Volvo Inshore Foiler--Technical Design Information

[Doug Lord]

17 years ago I designed, built and produced the worlds first production RC sailing foiler-the F3. It used dual, independent, wands that allowed the foils to produce all the RM for the boat. I learned about downforce and using foils for RM from those boats, Dr. Sam and Greg Ketterman. In your post above you ignore what I said in my previous post and discount the experience I have with Fire Arrow and F3 in addition to many hours with Dr.Sam including providing him two trimaran test platforms he used(with my help) to test the foil configuration for the 40' SKAT. In your post above you make uninformed statements that don't reflect the facts. Mainfoil downforce on Fire Arrow is not a theory, it is a proven fact demonstrated repeatedly during testing. The wand is mechanically connected to the flap-the zero lift point also corresponds to the designed main hull flight altitude-if the mainhull is lifted higher than this point by wind pressure on the rig, the flap will rise above the zero lift point and there will be downforce-no theory to it-just indisputable fact!
The wand controlled center foil augments pitch stability* working with the rudder t-foil. Pitch stability of the boat is superior to any normal trimaran without a wand controlled center foil and the likelihood of pitchpole much less. On the second day in overpowered conditions I intentionally partially pitchpoled the boat-it recovered automatically from a 45 degree angle. But that partial pitchpole took some effort because of the resistance of the mainfoil + rudder t-foil to pitch. As the boat begins to pitch the wand lowers the flap increasing lift on the main foil which, in combination with the rudder t-foil, resists pitch increase.
*see post 22 this thread
Here's what Ketterman says about his foil system, downforce+lee foil lift:

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."
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* On the Trifoiler the entire foil was moved to control RM, lift and negative lift(downforce) hence the term "incidence controlled foils". On the Rave and F3 the incidence was fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil(Rave only). 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.

Flyer 3 platform designed and built by me and modified for foils by Dr. Sam Bradfield for a test project he and I worked on for the 40' SKAT foiler:

 

[OzFred]

…In your post above you ignore what I said in my previous post and discount the experience I have with Fire Arrow and F3 in addition to many hours with Dr.Sam including providing him two trimaran test platforms he used(with my help) to test the foil configuration for the 40' SKAT.

This discussion is about your claims in regard to a central foil providing useful downforce, nothing else. I directly quoted your posts and your own sailing diary, where 1 hour 50 minutes of sailing and barely 1 minute of foiling was expanded to a "month and half testing".

There are many voices tempering your claims in regard to the efficacy of using foils for downforce (Tom Speer, Basiliscus) and noting the negative aspects of foilers that use downforce (Gary Baigent).

In consideration of all that, and that the AC50s used downforce from their windward rudder (but they also didn't have an option of using adjustable ballast instead, e.g. pumped water, which might have been a better solution), it's clear that in certain conditions downforce from a foil on the windward float is an advantage. But you can't extrapolate from that that downforce from a central foil is ubiquitously advantageous. Simple mathematics will tell you that the leverage is halved, so you must double the downforce for a given RM which way more than doubles the drag from the RM–inducing foil.

That is the very simple proposition that you have not addressed. You only repeat the same theory and comparisons with boats whose foils are in the windward float.

You have also never shown how the wand manages to change the flap to go from zero lift to any amount of down force you wish to imagine without changing the ride height (as requested again by TANSL last week). To create more downforce the flap must come up, so the wand must go down, so the hull must go up. I.e. the boat can't generate downforce without also healing to leeward. The effect of roll on a wand controlled central foil is halved compared to one on the windward float, so it will need at least twice the gearing to raise the flap faster for a given amount of heal.

But the effect of pitch and altitude changes are equal to a float–mounted wand, so you don't want any change in gearing for that.

 

[Doug Lord]

The Fire Arrow is designed to sail at between 10 and 17 degrees of heel. Maximum downforce is available at about 12 degrees angle of heel from the 10 degree setting*. The Fire Arrow is an over-square trimaran and the extra beam allows downforce to work efficiently-a point I have made many times and which is a major feature of the boat.
Kettermans Long Shot set several records using a foil system that relies on downforce. Doug Schicklers Volvo inshore design-the boat this thread is about- uses a system that is specifically designed to use a foil system that develops downforce on the windward foil (plus matching lift on the lee foil/ see post 21). Dr. Bradfield designed and built a 40 footer that incorporated the basic Rave system and was designed specifically to allow downforce from the windward foil. The fact is that downforce works and works well as pointed out above by Greg Ketterman.

