Gitana 17-100' Trimaran Foiler-Launch 7/17/17

Thanks Dolfiman, what I was not able to see is that the ship really "flies" leaning on the "B" point. So, if I do not get it wrong, the downforce does not exercise any moment, neither righting nor heeling. They only produce moment, the sails, the weight of the ship and the lift of the central foil. Am I getting closer to understanding how trimarans work with foils?
Thank you very much, also, David for your explanations.

 

Thanks, you are right of course, sorry for my bad English.

QUOTE "...it will inevitably come clear of the water on occasions and tip the boat over sideways at much greater speed than normal, giving less time to catch it. I'm sure these big tris will only be using lift upwards to reduce drag by keeping the hull clear as much as possible, and that means it's adding to the heeling."QUOTE
Yes, there is also this risk, a system which can delay the fatal rotation but if its contribution is overcomed, the heeling rotation can be at greater speed. Nevertheless, Gitana 17 and likely Banque Pop (central foil with flap) has this foil with downforce capability, I suppose tests are currently (or will be) done : learning by testing period , in french, quoting G. Verdier "On défriche".

Our debate is of first importance in view of the preparation of the round the world singlehanded race in 2019 : the "Ultim collectif" who has set an architectural frame in 2015 continue to meet and think about the final rule for this race, as regard security in particular.
ScanVoile: Ultime / La liberté a ses limites, le Collectif Ultim se dote d'un cadre architectural http://www.scanvoile.com/2015/06/ultime-la-liberte-ses-limites-le.html#.Wgn5bbpFyyL

 

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Downforce from the main foil at X distance from Point "B" certainly does create a moment that adds to righting moment and to the load on the foil at Point "B".
Vertical lift on the center foil creates a moment that subtracts from max righting moment. But vertical lift at point A also occurs due to the wind force on the rig. So the center foil is primarily used for vertical lift in light conditions to help get the main hull in the air when the wind pressure is too light to do it. As the wind picks up the need for vertical lift diminishes(except for roll and pitch control in waves) and the need for RM from downforce increases.
On the Test Model-the very first trimaran to use this type of system-control is 100% automatic based on input from a mechanical surface sensor(wand).There hasn't been much said about the exact method of control on Gitana and Bank Populaire but on Maserati a crew member was tasked with making foil adjustments as required by moving the whole foil . Electronic adjustment of a foil flap would be relatively simple and since it is legal for rudders I would think it would be legal for main foils as well.

 

You need to be cautious considering a central foil to increase righting moment on these boats. They are big, with a lot of inertia, and are not righting moment limited on many points of sail, so increasing righting moment off the wind won't help much. Even going upwind, if a gust hits and the central foil develops downforce, then the lee foil must develop an equivalent upforce, or the lee hull will touch down. In either case, the drag increases dramatically, slowing the boat, which brings the apparent wind aft, which then increases overturning moment.

Thus, although you may think righting moment is increased, it is actually decreased relative to the overturning moment, and of course if the central foil cavitates or ventilates, you have a sudden loss of righting moment with still a higher overturning moment, with disastrous consequences. If you want to use a foil generating downforce to increase righting moment, then the foil separation must be maximised, i.e. have the foil on the weather hull, and increase the vessel beam so that the drag penalty is minimised. Even then, it is very difficult to make it pay, as it also comes with increased structural loads, and hence weight, and hence drag.

The key to the design of these boats is drag reduction, not increased righting moment, and the central foil has a role to play in preventing the main hull from touching down, thereby preventing the resulting drag, and providing a broader window of stability with the main hull flying. This broader window is the best reason for these short handed boats, as the crew cannot give the roll balance of the boat their constant attention.

The other issue with these boats, compared to models, for example, is that they are big enough that they can already achieve near to the cavitation limiting speed without flying at all. Hence any gains from foiling are already marginal at best. With a small boat or a model the displacement speeds are much smaller than the foiling speeds, and hence the foils provide a proportionally much larger speed differential, and hence are much more worthwhile. This is the problem when trying to scale up results on a smaller vessel to a larger one, as Gitana has found. Their foiling MOD70 went from a 35kt boat to a 40kt boat when they added flying foils, but the Ultime is already a 40kt boat without the foils, and only marginally faster with them over long distances. Spindrift 2 is already a 45kt boat without foils, and can do these speed with the main hull in the water, so would not benefit from flying foils at all.

