I'm not exactly sure, but i think that using a bigger fin to create more lift and earlier planning, rather than increasing sail size is better because you have more control of your rig and board with a smaller sail than you do with a larger one. Increasing fin size helps you have more lift and earlier planning meaning you can use a smaller sail, therefore having more controll which makes everything much easier.

I think that is correct from what i have learnt off this site. Please correct me if i am wrong.

bubs

yeah i was spinning out at least twice on each run last week,

It was a small fin and i was just overpowered. Sounds like i needed a slightly bigger fin and slightly smaller sail?

A windsurfers power comes from the relative speed between wind and water, sail in wind, fin in water.
Lift of sail has to be balanced by lift of fin.
Normally we only think windspeed, because that's our enviroment, but it doesn't give the whole story.
If you can think of extracting energy from the difference in speeds of the 2 mediums, then it becomes more obvious that fin and sail should change size together.

Anybody know the relationship between speed of water flow over the fin and fin lift??
Is it logrithmic or linear???

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If you can think of extracting energy from the difference in speeds of the 2 mediums, then it becomes more obvious that fin and sail should change size together.

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Niiiice... I can understand that, makes a lot of sense!!

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Anybody know the relationship between speed of water flow over the fin and fin lift??
Is it logrithmic or linear???

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Proportional to speed squared. So a small increase in speed means a large increase in lift. Unfortunately drag increases at the same rate...

One a related note, it is often useful to change your fin size before you change your sail size. eg: in 17 knots you might sail with a 6.0m + a 32cm fin. If the wind picks up to 20 knots, you could/should probably go to a smaller fin, say 30cm but stay with the 6.0m.

As others have said, fin lift is proportional to speed. Modern sails can have about 10-15 knots of wind range, but most fins have a smaller range. Changing fin size helps to control the board as the wind strength changes, or even due water-currents or sea-state.

The best part is that they are significantly cheaper than having multiple rigs. ie: for each sail, you would use 2-3 fins.

I have been toying with the idea of getting a smaller board for the strong days of late and then re-read an article on fins on a UK Boards mag from last year.

Now I might be tending on getting a smaller fin for my board. My small board should be able to handle the stronger conditions, but it just lifts up on the fin and becomes unmanageable. The sail size of 5.25m is the smallest recommended for the 105ltr board, but the fin that comes with the board is 32cm and is a compromise across the sail range of the board.

The mag ready reckoner suggests 26cm, although I think that may be a little on the small size.

The general theory of smaller fin as wind strength increases is true to a limit. There is a finite size of fin that any board can handle.
You will find that too small a fin will cause you to spin out no matter what the conditions and too large a fin - lets justs say nebbian is right on the money "SCARY".

Fins are actually very important but usually neglected - you have to consider stiffness, chord length, thickness of the foil, rake - and then whether or not there are any minute nicks. The above thngs are important for slalom / speed fins making sure you go fast, don't spin out, easy to gybe and handles the chop. Not so sure about wave fins though.





The following is an extract from bluefinz windsurfing in singapore - pretty much nails it.

Matching the right fin to the board and conditions can make a huge difference to sailing performance. What keeps your board flying over the water? It is two wings, one in the air (your rig) and one in the water (your fin). Sailors will often happily spend a lot of money on a rig and neglect that little thing under the tail of their boards. The right fin can transform a board, making it easier to plane, smoother to gybe, less prone to tail walking etc. Selecting the correct fin however, seems to remain a black art to most sailors.

In order to keep the principles simple, the assumption here is that the boards in question are course, course/slalom or freeride boards with a pointer style fin attached. How do you go about selecting the right fin?

The single most important factor in determining fin size is the tail width of the board. A standard measure used by board shapers is the “one foot off”. Literally this is the width of the board measured twelve inches from the tail. Why is this so important?

Your fin generates lift. Without going into the physics of it, the fin’s movement through the water produces lift in the same way the sail does through air. For any lever, the maximum torque is produced at 50% of the length of the lever. Lets say we have a fin that is 50cm long. According to simple physics, the maximum torque will be at 25cm. Now lets slap this fin on a board whose one foot off is 40cm. The distance from the middle of the board to the rail will be 20cm. Therefore the fin will be generating a force which lifts outside of the rail of the board’s tail. Unless the sailor is exceptionally strong or heavy, the fin will now be difficult to control. This is why boards like the Starboard Formula have so much tail width, the rail to rail distance at the rear allows for leverage against very large fins and makes it possible to ride fins as big as 70cm or more.

The converse would also be true. If too small a fin is used, it will not generate sufficient lift and the board will feel dead in the water. Thus a simple rule of thumb is to use a fin equivalent to the one foot off measure, if you are using just one fin.

How does this affect sailing performance? To get on the plane, your two wings (sail and fin) must generate enough force to lift your bodyweight and the weight of the board and rig off the water. If you go too big in the hopes of planing earlier, you will be faced with a board that snakes through the water. One easy indication of a fin that is oversized is if the front of your shins start to hurt. This means you are now physically resisting the lift of the fin and you should downsize.

Too small a fin will give the opposite effects. Insufficient lift from the wing in the water will make the board slow to plane, “sticky”, it will not power out of gybes cleanly and may become very prone to spinouts.


This hopefully gives an idea on how the fin works with the board. As others have stated earlier the faster the fin moves through the water the more lift it generates - thus you can move down a size. There is so much more on how the fin can influence board performance. I always go for custom fins the stockies that come with the board are OK but don't allow you to really tune the hell out of the board.

As for price custom fins are not a very cheap start at $200+ for select and go up to $400+ for deboichet. Painful but worth it - a good fin truly unlocks what your board can do.

My understanding regarding stiffness is that a very stiff fin reacts like a higher percent carbon mast reacts to flex very quickly, thus i think they can get on the plane earlier. It does not handle chop very well and makes the board uncomfortable, also can be prone to spin out.

Regarding the likes of deboichet it's like buying a porsche or a WRX both very good performance cars one just costs a lot more. I believe the deboichet's can make coffee also

Regarding a quiver of cheap fins - you will get the greater range and some improved handling by having the different sizes - but the good custom fins should give a greater performance when compared to the cheapies performance. How much? Unsure, but it does exist - I've noticed changes from the stock fins to customs on my slalom gear. ultimately it is a price thing and a personal sailing preference, just because the local fast guy uses one type of fin does not necessarily mean that it will work for the type of sailor you are.
Experiment, experiment and experiment.

Wave fins, are a very mixed bunch, and you really need to choose according to your own riding style.
Some are very flexi, great for doing sliding type tricks, but not good for getting upwind.
A smaller fin makes the board more maneuvarable, so some wave riders use the smallest fin they can get away with.
I don't think that formula that relates tail width to fin size is all that aplicable to wave riding, wave footstraps are a lot further inboard, so rider weight doesn't have as much leverage over fin lift.

A lot of wave fins have the trailing edge at the base cut back, to help stop air getting sucked back from the tail. Most of them are fully foiled thru this section, but there's a few around where the trailing edge at the base is a square cut off several mms thick, I'd stay well clear of these, as the cut out instead of stopping spin out probably causes it.