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The Proas of John Pizzey - Part 3

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Pi - an early proa with a force balanced configuration - the windward canted rig balances the overturning force.

I started writing for Proafile because over the years I had seen several comments about this mysterious Mr. Pizzey! No mystery really, just a lack of information, so I thought I should rectify that and record my proa adventures. By the time I had finished putting pen to paper and thinking about what I was writing, my new design became clear as it had to meet the objectives I put down in writing. The design has been done conceptually and I will soon post a full description.

I have come to the conclusion that my next proa should be a flying proa, with a relatively narrow beam, with the rig laid off to leeward and with a soft headsail only and a canted central board in the float. Why?, because:

  • flying proa with board centrally in float so it sits preferentially with the sails and main hull downwind;
  • narrow so it lifts the float easily and minimizes everything, a bit like a monohull and one can’t question their success;
  • canted board so it assists initial lift of float and enables unskilled float-up sailing;
  • rig laid off to leeward so it works with the canted board and makes the boat instantly self-righting from a knockdown.
  • headsail type sail only, for simplicity and at one end only for spinning boat around when caught aback, with sheet on or off.

I want a simple, compact, user-friendly, pleasurably fast craft which is welcome at marinas. Approaching 70 my wish list is a little more conservative than previously.  I appear to be naturally asymmetrical with fast sidecars (self-designed and built of course) being another passion of mine.

Below are a series of drawings comparing a proa with a leeward canted rig as per Piawatha and Flight and a vertical rig.

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As a monohull heels, two things happen, the heavy keel pivots out to increase the righting moment and the force from the sail diminishes as the boat heels.  From what I have observed most classical yacht design does not consider this sail force reduction during design, they use historical data to determine the sail area v righting moment.  There are too many factors otherwise.  However it is generally agreed that the sail force going to windward diminishes as the square of the cosine for the angle of tilt. If the sail force is 100 units when upright it will be 80 units when tilted 15°, 57 units when tilted 30°,  and 35 units when tilted 60°.

Thus if the sail is laid off 15° in your proa, you will only have 80% of the sail force to windward when heeled at 15° but when heeled 30°  it will be reduced to 57 units versus 80 units for the vertically rigged vessel. When the sail is horizontal there is no angle of attack with the wind, it is effectively feathered into the wind, this make commonsense and mathematically (cosine 90)² = 0.  You will still have the full power of the sail for all other points of sail other than close winded.  Thus a big headsail (20% oversize) wont overpower you to windward but will certainly assist on a reach and downwind where you make your best speed.  Of course if you made this angle adjustable more extreme figures present themselves.  It would also enable the mast to be pivoted inward for more friendly rafting up with keelboats and multis.

In the comparison the vertically rigged vessel is on the left in the left hand column and at the top in the right hand column. The larger view shows the inclined rig vessel flattened with the centre of buoyancy shifted out and illustrating that a positive righting moment is retained from the weight of the vessel through its centre of gravity. Thus it can be designed to self right.

Both vessels are paired at the same angle of heel so both have the same righting moment at all comparisons.

Having well flared topsides on the main hull ensures that the centre of buoyancy of the knocked down craft shifts outward toward the cabin so that the righting moment increases to assist self-righting from a knock down but definitely not from an inversion, which you should try to avoid. Many, such as Russell Brown use a side pod for this purpose.  However I prefer to use flared topsides for reasons already expressed together with the canted rig. You can sail this configuration at extreme angles without dragging a pod in the water which I imagine would tend to slow you down and skew you downwind. Interestingly, having a masthead offset to the outside of the main hull may just provide the starter for righting from an inversion, a flooded float and a buoyant masthead, worth exploring?

