compsand thruster fin placement - how does design play into placement (quad/5 fin)

General Discussion
1

OK after a long hiatus I am returning to my 6’8" balsa compsand thruster project.

I’m setting balsa inserts in for the finboxes and need to know where to place them.

After sifting through the archives it seems there is a fair variation on fin placement suggestions ranging from 3-4.5" (center) and 11-14" (sides)…

This got me to wondering, has the compsand crew found that with the more flexible tail resulting from the construction method call for a change of fin placement?

Or is Burt’s further forward fin setting more a result of outline and rocker?

Bert talks about pushing the fins further forward for small wave boards particularly if the tail has not been made wider. This is not the case on my board, as it has a wide tail.

This board is a 6’8" small wave big guy thruster based on the old 6’8"R clark rocker. It will mainly be ridden in OC, LA and Ventura (SoCal).

The board will be 6’8" x 13" (n) x 22" (c-4") x 17" (t) x 7" (p) x 2.5" (before cutting a 1/2" concave deck) squash tail.

I’m 6’2" and 190lbs.

I using the s-curve tail Bert talks about (Greg called warp tails), were the tail curve is reversed in the last 6"-14".

Does the placement of this reverse curve match fin placement as well? In other words if I’m starting the s-curve at 14" should the front fin also be placed further forward to match the reverse curve? If so where relative to the hip caused by the reverse curve? front, middle or back of the fin at the hip? If the s-curve ends at 6" from the tail should the center fin also move forward to correspond with the s-curve?

I’ll be using pro-boxes so I will have some adjustment (both in front/back and cant).

Note I have not picked fins out yet.

other board specs:

1# eps

1/8" balsa

2.3oz glass under/inside

1/2" balsa rails

flat bottom

2

I have heard that the rockers need to change on a very flexy board.

Your build sounds a little stiff for small waves, you could go 1/16 bottom skins and 3/8 rails (my 2c).

Just add some glass patches in traffic areas.And tails get a little weird after 15.5"

I would recommend that you make your inserts out of some thing water proof.

Pvc,PU,or xps foam.Balsa inserts are a problem area.

I am not so sure on the fin placement.Maybe just behind the hips,a lot of personal preference.

CJ3 made a board like that and went with a 5fin set up.

He is a little bigger than you and loves it.

Try asking around at Compsand.com

Have fun,

Ian

3

gday 4est

that build schedules sounds perfect

just thin out the tail so the back box touches the deck skin

glass ons are a good option to get the tails thinner

or use fcs plug at the back

thicker skins can make better boards imo

when using a low density core,flex is primarily influenced by blank thickness and rail shape

then skin material longitudal stiffness

then material density

and so on

ive built plenty

the thicker the wood the less prone to delamination and denting

ive never had a dent on the deck nor a delam from normal surfing

also the thicker skins allow for a far more snap proof board

the secret is to get the tail to flex is to shorten the rocker

so all you need to do is thin out the tail and keep the rails boxy at the tail

down rails will stiffen up the tail considerably imo

as does deck dome

seal the inside good and your laughing

my boards are 1/8 or 3mm

and rails are 15 mm which is 5/8

and they flex great

id go 1/4 inch thinner overall on that schedule

anything over 2 1/4 requires careful consideration wrt skin type,thickness and density

loose the deck concave (you may want to sell the board later)

chuck a layer of 4 or 6 oz over the top

6 if you want durability

cant comment on fins really

guess its personal

standard works

although i agree with Ian

over 15 1/2 in the tail is the point of diminishing returns

and requires careful fin placement

i haven’t given up on wide tails yet though

will try one more that is super thin as a quad

4

Silly, Thanks for the general compsand design feedback.

Have you found you have changed your fin placements since you went to compsands? If so what are you running now and how did it change? What design characteristic do you contribute your fin placement change to?

Any thoughts on how the placement of my s-curve warp tail should impact my fin placement? Should the back of my front fins fall on the leading hip of the s-curve warp tail curve?

thanks

5

That is a fairly unique planshape.

With that wide of a tail you may do well to use a five fin setup like Griff uses. The board that I made that was close to those dimentions was a compsand 6’5" x 23x 1 7/8" 13"n 16.5 t 5" nose rocker and 1 3/4" tail rocker

I have the option to go 4,5, and 3 on it and 5 has been the best by far. If you are set on 3 you can always add some fcs plugs and inserts after the fact and make it a 4/5.

As for where to place those front fins, If your hip is at 14 then the fronts should be right around 11 3/4 - 12". That is going to be your pivot point no matter what the construction is. The back should be around 4". Use an adjustable rear fin to fine tune it. You will need some big side fins for that planshape in the thruster mode

Forget about the balsa like lavarat says and use some high density foam

Good luck it is a very worthwhile persuit.

Christian

6

Here is a photo of the 6’5"

Christian

7

Fin placement is a personal thing so Bert’s sets might work for him but maybe not you. It depends on where your foot is likely to be…the sweet spot. Your used boards will guide you there.

