making my first SUP: how about a keel?

General Discussion
1

I am getting ready to build my first SUP. I have been devouring all of the good information on these forum pages. I have a weird idea that I haven’t seen discussed:

Instead of a fin or two, why not a keel? I am thinking maybe a 4’ to 6’ long strip of wood, 1/8" wide, protruding from the bottom center maybe 1/2" or so.

It seems like it would provide good tracking with minimal drag. It should be cheap and easy to build. It wouldn’t be exposed to the kind of breakage that "real"fins are susceptible to. And it would make it easier to carry the board around.

I am sure not going to be doing any racing so I don’t need to turn easily, and I really want all of those other characteristics (good tracking, low drag, sturdy, cheap and easy to build).  What am I missing?

2

Because it would no longer be a SUP. It would now be a SUB (Stand Up Boat). Just joking. Swaylocks has always been about innovative thinking & sharing of ideas so don’t stop thinking outside the box. With that being said, what your proposing sounds problematic to me. What is your intended use? You said your not planning on racing, where are you using it? A lake? the open ocean? in waves?

With dimensions of 1/8" x 1/2", I doubt your going to see any improvement in tracking & without a fin (s?) there will be very little lateral resistence to your paddle stroke, so the board might actually  track worse. In addition, building it is probably harder than you think.Laminating it without bubbles will be a nightmare, & being that thin will probably make it easily damaged from contact.

There are design elements that can be added to a conventional board (that has a fin) to help you with tracking. Trying things like decreasing the tuck of your rail, making it harder & more “boxy” & carrying this element farther forward from the tail of the board. You could also try “straightening out” the outline curve by making the nose & tail wider. A third idea would be to add channels under the length of the board. There are probably other things you could incorporate as well.

A final thought is to practice your paddle stroke. Beleive it or not, there is a great deal of technique to “the stroke” and can make a big differeence.

3

Parthenon, thanks for the feedback.

“Where” - the upper Niagara river, where I live. It’s fairly smooth to choppy, depending, with boat waves. 

You talk like I might have a paddling impairment and need a remedial design! Which may be the case: I have spent less than 2 hours SUPing. But I know from my experience with canoes and kayaks that I like a boat that tracks well. 

My suggested dimensions are pure guesswork, and I could just as easily extend the ‘keel’ the full length of the board - 11 feet - and/or make it deeper. 

Two of the points you raised are definitely valid: the fragility of a 1/8" wide keel, and the difficulty in glassing it without voids.  Maybe I could address both of those by running a fillet of epoxy with microbeads along the keel, turning it into a V. What do you think of that?

Has this been tried before?  Or are you saying it’s just not necessary for people who know how to paddle?

I am planning on making a series of boards. This first one is intended to be a beginner project, both with construction and use. It will be wide and fairly flat on the bottom. I expect my next project will be longer and narrower - more of a touring design. Anyway there are plenty of ways I might screw up this first board, so it seems like a good place to experiment, and if the keel makes it track really well then it’s a good beginner board. And because it’s a learning project, I am looking for ways to keep the cost and complexity down, so it would be nice to not have to buy & install fin boxes.

4

Lee, I am by no means suggesting that you don’t try out your ideas. The only way you will learn is from trying, and then you’ll figure out what  works and what doesn’t. My opinions come from my experience riding out in the open ocean & waves, so your application is probably different.

As for your comment about your experience w/ kayaks & canoes, step back &  look at their design. Knifey & pointed @ the nose (like the bow of a BOAT) to track thru the water. Its’ size is a major portion of the hull design. A litttle 1/2" tall “runner” on a board that is probably 28" to 31" wide is insignificant. I think that your runner would need to stick down 3" to 6" to have any  effect. At that point, wouldn’t it just be easier to install fins? Your going to have to learn to install fin boxes at some point.

As for your paddling comment, I never said you had an impairment. It sounded like you didn’t have much experience and should learn better technique(that comes with experience).

Good luck with your project, and make sure to post pictures of your build.

5

parthenonsurfer, no offense was taken. I appreciate your answers, and I really am inexperienced and unskilled in paddling. That’s the whole point.

