Trimarans: Angle of Heel at Main Hull Takeoff

That is not true, Hussong! Just the opposite in fact-lifting foils are used to reduce wetted surface of the ama when flying the main hull and also help to prevent pitchpole. Orma tri's used the ama foil to lift up to 70% of the boats weight-hardly insignificant. Most modern large tri's (and many small tri's) use curved lifting foils including the new Mod 70, Banque Populaire, Groupama, Idec, Sodebo, USA-17 and on and on-they do it because it is faster and improves handling. And the video's and pictures of the M23, Sea Cart 26 and Catri 23 show just what a difference in ama immersed wetted surface they can make.
The lift from the foil keeps the ama higher than it would otherwise be allowing a lower angle of flying the main hull if the boat is designed to do that.

http://www.wired.com/wired/archive/14.05/sail.html?pg=2&topic=sail&topic_set=

 

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Hussong, you said in the previous post: "Ama foils play an insignificant part in the angle of heel when the main hull leaves the water".

Maybe this will help:
1) the foils lift the ama substantially higher out of the water than it would be on a boat without foils. That is clearly shown when the M23 video(post #2 of this thread) is compared to the Sea Cart 26 pictures( post # 5 of this thread).
2) if you lay out a tri(say, a Sea Cart 26) on paper showing the angle of heel with the main hull 1" above the water when using ama foils compared to the same boat not using foils the angle of heel using foils with the main hull 1" out of the water will be less. Thats because the boat without foils has to make up with buoyancy the substantial vertical lift generated by the foils causing the ama to sink lower and the angle of heel to increase.

 

Hussong, you are wrong with your interpretations of foil lift ... it is immediate; as soon as the boat moves, lift is generated. It may not be great at slow speeds but it is there and of course, the faster you go the greater the lift. You sound like many who make comments based on no actual experience.
However I'm in agreement about curved foil construction, it is not easy ... but with careful application it can be achieved. However again, straight, angled foils are no more trouble or more expensive to build than conventional boards, centre or daggers - as Bruce has pointed out. If we're not talking about Nomex, heated carbon laminates, even amateur backyard crazies can quite easily achieve building lifting foils out of standard epoxies, wood or foam core, glass and carbon ... it has been going on and has been successful for many decades ... although done quietly and without media fanfare.

 

Gary,

Exhibit A in this twilight zone is:

Joyon's IDEC trimaran, which recently blew right over on its side, flying its main hull to the max. The boat is equipped with lifting foils and I am sure that Joyon had them deployed as he was in the middle of a record run across the Atlantic when this incident took place. Best of everything boat. Latest in ama foil design. Hyper experienced skipper of unquestionable talent. Boat over like a turtle before the foils could do anything to resist the heeling moment.

Yes, I'm exaggerating for effect.

For ama foils to do any work at all, the boat must be moving forward. It must be moving forward fast enough to encounter the lift function of the foils as designed. We all know that ama foils are designed for a specific range of boat speeds for best lift production. Below that region and above it, they are decidedly less efficient, as in stalling, or ventilating. This is typically so for the lower ranges of boat speeds at the outset of movement forward.

So, what caused the boat to move forward and what happens when that force is brought to bear on the boat. The boat immediately heels and the amas resist that action. Some of the wind is applied to the sail in the form of thrust along the vector lines of the boat, but that does not happen immediately and foil lift also does not happen immediately and when it does, it is typically less than powerful, as discussed above.

The result is that the ama flotation volume coupled to the length of the aka beam provide virtually all the initial resistance, which lifts the main hull long before the foils can ever begin to provide lifting resistance. It is not impossible for a sufficently light and specifically designed trimaran to fly its main hull long before the optimal lift mode of the ama foils has been put into play.

 

Curved ama foils or straight angled foils do not add to RM- ever*!( ERROR-INCOMPLETE-see post #25) They actually cause a slight decrease in maximum righting moment because the combination of ama center of buoyancy and foil vertical lift is inboard of the lee hull.
They do ,however, reduce the angle of heel of the boat at takeoff as described previously. They also can benefit the boat with enhanced pitch control-depending on the design. And they lift the ama vertically.....
*See post 25 for more but to sum up: a foil on an ama that has less than 100% buoyancy can add lift up to the total weight of the boat. However, boats that use ama foils generally rely on a partial immersion of the ama for pitch control-if the foil supports the whole weight of the boat on this type there is no reserve for pitch control and another foil or two will be necessary.
It is really not correct to say "the foil adds RM"-because the foil adds vertical lift.....

