"Mayfly"-A Class Catamaran-Fischer

Looking forward for your workpaper

Hi Everybody, Hi Martin,

As most of this forum attendees, I will read your paper with great pleasure, I feel confident, you will fix the problem with time on the design board as well as on the water.

If you wish to share your ideas regarding your A-Cat mast concept (I saw somewhere it was a 30% chord mast??) Do not hesitate It will be a pleasure as well.
Do you want to manage bubble ramp at hight AoA ??

Cheers Mates

EK

 

Fischer sent me his report to publish on these 'off blogs' ...

Simon: all the data published in Catsailingnews is directly from the source, Fischer, so is not some third hand info.
You can rely 100% on any third party info published there, like Landy's views or others (that are sent directly to me), which whom I have direct and fluid contact as you have with Chris or others in Australia.

Martin decided to publish his report again there, and not directly on the forums, so that is why you can trust what I post there as original data/opinions from those quoted (besides you agree or not with them) :

http://catsailingnews.blogspot.com/2011/09/class-mayfly-update-by-martin-fischer.html



Cheers,
Martin from CSN.

 

MayFly

Here is an excerpt from the report-read the whole report at the link above or the complete report pdf below:

What went wrong with the Mayfly?

So in theory all seems to be fine and the question remains what went wrong with the MayFly? Over the last weeks I went again through all the number and I think I identified the main problem: the effective dihedral angle of the foils.

As I pointed out in the previous section the dihedral angle is very important for the dynamic stability of the boat. But the dihedral angle has also a strong impact on the total drag of the boat. For a given dihedral angle the ratio of side force to drag goes down (more drag for a given side force) and at the same time the boat is lifted a bit out of the water, which decreases the hull drag. So in terms of drag the dihedral angle of the foils has effects: a positive impact on hull drag (less drag) and a negative impact on side force related drag (more drag). For each dihedral angle there is a characteristic boat speed at which the breakeven between these two effects occurs. Or expressed in another way, for each boat speed there is an optimal dihedral angle. Hence the dihedral angle of the foils is a very important design parameter with a big impact on the performance characteristics of the boat and its control is crucial during the design process. This becomes even more important if you leave the usual “design space” as it is the case with the MayFly.

During the development of the MayFly I ran the computations under the simplifying assumption that the geometrical dihedral angle is a valid approximation for the effective or dynamic dihedral angle. The geometrical dihedral angle is simply given by

tan(dihedral) = A_horizontal / A_vertical

where “A” horizontal / vertical are the horizontal and vertical projection of the total surface of the foil. This angle can be easily computed from easy to measure variables. However, the relevant dihedral angle is the dynamic dihedral which is give by

tan(dihedral) = F_vertical / F_horizontal

where “F” vertical / horizontal refer to the horizontal and vertical component of the total force created by the foil. Due to the complicated geometry of the foil the local angle of attack at which the foil operates changes significantly along the span and hence the load distribution along the span differs significantly from the chord distribution along the span. This makes the computation of F_vertical and F_horizontal more complicated. During my re-analyses I (finally) computed the dynamic dihedral angle and compared it to the geometric dihedral angle. The result is that in the case of the MayFly foils there is a very significant difference between the two. The dynamic dihedral is significantly larger than the geometric dihedral and therefore the foils on the MayFly created significantly less side force than they were supposed to create. This pushes the breakeven point between the positive and the negative impact of the dihedral angle to significantly higher boat speeds than originally anticipated which is what we observed at the A-Class Worlds in Denmark.
I still have not fed these data into my VPP to quantify the impact but simply looking at the additional drag at low speed due to this lack of side force strongly indicates that this is the main explanation for the poor light wind performance during the Worlds.

A side effect of the lack of side force created by the foils is a poor tacking performance of the boat. If the foils create less side force than required the leeway angle of the boat increases and hence the portion of side force created by the hulls increases. The centre of effort of the hull is much further forward than the centre of effort of the foils which creates additional weather helm and therefore makes the boat very difficult to tack. I am living in New Caledonia and I have been running tests here with an A-Class with the same foil set-up – same rudders, same foil position - but with a set of S-foils with less dihedral angle. I cannot assess the speed relative to another A-Class boat (there are no other A-Cats here) but tacking is definitely much easier than with the boat I sailed in Denmark.

