What does decreasing the size of side fins do on a thruster?

General Discussion
1

We talked about this in an earlier thread, but it mostly had to do with dropping the back fin altogether and increasing the size of the side fins on a thruster. Interesting topic.

So, this morning, while looking at my boards, I noticed that my 7,6 pintail tri-fin (semi-gun) with three fins of the same size, has SMALLER side fins than my 6,10 fish/hybrid thruster. The fish-inspired board (Rusty Pirannah) has large side fins, like a twin fin, and a small three inch trailer in the back.

SO, my question, finally, is: How will reducing the size of the side fins affect performance. I plan to leave the three inch trailing fin in the back. I don’t want to go drastic, but drop down to smaller fins. I only use the Pirannah in head high or smaller surf. It spins out a bit in 7-8 foot surf on backside turns because the tail is pretty wide (15.5 inches).

What are your thoughts?

Thanks

2

Reducing the size of your side fins will change the emphasis how you need to ride your board. The biggest benifit will be that you’ll have less fin drag when paddling. But, you will need to rely upon your trailing fin more. On a narrow tailed gun, that’s not as much of an issue because as you get on rail less of your trailing will be lifted from the water. But, on your wider tailed hybrid this can become problematic unless you’re over finned now. To keep more of your trailing fin in the water on your hybrid you’re going to need to weight your back foot more and get on rail less. This means less trim more drag and less umph generated out of your turns in weaker surf. This is not a good thing for hybrids.

3

Hi Fairmont,

I’ve been a big supporter of using smaller rail fins on my big guy thrusters. Basically the smaller the rial fin the less drag you’ll feel, but you lose drive/pump and some tight turning response. I find it very helpful in weaker surf when you just want to trim cruise more. Basically it feels a bit more like a 2+1 than a std thruster. Better suited for trimming than pumping.

The smallest fin I use in the front is an FCS G3, but mostly use G5…my boards are about 6’6 and I weigh 175lb. Hope that helps.

4

Rail to rail transistion is slower with larger size side fins. Larger sides give a bit more drive but add drag so accelleration is better but speed may not be increased. Or it may be. Also larger sides increase resistance to the wave face, kinda like a thicker rail accept only where the fin is. This changes the balace of the board in a turn. Generally, I use larger more swept side fins with a smaller back fin in flat faced waves and smaller more upright side fins with a larger back in hollow stuff. There is no right or wrong. It’s all about balacing fin aspects for the conditions. If you feel the board hanging up, or it feels sticky, put on smaller sides.

5

Well, it’s an interesting situation because I haven’t ever surfed a shortboard this close to perfect in my life. In the early spring, when we had those late north swells (5-6 foot) and those early, cranking south swells (ahhh, remember those days? Hot air, glassy surf, overhead. Again, I’ll say aaaaahhhhh).

I digress: In the early spring, with head high to overhead surf, I was surfing better than I ever have in my life. The board was so responsive. It would do anything I wanted (except air, which I’m afraid to do because I don’t want to break anything—the board or me).

At the time, the board was set up like it is now. Two large side fins (5.25 inch) and a three inch trailer. Great combonation. Loose and fast, especially in hollow surf.

But, with the surf being about two to three foot, It IS sticking a bit. If the waves have punch, like they did the last couple days at Grandview—waist to occasional chest high—and steep, the board takes off like a rocket, front side or back, but in turns it sticks, so I can’t get my cutbacks all the way back around to where I’m back up and going down the line. I’m crashing into the whitewater before I can lift my nose enough to bank off the whitewater properly and bounce back. Therefore, I am crashing into the whitewash and eating it.

I don’t want to tame the cutbacks. It’s my favorite move and, frankly, the only move I’m pretty good at, frontside.

So, since I am pretty confident the board will continue to move quickly, I’m hoping that putting in smaller side fins will loosen it up a bit without losing speed in the turns.

Part of the problem is that it’s a 6,10 and I’m currently riding it in two to three foot surf. I should be on a 6,2 or something, but I don’t have one and probably won’t get one in the near future (except my 6,1 fish that’s still in the glass shop getting, well, glassed).