 

When to foil?, When to swim?

So we have all observed and agreed? that some foils work some of the time. In my VERY limited trial, I tried motoring straight up wind and moving my "ballast" (me) from the foiling side to the standard float. The lift was quite apparent almost as soon as the boat started to move and was close to lifting me by 5 kts, and was also very noticeable in the helm position. Both as expected. The question then becomes, when and how much to "foil". My boat has about an 8-1 main hull and a 12-1 float beam to waterline, and in the past, was faster with the float loaded with the crew weight instead of the main hull until the speed really picks up. The Buc, like many lower performance tris, does not lift the main hull in most conditions, so when should the change over occur? Anybody have some guidelines, I am in new territory Hussong, from my beach cat sailing days, a hard gust while moving slowly always has the chance of overpowering your boat, and of course, a foil is not going to help, but but that doesn't mean that foils aren't useful in many other conditions. The C-31 with the foils has put the crew in the water twice, but they have not quit using their foils, just learned the boat's limits somewhat better B

 

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Tom Speer, once said that for the lift to outweigh the drag with "foil assist" the wetted surface has to be decreased by 4 times the planform area( the topside of the foil-not both sides) of the lifting foil. I'm not sure thats too accurate but you could keep it in the back of your mind and run some with and without trials and see what you find-and ,of course, tell us!

UPDATE-9/13/11: planform area on a 45 degree angled straight foil would be 50% of the actual topside foil area......

 

Hussong, I realize your analogy of Idec's capsize was intentionally over top - but in that situation foils would have done little or nothing to change the result; Joyon said the boat just got literally blown out of the water, wrong place, wrong time, power of the elements once again revealing how inadequate we are. This is beyond any question about foils.

 

Just how do they (curved, or angled foils) do that if they do not enhance righting moment?

The physics would indicate that any device that enhances righting moment would increase the chances that a center hull would fly. This would include increased ama buoyancy especially. Now, you say that they actually reduce righting moment and yet, they also increase the early flying of the main hull. Perhaps something is fundamentally wrong with one of those observations?

Fix this if it is wrong, but I see things as this: Wind hits sail rig. Boat heels obediently due to induced heeling moment. Ama is depressed resisting heeling moment as it is x buoyancy measured against the overall weight of the boat that would be brought to bear on the resistant element (ama). As heeling moment increases, more and more of the ama is depressed until its buoyancy equals the overall weight of the boat. At that point, the main hull begins to lift, being leveraged by the resistant forces of the ama and it continues to lift as long as wind strength is sufficient to do so. At some point, dependent on design, the ama resistance (buoyancy) coupled with the righting arm (aka) has lifted the main hull clear of the water surface and you have a flying main hull as long as the conditions remain constant.

With an ama equipped with a lifting foil, I see things entirely differently, yet still the same with some slight variations on the theme. The foil lifted ama is partially, positively enhanced in its response to being depressed, provided that the lift of the foil is coupled with the wind and boat speed present. This would also allow the main hull to be lifted, but it happens at a later part of the lift cycle, especially when the design has not been executed to include sufficient buoyancy to resist the physical weight transfer at the outset of the rising wind strength.

Small amas with foils are highly dependent on the foils to provide lift functions throughout mid to late in the sailing cycle, while larger amas, such as one sees on big, fast ocean-going trimarans, or boats also designed in that style, are much less dependent on ama foils for the lift necessary to fly the main hull. A boat with small amas and foils is still very susceptible to over-turning moments when the foils have been overwhelmed with heeling moments that are far beyond their ability to create equilibrium in the design.
Larger amas of the 200% variety and not equipped with foils will always have their full resistant buoyancy available, no matter the heeling moments present, because they are designed to handle a much wider variety of conditions.

In short, dependent on design, ama foils can provide a small heeling resistance that is beneficial to the lifting of the main hull. Conversely, larger amas without lifting foils can do this much earlier, as the buoyancy is always there, no matter the heeling moment, or the gust factors present.

Design issues that lead to early flying of the main hull have nothing to do with the presence of ama mounted foils in the larger game before us. It has already been stated by another member that foils actually reduce righting moment.

So, the only way to actually get the main hull flying early is through design, rather than through ama mounted foils. Righting moment alone flies the main hull. If that is an early function in the overall sailing scheme, it is through design that it happens. Several issues come into play in the process such as: dihedral, overall weight of the complete platform, rig size and placement, main hull draft and ama buoyancy when coupled with the overall beam of the boat.

