When you began designing Flying Fifteens, and you had already done quite a lot in other Classes, what were you aiming to produce?
A boat with a flatter rocker, because you then lengthen the waterline and narrow the waterplane: what goes through the water is then long and narrow as opposed to short and fat.

You once said the waterline length was increased by about 1'6"(450mm) over a round rockered boat...
Yes, I think so; but if you compared the two in choppy water and with a big heavy crew the difference would probably only be about 9".
What you are doing is to keep the ends of the waterplane in the water for a longer period of sailing time, because the boat (the rocker) is less banana-shaped. When the boat is in motion, when the bow goes in the stern does not come up so much as the old boats because it is flatter.

I don't think the long narrow shape points higher, but of course it does have more lateral resistance, resistance to leeway.
As for planing, the waterline being longer you have a higher speed before you start to plane, and if you are going faster you are more likely to start planing than if you are going slowly. I think the modern flatter boats plane better anyway. All dinghy classes are flat now if their Rules allow.

Do you alter the waterline by where you sit in the boat?
You don't need so much crew weight in the boat now; the old boats with a lot of rocker needed more weight to lengthen the waterline (and with the amount of weight they carried, the crew tended to sit right forward going to windward, for instance - then it would make a difference!). But nowadays, with the flat rocker, crew weight isn't nearly so critical. A lot of light people do very well, and people don't alter position fore and aft nearly as much as they used to.

We all have the same displacement, more or less, so how does one Fifteen have a lower wetted area than another?
The wetted area is usually the after end of the boat (the after end is the mass of wetted area); on evenly-rockered boats you can easily move to alter your weight distribution forward and lift the after end out (like a Laser) but most modern boats (in classes which allow shape adjustments) have got a flatter rocker and longer water-line, and reducing wetted area is more difficult. You are stuck with it really.

We've had many examples of round-rockered Fifteens doing very well inland, but comparatively poorly at sea, in choppy water. What is the explanation?
They are very good at quick-tacking: the shorter waterline means they are quicker turning around. Also they are able to adjust to conditions more quickly than a longer waterline boat: you can luff and bear away much quicker, all turns are easier.

When you flatten the rocker, what adjustments do you make to the sections?
With a longer waterline you have less waterline beam and you have a very wet boat, like my Mould II boats. So to overcome that I then made boats fuller in the bow, more U-sectioned aft, and slightly deeper aft. With the longer waterline you have less of the mid-ship section in the water - the waterline is lower at each section so the angle of the section to the waterline changes. This is rather more noticeable at Station 4 than elsewhere (Diagram C) so I have made that slightly finer at the waterline and above. Taking the underwater sections of the ideal boat they would be something like Diagram D, whereas the Fifteen tended to be like the dotted line. The angle of each section to the waterline changes, and getting the boat to look right was a problem! Of course you can only do a little bit because of the Rules, because of the drop measurements; without those measurements you could do a lot more.

At the front you have a deeper foot?
Yes, because the boat has got that raking forward bow you don't alter the shape of the bow section when the entry is further forward. In Diagram E, this section on an old boat is further aft than a flat-rockered boat (when the bow sinks in deeper), but the section shape is the same.
But of course the further forward the waterplane extends, the finer the bow; the boat has a given beam on the waterline but if the bow meets the water further forward, the beam is narrower. It is a finer bow.
With the narrower longer entry you can afford to put more underneath, have a deeper forefoot. For instance, a section through the boat just aft of the front of the waterline (say at Station 21/4!) is more U'd now compared with my earlier boats. And while the rocker just in front of the keel is 1" above Uffa's lines, I have slightly flattened the rake of the bow by coming Vi" below Uffa at about Station 1.
The effect is to make the boat drier.

And what is the effect of various bow shapes on the way the boat handles?
If it is too narrow you get a lot of weather helm. But if it is too U'd, and perhaps too flat underneath (too straight on the rocker-line from the front of the keel forwards), the bows will just bounce around and the boat will be very tricky to sail: IOR boats to this shape tend to have thumping great keels (compared to FFs) giving lateral area to drive the boat along!
The Fifteen is quite an old-shaped boat; at Section 3 it is quite V'd. Some designers used to build chicken-breasted boats and make them hollow in front (a little concave curve in the section) - to grip the water, and partly the idea was to push the water aside.

