18' Whisper Production Foiling Cat

[Boink]

Actually on a cat with 8 feet of beam and twin trapezes, you need to add between 150 and 200 kg of additional RM from the crew, who when fully extended apply this load an additional 4 feet to weather of the windward gunnel - a considerable lever arm. Based of the CofE of the average 6ft crew being through their chest at about 4 ft from their toes. A trimaran might be as wide as it is long - but many are not - and the central hull is only contributing its mass at half beam point. In fact until windward RM is developed (read on why this is unlikely) its RM is generated by a lever arm of only half its beam (9 7 half ft in this case) The additional RM of a Tri needs to be compensated by the additional engineering and structure it carries to achieve the same platform rigidity. So a similar length boat is often much heavier (3 hulls to lug around rather than 2 etc etc) to achieve the same rigidity. Heavier structures are affected proportionally less by their crew weight and position than lighter ones - think Moth (30 kg plus Sailor) vs. MOD70 (4.5T plus crew of say 500kg)

Even Oracle have demonstrated that platform rigidity is highly valued in performance multihulls - both between v.1 & v.2 of their AC72's - but most tellingly with their 45ft test mule that they have subsequently developed which is super rigid in all directions.

Secondly, the only boat to successfully deploy windward RM was SailRocket - but remember that this is a one tack boat only and when it didn't work it nearly killed the pilot. Look for the video of it somersaulting at 50 knots. It also was not hydrofoiling but sailing on very small planing sponsons. Its rig needed to be heavily canted to align its forces with the water foil - and it had some interesting steering characteristics........

This is the reality of text book physics as opposed to real life sailing.

Currently foil developments are successfully reducing drag - and the instability of both wind and water conditions make chasing additional righting moment problematic and potentially dangerous. It would also lead to adding drag back into the equation and loading the structure up. Hydrofoiling is more likely when the structure is feather weight light and rigid. There is enough complexity to attempt to understand and control, without adding the reverse scenario of adding down force from the weather foil.

Using the parallel example of say an F1 car which rolls on a fixed immovable surface (the road or track surface) where the amount of down force is firstly related to speed and secondly limited by the track surface - a very useful hard barrier that resists the car burying itself into the ground unlike water where a boat would fly its windward hull back down to archimedian position (or worse) where the deceleration caused by touchdown would not only spoil the hydro foiling experience - but more likely manifest itself in a violent round up to weather (remember the leeward hull would still be flying and now trying to overtake it' windward brother) or worse...... All of that would (or could happen in a split second) so some very clever active management of flight characteristics would be required - and for many (myself included) would be a step too far from the characteristics of sailboats - it would be akin to putting traction control and ABS of cars on sailboats. I want the hairs on back of my neck stood up for the right reason - not because the on board flight computer has shorted out in its carbon fibre housing.

Bare in mind that F1 have recognised that all the downforce which is great in corners is performance reducing down the straights - so they now back this off in certain sections of the track.

Armchair commentators need to recognise that there is a lot more going on than lawn ornaments will ever demonstrate...........

 

[Doug Lord]

With any design experience one would know that for a normal 6 footer the crew CG on a trapeze is about 3' from the gunnel, not 4'. The Osprey trimaran (18' LOA, 22' overall beam) weighs about the same as an F18. Downforce from the windward foil is a critical design feature of boats like the Hobie Trifoiler(Long Shot), Skat, Rave and Osprey(see Note) and others which do not use movable ballast or have limited movable ballast.

(NOTE: The Osprey allows the crew to sit to weather-no trapezes) Not all wand based boats require RM from the foil system if they, instead, rely on movable ballast).

This is something Greg Ketterman wrote about his system and it pretty much applies to the Bradfield system as well:

