The oceanic lateen

This is one case where Wikipedia betrays its western nature – it makes a statement directly in contradiction to that of Hokulea/Hawaiians/Polynesians.

http://archive.hokulea.com/evolution.html

A most interesting article in itself but for me, the key question was – does a sail with most of the area high up tip over more than a western sail, either quadrilateral or Bermudan triangle?  My western-ness says yes.

This is one sailing, there are many others in the folder:



And yes, they certainly lean over or heel, as it’s called. The name used for that force is heeling moment.

Yes, logically, of course there will be heeling moment with any sail heading upwind – I don’t want to get into Bernoulli here because it’s more complicated than that but there are vectors in a few directions and the ancients certainly went with these sails over long distances, long voyages.

Centre of effort

There are not all that many key rules of thumb in sails – three of them are centre of effort, centre of lateral resistance and heeling moment. CLR is not critical to this particular calculation as sideways resistance is fairly constant on my boat:

The operative CE calculation for me is how far above the mast steps is the CE for my designs? I want it within 14 feet of the mast step.

So I went back to my other designs and measured those, with the CEs coming out fairly constantly around 13’6″ to 13’10” on every design except one unisail version ditched long ago.

Difference was that on all the other designs, the area dropped off as you went upwards, the oceanic lateen’s [OL] widened.  Plus they carried a large crew of 12 to 15 on those boats but those boats were 60 feet to 100 feet long, mine is 41 feet, with 27 foot masts.

From my own sailing days, I recall my 29 foot wingmast on the A Class, which I carried on my shoulder – wasn’t light, wasn’t impossible.  Impossible now though – hence I can’t put to sea without a crew.  I wouldn’t anyway after my 2017 event.

Back to the technical things.  if you look at my sailplan there, it doesn’t actually widen as you go up, it widens as you go, so to speak, north east, it’s lower than the Polynesians’/Hawaiians’.

The CE calculation, joining middle of bases to apices and the two sails being of equal area makes it easier to calculate, though they have variable rake [leaning fore ‘n aft].

An immediate advantage becomes apparent – variable rake, where CLR is fairly constant [mine], combined with a moveable lee board from a number of places on the lee of the windward hull [shall get into why on the lee of the windward hull in another post] meaning that weather or lee helm is variable too – you can arrange thrust where you want it.

‘Helm’ mentioned here means the tendency of the CE and CLR not to align, resulting in the boat wanting to sail into the wind as wind increases [safer, more desirable] or downwind [less desirable].  On these designs, you make it what you want it to be, given the conditions at the time.  Major plus if your masts are moveable, which they would be if all stays were running stays [with cleatable blocks or pulleys at the end].

And the spinoff is that if the mast is light enough to start with [which 27 feet is] for two burly crew and can be lowered to the deck, then this is a boon when at rest in harbour and/or at anchor.

The traditional end of day furling they used was for brails to haul the sail to the mast upright and I can see that being used for temporary stops in good weather but for any length of time, such a small mast can be lowered by means of two crew on the forward stays [block and line] and the skipper behind taking and lowering to deck.

Which does get to one aspect I’ve not decided on yet.  I’ve seen the leading yard as a mast in itself, i.e. it is raised and lowered whole, braced all around during sailing … or the alternative – a short, stubby mast to the head of which the triangular OL is raised and lowered.

One advantage is that with variable mast steps, you pretty well have total control.  I envisage the heavy weather rig being deployed more horizontally [meaning lower], with the step being further forward [one of the four or so steps].  Those steps are easy enough to build in.

Reefing and furling

Furling I see as the sail being collapsed completely, either onto boom or to forestay in the case of jibs, whilst reefing is just reducing sail.  These sails do not reef – at least the ancients didn’t.

What they did was use a system of brails across the cloth and it gets into the nature of the sail now.  These sails are pretty hopeless when curved, that is – they kill thrust, which spins off into less heel [desirable], less forward motion [sometimes desirable], a desirable safety valve in other words.  I’ve read much on this and it was pretty clear that they did not seem to feel the need, even in big seas and storms, to actually reef as such – they tightened the brails and those killed thrust.  And they’ve had thousands of sailing hours to test that.

I need to explain further.  For small amounts of thrust kill, just release the sheet a small amount, but for large amounts of insta-kill, the brails are applied.  That’s clearer in this photo:

And here:

For total kill – drop the yard.  I like it.

Stop stop stop! Why do it at all?  Why this sail?

In other words, why go to that trouble when there’s a perfectly good western system available?

Well there is but check out the price of just a single block and cleat again and you’ll get the idea.

That’s not in itself insurmountable – I can make my own, using jam cleats, my own blocks involving buying only the nylon sheaves – no, the real reason has still not been emphasised.

The modern western rig is under high pressure wherever it is bounced down, the huge pressures on the sailcloth and mounting points are well known on modern boats. I wish to take much of that pressure off mounting points and off the cloth itself.

One way is the Chinese lug but this has almost no forward thrust without complicated camber induction [posts passim].