When caught aback the heavy hull is upwind and if the sheet can be released the sail will flag harmlessly. If the sheet can’t be released the sail pressure will spin the craft around to a float upwind attitude from which you can move off again by sheeting in.  Don’t forget that in normal sailing mode the craft is relatively tender by comparison with a typical multi, so when caught aback the craft is put into a much more stable mode than it was while being sailed because the heavy hull is now upwind and well able to resist the overturning force generated by the backed sail. Thus overturning is most unlikely unless there is a sudden bullet just as you are caught aback. Even then it’s unlikely. Rotation about the centreboard back to a normal sailing attitude will also occur, but less quickly, with the sail flagging, and of course you can always sheet in to hurry it up.

If the windward hull is lifted when caught aback, the canted rig, or any rig for that matter, may catch you out by elevating rapidly as the hull lifts, rotating about the downwind hull, coupled with decreasing righting moment, an unpleasant characteristic of a multihull not shared by a monohull. As the rotation progresses the underwing becomes exposed and it may all be too late. It may never happen and probably won’t especially if the old multihull maxim “reef for the blows and not for the lulls” is followed, although I always had difficulty with that.  It can all be designed around, but if you want light weight and consequent cheaper construction it is a factor to consider. In my proa with an inclined rig you can ignore that maxim and reef for the lulls and I like that very much, it take most of the anxiety out of sailing and it is great fun heeling way over in puffs. See photo of Flight sailing.

There is no perfect solution to this quandary, any multihull can tip over, some proas can recover from a knockdown and of course these considerations do not address a stern over bow inversion. Design is a matter of considering all aspects and coming up with a good compromise and there are many factors not discussed here.

The difference with my configuration being that you will be sailing in conditions that will only tip a relatively tender vessel as opposed to sailing in a stronger wind needed to tip a very stiff vessel.  Thus when it occurs everything is calmer and less dramatic. This characteristic may suggest a submersible float is desirable so wind is spilt from the backed sail before inversion of the craft occurs, again the Polynesians knew what they were doing with their log on stand-offs, their designs being no fortunate accident, but then a submersible float can also be tripped over. So ultimately “ya pay your money and ya take ya chances” but it’s great to have good reasons to be confident in your selection.

Speaking of submersible floats, I have sailed Pi (float downwind) on many occasions in strong winds with the float completely buried about a foot below the surface. This usually started by punching the float through the back of a wave. This did not appear to slow her down that much, in fact we were usually flying when this happened.  We never released the sheets or rounded up, just kept trucking and eventually the hull would pop up again some 100m or so along the way.  We would all just look excitedly and with delight, watch and wait. The inclined sails certainly damped out fore and aft oscillations which I believe assisted this characteristic. I would love to see a light version of Pi, I think it would be a screamer.

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Prior art: diagram of windward canted rig vs. vertical rig.

It seems natural that we all want extremely wide multis for power and accommodation. However, one should avoid equating narrow beam with slow speed. In a narrow proa, the resistance when sailing on one hull is minimised and there is plenty of volume in the float to add water if you want that extra stability. Narrow beam also enables a lighter weight to be achieved and with careful design you can spend a lot of your time on one hull while relaxing (not worried by a capsize) at the controls and I really like that. When under kite with the head of the sail right back I suspect you won’t be relaxing, (I am ahead of myself here but you will understand when you see my next design). In strong winds it will be your nerve which backs off before other factors intervene. So overall you may not have that rocket speed, although with kite up ??? but your average in all the conditions you choose to sail in should be very good.

Don’t get me wrong, I love fast outrageous boats that work, that’s my history and if I could have an updated Pi tomorrow I would probably jump at the opportunity. However my conclusions are for a craft I would like to own now having been through all that experimental stuff.

