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PostPosted: Sat Jan 11, 2014 9:13 pm 
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Ok lets go back to Feb 2010 Giz magazine had an article about the first Tribrid (human-sail-electric) watercraft in history ( http://www.gizmag.com/evolve-electric-m ... yak/14106/ ).

As an Engineer/inventor this article intrigued me so much I went out and bought the first TI I could get my hands on. Here is the video from that article that got it all started for me (warning: it's kind of boring but what he says is very significant)
[youtube]http://www.youtube.com/watch?v=241QmxFcbuc#t=150[/youtube]

We all know the world is running out of oil, and the days of raw power boating with giant gas engines (guzzling huge amounts of fuel) are pretty much done except for the few rich people who can afford to squander thousands of dollars in fuel per yr (at $4.50 per gallon). Or multi million dollar sailboats, this pretty much leaves us common folk who also like to get out on the water and actually get somewhere and do things out of the game all together.

I’m a little frustrated right now because since that article was posted I have seen nothing written on the subject since then. I just did an internet search and the only things that come up are that original article and my own crap on the subject, "AM I MISSING SOMETHING".

Now 4 yrs later the Hobie Adventure line of boats are one of the most popular boats on the market today, but not necessarily for the reasons outlined in the above article (they are just plain fun, and probably one of the best family boat investments you can make today, as we all know).


Some things that I interpret from the Giz article that need work.

The guy said in the article that you can go up to 8 hrs at 2 mph with the evolve. This guy is obviously not from the keys or south Florida where distances are huge if you ever want to get anywhere, and the currents are 5-6 mph in many areas. Two mph will not cut it (which would equate to around 8-10 mile range at a snails pace), realistically you need to be able to maintain 6-10 mph with a range of minimum 30 miles or so regardless of wind conditions for it to be of any use at all in my opinion, and where I sail (Ocean and destination sailing). Basically we want to use our TI as a family SUV and do everything we used to do with our very expensive and fuel hungry powerboats.....

Don't mis-understand what I'm saying, in my opinion the evolve system is light years ahead of anything else out there, and for what you get, is a very good value, and for lakes and some rivers (ie..short distances in relatively safe areas), it does the job extremely well. "But I want more".

One thing that I feel is significant that the guy doesn't really talk much about is, what if you use all the available power sources together as a total package.
For example what if you use the evolve and pedal power while sailing to increase your speed and increase your range, as well as give you acceptable performance regardless of the wind and direction. If anyone has tried to sail their stock TI in 5mph winds, typically your only going to get 2-3 mph performance, this just doesn't cut it for me (The stock TI sails at around .6 of available wind speed),Unfortunately when the wind gets over 12 mph, along with that wind you get lots of waves and chop and it becomes quite a wet and uncomfortable ride in any small craft or sailboat (that’s not for me, I try to stay out of those conditions). Alternately how many have been out on a sailboat 15 miles from launch and the wind dies, 'basically your screwed'.

Now lets get to reality on electric power and the state of the art. Fact of the matter is we are 20 yrs away from having a viable electric battery replacement for fossil fuel (if we're lucky). As an example the state of the art battery in your cell phone will power that phone for about 3-4 hrs usage (not including stand by). The same energy demand if powered by 1 gallon of gasoline would last 30,000 hours ( I saw that on a TV commercial recently). And solar power is pretty much limited to charging your small electronics over a long period of time, and that’s about it (nothing on the horizon there either).
Basically what this means is the energy density of a standard phone lithium ion battery is around 100 wh/kg whereas gasoline has an energy density of 12000 wh/kg. Which is why gasoline is so important to our society, as there is nothing now or on the drawing boards that can replace it power-wise pound for pound. This is the reality of the facts about the world we live in, we all wish it was better but fact is it’s not, with the evolve system being the best I have seen out there.
Does this mean we are basically screwed when it comes to be able to make a tribrid boat that has the capabilities that I outlined above and have to wait another 30 yrs for battery technology to catch up. Should we just stay home... or only go out when the conditions are just right which is only a few times a year around where I live, I prefer to be out there every weekend if I can (with the best time to go out being when there is very little wind and nice calm seas, which is ten months out of the year here where I live).
What are our choices:
1. Forget about sailboats, and go buy a $60k powerboat, and spend a hundred bucks a week on fuel ( I don't think so).
2. Just stick with kayaks and day sailers and be happy going short distances in protected areas (what most of us do now) ……….