6

My observations of fin configuration relative to fin size have led me to conclude that there’s an optimum total fin area that’s appropriate for 2+1, tris, twins + trailer, and singles (and probably twins, 'tho my experience here is limited). My optimum total is 13 to 14", factored using fin depth only for 2+1’s and tris, and depth + base for singles. I tend to configure at 13" for smaller waves and 14" for large. Examples would be…2+1: sides at 3 1/2, center at 6 1/2 or 7".

           tris: each at 4 1/2 to 4 3/4". 

           twin + trailer:  5 1/4's + 3". 

           single:  9" x 5".

If I go larger with the center, I go smaller with the sides, and vice-versa.

The conclusion is drawn primarily from beach break conditions with wave faces from 3 to 15+', all over the east coast and Pacific side north Costa Rica, and 1 summer trip to south shore Hawaii. All my boards have Futures sides with center box. Whenever I move outside this “optimum”, board performance seems problematic, the most glaring example being my inability to use any of my boards as twins because the Futures sides max out at 5 1/4" leaving me underfinned.

Anyone have experiences that are different from mine?

7

No offense intended but when you start talking about fin depths like there isn’t any other way to measure a fin it drives me crazy. Factory Future Fins may max out at 5.25" but I’ve made them way bigger.

My take on decreasing rail fin size (over all surface area) is simply this:

If you keep the center fin the same size then you know what you’re talking about. Once you change it the whole equation changes. So with that said if you change the size of you rail fins you change the amount of rail drive you have. It’s that simple. You can give a board too much rail drive and slow it down. You can give it not enough rail drive and slow it down.

When fins begin to interfere with each other you know that either the rails or the center is too big for the board.

Some boards, particularly boards with relatively more tail rocker usually require more rail drive to work well. Flat rockered board are a different story. Some want lots of rail fin drive some don’t it depends on how they’re configured.

Putting the right fin set on a board is best done by a very experienced shaper IMHO. Or one of the legends like George or Curtis.

Mahalo, Rich

8

Well, I certainly don’t want you crazy now, eh? I’ll do my part for your cure but the burden is, of course, on you to control your misinterpretations and reactions. I’ll be pulling for you in your struggle with the formidable bear of emotional distress. No need to fret about offending me, I don’t grant you the power…and I wouldn’t want to deliberately add to your baggage.

Your response to my post is a non-sequitur. Re-read the post. I’m proposing what my experience leads me to conclude is a quick and dirty fin selection process using only depth as a guide. It’s both explicit and implicit that I’ve ignored numerous variables in multiple categories in the premise. It’s optimum, not instant-case, globally perfect.

Bear in mind that if we all relied solely on establishment, “expert” opinion, there would be no reason for you to make custom fins. I’ll further note that I can’t recall any shaper/expert who has suggested or supplied a production fin configuration outside the parameters I specified.

You’re a custom finmaker whose opinion could provide me with input I don’t currently have. Feel free to respond, provided that at least part of the response addresses the premise in a relevant manner.

9

I have never figured out why anyone wants side bites, when singles shaped properly ride just fine. I have never found side bites to add much.

Hello Laconic. Good getting to meet you this past weekend. Let me know about the EX and the board. We will catch waves next time and I am still expecting steak instead of chinese sonny boy. I want a full report when you get a chance to ride it. I think as much as size is the rake or lack of rake and rail shape.

10

Hi-Ho, Solo, good meeting you. You’re OK, and a man of your word. I’m stoked about the “real McCoy” you got for me and can’t wait to give it a go-truly a visual work of art. Sunday in Mel Bch was not up to Saturday’s high-standard and I returned home early. Did your chase of “Alex” surf Sunday in Jax pay off? You’re welcome to flop at the Mel Beach bungalow anytime…'tis the season! I’ll e-mail you when I’m planning to be up there. Drop in!

Re singles, I’m a bit conflicted-I reckon there’s a time/place for them, since my last 2 ordered boards have been singles. If you need convincing about the versatility and efficacy of 2+1 set-ups I’ve got 2 stix in my quiver that weren’t in Mel Bch last weekend for you to try: 7-6 Loehr half-fish custom epoxy that works super in stomach to a bit overhead as a 2+1, twin + trailer, and tri; and an 8-6 custom epoxy Pointblanks/Patagonia “GG”, best in 4-15+ as a 2+1 or tri. At 2 7/8 thick, the Loehr is likely marginal for you at your current weight, as is the Pointblanks at 2 5/8 thick. Cabbage-patch it , dude!

