Select to expand quotesailquik said..
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.
I agree. The fin only has to balance the side force of the sail. And that's more or less dictated by the riders weight and length of his arms. ( If anything, small sails can generate more lift, they're shorter so have less leverage over you) Using a smaller fin in rough water, where you're expecting to go slow, is a bit counter intuitive.
But a small fin operating at too much angle of attack is slow. Using a sunken windward rail to generate lift, rather than 100% fin is slow. Not hiking out fully is slow. So I'd guess a small fin in rough water is more about going slow than any fundamental control issue.
I go for biggish fins in the rough. I find a big fin does allow you to fully hike out at slow speed without fear of spinout. This is a more comfortable way, easier on the knees, of negotiating rough patches. You can use other tricks in the smooth patches to keep your speed under control.
Select to expand quotemathew said..NCUSAGUY said..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.
So... rigging a smaller sail is suitable for stronger wind, but not a smaller fin ? Hmm... I like your version of Physics, your ideas are intriguing - I wish to subscribe to your newsletter. 
You are on a smaller sail which implies it is windier. Get yourself a GPS and measure it - I would put money on it that - even with a big board and fin -> you wont be the same speed, you will be faster.
So a smaller fin (up to a limit, of say spinout) will then give more control. Which means you wont be on the verge of crashing.
I have been windsurfing for 35 years and have carried a GPS on EVERY outing during the last 9 years and recorded top speed, average speed, distance and moving time. While I am not a speed sailor, I rig for control and if at times, the chop lessens and the wind is strong, I will bear off for a bit to see how fast I go. I don't rig bigger than needed, or do I use smaller fins or smaller boards just for speed.
I seem to have a limit on speed, with skill, age, equip., and venues being the limiting factors. I have achieved speeds between 30 and 31 knots 18 times with 31.02 my best (3 different times). Most of these were on my Starboard IS 111 with either my 7.6 or 6.6 race sails. The remaining were on my 105L Hi Fly bump and jump with either a 6.0 or 5.2. The site where all of these speeds were recorded has a small bay (200 meters) that is somewhat protected and at the end of a 1/2 mile reach. If the wind wrapped around into the bay, occasionally, I could get a short speed run.
My version of physics is that if my board with an overpowered 6.0 sail (COE stable, but hard to hang on to without sheeting out) is at speed, and is pounding me to the point of loss of control at around 25 knots of board speed, I need to do something. So if I change down to a 5.2 sail on the same board and fin and could achieve the same speed with more control (able to sheet in and close the gap with more pressure on the mast foot), then why use a smaller fin? Same board, same speed, same water state, (smaller sail) = same fin. I don't think changing to a smaller fin if overpowered on the 6.0 would accomplish anything, unless the fin was too big to begin with. More speed doesn't equal more control. I raced long boards and formula for over 30 years, so I know about big fins and lift. When a board becomes "squirrely" because of a too large fin, I know what's happening. For a while, I raced a Bic Techno 283 and used three fins for different sails, but I was racing upwind and downwind courses and need the max upwind potential without losing control on the reaches or downwind. I used sails from 9.2 to 6.0 on that board.
If the chop is comparatively small in strong winds, and I wanted to achieve faster speeds, then a smaller fin (less drag) and a somewhat overpowered sail would make good sense. Anyway, we all do what works for us and there is no perfect answer.
I used three different fins today on my 116 litre Manta with a 6.2m bagged out in only 15-20 knots.
I was racing Greg F across the river countless times.
First tried my carbon 35cm Sonntag SLR - great lift,no feel of spin out and exceptional upwind and thru the lulls.
Downside - it lacked control in the 20 knot gusts.
Next I tried my 32cm extra wide G10 Volt.
It was clearly the fastest in the gusts of the three but the feel was not good,like I could spin out at any time if underpowered. Upwind was poor and thru the lulls poor!
Lastly I tried a G10 Vector VX 38 cm.
It felt good as in no spin out,went upwind well but a little short of the Sonntag and in the gusts it kept the board down better than the Sonntag but nowhere near as good as the 32cm Volt.
Overall I prefer the Sonntag.
Not going to use my 32cm Volt in this board again. It's too small for it.
Smaller fins off the wind powered up are unquestionably faster but for overall performance are not good.
Hope this helps.
Select to expand quoteNCUSAGUY said..
.... So if I change down to a 5.2 sail on the same board and fin and could achieve the same speed with more control (able to sheet in and close the gap with more pressure on the mast foot), then why use a smaller fin? Same board, same speed, same water state, (smaller sail) = same fin. I don't think changing to a smaller fin if overpowered on the 6.0 would accomplish anything, unless the fin was too big to begin with.
The "too big to being with" is a function of water-state, wind-speed, state-of-mind, etc. so that statement is patently false.
Let me get a better understanding of what you are saying here -> it reads here as though you have never actually tried to use a smaller fin to give more control (in 35 years of sailing no less!) and thus saving you from changing down a sail size - yet you dismiss it. Is this correct ?
Mathew,
In real terms, I have four boards and change down as the wind increases. Each board has one fin, but 2 or 3 sails each. So as the wind/chop builds, I either change sails and or boards. So in a sense, I change fins, but the board comes with the fin change. When I decide to "rig" down, it's normally because my board speed in the existing water state pushes the limit of control. And yes, a too large fin can be the issue, but that's not my issue. For me, it's speed vs water state that is the control issue. Changing to a smaller fin does nothing to slow the board.
Venues play a roll here, and in a few places that I sail, a smaller fin can play a roll. In relatively flat water, my fin on my iSonic 111 it a tad small for my 8.4, just right for my 7.6 and a little big for my 6.6, but if I am over finned on my 6.6, I change to my 105L bump and jump and a 6.4 or 6.0. The 105L board is a lot more fun than the iSonic. That's in smooth water, but when the chop builds in other venues, it's too much speed for the chop that motivates me to change, not the fin.
So you are right that changing fins can help with control, but not in all circumstances. That's my point, one size doesn't fit all.
Select to expand quoteNCUSAGUY said..
So you are right that changing fins can help with control, but not in all circumstances. That's my point, one size doesn't fit all.
By not changing fins, you are literally saying that one fin fits all situations for that board's usage.
PS. If you want to go slower, just sheet out.
There was a good example of this yesterday. Fangy has designed and had cast some great heavy weed fins, (both weed and fins are heavy).
But for a start there's only 3 sizes, 20, 24 and 28. Swindy has asked for a 22 as the 20 was too small and the 24 too big for one of his boards.
Fangy had the same experience yesterday himself, so a 22 is getting thought about.
Fangy's write up is here.
www.gpsteamchallenge.com.au/sailor_session/show?date=2017-12-27&team=2
Just ignore any derogatory remarks addressed to "cap'n calm"
Select to expand quotemathew said..NCUSAGUY said..
So you are right that changing fins can help with control, but not in all circumstances. That's my point, one size doesn't fit all.
By not changing fins, you are literally saying that one fin fits all situations for that board's usage.
PS. If you want to go slower, just sheet out.
In my case, most of the time - Yes.
If one stays with one board for a broad wind range, then changing fins make more sense. I have done this as stated earlier.
Sheeting out will slow the board, but if one has really good speed, sheeting out takes the pressure off the mast foot and the board becomes less controllable. Power on is the best way for control, assuming your sail isn't too large for the wind. At times, I am forced to sheet out (or head up), but that's the time to change down to a smaller board or sail.
