September 13, 2014


To my disappointment the presentation of Yrvind Tens slotted leeboards was not universally met with rapturous applause. Some people think they will cause too much drag and not be strong enough. Therefore I valiantly feel obliged to come to their defense.

Aviators did invent the name but not the flaps, navigators did. The classical rudder attached to a long keel is a flap. If the tiller is moved a few degrees to weather the boat becomes more weatherly. That the rudder in that position creates a bit more drag is a minor point as long as the boat wins races. On the other hand, going to the extreme by moving the sail area forward so that you can keep the tiller 10 or 15 degrees to weather will increase the keels lifting force even more but then the drag will be so great that the boat has no chance of winning.

If the boat is propelled by sail and not an airplane engine the flap can and must of course be designed accordingly. I also like to point out that the leeboard is overlapping the flap. Therefore it will be attached not on the foils weak trailing edge but more forward in a position, strong enough.

From a fluid dynamic point of view the constellation mainsail and overlapping genua is a slotted flap as A. Gentry has shown. Land yachts use slotted wings and they are very fast. To my mind if slotted flaps can be used on sailing boats for rudders and sails I see no reason why I should not use them on my leeboards, its just a question of good engineering.

Why do I not use a long fixed keel as is suggested? Well it is certainly quite OK for heading into gales, but in my opinion, a centerboard or leeboard placed well forward the sail area is a much better option, and like I have said before, when retracted less likely to capsize the boat. It gives you a choice.

1989 I sailed the 15 feet Bris-Amfibie from Ireland to Newfoundland against the prevailing westerly’s. The mast was ten feet long. In heavy weather I had a 1 square meter jib and a 2 square meter triangular main. Between 6 and 18 of June we had continues contrary gales. The boat had a centerboard. It was placed well forward of the mast. Its area was about ¼ of a square meter (0.6 m deep x 0.4 m cord). Finally after 12 days the sun came out and I got a chance to fix my position. To my joy it was more westerly then when the gales had started.

We arrived in St Johns after 47 days. It had been a stormy passage but no capsizes, nothing broken, all the plastic jerry cans topped up with rainwater.

Before leaving Sweden I gave some yachtsmen lessons in celest navigation. They where bound for Nova Scotia.

“We might see each other on the other side” I suggested.

“I am not so sure” their captain said.

“Your boat is so much smaller it will take you longer time and we cannot wait for you.”

It was an argument plausible enough. After all they had a modern powerful 35 feet boat equipped with a strong diesel engine, big fuel tanks, radar and all the other stuff. I had my oar and sextant.

In the end we never meet them. The North Atlantic had been too stormy that year, they later told me. After two weeks they had turned around and run. Everything was a mess in their boat. Later they sold her.

The problem with a long fixed keel like the one Manie B is planning to have on his ten footer is that going downwind he will get weather helm and a lot of drag. On a circumnavigation there will hopefully be much downwind sailing. Leeboards on the other hand, they have even less drag than centerboards because the smooth bottom will cause no turbulence. In strong following winds I will use my twin rudders like a snowplow giving her a lot of inherent course stability.

All my experience and all my knowledge speak against the long fixed keel. Please do not use it. I never go back to it.

Below are some pictures of Bris Amphibie. The centerboard can be lowered further had not the floor been there. Click once or twice to enlarge.

Above Bris Amphibie sailing her main is 2 meter square her jib 1 meter square.

The boat.

A bit of the centerboard can be seen.

Regards Yrvind.

September 4, 2014


Every sailor knows, or ought to know that the force created by the flow over a foil increases with the square of its speed.
If Yrvind Ten would make three knots through the water hard on the wind in ideal conditions I be very happy.
If she will make half a knot in gale conditions I will be equally happy.
The heart of the matter is that the power of the foil to resist leeway has in the gale case been reduced to 2.8 %, very unsatisfactory.
(3 divided by ½ = 6. Six squared equals 36. 1:36 = 2.8 %)
To make things worse the need for lateral force in a gale is much bigger than in nice weather.
Airplanes have a similar problem. During take off, when they travel relatively slow, they need lots of lifting force.
In order to become airborne they employ flaps. That is, the rear portion of the wing rotates downwards. Flaps are high lift devices. They also reduce the stalling speed.
I have decided to use flaps on my leeboards – detachable flaps.
I will have an asymmetrical leeboard on each side, asymmetrical because they are more efficient.
I will use a slotted flap. They allow fluid to pass between the foil and the flap. That way I will have no problem to seal the gap.
I will place the flap not in the same streamline but in the way a jib and main are positioned in relation to each other, with a bit of overlap.
In good conditions the flap will stay on deck because the leeboard itself will be sufficient to prevent leeway. That way drag is reduced.
I only need one flap as I can flip it bottom for top using it on both sides.
When I put the flap on and take it of, the leeboard will be out of water. That way I can easily do the work from the hatch.
In nice weather I increase the sail area and reduce the lateral area. In rough weather I increase lateral area and reduce sail area.
Before I add an object to the boat I ask myself, do this thing earn its keep? In this case I believe it does, for the following reasons. Not many small boats have been cruising the roaring forties and not many people live there. What we know about the weather there comes mostly from big boats. But, big boats are not stationary observers, often they voluntary travel with east moving weather systems. That way they can enjoy and report strong westerly winds for long periods of time.
Compared to bigger boats Yrvind Ten with her speed of about two knots can be considered almost stationary. She will experience completely different weather patterns. She will be exposed to two or three lows each week. When a depression has passed there will often be nice easterly winds – not adding much to the progress. From a stationary observers viewpoint the roaring forties is not a strong trade wind.
To complicate matters there are also strong easterly gales. Once, when I was less experienced I rode one out in comfort to a sea anchor. When the weather had moderated and the sun came out I fixed my position. I had been set back 150 miles.
Some navigators may think that’s a good deal, not me. My present boat will hopefully be able to fight gales head on. The new strategy to deal with contrary gales is to dig in, to hold on stubbornly – at the risk of being capsized – until the wind swings over to a more favorable quarter, then I will ease the sheets and run.
Slotted flaps are high lift devices. They reduce the stalling speed. I am betting on that they will earn their keep.
However one must not forget that in stormy conditions flow around foils is very turbulent. One reason for that is the circular movement of the water particles in a wave. At the top of the wave they move in the same direction as the wind at the trough in the opposite. In a gale the speed difference is often more than six knots, a speed far higher than Yrvind Tens. Accordingly, theoretically, the flow around the foils will vary as a sine curve, sometimes being positive, sometimes negative.
In reality turbulence will create much chaos. Then only a lot of lateral surface prevents leeway, a bit like a parachute. So in stormy conditions Yrvind Ten needs all the lateral area she can get. The slotted flap will not only increase lifting force when there is flow, it will also at the same time increase lateral area. The leeboards itself are already as big as I dare to make them.

Below are some pictures. Click once or twice to enlarge.

Above the slotted flap.

The flap laying on deck.

Slotted flap on healead boat. Front vieuw.

Top vieuw.

To be continued…

Regards Yrvind.

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