The tyranny of design [1]

Most people I know have at some time harboured thoughts about a little yacht if they could afford it – to take out work colleagues or bosses, the wife and family or indeed, to escape all of these. Designing for such people is a challenge and a half.

There is a mathematical tyranny to the physics which says that if you don’t design sufficient buoyancy into the hull because you’d prefer sleeker lines, the boat ain’t gonna float. Same with rig, same with wetted area and drag. It’s annoying that just when you think you have the solution to an insoluble problem, physics comes along and:

1. defeats you
2. shows you there’s nothing new under the sun

Everything in sailboat design is a compromise and depends heavily on:

1. budget
2. the use you intend for the craft

… plus a host of other design considerations.


Size does matter and what you can get away with for a coastal hop is not what you need for an ocean voyage – that stands to reason.  There is a rule of thumb:

The largest wave you’re likely to encounter should be around 55% of length overall [LOA].  [Thanks, Mark]

If you look at the bar below the map above, you’ll see that the maximum wave [typically] is 40 feet.  Very roughly, take a bit less than that, say 35 feet and double it – that’s the length of boat you need – 70 feet – and in realistic terms, it’s a pretty good rule.   James Wharram’s Pahi is 63 feet:

… whilst Francis Joyon’s IDEC was 90 feet plus:

Naturally you don’t absolutely have to have that length boat – Chichester’s was 54 feet:

… and it was only poor design which made it a dog to sail.   We’ll come back to keel design later.   So clearly, money comes into it in a big way and most of us can kiss goodbye to round the world passages … or can we?   Abby Sunderland’s, Jessica Watson’s and Laura Dekker’s boats are all around the 34 foot or so mark.   So it is quite possible to sail one of these but in the really rough stuff – it’s going to be difficult.

What’s the median length?  Most people opt for around 26 to 28 feet – the best compromise of budget and crampedness in the cabin, using it for daysailing, overnights or weekends away, close to the shore or island hopping.   A rundown on what each length enables might be:

16-18 feet:  Trailer sailors …

… and that sort of thing are good for overnights for two people, typically husband and wife and maybe two small kids.  No more than that though.

22-24 feet:  This is most people’s entry level craft, usually off the shelf in a popular class.  I sailed on a Cal 22 on English Bay in Vancouver.

26-28 feet:  near the limit of most people’s budget and here we come to another rule of thumb:

Costs increase incrementally until about 28 feet and then you could expect to double that for every five feet thereafter.

So a $40 000 28 footer becomes an $80 000 34 footer.   Not always so and Ian Farrier can get you onto the water for around $50 000, ditto James Wharram.   A yacht-yacht though will cost.   There are all the extras, from pumps to heads [toilets] and engines – the inventory covers more than one page of A4 and it all costs.   A winch can cost $900 and you need four of them.

Pausing one moment at the 28 feet mark:

… this would handle 15 foot waves, which are the most you’d expect in coastal sailing and such craft have been known to cross the Atlantic and between Pacific islands.   As Ian Farrier points out though, these are small boats in ocean terms and wet.

By wet, he means there aren’t areas to shed wet clothing properly plus there is a thing called freeboard.   Freeboard is the distance between the waterline and the gunwhale or top of the hull [not including cabin roof].   Rule of thumb I go by, which is more generous than most designers:

One foot of freeboard for every ten feet of LOA.

Why do most designers not do this?   Very good reasons:  if you take the lines of the boat [the sheer], then if it dips to four feet on a 40 foot boat, at the bow it might be six feet and that’s a a lot of weight and also windage – the nose will always wish to head downwind.  More than that, if it’s a multihull, then the flipping danger increases incrementally.

On the other hand, I’m not sure about Wharram’s 34 footer – it seems way too close to the water for mine:

In absolute terms, I’d like to feel I’m about four feet above the water, so that allows me a 34 foot boat minimum.

What do you want it for?

An absolutely vital consideration because it is upon this that design begins.   In my case, I plan to cut corners.   There’s a derelict barge I’ve seen and if I can secure it and get it to a launching point [after epoxying], then the process which can take some years has begun.  Eventually I’d buy a pair of old catamaran hulls no one wants any more and they’d become my trimaran floats.   Eventually cosmetics can be added to the bow and stern to bring it into line with the shape of the rest of the craft.

Where would it go, once seaworthy again?  Anywhere, including broad canals in England which means the beam overall needs to be 14 feet or under.  Seven foot wide main hull [vaka], three foot six wide catamaran hulls [amas] and there is 14 foot wide for closed waters.   On open waters, the hulls opens to 24 feet as a trimaran.

