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PostPosted: Wed Aug 21, 2013 6:34 am 
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The "hacker" community that has sprung up around the AI/TI is amazing. Hakas, captains chairs, the BATboat, quarterdecks, the double bungie method, and spray skirts.

However, as I was sailing in 20knot winds yesterday I believe I had an epiphany. I may not be the worlds greatest sailor, however it seems that the TI is easily overpowered by its mainsail. This manifests itself as a buried leeward AMA. When the AMA buries, the drag produced bleeds tremendous speed. The solution as I understand it is to reef the sail, getting the leeward AMA back on plane.

The AMAs are a brilliantly simple design. Two areas of connection with bungies holding them in place.

Would it not be possible to dramatically enhance the performance of the TI by swapping the existing roto molded AMAs for a pair that provide positive lift? Couldn't the Amas be made of foam and fiberglass (like a surf board), with a shape that provides lift as the speed increases. Maybe with small foils? I'm not talking about turning the TI into a moth, but the amas seem to easily buried.

Couldn't hobie offer a higher performance set of amas? Or couldn't somebody (me maybe) make a set to test the idea. They would seem to be an easy structural retrofit.

What am I missing here???


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PostPosted: Wed Aug 21, 2013 6:51 am 
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I just saw this post regarding AMA design

http://seaonblog.blogspot.com/2007/05/t ... yancy.html


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PostPosted: Wed Aug 21, 2013 7:21 am 
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A set of planing amas wouldn't work too well unless you did the same thing for the hull. Even then I can see some problems that would crop up. The Weta I recently bought has a hull that planes, but the amas do not.

As is, the Island amas provide floatation and are shaped so as to keep frontal drag to a minimum under a pretty wide range of conditions and uses. Changing the material wouldn't really alter anything unless you managed to get the same volume for less weight or greater volume for the same weight. A different shape, maybe, but then you're going to give some type of performance on one end to gain performance on the other. Hobie probably wanted more of an "all around" type tri, which the Islands certainly are.

As far as actually creating lift you'd be back to foils and when one side lifted the other side would be depressed into the water which would then create lift and the entire boat would rise up out of the water, Trifoiler style, but without the leveling ability provided by the forward sensors. And then you're back to having the boat rise and sit, rise and sit. It really turns a boat into a one trick pony that only functions ideally under a fairly narrow set of conditions.

Not to mention that having foils on the amas reduces the versatility of the boat for beaching, etc., unless you had a way to retract them (Trifoiler again) which increases the complexity and the cost of the boat.

I don't like to furl my sail but realizing that flat is fast, once I began doing it a bit more often I found that by furling a bit in higher winds I was able to obtain a higher top speed than if I left the entire sail out. The leeward ama no longer buried as deep and therefore drag was reduced and less wind spilled off the sail than if I had left it all the way out.

In short, your answer to going consistently fast in high winds is simply to furl the sail a bit, finding that perfect amount that lets you get all the wind you can while still keeping the boat relatively flat.


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PostPosted: Wed Aug 21, 2013 8:42 am 
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For the AI owners, purchasing a pair of TI amas can accomplish some of these goals. Below about 13 mph wind you will maximize speeds with AI amas. With TI amas on an AI and winds 14 mph and above, you will see better performance both in stability and speed. Stability for obvious reasons but speed because you don't need to reef the sail now. You also widen that wind conditions that you can sail within safely.

Maybe a TI owner could strap on an AI ama onto the TI akas along with the TI amas and test drive the TI in higher winds just for kicks. Might get the same effect although more drag.


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PostPosted: Wed Aug 21, 2013 8:48 am 
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How about foil wings that were flush on the ama's hull providing just enough lift to counter the downward pressure on it so that it would tend to resurface instead of lift completely. That way, the leeward ama would "want" ride higher the faster it goes, keeping drag to a minimum.


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PostPosted: Wed Aug 21, 2013 9:11 am 
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I'm with Tom on this one... it is so simple to furl the mainsail by enough to prevent the top surface of the leeward ama from being buried, and you are then going as fast as you can in that wind speed and direction. The first time I put in a roll or two of sail, I was amazed how much quicker my TI went, and how much less drama was involved.

