The Secret Instrument on a Sail Boat

We have had a spell of dreadful weather, which is very unlike our usual serene Septembers here  on the South Coast of Norway. Yesterday though was more like a typical such day with light airs, blue skies and a sharp sunshine so typical here either end of the summer.

So we jumped at the chance of  a practice sail and seeing pretty much literally, where the wind would take us, in the 12 sq meter classic design. 

Upon reflection the sail made me think of three things – hull and keel speed, spinnaker work and learning to re-learn. Let us ponder upon the former.

On the Subject of Sailing Badly in Light Airs in Classic Boats

The 12 sq meter design is indeed a mini twelve mR designed as a training boat for youth of the wealthy on Oslo Fjord. It soon however became a favourite day boat of adults looking for an easily crewed regatta machine. King Olav had an early  ‘Fram’ 12 kvm indeed as a young man. On some of my previous outings in these elegant classics, much prettier it has to be said that most all of the UK’s one design day sailers, I have sailed very badly. Or been made to feel like my sailing skills were somehow thrown out of the window.

Coming back to the class after some spats and a very poor nationals in 2010 (the boat had mussels on the keel and when I first inspected her sails, GAFFER tape fixes on the spinnaker! ) I took these former failures as a challenge to learn the boat and prepare a decent example, and crew, for racing.

The burning light in the revitalisation of the class as a one-design with  our local centre of gravity, has been the boat builder and all round craftsman in wood, Petter Halvorsen. He like others before gave me a recent ‘heads up’ that the 12 kvm (kvadrat meter = sq. m) was so different that I  should throw out all I have learned in the Melges and a long line of boats, and rather learn the arts of keeping her moving.  

However there are certain principles of science at stake here, rather than leaving it all down to art. The boat may be very different in design from the modern regatta machines,  or the RORC tonne rule derived OD boats I sailed mostly before, but Scotty will tell you, a 12 kvm cannae defy the laws of physics, Jim.

Wave Goodbye to your Logs, and Say Hello to your Waves 

Now in likness to racing dinghies, the 12 kvm lacks any electronic instruments. You have of course a compass, a burgee or windex and the tell-tales to guide you in their analogue glory. However you also have another little known analogue instrument, your quarter wave.

Most of us learn about quarter waves when the advantages of planing vessels are being purported. Such a high performance sail or motor boat will ride up its’ own bow-wave and escape its quarter waver, leaving it as a vee long behind the transom. However the quarter wave departure is just as relevant to an 85 year old classic as it is to a modern carbon fibre dinghy.

To illustrate this I will actually use an odd source – the Tasar. Frank Bethwaite and Ian Bruce designed a production boat many years ahead of its time back in the 1970s, with a fancy wing mast and of note a very light hull which can plane not only ‘off’ wind, but given enough wind, on the beat too. The Tasar is the grandma’ of all the niner boats, whose design has then reduced that hull drag when climbing the bow wave in succession to the culmination in fact in the little known 39er/59er model which was so badly managed in commercialisation unfortunetly. The same little issue in light airs continues with these easily driven hulls – when to plane on the beat and when to just point as high as possible?

Delivered with each Tasar came an extremely illuminating folder ‘ Guide to Sailing the Tasar’ . In an addenum incluided in my edition, Frank discusses the issue of when planing pays up wind and when to point the boat.  (Which can be found on line at last I looked)

In brisk conditions over force three or so, depending on your all up weight, the Tasar will delight by picking up her heels on the beat and planing. This means she absolutely  flies well above the restrictions of her 12 foot water-line “hull speed” , and also makes less leeway because the foils create large amounts of lift. So despite the bow pointing lower, VMG is very much better. However in lighter winds the boat can be popped up on the plane somewhere near a beat, but in fact VMG will be poorer because the boat will need to be lower than actually optimal VMG. 

Various wind-speeds are quoted as being fairly great for planing up wind as the optimal VMG, 12/14knots seems to ring a bell for medium to large adults, as low as 9 for light crews. Below that though you have actually an analogue instrument which can tell you if you are pointing high enough or can come up on the wind. The quarter wave. In near planing but not optimal up wind planing conditions, it is best to get the boat moving rapidly on a tight reach and then work it up towards the beat. As you steer to the lulls and gusts in the wind, you have in fact a very accurate one point speedometer (sorry Log I should say ) – your quarter wave. 

