Sorm/Learning Sails
DonTyerman1:
Looking at the Ice-Opti rig it stikes me that (with some tinkering) it might be approriate for "storm" conditions or for teaching new iceboaters on days when the wind is "up a bit". A normally rigged DN is asking a lot of a new iceboater on a windy day.
Has anyone tried a low center of effort sail like the Opti's on a full-sized DN? Obviously speed will be compromised in a trade off for control.
The Opti's low center of effort appears to be well back hopefully eliminating the lee helm problem I've read about with other cut-down storm sails. I'm assuming the lee-helm was caused by cutting off part of the leech of a normal DN sail.
Any thoughts on this topic?
Geoff Sobering:
All the successful storm sails I know of don't try to shift the center-of-effort lower, but rather reduce the sail-area while keeping the CE at about the same location.
This always leads to basically cutting the bottom off the sail, but leaving the hoist the same.
An added benefit of this is that visibility is greatly improved, since the bottom of the sail is now quite a bit off the deck.
Also, with a much shorter foot, the boom can be made much shorter/lighter (I think there are some sails with basically a heavy batten at the bottom). This is also a safety improvement since there isn't a heavy boom swinging around.
Cheers,
Geoff S.
DonTyerman1:
Thanks for your thoughts Geoff
I've considered cutting the bottom foot or two of the sail off but the bottom of a DN sail is also the part that reaches furthest towards the stern. By removing this part of the sail you are moving the sail's center of effort forward which will produce lee helm. Lee helm strikes me as a dangerous force in any scenario but especially so in the hands of a beginner in an iceboat!
Somewhere I read of someone who cut the bottom of the sail off but also raised the sail higher on the mast. With the sail raised higher, the CofE moves backwards (due to the mast rake) reducing or eliminating lee helm. There's also no doubt that raising the boom (or eliminating it - replacing it with a stiff batten) would make a beginner (or anyone) feel more comfortable!
Of course a smaller sail will develop smaller healing forces but by moving it higher on the mast to counter lee helm those forces will also develop more leverage. This is at odds with the original goal of making the boat docile in windy conditions. That's why I suggested the notion of a completely different sail configuration - one that maintains the fore and aft location of the CofE but also lowers it by virtue of a sail plan that is both smaller in area and lower in profile - like the Opti's.
Speed and sail effiecincy is not huge concern as the goal is to keep the boat managable in strong winds that will make you go relatively fast no matter what.
The other route that might work would be to cut the top portion of the sail off - like the reduced sail rigs for Lasers. I also had an old windsurfer sail that was cut the same way. It was very manageable in strong winds - low CofE and, due to the low-aspect ratio, not prone to stalling. Of course with a windsurfer you automatically adjust the rig rake to account for changes in CofE.
Does anyone have sketches of their storm sails? I'd like to visually work it out before I get the scissors out!
Thanks!
Don
Geoff Sobering:
Quote from: DonTyerman1 on February 02, 2012, 09:24:16 AM
Somewhere I read of someone who cut the bottom of the sail off but also raised the sail higher on the mast.
My description was poor. That is exactly the design I'm talking about.
Cheers,
Geoff S.
DN 805:
Here are some issues you seem to have neglected:
Fore and aft position of boom blocks in relation to foot of sail and back deck of boat. I.e., the leverage of blocks aft of the clew of the sail will tend to tip the forward end of the boom skywards.
Mast bend and and how it affects the sail shape plays an important role in the control of the DN. Storm sail - little or no mast bend.
Stability is achieved with sail low, don't cut off the foot.
Best to stay home and read a book of conditions are not manageable.
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