Hobie Cat Forums

Just out of interest, here's a clip Cowsgomoo took of me a couple of years ago, close reaching in light wind, using my old rear seat platform. The boat was running very smooth and the bow is continuously out of the water.

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going from the AI to a TI, the new "centerboard" always messes me up. Since it is out of sight, I am always forgetting to drop it before unfurling the sail, and wondering why the rudder is not doing it's job. Sooo, sorry, but the mirage drive is NOT an effective lateral resistance, you really need the board down!

Hull speed is the speed of the wave that is as long as the hull. That speed varies with length, according to this formula:
1.34 times the square root of the length of the hull at the waterline in feet, gives the speed in knots.
for a TI, at 18.5 feet, the hull speed works out to 5.76 knots.
the AI, at 16 feet, gives 5.34

Planing involves getting over that bow wave and out-running it. Given enough power, you can exceed hull speed, and as you do, the stern will squat under the crest of the following wave (back end of the bow wave)
Increasing power eventually will push the hull up the crest of the bow wave and over it.
Easier done in a gas-eater, but try doing it slowly once, instead of just jamming the throttle, take it up slowly. That stern squat really becomes apparent.

Getting to 12 knots as a displacement hull requires an 80 footer!
So, somewhere around that, we would be planing in either Island boat.

Power requirements are harder to determine, but roughly, the power requirement increases as the CUBE of the speed. Which means doubling speed requires EIGHT times the power, and that does not take into account the power to lift the hull over the bow wave. That is some serious pedaling....

The question was never whether the drive could substitute for the dagger board, it's whether it can effectively augment the board.


And both boats easily exceed those speeds.

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The AI and TI are full displacement hulls, but fairly atypical ones. They have a large flat bottom (much like a surfboard) to act as a planning surface, but flowing lines at the stern that do not promote planning. They also have lots of surface area, which helps a boat get on plane but limits top end speed quite a bit.

As a general rule of thumb, the TI is limited to half the speed of the wind at it's best point of sailing. True planning (designed for the purpose) sailboats can do much better, traveling at the speed of the wind or even faster.

The TI only seems to plane for me when I sail solo, so I assume that weight severely constrains the ability of the TI to plane. I might make 8~9 mph tandem and 11~13 solo. It is very clear that the hull is plowing though the water when loaded and doesn't have the power to do more than slightly exceed hull speed.

The formulas to determine hull speed are well known and easy to figure, however the power required for a particular hull to plane is not so simple. Power demands do increase radically as one crests the bow wave, however they are extremely dependent on the weight of the vessel. This is why a loaded TI can't plane, it needs more HP than it's sail plan can deliver. It is also why once a TI gets up on plane it seems to suddenly accelerate, the HP needed to crest the bow wave is now devoted to speed.

Design limits will keep the TI to sustained planning speeds of 10-15mph, the hull has too much surface area, the hull has round chimes, the rudder is much too big at planning speeds, the amas are to small to remain afloat in higher winds needed to provide more power, and the sail plan is too conservative to provide enough power.

I guess the TI and AI could be considered to have semi-displacement hulls, which have a planning ability but retain most of the efficiency of a full displacement hull at hull speed. Semi-displacement hulls are extremely weight sensitive and won't perform without enough power or too much weight. So that fits also, but a true semi-displacement hull would have a square stern that would allow the hull to break away from the stern wave easily. The AI and TI have hulls that encourage the water to flow smoothly up and away from the hull, not an ideal situation for a planning hull.

cheers,

j

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2011 Golden Papaya TI with a 250 square foot spinnaker!
also a more manageable 100 square foot spinny...
&
the TI3 rear ama mod

Loved your comments and observations kayakman. They seem to reflect our experiences here trying to break the speed limit with AI's and TI's.

I have had the AI hull planing a few times (broad reach in 20-30 knot winds) and can confirm a sudden sense of acceleration, as if a turbocharger kicked in. It has only happened riding the tramps or the Hakas, where I was far off the hull, laying wide and flat during a big gust. Never more than 10 seconds at a time, maybe 12 knots, but it felt amazing when things started to fly.

We can reach and exceed those speeds all the time running downwind in swells over 3 feet, or surfing moderate breaks. The waves and gravity provide the needed horsepower.

It's all too temporary. At around 14 knots, the hull overloads, buries the nose and loses effective rudder control. The fins have a hard time adjusting to these forces and they will introduce cavitation and drag, even when held close to the hull. They tend to break too.

On top of that, it's very hard to control a tri at an optimal surfing angle to the wave. The leading Ama will turn you sideways in a flash and the stern will broach, Far easier surfing with a monohull, or even an outrigger, I think.

But back to the mirage drive/dagger debate, I offer everyone this picture. It may be helpful to compare the drive and dagger profiles as you consider whether a set of fins can be effective against leeway.

I just want to thank Scott GB2 for posting his tutorial and doing an AI sailing clinic at our local Hobie dealer. I put some of Scott's good tips to work yesterday.

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Regional Hobie Fishing Team Member

NOHUHU. two out of three of them have got hinges and flexibility...I would agree with you entirely if they were all as rigid as the daggerboard, but good luck trying to pedal with them in that case.

"Well, it may be all right in practice, but it will never work in theory." -- Warren Buffett on how the academic community regards his investment approach

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Tony - they do twist depending on the tension setting, and I could see how they would be less effective if the deflection range was roughly = the angle to the "apparent" water flow. But this doesn't take into account currents or waves or the fact that the fins don't just rotate straight around the mast, they twist, like our sails above water.

And the luff of the mirage sail (with the steel mast) never flexes. Only the leech, near the bottom really does.

Seems to me, as you move closer to a beam reach, there MUST be some lateral resistance.

I guess the real way to figure this out would be to drag a drive in the water as you sail and watch them. You can also turn them at different angles and see what kind of resistance the underwater sails really have. If you can turn them at 45º and they offer no drag, then you are right.

Either way, this debate is over a relatively minor effect. My preference is always to lightly pump the drives as you beat upwind. That makes an unmistakable difference.

If you Push the boat sideways in water with the fins down you can Feel the resistance! Anyway who cares?...lets Go!...