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PostPosted: Sat Jul 13, 2013 2:37 am 
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Hang on a bit! I didn't say they offer NO resistance, just that the resistance is bugger-all when close-hauled (eg where the leeway angle is smallest - probably single figures). I contend that when you are trying to point as high as possible, the flex in the fins will be enough to effectively rule them out compared to the daggerboard/centreboard.

Obviously there is some lateral resistance, wshich increases with the angle of leeway, reaching maximum on a beam reach. The single hull Mirage kayaks use this lateral resistance to enable them to climb (just) to windward, but at a very wide angle.

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PostPosted: Sun Jul 14, 2013 9:21 am 
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dpstivers wrote:
Great article! I have one question:
How do you lock the fins in the down position?
Dave not sure if you got the answer to your question but Tom Kirkman came up with a simple mirage drive lock made out of pvc. A few others weighed in with their solution too on this topic viewtopic.php?f=70&t=46358&p=199574&hilit=+Mirage+Drive+Lock#p199574

Scott - great article - thanks !
Barry

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PostPosted: Mon Jul 15, 2013 9:47 am 
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My apologies, Tony, for overstating your hypothesis. I initially thought, because we can't point all that high, that we would be dealing with reasonably large angles of sideways force. As you point out, though, we are not concerned here simply with the angle of the boat relative the wind, but the angle of boat relative to its movement in the water, and much of the time that angle, the leeway angle, will be reasonably small, much smaller than the angle to the wind.
Has anyone tried to measure this angle on an Island? It would require good measures of wind direction and boat track. The latter, over a reasonably long tack, could be obtained from GPS tracking, but measuring local wind direction within a few degrees, and having it consistent over the course of the course is beyond my instruments.
However, sailing yesterday, pointing about as I could with good telltale flow, I noticed that there was a fair amount of pressure trying to push one of my peddles backward. Since the drive had both fins, if this was sideslip pushing, I don't know why it would not have pushed more or less equally on both fins, and balanced out. But it made me pretty sure that if I ever conduct the one-fin experiment, I will find that that fin is pushing at almost all angles of sail. But, I don't have any good way to compare that with other forces resisting side slip.
I have sailed without the centerboard down (usually because I forgot to put it down :roll: ), and could make upwind progress, so the hull alone offers considerable lateral relative to longitudinal resistance.
While I am thinking about that, do you think the reason a daggerboard (or centerboard) is long and narrow instead of short and wide is only so that it is pulled entirely out of the water when the boat heels, or is there some other consideration? It would seem that maximizing lateral vs longitudinal resistance would call for minimal front surface and maximal side surface, best satisfied by a thin, wide, short board, but this is not what has been developed.


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PostPosted: Mon Jul 15, 2013 4:04 pm 
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I don't have any way to measure leeway either, but I have failed to spot any difference in leeway when fins are up of down.

As for the ideal aspect rsatio of the daggerboard/keel, Hobie have got entirely different solutions for AI and TI, but I don't know if that was driven by production requirements (small "footprint" of hole on AI, short housing for the swivelling centreboard on the TI). However, observation of compdetition yachts & cats tends to show high aspect ratio daggerboards to be the preferred choice...

I would love to see the outcome of someone adapting a Miragedrive plug to include a deep true aerofoil section daggerboard.

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PostPosted: Mon Jul 15, 2013 8:32 pm 
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Location: Adelaide, South Australia
Scott and Chris you are both right to a degree.
And that is the point, the degree that the mirage drives bend which varies over its length.

I have tried and not found much advantage to Leeway although it is hard to measure.

Regardles, excellent article Scott.
I wish I had that before I taught myself bad habits many years ago on hired cats.

For the information of any real newbies, my personal definition of a couple of terms used -
Whisker Pole - on an Island, light pole to attach to the clew (bottom rear) of the sail to push it out to
catch as much wind when running (downwind).
Barber Hauler - a light sheet attached to the clew which via an attachment on the ama, pulls the sail outward.
This has the same effect as a whisker pole.

My friend Rob has a barber hauler on his TI and I a whisker pole.
Both seem to work well.
If anyone needs more details, let me know.

Again, well done article.

Cheers, Brian


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PostPosted: Mon Jul 15, 2013 8:39 pm 
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Once I start, there is no stopping. :x

Chris, you probably clarify the difference between a tell tale and wind indicator at the end
of your article. If anyone needs further explanation, the tell tales are fitted either side of the
sail by Hobie.

An article on how I fitted a wind indicator is at this forum -
viewtopic.php?f=71&t=40451

Cheers, Brian


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PostPosted: Mon Jul 15, 2013 9:31 pm 
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A little tip I picked up here... if you cut a 1/4 ich notch in the end of your paddle, it can snag the line dangling from the clew of the sail, and the knot will catch on it. You can then let out all your mainsheet and prop the clew right out to the side, for the maximum downwind sail area.

Do you then call it a whisker paddle? :lol:

BTW, a masthead indicator is worth its weight in gold in my opinion.
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