This problem with the dihedral angle of the foils is relatively easy to fix, but it requires designing and building new moulds and foils which will be done over the coming weeks.

There were other issues with the boat, but these are in my opinion second order effects:

Problems with bearings of the foils and hence with the toe-in angle of the foils
Lack of stiffness of the main beam, which caused also a change of the toe-in angle of the foils

Last but not least the overall preparation of the boats and the crews was insufficient. We were aware of that but the alternative would have been not to show up at the Worlds and hence miss the best and toughest opportunity to test the boat under real racing conditions.Martin Fischer----Sent to publish in Catamaran Racing, News and Design.
at 7:27 AM
=================
Thanks to Martin here is the full report in pdf format(easier to study!) :

 

report on MayFly

Hi,

I asked Martin Vanzulli from catsailingnews to publish the report I had prepared about the MayFly. You may find the whole article on www.catsailingnews.com and the section that is directly related to the MayFly just above (Doug posted it here - thank you Doug). I am happy to discuss this.

cheers
Martin

 

Mayfly!

Martin, I understand that the A Class ,in an attempt to prevent full flying foilers, modified the rules in such a way to make flying theoretically impossible. How has this rule impacted the Mayfly design?

 

impact of A-Class rules on foil design

Hi everbody,

the A-Class rules have indeed a big impact on the foil design. Being obliged to put the boards from the top into the boat makes it almost impossible (or very difficult) to add winglets. Winglets would help to reduce the induces drag adn on boats without this restriction they are in general used.

A second and possibly more import factor is the limitation of the foil to an area outside 750 mm measured from the axis of the boat. Without this limitation it would be possible to reduce the chord length and increase the span in order to improve the aspect ratio and also to have a slightly more pronounced S, which would increase dynamic stability even further. I reckon both aspects would be a step forward but at least at the moment they cannot be realized due to the class rule limitations.
Another topic in the class rules is that they stipulate:

4. The extreme beam shall not be more than 2.3 meters (7 ft 6½ inches).
The beam shall be measured at right angles to the centre line of the
craft at the widest point and shall include hull appendages in all
positions (completely down and completely up flush with the bottom of
the hull).

Depending on the interpretation of that rule this also limits the curvature of the S-foils.
It is unclear what "in all positions" means in this context. Does this mean all positions while racing, or does it really mean all positions. If the latter is the case all current boat are not class legal since if the rudder is in an upper position and you pull or push the rudder tiller to the extreme the rudder is outside the maximum width.
This part of the rule was added after a recent ballot and in my opinion it is badly phrased.

cheers
Martin

 

Mayfly!

An excerpt from the report on pitch stability:

Pitch stability

In order to obtain a boat with self adjusting trim it is sufficient to look into airplane theory. The foil design must be such that if an external perturbation (wave or wind gust) imposes a change of the pitch angle the foils must create an angular moment such that the boat is pushed back to its original pitch angle. In mathematical terms such a “negative feedback” is characterises by the first derivative of the angular moment with respect to the pitch angle. This first derivative has to be negative and this “necessary condition” imposes a constraint on the position of the foils and most importantly on the size of the horizontal rudder foil! The position of the main foils and the choice of an L-rudder on the MayFly are a direct consequence of this stability constraint. Hence the primary purpose of the horizontal rudder is not creating lift – as many people say – but to create a negative feedback in order to ensure a stable pitch angle. And the position of the main foils on the MayFly relatively far forward was not chosen to improve the resistance against pitch poling – as many people say - but is imposed by the longitudinal stability constraint.

The lack of a negative pitch feedback is actually one half of the reason why boats with curved or canted straight foils and with no (or too small) horizontal surfaces at the rudders tend to jump out of the water once the lift created by the foils goes beyond a certain threshold. The second half is the flight height stability (see below).

To summarise: In order to obtain dynamic pitch stability (negative feedback) constraints on the longitudinal position and the relative size of the front and rear foil have to be respected.

 

Mayfly!

This is another excerpt from Martin Fischers report:

Flight height stability

Once the pitch problem is solved we still have to figure out how to stabilise the flight height. Surprisingly this is the more difficult part. To make the problem clear let’s have a look at a T-foil. This is obviously an extremely unstable system: if the boat speed is below the necessary take-off speed the boat won’t fly and if it is above the take-off speed the boat will completely jump out of the water. Only if the boat speed corresponds exactly to the take-off speed the boat flies, which is for a T-foil obviously a very theoretical thing.

With surface piercing hydrofoils the dihedral angle of the foils provides an efficient way to control flight height. By adjusting actively the dihedral angle a desired flight height can be achieved for different boat speeds. On Hydropthère (60-ft flying trimaran) they use hydraulics to adjust the dihedral angle of the foils, Steve Clark used on his latest C-Class catamaran “Aethon” a lateral adjustment of the upper foil bearing for that purpose and on Alinghi-V they tested a special foil shape for the same purpose. This is a possible way to go but there are limitations. If the vertical load is shared between the leeward and the windward foil this configuration provides a stable flight height regime (within certain limits of course). The stable regime gets narrower as the load distribution between the two foils gets more asymmetrical and if the whole load is on the leeward foil (sailing on one “hull”) the width of the stable regime gets down to zero and the system becomes neutral (feedback equals zero) in terms of flight height.
Thus this solution still requires active intervention from the crew, which is difficult during a regatta and even more difficult on a single handed boat as the A-Class. Furthermore it becomes unpractical for a relatively narrow boat like the A-Class which sails most of the time on one hull.

I therefore went for S-shaped foils. The idea behind this approach is to change the dihedral angle automatically as a function of flight height. The foils are designed such that at low flight height the system is unstable which facilitates “take off” and at higher flight levels the regime becomes more and more stable. At low flight height the dihedral angle increases (more horizontal) if the boat lifts. This creates a positive feedback which means instability. This instability helps pushing the boat out of the water as soon as it is possible. However, once the flight height goes beyond a certain limit the curvature of the S-shaped foils changes sign and the feedback becomes negative. This negative feedback causes the boat to sink back in towards a pre-defined position. This predefined position is close to the inversion point of the S-foil.
Hence the primary purpose of the S-shape is to provide a negative feedback for the flight height around a pre-defined “working point”.
This is not at all the same as the winglets on the curved foils of ORMA-60 trimarans or of the maxi-trimarans in France.
To our knowledge this is a new technical solution to the flight height problem for foiling sailing boats and we have submitted a patent on it.

 

All excellent stuff but ... surely the A Class with its measurement restrictions is the wrong platform for foiling. Foilers require NO RULES so as to produce the best fast sailing solutions and results. By all means keep the platform 18 feet length or so but from then on a foiler needs to be unrestricted. Making an A Class into a foiler is like trying to load a 7mm magnum cartridge into a .222.
Of course, many like the intellectual challenge of tweaking a tight fitting shoe to fit properly ... but it is a constipated project. Need a new class, cobras. Just imo of course.

 

Martin
I have to agree with you about the wording to do with rudders being badly phrased. I believe the real question is whether they are even "hull appendages" as discussed in the rule. For instance, you are talking about rule 4 - width. However, it describes 2 states "completely down" and completely flush with the bottom of the boat". I wonder how you get a rudder blade "completely flush with the bottom of the boat"!
Then you have to look at rule 8.2

It was first suggested to me that the "get out" is that we are talking about movable and retractable and that rudders aren't considered "retractable". However, clearly your rudders, being in dagger stocks, are retractable so do they need to retract into the hull? As an aside, your rudders cannot be inserted into the stocks from above. Does that make them illegal (I don't think so!)? Then there is the fact that the rudders are subject to their own rule about what happens if they are thicker than 76mm within 153mm of the bottom of the hull. If rudders are "hull appendages", surely the rule wouldn't refer to rudders at this point.

Overall, I have to conclude that for the purpose of the A class rules, rudders are not "hull appendages" because if they are, then many if not all boats are illegal. This then leads to a very interesting point. The 750mm rule only applies to hulls and hull appendages. Does this mean that rudders can have wings on them that go inside the 750mm or even ones that go past the 2.3m beam?

Having said all of that, I think that those of us who are really interested in these things understand what the rule is meant to say and as such, keep within the spirit of what the class wants.

As for your foil design, I love what you are trying to do even though I don't believe that you will succeed. At least you have had the courage to do it. Coming from a Moth background, the first thing I did was look at the foil rules and spent a long time discussing it with the top Moth designers. Our conclusions are clearly different from yours! I am also interested that I mistook the purpose of your rudder "wings". This was because I believe that for the way you are using them, they are significantly bigger than I believe are needed. I therefore concluded that they were there to provide constant positive lift. I actually have 2 different sets of winglets sitting in my office drawer for exactly this purpose but as yet, I still see too many other variables to test before these will come into play.

