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Frequently asked questions

Of course you send new questions to
1 Can a sailboat sail faster than the wind?
On the run: of course not (well, not without the engine and other cheats).
When sailing beam reach: if you have a boat with not too much drag this must be possible.
Surfers, ice-sailers and catamarans can easily do two times the wind speed.
With an "ordinary" sailboat you will be close to the wind speed, but probably go slower.
Perhaps you can understand that when you sail on a beam reach, with the same speed as the wind, The apparent wind is like on a close reach, and you sail just like on a close reach, so that is possible.

2 What is apparent wind?
Apparent wind is the wind you will feel on a sailing boat.
It differs from the true wind, what is the wind that you will feel if you stand still.
The difference is your own wind, which is the wind you will feel due too your own speed.
Your own wind is the wind you will feel if your stick your head out of the car window on the motorway.
Apparent wind is the wind which you get if you combine the true wind and your own wind.
Examples:
-If you suddenly go into an area with no wind, like behind a tree, the wind direction suddenly changes to more forward, because only your own wind is experienced.
-If you get an enormous wind puff when sailing on the close reach the wind changes to a beam reach, because the true wind is much more compared to your own wind.

3 How does the slot between jib and main helps by speeding up the air?
This slot working does not help and belongs to the useless theories.
The tale of the air speeding up in the slot between main and jib is that the slot becomes smaller, so must be speeding the air to have the same amount of air passing it.
Speeding up the air on the leeward side of your main must help to create a lower pressure there.
That there is a slot is correct. That the air speeds up is also correct. This results in a lower pressure on the windward side of the jib, and a lower pressure on the leeward side of the main, so cancelling out the supposed advantage.
Besides this the pressure needed so speed up the air is more or less blocking the slot, thereby resulting in less air through the slot.
The air is forced around the slot, and not through it.
This does not mean the main and the jib will influence each other, see the next question.

4 I 've heard that the jib produces 60% of the propulsion. Where can I find that on your site?
The foresail provides indeed a relative larger driveforce than your mainsail.
This because the foresail is in the faster air of the mainsail, and the mainsail in the slower air of the foresail.
Read air goes faster on leeward for an explanation how the air is speeded up.

Further the jib pre-bends the air for the main aft, so the main can not bend the wind as much anymore.

The air is being sucked from in front of the boat to the lower pressure on leeward of the main, so directing the air a littlebit more to leeward, and right there is the jib.
This means the main is making the jib operate in a wind more from a beam.

The jib does more because of the mainsail, and the mainsail does less due to the jib.

To complete the theory, The jib helps preventing stall of the main because the air is pre-bent.

5 Why do I make so much leeward way after coming about?
It has to do with the keel (and/or sail) being in a stall.
The keel has to counteract the sideways component of the sailforce. If you have no speed the keel deflect a little water a lot, so has a high angle of attack, and that is the enormous sideslip
If you adjust your sails (including the jib) to the winddirections while coming about and directly after it, and built up some speed before you point up again, it should become less.
Pointing up too soon will result in excessive rudder use, and excessive rudder does not help to regain your speed.

6 How does a wing keel work?
A wing keel draws the boat down, this downforce is pointed too the windward side at greater angles, and so helps preventing making leeward way.

In General the performance of a ship with a wing keel is worse as of a ship with deep keel, but that is not always possible (shallow waters)

7 When do I choose for the high aspect jib and when for the genoa?
If pointing is the most important you should use the H.A.
If speed is more important just choose the bigger genoa.
Why? The High aspect has less tipvortexes because of its aspect ratio.
This helps sailing close to the wind.
A genoa is just larger, and thereby produces more brute force (driving force and even more sideway force), excellent for speed.

8 Windmills always turn left, and my boat also sail better on the port tack, is this for the same reason?
No, windmills turn left because this is historical grown because grinding stones were made to turn left.
Actual only the windmills in the Netherlands and Belgium turn left. (source Informatie-XVI the gilde of voluntary millers, Evert Smit, which explains this very clearly in approx. 50 pages which I save you)

Probably you stand mast crooked, is bend, or your boat is crooked, or your weight has been divided crooked.

However, on the northern hemisphere the higher you get the more the wind will turn to the right and will go faster.
This means that windpuffs from above when you are on the port tack also have the tendency to come more from fore, so keeping the apparent wind direction more or less the same.


9 Do I go faster in deep water or on shallow water?
You will go faster in deep water as in shallow water.
This because the hull speed gets lower in shallow water.
You can see the drop in wavespeed due to depth in the coastal surf, where it becomes shallower, there the waves become closer to each other because their speed drops.

If it gets very shallow or very narrow you will also get suction.
when you sail along a narrow channel the water must be "pushed" below you through the narrow slit between the bottom and the boat, creating extra resistance.

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12 Someone has measured the pressure between leeward side and windward side, and it was zero. How is that possible?
Because The pressure difference between leeward and windward is not that much, and is thereby hard to measure.
It is in general much easier to measure the sailforce and then devide it by the sail area. The result will be the average pressuredifference.

13 In your theory of sail you mention that the sailforce is perpendicular to the boom, while in your tuning theory you mention that putting the draft more forward help directing the sailforce more forward. You make a mess of it!.
I wanted to keep the sail theory, the theory of couples and forces understandable for many people.
In general the sailforce is more directed forward as perpendicular to the boom. This because:
  • Twist, the upper part of your sail is a little more out, and so is the sailforce.

  • A more forward position of the maximal draft of the sail. The magnitude of the sailforce depends on how much you bend the air.
    The luff has the most curvature, so deflects the most of the wind, and thus delivers the most force.
    Exagerated example:

What you can learn from this is that you better can look to the sail as to your boom.
When explaining the forces and couples please keep it simple. It is already complex enough for many people.

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16 A tipped up rudder (still fully submerged) steers worse as a rudder nicely down. How is that possible?

tipvortexes are excessive and create loss of steering force.

17 That photograph of that plane which makes a slot behind itself in the clouds, is that real?

As far as I know the picture is real, however, the photographer was in an other plane I assume.(and thereby enhancing the downwash)
Photograph is of the company that build that plane (Cessna).

18 Can I copy content of your site?
Your welcome to do so. However, I would be pleased if you mention sailtheory.com as the source.


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