What if there was another option fulfilling the dream presented by that Giz magazine article from 4 yrs ago that inspired me so much :

Something that uses technology that is already available now with currently available off the shelf proven inexpensive technology,
Uses energy wisely, and is extremely inexpensive to operate, basically you can go out all day and have fun sailing at desirable speeds (6-10 mph) even in very light winds, get to destinations whether the wind is blowing or not, with a typical 3-4 hr trip costing about a dollar in fuel.
The actual wind direction is no longer a critical factor in where your going . (can easily sail upwind, or in almost no wind) .
And best yet, extremely easy and simple to operate, you don’t have to be an expert sailer to operate.

No mirrors or tricks here, I have nothing to gain by promoting this, just a desire to fulfill what that article promised us 4 yrs ago and make it reality. So I can use the stuff myself…….

I got tired of waiting for Hobie to bring what I wanted to market so I ended up building the thing in my garage, it’s not rocket science and there is nothing special here, everything is off the shelf, basically what you need is:

A Hobie Tandem Island (off the shelf, no mods to the boat itself needed at all)
An off the shelf quality 4 stroke gas outboard ( I recommend the Honda 2.3, it’s only 27 lbs, and is very well made) (…we have to use a gas motor unfortunately until battery technology catches up to our real needs). Of course the Evolve electric propulsion will give you equal performance and is available now, but because of the energy demand (300-375 watts, ie.... enough power to propel the TI to 3-4 mph (with no sails up) also will work but your range is severely limited by the battery technology (a few hrs at best)). Alternately if you have two really fit peddlers you can forego the motors altogether. Unfortunately I’m not that fit, and need a little supplemental power in order to make the chain reaction work.

What’s the chain reaction you ask that makes all this work….. This is the apparent wind that blows past the wing sail generated by your forward motion, this creates a vacuum in front of the boat that propels the boat. Basically the combination of the wing jib and the mainsail works like a giant amplifier making the boat go fast with minimal energy input. A secondary affect of the wing jib is it accelerates the air going over the mainsail, turning a regular Hobie TI mainsail into a super sail.
The only special component is the custom made 33 sq ft wing jib sail that I ended up having to make my self in my garage (since I couldn’t buy it anywhere), it’s a really simple design and cost me about a hundred bucks in components and material to make. No rocket science here, it’s just a wing sail.
That’s it that’s all anyone needs, I’ve been running mine for a while now and I have to tell you I am impressed, this boat fulfills all my requirements and the hopes that that article inspired.
If we can generate enough interest hopefully Hobie will start offering a ready to go package (they already make 99% percent of it, we are just asking for that extra 1% from them to make it into something commercially available as a package. Otherwise we have to build the darn things ourselves.
For those that know me, I’ve been chasing this a long time, and it is finally here (well I have one anyway, but I had to build it myself (which sucks))
I have posted my exploits on a couple threads on the forum with the same video as shown below.
Ultimate Tandem Island thread, basically all my designs and improvements, but it's really long and boring reading (just watch the movie LOL) ( viewtopic.php?f=69&t=33720&start=285 )

or Tri-power capabilities of the Tandem Island (the movie) ( viewtopic.php?f=70&t=50169), basically showing off my new Go-Pro camera and presenting the same movie as below but without in depth explanation.



No tricks or mirrors here, this is just me sailing my tribrid TI, the one promised in the Giz magazine article 4 yrs ago.


I am probably way out in left field, but my opinion is everyone would enjoy a TI like mine, but Hobie will never produce it unless there is a demand, and people start asking for it.

Thanks for listening (and sorry for the long post, again)
Bob


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PostPosted: Sun Jan 12, 2014 1:16 am 
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isnt it a trybrid as stock??
pedal
paddle
sail

so add a fossil fueled charged electric engine, you have a quadred.



OFF TOPIC
the entire argument of electrical motor power vehicles is already antiquated and redundant, when you turn on the power switch, while you might not smell it, its actually made from the burning of fossil fuels.......or worse.... nuclear.
the future to replace the combustion engine is, hydrogen. toyota are ready to sell hydrogen fuel cell cars by 2015, as are honda, just the infrastructure to supply the fuel is slow to catch up, with california leaading the way.
an electric car that takes a night to charge to get 300km is useless, as the cost for electricity is skyrocketing each year here in australia, for example, the state i live in, just had a rise of 22%. making running a home roughly cost (if you are power conscious) is between $200 to $500 a quarter.
as well as you just swap on fossil fuel for another.

solar, wind and kinetic enerdy is the future of powering houses. just my thoughts.


