From WindsportAtlanta.com: Wiki
Spin out is problem experienced by every sailor at some time. The causes and solutions can be quite complex, but l will try and give some guidance. Spin out is easily recognisable because it feels as if your fin is broken and your board goes side ways in and uncontrollable manner. What is actually happening is the water flow detaches itself from the fin, causing cavition, ventilation or what ever you want to call it. The actual theory of this, l will leave up to the aeronautical experts. Spin out is nearly always caused by pilot error. Some of the situations that lead to spin out are as follows.
Low speed spinout
This occurs when you are just getting onto the plain and the fin suddenly lets go. The water flow over the fin is not fast enough to generate enough lift to balance the large lateral forces that can be applied to the fin at this stage. The faster the fin moves through the water the more lift it generates and then more force can be applied to it. If the problem persists
- Use a larger fin with greater lateral resistance.
- Use a fin with a higher lift foil shape or one that works well at lower speeds. (See section on fin selection.)
- Don't load up the fin until board speed has increased.
- Sail more off the wind until the board speed increases.
- Check rigging of sail.
High speed spinout
This usually occurs suddenly and is caused by a rapid change in flow conditions over the fin. It most commonly occurs when the fin is operating at a angle of attack that is too high or in other terms you are pointing your board too high into the wind. In this situation the fin stalls because the water flow over the fin becomes detached. Unbalancing the fin or sudden changes in direction such as reaching off the back of a wave can also cause spinout. Once again this is usually pilot error. Whenever the board leaves the water it must be landed in a down wind situation where the board and fin are travelling in the same direction and your weight must be over the fin. This will maintain a smooth water flow over the fin surface. The same is true for wave jumping where the board must be turned down wind when landing. Whatever the cause of spinout it can be corrected by.
- Having the correct fin for the conditions you are sailing in. It is no good trying to use an 11 inch speed fin when you are slalom sailing with a 6.0M sail. Generally if you are having problems try a bigger fin.
- Having the correct rig balance. Sailing is a matter of balance. The rig, board, sailor, and fin must all be balanced correctly. Rigging is one of the first areas to consider. A sail not rigged correctly will exert too much side ways force and not enough forward motion. Plenty of downhall will help correct this. Mast position will change the center of effort of the sail which changes the whole balance of the board. A small change even half a inch can make a huge difference. Check board manufacturer specifications for correct positions. Boom height and harness lines will also effect rig balance as well the sailors stance. To avoid too much back foot pressure try to keep your body weight on the boom , this helps to keep the fin powered up. In conclusion if the balance is correct you can sail on the edge of control with minimal spinout.
Place your weight over the center of the board , take your back foot out of the footstrap and push down on the leeward rail to reattach the water flow to the fin.
Choice of a fin can be difficult especially if you are not familiar with different design philophysis. Even with a good knowledge the choice can still be difficult because minute differences to a foil shape can make a large difference in performance with many fin brands on the shelf your local retailer may also be of little help useless they have actually used the product. The best person to ask in the fin manufacturer who has extensive knowledge of their own product. My advice would be to avoid hand foiled fins, so matter how good the foiler thinks he is it's impossible to reproduce fins accurately by hand. Believe me I have foiled thousands of fins back in the days when little was known about foil shapes. My fins are designed on computer and machine foiled to very close tolerances. Which is impossible to achieve by hand foiling.
When deciding on a fin size of type there are a number of things to consider. The area, plan shape, cord length and most importantly the foil shape all have a bearing on performance.
The foil shape is the most important consideration. two thirteen inch fins with the same plan shape and area but with different foils may look the same but will perform differently. Probably the easiest thing to consider is the maximum thickness as a percentage of the cord length (width of fin). This will give a rough guide to the foil type, for example a fin with 100mm cord length and a thickness of 10mm has a thickness ratio of 10%. If you compare this with a fin of 80mm cord length and 10mm thickness the ratio is 12.5%. Generally the higher the percentage the more lift the fin generates for its area. The thicker foil of 12.5% although it generates more lift it generates slightly more drag, but the efficiency can be increased or drag reduced by shortening its cord length, so our fin of 80mm cord length generates as much lift as the 100mm cord length fin because of the shorter cord length and reduced drag, essentially we have increased the fins efficiency there are also other benefits. A shorter cord is easier to jibe and less likely to overpower in the gusts. It also flexes more which gives better control in the chop and in gusty conditions. So are there any advantages with the 100mm cord length fin? Yes, it has more lateral resistance which gives good performance in light winds but once its speed increases its efficiency decreases and it becomes slow compared with the shorter cord fin. It will also overpower quickly and tend to drive the board up out of the water and will be more difficult to jibe.
Lets go back to our fin with the 80mm cord length and give it a thickness of 10%. Now you will find that it does not generate enough lift until it is travelling at high speeds. It will be very fast with a good top end and excellent off the wind. But you will probably require an extra inch in length, to have enough holding power. It would also be very hard pointing high up wind.
A property frequently overlooked , stiffness is the most important after foil shape. Two fins of identical shape but different flex characteristics will effect the performance of a board in the following way. A fin that is too stiff will give the board a rough ride and in choppy water will overpower more quickly. It will be more prone to spinout and will give little warning of impending disaster. A fin with too much flex will handle the chop well but will be hard to get on to the plane and will have a lower top end speed. Ideally the fin should be stiff in the base and be flexible in the tip.