 

The primary benefits of the central foil is getting the main hull clear of the water earlier than would be possible with wind force on the rig and increasing roll and pitch stability. I talked to Guillaume Verdier about using downforce on Maserati and his comment was that it must be done very carefully(!).On the test model or any relatively small sport foiler using this system downforce could be an important asset and is the main reason for the oversquare beam on Fire Arrow.
Automatic mainfoil control using a wand works well on smaller boats but I would think electronic control on the big boats would really help maximize the benefits of the foil system.

 

You describe a scenario of divergence that can be possible but the scenario of convergence can also occured as well I think : at this point of interactions complexity, only :
- either the academic approach : the dynamic second-degree equations with supposed exact parameters and various scenario of external forces can give solutions and show when there is convergence and when there is divergence.
- or the full scale testing approach : now possible as far as there is someone with the mainsheet in hand ready to ease.

I agree with you that above 40, let'say > 45 Knots, foils faced the cavitation issue. But in the range of let'say 25 to 40 Knots, foils help jump to the 40 knots zone more rapidly, not to mention some advantages of stability in flying over the waves (for sea state of course, not all). Here are the advantages of foils I think.

 

Thank you @hump101, @Dolfiman and especially @Doug Lord, for your patience with me, answering my questions and clarifying the doubts of an ignorant like me. I think I now understand better how these "machines" work, although, of course, I still have a lot to learn about this exciting subject. Thanks again.

 

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You're welcome! If your interest has anything to do with small foilers there is a good practical book: Hydrofoils Design Build Fly by Ray Vellinga-probably still available through Amazon. Good Luck!

 

Please remember that this discussion is entirely theoretical. It assumes a perfectly flat sea state, perfect operation of theories and isolates individual effects with no consideration of correlated effects and reactions.

It's worth noting that downforce from the central foil must be countered by increased lift from the float foil, that the forces are huge (in the order of many tonnes) and that they massively increase stresses on the boat. To deal with the stress, more substantial structures are required, increasing weight.

I also don't see anywhere here a comparison or evaluation that is even remotely scientific, it is all pure conjecture.

Where is the analysis showing that creating RM from the central foil provides an overall benefit? Where is the calculation of additional lift from the float foil, drag from the central and float foils, weight from the required stronger structures, additional control engineering, etc. Where is the evaluation of the speed and conditions required for this approach to be better than alternatives?

For example, it would be far more effective to increase righting moment using force on the windward float (either with a foil or sea water). It has twice the leverage of the central hull and creates forces more aligned with the typical load bearing structures, so overall is far more efficient and may require less additional strength, less additional lift from the leeward foil, etc. What analysis has been undertaken to evaluate the two approaches? When would one be better than the other?

Also, much is made of "early foiling" without any analysis of when this might actually occur and prove beneficial in a race compared to the reduced drag and weight of not having a central lifting foil. It's just presented as an axiom that a full foiling trimaran must be faster than a partially foiling one without any analysis to prove the point. It has not yet been shown to be so in practice over the course of an ocean race. And that is the whole point: not whether it's theoretically faster in some idealised world, but whether it will win races in real life (RL).

So please, take the theories that you read here with a huge grain of salt.

 

You're going to upset the toymaker with this logic, OzFred.

 

It's a massive reality check that Gitana sailed 75% of a 4,500 nm full–on ocean race over 8 days with its float foils retracted, yet still finished less than 2 hrs behind the winning foil assist boat. That's an average of less than 0.5 kn slower (over 3,400 nm) without using its float foils at all (though the central foil may still have had a small benefit for heave stability and certainly the rudder foils would have helped pitch stability).

 

Maybe that combination of leeward rudder foil and main hull foil is the better one compared to the multiple foil setup; happy accident, less drag? Anyway their float foils were obviously incorrectly constructed, therefore deserved to lose.

 

@OzFred, it seems that one should agree with what you say in post # 69.
By the way, according to one of the great designers of this type of vessel, one of the greatest advances achieved in this whole matter has been the use of elements, hulls, hardware, ... built with lighter materials.

 

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The analysis you refer to for Gitana 17 has surely been done by Guillaume Verdier or he would never have proceeded with basic Foil System.
See here: High Performance MPX Foil/Self-righting Trimaran-The Test Model https://www.boatdesign.net/threads/high-performance-mpx-foil-self-righting-trimaran-the-test-model.36058/page-176#post-818121
for the advantages of the Fire Arrow Foil System on an oversquare full size platform.

 

Damaged Gitana UptiP ama foil: