Trimarans: Angle of Heel at Main Hull Takeoff

[Hussong]

From sailing Anarchy Forum at noon today:
http://forums.sailinganarchy.com/ind...=114946&st=200

"Posted Today, 12:01 PM

As part of the project my qualified guess is around 12-16 deg. heel depending on displacement, speed and curved foils in or out / with foils fully down (more righting moment) it make you lift the mainhull higher up in the wind range and speed but at lower heel angle."

From mononowmulti, who is a part of the development team for the Seacart 26.

Ta-Da!

This fits with foils that are dialed in for the upper region of the boat's speed range and not for foils that start providing significant lift at the lower regions of wind and boat speed.

First post on Page 3 of thread

[Doug Lord]

Originally Posted by Hussong

From sailing Anarchy Forum at noon today:
http://forums.sailinganarchy.com/ind...=114946&st=200

"Posted Today, 12:01 PM

As part of the project my qualified guess is around 12-16 deg. heel depending on displacement, speed and curved foils in or out / with foils fully down (more righting moment) it make you lift the mainhull higher up in the wind range and speed but at lower heel angle."

From mononowmulti, who is a part of the development team for the Seacart 26.

Ta-Da!

This fits with foils that are dialed in for the upper region of the boat's speed range and not for foils that start providing significant lift at the lower regions of wind and boat speed.

First post on Page 3 of thread

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Not right: In the Sea Cart 26 case the foil lift replaces the lift generated from bouyancy(when the foil is retracted)-it does not increase RM. In fact, as shown below(post 33), the foil version actually has less RM than the retracted foil version since the lift from the foil is inboard of the ama CB. Both versions provide similar total lift. However, if due to speed, the foil lift was to increase beyond replacing a portion of the buoyancy of the ama( when immersed with a retracted foil) then the greater foil lift could result in a decrease(perhaps catastrophic) in the pitch control of the boat, but no increase in RM. In other words, the foil cannot lift more than the weight of the boat and an increase in lift that eliminated the contribution of buoyancy from the ama would simultaneously eliminate the pitch control of the boat! Thats why it is so important to configure the foil for the conditions.

I just replied to the guy on Sailing Anarchy the following:

I'm curious why you say that there is more righting moment with the foils deployed than with them retracted? With a curved foil the center of lift of the foil is inboard of the ama center of buoyancy so the combined vertical lift of the ama buoyancy and foil would be slightly inboard, slightly reducing the righting moment, correct? Therefore, with the foils retracted the ama would sink more but the righting moment would be slightly greater.
The difference, with the foil deployed, is that the ama is higher out of the water with significantly less wetted surface and the takeoff angle of heel is less, correct?
I appreciate your help here-thanks!
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Assuming, of course that the Sea Cart 26 ama has more than 100% buoyancy.

[Doug Lord]

Just did a very rough but accurate couple of sketches illustrating what happens with and without foils on a tri that can fly the main hull with or without foils:

1) top sketch shows main hull just clearing water and ama foil deployed.
Righting Arm(RA) is 7.71', angle of heel is 16 degrees.(slightly less RM)
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2) bottom sketch shows main hull just clearing the water and ama foil retracted.
Righting arm is 8.25', angle of heel is 19 degrees.(slightly more RM)
===============
With a trimaran that can fly the main hull with or without lifting foils, these rough but accurate sketches illustrate the following:
a) the take-off angle of the foil equipped boat is less,
b) the wetted surface of the foil equipped boat is less,
c) The Righting Arm, and therefore the Righting Moment of the foil equipped boat is less than that of the boat with no foils.
d) The portion of the ama hull available for pitch control is greater on the foil equipped boat, which, along with foil placement, will reduce the potential for pitchpole.
==================================
The same princible-a boat with curved foils having less RM than a similar boat with "normal" foils-is well known in the cat classes that use curved foils.

click on image:

[Hussong]

Again, the duscussion is not about what happens should the boat reach and hold a very high rate of speed. That status was shared by the member of the Swedish Seacart 26 development team when I quoted his comments earlier from Sailing Anarchy. He very clearly said that in the higher speed ranges and wind strengths, the foil allows for a lower angle. All the rest of the speed regime and wind strengths, it is not as effective, by his inference.

That pretty much sums up what I have been saying all along and for me, the discussion has reached a zenith simply because this man has actual sailing experience aboard the boat. The rest of us, with some very exclusive exceptions, do not.

Moving on from the resolved issue, what is the actual need to catalog all the main hull flying heeling angles?

All around the world through several discussion forums where I read, as well as direct contact with several French multihull enthusiasts, this is not a driving issue for the sailors out on the water. Quite the opposite seems to be the general opinion as they continuosly say the their boats are faster when the main hull is not flying free of the surface.

