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There are two stages in creating a successful catamaran crossbeam solution.

First you must DESIGN the structure, only then can you CALCULATE it. The former is usually the more important and certainly the one most people get wrong.

There are several factors to consider when designing crossbeams:
First, you need stiff crossbeams, not just strong ones. Fortunately stiff beams are nearly always over-strong. By stiff I mean one without any obvious deflection. Engineers normally consider that to be 1-2% of length.

Why a stiff beam? Well imagine crossbeams made out of rubber. They would never break, but would be so flexible you could never keep the two hulls in line and the mast would fall down as the rigging flexed.

How can you make a stiff beam? Well, actually it’s not just the beams that you want stiff, rather it’s the boat as a whole. I’ve found that the best way to do this on an open catamaran is to have two crossbeams plus a separate one to take the mast loads. The actual positioning of the beams is also very important.

Although crossbeam size and placement is often complicated by rig and accommodation considerations, the beams must take priority! If they are too near the middle of the boat then the bows can flex up and down and you cannot keep the rig tight. If too close to the ends (especially to the bows) there isn’t enough boat to take the loads and, furthermore the beam cantilever is longer.

Having the first crossbeam almost at midships was the most spectacular mistake the Team Phillips designer made. Pete Goss and I are members of the same sailing club and other members were upset when I expressed concern about its design. After the breakage they realised what I was on about.

Once you’ve designed the structure it’s really a trivial problem to calculate the necessary scantlings. If you use a strain energy analysis you’ll find that the loads will dissipate quite quickly into the hull. Indeed it’s extremely rare for beams to break off the hulls. Usually the problem is the beams themselves breaking.

Fortunately it’s very easy to check the strength of catamaran beams once you’ve built them. You simply jack the boat up with a support under each bow and each stern. Then take one of the chocks away. The boat shouldn’t move appreciably. It looks scary, and is certainly a load that you wouldn’t get at sea. But is very reassuring all the same.

You can see a photo of our Merlin Tucanu surviving this test on the “Review of 2007" page. The Merlin and the similar Strider design use two aluminum tubes with inertias around 500cm4. I usually use ply and timber beams as they are easy to make and to attach to the boat (and of course to attach boat to them), but they are heavy (approx 1.5 x the weight of aluminum tubes).

It is not just open deck boats that need good crossbeams. Unlike most designers my bridgedeck catamaran designs also feature big beams. Apart from a big netting beam there will be one under the mast and one under the aft end of the cockpit. I design the anchor lockers and forward end of the bridgedeck to act like another beam. By adding theses beams I ensure that the boat is extremely stiff and that it is not a problem fitting big deck hatches or large companionway doors. I have seen many production bridge deck cabin catamarans that rely solely on the hull/deck mouldings for strength flex, and even crack, bulkheads because they are simply not stiff enough.

On a large, say 40ft, 7ton boat, beams that are strong enough may weigh 400kgs. Ones that are too weak will still be heavy - they may weigh 300Kgs. It’s NEVER worth trying to save weight in your beams. Carbon beams may weigh 250Kg at a huge cost. Is it worth spending that much to save 150Kgs at best? I’d rather spend the extra money on better sails and deck gear.