Which fin to buy and use

Our dealer told us they are on back order too.
Sounds like we've got a few weeks wait
Mickey

Having done one summer with stock fins, one summer with ST fins, and now a winter (only one outing so far) with turbo fins, my thoughts are this...

Stock fins: OK to start, but definitely lacking something.

ST fins: Gave me what the stock fins were missing, but still allowed the shallow water gunkholing (exploring in shallow back water coves, creeks, & inlets) that we like to do on the Chesapeake.

Turbo fins: Lot more kick than stock, and a good bit more than ST fins as well. Easily maintained 3.5 - 4 knots with relatively little effort. Will see over time how the extra depth requirement impacts our exploring.

It all comes down to how you use your boat.

Happy Sailing,
- Jim L

Extra depth requirement? A common misconception. Turbofins require no additional operational depth. Here's an example of an AI (Adventure mode) with Turbos exploring a shallow creek -- the depth here is about 6". The fins are operated with one foot forward with shallow strokes so that they "flutter" just under the hull (remember the fins can come up slightly beyond horizontal with the AI).


There are very few places you can't go with Turbofins and a little technique, as long as you're afloat!






ST's are excellent fins and might be a first choice if you're a old cyclist and want to "spin" your Drive. Standard fins are probably best for places where the fins are likely to get cut up (oyster bars). But for me, Turbos rock -- I don't go anywhere without them!

Hey Roadrunner,

I read your seminal post on clew tension with the Turbo fins and it got me to wondering: were the speed tests you conducted over a standing or a flying 100 yards? To use an auto analogy, if loose tension roughly equates to a low gear and high tension to a high gear, you wouldn't do too well over a standing quarter mile starting in third gear and you would do even worse over a flying quarter mile staying in second. Were you assessing maximum speed with your Pitot tube or the overall time to complete the course?

It seems to me that the job of the Mirage drive and fins is to convert work done by the legs into speed through the water. The main source of inefficiency (energy loss) would be from turbulent flow in the water. The problem with the standard fins seems to be that they generate so little resistance, that you have to paddle too fast to generate a given amount of work. I haven't used Turbos, but from what people say on the forum, I gather that when they are screwed down tight, they offer too much resistance for many people to pedal comfortably at speed. Having just acquired a set of ST's, I am wondering if screwing them down tight would generate performance comparable to Turbos set to loose, given a comparable amount of energy expended. Intuitively, I would have thought that tighter fins would generate less turbulence than looser ones, since the fin is pointing more in the direction that the boat is moving.

Obviously, whatever the theory, the proof of the pudding is in the eating, so I will have to try the ST's at different tensions. Unfortunately, I dont have the opportunity to compare them with Turbos at present.

Chris

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Lots of good questions, Chris. Here are some random thoughts and attempts to answer your questions.

Standard fins stall out at higher boat speeds because they don't have a good aerodynamic shape along the cord or cross section. If you look at the ST fins you'll notice the two different colors of material. The firmer black protects the edges and also fingers into the softer gray zone from the rear much like battens. This is not decorative; it gives the fin a better aerodynamic shape much like a wing or sail where the maximum camber is forward of the middle. This shape is able to provide thrust at a higher boat speed than the amorphous standard fins.

I don't subscribe to the first, second and third gear theory. IMO, with Turbofins, the instruction about tightening the clews for more speed is not valid. Sorry to disagree with Hobie here, but I've done fairly detailed comparisons with front and rear fins separately regarding clew position and I don't get the same result. The concept sounds good in theory using a fast swimming tunafish as an example, but the Mirage Drive is set up a little differently than a tunafin. Again, just my opinion (not being an expert on tuna propulsion).

If you think about a couple of boards attached to your Drive instead of fins (a rather extreme example), you'll see my point -- more like batting water back and forth. Look at a propeller and note the increasing twist vs. distance from the hub. This is what we're also looking for in fin shape. The fin needs to twist or warp to thrust the water backwards; the faster moving tip needs more twist than the base to maintain a similar angle of attack with the water. Also, the faster the RPM, the more the twist. The looser clews facilitate this twist or angular change with distance from the sprocket shaft. I've even gone to the extreme and added articulating links to let the fin base or root angle out with even better results.