 

Hi Simon,

thank you for your comments. I looked at the rules in detail and discussed it also with A-Class measurers and with members of the A-Class technical committee.

1/ hull appendages: The ISAF rules are very clear and very specific on hull appendages (just check the definition section in the ISAF rules). According to those rules rudders are clearly hull appendages. But attention they are talking about rudders and not just rudder blades.

2/ Yes, I agree with you that currently all rudders in the A-Class - except mine - are not class legal if you read the rules word for word.

3/ In the A-Class rule there is no such thing as "the spirit of the rule". So we have to stick to the wording they used.

4/ Width of the rudder (rule 3 of the A-Class rules): there are two possible interpretations: Width is in general measured perpendicular to the longitudinal axis of the boat. The horizontal wings on our boat are not exactly horizontal, but slightly tilted. If you look at horizontal cuts through the rudder - hence perpendicular to the longitudinal axis of the boat - at not level the width is beyond 76 mm.
If the interpretation is that a global width within 153 of the stern is taken as reference measure than this is also easy to fix: You put a black band on the rudders and while racing you do not pull them up further than that measure. Thus the winglet remains always outside these 153 mm. This is what the measurers at the World in Denmark decided to do, so we have even a sort of written confirmation that this is a possible interpretation.


As of the technical points regarding the minimum horizontal surface at the rudders to obtain pitch stability. According to my calculations the surface I chose is very close to the required minimum. So if my theory is correct, there is very little "leeway" to make them smaller.

 

So, Martin, are your rudders (hull appendages) movable and retractable. I would suggest that they are. If they are, they need to be inserted from the top and be capable of being fully retractable into the hull. Don't misunderstand me. I agree with you that the rules are badly written!

As for the size of your rudder wings, we will have to agree to disagree. Maybe there is some effect due to your centreboards I have not considered. I first used winglets on rudders about 25 years ago and have used them on 4 different development classes. Most of the early stuff was done by trial and error but the one thing that was always apartent was that you actually need much smaller winglets than one would imagine. And if there is one thing I am beginning to become more and more convinced about and that is that when it comes to these types of "horizontal" foils, calculations only get you so far. I would respectfully suggest that your own experiences with the first iteration of your Mayfly foils reinforces that suggestion.
Good luck with your next iteration. While I remain sceptical for many reasons, the great thing about the class is that anybody can try anything within the rules. I get a bit nervous as to how people will react if you succeed, in as far as many here in Oz seem to think that if you do there is a need to change the rules again! I also read on other forums a similar view from some Europeans. However, I think their reasoning is invalid - you cannot change the rules to stop something just because it makes older designs obsolete.

 

Hi Simon,

just to short remarks on your post:

1/ retractable hull appendages: The ISAF equipment rules give the following definition of a rudder:
Rudder: A movable hull appendage primarily used to affect steerage.

To my understanding this refers to the whole rudder, i.e. the complete unit of rudder blade and rudder head. On my boat the complete rudder can be detached from the hull, hence it can be flush with the hull (as all the other rudder systems currently used on other boats). Furthermore it is put from the top onto the rudder gudgeons.

2/ As of the size of the horizontal rudder wings. I think you must distinguish between a fully foil borne boat and a boat that is still - at least partly - in displacement mode. If you just want to stabilize a boat in displacement mode the winglets can be indeed smaller. If you want pitch stability for a foil born boat or for an almost foil borne boat the situation is a bit different.

 

I've looked for a picture of these for awhile: the rudder foil on Landy's A Class
compared with those on Mayfly. From a paper by Greg Ketterman the foils on Mayfly are more efficient because they have a more effective aspect ratio than do the t-foils-esp. upwind. Martin, is that assesment correct?

Pictures: L=Landy's T-foil; R= Mayfly "L" or "J" foil

click on image-

 

Thanks Dough,

It is a while, I was wondering how were the famous Landy's foils.

It is surprising how small they are compared to my expectations
The addditional drag may worth the gain in pitching providing
more stable air flow on the rig.

A logic step forward after the pierce-wave hull shapes.

Regards

EK