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PostPosted: Sun Jan 12, 2014 5:34 am 
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Nice story, Bob. BTW, when we look at your posts, we know we are committing.

Oz, I like that. Here in Miami, when I was at the University of Miami, I had an engineering friend who was trying to develop hydrogen gas as a energy source for cars--that was about 30 yrs ago! Hydrogen as fuel is a great idea. I hope it can be worked out. When hydrogen burns you get water--how can you beat that?

I always thought of a platform out in the middle of nowhere with a nuclear plant. The nuclear plant is generating huge amounts of hydrogen by electrolysis. and the hydrogen is shipped back to the states to provide pollution-free energy for our cars. Nuclear power is probably a non-starter today, but I like the idea. Of course, the side product of hydrogen production is oxygen. Maybe situate the plant near a dead spot in the ocean, you know, where all the garbage collects. And, then, pump the oxygen into the dead zone to help digest the garbage. I'm sure people, Toyota?, have thought a lot about this in the last 30 yrs.

Keith

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I sail: Biscayne Bay, Everglades to Cape Romano, Ft Desoto, Cedar Key

"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


Last edited by Chekika on Sun Jan 12, 2014 8:34 am, edited 2 times in total.

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PostPosted: Sun Jan 12, 2014 6:48 am 
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Besides the wind being an abundant power source on this planet, another is the sun.
I realize that solar cells are still in an expensive item compared to power output, however a sail made out of solar cells embedded into the sail fabric on both sides of the sail creating a "solar sail" may also one day be considered a cheaper method to power boat's motors as an integrated power system.

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PostPosted: Sun Jan 12, 2014 8:12 am 
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OZ:
you got me there, I forgot about the paddle ( I can't remember the last time I used my paddle though LOL).

I guess the main reason I am so sensitive to this is back in 2010 I had just spent 5 long years of my life trying to help the oil industry develop one of my accidental inventions ( I'm an inventor/engineer) which I was hoping would extend our oil and natural gas supplies for another hundred years or so (buying us time). My part was small and insignificant (the original concept/design), it took a huge team to actually make it happen. So I'm pretty aware of what's out there and the dilemma we are in.
I worked for quite a while trying to make my system work with solar/electric/fuel cell technology, but always came up short on the power requirements (battery life). I even developed a fuel cell to try and supplement the power, but ended up coming up short on range and safety factor (at least for right now, ie.. further development is needed)).
My final setup (the one that works) takes the fossil fuel (either gas or propane) and maximizes it efficiency, the main propulsion is the pedal/auxillary propulsion)), the Pedal/auxiliary propulsion is used to create the forward motion needed to create our own apparent wind which when it passes over the wing works as a giant amplifier (the chain reaction I described earlier). It doesn't matter where the auxiliary propulsion comes from, whatever source is available (evolve, hybrid gas, or human power) or any combination of the above.
I understand that it's a difficult concept to understand since it looks like a sailboat but it's really not. The whole concept is based on a different concept altogether from traditional sails from the ground up. Traditional sails as a primary propulsion system redirect the existing wind (you have to have wind to begin with, which has to be coming from a usable direction), by re-directing that wind into a propulsion force (same concept has been used for 3000 yrs with little change). This system doesn't do that at all, it works on a totally different concept and principles.

With this system you provide the initial propulsion force (via motor, electric, pedal, etc) to create a forward motion. As your forward motion increase this creates apparent wind that passes over the wing (creating lift). This is not dependent on the existing wind at all since you are creating your own wind via your forward motion as you go. The more air you can pass over the wing the greater the amplification, this is why the boat can sail almost directly up wind (actually faster than the wind in light winds), and also works just fine in almost no wind at all. Currently the boat can easily go 1.5 times the windspeed downwind ( I could never do that with my huge spinnaker and conventional sails).