*The boat is designed to sail at an angle of heel specifically to allow the UptiP ama foils to be able to remain in position without the need to raise and lower each board every tack or gybe as is required on every other boat that uses UptiP foils-a major breakthru in reducing crew workload while having the advantages of an UptiP foil on each ama(automatic altitude control).
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This or words like this have been used many times in the Fire Arrow thread indicating that the main hull has to lift(but not very much from the no lift point which corresponds to the design flight altitude/angle of heel) to start downforce-this was posted last week in response to a question from TANSL-edited today to be more precise. Previously posted here unedited in post 28 and initially posted in the Fire Arrow thread, post 2657 :

Downforce may be hard to understand for some because what isn't shown in the sketches is the LIFT of the main hull due to wind pressure on the rig*. As the boat begins to fly the mainfoil is unloaded due to this pressure. Just before downforce begins the mainfoil is at zero lift. Lift on the main foil is automatically controlled by a wand surface sensor. As the wind pressure increases the boat heels a bit causing the wand to reduce the down-flap angle until the flap reduces the foil lift to zero (this is the designed flight angle of heel of 10 degrees). Downforce is caused when the wand raises the flap above the no lift point(due to a small increase in the angle of heel from the 10 degree point to about 12 degrees). At the no lift point, the mainhull is entirely supported by the wind pressure on the rig. As wind pressure on the rig increases further the wand causes the flap to rise which generates downforce.
*The mainfoil can lift the main hull BEFORE there is enough wind pressure to do so.

 

[Doug Lord]

But the effect of pitch and altitude changes are equal to a float–mounted wand, so you don't want any change in gearing for that.

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Thats not true on Fire Arrow or probably on any other boat using their own version of the Fire Arrow Foil System. The boat has two independent altitude control systems. The lee ama UptiP foil has its own intrinsic altitude control system but the ride angle of that foil is controlled and maintained by the center mainfoil working with the rudder T-foil regardless of the angle of heel of the boat. Altitude change of the main hull has zero effect on the altitude of the ama. Pitch changes of the main hull can affect the altitude of the ama.
Pitch change of the main hull could be counteracted by a wand controlled ama foil maintaining ama altitude even with a main hull pitch change albeit with more ama foil drag due to more flap down.

 

[TANSL]

.....Altitude change of the main hull has zero effect on the altitude of the ama...........

From respect for your work let me comment that all that is very confusing since the words are elastic. You can put many together to form a sentence without being able to say anything or, at least, being very difficult to understand (for the little experts like me). Someone said that an image is worth more than a thousand words. Could you, please, show us an image that makes us understand the transverse movements of a multihull, and the forces that act on it, when it is flying?
On the other hand, after 17 years making models of foilers, you must have an impressive database and you must be able to show us clearly how your designs have evolved and why. It would be very intersante and sure that it will help, to the neophytes like me, to understand many things.
If you feel that you do not have to share your experiences, I will understand. Many thanks.

 

[TANSL]

I have tried to answer every question you have asked to the best of my ability.

Yes, I must admit that that is totally true, to the best of your ability.
I have also expressed on occasion what their schemes seemed to me.
I do not want you, who has been very kind, to be angry with me and, therefore, I will not continue to mock you.

 

[Doug Lord]

I didn't realize you were mocking me. Good to know........

 

[TANSL]

I do not know if that word is not the right one but I do not want you to interpret it in an offensive way. I wanted to say so as not to bother you anymore.

 

[OzFred]

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Thats not true…

I think you've misunderstood my comment. For a given amount of roll, the effect on a centrally mounted wand is roughly half the effect on a windward float mounted wand (i.e. for a certain amount of roll, the windward float will rise or fall more than the central hull so the wand will also move further).

Your wands are somewhere between the main hull and float, I don't know how you balance their effect on the flap, it may be that the leeward wand is dominant so the effect of roll is even more attenuated, maybe even by 75% if it's half way between the hull and float (compared to a wand on the windward float).

So a wand on the float is more sensitive to roll and a foil on the float is much more effective at creating righting moment (RM). These are the reasons that you can't directly compare the effectiveness of central wands and downforce on your boat to float–mounted systems on other boats. Your foil is optimised to lift the central hull, it is not as efficient or effective at producing RM.

One aspect I hadn't considered much is the pitch control aspect of a central foil. By stabilising fore–aft attitude, it keeps the AoA of float foils constant so they can be accurately trimmed for just the right amount of lift. If there's a risk of a large pitch down, the float foils need to have some reserve AoA so they don't create down force at the worst possible moment.

 

[Doug Lord]

I think you've misunderstood my comment. For a given amount of roll, the effect on a centrally mounted wand is roughly half the effect on a windward float mounted wand (i.e. for a certain amount of roll, the windward float will rise or fall more than the central hull so the wand will also move further).

Your wands are somewhere between the main hull and float, I don't know how you balance their effect on the flap, it may be that the leeward wand is dominant so the effect of roll is even more attenuated, maybe even by 75% if it's half way between the hull and float (compared to a wand on the windward float).

So a wand on the float is more sensitive to roll and a foil on the float is much more effective at creating righting moment (RM). These are the reasons that you can't directly compare the effectiveness of central wands and downforce on your boat to float–mounted systems on other boats. Your foil is optimised to lift the central hull, it is not as efficient or effective at producing RM.

One aspect I hadn't considered much is the pitch control aspect of a central foil. By stabilising fore–aft attitude, it keeps the AoA of float foils constant so they can be accurately trimmed for just the right amount of lift. If there's a risk of a large pitch down, the float foils need to have some reserve AoA so they don't create down force at the worst possible moment.

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The Fire Arrow wands are close in on either side of the main hull with the leeward wand in the water since the boat is designed to sail at an angle of heel.