A third post on biplane catamarans

Haddanuvva breakthrough …

Long ago in the mists of time, two posts went up on biplane cats:



The main two reasons I shelved the idea were:

1. I went tri, not cat

2. I read a report on a hi-tech model which flipped when a sudden gust came down off a hill. I started to wonder why and then it became clear.

If you put one mast and sail in each hull, then a wind coming in obliquely is going to hit the windward sail first and the only way then for that hull is up and over, especially if there is a board down from the lee hull. It’s a fulcrum.

There is also the question of blanketing or wind shadow.

There is also the issue of the area being split equally in half and so that’s still a massive area up on the windward hull, which is already struggling to stay in the water. Therefore, I decline to use a biplane arrangement.

My solution

Run six sails, not two, on unstayed masts embedded through the hulls, which might seem counter-intuitive at first and seems to compound the issue:

1. 100 sq ft is vastly easier to handle for an ageing man.

2. That area allows for dinghy sized masts [14′ x 3″] and other rigging.

3. Each mast is in a separate compartment, so that if there is a sudden squall, a mast causing damage does so in one sealed compartment. Yes, there’s always the chance they’d all break at the one time.

4. Removing the masts from the cross beams of a cat removes stresses but does transfer such stresses to the hull, which is more readily braced inside. It’s assuming unstayed masts here.

5. The added weight of six small masts is better placed on/in the hulls, not on the beams. Cats of this size need weight in the hulls to resist capsize.

6. Using modestly cambered junk sails takes advantage of the instant reefing. Junk sails are heavier with those battens and once a halyard is released, they come straight down into cradles, something which can be done from the cockpit.

7. Six sails give an enormous number of permutations on each point of sail. You might run one windward sail and three leeward upwind, you might use five or six downwind. There’s a rule of thumb that it’s better to have the area forward going downwind in big seas, hence many of the old ships ran their square sails from the foremasts. You can partly reef each sail to fine tune the centre of effort against the centre of lateral resistance as you go. And so on.

8. For an approaching storm, the masts lift out and are lashed to the cross beams, which gives added strength anyway. Each of these masts is about 18 feet long by 3 inches all up – a job for two crew.

In that situation, a 7′ x 3″ stayed staff does go up on the centre beam for the storm jib.

Redolent of but not the same as this:

16 comments for “A third post on biplane catamarans

  1. dearieme
    September 19, 2017 at 14:48

    Your mast length has increased from 14′ to 18′, a product of your enthusiasm no doubt. Would your storm mast be permanently erect? Presumably not. Will it be folded down for everyone to trip over?

    Anyway your sketch of China meets Polynesia is rather fine. But surely it would be a bit of a bugger to juggle six sets of sheets at the same time?

    Have you thought of an active ballasting system? On detecting that one hull is in danger of lifting so far that it will cowp you over, a pump would impel (say) mercury from the leeward hull into the windward hull. Wotcha fink? Or you could tow a heavy torpedo-shaped ballast weight beneath the two hulls at a depth of (say) four fathom – this is for ocean sailing. You’d rig it so that by tautening and slackening the chains that hold it, the weight would bear on the hull you want to keep in the water. You’d want the ballast to be pretty dense so that the “torpedo” isn’t big. Depleted uranium might be just the job.

    What sort of madness, you might ask, is a proposal to use mercury or depleted uranium? No madder than someone using a petrol engine when a diesel is available.

    Actually, there’s an idea. Let your “torpedo” hold your diesel engine, with ducting for the air and exhaust. So when you are causing under sail over the briny your engine is still doing a useful job stabilising your craft. For shallow water you just haul it up. Brilliant.

  2. September 19, 2017 at 15:03

    14′ above the deck, 18′ head to keel.

    7′ storm staff hangs from roof inside the open area, just gets taken out there when needed, sail attached. Three stays.

    Just been in the yard talking to the boss about the space and how it is to be done.

    Surely it would be a bit of a bugger to juggle six sets of sheets at the same time?

    It would be were it not a junk – junks are self-tending.

    Wotcha fink?