I was out again putzing around on Friday, the winds were very low (around 6 mph and steady), the water was very flat (which I prefer).
He is my GPS log from Friday (I was out for my weekly pedaling exercise)
Image

I didn't want to bore everyone with more video, but I was out about an hour and the winds were light and steady around 6mph. Upwind I was able to sail within 15-20 degrees off the wind (almost into the wind) at over 6mph (faster than the wind), then on the downwind leg I was able to get almost to 1.5 times the wind speed. I checked and measured my fuel consumption when I got back home, and had used less than 1/6 of a gallon of gas (under $.50 cents worth of fuel). My knee was bothering me a little so I couldn't pedal as hard as usual, if I had pedaled harder my fuel consumption would have been $25 cents for the day.

I think the problem is people watch that video of some fat guy out sailing his boat to creepy music (my family jokes me about it) without understanding the concepts of what is really going on, it's not really a sailboat at all.......

Sorry for the long post (again) it's just really hard to get people to understand a new concept.
Bob

edit:
Give me another year and I will have designed and perfected the wing sail replacement for the mainsail, at that point we will be able to eliminate the auxillary propulsion altogether (ie.. gas/electric or whatever), and the boat should have equal performance with just the pedal drive (human power). In the meantime this is the best I can come up with.


Last edited by fusioneng on Sun Jan 12, 2014 8:46 am, edited 2 times in total.

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PostPosted: Sun Jan 12, 2014 8:28 am 
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Bob, you don't have to apologize. Great idea. I get it, and, if I get it, probably everyone else can.

Keith

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I sail: Biscayne Bay, Everglades to Cape Romano, Ft Desoto, Cedar Key

"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


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 Post subject: Wing Jib?
PostPosted: Sun Jan 12, 2014 9:57 am 
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Fusioneng:
I put a standard small jib on my TI and quickly no longer used it. It did not develop the lift I thought it would.
I decided it wasn't worth the trouble connecting shrouds and running control lines.
From your video(s) it is quite apparent that a wing jib is the answer.
I play with several kites, up to a 12 foot Rogallo, but find that the one I use the most often is an 'inflatable' wing design with forward
air slots. It is easy to setup and store. After home-made bow splash guards were first shown, someone began to offer them for sale with professional quality.
Hobie still doesn't sell splash guards.
You said that you were a sail maker. Any plans on marketing the sail? Or, I wonder if you could sell your design
to a commercial sail maker. I am ready to be an early buyer.


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PostPosted: Mon Jan 13, 2014 9:15 am 
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Tidalwave:
Unfortunately pretty much everyone that has tried to add a jibs to their TI's gave up on them ( including Hobie). I've posted pretty much everything I have done all along on all my mods (including design and construction) on the Ultimate Tandem island thread.
Wing sail technology is very new and there is very little written about the technology, basically a sail maker would have to throw out all his existing sail knowledge because it works on different principles and to be honest there has been to date almost no acceptance at all from the mainstream sailing community regarding wing sails, I can only assume they don't want to interrupt their bread and butter, actually the entire industry is downright hostile about the subject. But hopefully with the success of the Americas cup boats things will start to change (slowly I'm afraid). With the one class design stranglehold on the industry, all based on ancient sail designs basically there will never be any new innovation or wing designs in the mainstream market (that's their bread and butter), they have made that very clear. I'm sorry and don't mean to be critical of the mainstream sailing industry, but when I go out and see exactly the same sunfish boats that I sailed back in the 60's and hated at the time, I think to myself WHY??? (but that’s just my personal opinion)

There is really nothing special about my sail, it's just something I threw together in my garage for a couple hundred bucks in material over a few weekends, the only reason I make my own sails is I can't afford to buy them. If you show the video to anyone knowledgable in sails and sailmaking they can duplicate and improve on my design, Since mine is a very rough and crude prototype I would expect a well designed and built sail of similar construction would have double the performance of mine.
Same goes with the evolve system, if I could have afforded one, I would have developed everything around that.
Considering that 99% of what I have revolves around standard off the shelf Hobie products, I doubt very much anyone else would be willing to pick something like this up.

Once people start accepting wing technology there are many more uses for it that can be quite valuable in my opinion.