What about the shape further aft?
Near where the helm sits (Station 6) the boat tends to squat, and more rocker and a more U'd section is a help with weight carrying.
Aft of here, the waterplane shape can be straight or more curved (Diag.F). The curved shape is built out sideways giving a shoulder which helps to hold the boat upright. I think the narrow shape (the straighter waterline one) must be good in marginal winds and for lightweight helms.
With the rounder shape the wetted area is reduced quite a lot (On the Mould IV, for instance, I put more round under the water - by dropping the rocker at Station 6 and continuing the depth sideways to near the water-line). The planing surface comes to a point at the back, the water coming along, round the curve and back towards the centre line. On the straighter shape, the flow goes round the wide part of the boat and continues straighter instead of curving the whole way.
The new boat, the Mould X, is narrower here compared with Moulds IV, V and IX, and without shoulders you do have to sit in quite quickly at times! As well as sitting out!

Yes, what about heeling the boat, Roy? I remember sailing Championships with you when you have moved all the extra weight (anchor, lunch, drinks) into the centre of the boat, but you don't worry so much about heeling?
Well, perhaps I tend to heel a boat too much.

But when YOU do it, the boat still goes very fast for you...
If you heel and you are going all right, it's all right, but if there's one boat more upright and going faster then you have got to try and match him a different way.
When you heel, the leeward water-line becomes much fatter than the windward waterline, so the water has further to travel round it and the pressure difference means you tend to make leeway. But I think if you are heeling over in light wind then you can point a bit higher, and have a little bit less wetted area.
Ideally you should not let the boat heel, except to get speed on the boat, you hold it upright more and more.
It is all about getting the feel of the boat.

Do you think weight distribution matters? Can you make a difference to performance by keeping weight out of the ends of a Fifteen?
I tend to allow for this in the shape. For instance, one of my moulds with a very fine bow had some boats built in it of a special lightweight material; and when the builders put the first boat on the scales not only was it 30kg underweight but also the bows (which had somehow got less resin in than was intended) pointed right up in the air. Now that boat (with 30kg added of course!) was incredibly fast. Boats from that mould are very fast anyway, but they find they go even better with a light bow.
So I allowed for that effect when I was doing Mould IV, putting more buoyancy in the bow and making it more U-shaped. You only need to alter the bow a tiny bit (2 or 3lbs extra lift in the front): think of when the boat is afloat, you can put your hand underneath and lift it easily. Also, if it is more rounded in the bow you don't need so much material there anyway because the shape is stronger.

What about weight distribution in the keel? Do you want to keep the weight up high, to reduce pitching moment?
No, I don't think pitching moment matters as much as other factors. Above the waterline you try to have as little weight as possible, below the waterline you try to keep it heavy. For stability. So you can hold your sail area better.
With the weight down low you have more power, more force to keep going when you hit a wave. The main force of the keel lifts, and goes on moving over the wave. You carry your way, but acceleration is slower than with a more weight-up-top keel.
You must also think of keel shape: the more of the weight is in the bulb, the greater the lateral area. A small area is OK for running, but going to windward you need as much area as possible.

What about the angle of the bottom of the keel to the waterline?
If you look along the bulb from aft, the shape is like a triangle getting smaller towards the tip. You want to get the whole triangle, not just the bottom, going straight through the water. This means making the aft tip point up a bit in relation to the waterline. But then you are losing draft (see Diagram C), so it has to be a compromise. Modern metre boats tend to have a rocker on the keel, losing as much as 6" depth at each end over a length of 6' - following the shape of the bottom of the boat - if draft was critical, they would keep the full depth of the keel.
Looking at our rocker, beyond the front bottom of the keel the boat (rocker) starts to curve upwards. (Some designers have kept it flat at this point, but that seems to have caused quite a few problems in performance). Now: water pressure on the boat is at right angles to the rocker all the way along; water flow follows the line of the hull all the way along; and the depth of water that moves is seven times the draft of the hull, about 42" for the Fifteen. What you are doing by having the aft tip of the keel upwards is following the line of flow.

Finally, Roy, what do you think makes the most difference to how fast a boat goes?
Well, I've always worked on it being the hull shape! But the sails are the real difference, that's why sail makers win; the good sail makers know what makes a sail go fast. But everything's got to be right: the boat's got to be right, the helm's got to be right - light and confident! The sails have to be right, and the mast to take those sails.

This page is based on an article by Roy Windebank and Sara Flower published in the ff World magazine in 1990.

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