Are there any rules of thumb for making a weed fin out of an upright fin. I have a 42 cm slalom fin that I'd like to convert to a 35 cm equivalent weed fin.

G.day Shifu, A weedie at 45 degrees will be (from memory) the 42cm divided by 1.41 giving you about 29.7ish. You can mess with the amount of rake needed, depending on the type of weed in your area. We suffer from heavy weed infestations and require a greater rake to shed the stuff, sometimes as much as 53 degrees, nearly as much as the delta. In this case the fin needs to be moved forward of the box to move the effort closer to the rear foot. Just remember, if you can flex the fin afterwards, start again with a thicker foil. Go forth and cover thy neighbours house in dust But, wear a good dust mask.

Also, if you find that the box is a ****teenth too large, you can use some meguires release wax on it to seat it, then it will smell a lot better than fish

Inside every slalom fin there is a weed fin waiting to come out

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elmo said..
Inside every slalom fin there is a weed fin waiting to come out

ONLY if it's stiff!!!!!!!!!!!
A flexi slalom fin is useless as a weedy. Put it over your knee, if it flexes more than a couple of mm, lokk for a stiffer fin.
Also a point to remember, is that the thickness to chord ratio decreases by about 20% when a fin goes from upright to 45deg.
If it's already a thin foil, it may be too thin as a weedy.
So a thicker slalom fin will be less flexi, and be less likely to end up too thin.

I've successfully converted thinner, flexi slalom fins, by adding a couple of layers of carbon to them, it thickens and stiffens them. But they need bogging and reshaping, a lot more work, than just reboxing.

Thanks guys.

So the plan is to take a surplus 42cm Black Project Type R and re-box it to around 40 degrees to make a weed fin that will give me the same lift as a 35cm upright fin. The Type Rs are reasonably thick and quite stiff so it seems it will be an ideal candidate.

I want to use this fin for blasting with my medium gear (6.5m + 104 litres). We don't have weed at our local, but we have heaps of Eagle Rays hovering under the surface. At the moment I am having massive collisions every few weeks and I am wrecking gear and slaughtering the wild life. I even hit and killed a Sea Snake a few weeks ago. I am hoping to make a fin that will give me the performance I like but will lessen the strength of the impacts I'm having and maybe give the rays and sea snakes a fighting chance.

I have re-boxed fins before so there are no workshop challenges, but I am uncertain about what depth I should aim for to get the same lift as a 35. I took a look at the chart on the Black Project site and it seems a depth of around 28 or 29cm would work.

I'd like to know if that seems reasonable.

You have to be very careful with black project fin lengths, sometimes they measure actual depth other times it's fin length.

For most situations, you need a similar depth weedy to the upright, depending on your board, it's not just lift that matters it's also leverage. If you have outboard straps on a wide board, that leverage is necessary to keep the board trimmed, if there's not enough the board will want to sail windward rail down, so you'll either be slow, or have sore ankles from keeping the windward rail up.
going down to a 29 when you want a 35 upright seems too big a jump to me. An upright reboxed to 40deg is going to be less powerful, (less thickness to chord ratio), than the original and have more induced drag, (it's easier for the high pressure side to travel with the flow, along the fin to the low pressure side).
But I always sail with weedies, you need to someone who regularly swaps from one to the other.

And you can still cut a ray in half with a weedy, I think Crashers done it twice now.

Check this thread
www.seabreeze.com.au/forums/Windsurfing/General/The-making-of-Weedzilla

Just for comparison, with my 105 Gecko and 6.5 Cheetah for comfortable B@J sailing NOT speed type sailing I usually run a 36 upright fin and to get the same feel with a weedy I have to run a BPWeed 42 which is 33.6mm depth
But the Gecko is 6cm wider than your Cross

Your BPR 42.5 has an area of 344cm2 is 11mm thick and 106mm wide at the head
The BP@Weed 42/33.6deep has an area of 352 cm2 is 12mm thick and is 150cm wide at the head

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ducati said..
Just for comparison, with my 105 Gecko and 6.5 Cheetah for comfortable B@J sailing NOT speed type sailing I usually run a 36 upright fin and to get the same feel with a weedy I have to run a BPWeed 42 which is 33.6mm depth
But the Gecko is 6cm wider than your Cross

Your BPR 42.5 has an area of 344cm2 is 11mm thick and 106mm wide at the head
The BP@Weed 42/33.6deep has an area of 352 cm2 is 12mm thick and is 150cm wide at the head

Thanks for this awesome info ducati. This will be a big help when I finalise my plans.