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


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PostPosted: Wed Aug 21, 2013 9:34 am 
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Plz don't forget - The higher the winds, the larger sail area = greater forces on the mast and mostly the bearings.
Using Haka's to offset the forces so the boat doesn't heel as much (therefore a greater speed) is putting a larger force
on the mast! You are pinching the mast at the cup/bearings, so the mast strength is the limit, not better Ama's!

I agree with Tom K, by keeping more sail out and leveraging the Ama's, mast just bends and Spills the xtra wind force over the top!
By sailing this way, you place maximum stress on the mast near the bearings too!

As he states, furling the sail, keeps the mast upright, keeps the speed up, Without harming the mast....


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PostPosted: Wed Aug 21, 2013 12:53 pm 
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:wink: A 'soft' T-shape (or triangular) frontal profile of the amas would provide a small wetted area when the ama just rides on the water. When the force down on the ama increased the wider upper section would provide more positive displacement. If the longitudinal profile were angled higher at the bow, the upper section would provide a wing-like lift to the front of the ama. If the top width at the front were more sharply narrower than at the rear, the frontal area wouldn't present a blunt bow to oncoming waves.
I have owned several pontoon boats. The first pontoon boats had round pontoons because they were really aluminum tubes designed as aircraft parts in WWII and were cheap to make with war surplus machinery. As soon as weight or wave height increased, the water surface would go above the midpoint and there was very little additional displacement but increased drag. A very unstable situation. My second boat had hexagonal cross-sections. As water height increased the 'flared' sides added both additional displacement and also lift. Both boats were nearly identical in weight but the hexagonal type had a nearly 50% higher top speed using the same horsepower motor and increased weight carrying capacity. .


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PostPosted: Wed Aug 21, 2013 1:30 pm 
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Drewyaker wrote:
How about foil wings that were flush on the ama's hull providing just enough lift to counter the downward pressure on it so that it would tend to resurface instead of lift completely. That way, the leeward ama would "want" ride higher the faster it goes, keeping drag to a minimum.



There could be an interesting idea here, although it may still not be practical. Anyway, if some sort of wing were molded onto the side of the ama, near the top, so that once submerged to a certain point it would create lift and bring the ama back up. And, once the ama rose to a certain level the wing would then be back up and out of the water and no longer produce lift. So you'd avoid the back and forth roll you can get when two competing amas are working against each other on opposites sides of the boat (I forgot to dump the sensors once on the Trifoiler - talk about a wild ride!)

Both wings could never be in the water at the same time and whichever one was would only produce lift beyond and up until a certain point.


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PostPosted: Thu Aug 22, 2013 7:18 am 
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leelanauX :

Of course you can hike out to help balance the boat (like on every other sailboat made), this helps, but you still get severe weather helm at higher speeds because of the tiny rudder on the TI when you have bigger sails. And because the hull is a displacement hull, even hiking out only gets you so much speed before the horsepower required to drive the boat faster goes thru the roof. An lets face it, 90 sq ft of sail is very small for a boat this size.
That problem is kind of a catch 22 with the TI. I have massive sails on mine (265 sq ft) but really can't use all that sail in winds over 12mph safely without tipping over or risk breaking something (though I do it all the time anyway). What I have is really rigged for the typical 5mph winds in the Sarasota area. Plus the weak and smallish rudder isn't really designed for higher speeds. I have had my TI over 18mph on several occasions (in 20 mph plus winds), and if I crank the rudder at that speed the rudder just snaps off. I have a similar problem with the AMA's if I'm screaming along at over 15 mph and a gust hits, if it buries the AMA underwater at that speed the nylon sheer bolts on the AKA braces and the AMA just folds in, I have nylon ropes bracing my AMA's to prevent this problem, but there isn't much I can do about the rudder pin problem.
Because the TI's mainsail is boomless, it tends to put more healing moment on the boat than some other sail designs ( healing moment is the force trying to tip you over). The easiest solution would be to extend the existing AMA's out another foot or so per side (making the boat 12ft wide), by doing that you should be able to increase the sail area about 15% to 20% ( instead of having to furl the sail at 12mph winds, this would now be around 15-16 mph). Of course if you hike out to help balance the boat, all the figures increase even more. "The best solution is to hike out and never let your AMA's touch the water". Since most people sail from the back seat, hiking out from there is pretty tuff to do. I only sail from the front seat, but I have a large bow sprit to get my fore sails further forward, and they are angled so they also provide lift for the front of the boat, this lifts the bow out of the water (reducing wetted area), and helps you get on a plane (actually it's a semi-plane).
I noticed that anytime I have been able to get really good speeds, the tip of the bow is typically out of the water 4-6 inches or so (just an observation on my part), and my AMA's are not touching the water except at the very back, I hike out to keep the boat balanced. This brings up another design issue (the hull) the TI hull is not designed for speed, because it's pointed in the back even when semi-planing, it still drags pretty bad. If the rear of the boat were shaped with a rounded rear (like the Weta), the hull would be able to semi-plane easier. If you ever watch a video of a Weta really pushing hard, you will see the bow in the air, and the helmsman hiked out enough to keep the AMA's from touching the water, if you look at the AMA's on a WETA, they are tiny, non-planning, and not really designed to keep the boat level, you have to hike out (as with most every other boat out there of that style). Also I think when a WETA is really hauling the front half of the hull is out of the water.

