Longboard Fins for Sailing Dinghy Frankenstein

General Discussion
1

Hi all!

I’m looking for huge longboard fins to use in a franken-dinghy that I’m building and I hope ya’ll might have some suggestions for inexpensive/not awful quality fins.  Preferably the most massive fins that will fit into

The deal is:

  1. I have a small 12' [ply]wood rowboat that I'm setting up for sailing
  2. Normally one would install some kind of trunk in the middle for a centerboard/daggerboard.  That's a lot of work and takes up lots of space in the boat and I'm not into that
  3. It's totally OK with me if it sails bad or not at all, mostly this is a fun science experiment for me
  4. I plan to use epoxy putty & glass to attach a pair of longboard boxes to the bottom and then use a pair of longboard fins in place of a center/dagger board
  5. Of course, windsurfing fins are a thing, but windsurfing boxes (tuttle/deep tuttle/power) are deeper than a regular US/banhe/longboard box and since this is going to be frankensteined onto the bottom of the boat, that's not ideal
    1. Also those boxes attach the fins by way of a screw through the top, which in my case would be a screw through the bottom of the boat, not ideal
    2. I also hope to use one of the breakaway longboard fin screw tabs so that hopefully I don't rip a hole in the boat if I ram into a log or beach.

Fin wants/requirements

  1. **size**.  Back-of-the-envelope math suggests that a pair of bigger D-fins will provide a similar amount of surface area to the centerboard that you're *supposed* to build for this boat.  Probably the bigger the better.
  2. **strength**. These are going to take mostly sideways load because their purpose is to resist sideways movement from sail action.  So probably solid fiberglass?  Also would be great if they could survive maybe getting crashed into on occasion.
  3. **cost**. I have to buy two of these and really don't know if this plan will work at all so buying super-nice fins is probably not sensible.  I feel comfortable re-selling them if it doesn't work out, but I'd rather not dump a bunch of $$ into nice fins just to discover that this plan is, in fact, crazy.

So anyone have reccomendations for durable, big fins that aren’t too expensive? Or have a pair of matching huge fins they want to sell me?

Thanks!

2

Surfboard parts is for surfboards,

Sailboat parts is for sailboats.

I would spend the extra few hours to cut out, foil, and glass worthy  parts.

I would consider a leeboard that is mounted externally on a pivot bolt and a rudder/tiller combo with proper hardware.

My 8’ sailboat uses an external  12" x 48" leeboard that is about 30" immersed when pivoted down.

The rudder blade is 9" x 30" with about 24" immersed.

Both are basic plywood foils that are glassed.

Plans for these are mostly based on the ‘catbox’ version of the puddle duck racer:

What are your plans for a sail?

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3

With Jrandy here. 

 

The LB fin boxes adhered to hull is not a Plan I would pursue in this application.  The fin boxes are likely not strong enough for lateral loads when not recessed into something, and the biggest  of LB fins might not provide enough surface area.

  Something else can be done better for cheaper and be more effective

4

I’ll chime in alongside jrandy - 

You have some problems in the basic concepts here:

First, if you’re going with a lot of fin area and real strong fins, you need enough beef to hold them on the boat, no? Making the attachment light and breakaway is a little contradictory, right? 

Something like a bottom patch of two layers of 3/4" ( 20mm or so) marine plywood well and truly glued and screwed to the bottom, edges well faired, glassed or painted really well so they don’t delaminate on you. Then, rout a close-fitting series of slots in it, a series so you can move said fins around and find out what works, And not small patches, You need to spread the load out. 

You need this sort of beef because the load on said fins isn’t minor. 

Epoxy putty and glass isn’t gonna come close to cutting it. 12’ boat, how thick is the ply on the bottom? 1/4"? 3/8", possibly 1/2", with little or no framing, right? You would hope said boxes break away, without taking a large chunk of the bottom with it. And that’s just sailing the thing, running into things, well, yeah, won’t be pretty. There is a reason centerboard boxes and their structure are kinda beefy. 

Surfboard boxes are probably not going to cut it either, they are not built for this sort of loading. The thicker, beefier windsurfer boxes are closer to the kind of loads you’re looking at. I say closer, but again if you jack up the size the loading increases and not just a linear increase. They might be too weak for this. Also, you want fins with a long, narrow shape- they are more effective if they go deeper, close to the surface/hull not so much.

jrandy’s suggestion of leeboards, something that hooks onto the rail on the lee side, that’s a good one. Set up so you can move them, again to see what works. The rails/gunwales and the sides themselves are a helluva lot stronger than that poor innocent plywood bottom. 

Now, jrandy had a Really Good suggestion that I’ll also agree with. Puddle Ducks. A bunch of us up here ( the surf shop softball team, actually) built several. We have a freakin’ regatta, no less. No two alike, salvaged Sunfish rigs, mostly, but by no means exclusively. One guy built a square rig version. Really. Had cannons on it. Building skill ranges from excellent to ‘oh, this is gonna be entertaining to watch’. 

Ridiculous amount of fun for not much money, all standard lumberyard wood. And we are not talking a major woodworking shop or serious cabinetmaking/boatbuilding skills here. 

http://www.pdracer.com/

Have at it. hope that’s of use

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