Your design is very wide including the tail. It will rail surf way better with 4 or 5 fins.

Ive tried a super wide one too (20.5") and I give it 7 out of 10. So my future builds will be under 20". My biggest gripe is loss of rail sensitivity (cuz the rail is further away) and also slower rail to rail action. Its just trade offs.

One thing with the multi-finners…you’ll need to experiment heaps to dial it in. It took me 7 or more surfs with different sets to get it right. Im still experimenting on a finer scale. Its a shame BB has written off 4/5 finners…imo wide compsand were meant for them.

8

all great tips and advice…

part of my challange is that I’m a longboarder and with the occasional exception of a brief in the water board swap (couple of waves) I have not riden a thruster in 25+ years. For me this is a narrow board… My traditional fish and LBs are all 24"+ with the exception of my short board (9’ LB) and that is 23"+.

So I don’t have a sence of my foot placement. I’ll be learning that with this board.

The idea behind this board was a sort of small wave performance pointy nose “Fun Board”. The width was chosen in hopes of making it a early entry wave catching machine. I figure with that big tail it should get in nice and early.

The dimensions were somewhat modeled off of BBs magic carpet. So maybe it makes the most sense to use BBs fin placement…

As for multi fins… I’m game for that as well. Do quads and 5-finners generally have unique outlines/rockers or are they similar to thrusters at least in profile and rocker?

I guess one issue with tuning of a multi fin is this will be an occasional use board so unlikely to get a lot of tuning time. The thought was it would be a short pointy LB for when I go surf with my non-LB crowd.

Would love to hear more comments both on my design and how to fin it, and on how design plays into fin placement.

As a pro-waterskier, I understan the importance of fin settings. In waterskiing we adjust fins to the 1/1000th. With a water speed of 60-70+mph it makes a difference. It seems like the surf industry as a whole found a fin placement (3.5, 11) that worked and moved on. Seems to me with new designs (flex, materials, thinner/wider boards) that placement should be rethought. Guess that is part of what drove the question. Have the compsand crew found that the old numbers are sub-optimal.

thanks

9
Quote:
Forget about the balsa like lavarat says and use some high density foam

What is wrong with balsa inserts?

I plan to put the inserts in before the bottom skin goes on with glass under and over the inserts.

I had planned to add about 1/2" of insert in an effort to avoid tieing the bottom skin to the deck.

Should the leash plug go into an insert as well?

or will the 1/8" skin be adequate?

thanks

10

id recomend CJs advice on your fins setup

12 and 4 seems to work well on wider tails

you want waterproof inserts for plugs and leash plug

all plugs leak

use foam and preferably seal the hole before installing

the idea is to fully insulate the core

balsa is like a wick

11

part of my challange is that I’m a longboarder and with the occasional exception of a brief in the water board swap (couple of waves) I have not riden a thruster in 25+ years. For me this is a narrow board… My traditional fish and LBs are all 24"+ with the exception of my short board (9’ LB) and that is 23"+.

Okay makes sense.

Can you pump a board? Do you want to? Its kind of important.

If not, a 4/5 finner will likely suit you better.

But like CJ3 said, with all the fin plugs you can try it any way it works best…2 fin, 3, 4 or 5, bonzer 5…its a beautiful thing.

What can you tell us about the fish you have? There should be good design input there.

Yeah like Silly said, forget balsa inserts - no good. Hi density foam, preferrably non absorbing like divinycell et al. You may be able to scrounge some at a local marine boat repair facility. Better yet, if youre not poor, buy a 1" thick sheet (~$150). Fin inserts for years.

12
Quote:

Can you pump a board? Do you want to? Its kind of important.

If not, a 4/5 finner will likely suit you better.

But like CJ3 said, with all the fin plugs you can try it any way it works best…2 fin, 3, 4 or 5, bonzer 5…its a beautiful thing.

What can you tell us about the fish you have? There should be good design input there.

Yeah like Silly said, forget balsa inserts - no good. Hi density foam, preferrably non absorbing like divinycell et al. You may be able to scrounge some at a local marine boat repair facility. Better yet, if youre not poor, buy a 1" thick sheet (~$150). Fin inserts for years.

{Hum… the post did not come through… I was wondering why I had no reply…let me fix it}

I guess I’m going shopping for some d-cell.

As for pumping, yes I can pump but not the most proficient, so maybe I’d be better off with a quad or 5-finner.

My fish is 6’2" x 18.5 (n) x 23.5 (wp at C+6") x 18 (t). The tail is definately too wide for OH though seems to work nice for knee to waist plus. On over head I have to take the drop and bottom turn as a knee board and then stand up as I come down the second time. It screams down the line with a little pump. It has lockbox keels. Other than being oversized it is a pretty traditional Lis style fish. Except for the deep concave in the chest area going to double concave out the back.

