Fins with horizontal wing area aligned parallel to the surfboard bottom do not work well on boards with anything other than very low tail rockers, regardless of what the manufacturers say.
Such fins need suitable boards and they are only a small percentage of boards.
Selling them as a bolt on to any board which will improve performance is misleading advertising in my opinion.
Also, I think that it's useful to remember when talking about drag that it's the fin's lift/drag ratio that counts more than just the fin's drag. For example if the fin is increased in size in order to do some of the lifting work usually done by the surfboard hull, the overall surfboard/fin drag can be less even though the fin drag is higher.
Often these days we are fed the idea ( recently by Skip frye for example ) that more fin equals more drag and therefore a slower board, but that is an erroneous assumption.
An example is hydrofoil tow in boards which obviously have lower drag than conventional boards but are all fin and have more fin area than most conventional boards ( by conventional here I mean planing craft )
Ray F:
Regardless of planshape & foil, more of any fin will result in more drag. As you point out, however, all planshape/foil combinations are not equal in the lift they provide in comparison to the drag they impose. This being said, all hulls require certain amonts of lift from there fins, in order to turn in the desired fashion. So, in essence, the goal is to provide enough fin for the desired effect, but no more.
I don't believe I'm fed this, as much as conclude it from basic physics, logic & reasoning.
Do you not agree, Roy?
Hello Ray, no I don't
I'll try to explain more clearly.
You say that fins need to produce a certain amount of lift in order to turn but no more. That's not true . Fins also in many cases produce upwards lift. This lift does the same job as planing based and buoyancy based lift from the hull of the surfboard. I illustrated the extreme case of such lift by using the example of a hydrofoil board which is lifted out of the water.
When a fin produces lift which takes pressure off the bottom of the surfboard it reduces the drag ( and the lift ) produced by the hull of the surfboard. Because of this it is not te case that larger fins necessarily add to the overall drag of the surfboard, and in fact larger fins can reduce the overall drag of the surfboard by producing lift more efficiently than the surfboard hull does.
Obvious examples of fins which do this are ones which have horizontal lift area. These include all canted fins, and fins with horizontal winglets, as well as tunnel fins. As explained here previously however, vertical planar fins are also able to produce the kind of lift I've been describing in some circumstances.
By the way it's also not true that "Regardless of planshape & foil, more of any fin will result in more drag" even if just talking about fin drag without taking hull drag into account.
The myth that more fin area necessarily slows a board down can be found all over the surfing industry, and it's become much more common recently due to the finless craze. The myth is supposedly based on physics as desribed by yourself, but it is an incomplete explanation and therefore not always correct.
Rob Lion:
totally agree with Roy on this line of thought. Well said!
I think there is something to be said for eliminating/reducing fins all together, but that requires a different design manifesto, and will have to be debated on a different thread...
Having ridden longboards for a long time, I think it is silly to think that more fin means more drag. It does stiffen the board which makes it feel slower, but if the fin is foiled correctly, it could create more lift and stability. Some of the fastest logs I've had have huge fins, some fairly crude(old 60s boards) and some more refined. More importantly, as has been said, the bottom curve of the board has to present the fin to the oncoming water at the correct angle or you're asking for trouble.
This more fin argument can also be debated by the example of the traditional fish crew. Some of those things have a 10" base, 6" deep. That's a boatload of fin/keel, yet they are faster/looser than a lot of "normal" boards by a mile...
I think there is something to be said for eliminating/reducing fins all together, but that requires a different design manifesto, and will have to be debated on a different thread...
Having ridden longboards for a long time, I think it is silly to think that more fin means more drag. It does stiffen the board which makes it feel slower, but if the fin is foiled correctly, it could create more lift and stability. Some of the fastest logs I've had have huge fins, some fairly crude(old 60s boards) and some more refined. More importantly, as has been said, the bottom curve of the board has to present the fin to the oncoming water at the correct angle or you're asking for trouble.
This more fin argument can also be debated by the example of the traditional fish crew. Some of those things have a 10" base, 6" deep. That's a boatload of fin/keel, yet they are faster/looser than a lot of "normal" boards by a mile...
I understand the concept behind hydrofoils, but I'm not making the connection. The horizontal surfaces of hydrofoils (with a certain amount of angle of attack) provide an incredible amount of surface area to which exert verticle lift by means of Bernoulli's principle. This not only pushes from below, but more importantly lifts from above.
