The Difficulties of Designing Proas

Over Christmas I finally had the chance to sit down and work on my boat design again. As some of you probably know, I’d been working on a 9 meter proa design, on and off, for most of the last year.

Proas are unusual amongst sailing craft in that they always keep one side of the boat, usually the ama (float), to windward. To change tack, this requires having a bow on each end of the boat, being able to rotate the mast / rig 180°, and having duplicate steering gear, one for each direction (I’ve written more on that here).

Shunting a proa, going from starboard tack to port tack.

Doing this has a number of advantages, most importantly that you get a drastic reduction in the loads on the beams connecting the two hulls, and the ama can be made much smaller, allowing large weight and cost savings. While the righting moment of a proa is lower than on e trimaran or catamaran (i.e. you can carry less sail), the boat also has a lower resistance, so the overall decrease in performance is not that large. The smaller rig is also less expensive, so you end up with a boat which is maybe 10-20% slower than an equal length trimaran, but at perhaps half the cost. The downside is it’s a bit tricky to handle (especially when there’s a lack of sea-room), the steering gear is more complicated and you need two sets, if you want to have a normal bermuda rig then the standing rigging is pretty tricky to get right, since the mast has to be able to rotate almost 180°. You also have to position the maststep far enough to windward so that the staying angles are acceptable (minimum 7-8°), so that the rig won’t fall over even if you do accidentally tack one day.

While I thought the design was pretty elegant, the complexity of these problems as well as the sheer size, workload, and cost associated with a 9m proa, proved to be quite a headache. It was becoming more and more apparent that this wasn’t going to be the solution I’m looking for. It didn’t make me too happy, but four years in from my very first musings about designing my own proa, I decided to take a blank sheet of paper and start thinking about other options…

Leave a Reply

Your email address will not be published. Required fields are marked *