{ see pic of fish in next post }

The template tracing is 1/2" wider than the final planned outline, though it gives the general idea. It is a little tough to see the s-curve though it is the BB design were the template is reversed from 6"-14". First the standard curve was drawn and then the reverse curve so if you look close you can see both curves in the tail. Yes it does look a little scarry wide… though it is still narrower than BB’s magic carpet. The planned outline is 6’8" x 13" (n) x 22" (wp @ c-4") x 17" (t) x 7" (pod).

{ see pic of template in next post }

Now for the quad/5-fin setup? Will I have a reasonable setup if I install the thruster option at 4" and 12" and then comeback and install the rear quad with in the thruster cluster?

Thanks

13

and I hope here are the pics that did not come through with the previous post…

my fish:

my new template:


165×549 24 KB

14

Not to disagree with the advice of those who have taught me so much… but I say go with the surftrux for fins. If you have a longboard background you will love the ability to take a step forward on the board and glide in trim, and with a board that wide it shouldn’t be a problem. Less drag when not actively pumping = more fun for me, anyway. Just something to throw out there.

Pat

15
Quote:
Its a shame BB has written off 4/5 finners...imo wide compsand were meant for them.

OK, now that I think you guys have all sold me on buying more fins :slight_smile: and going with the 4/5 fin option. What is is that make cmpsands such a good match for the multi fins? Is it that the compsand lends itself to the wider board out of the desire to keep it think for flex? or is there something else there?

Would love more feedback on the question I posted in my last post w.r.t. placement of a thruster vs quad vs 5 fin option.

thanks

16

heya 4est, long time no! Big Sur this year? Check the Mckee site for fin placement; http://www.mckeesurf.com/brucemckee/multisystem.htm

My next two balsa comps will be a 7’6" and 6’6" (both ~ 23" wide x 2 1/4" thick at the rails concave deck at 2") using Mckee’s formula for a 5 fin placement . Probox set up. I route out the 1lb foam and insert 2lb plugs for the boxes.

Good luck!

17

I’m not a fan of the 5 fin setup (at least for longboards). So far I’ve tried them with 3 different fin setups and I’m not seeing the magic. I have one more five fin setup I’m going to try, but so far none have equaled the two big side fins with the small trailer. However, I am open to the idea of the 5 fins working on smaller boards. On my 8 footer the big side fins don’t create magic and I would be interested to try more fins. I might end up putting in the extra plugs (I already have the cores in the board) to see how it goes.

18

sorry been busy today…

set your thruster set as std,

then add two more rear rails where std quads go.

its that simple.

wrt my comment…ALL wide boards benefit from more than one fin at the rails, not just compsand. But BBs boards are REALLY wide, thus even more applicable. Over the years, BB has discussed at long length the benefits of fat foils and cutaway fins etc etc…with one of the objectives having more speed coming out of turns. Thats what 4/5 fins do really well. There’s just more speed on rail, period. IMO, BB is totally missing out.

wrt to your design intent…you will shocked…the question will be whether youll be pleasently shocked, or disturbingly shocked. BBs boards have a very sig bias towards extreme performance…your surfing will dictate how far you can take it. For you, I would recommend a cleaner outline over one that is hippy, esp if your surfing more down the line rather than in the pocket. Too many variables to judge at this point.

The first build is usually really really bad (too stiff), or really good. I lucked out (by mistake) and got a good one first time at bat…havent looked back since. However, Ive yet to build one like BB does…Ive got my own way that works best for me. Just keep in mind…its a journey.

19
Quote:
heya 4est, long time no! Big Sur this year? Check the Mckee site for fin placement; http://www.mckeesurf.com/brucemckee/multisystem.htm

My next two balsa comps will be a 7’6" and 6’6" (both ~ 23" wide x 2 1/4" thick at the rails concave deck at 2") using Mckee’s formula for a 5 fin placement . Probox set up. I route out the 1lb foam and insert 2lb plugs for the boxes.

Good luck!

Tim, not likely to make it to Big Sur this year… knee surgery 9/20 and then lots of rehab. Hopefully I’ll have a new quiver of boards to ride when I can get back in the water.

As for McKee’s formula… yeah I had looked at that when going through the archives. Seems a number of people feel that he sets his rear fins of the quad config too near the stringer. Anybody care to comment hear on rear quad placement w.r.t. the front fins? I plan to set the thruster config at 4" and 12", 1.25" from rail and prob 1/4" toe. Going to use pro box so I’ll have some front/back adjustability and adjustable cant.

20
Quote:
...For you, I would recommend a cleaner outline over one that is hippy, esp if your surfing more down the line rather than in the pocket.

Craftee,

Please explain/expand? Are you suggesting that the outline I have drawn above is too hippy? Are you suggesting that I avoid the s-curve? Thought was it would help that wide tail go rail to rail. I prob tend to be a down line surfer though decided to build a thruster (now a quad/fiver) to do some more top to bottom pocket surfing. Guess we’ll see where I end up.