As an example a simple symmetric furling wing like this one shown below would be a real boon to kayak sailers, it's much more powerful (50% more efficient) but without the heeling force (the force trying to tip you over). The sail pivots freely 360 degrees, if the wind picks up you just let it go and it goes to idle (no driving force and almost no heeling force). To test mine out I went out and peddled figure 8's in 25 mph winds (20 ft tall above the water is a little much though for a kayak sail, and yes it did scare me a liitle), the other problem with a kayak sail that tall which I never considered till after I tried it, is trees always overhang the rivers and you have to take it down too often (DAH, not one of my brightest ideas LOL).
Image

That's the same sail I use as the jib, when I designed it, I designed it so it can be used as a kayak sail in kayak mode, or as a jib. It actually works well, that 33 sq ft sail is almost as powerful as the 90 sq ft mainsail. However at almost 20 ft tall you have to add a 60# weighted keel to use it in kayak mode.
It would be extremely simple to design a small square top symmetric furlable wing (about 20-24 sq ft) that would out perform the standard Hobie kayak sail by a significant margin but with way less heeling moment (wing sails have about 1/4 the heeling force of standard sails), but same situation applies here, nobody else is going to design and manufacture something specifically for Hobies exclusive products. I sold all my other Hobie kayaks and only just use the Island anymore, so I have no plans to make anything, I'm just sayin it would be cool.

There is nothing to sell design wise, it’s just a sail, anyone that looks at it can easily duplicate it. Plus this is only a hobbie for me to occupy my mind and provide some exercise on the weekends, I have no intention of ever selling anything, I just love to go out and use it on the weekends, and thought others would like to do the same.
When I read that article in Giz magazine back in 2010, what I have now is exactly what I imagined the Hobie Tandem Island to be, it just took a little work to make it come true for me personally.
I love my Hobie TI, it fulfils everything I ever wanted.
Who knows the Hobie team are some of the brightest people I know, I'm sure they are paying attention. If it makes sense to them, I'm sure we will see something.

Bob


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PostPosted: Mon Jan 13, 2014 9:38 am 
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Bob, what aerofoil section does your wingsail have?

BTW your influence spreads...
Image

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Tony Stott
2012 Tandem Island "SIC EM"
www.scenefromabove.com.au


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PostPosted: Mon Jan 13, 2014 1:03 pm 
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Tony:
I just went back to my cad drawings and tried to make sense of them, I suspect I winged it (no pun intended) quite a bit of it once I got started.
I started out with a naca 0012 300 airfoil form.
However I wanted to be a little conservative so I made it a little thinner than the spec (if I remember correct the widest point was 15% back from the leading edge and the widest point was 12% of the length (the widest point should have been around 15% or 16%, I regret making it thinner now).

Here is a drawing of spar 13 (17 spars from top to bottom)
Image

Here is an overall outline showing the spars that I actually drew up

Image

Here is a closeup of spar 13
Image

Here is a pic of the boat on my cad system with the sail in place (edit: Bonus pic of the future jointed wing reef-able rigid wing mainsail- didn't realize that was in there)
Image

Basically what I did was establish the outline of the sail (aprox 3.5ft x 15ft) I then measured the length needed for each spar (17 batton spars altogether). I then multiplied that length by .15 (15%) to get the location of the widest point. I then multiplied the LENGTH x .12 (12%) to get the width. I then struck lines from those points to the od of the mast and a point 3/4" from the back edge. I then made a few control points at regular intervals (usually about 10") to be use as the tie point. These points are where the two sail surfaces are tied together. I just used blocks of foam 1/2" x 1" foam in a cloth sack (kind of like they contain bed springs), simple but it works and doesn't interfere with furling. At the widest point on some of the bigger spars I used 2 pieces of 1/2" x 2" wide foam mounted side by side (so they can collapse like an accordion when the sail is furled). I just used that green cushion foam I bought at the fabric store. Before starting I clamped a chunk of that foam in my vice for a month, and it popped right back to it's original shape.

All the batton spar lines and the control points were measured up and marked onto the inside of the sailcloth (I just used cheap ripstop nylon (it's just a prototype))
It takes pretty good sized layout tables, and you have to be pretty precise with your measurements.