    That’s the principle behind some bilge keels in monos. Seems to work all right too. This is an unballasted cat. No keels, two leeboards.

    Depleted uranium might be just the job.

    It might just, were there ballast needed.

    I already have the outboard – 15hp, will push along 3.7 tonnes. Boat is 3.5 tonnes.

    Interesting that you mention Chinese/Pacific as that is very much what it is redolent of. I’ll do a sketch now of the side elevation [in rough] and put it at the end of the post.

    Give me a few minutes.


  3. Distant Relative
    September 19, 2017 at 15:54

    “Or you could tow a heavy torpedo…..”

    A la “African Queen”

    Ooh! Mr Alnott 🙂

    • September 19, 2017 at 16:04

      You’re booked as the entertainer – many perks.

  4. dearieme
    September 19, 2017 at 18:23

    You’ve not mentioned how you propose to organise the supply of floozies.

    • November 12, 2018 at 09:54

      The African Queen had just the one floozie, and she was well past her use by date for any but a knackered old shyte. 🙂

  5. September 19, 2017 at 20:09

    If ease of sail management is a primary concern, why not a traditional sloop rig with a self-furling jib and a battened main dropping into lazyjacks, all of which can be managed single-handed from the cockpit?

    • September 20, 2017 at 03:58

      For so many reasons.

      Reefing is first – waiting to go wrong, especially in-boom and in-mast, the junk reefs itself once the halyard is released and on any point of sail, just falls into lazy jacks and there it is. Bermudan needs rounding up in a blow to reef, needs hauling down by a crew, demanding, even with good batt cars.

      Vastly too much area in one sail, say 250 for’ard of 350, concentrated on a limited number of points, stressing the beam and staying points.

      Expense beyond my wallet.

      Super high mast waving about, spreaders, stays and lines everywhere, e.g. vangs, travellers, eac one potentially jamming, expensive to replace Harken blocks etc., winches need servicing.

      No back up when mast bends or snaps, no sailmaker at sea but a junk sail easy to make and replace at sea. Low tech also means patching is viable.

      Problems of CE and helm when the area is focussed on one vertical line over a fin keel, whereas the lateral resistance is elongated horizontally and is shoal. Eliminates the need for a keel – shallow water easy.

      High heeling moment upwind, requiring constant cranking down of boom, which I’m trying to avoid at all costs, trying to keep it low stress and low tech.

      Most cruisers spread their area to at least ketch/yawl. The modern junk with the cambered panels has some upwind drive but not at Bermudan levels and it’s drive we’re trying to avoid, as drive means stress points plus heeling moment. Drive in a cat further means having to trail a rode to slow her down.

      Junks come into their own offwind, eliminating messy handling of spinnakers with small crew of three and far more efficient than the Bermudan outside the narrow range of tight reach to tight reach but not as good into the wind, which is desirable. Bermudan sloop is fine for coastal sailing with much upwind work but is not the rig of choice for voyaging on a limited budget.

      The overriding consideration though is safety. Release the halyard on a junk and it reefs itself in seconds on any point of sail, even running. That’s mighty handy at sea shorthanded and can’t be easily done on a Bermudan.

      Finally, aesthetics. The junk or gaff ketch, even the lug, suits the lazy double canoe look better, it’s more exotic, whereas the western Bermudan is all high tech and efficiency, clanging metal at night, whereas the junk/voyaging canoe is wooden and low tech, with the clunk of wood at night, not metal.

      There’s a post coming up later with some pics to illustrate this.

      • September 20, 2017 at 05:08

        Well, without knowing your budgetary restrictions, and I wouldn’t be so rude as to ask, it’s hard to comment further on those points.

        However, you could be off on the journey next spring if you had £20k-£25k available. This is one example; https://m.apolloduck.com/boat.phtml?id=534749&rid=gb&cid=2&z=gb2

        But there’s at least 5 more of the right size for the French system on that website.

        We spent 7 months on one of those, family of four. Spent €50 a day all up for fuel, marinas (where we had no choice), food and wine.

