Changing Fin Size

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Ashevillejanes
Ashevillejanes
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Joined: 07/10/2018 - 09:23
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Changing Fin Size

How important is it to change fins size with sail size? I don't like to make a bunch of changes every time I go out. Different size sails is an obvious option. I am using a 38 cm freeride fin on my Starboard 126ltr S Type. I have used 7.5 down to a 5.3 in all wind conditions and had some great sessions. Ignorance is bliss, because I'm not seeing the need to swap out my fin! Educate me! Thanks

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nitro's picture
nitro
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Re: Changing Fin Size

I regularly sail with a guy who uses the same fin on his board from 7.5 down to 4.2. He is having plenty of fun, but it not concerned about jumping, jibing, carving, etc. Me, on the other hand, use a specific fin for each sail I use. I can sail faster with more control, jibe better, jump better, spin-out less etc... when using the 'right' fin.

So if you are having fun and not worried about getting that last 10% out of your board, don't worry about it. But, I would be on the look out for a used free-ride fin in the 30-32cm range for your smaller sails. Used cheap fins are not hard to find.

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FoilDodo
moredownhaul
Ashevillejanes
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webguy
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Re: Changing Fin Size

Fin size is more related to board size - one foot off OFO (or 30cm) the tail really than just max width - and wind speed than sail size but sail size has a correlation.

The width of the board at the back strap (OFO) determines the approx max fin size as if the board is too narrow, the lift of the fin will overwhelm the leverage you can put on it. Conversely, put a too small fin back there on a wide tail and there isn't enough lift so you end up sinking the rail. Of course, you could move the back strap in but a) that's a PITA to do and b) you'd likely be on a smaller board for reasons (more control, less drag, etc).

But lets back up a moment before we go in a circular argument, why design a board at a certain OFO or total width to begin with? It has to do with intended wind range. A board intended for 10-15 will differ for one designed for 25-40. In lower winds, lulls matter more; the low wind speeds restrict how fast I can go (say I top out at 1.5 times wind speed) and the lift the fin can produce; and the water is flatter - a wider board will still be controllable. Remember, the board produces "lift" as well. It's lift -more pressure underneath than above - is what makes it plane. A long narrow board is essentially a low aspect lifting surface and a shorter, wider one, more high aspect. Lower wind and board speeds mean I need more lift from my board.

As the wind picks up, reducing drag and increasing control with a smaller/narrower board becomes more and more important. The reason they didn't make wider boards* (there were a few renegades) earlier than around 1997-98 is they couldn't figure out how to make a wide board that had a decent wind range as the prevalent design philosophy was that flat bottoms were fastest. The willingness to put in some vee in the bottom plus the introduction of flaps and cut outs (which limit tail lift in overpowering conditions) helped significantly.

So... each board has a range of optimal fin sizes. Say, it's 38-42 cm. If I need more lift (ie lower wind speeds, more lulls), I use the 42. If I'm plenty powered and 1.5 x wind speed is now 30 instead of 20, than I need less lift. Sail size connects because we usually rig the 7.5 in lighter winds when we aren't either as powered or going fast at max. If the wind picks up, we rig down as we now have an abundance of power, fin lift and are more interested at that point in limiting both fin lift and drag. Of course, if we've rigged small and the wind lightens, we can simply put in a bigger fin instead of changing sail size. Same with a big sail when the wind picks up.

So, when should you change fins? If you are having trouble planing or spinning out at low speeds, go to the bigger end of the suggested range. If you are well powered and the board wants to tail walk, or you want to minimize drag, consider going to the lower end of the suggested range.

*Longboards intended for racing were limited in width by international class rules, iirc, around 69cm.

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moredownhaul
FoilDodo
Ashevillejanes
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FoilDodo
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Re: Changing Fin Size

What webguy and nitro said. Good

We had this discussion years ago and as I recall Radny had some insights that sounded legit (but I still didn't fully understand). The conundrum for me is - how does your fin know how big your sail is? If the wind picks up and you rig down to keep the forces above the water more or less equal, the same fin should work, right? Well, it doesn't. The 'right fin' really helps getting dialed in.