The inclined rig as per Pi also makes an interesting study. When caught aback the sail force is downward within the bounds of the overall beam,  or close to it from memory, so it was never going to be overturned by being caught aback.  I don’t recall ever being caught aback but it obviously happened, she was just so easy to get back on course that such events weren’t scary and haven’t left an indelible mark in my memory. Her limitations were that it took more than a zephyr to pop the sails upward to their operative airfoil shape, especially if made heavier with rain, and they were heavy, 12oz from memory, because we hardly, if ever, reefed them. Thus she was a dog to windward in really light airs, but good on all other points.  In addition the sail area is limited by the beam, so a wide overall beam is required and it has to be very strong to support the rig.  Pi had 2 x 300sq. ft. sails and weighed about 6500lb.

imageAnother difficulty in designing proas is trying to get a symmetrical appearance that puts a smile on your face when you row out to it or glance back at it just one last time when departing.  This is especially so when incorporating a diesel auxiliary which is extremely desirable in a cruising boat.  Incorporating an auxiliary engine which provides electrical power generation and/or gas circulation for refrigeration is another great quandary in a sailing proa, but there are now many inexpensive options available.

As always, my new design is a compromise because It will be made by extending my 24’ Flight to 30’ or thereabout. My initial Pi design was very spaceshippy.  In reality I managed to convince my great mate John Gross of Fastback Catamarans fame, not to mention his home designed and built jet aeroplane, to mould out sections from his moulds to create Pi as per the attached photo of a convoy delivering his moulded parts which we floated through the mangroves to the front of our house where they were assembled by John.

 

Posted: 16/Feb/2013

5 Comments

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Luomanen said:

This is super fascinating, John.  I have never thought about the idea of a stable knock down in a multihull.  I can’t say its particularly comfortable thought, but its certainly one I never considered before.  Does the tack of your jib change ends when you shunt, or do you hoist another jib?  Also, I’d love to hear more about what kind of steering this hypothetical proa might have.  Thanks for sharing your experience with us!  It is much appreciated.

Posted: 19 Feb 2013 - 5:15
Pizzey said:

Hi Chris, my isosceles triangle sail has 4 lines only, the hoisting halyard for hoisting the sail to the masthead, the flail line threaded through a few central eyelets in the sail and which passes from a shackle slidable about a longitudinal strop or track to a further halyard so it can be tightened or slacked off as desired, and a sheet from each lower corner. Pull the front one tight to tension the leading edge of the sail as required for a luff and use the rear one as a sheet.

In Flight I had ropes connected to the pivoting mast stays so that the mast movement from one end to the other raised the front rudder and simultaneously pulled down the rear rudder.  The sheets from the sail passed through blocks on strops associated with these ropes so that as the rudder was pulled down the rear block was pulled out to achieve the desired sheeting angle.

I don’t think it would work very well with a fixed mast,  as has been previously suggested but I could be wrong, as the rotation of the sail during pivoting of the mast puts the rear sheet position more or less in transverse alignment with the tack position at that end.

Posted: 19 Feb 2013 - 6:17
multihuller said:

Hi John,
as an extension to the leeward canted rig I would install a telescoping mast. The upper part is hold by bungees which allow the rig to heel. It should be possible to arrange the forces, that the ama ‘skim’ the waves without flying to high.
http://www.multihull.de/proa/proakent.htm

Posted: 21 Feb 2013 - 7:33
Peter Fynn said:

Hi John,
Fascinating ideas - thanks.  What do you think of the following concept:
Create a deck for the float (ama) with a bath shape in it.  The bath would have an open drain to the sea.
Bad weather will fill the bath with water providing a greater righting moment (water ballast).  When the bad weather passes the water drains and the float reverts to being light again.
Clearly the bath volume and the drain size would have to be established by experimentation.
Passive water ballast!
Peter

Posted: 23 Feb 2013 - 11:30
brock1007 said:

Thanks for inspiration, John; concerning the rudders I´d like to suggest the following solution, which works quite well on my 6 m proa: I fix two asymmetrical foils ( thus providing lift to windward and also lifting the main hull up, as standing oblique ) on the leeside of the vaka by each two silent blocks, usually used for combustion engines´reducing vibrations. These rudders are connected with extension-bars on a traveller in the middle of the deck. They are working fine and precisely; they don´t break or are severely damaged, when ground contacted, and they are easily lifted before beaching.
Kind regards
Dieter Brockschmidt

Posted: 27 Feb 2013 - 2:38