MR3133 is getting a little 'long in the tooth' and I believe starting to flex.  I was considering adding a simple space frame (connecting shrouds to stem) before she folds in half in the next blow.  Does anyone have experience of having designed and fitted one of these things?  I am assuming the best way would be to do a full size mock up with plastic plumbing pipe and then taking it to a workshop to have it fabricated in aluminium tubing?  The mast is hog-stepped and the buoyancy tank is the high version.  What shape (for strength)and how best to secure without restricting spinnaker movement or spoiling the looks of the boat?

TRy Laurie Smith of Southampton who had a series if these things on his 505's however its probably cheaper to get a new boat.

Andy Hayes is the man to contact - do you remember reading his article in the magazine a while ago?

If you weld a frame outside the boat, I'm not sure you could slide it into the gap, whole! Perhaps better to built it in situ, bolting together? I'm sure you could do that at home.

Key point to remember is that you are aiming to isolate the forces imposed by the rigging from the hull structure by triangulating them into self contained stronger structure. Some of the forces are compressive (the links from the shroud attachments to the mast gate position) and some are tensile (the ones from the shroud attachment points to mast step on the hog) so different materials and methods can successfully be used to ensure the final structure is light, but adequately strong.

Rather than attempt to make up and install a complete frame, you would be better advised to beef up what structure there is in the boat with strategic tubing or wire.

A key consideration is to ensure the space frame stays in the hull... I well remember Andy Fitzgerald and Bruce Grant installing a space frame in one of their early Int 14s.  They were sailing well and I recall watching the whole rig and space frame peel out of the hull and fall overboard, leaving the hull completely intact and bobbing upright around Chichester Harbour attached only by the sheets to the errant rig.  More glue required!

GGGGGGGGGG

I dont want to pee on the bonfire but..I looked seriously at doing exactly this to 2739 and I worked out that it would actually be cheaper (with everything taken into consideration, in the long run) to buy a newer boat.

By all means go for it if you have the time and finances but ask yourself the reasons for doing it.

Good luck.

Were I doing something of the like to a Cherub (in fact I did 2 years ago albeit to a foam boat) I'd be considering carbon/foam sandwich integrated with the hull structure rather than a bolt on piece of aluminium. If you've got a deck stepped masts or ambitions to change to one then (if your rules allow) a carbon bulkead from mast post to stem and one from mast post back to each shroud anchorage would stiffen up the boat enormously. If you're into doing that sort of work to your boat it would be shed loads cheaper than an alloy frame and a better result. Probably lighter too. Then a nice varnished foredeck back on top... And I suppose if one were really going for it one would vac bag a sapele or whatever veneer onto the aft face of the transverse bulkheads too if there were a few ounces to spare.[grin]

I am considering deck stepping my boat and I am looking into carbon/foam sandwich bulkhead or carbon tube space frame (see other thread 'needlespars', and thanks to Dick Holden for the bulkhead pics)

Question; would a carbon bulkhead be considered part of the hull and would it be legal?

Thanks to everyone for some great suggestions/comments.  As for purchasing a new boat or looking at other Merlins in the dinghy park, I am one of the very small handful (2...possibly three)of MR sailors in the US,  so I am stuck with the tired boat I have.  I was noodling on importing a second hand boat (and might still do so one day)but am not sure when I would have the time or $$ to get on a plane and head for home so I can spend a week searching...and buying unseen is a little risky. Shipping costs are actually reasonable (about $800)  I have just purchased a new (well used!) set of sails (thanks to the MR web site) so I should probably stick with 3133 for a while longer.  I did crew in the 1990 nationals ( Buttercup Trophy..and proud of it too!)and recall an older boat with the same problem being repaired (by Mr Jackson I believe?) with aluminium tubing.  Perhaps the suggestion of beefing up the existing structure (doubling up some frames perhaps) might be the simplest.  For anyone out there in cyberspace with nothing else to do, for the last five years I have been sailing 3133 on Lake Mohawk in New Jersey - see the attached link for some info on this rather unusual sailing club - http://www.lakemohawksailing.org/
We are moving back to North Carolina this semmer so the line up of boats to compet with will be very different.

You could consider a bulk head using 4mm ply, 12mm closed cell foam, 4mm ply, cheap light & strong.