What governs the size of the boat?

Decided finally on a round stern – not a canoe, not a pointy but more of a rounded. Rudder is ready to go on once I can get the pintles and gudgeons.


Anyway, thought I’d give you a bit of the thought process in deciding why the boat is the size it is and the shape.

a  Breadth. This was already determined by Canal authorities – it had to be 6’10” wide. Now I don’t know a yacht of any size which is 7′ wide roughly. They start about 8’6″ and go to about 12′ for my size yacht.

So obviously, for it to fulfil one of its three purposes – sailing boat – it needed pontoon type thingies as close to the 6’10” limit as poss. In other words – straight sided at that breadth.

That, on the other hand, allowed me to run a 3′ wide centre hull, not unlike a trimaran and also like a trimaran, the greatest headroom would be down the centre – 6’2″ to 6’10”. On either side would be the armchairs, benches, beds etc.

The deep V down the centre [for seakindliness] then sprouts wings to the outer edge and as the boat heels [leans over], the windward wing would actual be out of the water. All the stress would be on those two pontoons on that side, flat against the sea and acting as keels.

Because they’re so critical, I’m building them using 8×2 structural timber with glassed ply over that and then cladding over that.

The frames of those pontoons are every 1′ and they stick up above the pontoon to the gunwhale as uprights. So they also serve as buffers. The overkill in the pontoons contrasts with the lighter build elsewhere, e.g. internal walls.

On the hard standing, the four pontoons and the centre keel [12×2] support the boat.

So, given all this, there are many factors determining the length.

b  A rule of thumb says a 75′ yacht is a luxury, a 60′ will take most everything thrown at it at sea, a 50′ is what the investment banker might afford, a 40′ is still fine at sea if it’s designed and outfitted properly.


c  There’s a rule about waves. A wave in the open ocean which is 55% of the boat’s waterline length is going to do damage, if it breaks. Therefore, a 75′ boat can take around a 40′ wave, which is about all it’s going to generally meet. So that’s a good length.

A 40′ boat can take a 22′ wave and as most oceangoing boats are plus or minus around 40′, they’ve shown that most conditions can be handled by 40′. You just wouldn’t want to do Cape Horn on a bad day or south of Australia.

d  There is load carrying involved. It’s not that a small boat won’t get across the Atlantic, it’s how much water and fuel it can carry. And that’s all weight which has to be calculated in, plus it loses that extra ballast as the trip goes on. You’d want it not to make a big difference to the overall weight and you can only do this with a larger boat, say 50′, preferably heavy.

e  Money. I was never going to be able to afford a 40′ yacht, I’m not in that league, not these days. However, remember that such a boat is around 12′ wide and mine is 7′ wide. The materials saved, especially if I build it myself, can extend beyond what I could afford normally.

So what could I afford to build normally? With the lump sum, about 26′ of orthodox boat for coastal sailing. But if you do a quick calculation – let’s say that boat is 26′ by 10′ wide. To build it oneself, lop a third off the cost and put that on the length. That gives me around 39′.

f  The rule of thumb for length to beam is about 12 or 13:1 for an off the beach catamaran per hull, i.e. fast. Apparently it doesn’t help to up that ratio any further. Mine is 13:1 for the centre hull and 5.59:1 if the wings are included but that’s sitting flat in the water. As it leans, it goes back to around 7:1. For a cruising monohull, that is a fast design.

Common cruiser ratio is around 3.5:1.

g  Headroom. Working on 6′ of boat for 1′ of headroom, a 36′ boat gives you 6′ HR but don’t forget supporting beams for the deck. 38′ gives most humans room to stand full height.

h  Freeboard or the distance from flat sea to gunwhale.  Rule is 1′ for 10′ of boat. So 39′ needs 3.9 to the gunwhale, leaving about 2.3′ cabin roof or deck.

i  Cockpit drainage. You ideally want the cockpit 1′ above the WL – mine is about 10″. It’s not ideal but acceptable. A 26′ boat would have the cockpit about 7″ above the water.

plumb stem

 UK Regulations. In the UK, any vessel under about 45′ is considered a pleasure vessel and does not come under a different set of commercial regulations. It excuses you from so many regs and fees that it’s crazy, IMHO, to have a UK reg boat over 45′ on small money.