Another problem preventing adding more sail area to the TI is the mast holder. If you look at it closely, the only thing bracing the mast forward to back is a tiny 1/4 inch stud in the hull bottom. The hinge point is one foot above, and the sail is 18 ft tall, so the leverage on this stud is tremendous (17 to 1 leverage), basically if you put a brick under an 18 ft pole one foot from the end you can easily lift your car. As a test you can extend your sail fully by pull it tight, then give the control line a swift jerk, you will snap that stud off. So unless you have your hull re-enforced (like I do) and have a rear stay line (like I do), then adding large sails to your TI is a recipe for disaster.
I have designed and built 3 or 4 different versions of hydrofoils for my TI, and they all run into the exact same problem. Before getting up on the foils you have the drag from the hull plus the drag from the hydrofoils (double drag) so without enough sail area or more efficient wing sails, you simply can't generate enough speed to get onto your foils, so you end up just dragging them around all day (not worth the effort).
One of my foil designs was something along the lines of what Tom was describing. The foils are 3 ft long and 4 inches wide, they are mounted at a 20 degree angle outward (pointing down and out) The end of the foil is about a 1 1/2 feet past the outside of the AMA (passing under the AMA), when the boat is level just the tips of the foils are in the water. As the boat tips more foil submerges increasing the lift. Also when the boat is tilted the other foil is completely out of the water. I had 4 foils (one mounted to each AKA cross bar. The foils were design to remain in neutral (no lift) until you hit 7mph, at that point the lift overcame the torsion bar and allowed the foil to tilt to 8 degrees attack angle. The foils did work to help keep the boat level, but because of the hull design (displacement hull) I was only able to get to foiling speed a couple times, the rest of the time I was just dragging the foils around all day long trying to get up to 10mph. That was 2 1/2 yrs ago the foils have been sitting in the garage since waiting for my wing sail solution. I've done all my calculations, and with a 100 sq ft wing sail main, and my 33 sq ft wing jib (already completed) I can get the TI up onto the foils (fingers crossed).
Conclusion:
The TI is a great boat, and extremely versatile, you can do almost anything with it. But the displacement hull and sail design pretty much excludes anything high speed. If you wanted to be creative with foam and glass, you could build a cap to go over the back of the hull to remove the point, and make it look a little more like the WETA's rear (squared off), so you could plane. At the same time add a transom and swing up stronger and bigger rudder like the one on the Getaway. This would also add more floatation to the rear (mine is nearly always almost under water now). When I'm really moving along presently, the rear of the hull is completely submerged under the wave created by the hull. Basically I think I'm applying massive horsepower with the sails to basically drag the boat thru the water, definitely not the way it should be done.
Hope this helps
Bob


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PostPosted: Thu Aug 22, 2013 7:58 am 
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Very insightful Bob. Your experiments are very interesting.

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


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