Attaching a skeg/sword/tracking fin on a surfski?

"Ability before stability" ..
means start from a start beginner surfski and work yourself up through the models when you improve and use seat pads to make yor comfy craft more unstable to train the upgrade to the next less stable model.

So I was thinking of making my own weed fin (those small ones in front of the rudder),
but then wondered,
why not - for the fun of trying random stuff - place a tracking fin (like a sword for stability) under the hull (under/behind the seat) for making a tippy ski more stable, eg increasing the force needed for rolling.

This would be the other way round compared to adding sead pads to a stable model in order to gain instability for training, as here one would go for the instable model ealier but then reduce the length of that "sword/skeg" when one improves, until in the end, one would remove it completely or put it on again in conditions when one requires extra stability.

For playing/testing this out, one could use the industrial type strong velcros of tesa and attach the fin(s) to the hull. Besides easy to cut-out DIY shapes for testing, alternatively many clip-on tracking fins are around for SUPs, surfboards (like those rubber fins) or inflatable paddle boats.

... or use two fins parallel off center towards the sides like twin keels or bilge fins (not to such an extreme extreme to the outside tho and short, like those short" tracking fins", see google) ... or ... or ...

I wonder if that is a completely stupid idea? Has surely been tried out several times over the decades I'd guess and there'd be reasons why one does not see surfkis with skegs/swords/tracking fins. Forces on the hull in ocean conditions might be a concern, but making a tippy ski more stable for flat water fitness racing to skip the model upgrade journey until stable? Wondering.

before anyone complains that sailboat swords are to large for surfskis , google "tracking fin" with image search > regarding the shape, I mean those small clipon-on style fins

Funny you should bring up this topic. Having bought a V14 recently, I have been advised to get a surf rudder to reduce some of the initial twitch of the ski. I think a skeg would probably aid greatly in reducing the roll. I also own a SUP, and a very effective way of reducing roll is to put the right fin on it.

I say go for it. Would really be curious to see your results.

I don't think it's a stupid idea. It's done all the time in stand up paddling. Here are couple of things I can share on the subject:
1. Velvro may not work so well; probably too much drag, and it may come off at speeds in excess to 18km/h (when catching a wave)
2. When using a rudder and a skeg combination, try to go with a weedless rudder (it's usually not as deep and therefore makes the ski a little more forgiving when hitting the pedals).
3. You could place a FCS fin box, but I would personally place it somewhere between feet and seating area (obviously on the hull). Should you go against the idea, filling those two holes will be a breeze.
4. FCS type boxes also give you ample fin choices
5. Having tried a fin (thin dagger) well in front of a rudder on a SUP, I can tell you that the dagger acts as a pivot point for the rudder to go around: it actually makes the board more responsive to direction changes. Unfortunately, I haven't tried the system DW.

Thanks, Kwolfe

Yep, using a larger surf rudder for adding stability to a tippy ski has been discussed here already, even before your thread as the search function shows. As mentioned there, it has the drawback that that if you hit a pedal unintentionally to much you might make a too hard turn and tip over. Thinking of it, it's likely your recent thread fertilised my subconscious.

So why not keep a small marathon rudder but add one of these small surf rubber fins
(max. height 10cm, length 10cm), means not a skeg as large and as located as in the link below, see right column:
www.axiskayaks.com/kayaks-and-surfskis/options1.htm
Note: read down, retractable skeg, they say use either one, NOT using both at the same time.

I don't want a skeg for tracking, but test if fins could hinder rolling but keep manouverability. Hence, I'd even prefer to add two very small rubber fins (6cm height, 10 cm length) in parallel (e.g. as twin fins) and off-center, somewhere behind the seat, just for the fun of testing.

...or ... when not ashamed of anything, attach two long T-shaped rubber profiles along the half the length of the ski, ahem, to make my own "clip-on chines" for whatever effect that has (other than slowing down the ski like additional fins ...).

I'll gues my current compositie ski is not tippy enough for me though for guestimating on the effect of such fins, but I'll then just raise the seat ...

As I wrote earlier, it's just for the sake of trying randon stuff for the fun of it or simply having silly ideas while not caring at all if it has done before and deemed stupid.

When done, I'll post pics and a scientific analysis (NOT) proving my deep understanding of fluid dynamics (NOT).

Spacehopper w.b. the guy to know

photofr tanks for the input,

"Velvro may not work so well; probably too much drag, and it may come off at speeds in excess to 18km/h (when catching a wave)"

Yep, with "For playing/testing this out, one could use the industrial type strong velcros of tesa" ...
I rather meant for testing where to plave the fins and if it really has an effect on rolling. Clearly woud not withstand open ocean downwind.