Looking forward to seeing you soon for a sesh. My treat for a great, pig-out, full rack of rib + half-a-chicken dinner at THE primo area BBQ joint.

11

Laconic,

My big old self rides a Forstall Cheater that is 2.5 thick and 22’’ wide x 7’2’’ and I have no problem paddling it. It’s timing. I have had a two plus one a few times and did not really see a diff. I could ride that point blank of yours without much trouble…I’ll keep my 9’0’’ stall for me though. I am curious about Geoff’s dome on a 9’0’'? I still think rather than decreasing fins on the side it’s better to simply go single or keep with regular thruster set up. I don’t think really small small sidebites make that much diff. but then again you know about opinions…

Yes we will catch some waves and no the surf in Jax was only waste high on Sunday. Skunked.

12

Hey Laconic1,

I’ll do my best to be relevant here but your definition may be different than mine.

Certainly we are in agreement that if we all relied on so called expert opinion we wouldn’t make much progress. I also agree with you that some shapers are very progressive when it comes to fin selections.

I’m going to take the risk of assuming that the over all point of you post was one of balance and that center fin drive and rail fin drive has to be balanced, that too much of one and not of the other make for very bad performance, that a given amount of fin surface area will produce a given amount of drive and drag if foils and fin templates remain constant are the perimeter that your expression is in. All of them are note worthy and accurate.

So there is merit in what you say and in retrospect I believe your observations are in some ways revealing. I’m sorry for if the spuds were not quite done when I served them. Balancing a fin set-up is a most worthy one point indeed.

My point is simply that there are lots of other variables and certainly I might have said things better.

You said, "It’s both explicit and implicit that I’ve ignored numerous variables in multiple categories in the premise.

It’s optimum, not instant-case, globally perfect."

The first sentence is an accurate accessment for me but I have to depart from you with the justification offered in the second sentence. For me optimum and globally perfect is a bit of stretch.

If I get a little sideways on you cause I get too far forward on my board and break my fins loose it’s cool the wave will catch up. My fins are probably set-up too stiff and it’s the only way I could turn.

You may feel that my answer was reactionary and I respect that. My point is that to make fin sizing purely arithmetic can be very misleading. It’s a response that comes from experience, some what emotionally charged I must admit, that wants to say, Wooo!" It’s about passion. I know mine is showing and I respect yours.

What I’m getting at is simply that there are many different fin templates and one four inch fin can be very different from another.If we’re talking generic fins, and if there is such a thing, then I really don’t have any big problems with your equations because, again, in the end it think your point is one of balance. That your looking very carefully at how fins play off each other is a very important part of what we’re looking at. Over all fin area does play in this no doubt, but I’m sure you’ll agree that there are more fins out there than a G5 or an X5 and it will take a few more strokes to paint a complete picture.

Consider just one issue that tends to skew things. It is center fin placement. One might move the center fin fore and aft to tune a given set-up to the conditions and find an amazing difference in how a board will surf as one slided a relatively small center fin aft when the board has sufficient rail fin power to hold a good line. What I’m getting at is the a center fin has more power as you move it aft, which tend to skew the arithmetic picture. There are other issues but I think one will suffice.

Not to worry, no baggage here, the water’s so warm all I need is a spring suit –

If quick and dirty works for you go for it man. I’ve seen guys out on sponge boards that can surf circles around me on the best equipment I have. (oops – did I say that?) Some guys just rip on everything and a quick and dirty coupling can be fantastic, Oops – did I say that? No offense (-;

Good Surfin’, Rich

13

Rich,

True…True…it can be very nice indeed!!!

A waterman doesn’t need no stinking fins. Some of the most fun sessions I’ve ever had in my life we’re on a 11.625" X 14.25" X1.5" self-shaped-packing-foam-softee…Argghhhh.

And my day’s spent riding a plywood Paipo…Hmmmm I wonder if?..Hey I don’t need you guys! All I need is a sheet of Plywood! OK - no more custom stuff for me…

daddio

ps. How are the Quad and Thuster set you’re making me coming along?

14

I wrote this some time ago for ESM magazine. It’s a bit simplistic but these are the findijngs I’ve found with fins over the years.

