Hobie Cat Forums

I wrote an article on sailing the Hobie AI for beginners. Let me know if I missed anything important.

http://www.norcalkayakanglers.com/index ... ic=39153.0

GB2

Excellent article !!! Well done.

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Jim

Thank you for the great article. I have printed out the Word version as we will be having lots of family and guests using our TI at a family summer cottage. This will probably be required reading.

Great work Scott!

Great stuff. Thanks for posting the right of way rules.

Great article! I have one question:
How do you lock the fins in the down position?

I keep my feet on the pedals going upwind so if you keep your feet next to each other the fins are pointed down.

Just remember that the fins cannot contribute anything but drag if they are not being pedalled. This is a fundamental of their design, as the flexible part will simply line up behind the stainless steel mast in the water flow, thus contributing zero sideways force. I believe you would probably get less drag with them flat up against the hull, or even better, removed from the water until needed.

Tony, if that were true, they would be incapable of propelling the boat forward. They are attached to the drive at the clew to prevent them rotating around the mast.

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The purpose of the fins being down going upwind is to provide lateral resistance like the daggerboard.

Chris, when you apply pressure by pedaling, you force the fin masts across the water, and the flexible part of the fin bends behind it and then takes on the shape of a propeller blade, creating forward thrust. This is the whole secret of the miragedrive. Its super efficiency comes from this thrust being available all the way through the pedal stroke, and then, on the "return journey", the fin flips over to the other angle, effectively "reversing the pitch of the propeller" so thrust is generated on the reverse stroke as well.

GB2. For any sideways resistance to be available from the fins, they would need to be rigid, which as described about, would render them useless for forward propulsion.

But Tony, the whole point about the fins is they move sideways (drag) and generate forward thrust (lift). They are like any sail - if there's no drag, there's no lift. A propeller is just a rotating sail. If the fins didn't encounter lateral resistance, you would be able to pedal with zero effort.
If you remove the pins from the clews, the fins will do as you say and rotate around the masts and offer no lateral resistance. They will offer no forward movement either. They will be like a sail with zero angle of attack or a propeller with zero pitch.

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Chris, you are pazrtly correct, but remember, the drag you speak of is generated by the fin being forced sideways by the pedal action... if you don't pedal, the flexible part of the fin will just bend slightly to follow its mast through the water. The act of bending the fin might create some tiny amount of drag, but this would be microscopically small compared to it when it is being forced across the water when pedaling.

If the centreboard/daggerboard is down, leeway is going to be in the order of 10 degrees or so tops, so if the rear of the fin bends only 1/4 inch or so, it will be pointing directly in the water flow, and the sideways resistance generated by the fins will be more or less nothing.

The bottom line is this... the fin can't have it both ways.. if it flexes, it generates forward drive when pedaled, but it is crap at generating sideways resistance.

If it is rigid, it is crap at generating forward drive, but it is good at generating sideways resistance.

The laws of physics at work.

Tony, we could debate physics till the cows come home . Can I suggest one experiment? Hold the drive in your hands over the water, with the pedals held together. Dip the fins in the water and sweep from side to side. See (and feel) how the fins behave.

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Chris, maybe my explanation was unclear. OF COURSE the fins bend, that is the whole point of the Miragedrive, but you DON'T sail sideways; almost all the time, the Island is going just about straight ahead as far as the water is concerned (unlike the wind). When you pedal, you force the fins to go sideways in the water, and the flex in the fins generates forward force. If you stop pedaling, the only possible force able to flex the fins is leeway, which is minimised by the centreboard/daggerboard plus the narrow hull and amas, which will all resist sideways motion. At that point, the flexible part of the fins, being flexible, will NOT offer any resistance to the relatively minute leeway. Sure, at the very top of the fin, movement is slightly restricted, but even here, the "slop" available would come very close to equaling the angle of leeway, so even up there, sideways resistance would be minimal

Waving a miragedrive sideways in the water obviously bears no relationship to its behaviour when just hanging down under the hull

Sorry, but the laws of physics cannot be overcome.