        As for reefing the main in a blow, I recall what my instructor replied to my question, “how do I know when it’s time to reef the main, skip?”. “When you even mildly consider, ‘is it time to reef the main?’, it’s time to reef the main”.

        • September 20, 2017 at 09:58

          The Catalac is certainly a way to go for extended hops with family. Family is a major consideration in this as they can fit into space and have things organized in a way non-family can’t. Husband and wife sailing is ubiquitous and a good thing too. So many retired couples among them.

          There’s also the question of whether it’s your home, whether you’re doing the global run or island hopping, whether doing Europe/France/Med with much canal work, hot or cold climate, whether you’re in your 40s or 70s, also avoiding marinas.

          Your 7 months, William, sounds just the ticket for that purpose. This thing is horses for courses and the Catalac or similar is perfect for that job – will take on the channel and Med, good stuff.

          My criteria was a sound boat for the long haul [say 15 years], which would do the English canal system [6’10”] plus the Southern Ocean, two contrasts, on a ludicrous pension [due to my own poor choices over the decades], without being the equivalent of a shanty town hovel. Wood in general and white interior paint, by the way, does wonders in that respect.

          Also one I could build and repair to my specifications and which afforded a great deal of space within that formula, with me making and repairing the sails, the masts, the skin – the only things done commercially being a sailmaker storm jib and engine maintenance, plus of course the nav lights, radio, EPIRB, GPS, flares, fire gear, propane etc.

          I do see where you are here and am by no means knocking it.

          With me, there’s also a labour of love aspect, having built virtually everything myself. The urge to build is strong, as it was in my father, who built our house and my first and second boats – not many do that these days and I’m not of his skill level but am Ok at it.

          • September 20, 2017 at 11:00

            Making your own is admirable and on a budget, more so.

            However, I will give you three pieces of advice;

            1. You’re a long time dead. I have lost count of the people I knew who had a big retirement trip planned and either never made it that long or their knees, eyesight, general health didn’t. Carpe Diem on whatever vessel exists now (or next spring) that fits the key criteria.
            2. Get the smallest boat you can be comfortable and safe enough in. Don’t kid yourself that by building something you can have a bigger boat. If you’re going across an ocean, whatever you build will be more expensive in either money or time than ANYTHING currently available on the secondhand market.
            3. Beware of hubris, arrogance and the Dunning-Kruger Syndrome. Those boats on sale right now have been proven in the conditions you’re currently designing and building for. Every boat is a compromise in some way. Better a slightly-substandard proven vessel than a theoretically-perfect unproven one.

            Lastly, I struggle to reconcile a planned journey on the Southern Ocean whilst avoiding the Bay of Biscay to the Med. Seems a little, well, Walter Mitty-esque. Sorry.

  6. September 20, 2017 at 12:15

    Ah but you’re addressing someone who has built boats before, not some starry-eyed dreamer with a syndrome. I’ve learnt which are suitable and unsuitable and am still learning after five decades at it – for example, the limits of epoxy and at each stage, I listen to those in the business.

    In building terms, it’s how much wood and glass is used, rather than the dimensions. Given X amount of materials, then one makes the largest boat those materials safely allow, the aim being longer and narrower in preference. There’s nothing in the least experimental which has been presented here – they’re all traditional methods.

    The boat goes through two tests before getting to the water – the BSC for the canal and the RYA tests for sea. Far from any syndrome, such things determine what it finally hits the water as.

    As for second hand, that is where most of the problems for dreamers come from – buying a boat which is nearing its useful end date, the GRP starts parting, keel bolts start to have issues and so on. Good money after bad.

    Commercial boats are not the summum bonum at all, buyers are often starry-eyed about this – these boats are the least it’s possible to give the buyer for some years without problems. After that they become a trap. Comes out all the time on fora.

    Healthwise, that is precisely why this design – it is easier on the health and will only be going anywhere with crew. It’s also within the dimensions the haulage company specified.

    Bay of Biscay – of course it will be avoided. If I do that run at all, it will be through the canal and Gibraltar. I might stay on canal and river, see what the future brings … but the boat itself would be capable of ocean, even if I’m not by then.