 

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Well, I was wrong to say the lifting foils don't increase RM* ever-- I was thinking of amas with enough buoyancy to fly the main hull without foils.
Amas that have less than 100% buoyancy can still fly the main hull if equipped with foils designed for that purpose. And in that application the foils do increase vertical lift that can result in an increase in the overall RM of the boat. My apologies for letting that get by.
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*Note: what the foils can do is increase the lift on the ama so that it doesn't submerge, allowing the combination of "X" ama buoyancy and foil lift to allow the main hull to fly by supporting the entire weight of the boat. RM is the Righting Arm times the weight of the boat. The Righting arm is the distance of the center of lift( ama CB + foil lift) from the boat center of gravity. A boat relying on an ama of less than 100% buoyancy plus foil lift to fly the main hull is pushing the edge relative to pitch control and may require a rudder t-foil or some other type of foil on the back end of the boat(Catri).
-------On a tri that has sufficient ama buoyancy to fly the main hull with or without foils the RM without foils will be slightly greater than the RM with foils but, with properly designed foils, the total wetted surface of the ama will decrease substantially even though the RM is slightly less with a working foil. And the pitch resistance of the ama will increase with foils as well not only because of the location of the lifting foils, which is very important, but because of the greater reserve buoyancy of the ama(since it is physically lifted up).
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You have to understand that ama mounted foils begin to work when the boat starts to move. As an example, on Martin Fischers new "Mayfly" A Class catamaran the lee hull foils(main + rudder) support 40% of the boats weight in 10 knots of wind. A foiler Moth takes off between 6 and 7 knots boat speed with the foils supporting 100% of the weight. It is a matter of designing the foils for the best combination of lift and drag for a particular boat. On an ama hull with a high L/B ratio it may be better to keep the foils retracted in certain light air conditions if the boat also has a dedicated daggerboard. Then deploy the foil at "X" speed.
Foils can be designed to work throughout the speed range of the boat-as on USA-17 and others. On any boat designed to fly the main hull, that also uses foils, the foils will be developing near maximum designed lift as the main hull starts to fly because this is when they do the most good-it is essential that they are 100% effective at this point in the speed range(main hull 1" above the water-as soon as the main hull clears the water).
On early foil applications the angle of incidence of the lifting foils was not adjustable-on most new tri's using curved lifting foils the angle of incidence is adjustable. This is important because on earlier incarnations the boat could go so fast that the ama lifted clear of the water (with the main hull already flying). This, of course, removed the only pitch stability the boat had an invariably resulted in a crash. Curved lifting foils can adjust the angle of incidence of the foil by angling the top fore and aft with no effect on the portion of the foil providing lateral resistance. In addition, the foils can be retracted reducing their lift area. On a high performance lifting foil equipped tri it is essential to configure the boat for the conditions-not just for speed but for safety.
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Back to the thread topic: it seems as though, so far, that those boats designed to fly the main hull AND use lifting foils have less of an angle of heel at the critical point-mainhull just kissing(1'' clear, more or less). This can be important for resistance to capsize, utilization of maximum power, resistance to pitchpole, and lower wetted surface resulting in a probable greater speed potential. I'll keep collecting info and posting it here and hope anyone else that runs across pertinent info will post it here.

 

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Hussong, on a boat designed to fly the main hull that also has the buoyancy to fly the hull with or without foils the foils do not enhance the righting moment of the boat-as I mentioned earlier they slightly reduce the righting moment. But they definitely reduce the angle of heel at the critical point(just when the main hull leaves the water) because they physically lift the ama vertically. You can see how the boat would rotate if the ama was more or less immersed and the main hull stayed 1" above the water: more immersion = greater angle of heel---less immersion due to foil lift= less angle of heel.

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Foils could allow the boat to fly the main hull slightly earlier than if the same boat(as above) didn't have foils because the resultant center of lift(ama center of buoyancy + foil center of lift) is slightly inboard of the ama center of buoyancy w/o foils. So a given wind speed would tend to fly the main hull a bit earlier because of the slightly reduced RM.
Hope this is helpful.....

 

Helpful, maybe. Accurate, not at all. I get a strong feeling that you have not grasped the full story here. That is supported by your self-correction comments.

Thanks for your time.

 

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Interesting. I was accurate for all trimarans at ama buoyancy of 100% and above-most trimarans. I made the correction to include the increase of RM possible with trimarans of less than 100% ama buoyancy.
Please let me know if you have found any further inaccuracies-I would be most grateful.

 

my new 8.5 tri wil be 20 degrees heeled when main hull comes out

 

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Thanks, very much Sam!