Fin placement for a single fin mini - Now a build thread!

paulr · February 29, 2012, 11:24am

Hi guys

I’m planning a little 5’2" mini-sim-esque single fin, but I’m struggling to sort out the fin placement. In the archives, I’ve found references to a rule of thumb about placing the fin where 25% of the length overhangs the rail, but my problem is a 7in fin doesn’t actually overhang the rail at all. The below picture is to scale and shows where I’m thinking of placing the fin (about 7in from the tail).

I’ll be using a fin box, so at least I can adjust it a bit, but the 7in box I got is now looking too short for any real adjustability, so I was thinking of using a 10.5in box (which is what is shown in the image).

Any recommendations of the size/style of fin and where I should place it?

(FYI I’m making this out of a homemade hotwired 1.3# EPS blank, so I need to sort out fin placement early so I can put in a high-density PU insert before I vac-bag the bamboo veneer).

Cheers
Paul

thrailkill · February 29, 2012, 2:29pm

Use the 10.5 inch box. Set the box 2 to 3 inches up from the tail. The measurement to the base trailing edge of the fin is meaningless. The measurement to the base leading edge is the measurement to pay attention to. I'm curious as to why you chose to go single fin. Nothing wrong with the choice, just curious. I made alot of boards like that in 1970/71. Just not that short, but in the 5'10'' to 7' 0'' range.

sanolocal · February 29, 2012, 2:58pm

A old rule of thumb was whatever your base is, is how far you place it up. It would have a lot to do with the template. The other point to base from is a six foot board 6" a seven foot board 7". Now as I said before it really would have to do with the template I was using. If your tail is all foiled out move forward if your tail is thick move it back. I would run a Greenough style fin and a small box set a base forward. I would even think about dropping some side bites. That would help hold the tail in.

Fun looking toy

pridmore · February 29, 2012, 8:52pm

dunno bout the singley on a mini, I reckon the twins or quads are spot on with that wide tail myself, but please post pics and a report once ridden…

beerfan · February 29, 2012, 9:08pm

Needs a bigger fin IMHO.

RDM · February 29, 2012, 9:16pm

I'm with beerfan - I think a 9" fin would be a better match.

Bills box placement suggestion is also advice I would heed.

lankameese · February 29, 2012, 10:22pm

[quote="$1"]

I made alot of boards like that in 1970/71. Just not that short, but in the 5'10'' to 7' 0'' range.

[/quote]

ahhh ha!

now I'm starting to get it.

Warren

paulr · February 29, 2012, 10:30pm

Thanks for the advice Bill, it’s much appreciated. I’ll place the box as per your suggestion.

As for why I’m going the single, well there’s a little bit of a story behind that…First of all, this will be the board that almost wasn’t. I bought a big block of EPS a year ago and cut four boards out of it. I was going to throw the offcuts away, when I realised I could glue the largest offcuts together to make a blank just big enough for a mini-simmons. So I made up my blank, with the intention of making a quad. Then while I was researching outlines and rockers for a mini, I came across an old single fin in someone’s rubbish that looked just like a mini-sim (complete with wide tail!). It was in a bad way and not really worth repairing, and had some “interesting” design attributes, but the overall planshape with the single just looked right, and looked like a whole lot of fun. So I decided to try my mini with the single fin. My outline is not the same as the board I found, but it did prompt me to make a few tweaks to the design. I will still likely put quad boxes in it anyway, to give me more options, but I like the idea of trying a single, especially since I’ve mostly only ridden HPSBs.

Cheers
Paul

paulr · February 29, 2012, 10:33pm

Thanks for your input SanO. If I place the box like Bill said, I can start playing with fin positions as per your suggestions. Glad you mentioned the Greenough-style fin, because I was starting to think along that path.

Cheers
Paul

paulr · February 29, 2012, 10:40pm

Thanks pridmore, beery and RDM, I think you’re right…the more I look at it, the more I know it needs a bigger fin. At this stage, I just need to get my positions right so I can install the high-density PU inserts. The fun of trying out different fin setups will come later!

You’ve all given me enough info to progress (this project has been sitting idle for a couple of months). If I can get some shed-time this weekend, I’ll cut the outline, install the inserts and post up some pics!

