Ian K said...


So, just confirming, you do measure the foil along along the flow line?

Yep I do

I'm asking because it's been previously claimed here that the "water" sees the foil at right angles to the leading edge. Maybe that's just mathematical jiggery pokery that gives the same answer.

Yes I've seen that and it makes no sense at all to me. I don't have the maths though to give any definite reason.
But I've also see it written elsewhere, that raking a foil backwards decreases it thickness to chord ratio.
I just do what "feels right" to me.

So all your percentages thicknesses are quoted along the flow line?

yep.

Which raises another point. Why do we rarely see fins with any significant length of span where the leading edge is parallel to the training edge? A section of constant chord I suppose you could also describe it as.

2 reasons why I don't do it, a parallel fin puts all the load and flex at the base, not a good construction strategy. And induced drag becomes a big problem with parallel fins

I'm thinking that tapering a foil throughout the full length is maybe a hangup from aeronautical applications where wing strength vs weight needs to be more finely tuned.

Don't forget those pics of fin flex, there are large loads on a fin.
And this thread is about timber fins, although the carbon skin has huge tensile strength, the timber can still compress at the base where all the load is.

Looking at the Seabreeze graph for Mandurah....looks great



I hope the X2 did the job on the 43 :-))

As barrel nuts are very hard to obtain, I'll see how well this stuff taps.

I use Acetal rod for barrell nuts.
Get it from the local plastics supply.
It seems to be similar to the stuff they use in Nylock nuts. Never going to lose a bolt.
I use 1/2" (12.7mm) rod.
I drill the fin head holes to size leaving a 4mm section in the centre drilled at at 12.2 and press the rod in then drill and thread for the bolts after its fitted.
The bolts are always tight in the thread.
Great stuff, never stripped any in dozens of fins and it's cheap.

Te Hau said...

As barrel nuts are very hard to obtain, I'll see how well this stuff taps.

I use Acetal rod for barrell nuts.
Get it from the local plastics supply.

>>

Thanks, I'll see how easy it is to get it here and give it a try.

Waiting4wind said...

Very informative and great thread. I was thinking about making a weed fin. Now that I know what's involved I'll just go out and buy one!

Des you dont have to make a weed fin all you need to do is rebox an old slalom, sometimes the older the better because to old fins were wider at the base and make perfect weed fins reboxed , you mite need to take 5ml off the leading edge box at 45 to 50 and presto ,
after saying that you've got 3 days to make it and ill cya at illawarra on saturday, the words out, a couple mexicarno's and a gringo is makeing the trip from south of the border

Thanks guys. I do have a weekend project ready for some on the water testing... Just waiting for my knee to heal before I can give it a test run.

I found an old, fat and ugly Finworks A box weedy. I put one of those Multi fit bases on and if it works I'll get a proper base on it.

Depends how much spare time and spare cash you have, being retired I have much more time than money.
spending several hours and a few dollars makes sense from where I sit.

Got the first layer of bog on today.




Used a thick mix of q-cell and epoxy, spread it on the fins with the squeegee then smoothed out with a thin piece of plastic cut from an ice cream container.
That's the yellow rectangle on the box. Holding both ends of the plastic tight against the fin edges, using a smooth dragging action from head to tip, can produce a finish that doesn't need huge amounts of sanding.




Next step is cooking in black plastic to fully cure the epoxy, then more sanding and comparing with the printouts.

decrepit said...

Sanded the bog down to the foil printout, then squeegeed on another light layer to fill any low spots.
Just a light sand and they should be ready for boxing.

its going well decrep wheres the bog

decrepit said...

Well a lot of the previous layer has been sanded of.
The new bog layer doesn't show up well because it's fairly thin, especially over the carbon area.
I made the mistake of allowing for the thickness of 3 carbon layers, then putting 5 on, so I've actually sanded into the carbon a bit here.

grteat job carbon is stiffer than bog

Today was time for the bolt holes and threads.




From the left, fin out of mold, drill to fit board fin holes, set square vaseline, fin bolt and clearance drill, pencil, dowelling jig with small drill adapter, taps with wrench.

There are several methods of getting a thread in the fin, barrel bolts are probably the most common, but due to a severe shortage of barrel bolts, I'm trying two other methods.
Where there is a solid substrate I'm tapping a hole in the existing material. Where there's only high density foam I'm having a go at molding the thread in.

Whichever method you use, the hole positions need to be accurately marked for the holes in your board.
(I don't trust tuttle standards, there seems to be a lot of variation between manufacturers ).

Here I use a clearance drill for the holes in my board to mark the fin. Don't use an electric drill, just a bit of hand pressure twirling the bit.




once I have a clear mark on the fin base, I use the set square to draw a line across the bottom and down the side of the fin.
The dowelling jig can then be aligned on the pencil line, set the jig to half the thickness of the base.





for the tapping hole I use the small drill adapter in the jig, wrap the drill with some masking tape at a depth that doesn't go all the way thru the fin. As these threads are going to be weak, I want them fairly deep so I can use longer bolts and get a lot of thread engagement. I remove the small drill adapter and use the jig to hold the tap square.

Fortunately the standard hole in my jig is a nice clearance hole for the fin bolt, use a neat fitting drill in the jig and drill the rear hole.

Thoroughly coat a fin bolt in release agent, (vaseline here), work a microfibre mix, (not too thick) into the rear hole. slowly push the bolt into the hole.





Tomorrow will be the test, how well the release agent worked!!!!

I'm not worried, if I can't get the bolts to screw out, I'll heat the bolt with a soldering iron, that softens the resin, trouble is it'll probably ruin the threads as well.
If that happens I'll have to make barrel bolts out of a large stainless steel bolt shaft, unless I can source acetal rod.

An easier way if you're doing lots of fins, is a dedicated drilling jig, fits neatly over the base and has tubes where the 4 holes go. If you have barrel bolts it's just drill 4 holes, a nice press fit for the barrel bolts and you're away.

A bit more tidying up a coat of sanding filler paint and fine wet and dry will finish then off.

Windxtasy said...

Now I know why fins are so expensive!

I sure production fins are made very very much faster, my methods minimise material costs but are heavy in labour, suits my time rich retirement.
And so far all my fins have been 1 offs, no two are the same, so production methods are more problematic any way.

Thanks Sam,

Well the vaseline did it's job, bolts undid as if they'd been done up fairly tight.
Sanded the front and back of bases for a good fit in the boxes and tidied up a few holes in them today. Tomorrow it's out with the spray paint.

, much easier to find fins on the surface than on a muddy bottom.

Hmmm, yep

Mike that looks super great posting and a great job