Cost?   Well as I intend this to be my home, this is an ongoing thing and doesn’t have to be spent all at once.  Plus I’ve already found my mast and gaff.   For most people though, it’s not their home and so they want it designed “properly”.   So let’s get designing.

What do you want it for?   Let’s assume you don’t know or perhaps it’s for crossing the channel to get cheaper booze and fags, perhaps for weekend trips.


Let’s settle on 37 feet, which is what mine will be.   That usually carries a 12-14  foot beam in a monohull and that’s the number we’ll hover around.  Now we can run a monohull, a traditional yacht, 14 feet wide and it’s superb – bags of room inside but it lacks privacy.  This is one of the key criticisms I have.  I want my private quarters and though one can’t demand such in a boat under 60 feet, I want them – so there.

Plus, a monohull leans over and I’ll be damned if I’m going to sail around at 30 degrees heel [leaning over].   Plus I’d need a deep keel of lead and that adds weight and cuts where I could use the boat – shallow water capability is a big plus.   Many old British designs use a shoal keel [or a shallow keel running the length of the boat] but lose righting power [the ability to stay up and down] in the process.

The motion is not as nice in a monohull – it tends, with its tubby lines, to slam down on waves and either dislodge teeth or give seasickness.  Multihulls reduce this if designed well.

However, in northern seas, such as around Britain, the sea state and conditions are often so dire that the chances of being knocked down are high.  A well found keelboat [meaning a safe boat for the sea] has an advantage here – it can be slammed down but it will come back up again.    Any multihull will turn right over and that’s where it stays until a rescue craft comes along.

Unless you plan against it, of course.  More on that later.

So, on sheer ability to meet the waves, track nicely and sail without constant slamming of the hull on the wave ahead, with minimum chance of flipping if sailed well, t’s hard to go past a trimaran.

There’s an issue with just taking a conventional boat and tacking amas on it.

The whole point of a tri is minimum wetted area, so usually they have two narrower, useless hulls at the side that you can’t live in [see pic above], plus a narrow central hull down at the waterline, broadening out higher up and the accommodation is cramped and spartan.  Most tris have a central fore and aft area walkway, with collapsible table and then berths either side, up high near the beams.

If I was racing – fine – but I’m not and I don’t like the inside of a tri.   I’d rather sacrifice speed – and incidentally the chance of pitchpoling [end over end] by bringing the rig down to my rule of thumb:

Taller mast is no higher than 0.9 [above deck] of the LOA and there should always be more than one mast.

This spreads the load along the hull more evenly, meaning less stress points, less breakage, more easily handed sails.   Which brings me  to the next rule of thumb:

At my age, I don’t want to have to handle any sail over 250 sq ft, despite winches, despite all mod cons.   Anything above that is a big sail for me to handle alone.

A 37 footer needs between about 650 sq ft of working area and about 1100 sq ft for racing.   I’m pitching for about 800 sq ft.   So I can run that as 4 x 200 sq ft or maybe as 150 and 150 up front, then 270, then 230 mizzen [at the back].   Eminently handlable and allowing of a freestanding pair of masts, resting inside the cabin on the keelson.   The presence of a dirty great mast in the middle of the cabin might be a horrific thought unless it’s designed to be up against a partition between two living areas.

In fact, I’ve designed the living areas around the position of the masts.   I placed the masts first along the 37 feet, then did the rig, then took care of the living spaces and cockpit after that.   So not only does the mast not get in the way, you can attach things to it, hang towels etc.

Which is one of the reasons not to go for a catamaran – with a cat, the mast balances on a bridgedeck crossbeam.   Here are some other reasons:

1.  Ian Farrier says that under 40 feet, tris are the design of choice but over 40, the cat has much going for it.   The reason is that the hulls of a small catamaran are too narrow, too low to contemplate living in the hull.   Over 40 feet, it’s possible to design standing headroom [6’2″] into the hulls without them looking too highsided and ridiculous, let alone the windage.

2.  Plus one other thing.  Cats under 40 feet are quite lively.   Whereas a Pahi is so heavy it just will not lift a hull, a small cat will, in a gust, lift a hull alarmingly and if the wind gets under and catches the underside, over she goes.   Even large cats do that when racing or speed record chasing:

I for one am not prepared to risk that.

A trimaran works on a different principle.  In a gust or knockdown, the first reaction of the leeward float [or ama] is to be pushed down into the water [see the pic of the old French tri in dirty conditions below] but as they’re buoyant and designed to do that, the boat springs up again once the gust passes.   Sure they can go past the point of no return but this is very rare and tris are rated among the most seaworthy, certainly under 40 feet.