Now the Tasar is a very easily driven boat so it will start to climb its bow wave well before full planing happens, and as you may know from motor boats, semi planing is vastly inefficient. You push more and more drag as you climb and although motor boats designed to be semi’planers sit very comfortably in the sea, they use a lot of power and fuel to maintain top speed.Most often in a planing motor boat you can simply back off the power when you get planing and ease up the speed with little use of gas pedal. The same is true in the Tasar in lighter winds. If you try to plane upwind, you either end up sailing too low for effective VMG, or not utilising the power in the wind well enough in taking a middle road, low of pointing. 

If you know then that say 12 knts wind is the critical point for you, and it is blowing 9, then Frank’s advice is to point the boat higher and sheet in for optimal VMG, and he went out and pretty much proved the point with some empirical data collecting, as features in ‘ High Performance Sailing’ his great body of work in one book. You can tell you need to point the boat because your quarter wave starts to exit the boat from aft of the hull, often at the rudder. The critical speed is too high as you start to semi plane, and you can then steer up and sheet in until the qaurter wave attaches to the hull again. 

What Have Planing Boats Got to Do With an 85 Year Old, Long Keel Classic 

Now that was well and good for the Tasar in the last section and planing, but how does this relate to heavy displacement craft. ?  Well it was from my experience of sailing the Tasar in very light winds when just ghosting along.

I took the theory of the qaurter wave being attached further. In very light winds in a dinghy you positioin your own and the crew’s weight forward, with the crew even ending up on the focstle, bow down. Here what I did in the tasar was to try and get the hull out of the water to the point where the qaurter wave exited, which in light airs was nearer midships.So you can either match the ‘wetted area’ to where the wave is coming off in order to optimise drag, because after that point the hull is in turbulent drag, or you can foot-off to gain speed and get the wave  back to the quarter or even transom. Here you have then a new vital instrument, a green light which suddenly flicks on infront of your eyes, only if you bother to look and use it!

The two waves of a boat, or wakes, are vortexes of turbulent water. The qaurter wave represents where the movement of the water detaches from its’ flow along the hull into a wave, a rolling body of water.  Now we know that wind in light airs will cause a nasty bubble on a deeply set sail, it will not be able to transit the full legnth and rather will give up its energy and actually create drag along the windward side in particular, and fail to give optimal power from exiting the leech. We know this too of keels, that they need enough speed to ‘fly’ otherwise the flow will remain turbulent and not nicely attached ‘laminar flow’ . It is the same fluid dynamics working on the hull too. 

Now to our classic, long keeled boat. These are typified  by long overhangs and a water line length often only 2/3rds of the LOA. The keel is relatively close to the this LWL in fact in terms of its mid chord, and again relatively speaking when comparing them to a tonne design spade keel, or modern bulb footed short chord keel like in a Melges or Whitbread 60. So we can use optimal hull speed as a near index for keel speed, and aim to reach that speed in order to ensure our keel is flying on the beat and reach, and thus we arent going too much sideways ie making adverse leeway.

Now we come back to my dreadful earlier performance in 12.5s. Often I had experienced being rolled over on a beat by boats which seemed to have their own private wind band, if not them being out right scoundrels with a hidden electric motor on board!  Most ingracious and unbecomming of a man with 25 yachting years and more !

Now those overtaking boats maybe did have a better bit of wind, or more likely were on the right side of a shift when it came, thus getting the wind earlier. But I was a sitting duck because I hadn’t achieved keel speed or anywhere near it, so any motion in the boat would result mostly in drag and leeway. Like our planing breaking a barrier, we also have to push the boat to sail near hull speed in order to get lift from the keel and counter act leeway optimally.

So this is where Petter comes in , saying you have to keep the boat moving all the time, and that is the art of it in fact, with the science explained. Yes you can bow down a little in a classic boat to reduce the water line and wetted area, but it is more important to get the boat moving and keep it moving with the qaurter wave as long back as is needed to fly the keel. This is then on the leeward side for the most you want to look in a classic, as you heel a little to lee and have a longer water line there to attach that flow to. 

Now in the spirit of the late, great Frank Bethwaite, I would like to get out with a drone and follow some well sailed and purposely badly sailed 12 kvm, and other boats if I can. From drone footage we can see the nice Vee shaped wake and where it exits the hull exactly, plus the relative height to the wavelets , and the windex in shot I hope, plus also all importantly, the leeway being made, especially relative between well and poorly sailed boats (speed too low that is) 

Not a pencil laid on graph/paper in algebraic anger, nor  a ahem, slide rule raised to war. You could well of course work out keel speed from a design draught of your boat or hull speed for that matter. But there we have it, a little like that clever little top tell tale which informs you about so much physics going on up there in such a simple way, we have a little secret watery tell-tale over our shoulder.