All other things being the same, a 10" deep by 1/8" wide vertical fin would require an expotentially greater amount of relative flow in order to receive a much, much less amount vertical lift. Because there is no upper camber (as in a horizontal wing), the low pressure area doesn't exist to assist in vertical lift. This being the case, only hydroplaning (less efficiency) is taking place...and that's only being applied to a 1/8" wide by 10" deep area...depending on rake.
To exactly which area of a vertical surfboard fin is force being applied in order to induce significant vertical lift the hull? What am I missing?
Rob are you saying with all other things being equal, a longer fin does not create more drag than a shorter fin with the same exact foil & cord?
Rob Lion:
Ray, from personal experience, I have a few longboards with boxes in them, and have tried loads of fins from hatchets, to hull flex fins. I have yet to find a fin that makes the board faster overall. The difference between the hatchet and hull fin is when I hit the bottom turn, the board reacted "faster" with the hull fin, but it didn't start beating sections (just trimming) that I could make with the hatchet. The hatchet is 10" and the hull flex is 9" FWIW. So, the hull fin is smaller in both depth, and area, but was no faster in trim, due to the limit of the board's design. Turns however, were much snappier and smoother with the flex fin, where the hatchet naturally wanted to pivot/stall turn.
And that brings me to my point, that I think what a lot of people overlook when talking about fins and all that is a fin's design will cause you to ride the board differently from one design to another. This can create that feeling of radical speed and maneuverability. If you have a fin that is lively, and turns sharply, you are going to be cutting it up, always on your rails - which alters the discussion as you are now taking all sorts of other factors into consideration. If you have a fin that wants to stall or pivot, you are going to go in a "straighter" line, and ride mostly on one rail which feels slower since it is less active(not really but you know what I mean.) So, which "feels" faster, the one that is always turning, or the guy blazing down the line in trim?
My money is on the bet that neither one is actually "faster" but the impression each approach to riding is altered. After all, a boards design will only go so fast no matter what fin you put on it, unless that fin creates some sort of vertical lift that presents the hull to the water at a different angle - which is what I would imagine one of Roy's tunnels would do, for example.
Ray F. wrote:
I understand the concept behind hydrofoils, but I'm not making the connection. The horizontal surfaces of hydrofoils (with a certain amount of angle of attack) provide an incredible amount of surface area to which exert verticle lift by means of Bernoulli's principle. This not only pushes from below, but more importantly lifts from above.
Hi Ray
Actually the lift produced by immersed foils can be explained equally well via Bernoulli's principle or by Newtonian physics ( the Coanda effect )
The amount of area used is irrelevant to the principle involved. For example the small wavegrinder winglet will provide some lift against the bottom of the surfboard, thereby relieving some of the load on the bottom, in the same way that a hull lifting hydrofoil will.
The main point I was making about increases in fin size was that if the lift produced replaces hull lift then there is an opportunity to reduce the overall drag of the surfboard.
Even completely flat plates produce lift in the same way that cambered wings do. The low pressure areas still exist. inverted wings ( i.e. aircraft flying upside down ) also produce lift which can be explained via Bernoulli's principle.
In fact extremely thin fins with low chord ratios are known to have good lift/drag ratios in low angle of attack situations.
Vertical fin area provides vertical lift whenever the board is rolled away from a horizontal position. This is noticeable when the board is in trim on steep parts of the wave. This lift component is vertical i.e.it is at 90 degrees to the surface of the earth, rather than at 90 degrees to the surfboard hull.
Firstly one needs to ask whether the fin you are discussing is vertical or horizontal, as that makes a big diference, as explained. If it is horizontal then any increase in area prior to sufficient lift being produced to lift the hull clear of the water is likely to reduce overall drag.
Secondly ( assuming a vertical fin ) increases in fin area can reduce overall drag by reducing load on the hull in some situations as explained. Larger fins operate at lower pressure so are less likely to enter inefficient stall situations.
Thirdly we are not restricted to "all else being equal" . . . some fin configurations have less drag than others due to their shape, foil etc, so can be used in larger sizes without a drag penalty.
One has to be careful talking about straight lines in surfing, as some people erroneously believe that trimming across a wall is a straight line situation. In reality surfing in a straight line only occurs when moving at 90 degress to the wave front which is very unusual. Furthermore from the point of view of fins a board which has toed in side fins is NEVER able to travel ' in a straight line' as one of the fins is always under load. Likewise a board with horizontal fin area ( including those with canted side fins) can never be considered to be travelling in a straight line from the point of view of the fins except when airborne.
Regarding 'quantifiably faster' I measure surfing speed on occasion but the vast majority of manufacturers and surfers do not. So one should be careful drawing conclusions about what is quantifiably faster.
