Further thoughts on the biplane cat

It would take a long process for me to get my design from paper to this computer, involving going into town to the local library for an hour and a half, so it is not going to be possible to post it here.  One day I shall.  This one’s as close as I can find – just take off the bridgedeck and have a trampoline.  Also, my rig is a bit different.  Masts are in the same place, though mine are far shorter.

It always worries me when I design a boat or a system and can’t see anything wrong with it. Much better is to be put down for the design. For example, it took me weeks to come up with a solution to the junk sail dilemma, I finally did it, sent it to the JSA, only to be be advised not to reinvent the wheel.

Aaaa … yes.

The good thing was that at least I’d got it right – it was just that a man in Norway had also come to that conclusion and built the rig as well, in 2004.

Some designs of mine, e.g. a reefable crab claw – reefable meaning the area can be reduced – looked good and it opened up all sorts of possibilities. I came back in two days and immediately saw what was wrong with it and that conventional wisdom was right – it can’t actually be reefed.


Having stumbled on the biplane catamaran, thinking this a radically new idea, it was jawdropping to see that it had been done by many over the years and not only that, it was workable. So why did designers run with it for some time and then move back to conventional?

I think the answer lies in the radical look of the craft – some might say it offends the eye in the mast[s] not being where most people would expect the mast[s] to be. After a long time with such a workable and sueful rig, safe too, you might just hanker for a more “boaty” boat, in the classic style, despite its design limitations which the biplane overcame.

I can see that and yet, as an aging man designing for similar aging men, with a view to keeping them on the water longer, the biplane certainly has much to recommend it. But let’s start with its limitations.

1. It needs a thicker and therefore much heavier mast or else it needs hideously expensive carbon fibre.  We can get around that flexing [to spill wind in a gust] by putting on a balance lug rig which autospills anyway.

2.  The stresses are completely different.  Cats have the advantage that if you place a mast on the centre beam, you can run stays either side, out wide and that means you can use a thinner section.  Not only that but you can have a rotating wing which takes even less stress.  Downside is the enormous stress on the main beam and the twisting overall between the hulls.

OK, take away the house in the middle, lengthen the hulls, widen them and make the section rounder, move the masts back to 45% from the bow, put in vertical stemposts, increase beam, lower sail a foot at the boom and lower height overall and it’s not far away.

Downside of the freestanding mast with a cat is that, as the cat hulls are by definition narrower, placing a mast in each hull precludes stays – they’d be too close to the hull on the outer side.  Freestanding seems the only way.  Now, that places enormous pressure on the step [where the mast sits] but that’s Ok because it would be on the keel – a strong point.  Where it comes unstuck is in the supporting partners at deck level.

Essentially, what you have is a giant cantilever, with maybe 500 sq ft of sail pressing the mast against maybe 6 feet of embedded base.  So that severely limits mast height, given that we’re going to use wood.  In turn, that limits lower size of the boat – in other words, it would be pointless below a certain length of boat, e.g. 40 feet because the way sail area does not increase in a linear manner for increase in mast length but exponentially.  So one finds the lower limit very quickly.

Besides, cats aren’t the best option below 40 feet and if you run a trimaran, you don’t really need a freestanding mast – stays on a rotating rig are fine.  So sheer physics is determining that the boat needs to be, say, 43-47 feet, with around 550 sq ft in each sail.  Any longer and the mast needs to also be longer and thicker and heavier and it becomes impossible over 50 feet of boat.

Therefore the optimum is to design a rig for a 43 foot boat and then just lengthen out the ends to make 47 feet overall.  That’s a nice conservative rig, still with some speed.  I noticed one design with 1380 sq ft all up – that’s way OTT for an aging sailor and was clearly meant for a crew.

I settled on a 40 foot mast overall [34 feet above deck level] which is very, very short for a mast on that size boat.  Now it also means the hull must be minimum 6 feet wide, to allow the partners to support the mast.  In cruising terms, you actually need wider, to allow you past the mast either side.  That indicates 7 feet wide for each hull and 3.5 ft at the waterline, giving 13:1.  As the ideal cruising cat hull has between 10:1 and 14:1, then that’s good.  In fact, we could widen at waterline a bit and that equals comfort inside.