HYDROFOIL SAILBOATS IN GENERAL
"Hydrofoil boats can be categorized into two categories; 1) Incidence controlled hydrofoils* and 2) surface piercing hydrofoils. The difference lies in the way the boat maintains the proper altitude above the water surface. A surface piercing hydrofoil boat maintains proper height by varying the amount of foil submerged. The boat raises up as the speed increases and reduces the amount of foil submerged and therefore the lift. The boat finds equilibrium at the proper altitude. An incidence controlled hydrofoil sailboat has a mechanism that controls the angle of attack of the foil to maintain the proper altitude. It is generally believed that surface piercing is simpler, but incidence control is more efficient. In reality, it is the method that works with fewer problems that is simpler.
From the beginning it was felt that incidence control was better suited for a sailboat even though most of the existing hydrofoil sailboats were of the surface piercing type. There are many advantages of the incidence controlled foils; however, the most important is what I call the DLA (dynamic leveling affect). This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited.
Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise. The dynamic leveling affect not only produces a dramatic increase in top speed, but is also responsible for all the other key features that this stability provides.
The other major advantage of the incidence controlled foils is they are less affected by the waves and other surface affects. Drag and losses associated with the surface are the major reason incidence controlled foils are more efficient.
All hydrofoil sailboats have problems with ventilation; however, surface piercing foils have larger problems, because the foils are piercing the surface at a smaller dihedral angle which makes it easier to ventilate."
------
* On the Trifoiler the entire foil was moved to control RM, lift and negative lift hence the term "incidence controlled foils". On the Rave the incidence was generally fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil. Lift and negative lift on a Rave foiler is generated by the wand (designed by Dr. Sam Bradfield), a surface sensor(dragging in the water) and attached directly via linkage to a flap on each main foil. The wands are independent just like the trifoiler "incidence controlled" foil sensors.
==========================
Just a note: The efficiency of using foils to create RM is not universally hailed as a great solution because it requires a boat of great strength(weight) and because, before the Osprey, this type of system was known for being poor in light air. The Osprey takes off in the same wind speed a Moth does-6-7 knots. Another important consideration in using foils in this way is using an oversquare platform( Osprey 18' LOA, 22' Beam) because it helps to unload the foils making earlier takeoffs feasible. Not used in every case but a viable way to improve low speed take off characteristics and reduce drag.

Both the Hobie system and Bradfield system have been used on record setting foilers and are known for their top end speed. Osprey is one of the first foilers of this type to have excellent light air speed.

 

[Boink]

douG - just because yOuR centre of maSs is at 3 ft - doEs noT meAn everYoNe'S is.....

For normal people (who actually sail trapeze boats) rather than dRiVe cOucHeS, have correctly shaped bodies.......

Do not quote esoteric one off boats as if they are commonplace and representative of the norm.

 

[Doug Lord]

There are several times more of the boats I quoted than the total of other wand systems on multihulls. They are more representative of the "norm" than any other wand based boats now. But the important thing, from a design perspective, is understanding the difference between, say, a Rave foil system and a Whisper foil system. And between a Whisper foil system and an S9 foil system.......

PS-It would be real interesting to see somebody with their CG at 4'- what-8' tall?!

 

[SimonN]

Doug (and Boink)

Why am I not surprised that you can't even get the CoG position of a person correct? While i agree that 4' is wrong, so is 3', even as a rule of thumb. There are 2 well known rules of thumb and from them we learn that the CoG of a person with arms down the side is between 55 and 56% of the total height, measured up from the ground. However, as people do not tend to trapeze with their arms down by their side, most use a figure of around 60%. So in a 6' person, you would consider something like 3'6" to be a good approximation for CoG. If you want to get really technical, a woman would be a bit lower and a man might be a bit higher, but for righting moment, it's good enough.

 

[Boink]

Plus those of us that actually do trapeze - do it on our toes, not flat footed - so that adds as well..... I'm sticking with my reply which was to highlight that the differences between a tri & cat are ultimately closer than you suggested.

But you use "one offs" and non mainstream "oddities" as being representative; and wow us with your accuracy within a thousandth of an inch.

Come on cOucH PoTaToe - split some more hairs and twist your stats.

 

[Rantifarian]

Centre of mass of a human male is usually about 55% of their height with arms by their sides. It will be higher for athletic populations due to increased muscle mass in the upper body having a stronger effect on c of m distribution than the increased mass of upper left and glutes. The arms on chest or above the head position of a racing sailor will also increase the height

 

[hawk 808]

Plus those of us that actually do trapeze - do it on our toes, not flat footed

That's bull... Most of us aren't even totally stretched (the head is usually tilted)

http://sealaunay.photoshelter.com/gallery/EUROCAT-2011-F18/G0000M36QqhzMycU

But we usually wear some stuff around our chests which weights a bit. And more if wet...