Cheers
Paul

ambrose · February 29, 2012, 11:13pm

CLR center of lateral resistance

not further forward than 1/3 comes to mind

I asked my majik 8 ball shook it and it said that.

one third should be based on the template

rail lines extended to their logical conclusion

(like the immaginary pintail that all simple foils are)

you can do this on a piece of paper

or in the sand or just guess.

beyond that far up it gets squirrely

when your foot gets behind the CLR

extended bases or area fins become

real efficient stablizers on wide tailed

single fins.this is why the wide singles like

the crawford kneelos wern’ t co opted by standing nazis

and tri fins were,the angle of bank of a cornering single fin

exploding out of a g-max turn just would sweep a standing rider

overboard.If you are gonna rail ride it too much fin area

will make the transition from fin to rail a spin out.

greenough flexy poke eye flexys transition from

fin to rail quite subtly .

…ambrose…

am I nuts?..yes

and suffering the effects

of pineapple juice.

paulr · April 18, 2012, 12:13pm

Now this thread becomes a build thread! With pictures!

The motivation for this board was the EPS offcuts I had lying around from the last four boards I made. I was cleaning up my workspace, throwing out most offcuts, when I found three pieces that I realised could be glued together to make a small blank. Hence my 5’2 design made to fit the biggest blank I could make!

First I had to square off the ends of the EPS offcuts, using the hotwire.

Then I glued the pieces together using an EPS-safe contact adhesive.

I did the usual print out from boardCAD, stuck the A4 pieces of paper together on some leftover masonite, and cut out two copies of the rocker template. These were cleaned up on the belt-sanding.

I fixed the rocker templates to the EPS block using some 2" wood screws.

The top of the blank was cut using a hotwire.

The vertical glue line cut cleanly with the hotwire.

The horizontal glue line didn’t cut so cleanly, but I can fix that up later.

The bottom of the blank was then cut with the hot-wire. I had some overheating issues near the nose that caused a bit of melting, but I’ll get to that later (nothing that can’t be fixed with a bit of spackle!)

paulr · April 18, 2012, 12:29pm

Once I started with the notion of creating a blank out of otherwise scrap EPS, I thought I’d try to extend the idea to doing as much of this board as possible with just the bits and pieces I had lying around. Again I stuck together a bunch of A4 sheets from a BoardCAD printout, but this time just on a big piece of 200GSM card that I had lying around, rather than go out and buy a sheet of masonite. I traced the outline onto the blank.

To cut the outline, I used my 90-degree right-angle hotwire cutter.

The outline is always a bit wobbly using the hotwire because there is very little resistance to such a thin wire. But as long as I stay a couple of mm on the outside of the line, all is good!

I then used my 90-degree right-angle sanding block to clean up the outline.

The outline came out nice and clean using the sanding block.

And that completes the outline of the board.

paulr · April 18, 2012, 12:33pm

That’s about all I can be bothered posting tonight. I’m a fair bit further along the build process, but it takes ages to post up all the photos on here, so I’ll leave some more for later.

Cheers
Paul

ambrose · April 18, 2012, 7:08pm

wow bitchin deluxe kudos award

goes to you pauly boy.

spring inspires us

a little work every day

will get you there.

I love your 90 degree hot wire cutter running plate concept.

and the simplicity of your template tuner too!

…ambrose…

so nice to be infused with

genius when waking up

in the morning,it

contributes to the feeling

it will be a fine day.

LeeD · April 18, 2012, 7:44pm

I'd place it as Bill said, look for a 10" swept log fin ASAP, and plan to go longer but narrower.

Think CheynHoran. His single fins had 17-19" wide tails, measured a foot forward of the tail block. I remember his fin sizing around 13". He can turn, of course.

We made plenty of 13" swept sabre fins, with narrow 4" bases, and they seemed to hold in much better than wider, but shorter fins.

Worse come to worse, augment it with some sidebites.

Foil and rider placement, wide point, all affect where you stand, which affects where and how big a fin to use.

paulr · May 30, 2012, 12:55pm

Thanks for the kind words Ambrose. I really can’t take any credit for those tools, as I’m sure I saw similar ones on Swaylock’s at some time or another…

Cheers
Paul

paulr · April 19, 2012, 4:47am

Thanks for your input Lee, I’m certainly taking Bill’s advise on board (well, mostly anyway! I’ll get to that later…). I like the idea of a long and raked fin, but I didn’t realise that Horan had such long fins. Someone else suggested a Greenough style fin, and through some PM discussions I’ve had, rail design in combination with fin choice have been bouncing around in my head.