Ray you appear to be assuming that a board which is 'faster in a straight line' ( whatever that means ) is less maneuverable than other boards which are slower in a straight line. This is not necessarily the case.
Wetted surface area drag is only one of several kinds of drag produced by fins
and hulls.
Some fins are lower in overall drag even though they have higher wetted surface area.
Also, as already explained, if the fin reduces load on the surfboard hull it can reduce overall surfboard drag by reducing hull pressure and/or wetted surface area
Ray F:
To use Roy's example of a hydrofoil, the foils aren't 5 times wider than the craft. Why? Because after the required amount of lift is achieved, the extra surface area is needless drag.
Yes that's right and it also follows that in the case of foils which reduce load on the hull, the potential to reduce overall drag ( by increasing fin area ) exists up until the hull lifts clear of the water.
Also large fins can be more efficient than smaller ones due to the fact that they can operate at lower pressure .
It all depends as they say, and the 'less fin is always faster ' mantra we hear is false ! Keep in mind that this mantra is not qualified by "once the most efficient fin size has been achieved any further fin area is detrimental to speed' or anything like that, no it is used to try and prove that finless boards are necessarily faster than all finned boards . . . which is obviously NOT the case. . . finless is just a different approach and is not necessarily faster !
Ray F:
Hi, Roy. Just to establish some common ground, we need to differentiate the relative lift of a foil. It's commonly accepted that "lift" is the relative force that takes place along the camber of a foil. In a common vertical surfboard fin, it provides the ability to pivot left or right. I specifically state "vertical lift," to describe any exertion of force that moves the board to a higher location in regards to the waterline. The same effects of lift that occur on the common vertical surfboard fin (pivoting the board right or left) create are taking part on a hydrofoil when the hull raises out of the water. In both cases, the foil has a 2 cambers (either left & right or upper & lower. Bernoulli's principleapplies in both cases.
Where Bernoulli's principle doesn't apply is providing vertical lift to a surfboard with a common vertical surfboard fin. There is no upper camber to provide a low pressure draw. It can only be pushed (or hydroplaned) off of a raked fin. At the average surfing speeds, the amount of force exerted on the leading edge of a raked fin cannot raise a board out of the water with any significance. I don't argue that it is impossible. I just stand firm that under natural conditions, it's not enough to provide a noticable difference.
Firstly as mentioned my previous post fins do not require 'camber' in order to provide lift. The low pressure area exists with uncambered flat plate fins and with inverted foils ( e.g. aircraft flying upside down still creates a low pressure area above the wing ). All fin and wing based lift can also be explained by Newtonian physics ( The coanda effect ) . .. the explanations amount to the same thing. People who were taught the bernoulli explanation often mistakenly believe that camber on the top or low pressure side of a wing is required in order to produce lift, and also commonly believe that more camber is required on top of a wing than on the underside in order to produce lift. Both assumptions are incorrect.
Secondly I'm not suggesting that raked leading edges provide significant vertical lift. Vertical fins provide vertical lift when they are canted by rolling the surfboard on to the rail. We are using the term 'vertical lift' ambiguously, it can mean vertical in relation to the surfboard bottom or vertical in relation to the earth's surface i.e. opposing gravity. When a vertical fin is canted during trimming, angled drops or top turns it takes load off the surfboard bottom by lifting against gravity. Such situations comprise the majority of the time spent on a wave, and thus vertical fins can be designed to use this opportunity to generate vertical lift, and in doing so larger vertical fins are an advantage.
Ray F:
I'm sorry. I've reviewed my posts and cannot figure out how to make it any clearer. I understand the concept of reducing the hull's surface in the water in regards to reducing drag. I believe, at this point, the intent of this thread is beginning to derail. I apologize if I have confused anybody, but I was only reviewing a fin. I did not mean to segway so deeply into hydrofoils & theoretical planshapes as much as address practical (and available) fins from a physics standpoint.
The wavegrinder fin is a 'hydrofoil' fin whether we like it or not because it has horizontal wing area.
Due to its depth below the hull it will lift against the surfboard bottom in the fin box area even though it is set up parallel to the bottom of the surfboard in that area.
The point about vertical fins being used to create vertical lift, and the consequent use of larger fins is one of the cornerstones of my surfboard designs and I have never seen anyone demonstrate understanding of it yet, even though it is esentially very simple.
Rob and Ray I hope that you have enjoyed the discussion so far, I certainly have.
http://royalsurfboards.blogspot.com/
http://magicseaweed.com/community/viewtopic.php?f=13&t=22477&p=313341#p313341