 

[Doug Lord]

Doug (and Boink)

Why am I not surprised that you can't even get the CoG position of a person correct? While i agree that 4' is wrong, so is 3', even as a rule of thumb. There are 2 well known rules of thumb and from them we learn that the CoG of a person with arms down the side is between 55 and 56% of the total height, measured up from the ground. However, as people do not tend to trapeze with their arms down by their side, most use a figure of around 60%. So in a 6' person, you would consider something like 3'6" to be a good approximation for CoG. If you want to get really technical, a woman would be a bit lower and a man might be a bit higher, but for righting moment, it's good enough.

I was wrong about the CG of a "normal" 6 footer being at 3'-if male it would be 3.37', if female it would be 3.3'. However, from a design standpoint you can't assume you have 6' males and/or 6' females on the wire-you have to use a number that gives the best RM for the widest range of people-tall, short ,male or female. And that number is 3' from the gunnel to the design crew CG.

If you go to Bethwaites "High Performance Sailing", page 178 and work out the example he gives, the crew CG from the gunnel is 3'. That number is a compromise that fits a wide range of potential crew and works real well.

 

[SimonN]

Doug (and Boink)

Why am I not surprised that you can't even get the CoG position of a person correct? While i agree that 4' is wrong, so is 3', even as a rule of thumb. There are 2 well known rules of thumb and from them we learn that the CoG of a person with arms down the side is between 55 and 56% of the total height, measured up from the ground. However, as people do not tend to trapeze with their arms down by their side, most use a figure of around 60%. So in a 6' person, you would consider something like 3'6" to be a good approximation for CoG. If you want to get really technical, a woman would be a bit lower and a man might be a bit higher, but for righting moment, it's good enough.

I was wrong about the CG of a "normal" 6 footer being at 3'-if male it would be 3.37', if female it would be 3.3'. However, from a design standpoint you can't assume you have 6' males and/or 6' females on the wire-you have to use a number that gives the best RM for the widest range of people-tall, short ,male or female. And that number is 3' from the gunnel to the design crew CG.

If you go to Bethwaites "High Performance Sailing", page 178 and work out the example he gives, the crew CG from the gunnel is 3'. That number is a compromise that fits a wide range of potential crew and works real well.

Don't care if you are taking it from Bethwaite's book, The figures you now come up with fits the most pessimistic way of calculating and does not take into account the activity taking place. even ignoring Boink's idea that we trapeze on tip toes, you never trapeze with your arms extended down your side. Maybe Frank was being conservative, but the correct figure is significantly more than at 50% height. If you take two 6' males weighing, say, 12.5 stone, you are under estimating righting moment by 175ft.lbs. That is significant.

 

[Doug Lord]

From a design standpoint, if you design for the RM of two 6 footers at 180lb, then you will be overpowered with any lighter crew or shorter crew. Makes a lot more sense when designing a boat to be sold to the public to have a more realistic figure to base maximum righting moment on. Especially if your target market is male/female couples, kids, women etc.

 

[Speng]

what a bunch of nitpickers...

My question still stands: when the crew's fully hiked out and the boat wants to heel more but the wand makes the weather foil pull down harder what is there to limit how hard the f#cker pulls down? I hope it's not the foil or some other expensive carbon bit breaking...

 

[Doug Lord]

I answered you: the limit is structural. The foils ,if allowed to operate completely independently, are able to break the boat. I'd guess that on a boat like the Whisper, when the the windward foil starts to pull down*, you would depower to prevent overloading the structure. Dr. Sam told me of an incident with a Rave-I think it had a never exceed speed of 30 knots-and the guy exceeded that quite a bit and broke the single forward crossarm. That's a damn big aluminum tube and the forces must have been huge.

The whole advantage to using movable ballast is so that the foils don't have to provide RM. Another disadvantage to letting the foils develop RM on a relatively narrow boat like the S9 or Whisper is that it would be draggy because the foils aren't separated enough to be unloaded sufficiently.

* you can tell by looking at the flap or wand angle though there is probably some indication on deck.

 

[SimonN]

I answered you: the limit is structural. The foils ,if allowed to operate completely independently, are able to break the boat. I'd guess that on a boat like the Whisper, when the the windward foil starts to pull down*, you would depower to prevent overloading the structure. Dr. Sam told me of an incident with a Rave-I think it had a never exceed speed of 30 knots-and the guy exceeded that quite a bit and broke the single forward crossarm. That's a damn big aluminum tube and the forces must have been huge.

