Wondering what causes a fin to cavitate. Every time I use a fin smaller than 52cm it occasionally cavitates, usually just as I'm transitioning to planing. Took my 140 L f race out today with a 7.5 gator and 48cm pointer fin which I thought was pretty big for the sail size and sure enough I cavitated twice in 8 laps. Other guys use smaller fins and never cavitate them. What am I doing wrong, is it because I tend to turn upwind a little to get in the straps just before planing ? do I need to wait until planing till jumping in the straps, turn downwind right before planing or both? or maybe putting the back foot in too early. It doesn't happen to often its just annoying having to take the feet out and get going again. I must of skipped along sideways for 150m today staying in the straps turning downwind trying to get the fin locked in again but the only way is to get out of the straps and start again.
What you describe is almost certainly 'stalling' and/or 'ventilation', not 'cavitation'.
Cavitation generally only occurs at quite high speeds or extreme loads. Unlikely on a large fin at low speeds.
As for your issue. Try putting you back foot in the strap first so you don't upset the board/rig CofE balance and round up.
It's also better to run slightly broader (off the wind) when accellerating onto the plane to unload the fin somewhat and increase your acceleration.
Select to expand quotesailquik said..
What you describe is almost certainly 'stalling' and/or 'ventilation', not 'cavitation'.
Cavitation generally only occurs at quite high speeds or extreme loads. Unlikely on a large fin at low speeds.
As for your issue. Try putting you back foot in the strap first so you don't upset the board/rig CofE balance and round up.
It's also better to run slightly broader (off the wind) when accellerating onto the plane to unload the fin somewhat and increase your acceleration.
That makes sense I'll try that I've always put the front foot in first thinking that's how it's supposed to be done sometimes I find it difficult to get the back foot in second on formula boards and short wide boards
This is about spinout, so I'll copy and paste something I wrote elsewhere.
Most people blame their fins for spinout before they blame themselves but it's worth understanding what is happening.
Spin out happens when the sideways load on the fin is greater than the lift it generates.
To reduce that load you first move the harness lines back on the boom. Secondly you adopt a more upright stance when sailing, and that means shifting the mast foot back( yes, back) and you can also move the front foot strap to a more forwards position.
Both those changes bring the mast more upright, fore and aft, and the board is then driven off the front foot more.
Fitting a bigger fin is usually the worst thing you can do because that keeps you in the stance rut that is causing the problem in the first place. You'll find that beginners often sail with fins that are too big, due to their poor stance. When you are trapped under a raked-back rig you inevitably overload the fin via your back foot.
I think the answer is far more simple here.
Outboard foot position loads fin sideways. Its designed to.
You are jumping back into the straps before planing (ie: before the fin is making enough lift) and loading it sideways, so it slips. Deliberately turning upwind will load it more and exacerbate it.
So yes, planing then straps.
All of the above is correct. Sounds like you're loading the fin a little bit early, and its ventilating.
cavitating a fin is actually much harder to do. And much easier to correct.
A bad fin cavitates all the time.
A great fin is very hard to cavitate and is self correcting when out.
Select to expand quotemob dog said..
Wondering what causes a fin to cavitate. Every time I use a fin smaller than 52cm it occasionally cavitates, usually just as I'm transitioning to planing. Took my 140 L f race out today with a 7.5 gator and 48cm pointer fin which I thought was pretty big for the sail size and sure enough I cavitated twice in 8 laps. Other guys use smaller fins and never cavitate them. What am I doing wrong, is it because I tend to turn upwind a little to get in the straps just before planing ? do I need to wait until planing till jumping in the straps, turn downwind right before planing or both? or maybe putting the back foot in too early. It doesn't happen to often its just annoying having to take the feet out and get going again. I must of skipped along sideways for 150m today staying in the straps turning downwind trying to get the fin locked in again but the only way is to get out of the straps and start again.
What fin are you using? Many good fins out there nowadays but some don't to anything else but spin out if loaded ... some old ones (like Drake fins 2008-2010) I think were designed to spin out 
Select to expand quotesailquik said..
What you describe is almost certainly 'stalling' and/or 'ventilation', not 'cavitation'.
Cavitation generally only occurs at quite high speeds or extreme loads. Unlikely on a large fin at low speeds.
As for your issue. Try putting you back foot in the strap first so you don't upset the board/rig CofE balance and round up.
It's also better to run slightly broader (off the wind) when accellerating onto the plane to unload the fin somewhat and increase your acceleration.
I agree with sailquik ,its more like early spin out ,and at low speed ,I have the same board 140 F RACE ,i use a 45 ,that is more swept back but still a race fin ,how ever if I use my very straight pointer fin (slightly thinner ).a 46 ''..yes you have to get going first before in the straps, as it does the same thing ,spin out ,but only at low speed and takes ages to kick back in ,its a love hate with that fin ,but boy does it fly up wind ,so I would say its your fin and needs a slightly different approach to sailing .or get going ..you have to baby it before you put the feet in ..I find heading down wind helps .it hates angles at low speed .
Thanks for all the comments guys I can learn something from all of them. I'll see how i go today. wind will be 23kt today so i'll be using an ezzy cheetah 7.0, I realise the fin is too big for this but its the smallest deep tuttle fin I have at the moment. Below is the fin its the one I got with the board and it wasn't humming just releasing all of a sudden, its definitely my technique, i'll work with all these tweaks and im sure it will be corrected.
give the fin a sand with wet and dry paper 600, put it under running water before and after and you should see the flow much smoother. Also make sure there is no grease, wd40 or sunscreen residue on the blade
Mob Dog you can fit a standard tuttle fin into a deep tuttle box, not an issue you just need longer screws.