But to be even more specific, where the sail attaches along its edges is critical. Professional sails use boltropes sewn to the edges of the cloth and I can’t sew. I have neither the machine nor the expertise nor the patience, nor the space to lay out a sail like that.

There is another way and I’m hoping you can follow my poor attempt to word sketch. Imagine three strips of batten as long as the yard needs to be. The outers are one inch by three inches in profile, the inner, around which the sail edge triple wraps, is either one inch or three quarters – have to think this one through.

Straight edge of sail is tacked to the middle batten [part of the yard now] every three inches. Roll batten [with help at the other end] 360 degrees – maybe even once again if the leading edge, cloth is cheap, firmness is critical.

Lay the batten on one of the outer battens [on deck], lay the third batten on that as a ‘sandwich’ and now the securing comes in. I envisage small bolts with wing nuts but screws might do the job, even nails.

Repeat on the other edge.

Meanwhile, the upper, free edge does require painstaking sewing. I can sew using wax thread and awl on folded edges, every inch – it’s just doing all three edges that way which I baulk at.

Now can you see why the total cost of around £200, as against £3000 is enough to tip my hand, let alone the saving in manhours?

But there’s one more spinoff – at sea, let’s say a sail rips. With this system of a short stubby mast most like, leading edge yard is lowered to deck, screws are unscrewed, cloth replaced by a new triangle, rescrew as before.

Cost? Each sail is half a rectangle of cloth, which retails at £80 the square, inc. VAT. I have two others in storage onboard, precut into the triangles.

That does seem to me eminently practical and within my budget. Downside is I’d need a second pair of heavy weather sails [smaller] on their own yards [around fourteen feet] but remember this is a double hull with beams – the sails strap down to those beams, plenty of space.

Reefing again

It’s certainly workable, it worked for them, it does today [some photos were from 2015], necessity was the mother of me exploring but I can see why westerners would not look at this rig.

I feel there’s no choice for me. I’d say I’d get one season out of these sails, while the professional gives me 5 seasons.

Reefing – seems to me it’s fine having the N2 rig on deck, strapped down while sailing, or even up while the N1 is strapped down on deck – not an issue, covered or boxed against UV.

That bit’s ok, what worries me is that it’s a large jump down from 410* square feet to 210. There’s going to be a time I’ll want 300 up.

Now there is a way – I noticed in the lower apex that they allow the lower, pointy end to be loose. I wondered why. Then I saw the possibility. If that pointy end can roll back like a roll of cloth or maybe a condom, and if there is a place on the side of the mast two feet up, a boom step so to speak, that lower yard or boom can roll the sail and take 50 sq ft off each sail – there’s the 100 reduction.

However, that’s a further complication and it might even be worth, on a double hull craft, just having a third set. One maybe at 420 sq ft [the pair], one at 300 [the pair], one at 180 [the pair] and as there are six sails in that – if they’re used most days, then that’s less life deterioration for each sail. Plus using the variable mast steps, a single sail of 90 sq ft would do as the storm sail.

The bottom line is that this sail construction method is the simplest and cheapest, yet quite strong at those sail areas – the largest sail, at 205* sq ft, is hardly going to break anything in light to moderate breezes.

Stubby mast and yards or just yards?

I’ve seen both being used and both have their points. With two stubby masts and variable mast steps, the sail can actually be ‘hung horizontally’ from one mast or the other.

Against that, the leading ‘yard as mast’ is that much less woodwork, plus there is no stubby mast or stays to get in the way of the sweep. That’s attractive.

The more orthodox way, still requiring the lowering of the yards, is the stubby mast and halyard. You know – on a cruising boat where the woodwork and sail aloft is not all that much when you look at a 19 foot wide boat, 41 feet long, it does start to seem to me that the stubby mast is the way to go. It is far easier to lower the yard to deck using the halyard.

Plus one major factor. It’s far easier for me to use this triple batten system as yard and one plus is that the yard can be shaped, the stubby mast need not be.

Stubby mast it is then [within the space of this one post, ha ha], because the yard is then able to swing, rotated, in line with the apparent wind, free of the mast on the leeward side. That’s efficient going upwind and unstresses the woodwork sideways.  Yep, nice.

One other criticism – tightness or the sail on the leading edge. Yes, it does require plywood sandwich head and foot at the apices and fine poly cord lashing to the yard at either end. It means that during sailing, the tension cannot be adjusted, only when back on deck. However, poly has little stretch in the first place, which rather precludes canvas.

I’m thinking this is the way to go and this post is in the nature of thinking out loud at you, but also keeping the interested up to date. I can’t make the sails yet because the hulls need constructing first, in order for the sail to be made on top of the hulls, wrapped and tied down. I can put in the mast steps early though, plus their bracing/strengthening points within the hulls.

*Sheeting these sails

The largest sail is 210 sq ft.  420 sq ft seems barely usable in pushing the boat forward.