CHANGING FINS

INTERCHANGEABLE FINS

The concept of interchangeable fins is probably almost as old as fins themselves. With the exception of a short period during the Ô70s fins for the most part have been glassed on. Now with the advent of the first truly lightweight dependable fin box system for thrusters, made by numerous fin companies fin systems, we can now take advantage true interchangeability.

ARRANGING FIN SETS

Arranging fins in sets that make sense to your individual surfing style and to changing conditions can take a lot patience and experience. The purpose of this paper is to make you aware of the significant advantages of interchangeable fins and to help you get a feel for how fins work.

HOW FINS WORK

The front fins create lift, facilitate faster turning and give the board more speed out of turns.  But they also slow down rail to rail transition giving the board a ÒstickyÓ feeling like its about to catch an edge if theyÕre too big.   

The back fin is for control.  It has the effect of pulling the tail of the board down and anchoring it to the wave face.  The bigger the fin is and/or the further the fin is placed towards the tail of the board the stronger the anchoring effect.  The disadvantage is they can also create drag and make the board bog.

WAVE SIZE

Small waves are generally slower than large waves.  Therefore for small waves you will need as much lift and maneuverability as possible while control will not be as much of a concern.  Large side fins with a small trailer fin will create this effect.   For large waves you will generally want more control and need less lift.  So you will probably want more back fin and possibly less front fin than you used in small waves.

WAVE SHAPE

Flat faced or mushy waves call for a board with more lift and glide.  Hollow waves call for  quick rail to rail transition and good hold.  For mushy waves a regular set of side fins with a small trailer fin will probably work better.  For hollow waves small side fins will allow fast rail to rail transition and a larger back fin will anchor the tail securely in those steep sections.

SURFER SIZE

Larger surfers should generally use larger fins. Smaller guys arenÕt generally as powerful and need less fin. This is mostly regarding the front fins.

TROUBLE SHOOTING Q&A

Q: My board spins out off the top and slides out on cut backs. Should I use bigger fins?

A: A larger back fin will help your hold but your problem is probably that your side fins are too big. First try a smaller set of side fins and if thats too loose and slides to much add a bigger back fin.

Q: When IÕm cutting back my board seems to stick right in the middle of the turn. I canÕt seem to complete a full roundhouse. What gives?

A: The problem is that when your turn is in transition from a carving turn at the beginning of the cutback to a pivot at the mid point of the turn, the fins are fighting each other. Fins must work as a unit. A smaller back fin will restore harmony and solve your problem.

Q: Help!! I canÕt seem to get my board planing. IÕm always bogging and going slow. My friends donÕt seem to have this problem. Please help me.

A: You have a very common problem to which there are two possible solutions. The first solution is a smaller back fin. This will reduce drag and enable the board to plane more easily. If this doesnÕt solve your problem it probably means your board is too small. Ask yourself these Questions:

(1) Are you bigger than your friends and trying to ride the same size boards they do? (2) Do you have trouble catching waves and when you do catch them do you have trouble getting up? If you answered yes to either of these questions then your board just maybe too small. There is nothing wrong with riding a board that is the correct size for you regardless of what your friends may ride. After all if your friend wears a set of size 28 pants IÕm sure he wouldnÕt expect you to wear the same size even though your waist is 32. Surfboards arenÕt any different. Get yourself enough board!

Q: IÕve been riding my board with just the side fins and it seems to work fine. Is this O.K.?

A: You bet it is. IÕm for any fin set up that works for you. In fact there are a number of really fine surfers like John Holeman who use this set up in small waves all the time. You may want to keep a trailer fin around for those hollow days though.

Q: IÕm using a regular set of thruster fins in my board for bigger days and I just canÕt seem to make it around some of the mushy sections that I feel I should. Could you suggest a possible solution?

A: A lot of surf spots here on the U.S. mainland tend to get mushier and more flat faced as they get bigger. If this is the case at your local break then the solution would be to use a smaller back fin that will reduce drag allow the board to build speed more easily and glide a bit more. This will also allow the board to have a bit more down the line speed, allow it to get a little further onto the flats in front of the wave and get you around those sections.

Q: I like to noseride but my board pearls and sometimes the fin comes out when IÕm riding forward. Can fins help me noseride better?

