what fin size for a 6.8m on a S type104

[drzone]

Just like the Art and Science in medicine, fin making and tuning is just as much Art as it is science. In this case, probably more like magic. No one mentionned the thickness of the fin, the water state, how much foot pressure you intend to push on the fin, water temperature and salinity.
And it's not like medicine, which is already 'grey', where you either have cancer or you don't.
Here, you are trying to define what is the right fin...what is right.
May as well answer 'to be or not to be'.
Kills me when people introduce formulas like the gospel truth. Just like the gospel, it's only a guideline. I would agree that if SB wrote down the aspect ratio, fin length, thickness and surface area, it would be a good guideline for all.
Cheers.

[steveC]

Hi Ola,

Earlier you made the following statement:

"In any case, publishing the area of the Drake fins can't be a bad idea. Then people can do with this information what they want."

This is getting to the heart of the matter that I commented on earlier. Information is a good thing, and it creates broader base or dimension that folks can reference. Like Del Carpenter, I'm not one with a scientific mind that gravitates to formulas to figure things out. Nonetheless, I still like specifications because they tell a story of sorts. A wealth of simple facts can help us ferret out a path. Personally, I find inituition one of my best guides, particularly when the visual shape of something is revealed. Still a lot of trial and error is possible in conclusions, but there are lessons in that.

In parting, I think drzone is right on in suggesting that Starboard outline aspect ratio, fin length, thickness and surface area. The only other additions I would suggest would be fin material, to include photos/drawings.

[Phill104]

Do you buy a board because of the fin supplied. The supplied fin shoud give a reference point as to where you should start to look when adding new fins to your quiver. If you spin out on a given sail/board/fin then try a different fin, if you feel over finned, change again. There is usually someone with similar kit around that will let you try a fin of theirs. This kind of trial and error teaches far more than any formula could. Starboard alread show a suggested fin range for each board so that also provides guidelines. On top of that the fin manufacturers are very helpfull when choosing a fin so an email to them also helps to make an informed decision far better than any formula could.

I did sit and think about the formula thing for quite a while a couple of years ago but there are just too many variables to get even close to a working system. Even trying to simplify it gives far to vague a system.

It would be good if starboard would state whether the supplied fin is intended for the top,mid or bottom of a boards sail range and each fin name/size in the boards descriptive data to link directly to the relavent fin on the drake site.

p.s. a list of fin manufacturers can be found here http://www.windsurf.me.uk/Links/links.html

[Ola_H]

Phill104 and others:

I have actually not asked exactly how the stoch fin choice ot the different Starboards is made, but from my persoective it seems like each board get a fin which is in mid sail range for the "typical" user of that board. For example, the Kombat and Flare 97 comes with the same fin (Drake Crossover) but the Kombat comes with a much bigger fin (given boards size/sail range). This is (I think) because a Flare is intended fo Freestyle and a dedicated freestyle sailor will want a much smaller fin for a given sail size than a B&J sailor that uses the Kombat would feel comfotable on.

That said, both a Flare and a Kombat can be trimmed towards more B&j (Flare) or freestyle or wave (Kombat), but then you need to look at different sizes and/or different types of fins.

[Dolf]

Hello Ian,
Following is my entry for your challenge to provide a formula. These should all be added to your new Windsurfing 101 section of your website. If my entry wins please send the prize c/o HSM, Maui. Thank you.

REQUIRED FIN AREA
A general guideline for required fin area in normal sailing conditions is:
Required fin area (cm2) = 0.53 x sail size (m2) x sailor weight (kg.)
A smaller fin will result in spinning out and difficulty to point upwind.
Larger fins are unnecessarily slower from additional drag and if much larger result in instability and lack of control.
You can modify the coefficient to make it suit your personal preferences, which will then give you a reference for different types of boards, sail sizes and conditions.
As fin area is very sensitive to speed, experts and racers can use smaller fin areas with a coefficient as low as 0.36*. (Ian please feel free to modify and add to.)