What I’m relying on here are the tank tests which showed a coefficient of 1.4 to these sails when the leading edge is round but 1.6 or higher when shaped [by sanding]**.  Also, a certain tapering towards the ends is apparently efficacious.

Do the calculation.  My boat requires 600 square feet of regular sail, at a coefficient of 1.0.  Apply 1.4 to my 420 sq ft = 588 sq ft [adjusted].

That is actually overpowered, as my boat is superlight and narrow [hull strength from the multi-chine construction].  I could even reduce sail from 420 sq ft. As an older man now, I’ve been looking at 540 sq ft regular, no more, plodding along upwind at around 7 knots, downwind a low 14 knots.

I believe I could get away with 400 sq ft total with the Oceanic Lateen.

** Warning – these coefficients apply only to the lower apex secured to the bow of a mono, they do not apply to a cat.  The theory is that the wind coming up from the bow and sides vastly increases flow over the leeward side.

Maybe that can be partly induced with planking immediately aft of the mast step – that’s for later.

The sheeting, yes. My rule of thumb on normal sails, end boom sheeted, is 15 sq ft per purchase. It’s not an official calculation, using pounds and footpounds but it works for me.

Thus, on a sail of 150 sq ft, at 20 knots of wind, ideally I want a 10:1 purchase. But this is a cruiser, where one reduces sail, so for the 210 sq ft, I would run a 14:1 or even 12:1.

The 12:1 is attractive as it precludes winches, plus it means a continuous sheet in a triangle, running from sailing station to boom [looking from the stern], back to that hull, back to boom, out to other hull and back and so on, eventually cleated at the other sailing station in the other hull, three mounting points on the boom, spreading the load. 12mm sheeting [soft modern poly sheeting] would be fine, through 14mm sheaves.

And this design sail reduces clutter, all lines leading upwards, not cluttering the deck.

Repeat for the other sail. KISS, no mechanical parts needing servicing. The boom attachments are snap hooks to bridles, unclip when changing rigs – N1, N2, N3. Quite workable with crew of two plus me.

Incidentally, there are four cabins at each corner of the craft [another post]. Nuff for now.

Er, one last thought. Having read that Hawaiian article more fully, the idea of the scalloped trail edge, not only beautiful but functional, along with extended, tapered tips, does seem indicated.

4 comments for “The oceanic lateen

  1. woodsy42
    May 23, 2020 at 15:20

    I know nothing about sailing James. But used to enjoy applied maths, so an interesting puzzle that is way beyond my abilities to solve! Just looking at the sail as a flat rigid triangle then my intuition says your ‘center of effort’ would be roughly where the center of gravity would be so the ulua upside down rig would be higher and heel over more. But the sail is not 2 dimensional and air pushing at the sail has to spill round it, intuition suggests the ulua design will spill more air upwards (as the unsupported sail edge points more upwards) than a standard sail. Would that affect the C of E, directional stability or maneuverability of the boat- better or worse? Also (as a non sailor) I wonder if the wind is stronger or more/less stable by having the sail spaced higher from the water and any turbulence from the boat hull?

    • May 23, 2020 at 16:44

      The CE in relation to the CLR and in its height can certainly change with altered sail shape through twist. If you have a modern monster where the boom is cranked down to a track using bracing at both ends, then the CE is not going to substantially rise and you pay £big£ for the privilege.

      The OL is an amateur sail where everything flexes, so it will rise. But as you say, it’s the air spillage or ‘exhaust’ which also counts and the flex can spill wind before the human hand can touch a rope. It’s better on a multihull as someone said.

      Tied in with this is underwater hull shape. If the boat tracks well and sits low, it will heel more quickly, the round bottomed craft will slide and it takes more to push it over. However, that side slip is countered by a centreboard or leeboard.

      So it’s not just the sail contributing to heel. Another contributory factor is sheer boat length. I’ve gone ultra-narrow and very long, which gives form stability. Ditto with the Hawaiians. Also better for big seas. My A Class was like that – 18 feet long, 7’6” wide, well under the recommended 2:1 length to beam.

      The nice thing with this sail is instant spillage by itself, followed by the brails, followed by the furling brail if in trouble. Low slung hulls help too. Downside is it will win no races, the foil is not kept taut. She’s a cruising sail or to be more specific – voyaging.

      One thing I’ve done since the post is scallop the trailing edge, which brings CE down further amd looks better anyway. It’s all a compromise but it’s not bad, this one.

      I’m not putting any self steering in, so one person will need to helm, which means short hop after short hop but it also means instant sheet release. I’d envisage at night running just one N3 sail and bumble along.

  2. May 23, 2020 at 16:54

    The white cat has vastly more sail up than the light one:

    https://youtu.be/LeKPRmhVuvM

  3. Doonhamer
    May 23, 2020 at 21:03

    Intuitively I think that wind speed at deck level will be less than that more than 10 feet above deck -friction turbulence etc.
    Therefore it makes sense to have a sail with minimum area close to deck, such as ones you talk about.
    Also less chance of being stunned or knocked overboard by a swinging boom.
    Thank you for your explanations.

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