SC65

changing rocker and shape of old glass sandwich catamaran hulls

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wondering if anyone ever has done this and what would be the easiest method to reduce a too pronounced rocker (causing a lot of pitching and hobby horsing) in an old V-shape/U-shape like glass polyester sandwich hull construction? hulls in question are 25 feet long and only feature one bulkhead under the front beam, which will require adjustment.

a first approach is thinking to reduce the rocker by cutting out a up to 20cm wide section (as it widest point near the daggerboard trunks, reduced to 0 when nearing bow and stern) along the centreline. In a next step to cut/mill the outer skin in several  staggered longitudinal lines along the previous waterline and the new centereline. the purpose of this is to then allow bending the otherwise stiff sandwich laminates towards each other until they meet in the center, hereby reducing the rocker (and volume) of the hulls. the idea is then to join them in centerline again and to fill the cuts in the outer skin with lightweight filler and add one or two fiber layers on top to the same thickness as the outer skin which was cut. 

another approach would be to cut the hulls off below the waterline, build a complete new underwater section, and to join them together again.

I would assume that the first approach is faster and less costly then the 2nd, however I am not sure how well it will actually work to bend an already built sandwich construction just by cutting a few cuts in the outer skin, so would appreciate anyone familiar with catamaran hulls as such and the proposed measure to comment on it.

I am aware that both measures will reduce the volume of the hulls and as such will affect how the hulls sit in the water (lower) but believe that the hulls carry enough extra buoyancy to allow such measures. I am seeking technical advise/ shared experience rather than opinions or debates of why to do something radical like this.

many thanks in advance!

 

 

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Its a bit of a piece of string question. I've done a bit of this sort of hacking on smaller boats, and some shells are relatively floppy and can be persuaded to change shape fairly readily, whilst others are rock solid and won't move.

Have you considered whether your desired aims could be met by addition rather than subtraction?

Could you just glue foam onto the existing skin fore and aft (if you want a reversible change) and add a new skin (which could be relatively lightweight as non structural) or even remove the outer skin and add the foam and then a new full spec outer skin? I've altered dinghies by both of those methods, and it has the huge advantage that none of the internal framing and structural components are changed, so the job is a whole lot easier and cheaper. If you leave the original skin untouched OK you pay a weight penalty, but it has the enormous advantage that if, after a season or two, you decide the change was for the worse, all that's required is to remove the new skin in the areas you wish to change again, reshape and reskin. There are no structural considerations at all. 

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spend you time taking weight out of the rig or adding winglets.  either of those will make an order of magnitude more difference than changing the hull shape.

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I agree with Scarecrow.

Could you remove the deck and spread the hull at the gunwales?  I'd fear that a cored hull is likely to be too stiff to deform much.

 

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6 hours ago, JimC said:

Its a bit of a piece of string question. I've done a bit of this sort of hacking on smaller boats, and some shells are relatively floppy and can be persuaded to change shape fairly readily, whilst others are rock solid and won't move.

Have you considered whether your desired aims could be met by addition rather than subtraction?

Could you just glue foam onto the existing skin fore and aft (if you want a reversible change) and add a new skin (which could be relatively lightweight as non structural) or even remove the outer skin and add the foam and then a new full spec outer skin? I've altered dinghies by both of those methods, and it has the huge advantage that none of the internal framing and structural components are changed, so the job is a whole lot easier and cheaper. If you leave the original skin untouched OK you pay a weight penalty, but it has the enormous advantage that if, after a season or two, you decide the change was for the worse, all that's required is to remove the new skin in the areas you wish to change again, reshape and reskin. There are no structural considerations at all. 

Hi Jim, thanks for the feedback. I actually did consider adding foam as you describe and actually have done something similar to one of my boats in the past with some success, but it does come with a weight penalty, I am not too fond off, considering how much I would have to add at the transom to get it anywhere near to my liking. Currently the boat is 7.65m long but the depth of the rocker is 35 cm and I think something like half of that would be far better to reduce pitching.

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There was an Island born (Nevis) trimaran skipper on St Croix who had an old Newick tri that he daychartered very successfully. But then he would kick butt with it on our local races! 

He pushed the boat hard and wanted more volume in the amas so he took a skill saw and made a cut through the deck from the bow to nearly the main cross beam. He then took wedges and pounded in the slot forcing the deck and topside of the hull apart for more volume and when it looked right to him he fit a long sliver of ply to close it all back up. Worked line a charm. Next season he did something similar to the bows by slicing down the keel. Eventually he built a new boat with the volume (and more) that he wanted.

Did I mention he pushed the boat HARD when racing? New boat.

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Bit more relaxed in work mode.

Image result for trimaran CHaris

Lew was THE MAN and you could never take your eye off of him on the Round the Island Race. He knew those waters like his own, well ...

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Pitch reduction in multis is best accomplished by having an assymetric waterplane area. So wider stern and leave the bow alone.

I tend to agree - add foam around the stern, glass over and make the last 1.5-2 m wider. My 12m Woods Meander had deep V hulls with lots of stern rocker and I seriously contemplated changing hull shape but pitch motion was never big enough to worry about in most sea conditions.

Also consider T rudders to reduce pitching. Gotta be strong for the forces though! 

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18 hours ago, SC65 said:

but it does come with a weight penalty, I am not too fond off, considering how much I would have to add at the transom to get it anywhere near to my liking

I know exactly what you mean, but worth doing the sums for how much weight though if you just add foam over the existing skin. Its not as if you need to make a full strength outer skin, it only needs to be strong enough to cope with handling, and the core can be low density too: its not as if its structural. And, after all, if after a bit of tuning you end up with a shape that meets all your requirements, you can always get the angle grinder out and cut off and replace the modified parts of your hulls to reduce the weight, and you can regard the Mk1 foam additions as being low cost prototyping. 

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We added about 6' of waterline on 27' floats fwd, left the sterns alone. Reckoned we added about 500 lbs vol for 50 lbs each. Used cedar frames glued and screwed to existing lam, filled voids with styro eps (sic), heavy glass job over. Seems pretty good. 

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