REQUIRED SAIL SIZE FOR WIND CONDITIONS
Your sail size for a given wind speed depends on the type of sail construction, your ability, preference to being over or under powered or if you are racing, but a good rule of thumb is:
Sail size (m2) = 2.47 x sailor weight (kg) / wind speed (kmph)
Sail size (m2) = 0.7 x sailor weight (lbs) / wind speed (mph)
Sail size (m2) = 1.34 x sailor weight (kg) / wind speed (knots)

REQUIRED BOARD SIZE TO FLOAT
For a board to float you in stagnant conditions your board volume (litres) needs to be your weight (kg) plus 25.
This allows 10kg for the board, 10 kg for your rig and 5 kg for your personal gear.
Any forward motion of the board will significantly aid in keeping you floating which allows you to reduce board size.

SAIL QUIVER SIZING
There is no specific correct answer to the right sail spacing as it is a personal choice of the compromise between bigger gaps that give a wider overall range of sails for more time on the water or a narrower range so you can size more precisely for the wind speed.
Generally a ratio of sail sizes of less than 13% is not significant enough to justify the extra expense of having more sails. On the other hand a ratio of more than 20%* can make it difficult to pick the right size.
(*This is our preference but Ola seems to think this should be a bit larger, feel free to change it based on your real data.)

MAXIMUM SAIL SIZE FOR A BOARD SIZE (This is an iffy one)
There are too many factors such as board type, shape, width, tail width to provide a formula for the maximum sail size for a board volume. On average this may provide an estimate:
Max sail size (m2) = 5.9 x Board size (litres) / sailor weight (kg).
Or conversely:
Minimum Board size (litres) = .17 x sail size (m2) x sailor weight (kg).

FOOTSTRAP POSITIONING
Add the info from your catalogue

[Dolf]

Hello Del,
?It was presented as a formula for determing the "minimum required fin area" needed. Dolf, do you have a formula for determining the maximum useful fin area??
Thanks for your comment which helped me change my wording as per the post to Ian above. The rational was that 0.53 is the magic threshold for my wife and I where spin out becomes unacceptable. Hence we pick the next larger fin above the 0.53 limit. There is not such a black and white threshold for maximum size, the performance just gets progressively worse as you go larger. And for a beginner at slow speeds there is no maximum.

?And does the length of the board change either formula??
For the 99% of us less than experts: No. This is based on empirical data that I will elaborate on to my future response to Ola when I get some more time.

?And wouldn't the board width in the fin area also change the formula??
No. The maximum fin size that a board can take is related to the tail width but that is beyond my expertise and it is a different subject than the strict determination of required fin area.

Please let me know how things work for you as my formulas are always a work in progress and can use meaningful feedback to keep improving them.

Thanks,

[Dolf]

Hi Ola,

I will emphasize again that the formula is not intended for experts. They do not need it. They could use it but it would have to be extended to a more complex matrix. I think that your example points out that the formula would work for you and Ian if you were to take the time to record enough data so that you could make the adjustments that work for you.

Now for the other 99% of us windsurfers: the formula was developed empirically using data from my friends with a range from beginner to very good (but I again emphasize not expert) with formula boards to wave boards and all kinds in between, with Drake formula fins and freeride fins to all kinds of brands of wave fins, from 9.8 to 3.7 sail sizes, over a 9 year period. I plotted the results and was astounded how consistent the data was given the huge range of parameters. That led to the formula, not the other way around.

It is easy to poke holes in any formula using extremes and exceptions, at the risk of loosing sight of the simple fact that this is a 9 year proven tool that provides excellent results for the vast majority of windsurfers in a huge range of applications. Again it would be fairly straightforward to extend the formula to cover the full range but that is not my intent The intent is to make life a lot easier for the huge majority of your customers and to keep it simple so that for those of us it is easy to use within our normal applications.

I could start poking holes in your arguments as it relates to my formula, but I think that it would be more effective if you kept track of the ?best? fin area for all your different conditions rather than looking at the possible ranges for the different set ups in conditions where you are not using your superior skills to make it work. An example is how you are able to use a terribly small wave fin that I would spin out like crazy, to get out to the waves, because that is the size that is ideal for surfing in, or in speed sailing when you bear off until you have sufficient speed to load the fin. Then if you find that there is not a consistent coefficient then we would genuinely have something to discuss. Or you could approach it from the other side and size the fins for your wife using the formula for a few months and see if she has any complaints.