  7. Markusik
    November 11, 2018 at 19:21

    I just discovered this website, led here by an image search for biplane catamaran. Bravo, sir, I appreciate your insights and ingenuity.

    I’m likewise investigating the feasibility of a biplane cruising cat using “new generation” freestanding balanced battened lugs. You probably already know, but a fellow of the Junk Rig Association had an excellent discussion of his hinged wishbone batten (monohull) rig on boatdesign.net https://www.boatdesign.net/threads/the-design-of-soft-wing-sails-for-cruising.49425/

    My cat is looking to be somewhat different than yours, with a large bridgedeck salon, and trying to make up for my lesser experience and knowledge of boatbuilding with a larger budget. Using tabernacles to increase the effective bury, and carbon fiber (prices are approaching reasonable, Rob Denny of Harryproa fame is a good fellow to ask about restraining costs), using the salon roof as an end cap, I picture two masts set quite fore, 15.2m high (~1.9m of which is buried in hulls, ~0.8m in tabernacle, 5.6:1 bury where 7:1 is generally considered the upper limit for CF) leading to nearly rectangular sails 12m by 4m, 96m2 total. Also investigating a traction kite for off the wind. I came to the same conclusion that leeboards are the way to go. Your thoughts on six sails were interesting, but impractical for my purposes.

    I see it’s been a while since anything was posted to this thread, so I hope you are off living your dream! Thanks again for the insights and enjoyable read.

    • November 11, 2018 at 20:05

      The biplane has proven fine with Wharram cats with deep displacement, especially with junk rigs but on light, rounded hulls, the sail in the weather hull proved to be a bit dangerous and various boats bottled or flipped.

      My own idea above was not practical when it came down to it, for reasons stated in comments. If you have a heavy cat, long, it may turn out ok.

      I’ve gone completely away from the cat to the mono with floats and Bruce foils, that’s what’s currently being built, a bit like a Viking longboat with training wheels.

  8. Markusik
    November 11, 2018 at 20:44

    Thanks for the reply, sorry you aren’t on the water.

    My (pre-design) cat is currently 14m LOA, 7m BOA. The draft is shallow, the center of gravity is high, and I was thinking about rounded bilge hulls. Various steps are planned to reduce the weight (composting heads, petrol outboards in bridgedeck wells, reduced water storage offset by lots of solar and watermaker), and keep weight low and lateral in hulls. Total hull and bridgedeck windage is ~2.7m. With 3-4 adults, 2+ children, and all the trimmings I have trouble picturing this less than 6500kg, even finished to a light and inexpensive standard.

    I’m unaware of biplane incidents besides the French racing cat, and unsure how good a comparison that is to a cruising cat. With some flexibility in the CF masts, and the tendency for junk rigs to spill when overstressed, I have trouble imagining a 48m2 windward sail with relatively low center of effort flipping this. It’s food for thought though. I’ll start researching biplane cat incidents. I don’t want to use up your spare time, but any research leads you could share would be greatly appreciated.

    Best wishes for your longship!

  9. November 11, 2018 at 20:58

    Some interesting things here, especially about the Bruce foils:


    14 metres is nice but there are so many factors, e.g. fullness of the hull profile, type of daggerboards etc. In big seas I imagine you would run the windward board, if symmetrical section, with the lee mainsail, which reduces both drift and heel to a point.

    The reason I drifted away from the biplane cat was lack of reaction time as I age, wanting instant gust compensation before I can even reach out to sheet off. The undercompensating Bruce foils hinge down from their slender pontoons, one sided foils themselves in the vertical plane, i.e. lateral resistance, with 4 degree toe-in. Not radical, it’s been done before, they don’t lift the craft, only assist to the degree I angle them in sailing.

    The CF mast seems to me a must with the biplane, which puts it out of my league but they do work. Not sure anout the wishbones though, would like to see more instant depowering.

Leave a Reply to dearieme Cancel reply

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

This site uses Akismet to reduce spam. Learn how your comment data is processed.