My impression, after reading dozens of these types of comments, is that a main hull that is just skimming the surface, provides for an end plate effect on the daggerboard, enhancing its efficiency and taking a big load off of the rudder.

Further along those lines, the immersed ama is also presenting a better sectional shape through the water and is much more efficient.

Please share your sense of importance regarding this particular issue.

[Doug Lord]

Originally Posted by Hussong

Please share your sense of importance regarding this particular issue.

===========================
Flying the main hull("just kissing"), on a boat designed to do it, is the fastest way to sail a small tri and many larger tris.It's true for the same reason that the fastest way to sail a cat is with the weather hull "just kissing": it gives maximum righting moment with minimum wetted surface. This is true whether the boat has foils or not.
Takeoff angle is important, especially on a boat without a canting mast-the more upright-the faster and the less chance of pitchpole/capsize.

[Doug Lord]

Originally Posted by Hussong

Further along those lines, the immersed ama is also presenting a better sectional shape through the water and is much more efficient.

==================
Numerous trimarans(and catamarans) including the ones I've designed and built have the ama canted so that it is vertical at the designed sailing angle of heel.


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Hey, Chris-just realized it was you! Took a little longer this time......

[Doug Lord]

Originally Posted by Hussong

Again, the duscussion is not about what happens should the boat reach and hold a very high rate of speed. That status was shared by the member of the Swedish Seacart 26 development team when I quoted his comments earlier from Sailing Anarchy. He very clearly said that in the higher speed ranges and wind strengths, the foil allows for a lower angle. All the rest of the speed regime and wind strengths, it is not as effective, by his inference.

================
You need to watch this inference stuff, H-c. Foil lift varies as the square of the speed, so at 10 knots the foil lift is 4 times what it is at 5 knots. At 15 knots it is 2.25 times what it is at 10 knots.
So in reality, foil lift(effeciveness) increases dramaticlly with any speed increase throughout the wind range. The foil lift can be regulated-that is, configured for the conditions- by reducing its immersed area and/or by changing the angle of incidence of the foil by adjusting the head of the foil fore and aft, if that facility is available on the Sea Cart 26 like it is on many other curved foil equipped boats.

[Doug Lord]

Originally Posted by Hussong

#1 Again, the duscussion is not about what happens should the boat reach and hold a very high rate of speed. That status was shared by the member of the Swedish Seacart 26 development team when I quoted his comments earlier from Sailing Anarchy. He very clearly said that in the higher speed ranges and wind strengths, the foil allows for a lower angle. All the rest of the speed regime and wind strengths, it is not as effective, by his inference.

# 2--That pretty much sums up what I have been saying all along and for me.......

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H or C, I feel like I should point this out as a matter of clarity: This (quote #1& #2 above)is not what you've been saying all along-in fact this whole discussion was precipitated by this remark in post 13 :

#3-

Originally Posted by Hussong

Ama foils play an insignificant part in the angle of heel when the main hull leaves the water.

My point then(see quote #4 below) was ,as it is now, that the ama foil directly influences the angle of heel of the boat when it flys the main hull-not an insignificant thing at all. In the first quote above you quote the Sea Cart guy(?) as saying nearly the same thing I did-you said the opposite in quote #3. Just for the sake of clarity.....
Thanks for your contribution to the thread-thats why I gave you the rep points....

Originally Posted by Doug Lord

#4The 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.

[Doug Lord]

Once again:
I'd appreciate all the help I can get from designers and sailors with getting information on the main hull take off angle of heel of as many trimarans as possible.
This is with the main hull "just kissing" (say 1mm above the water) .
I've run into wide variations and I'm interested in getting lots more info for which I will be very grateful.
Some that I know or have found:
1) RC model-slightly oversquare moderate main hull rocker: 13 degrees, (accurate)
2) My new design-wide main hull, little rocker: 10 degrees, (tentative)
3) USA-17: 13 degrees, (accurate from Oracle)
4) many other tris are much higher-one I measured here was 26 degrees!
5) Mod 70- about 13 degrees
6) Sea Cart 26 13-16 degrees-company estimate.
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Again, your help would be most appreciated!

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--At this point I don't necessarily need analysis of why the angle is as it is(beam, weight SA, dihedral etc) just the angle will be ok. But analysis is ok-even great- just not absolutely required right now. Thanks.
-
-- Estimates are ok-just mention that's what it is.....

Picture: Sea Cart 26-company estimates takeoff angle at 13-16 degrees

click on image:

[oldsailor7]

I'm loving this.

Pass The Popcorn.