So I have been “doing the math” on that SUP keel.

I think the effectiveness of a fin or keel - it’s ability to resist sideways movement - is a function of its size - its surface area.

Let’s take a hypothetical fin: 6" long, tapering from 4" wide at the top to 1" wide at the bottom. It has a surface area of 15 in2. If there are two fins, that’s 30 in2.

Now imagine an 11’ long SUP with an 8’ keel. To provide the same 30 in2 of surface area it would need to be 5/16" deep. At 1/2" deep it would have 48 in2 of surface area - 60% more than the fins.

That’s what I was thinking originally - that 1/2" of keel ought to provide as much tracking stability as a pair of fins. But now I realize there is at least one other factor. Fins serve two functions while paddling: (1) to resist turning while the rider paddles - that’s probably the most important factor most of the time. But when there is wind they also (2) resist lateral movement from wind.

I think, on the 2nd factor - resisting wind - it’s all about surface area, and the keel is 60% more effective than fins.

But to resist turning, the fin or keel is only effective if it’s far from the center - at the front or rear of the board. The portion of a keel directly underneath the rider doesn’t help.

So that hypothetical pair of fins has an effectiveness based on it’s 30 in2 surface area but also it’s location at maybe 5’ from the center of the board. Really going out on a limb here with math, let’s call that a ‘strength rating’ of 1800: 30 in2 x 60".

The 8’ by 1/2" keel would have a strength of 1176.  To get to that same 1800 it would need to be 3/4" deep. 

I am ignorant of the hydrodynamics. Is a long narrow fin just as effective as a short wide one?

If I have to put fillets on either side of that keel to make it feasible to glass it in, does that reduce its effectiveness?

If there is anyone here with the knowledge to think that stuff through for me, it would be nice to know.

Oh BTW it looks like I got really lucky and found a major EPS supplier in my back yard:

http://www.thermalfoams.com

6

Hello Lee-

I’d do the eleven footer with twin stringers, spaced w/ 1" of foam, and tuck in a couple Bahne boxes. Put one in the back and one in the front as a ventral for kicks. I think having boxes long term will give you flexibility in being able to dial in fin area  and a box between appropriately spaced stringers is a super easy (no jigs needed) and super strong install.

A 9-10" SUP fin will carry about 45sqin area and will be 1-up in the back. Ventral fins are usually smaller. The paddle will have around 100 sqin.

What density foam are you planning to use? I am liking 1.5PCF these days.

 

 

 

7

Jrandy, thank you. That’s a really interesting idea, I am going to have to think about that. I like the construction method. I like the idea of, put both boxes in now, use the front one later if you find you need it. Or start with both fins and remove one if/when my paddling skills improve.

I am waiting for a quote from a local foam vendor for a block of 1.5PCF foam - actually two blocks, each 3’ x 12’ x 6".  I really hope I can get that for a reasonable price. If not, it will be Home Depot and I will be gluing up multiple layers of that soft stuff to get the thickness I need.  Yuck.

Something I read, either here on Swaylocks or elsewhere, I can’t remember: With EPS cores you need multiple layers of 6 oz glass & epoxy to get the hardness (dent resistance) you need. That much glass and epoxy provides enough strength that you don’t need stringers.

What do you think of that?  It’s possible that statement applied just to the soft Home Depot EPS.

What do you use for stringer material?

8

Lee, my last couple big boards have been stringered with 1/4" plywood, pieced together with glass and resin.

I’d try to go for 1.5PCF block foam, almost 2x the density of beadboard EPS insulation and no glue lines to mess with your head. Hot wiring makes quick work of block foam.

My only SUP build so far was 3 x 4oz “E” + CF deck patch and 7.5 + 4oz on the bottom, single stringer and was a bit ‘bouncy’ on the stands until glassed. The glassing schedule was courtesy of Dwight.

Keep in mind I am beginner, so your mileage may vary. Take some time to search the interweb and make informed decisions.

PS: http://www.swaylocks.com/forums/design-review-big-boy-longboard?page=3 to see the box between the stringers business