On the FCS boxes, yep, saw them when googling, however I only want to play around for the "AHA"- or "OOPS"-effect not going machining the structure of the hull . An if it were to work, test barge wakes (10-14 km/h here) with it, lose the fins (might attach a fishing line to keep them lol), but if it made sense > then think of attachments.

Yep, I have a small marathon rudder in use anyway.

photofr,
I am in the decision making between a 560M (snug fit) and 560 (more comfy reg. stability) ... so you see where this idea might be coming from if Freud could squeeze the real motivation out of me ...

as in: be patient and raise my seat hight in the current ski and TRAIN vs. be impatient and attach a fin to the too-tippy-for-me-damn-it-I-should-have-known-better-and-waited-a-year model lol

ahahah... then get a 560 (they come with two bailers).

Take one of the bailers and screw-in a fin made out of wood. If you don't like it there (which seems like the near perfect location anyway) then you just remove the current bailer and use the other one that came with it.

WAIT: might be cheaper to use a friend's Elite ski and screw a wood fin on their bailer?!

photofr, for the latter I'd need to find another surfski paddler first, make him my friend and trust me,
make him lend me his ski ... even the first part would take too long lol.

But the bailer idea for one rudder seems good, thanks, although I fear any structural damage from side forces. Will think about it, thanks again.

What about strap-ons for testing?
One could then even use two instead of one fin and place them at different locations along the hull for testing ... ok, not fixing that thing it too tight, best with wide thin loop rubber band and an additionional fixture on deck so you don't lose that assembly when fixed somewhere behind the seat. Placed under the seat, one could use the side handles for fixing (if present and dep. on strength). Either all glued together from available rubber stock (with rubber fins sitting in cuit in pockets in the saddle) or .. when time I'll do a test casting of having all integral as one Piece (fins and rubber saddle).

Later, when one knows where to place those 1-2 fins, one could try to laminate a thin FRP layup saddle with slots for the fins. Well only, if this all makes sense at all.

strap-on-fins lol:

The strapping of fins (with straps) would work, but drag would be a concern and may mask what you "feel".

Speaking of feeling:
Try paddling on glassy water with and without your bailer open: you'll already notice less roll. That tells me that just like on a SUP, you do not need a very deep fin.

Two smaller fins in a twin setup would seem to be advantageous, but placing them properly would be another story altogether. A single fin setup is in my opinion "more than sufficient" to really help stability.

...

so, got the other fins today, made a pic with a dollar bill to get an idea on size ...
off testing now ...

result of the tiny and thin fins in a nutshell: tippy skis, here I come

How large was the affect? I would have thought that they were not deep enough but I guess I was wrong. Also, any noticeable drag?

Shallow is usually more efficient than we think regarding ventral fins. However, that usually means more than 2 inches in depth, but less than 5".

My best guess: the one longer than the dollar bill make not be deep enough.

photofr: you wrote "Try paddling on glassy water with and without your bailer open: you'll already notice less roll. "
I do not know how much bigger your bailer is vs. the smallest fin in the pic but yep, of the 3 fins tested, the smallest one had least effect on rolling. But you feel the small one once moving faster.

kwofle: additional drag ... " princess and the pea", she might have noticed, I didn't so far, except a bit for the largest fin, but boat also had a new feeldue to the new "pivot point", when turning , I had to assist more with leaning to like the feel of a jard turn, yep, noticeably more rolling torque needed compared to finless.

"Experimenting with Rudders
Thursday, 17 April 2008 08:26 | Latest Surfski News
I enjoy the science and design of surfskis nearly as much as paddling them and have experimented with a number of things including:

A low aspect ventral fin under the seat of the paddler
A winged rudder with an endplate
A hydrofoil located on the bow of the ski"

www.surfski.info/reviews/surfski-gear/ta...ebuilt.html?start=20

3 inch deep fin, placed about 3 feet from the bow: that's not theoretical, that's what I can share from experience with my 18' foot SUP board. It actually makes the board turn faster!

Uffilation wrote: Spacehopper w.b. the guy to know

You rang sir?

You've seen this (www.surfski.info/getting-started/tips-ot...31/radical-red7.html) I would assume? Be interested to hear from the author of that about his findings after making the mods.

Not sure if you've noticed with your DIY fins that it (should) reverse the roll direction when you steer? i.e. with just a rudder the ski rolls into the turn - with some sort of foil forward it should tend to roll away.

I suspect it would make the boat more nimble if it was a narrow/deep (i.e. high aspect) fin. Pulling the centre/daggerboard completely up on a dinghy makes it handle like a sack of spuds and needs far greater rudder angles (i.e. drag) for the same amount of turn.

For a given ability level I wonder if the added drag of the fin could be offset by making a narrower boat more manageable and the reduction in form/wetted area drag - plus better manouvreability.