Mirage Drive at slow speeds

Hello again,
It's been a long time since I've been on here but have a quick question. I had a fantastic summer with my tandem but ran into a slight problem when out in Georgian Bay. The wind was quite calm when leaving shore but quickly raised to about 20-25 knots when far from home. I was sailing on the tramps with full sail and the left ama completely buried when I broke a rudder pin. With the sail reefed in and sitting in the back I tried to make it back home with the paddle acting like a rudder but struggled with no positive results (can't believe I didn't have a spare pin). I furled the sail in and then started to use the Mirage Drive to start the long pedal home into wind. It ended up taking an hour of hard pedaling combined with paddling to make it home and believe me, I needed a drink when finally stepping on shore.
My question is, when fighting a headwind and forward movement is minimal, are the fins providing the same power than moving forward at 4 or 5 knots?

I think the answer is yes, the drives provide the same forward thrust in either case. It's just with the headwind you have to overcome the winds force and current before ever beginning to move forward. A simple test is to stop pedaling toss a piece of paper in the water and watch it. If the wind is blowing you backwards the paper will drift forward, if it's a strong current it drifts backward or stays stationary beside your boat (this means your caught in a strong current). It's near impossible to see currents offshore. Where we live and sail around key west the offshore currents are very strong (5-6mph) trying to drive you south away from the island (next stop Cuba). When the wind is from the north it impossible to sail north toward the island on a TI.
I have also been out there with a broken rudder and discovered quickly that trying to sail with a paddle is near impossible for me anyway (maybe others can do it but I know I can't).
Actually we were in a situation once where we were off shore trying to go north, we had pretty good wind and were sailing like crazy with the GPS showing we were traveling 3mph. When off shore you have no visual reference. Turned out we were traveling backwards at 3mph being blown offshore (that ended up being a very long day (lol). If I were in your situation, I would have probably dropped the mast, get as low as possible in the boat, the pedal/paddle back to safety. Currents are like rivers (usually narrow), I would steer closer to shore and follow the shore line back to launch, using the shore to block the wind if possible. Actually up here in sarasota we ended up having to walk our TI the mile or so thru big pass In knee deep water because we couldn't get back in from offshore against the 5mph current (this has happened more than once to us). It's more likely I'm just a really bad sailer (lol).
Hope this helps
Bob

Thanks,
Headwind and current aside, are the fins more efficient with a bit of forward movement? Is there a cavitation effect when pushing hard with no forward movement? The design is brilliant, but is there a similar effect to boat propeller or aircraft propeller at static?

raynabear:
The fins are hydrofoils and amplify force like an airplane wing would, so you are correct, if there is a little forward motion they amplify better. I personally haven't noticed any cavitation effect like you would on a boat prop (ultra low pressure (vacuum) created on the upper surface (creating a bubble)). Actually I think hobie has several videos of the mirage drives in action on the water, it would be interesting to watch those videos to see if a big bubble forms on the upper surface at startup (I suspect it doesn't).
I suspect the majority of the energy produced from a stand still is directional force (forcing the water back to propel the boat forward), with lift beginning to occur (acting as an energy amplifier) and kicking in once you have a little forward motion developed. What factor that lift has on the forward force, I have no idea, but I suspect it's a very small factor at low speeds (lift vs directional force) and increases with forward motion speed (basically becoming more efficient as speed increases (less energy in vs more out (amplified energy (like an air conditioner works))).

My TI is not a normal TI, as it's kind of rigged for speed. My typical cruising speed in low winds (4-8 mph) is 8-10 mph. I have a hybrid setup with motors, I pedal 100% of the time always, and find when I pedal I get around 100 mpg with my motors, if I don't pedal I only get 50 mpg. The pedaling doesn't effect my top speed at all, but greatly affects my fuel economy. This tells me the hydrofoil effect and lift created by the mirage drives while moving forward is a huge factor in propulsion of the boat (way more than most realize). If I could pedal fast enough I wouldn't need the motors at all, but we get pooped out after a mile or so when we just pedal (without the hybrid supplemental propulsion (engines))
Basically I'm really big into human power hybrid propulsion crap (wing sails, hydrofoils, and all that rubbish), basically trying to squeeze every ounce of energy I can from every available source.
I know boring stuff ( LOL)
Bob

In 20-25kt winds I have found it nearly impossible to make headway without the sail and a fully rigged TI. Without a rudder I don't see how you managed, as the wind catches the freeboard and just pushes the big ol hull wherever it wants. I've lost steerage close to a shallow reef in maybe 15-20 knot winds, and with myself and my daughter pedaling, and paddling/steering with the paddle, barely made it a few hundred yards to safety. Since then I've kept a steering blade with me and practiced sailing with the rudder up and using the blade.. Still not easy or efficient but it can be done.

-RH

_________________
2013 Tandem Island "Victory Lane"
Oahu, Hawaii
www.rotorheadsblog.blogspot.com

No. They can produce amazing Thrust / torque at a near standstill. This is also shown by their ability to tow larger vessels and win tug-a-wars.

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

I no longer carry a double bladed kayak paddle, but rather use a cheap, strong, flat-blade canoe paddle--better to use as an emergency rudder. If you are out in 20-25 kts wind w/o a rudder, you have a major problem. But, if you are out in those winds, you know that could happen, and you should always carry a VHF radio and a SPOT or PLB.

Keith

_________________
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

That makes sense for emergency steering Keith.
I still carry the double bladed paddle.
But I'm with Bob. My emergency steering is the black thing behind me.

_________________
Cheers, Brian in South Australia
Tandem Island -

Has anyone tried lashing a paddle vertically to an aka and using it as a rudder? Seems like that might provide some directional control in an emergency.

Just wondering. May be too simplistic to work.

Yes. Just use some bungee balls. Works fine on a beam reach and upwind, if you stay on top of it. It takes concentration and a bit of muscle.

Problem is, you may need to swap the paddle from side to side, to correct headings and navigate traffic or swells. This goes 2x when downwinding.

Give it a try. Use the sail and pedaling to steer too, if you can.