A: You bet. Noseriding can be improved by using a larger back fin. This will hold down the tail while you camp out on the nose.

Q: When IÕm tube riding hollow waves I always seem to ride down into the lip and get crushed. Its kind of frustrating and hurts too. What can I do?

A: Your fins need to hold you higher in the pocket so you wonÕt slide down and get crushed anymore. Increase the size of your back fin.

15

i must have missed something…

i did agree with greg on how he sets his fins for small waves and hollow waves , i use the same formula , and im assuming greg came to those conclusions the same way i did , go surfing , get good guys to test your boards…

what you said at the begining of your post greg, that was a bit more subjective and dependant on how the rest of the board is set,

going back to fairmonts problem of wanting to not lose the feeling of his favourite small wave board …

2 things come to mind actually three …

dont put smaller sides in , the board isnt set up for it, youll hate it …

if you want the nose to lift a little higher and get better small wave hooks and back onto the foam after cutbacks …

1 best option , i dont know how you could do it appart from custom fins , you need more cant on your front fins…

2 either move your front fins back 1/8 at the same time or the other option is move your tail fin forward 1/8 …

but option 1 will make the biggest difference…

regards

BERT

16

I agree. Not all fin systems allow for these kinds of movement and the article was directed to improving exsisting boards. O’Fish’L does make fins with two different cants now and I’ve found your number one to be right on. I also agree with the number two choice. In fact my personal fin measurements generally run 4.5 and 11. I find that the close cluster fits my personal style better.

17

Greg,

I guess this is my question: If you put a 8.5 or an 8.00 normal base normal rake single on a 9’0’ with a 14.5 tail; What good will small tiny sidebites do? It seems to me there is already enough fin in the water, why add more drag. Maybe on one of the really narrow singles, but mostly I see guys riding sidebites with pretty big center fins.

18

I’ve experimented with fins a lot and putting sidebites on does make the board accelerate a bit better and turn a bit shorter, like putting a toe in the water in the old days. They do slow rail to rail transition a bit though. Longboards are very different from shortboards when it comes to fins. So much speed in a longboard is built into the planing area and there is more drive from the long rail line. So you don’t see the drastic differences between fins that you do with shorter boards. I have had many customers who have smaller feet (girls mostly but some guys too) that much prefer a single because they aren’t able to put as much weight on the rail. That outside fin takes more to push into the water. With shorties so much of the speed is based on acceleration and the fins do so much of this in modern boards. So to your question, do they do any good. For me, yes they do. For you, maybe, but maybe not. The only way to find out is to try. And then realize that just because it did or didn’t work for you doesn’t mean someone else might get a different result.

19

Halcyon, I appreciate the reasonableness of your response…I anticipated some justifiable return flame. I’m familiar with the generally positive, informative, content of your posts. My apologies for the excessive use of sarcasm. It’s something I’m aware of and working on.

It’s clear that we’re in general agreement on fin permutations. My question, however, still has not been answered, and I now have to consider that I was in some way unclear.

New Q, 1 of 2: Does anyone regularly use production fins on any size board for 2+1, tri, or twin + trailer set ups whose total fin length/depth(only) measurement runs less than 13" or more than 14"?

New Q, 2 of 2: Does anyone regularly use production single fins on any size board whose total fin length/depth + base chord sum runs less than 13" or more than 14"?

I’m chuckling as I write this, thinking that I’m the lone sane person in the asylum which, of course, makes ME the sole candidate for the straitjacket.

20

New Q, 1 of 2: Does anyone regularly use production fins on any size board for 2+1, tri, or twin + trailer set ups whose total fin length/depth(only) measurement runs less than 13" or more than 14"?

Answer" On my 9’0"x23.0" Freeline I’m running 5.25" rail fins and a 7.0" center so that’s 17.5" total but because of the way the fins are configured the surface area on these fins is probably close to a standard set-up with 13.0" total inches.

With this said I must add that an 8.0" cutaway center and 3.0" rail fins work very nicely on the board. That’s 14.0" and fits right into your equation.

I have to say that it is extremely difficult to divine anything meaniful from these kind of numbers that are only associated with fin depth.

If surfcraft is held by arithmetic boundaries its design process will end up a sinking ship in a mechanical ocean – Never the less we wouldn’t be here if not for the digital world (-;

Good Surfin’, Rich