Why use a smaller fin?

Stupid question time. Why do you use a smaller fin when one fin can get you on the water and going fast? You need to stick to the water, otherwise you would drift everywhere whilst trying to hold your line of sail. Wouldn't a bigger fin be better for holding you onto the water? Yes I get the theory of cavitation and surface areas, drag, but maybe not most of it, still, how much difference does it really make? As I said, stupid question really, but why?

I have noticed that, when I get into the real speed spot. I do need to almost fight the board and sail to stay on my line, but I'm usually going faster than I have been (or want to go) so I just sheet in further and go faster! Obviously I'm in it for the speed and excitement, I can't even gybe yet, think I'm going too fast! Does your speed drop with the smaller fin? So if I'm on a downwind run back into the beach, south wester following me, would a smaller fin be better to keep the board on line?

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Scrondies said..
I have noticed that, when I get into the real speed spot. I do need to almost fight the board and sail to stay on my line, but I'm usually going faster than I have been (or want to go) so I just sheet in further and go faster! Obviously I'm in it for the speed and excitement, I can't even gybe yet, think I'm going too fast! Does your speed drop with the smaller fin? So if I'm on a downwind run back into the beach, south wester following me, would a smaller fin be better to keep the board on line?

Unless your doing speed runs all day at luderitz, theres a balance to find.

yes, a smaller fin will give you a greater amount of control and speed downwind. But generally youve got to sail upwind to get to the start of your downwind run. There will be a recommended fin size for your board/sail combo that will give good all round performance.

Think it's all been answered, but basically the lift of the sail has to be balanced by the lift of the fin, so ideally they should both increase and decrease together for max efficiency.
The main treason for going to a smaller fin is that the current fin is too big.
This can be evident in about 2 ways.
The main one being it's trying to break your ankle, lifting the windward rail, this will cause series control issues if it gets too bad.
The other one is the fin has just too much drag when you're trying to go as fast as possible.

Thanks all for your comments, better get a smaller fin! Pretty sure I'll still go fast

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decrepit said..
Think it's all been answered, but basically the lift of the sail has to be balanced by the lift of the fin, so ideally they should both increase and decrease together for max efficiency.

I tend to disagree with this and would say the fin needs to be balanced against the width of the board's tail. When the load of the sail gets too great you change down and the smaller sail puts the same amount of power down that the larger one did in less wind. For the same sail size you may use a larger or smaller fin depending how powered up you are but the fin range suits the board rather than the sails.

Edit: rereading what you wrote, you never said anything about changing sails that's just how understood it.

When a board is planning slowly, a larger fin will be able to operate at a lower angle of attack and have better lift to drag ratio. Smaller fin with need a greater AOA to get lift and therefore have more drag. It will also be closer to stalling (complete loss of lift when speed gets too low and AOA get too high to maintain lift) This is 'spinout'. Not good.

As you go faster, the angle of attack, to produce the lift required, will decrease. At a certain point you get to where a smaller fin can have less drag at a low angle of attack and still produce all the required lift.

At very high speeds off the wind, even a tiny fin can have all the stability and lift required, and a lot less drag.

So it is actually more about speed through the water than sail size, or even board width, although board width plays a big part in helping control fin lift at high apparent wind angles.

Note: The term 'Lift' here is used to describe the horizontal or 'sideways' force produced by the fin, not the force trying to lift you vertically out of the water, although most fins produce a small amount of force in that direction also.

Spotty was using a 23 KA assy fin(from memory) in his 80cm carbon art board at Lake George with no probs, I found that sailing on a lake I can get away with smaller fins on my slalom boards but when I use the same fin in open ocean sailing forget it the board spins out all the time, need to go much bigger in the sea because of swell etc

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choco said..
Spotty was using a 23 KA assy fin(from memory) in his 80cm carbon art board at Lake George with no probs, I found that sailing on a lake I can get away with smaller fins on my slalom boards but when I use the same fin in open ocean sailing forget it the board spins out all the time, need to go much bigger in the sea because of swell etc

Because LG is very flat water, board speed is higher so a smaller fin produces more lift.