I’ll probably end up making a couple of fins myself to play around with some options. I was thinking around 10", but based on your suggestion, I think I’ll do one around the 12-13" mark as well.

Cheers
Paul

paulr · April 19, 2012, 11:24am

The next step is to shape the deck of the board. Now I don’t pretend to be a shaper (this is board #006 for me), so rather than cutting into it with the planer, I need a more systematic approach that is accurate, symmetric and repeatable. Additionally, because I don’t have a dedicated workspace, but just build boards under an old tin roof by the back gate, I need to keep the area clean and tidy because my wife takes the kids in and out the back gate in the pram. So I need a to minimise the foam dust. The best way to do this is cut deck bands with a hotwire! My approach involves printing out a set of slices through the board using BoardCAD. These slices are every 150mm, and on the printouts I draw lines at 5, 10 and 20 degrees that just touch the deck surface. I then measure where these lines intersect a horizontal line drawn at the highest part of the deck for that slice.

I do this for a bunch of slices, and then produce a table that details the points that make up the deck bands.

I translate these points onto my outline template and draw the deck bands. I don’t bother trying to smooth out the curves, because the inaccuracies in the hotwire take care of that!

I translate these bands to the blank by overlaying the template and poking a pen through the key points.

I then draw the bands on the deck using the faint pen marks from the previous step. Thus I end up with bands that represent the 5, 10 and 20 degree cuts that I need to make. Sometimes the bands don’t look right, like in this picture, but I’ve learnt to trust them, 'cos they’re right! The reason they splay out towards the tail in this photo is because the deck rolls down towards the tail.

To cut the bands, I made an adjustable angle hotwire, which has preset angles of 5, 10 and 20 degrees.

I start with the outer-most cut, which is 20 degrees. It usually ends up quite clean.

So the 20 degree deck bands are cut first.

Then I cut the 10 degree deck bands.

And finally I cut the 5 degree deck bands.

This close up shows the hard transition between the deck bands. Note the glue line, which is a bit messy because it is horizontal and the hotwire had a bit of trouble cutting through it.

I then blend the deck bands into smooth curves using a hand sanding block with 80 grit sandpaper.

paulr · April 19, 2012, 11:45am

Now earlier I said I mostly followed Bill’s advise regarding the size and placement of the single fin box. I will be installing a 10.5" box, like he suggested, but no matter how long I stared at my template with the box drawn on it, I just couldn’t convince myself to put it at 2-3 inches from the tail. To me it just didn’t look right. But then I’ve only ever know the thruster era, so I don’t really have any point of reference for a single fin. In the end, I decided locating the box 3.5" from the tail was the right spot for this board. If I need to, I can always use a fin at the back of the box with a bit more rake.

I also shared some interesting PMs with some sway’s folk regarding other fin setups, so I also decided to add quad boxes as well, just in case I don’t like the feel of the single, or just for some extra options! The quads will be placed in fairly standard locations (rears at 5.5" from the tail, 1.5" from the rail with 1/8" toe, the fronts at 11.5" from the tail, 1.25" from the rail and 1/4" toe).

Now since my EPS is what we in Oz call “M” grade (19kg/m^3), it works out at about 1.3#. The beads are small and tight, and it cuts and sands very nicely with minimal tear-out, but I still use high-density inserts just in case. I route out holes for the inserts using a homemade router template.

Routing out the four quad boxes, plus the center box is pretty quick and easy, and I cut the high-density inserts to match. Perhaps an interesting aside is that the high-density foam I use is PU from the very first surfboard I owned (a 6’7 Gary Taylor from Trigger Bros.) I’m not particularly sentimental, but I think it’s nice to know a little bit of the board I learned to surf on is in every surfboard I’ve made. Maybe that’s why they ride so good!

I glue the inserts in with EPS-safe contact adhesive, and when dried, I trim and sand flush with the bottom of the board.

Now it’s time to cut the rail bands. Again in the name of minimising foam dust, I do this with a hand sanding block with 80 grit.

Then I blend the rails.

And that takes me to the end of the shaping process. Sorry, I didn’t take a decent photo of the finished shape, so you’ll just have to imagine how a board #006 should turn out.