Why not 8 or 9 feet wide hulls?  Weight, shape – it’s creating a tub which would wallow like two monohulls strapped together.  I’d like 5 feet wide but that’s impractical – 7 will have to do, which on 47 feet all up, is OK.

Now, running a sliding mast step on a shuttle trolley is not hard and that allows less blanketing of one sail by the other.  It is braced fore-and-aft by cables, the only complicated part on the boat.

The sail is a fully battened balance lug, set on the outside of the mast for specific reasons.  Firstly, it gives the greatest gap between leading edges, which means less blanketing.  It is well behaved [with battens] and easily reefed – critical for an aging sailor – by one continuous line from either cockpit – goes down into lazy jacks.  Of course, early reefing means even less strain on the mast.

One thing cats do, especially in light winds and swells, is to roll sideways and with two heavy masts at a distance like this, the tendency to rock sideways, back and forth, would be pronounced.  Once the wind started, it would be better but still, it means the overall beam can’t be under 18 feet [good for the beams] or over 25 feet [beams like matchsticks].  7 foot wide hulls indicates an overall centreline to centreline beam of maybe 22 feet, giving a total beam of 29 feet – more than enough.

With very little internal building, e.g. beanchairs and air mattresses plus watching which supplies come on board – much dried food, most of the weight in the water bottles, distributed over the length of both hulls, all the major weight in the strengthening at stress points, it would be just lighter than what you’d expect overall.


I’d expect, with light-medium weight, 12:1 waterline length to beam, 1100 sq feet in the sails, only a trampoline between the hulls, with three medium sized beams, that performance would not be in the Corsair class but would certainly kill the floating palaces which comprise most cruising cats.  Of course, one pays with fewer nick-nacks and creature comforts.  It would be quicker than, say, a 16 foot off-the-beach cat.  That’s enough speed in big seas.

With that rig, she’s not going to pitch, certainly won’t yaw with the cutaway shoal keel, is almost impossible to pitch pole and almost impossible to capsize.  If it did capsize, the ship’s boat, with outboard, is strapped underneath [now on top, upside down] and as it is self draining, it can be turned over and brought into play.


I’d imagine the crew would just stay inside while the storm lasted then, in calmer weather, the righting could begin.  This is going to be tough and for that sized boat, only the ship’s boat, with motor, could do the job.  The ship’s boat is hard [not inflatable] and the outboard [doubling as the cat’s motor] would have sufficient horsepower to do the job.

1.  Release both halyards.  When the boat comes up, sails will fall into lazyjacks.  Short masts also come into their own here.

2.  Using the righting ropes which are always in place but need extending now, this is to be done slowly, there’s all day to do it and energy conservation is everything.  Wife and/or family are in the ship’s boat, wife driving.  When lines are all ready, leading to one point, the line from the bridle on the ship’s boat is brought over and clipped on and wife takes up the slack.

3.  Boat slowly comes upright and man’s job is to check for obstructions, halyards jamming etc.  He’s on the tramp, near the gap where the ship’s boat was released from, on the underside, ready to climb through to the upperside.

4.  Wife now applies maximum power and the boat comes up [hopefully], then throttles off once it even looks like it’s coming upright.  Boat crashes right way up and wants to roll the other way.  Let it find its natural position.  Man scrambles over to the clip and releases it, wife motors round and under the boat [5’9″ clearance], ladder is dropped and they rejoin the boat.  Ship’s boat is hauled up.

5.  Rest of the day is spent repairing/setting straight/eating/resting.

4 comments for “Further thoughts on the biplane cat

  1. September 26, 2011 at 20:44

    Are you familiar with the Adastra Trimaran?
    We can dream.

  2. Hermann Otto
    October 15, 2014 at 04:57

    Like you when I decided to build a bi plane rigged,I was amazed how widespread the idea was..
    I chose the chinese Lug which can be shifted a bit fore and aft. It sailed on the wind ca 12odeg between tacks.

    I broke battens in gybes Herman(c:

    • October 15, 2014 at 09:27

      That’s interesting [sorry for the delay too, Hermann – comment got lost]. I always wondered about the full length battens in gybes and over time.

      I confess I’ve now gone ‘radical-old-fashioned’, with a few smaller square sails. More work but quite handleable.

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