I have tested ST fins but to a lesser degree and have conflicting results. They rotate faster and are shorter so what holds true for Turbos is not necessarily the same with STs. As noted, different RPMs calls for different prop pitches. Nevertheless, I believe in general, looser clews are advantageous with the STs as with Turbos. I've had some very good results articulating the clews of ST's however. This is distinct from loosening the clews as (again) it opens up the root of the fin as opposed to increasing the warp at the tip. An unfortunate byproduct of this though is the fins' tendency to have an annoying clacking sound.

In terms of efficiency, I think the ST's and Turbos are pretty close, with the Turbos having a small edge, probably because of their better aspect ratio (length to width). The rather significant difference in thrust potential is due to size difference more than efficiency.

I have done both acceleration and flying 100 yard comparisons. Everything I've reported though is with the flying start.

My recommendations for Turbos are for both cruise and sprint speeds. There is no circumstance where Turbos with a tighter clew has shown anything except more leg resistance, less speed and less acceleration. Do the loose clews top out or stall? I've been over 9 MPH on boat wakes with them still pulling well and over 11 MPH on a few occasions sailing the AI where again, they still feel strong.

Having adjustable clews is really nice though, especially with Turbos. There are two different Turbo mast lengths in circulation and two different fin compounds. Additionally, the rear fin feeds off the front fin and there is some evidence to suggest that a slightly tighter clew on the rear fin may improve efficiency a small amount. The down side to this is that the boat wiggles somewhat, which is less efficient. So I wouldn't recommend it, but encourage others to experiment further.

Oddly, when I look at pictures of Drives on the forum, most have much tighter clews than I would run. I'm impressed that these folks are maximizing the exercise value of the Mirage Drive -- glad everyone is not as lazy as I am!

There, you have more answers than you probably ever wanted!8)

That is a monumental piece of advice.

Thanks,

Keith

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2015 AI 2, 2014 Tandem

"Any intelligent fool can make things bigger and more complex ... It takes a touch of genius and a lot of courage to move in the opposite direction." A. Einstein

"Less is more" Anon

I would think that if anything the rear fin should be looser. Because the water coming at it off the front fin would be coming at more of an angle than undisturbed water, it would need to rotate further, not less, and more clew tension would mean less rotation, would it not?

Thanks for that detailed explanation Roadrunner. You've given me plenty to think about.

Chris

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You may be right Tom but here's my rationale. The leading fin sends a pulse of water over the trailing fin at a diagonal, with more of an aft component than the surrounding water. Remembering that the fin trails the leading edge as it sweeps across this pulse, a more "twistable" trailing fin will tend to align with the flow easier, taking a smaller bite. In any event the result is probably small as this is an intermittent effect, not a constant situation.

I have a Drive that is used for "racing" (whenever I want to go extra fast). The leading (new style) fin has a bit more flex than the firmer (old style) trailing fin. There may be one or two other mods incorporated in this drive, but it cruises about .08 MPH faster than my stock Drive. It's not a lot, but us old geezers need all the help we can get! This is not a case of tighter clew but rather firmer fin -- a different side of the coin. I place it there using the same reasoning. If one of your ST or Turbos is a little firmer than the other (not uncommon), try switching positions and see if you notice any difference.

On a related but slightly different matter, even with loose clews there is a rather significant lateral component of thrust with all fins. One can get a vivid demonstration by removing one fin (simulating a detached fin) and running the Drive on the one remaining fin.


To get the full effect, leave the amas at home. You'll really rock and roll and feel like the boat is going to turn over (at least with Turbos)! Within a few minutes though you'll adapt to the rhythm and be surprised at the speed you can cruise at with the one fin. Oddly, when a fin falls off (which is rare) most disengage the Drive under the assumption that it must be broken beyond use. You can lose any one fin PLUS the idler cable PLUS either fin cable and still operate the Drive -- almost impossible to completely disable it.

The Mirage Drive offers more performance potential and reliability than most people realize -- I'm constantly amazed by it!8)

First...a comment on removing the 'standard fins'...it took more than
just a couple of padded vice grips to remove the old fins...I thought
I had pretty good biceps...but couldn't get either mast to unscrew with
just vice grips. I ended up placing the drive in a well padded vice and
used a well padded two foot pipe wrench on the masts. Even then the masts just slowly backed out. I was careful not to damage the drive nor
bend the old masts (might need them if I ever damage the new Turbo fins!) The amount of dry Loctite blue on the old masts was awesome.
I liked the fact that Hobie includes a small tube of Loctite blue with the kit.

Second...how do you tell if the adjustable tension is correct? Thread comments say 'less tension' or 'more tension' on the front or rear fin.
Is there a certain amount of measurable 'movement' (flexure) that I should be aiming for?

