two piece held in compression, no welding.

clip_image002.jpg

clip_image004.jpg

*Not sure what to do with the photos?
Mrs TCM Diana
 
Hi Diana

Do you still have my email address? Email the pics to me, and I'll post them here for you. If you don't have my email, pm me, and I send my email

Brendan
 
The pictures need to be posted to a photo hosting site such as photobucket. Then copy the 'direct link' from them into the YBW post. It looks like you've just copied and pasted the image titles from your computer. If you send me a pm with your email address I'll send you an email and you can reply with the photos attached and i'll then put them in the thread. (click on my name, click on send message)


Oooops, wrote the same time as Brendan, over to you.
 
Pics here, very small though

clip_image004.jpg


clip_image002.jpg
 
These pics were taken last year and they might show what it is Matt has got trouble with.

This is the bowsprit tip with the gennaker attached (it has a furler) - it isn't a forestay furler; the bowsprit only holds the gennaker clew (foot?)
Bow02.jpg


This shows the top of the A frame with the wire attached to the bowsprit tip leading off to the left
Bow04.jpg


and this shows the arrangement of the A frame, the bowsprit and the transverse beam, "cross tube", to which the A frame is attached
Bow03.jpg


If I have understood Matt's description correctly, the A frame has been pulled down, or rotated toward the sea, by the weight of the gennaker being dragged through the sea and pulling the bowsprit down which, in turn, pulled the A frame with it and the A frame mounting plate off its seating on the main cross tube by about 5mm on either side.
 
Ah, seeing the whole A-frame helps and your last piece of text seems about right.

Still can't help though ! If Matt, as a competent grease monkey, can't decide what is necessary when he his is on the scene, I've got no chance as a ham fisted incompetent over 3000 miles away!
 
jings .. the danger might be of the welding completely detaching and the A frame falling away and taking the bowsprit with it? That might do some damage. If that is possible, I would find a way of pulling the top of the A frame back and down to hold it in position ...

I doubt there's any precision with this weld.
 
From my experience with catamarans, and from the pictures, the A_frame is not supported in any way from astern. The transverse wires on top of the A-frame seem redundant because they are not connected to the top of the A-frame but just roll over the top in a channel. Therefore it seems that the whole purpose of those transverse wires on top of the A-frame are simply to hold the A-frame down firmly to the cross beam. The legs of the A-frame are splayed for transverse support from sideways twist while fore and aft movement is prevented solely by the weld to the cross beam.

IMHO that A-frame existed on borrowed time. The lack of fore and aft support put all the forces from the bowsprit onto the weld.

TCM needs the advice from a Marine Architect to redesign the A-frame fore and aft support.

In the meantime if you MUST use the forward sail you will have to jury rig an aft wire.

If it was my boat, I would not use the forward sail until an Architect has examined the structure in case a similar event happened again and twisted the cross beam. Just my advice.

I would guess that the A-frame is really used as a spreader type thing to add strength to the cross beam and to tie the bows together using that heavy gauge wire. I have also seen such an A-frame and bridle to take the considerable forces from the forestay. Certainly a bridle system is often used. TCM's boat seems, from the pictures, to have the forestay connected to the cross beam. To connect it to the bowsprit seems to be an afterthought. To me it doesn't look right, but I am way out of date with modern catamaran design. Is the wire connection from the bowsprit to the A-frame a previous owner modification?
 
Yep. Don't stress the forward sprit. Just use the aft headsail. It may be that heavy lashing around the cross beam and plate will minimise tendency to split any further, but why risk it? It's not a a race.
 
It might not be clear from the first photograph in my post, but, there are three wires off the bowsprit tip; two very heavy wires run to either bow and are attached with substantial chainplates to the bow, the third lightweight wire runs to the top of the A frame. Therefore, the bowsprit has very considerable resistance preventing it from rotating up from three components: the bowsprit attachment to the transverse beam and, principally, the two hull chainplates. There is, though, virtually no resistance preventing the bowsprit rotating down other than the A frame welding. Under normal conditions this would not be a problem? That isn't to say that its a good design which is also your opinion?
 
Yes. I did see the bowsprit wires. The downward forces on the bowsprit should be absorbed by the front forestay just like any bowsprit on a monohull. The upward forces are taken by a wire to the hull close to the waterline which on a monohull usually has a "dolphin striker or martingale to give a better angle. This is the weakness of a catamaran because there is no centre hull so two wires at a poor angle must be used to the waterline of each hull. This makes the sprit "flimsy" which means that the tension on the FRONT forestay must be less than for a monohull which in turn means that less downward support is available at the end of the bowsprit.

It appears that a wire was added from the A-frame to the end of the bowsprit to augment the rigidity because of the design restrictions of a catamaran and the absence of the "dolphin strikers".

IMHO, the wire from the A-frame to the bowsprit is a bad feature because the A-frame is for the considerable stress from the forestay bridle system and NOT to support a bowsprit.

If that A-frame goes, then the forestay will follow and possibly the mast.
I would advice TCM to contact the ARC organisers to obtain the consultation of an Architect to examine the pictures. For my part I dont think that wire from the A-frame to the bowsprit should be there. But I am not an Marine Architect.
 
I would be careful welding the A frame mounting the to cross beam. You don't want to compromise the cross beam strength in any way. Maybe pull it back to where it was and then stabilise the A frame mounting plates with an epoxy bandage. This type of failure is a common problem with many prodders. They are designed to carry upward forces but not downward. Just a pity yours was so well supported by the cross beam A frame. For the future you could stabilise the A frame with a wire running back to the mast, chain plates or some other mounting point, or replace the wire between the bowsprit and the A frame with something that will break before the A frame mounts. Maybe the boatbuilders have developed a fix but given the number of miles you are doing, you are probably the "bleeding edge" owner.

Cheers
 
[ QUOTE ]
... the A-frame is for the considerable stress from the forestay bridle system and NOT to support a bowsprit.

[/ QUOTE ]
hmm .. I see exactly what you mean. Any upward force (exerted by the forestay) will bend the transverse beam but will be resisted by the A frame and the wire running over it and attached to either end of the beam.
 
My feeling is that the A frame has next to nothing to do with the sprit, and much more to do with keeping the cross beam in compression (like stays on a mast). The gauge of the cable running across the top of the A seems to imply that it has this function. Failure in the A may therefore threaten the function of the cross beam. I should stress that I'm not an engineer, and I hope that my suspicion is wrong. A phone call to the builder would be my first priority.
 
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