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Ashevillejanes
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webguy
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Re: Changing Fin Size
FoilDodo wrote:

What webguy and nitro said. Good

We had this discussion years ago and as I recall Radny had some insights that sounded legit (but I still didn't fully understand). The conundrum for me is - how does your fin know how big your sail is? If the wind picks up and you rig down to keep the forces above the water more or less equal, the same fin should work, right? Well, it doesn't. The 'right fin' really helps getting dialed in.

The answer to the conundrum is that the fin doesn't know the sail size but it doesn't care. It only cares how fast it's going because that's what determines the amount of lift it generates. For a 7.5 and 6.5 generating the same amount of power ie pull on the harness, either the 6.5 has to have more wind or if the wind speeds are the same, the 7.5 has more drag. So, at the same "power", the 6.5 is likely going faster. The fin doesn't know you've rigged a 6.5, only that you are going faster. A smaller fin at the faster speed of the 6.5 produces enough lift to balance the power as the bigger fin did with the 7.5. You get some added benefit because the lower drag of the smaller fin re-enforces the effect a bit.

So, if a smaller sail is faster, why do racers rig enormous sails? It's about the lulls. They trim the big sails flatter (reducing power and drag) but have the extra surface area and ability to trim fuller if there is a lull. The 6.5 may be slightly faster in a straight line but if it can't plane out of the jibe, it's game over.

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FoilDodo
Ashevillejanes
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FoilDodo
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Re: Changing Fin Size

Makes sense, except rigging down often means more chop so usually not more speed. Fully powered on 6.5 vs fully powered 5.5 seems about the same speed-wise but I've definitely felt over-finned without changing.

Quote:
the fin doesn't know the sail size but it doesn't care

I think my fins DO care. A lot. They love me and I love them. Good
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webguy
Ashevillejanes
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Joined: 07/10/2018 - 09:23
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Re: Changing Fin Size

Excellent info, as usual, I guess I"m in the market for a 30-32 cm Freeride Tuttle fin! Let me know if you have one for sale,

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Re: Changing Fin Size
FoilDodo wrote:

Makes sense, except rigging down often means more chop so usually not more speed. Fully powered on 6.5 vs fully powered 5.5 seems about the same speed-wise but I've definitely felt over-finned without changing.

Quote: the fin doesn't know the sail size but it doesn't care
I think my fins DO care. A lot. They love me and I love them. Good

Coming soon to Disney Plus Biggrin Because, not only does your fin care, we do, too. Give rose

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Ashevillejanes
FoilDodo
Ashevillejanes
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Re: Changing Fin Size

Or I guess another option is a smaller board with a smaller fin, instead of replacing the fin on the 126ltr, I can just pull out the smaller board that already has a smaller fin!!

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arrogantj
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Joined: 01/24/2012 - 14:11
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Re: Changing Fin Size
Ashevillejanes wrote:

Or I guess another option is a smaller board with a smaller fin, instead of replacing the fin on the 126ltr, I can just pull out the smaller board that already has a smaller fin!!

Lol

Yes, smaller board great option as you won't be bouncing around like a truck on a bad road. Unless you are a committed speed freak, consider a freeride or even FSW as you go smaller as it will turn more easily and be a bit more mellow. Still plenty of speed but you will have turning and carving options. Or, get both...

--- The Arrogant Jerk: Crabby and irritable since 1998.

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Ashevillejanes
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Re: Changing Fin Size

Ha, I kind of already have a garage full of boards! It might be a problem, or it might not!! Freeride for me for sure!!

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gene_mathis
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Joined: 05/17/2002 - 05:00
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Re: Changing Fin Size

Or...

Run the smallest fin which works with your largest sail and then use it for the smaller sails also. I run a 32cm fin on my main board, (Tabou Rocket 115 liters, 66 cm wide,,came with a 36 cm fin) which works with my biggest sail, 7.5 and also most of my other sails, 6.0, 5.2 and even 4.5. I do not change fins and this board is what I used probably 90% of the time (until I started foiling.) Supposiedly, the smaller fin is faster as it has less drag, but wont plane up quite as quick as a larger fin.

Gene

PS - I probably should have read all the posts above before posting and I am not the guy nitro sails with regularly.

Gene Mathis

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webguy
Ashevillejanes