The whole advantage to using movable ballast is so that the foils don't have to provide RM. Another disadvantage to letting the foils develop RM on a relatively narrow boat like the S9 or Whisper is that it would be draggy because the foils aren't separated enough to be unloaded sufficiently.

* you can tell by looking at the flap or wand angle though there is probably some indication on deck.

I don't believe that the limit is structural. The limit comes about from an issue of weight. It is possible to build structures that could take the loads, but you end up with a viscous circle. You strengthen the structure in the boat - in the case of the Rave you could use carbon beams which would, at minimum, double the strength. Then the failure point would probably become the rig, so you use stronger materials and you then have problems with the foils, so you make them better and finally the hulls need to be strengthened. in the cas eof the Rave, you can probably go though a number of iterations using more expense but better materials before you need to add weight. But then you get to a point that to make it stronger, it ends up heavier and you only do that if the extra power overcomes the weight penalty. You keep going around in circles on this until some day, somebody comes along with a smaller, lightweight version with smaller foils and smaller rig that is faster...... and we all give the knowing smile of recognising history repeating itself. Or in the case of classes with limited sail area, we see another limitation, because you get to the point where you cannot produce any more power to overcome the weight needed to make the boat strong enough.

The other thing to consider is that the foil that does adds righting moment on one tack has to provide lift on the other. therefore, you end up with a situation where adding area to get more righting moment might produce more drag for teh same lift, cancelling out the benefit.

And all of that ignores the price limitations.

One might argue that this is just the other side of the same coin that Doug is arguing, but I see important differences. And we are so far off the structural limits of the materials we have available that it's not yet part of the conversation.

 

[Doug Lord]

You can't look at windward foil downforce without realizing that the lee foil has to lift an equal amount more so as RM is generated by both foils the structural loads increase dramatically.

Again,and this is a major consideration: one of the advantages of NOT using the foils for RM is being able to build a lighter boat. It is important to realize that on narrow cats, as compared to wide boats like the Rave and Osprey, the separation between the foils ADDS to their loading and becomes draggy as well as structurally intense. The drag issue would affect this type of boat in racing against other types of foilers.

It would be interesting to hear what the designer of the S9 ,Michele , and the Whisper designer(s) have to say about this.

 

[Jim Caldwell]

But, the wider the beam the less RM the windward foil has to generate therefore less drag. But it will still be a design ballancing act.

 

[Doug Lord]

True-and you have to consider both main foils from a design standpoint. If the windward foil pulls down then the lee foil has to lift vertically more by the same amount. It's exactly like adding weight to the boat.

 

[Phil S]

Getting inreased RM from foils also adds hugely to total drag.

Beach cats with crew on trapeze have the centre of gravity of boat+crew combo at about the widward hull c/l or about where the windward foil is. If the windward foil pulls down hard enough to double RM, the leeward foil has to carry double the lift. So total vertcal foil force is now twice upward from the leeward foil plus once downward from the windward foil. The total induced drag is now 3 times what it was without the down force, or what the drag would be if the boat weighed 3 times as much. Probably more if the windward foil is a cambered section designed to lift only one way.

Sounds like a bad idea to me if the purpose is to go faster.

 

[Doug Lord]

I agree with you Phil. But foil RM can work at a cost of a heavier boat and at least a square platform. On boats using this their top end speeds are high but they don't foil until 12-15 knots of wind with the exception of the Osprey that foils in 6-7 knots. "Long Shot" still holds a speed record.....

It seems like a bad idea on a narrow platform like the Whisper and S9 that have movable ballast(crew) on trapezes.

 

[Doug Halsey]

You can't just say adding downforce on the windward foil is either good or bad. Like most things in sailing you have to be more specific about the conditions.

When I was doing preliminary design work on my foiling trimaran (Broomstick), I went through some of the iterations Simon was referring to & using a VPP code, I estimated that the downforce would be a hindrance in winds up to about 25 knots & a benefit in stronger conditions.

I suspect that most often a foiler designed to compete against existing floaters would do better relying on crew weight, but that one designed primarily for speed trials could benefit from the downforce.