Platform Flexure in Multihulls

It seems that most if not all of the multihull designs incorporate very stiff crossbeams (akas) to connect the hulls. My question is whether or not there may be some advantages to allowing a certain amount of bending in these members.

Based on my experience with full size & model iceboats/landyachts, the trend is towards allowing a fair amount of “softness” in the platform. The main advantage is that as compressive loads from the mast push the hull down in a gusty puff, the rig is automatically allowed to “slop off”, spilling air & preventing capsize. It seems to me that the same concept would work with multihulls. Another advantage could be that the outer float hulls (amas) would have a certain amount of independent movement, allowing them to adjust independently to localized wave crests.

There are probably some good reasons why this concept isn’t such a great idea. I’d be interested in hearing other people’s thought as to why this would or wouldn’t be worth a try.

Bill K

Hi Bill -

invariably in beach cats (full size) the rigs are left “sloppy” to some degree. This does allow the rig/mast to fall off slightly to leeward, and also the slightly loose shrouds help the mast to rotate toward apparent wind easier than if the rig were to be “banjo” tight. The mainsheet tension while sialing to windward provides both leech tension on the main, and also adds tension to the forestay to keep it and jib luff tight. As the mast leans aft, upwind with a lot of sheet pressure, the center of effort is allowed o move back slightly adding a bit of weather helm, and helping the boat to point higher, easier. Downwind, when mainsheet is released, mast moves forward along with center of effort, the downhaul is loosened and a positive inhaul is used on the clew to add a lot of camber into the sail for downind sailing (in most conditions)

As to stiffness, the Gougeon Brothers back in late 1980’s actually set their Formula 40 floats (amas) up to “float”. The front of the float attachment to cross beam was pivot point, and the rear was left to “hang” loose. A preventer line kept some control over the float, and also they engineered a kind of wooden spring to take up loads as float moved up and down, independent of either the main hull or the opposite float. The idea was designed to allow different angles of float movement through angled waves. The boat ADRENALIN was spectacular in it’s first few outings. It did lose one of the floats when the preventer line broke allowing the stern of the float to move sideways instead of up and down. Because of the success of this trimaran in the European Formula 40 circuit, it was legislated out of the competition the following year, and only catamarans were allowed. Too bad!

Along with the floating amas, the tri also had other unique ideas incorporated. Obviously it was the only all wood trimaran to compete and finish as well as it did. The tall rig had single spreaders but multiple diamond wires as I recall. Those were designed by Mike Zuteck from Texas - a Tornado sailor and Physics degreed individual. They referred to the rig as the “Zuteckilator” or something like that. Another innovation for the time, was the mainsail was large, but they only used a “ribbon” of a jib. Just enough to direct the wind over the main, but not so big to have to worry about trim. At the time, asymmetrical spinnakers were not being used.

On my MultiONE, the cross beams (carbon tubes) simply slide into a set of tubes mounted on the floats, and they are basically held in place by the side shrouds bridle wire. They are loose and slip in - no bolts or screws to hold in place. Here is a close up photo of the exterior end of a cross beam with side shroud bridle wire attachment.

Thanks for the very interesting recap on the Gougeon brothers. I was not aware of that aspect of the Adrenalin design. Man, those guys were (and still are) TRUE innovators. They really know how to think outside the box. I see they’re into small kayak type gunkholing sail rigs these days.

Anyway, do the carbon tubes on your MultiONE deflect enough under load to allow automatic rig slop & independent ama movement? I like your idea of allowing them to pivot freely within the attachment tubes. As I’m sure you remember from your architecture school training, a freely rotating “pinned” connection will relieve all the stresses & strains involved with a “moment” connection

No Bill -

there just isn’t enough weight in the floats to make a 1/2 inch diameter carbon tube bend very much at about 32 inches total beam. But, being the cross tubes can rotate in the floats, the starboard bow can be almost 3/4 inch higher than the port bow if waves force the issue. If you recall, my first mainsail was made with a flat top suspended on a rotating gaff from top of mast. (Can’t say Wing-Tip Rig for obvious reasons) This rotating gaff/crane supports the mainsail, but lets it twist off a good deal in strong (and not so strong) winds. I added a preventer line that can be adjusted on shore before sailing. It basically limits the amount/angle the support can rotate. Still fooling with that as it is an all-or-nothing adjustment and if wind gets heavy, there is no way to release without bringing ashore. Had thought about a bit of surgical tubing to allow stretch - much as the Bay City Landsailors are doing with their shrouds.

It’s not the weight of the floats that would cause the akas to flex, but instead it’s the wind’s lateral force vector on the sail rig (lift) which is resisted by the buoyancy of the ama (in combination with the centerboard or leeboard). That being said, I agree that 1/2" carbon tubes are going to be plenty stiff enough to resist such loads. What I am proposing is having more flexible akas, so they will essentially act as springs and absorb at least part of the force that would otherwise become heeling moment. As a consequence of this bending, the distance between the hound & the leeward chainplate is decreased slightly which allows the mast to slop off to the side & spill excessive air. In a way, it’s similar to using stretchy shrouds to slop off the rig, but my observations of that approach has lead me to believe that it’s much harder to control & can lead to excessive slop.


The concept of “flexing” platforms was tried here by the Billett family and seeing as they no longer are going done that path it would seem that for them it didn’t work.

I do however promote a small amount of “twist” in the platform and it seems to have no dire affects on the boat, except maybe in very strong winds where the leeward float can more easily bury and the boat can flip. (quick thumbs help here).

Overall though stiff platforms seem much better, here anyway.


Just a guess here, but experiments with different lengths (tension) of surgical tubing might be an alternative answer. Just insert somewhere along the side shrouds to allow the wind force to stretch, thereby allowing mast to tilt off to leeward. Might need it for forestay as well - however, when mast falls off, you do not want it to rotate forward. It needs to move backward to increase (slightly) the weather helm, allowing boat to point a bit higher in puffs. Unfortunately, allowing stretch in forestay isn’t good for jib luff shape, so that might be an issue.

I still have not ruled out using a uni-rig consisting only of a mainsail. They are known to point higher, it is easier to set angle of attack to windward, no backwind of main by jib, no slot issues, but definitely much more difficult to keep “heated up” downwind. Gybing is a necessity in all but survival conditions. This would be the “pants pucker factor” if it is really blowing - always on verge of capsize or pitchpole.