And if you have a righting system in place, then it’s not a disaster.  More on that later.  Tri designers will point out that if I make the amas 3 feet six wide [to keep the overall beam under 14 feet], then while not exactly like sleeping in a coffin, it’s not all that comfortable.

I beg to differ.   Remember there is a 37 feet boat, with 28 feet of usable cabin space, not loaded to the ends.   Inhabited amas give you an extra 21 feet of usable and livable space in each ama and that makes a total of 70 feet, which is the usable space of a 90 foot monohull.   That is awesome.

Then why don’t tri designers do it?

1.  Cost.   Three hulls cost almost three times what one hull does, so they say.   Actually, if the central hull is 7 feet and a normal monohull is 14 feet, then it stands to reason that the tri’s central hull will cost two-thirds or thereabouts.  Each ama is about a quarter the cost of the monohull, making the whole thing about 14 twelfths of the cost of a mono.

More but not three times as much and as I said earlier, the cost can be spread over time, as and when one is ready.  The boat will sail minus amas until then, if wide enough.

How?   Using low, long sails for now, totalling about 300 sq ft max.   Much less weight – much less sail.

Plus tri designers tend to design around orthodox notions of who sleeps where.   With three hulls to fill now, why should anyone sleep in the amas anyway?   Fore and aft cabins in the vaka, separated by a central cockpit and galley are fine – storage and sitting areas in the amas, plus the loo [head] transfer much of the clutter to the sides and improve weight distribution anyway.

There’s no perfection, only compromise and this is about as good as we can get a tri.

2.  If I use a long, low rig, then the underwater foils need to be shallow and long too, which makes the boat track well but ruins its manoeuverability, one of the tri’s strong points, as well as its fabled pointing power [it’s ability to sail into the wind].

Answer?   Two leeboards either side of the main hull [the vaka].   Leeboards are asymmetrical and take care of that problem.   Plus they don’t look unsightly as they are covered by the amas from view.

4.   The main objection though to my cunning plan is that the whole idea of amas is that they are small enough to absorb gusts and buoyant enough to push the boat back up – they are a safety valve.   By making the vaka and amas closer in size, it’s no longer a trimaran but a floating pontoon and the wind will treat it like a catamaran [the reduced central hull weight contributes to this] just as it does an Orma 60:

In the end, it comes down to aesthetics.   Some people just cannot countenance the idea of a beautiful traditional craft ruined by “training wheels” stuck out the sides.

It needn’t be like this.  No one’s taking a Colin Archer and altering it.   A Thames wherry is a far more amenable craft because it is long and straight:

If the fold-in amas are of similar shape and design, it wouldn’t look out of place, in the same way that a traditional Wharram design is authentically Polynesian:

… and not some aberration or hybrid.   In other words, designed from the keel up as a traditional shape, three hulls need not be an eyesore in the least.

More in Part 2.

2 comments for “The tyranny of design [1]

  1. July 28, 2012 at 17:24

    Fascinating thank you.

    (small point, I think you mean that the largest wave should be 55% of LOA, not the other way round?)

    As an idea regarding righting multihulls, you can get dinghies that self-inflate with a gas cylinder. Could something like that attached to the top of the mast at least get the thing on its side?

  2. July 28, 2012 at 18:14

    Indeed possible if it’s stored at the mast step and then forced down to the masthead by halyard. With two masts – twice as effective. That can be combined with the ship’s boat having an outboard.

    Most tris [won’t work on cats] have collapsible floats, either concertinering or swinging aft. Mine keep the floats vertical and slide across the deck, which is possible with 5′ struts and a 7′ vaka beam. So the floats slide up to the vaka.

    Now if lines are already in place [concealed for windage], the floats are brought into the hull, your idea of the buoyancy at the mastheads is employed, the rig is easily released [gaff or lug], the leeboard nearest the ship’s boat is down and the other up, then the ship’s boat has a relatively simple task of powering the boat back up again.

    As the instances of tris flipping is so small and only when sailing for high speed [instead of cruising], with enormous reserve buoyancy stopping it going right over, then if it does happen and you’re cruising, you allow yourself a few hours to have it all in place, then up she comes.

    If you fail, you get inside the vaka [designed upside down], eat well, sleep and next day try again. The boat will not sink but stay there, upside down, presenting an aerodynamic face to the wind.

    The flip [or turning turtle] is one of the two disads of the tri, the other being burying the bow but once again, it’s all in the design and conservative sailing, reefing [reducing sail] early and conserving what you have.

    Tris have an excellent safety record as it is, so with these large moves to safety features [which reduce speed and that’s why designers don’t usually bother all that much], then it’s more difficult for the boat to come to grief.

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