I look for two gouges for a standard set-up.
1. The back part of the fin should just fit in the clew outhaul slot far enough not to slip out -- about 1/8 inch or so.
2. Look for at least 1/4 inch wind-up space or free travel along the mast.


It doesn't hurt to dab a little Locktite on your clew adjustment if you find it turns easily. It really won't be hard to break free later if you want to change it.

Thanks for your input, RR.

Roadrunner,

I've taken the AI out with the ST fins twice now, once with the clews loose and once with them tight, both times in rather erratic gusty winds. With the clews loose, I found that, like with the standard fins, there was insufficient resistance and I had to paddle like crazy to compensate. With the clews tight, I was quite happy with the resistance - my legs were a little tired maintaining about 60 cycles per minute, but nothing too dire. What I really liked, and did not know to expect, was the performance I could get from peddling hard during strong gusts of wind which were overpowering the rudder and pulling the boat to windward. Rather than having to reef the sail , I found that by peddling hard, I could regain control of the rudder. It felt just like pressing the accelerator when a car is understeering into a corner.

i am not disputing your erudite explanation of the efficiency of the fin shape, but there are two machines at work here - the Mirage drive and human legs. Subjectively, I like being able to apply a bit of force when I need too. I am not sure if that would show up in speed trials. I will try to reproduce some of the work you did with Turbo fins, using the ST fins in calm conditions in Adventure mode.

Regards,

Chris

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Thanks Roadrunner for the fin photo.

A couple of observations on my newly installed Turbo-fins.

The new masts were carefully screwed into their female brass inserts until
gently tight.

The forward fin-mast has a 3/8 inch gap between the top of the fin and
the brass receiver (pushing on the fin...the fin will not slide any further up the mast). The rear fin-mast has a 3/16 inch gap between the top of the fin and the brass receiver (pushing on the fin...the fin will slide all the way
up the mast and touch the brass receiver). It appears that compared to
the forward mast...the rear mast has a longer hole inside the fin.

When both tension adjusters are set with about 1/8 inch in the slot (as suggested by Roadrunner)...

The tension in the clew outhaul slot, in which the forward fin tension ring pin sits, is very tight. That is, there is very little play on the rear of the fin when I wiggle it.

On the rear fin, there is about 1/2 inch of 'flop' when I wiggle the rear fin.

Should I tension the rear fin, so that it wiggles only the same amount as
the forward fin...or vice versa?

Chris, that is great information! As noted, I have substantially less experience with ST fins than the ST Turbos. Most of my ST data is comparing it with Standard fins at different speeds, and differential comparisons with degrees of freedom allowed at the clew (example below; note the added link) for which I was using the clew adjustment specifically to alter foot travel rather than fin twist. There is a lot left untried!


Since I use Turbos almost exclusively, my specific clew adjustment trials are with Turbos. Additionally, I have no performance experience at all with the newer more durable fin composition for STs. With Turbos this makes a difference, and would guess it also has an effect on the STs.

In looking over my notes, I can also confirm that a stiffer fin with STs may add more speed (but also a greater pedaling effort). This is exactly the opposite from the Turbos for the possible reasons I mentioned above. As with prop blade length and pitch, STs exhibit a different performance pattern.

I also reviewed my notes regarding Tom's comment. Specifically (again, using Turbos), as I tightened the front clew, speed change was negligible but pedal effort increased markedly. Resetting, then tightening the rear clew, speed change was also negligible and pedal effort increased to a lesser degree, but at the extreme, the boat started yawing with cadence (which was held constant at 40 CPM). My specific conclusion -- for cruising, given the same cadence, tightening Turbo clews does not impact speed significantly, but does increase effort, therefore fatigue. Over an extended time, looser clews will facilitate a greater range specifically due to the fatigue factor. At sprint speed, tighter clews actually slow performance by inducing a lower maximum cadence.

I have had slightly better one hour times using an older, stiffer Turbo on the rear, but this year I'll try Tom's idea with the stiffer fin in front and see what happens. Fin stiffness is simulated by clew adjustment, but there is some difference due to allowable degrees of freedom at the foot (more allowable foot movement gives more speed).

I think your comparison is excellent and it's great to see someone else taking the time to try some of this stuff. I hope more folks will experiment and share their results.

PS: As you may surmise, Drive performance is my favorite topic, but probably boring to most here on the AI forum. Sorry if I get carried away!