Once everything was laid out I sewed the batton pockets (1.5" wide fabric) into the sail (just following the lines 34 pockets altogether) and attached the fabric pockets at the correct points and put the foam blocks all in on one side only. I started out sewing them all down but got bored with that and ended up using waterproof fabric glue or a hot glue gun which also works.
I then lined up the edges and sewed the trailing edge of the sail with 1.5" wide fabric, I folded in some 1/8" dia fiberglass rod into the trailing edge on the top 2/3 of the sail, and 5/16 fiber down on that reverse tapered section down near the bottom of the sail (that reverse taper thing is kind of important because it creates a truss that keeps the sail tight).
On the leading edge I just sewed in a steel tape measure into the hem that keeps the front edge rigid (for the air inlets). It was an old 100 ft flat tape measure (now a 70 ft tape measure LOL).
I ended up using a 1" dia aluminum mast (from and old painters extension) as the mast. I just screwed the leading edge to the mast at the tangent of the radius near every spar (about every 10-12 inches) I actually sewed in attachment straps that the screws go into after running into problems drilling thru the steel tape measure.
On the really short ones up at the top, I just cut and glued foam in, instead of battons and batton pockets. Also later on I added 6-8 inch long foam wedges in the trailing edge of each spar to keep the trailing edge from bending too much. (basically 1" wide at one end and pointed at the other 1/2" thick foam 6 to 8 inches long glued into the inside of each batton pocket to keep the trailing edge from separating. I also sewed a blue canvas cover over the end of each batton pocket to prevent the battons from poking thru the back edge of the sail cloth (a lot of stress there when furling).

The battons themselves can be any material that you can furl, and then return to their original flat shape like fiberglass, I ended up just using half inch wide steel tape measure tape (the kind with the slightly curved steel so it snaps back into shape. I covered each batton with 3M packaging tape to keep water out. It's the natural curvature created by the springiness of the battons going through the control points that creates the b-spline curvature and gentle blended curved shape that will create the wing surface. Also keep in mind once the inside is pressurized the sail puffs out and becomes much thicker.

If you look at the video you will see I have 1/4" x 1" aluminum bar at the bottom, this is necessary to keep the sail tight and help form the rigid shape. It needs to be un clipped and removed to furl the sail.

That's it... the design is extremely simple and you can build in a weekend. The wind is what forms the parabolic airform shape automatically for you as you increase the angle of attack. You naturally create a vacuum on the outside surface and pressure on the inside surface as you change the angle of attack. This is what creates the necessary asymmetric form.
The wind fills the inside of the sail with air making it extremely rigid. The control points and the battons force the sail to maintain its correct shape.

I've been running it almost a year now and haven't had any rust problem on the tape measure tape except the one that I forgot to cover with tape. I dipped the ends if each batton in epoxy so the end is a nice ball shape (so it can't cut the sail fabric). The battons need to be about 2 inches short so they can slide in the pockets when you furl the sail up (otherwise they try to pop out the back edge when you furl the sail up). It's surprisingly durable and I have had it out in up to 30 mph winds on several occasions.

I found I don't use the furling function hardly at all (kind of a waste of time) since once it is up, if you release the sail it just idles (no lift/no drag) so I just leave it up all day, then only furl it when I get in and just lay it down on the boat for transport. I leave all the rigging and connections attached, basically just furl it and lay it down, that's it.

You will find that once you build it there is almost no heeling force trying to tip your boat over from this sail design, so it will work on your TI without having to extend your AMA's out further. Plus it's way more powerful than a normal jib (it's actually almost as powerful as the 90 sq ft mainsail itself). At least in my testing I was able to get up to 6-7mph in 12mph winds just flying the jib alone (no mainsail)

Hope this helps good luck
Bob


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PostPosted: Mon Jan 13, 2014 3:49 pm 
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Thanks for the detailed explanation Bob, I really appreciate the thought you put into the design. Just one question, how does the air get inside the sail to inflate it? I have a softform kite, which needs huge openings at the front to keep it inflated, but I couldn't see any openings in your design.

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Tony Stott
2012 Tandem Island "SIC EM"
www.scenefromabove.com.au


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PostPosted: Mon Jan 13, 2014 4:08 pm 
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Tony:
That is why the steel tape measure is sewn into the leading edge, one on each side of the mast. If you look at the video you will see the blue straps on the leading edge every ten inches or so. The air flows in between those straps in the little slits. Not a lot of air is required to fill and pressurize the inside chamber. When I originally designed it I wasn't sure I was going to need the slits so on all my development videos they are taped over. Without the slits and the pressurization the sail is not rigid and can flutter in higher winds. With the sits it's rock solid
Bob


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