k  If you’re under 12 metres [39.37′], you also get leniency with the required throw of lights, lights you must have, fire extinguishers, bilge pumps etc., so it’s not good to go over 39.37′ .

l  Canal regs also have a band from about 37-39 feet so this is within that region.

m  Sail. Ordinarily, a 39′ yacht will carry around 750sq ft working area. Even if I split that onto three masts, it still confronts me with 250 sq ft per sail. And frankly, at 7′ wide, the boat would need an amount of ballast which would seriously tip the ratio. Usually you want 30-40% ballast. With my much lighter boat, it can only carry a ton and a half and that, along with beam, severely reduces the sail area I can carry.

I worked out I could get away with 520 sq ft, which gives me about 173 sq ft per sail which is still over the limit which I, alone, can handle. Unless, that is, I run a Chinese style lug with battens, in which case, the halyard on a winch does all the work. Will that amount of sail power a 39′ boat? Not a normal one but this one it might.  It’s narrow and light but against that is the drag of the pontoons – a necessary evil.

It’s probably underpowered but I’m not getting any younger.

n  Outboard. These are so expensive. The firm I am using has 15hp as its top of the range. Remembering that this is only an auxilliary engine at sea and only needs go 3mph on the canal, the accepted rule is 3hp per ton. As my boat will come out to around 3.5 tons, I could have done with 9hp but actually traded it in for 15hp. More than enough.

25hp would be too much – too fuel guzzling, requiring larger tanks.

0  Linear space requirements. In a normal yacht, there is width to put things in but on a narrow boat, things get linear, so to speak. Using 6’3″ as a cabin length, i.e. the length of a bunk, also looking at a 9′ salon, 8′ cockpit and aft deck, 2’9″ loo and shower room and around 3 feet at either end, it comes out to around that mark again- 38-39′.


p  Privacy.  At any one moment, there are 7 separate areas – front deck, two points on the cabin roof, cockpit, two cabins and salon where you could be on your own, not counting the loo, in reasonable weather, reading a book, sunbathing.  With a crew of 3, this gives everyone above two choices at any time.

Privacy was a major factor with me. You want to know the main reason? When my guests wake up in the double cabin up forward, I want to be through three doors at the other end of the boat so they can’t hear me snoring.

So that’s the rationale.  It’s turned out to be a tad over 38′ long, which is plenty long enough for me and would be OK also carrying a couple with maybe a child.  That really is the limit though on voyages.  For day sailing, you could have 6 seated but I’d not carry that – for a start, I only have four lifejackets.

The ideal is three, to me, so that those two have each other’s company while I sail, two is good because it’s another pair of hands on deck, one is more dangerous but the systems allow it.  It’s OK but I don’t fancy it in channels and around other shipping.  On a canal, you’d want two.

This design of mine is specifically for two scenarios. One is me onboard and a couple up forward. The other is the design being sold [maybe down the track] as a husband wife combination – the most common crewing of cruisers around the world.

It would be dead easy to reverse the cabins and run a twin or foldout double where my cabin now is and a twin up front for children or a double up forward and a twin where mine is. I’d dare go so far as to say it would be ideal for those combinations and affordable too.

And no, I’m not touting – I have to see how it goes over a couple of years first.

5 comments for “What governs the size of the boat?

  1. The Jannie
    August 6, 2014 at 08:29

    Is it easy to insure a self-build? In so many areas insurance companies give a sharp intake of breath – and reach for the calculator with the big numbers on it – when faced with something non-standard.

    • August 6, 2014 at 10:06

      Needs a certificate. On the canal, that would be the Boat Safety certificate. On the sea, that would be the RYA blessing. Insurance company accepts this. They need certificate, dimensions and intended usage.

      Also, one goes to specific boat insurers. Slightly higher but they know the score and know what to ask already. Vast majority do it this way.

  2. dearieme
    August 6, 2014 at 12:54

    I sees that our lovely new aircraft carrier is too wide for the Panama canal. And, of course, its proposed new planes are so useless that they might never enter service. We should flog it off to the Russkies quick.

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