You have also brought up a related but separate issue. A huge part of selecting a fin is deciding on the type of fin. And within that criteria there are a gigantic number of variables that are all very important on the performance. These issues are very well covered on some of the fin manufacturers? web sites and you have given some excellent examples. It is a related but discreetly separate issue from selecting the fin area. As my experience is that this is where most of us have the most trouble, this is what I have focused on.

Regarding your PS, this is relatively speaking what I did within my resources, so what is your point?

Now my PS, This has become the wrong place to discuss this and it is way off the original subject. It should have been under its own heading in the Bulletin board, but we have gotten to this point so we may as well finish it here unless you want to move it.

[Dolf]

Hi drzone,

Other than the science of fluid dynamics has largely replaced the art of airplane wing design and behind magic there is usually a clever application of science or simply deceipt, I agree with you. So what is your beef against my formula?

[Dolf]

Hi Phill104,

My experience with fin manufacturers has been at both extremes, so I think it is usefull to have some knowledge in addition to their advice.

I agree that there a too many variables in fin selection but a managable amount if you stick to just the area selection. I know my formula is simple, but how do you find it too vague for the purpose and parameters stated?

It seems to me that the formula and the publication of fin area achieves your stated objectives so we have the same desired objective. What is your exact concern with my method?

[Ola_H]

Hi again Dolf.

I will reply shortly to a few point below, but first I want to say that I think its good work with the formula and a strive for simplification. I have ablsolutely nothing against that, I'm only genuinly interested wether the formula actually holds or not and if not, if it will hold under some certain limitations.

Dolf wrote:

Now for the other 99% of us windsurfers: the formula was developed empirically using data from my friends with a range from beginner to very good (but I again emphasize not expert) with formula boards to wave boards and all kinds in between, with Drake formula fins and freeride fins to all kinds of brands of wave fins, from 9.8 to 3.7 sail sizes, over a 9 year period. I plotted the results and was astounded how consistent the data was given the huge range of parameters. That led to the formula, not the other way around.

If you want to share this data, you can post it to my email. I will then have a look at it and see if I also find a (bi-) linear relationship. Or maybe if would just be enough if you describe the method you used to find the relationship.

Dolf wrote:
It is easy to poke holes in any formula using extremes and exceptions, at the risk of loosing sight of the simple fact that this is a 9 year proven tool that provides excellent results for the vast majority of windsurfers in a huge range of applications. Again it would be fairly straightforward to extend the formula to cover the full range but that is not my intent The intent is to make life a lot easier for the huge majority of your customers and to keep it simple so that for those of us it is easy to use within our normal applications.

I could start poking holes in your arguments as it relates to my formula, but I think that it would be more effective if you kept track of the ?best? fin area for all your different conditions rather than looking at the possible ranges for the different set ups in conditions where you are not using your superior skills to make it work. An example is how you are able to use a terribly small wave fin that I would spin out like crazy, to get out to the waves, because that is the size that is ideal for surfing in, or in speed sailing when you bear off until you have sufficient speed to load the fin. Then if you find that there is not a consistent coefficient then we would genuinely have something to discuss. Or you could approach it from the other side and size the fins for your wife using the formula for a few months and see if she has any complaints.

The examples I used was not choosen specifically to poke holes in your formula and what is interesting about them is that both the freestyle and wave fins are predicted to hold much bigger sails than I think they do in practice. One of my points is that although I could maybe make them work (with a 6.0), I would not encourage any non expert to use such small fins. So, I don't think my level has much to do with this (like one could argue if the formula had predicted what I consider to be to big fins).

I will look at my fin sizes for a while and see what this leads me to. However, even the three mentioned examples indicates that any data set of mine will show quite a lot of deviation from a linear formula.

Dolf wrote:
Regarding your PS, this is relatively speaking what I did within my resources, so what is your point?

Well, my point was that I do think some kind of empirical derived formula is possible, but also that sometimes its not that straightforward to find.


Cheers,