I will guarantee your issue has nothing to do with the fin it will be technique. Concentrate on not pushing on you back foot until you are planing. Fins need some flow to generate lift, if you push hard at low speed the fin is not generating enough lift to balance the force you are applying and will let go.
Also if interested in the difference between spin out/ ventilation and cavitation go to the Tribal fins website and there are a couple of videos where Chris Lockwood explains it really well.
As many people have said, spin-out is caused by excessive pressure on your back foot pushing the fin sideways.
The real fix, as applies to almost everything, is to get more skill so you can feel the pressure of the fin and adjust how hard you push.
A exercise you can do is to try and push your back foot down than just stuff your foot in the strap and load up. That will fix the spin-out problem and get you used to controlling the pressure and direction of load on your back foot.
Move your harness lines back. This will balance your weight more evenly across your feet and relieve excessive fin pressure. More downhaul can act similarly.
^^^^ Only a solution if you are talking about fully planing and proper setup.
If his lines are already placed correctly, and the problem is indeed too much back foot loading the fin when not yet planing, the above advice will cause further problems once planing (oversheeted)
As I said I think this is a much more simple answer than all the setup things - its just a basic stepping back too early.
IMHO
Select to expand quotemob dog said..
What am I doing wrong, is it because I tend to turn upwind a little to get in the straps just before planing ? (Yes) do I need to wait until planing till jumping in the straps (Yes), turn downwind right before planing or both? (Yes) or maybe putting the back foot in too early (Yes) . It doesn't happen to often its just annoying having to take the feet out and get going again. I must of skipped along sideways for 150m today staying in the straps turning downwind trying to get the fin locked in again but the only way is to get out of the straps and start again.
Seems you know the fix already.
If you have a lot of weight in the harness & push down on the front hand you will have less sideways pressure on your feet and can get away with putting them in the straps early but you need a good sense of feeling of what the sail & wind is doing otherwise you will catapult a lot.
Like Mark said, a board with outboard straps is designed to put pressure on the fin. But not too much at low speeds. Its also got a wide tail for early planing/big fins, but if you aren't careful that also puts a lot of sideways pressure on the fin.
Got to say a 140l*80cm board, 48cm fin and 7m sail in 23 knots isnt a good match unless you are very heavy.
I'm (78kg) only a recreational sailor, dont race, and would only use that board with a 8.5 or 9m sail in less than 15 knots average wind. I find Patrik's guide of being ok for M/L sailors in 15-25 knots a bit out.
patrik-windsurf.com/f-race-2020/
Edit. Forgot to add video. Weight in the harness and pushing on the front hand, bear away a bit and you can get into the straps fairly early.
8.5m, 125l*80cm wide, and wide tail 57.3cm ofo, 46cm Drake DW 46cm fin
Thought of this topic yesterdayI was moving along, going pretty quick, but on one reach I noticed I wasn't quite planing as fast and then, boom, the tail was sliding out a bit, causing me to push pretty quickly through the front strap to straighten out the board and get the fin working again. But, it happened a couple more times, but I also noticed a weird gurgling noise. Seemed to happen right after running over a small piece of chop, nothing big at all. I didn't think that I was possibly giving the rear too much pressure as I wasn't trying to carve upwind and I have gone with more angle and never spunout.
Well, I slowed down and dropped the sail and looked at the fin. Found a huge clump of seagrass caught up at the top. Pulled that crud off and got going again, no spinouts.
Select to expand quoteaeroegnr said..
Thought of this topic yesterdayI was moving along, going pretty quick, but on one reach I noticed I wasn't quite planing as fast and then, boom, the tail was sliding out a bit, causing me to push pretty quickly through the front strap to straighten out the board and get the fin working again. But, it happened a couple more times, but I also noticed a weird gurgling noise. Seemed to happen right after running over a small piece of chop, nothing big at all. I didn't think that I was possibly giving the rear too much pressure as I wasn't trying to carve upwind and I have gone with more angle and never spunout.
Well, I slowed down and dropped the sail and looked at the fin. Found a huge clump of seagrass caught up at the top. Pulled that crud off and got going again, no spinouts.
It's a good idea to spend some time looking at your fin wake and calibrating what it looks like when everything is going well. It is then pretty obvious when you have some weed, not only in feeling of drag, but the bubble trail behind your fin is more significant. Speed sailing in shallow weedy areas we pretty much have to do that each time we beach start. Once I'm on a plane, I look down at my fin bubble trail before I try any serious speed.
^^^ +1
Even a single small strand of weed on the leading edge an upright fin can knock knots off your speed and make it realy hard to go upwind. A clump is a disaster .
This why weed fins were invented. 
Select to expand quotemr love said..
Mob Dog you can fit a standard tuttle fin into a deep tuttle box, not an issue you just need longer screws.
I will guarantee your issue has nothing to do with the fin it will be technique. Concentrate on not pushing on you back foot until you are planing. Fins need some flow to generate lift, if you push hard at low speed the fin is not generating enough lift to balance the force you are applying and will let go.
Also if interested in the difference between spin out/ ventilation and cavitation go to the Tribal fins website and there are a couple of videos where Chris Lockwood explains it really well.
That was very interesting. I haven't seen it before.