Because the water is very flat, the board does not bounce, and that leads to far fewer shock loadings on the fin. Shock loading can greatly increase pressure on the fin and sometimes lead to a high AOA. A high AOA can result in the fin stalling. = spinout. very much lower chance of that happening on very flat water.

Also, the water in LG can be very high salinity, therfore denser than normal sea water, and the fin gets more lift for it's size and speed.

Anyhow, the bottom line rule of thumb is that when you are going faster on flatter water you are better off with a smaller fin.
When you are going slower, especially if you are also in rougher water you will need a bigger fin, or at least one with more lift and a higher tolerence for high AOA. But there are a whole lot of qualifications, for this and that condition, that will lead sailors to using specific styles of fins within those basic principles.

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NCUSAGUY said..

I have a 105L board that I have sailed with 6.4, 6.0, 5.2 and 4.5 sails all with the same fin. Clearly, as the wind builds, I go for smaller sails to achieve relatively the same board speed with the various sails. The power transferred to the board via the sail is the same over a range of wind and sail sizes. Why change the fin? This is for free riding with some bump and jump with a goal of maximum speed without sacrificing control.

A larger fin would help get the board planing in lighter winds, but it's not necessary with good technique and could hamper control once fully powered. So my fin choice is geared to what will be best at speed (the same speed with either my 6.4 or my 4.5).

Because.... instead of changing from a 6.4 to a 6.0, and wasting time while the wind is blowing, you can just change the fin. It will make the board easier to control - which allows you to keep the bigger sail. All combined it should allow you to go faster (and thus the same lift with a smaller fin).

A larger fin always wins the race vs good technique... why do you think Formula sailors use a 70cm fin ?... they would use bigger if the class-rules allowed.

If you are changing down from a 6.4 to a 4.5 -> it is much windier. Yes the water is probably rougher - but you will be faster. If not, then you likely need better technique. [ Using a GPS regularly will help you gauge how effective your technique is. ]

Yes Andrew, and as the wind picks up it just gets choppier, so staying with a bigger fin makes some sort of sense.

There's just too many variations in this sport to make hard and fast rules.

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sboardcrazy said..
Depends on how deep the water is too..

Good one Sue, if the fin is scraping on the bottom, go to a shorter one. just don't get catapulted at speed.

Freestyle boards use tiny fins, formulas large ones.
I find fins to be quite a bit more simple than I originally thought.

What do you guys think of this?

Length = leverage + lift
Width = power + drive

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sailquik said..
Mat, I don't think NCUSAGUY's goal is to go faster. I think his aim to to keep speed down to enable control in the rough water.

Yes, exactly my point, which is the same point I tried to make to the "wingsail" guy - one sail isn't going to cover 15-40 knots. Chop in 6.4 (17 knots) wind is minimal compared to 4.5 wind (25 knots). I go for speed, but if I am on the edge of crashing, then I rig down for more control. It's not lift from the fin that's the issue, it's the speed and pounding in the chop/waves where at some point, there's a high potential for a crash.
Actually, I use my 105L board with my 6.4, and 6.0. I jump down to a 96L board for my 5.2, 4.5 and 4.0. The smaller board has a smaller fin. I have two larger boards and each has it's one fin, so I basically cover 10 knots to 30 knots with four boards and four fins.

Just to confuse a few, one board ( 111 Lt) , one sail 7.9 Karace, fins C3 40 , Vector 38, Lessacher 38 & 25 dual assy weedies. Wind 10kts to 25kts measured by automatic wind gauge BOM. Water state flat to crazy chop.
Much easier to change the fin and a small sail just doesn't work on a large board with a large fin

Regarding rough water, you also have multifins.

I used to sail a single fin freewave and I always spun it in rough water. An expensive carbon fin made little difference. I could always recover, but it was annoying.

Now I sail a thruster wave. It never spins. I almost forgot the feeling. I have to make a real effort to spin it and it's very mild compared with the single fin and is almost "self-righting".