Luiz,

I saw the pics of you catri being produced on the website, looks good.

Hopefully you can sail it rather soon.

>>The new owner of the first Catri 26 prototype, from Sweden, told me that he likes to sail singlehanded with one or two rreefs in heavy winds. Under this conditions, he says that the boat sails totally airborne (supported by foils only) and is the only boat around that can rival the windsurfers.

The thing is that the arrow principle is used to set the angle of attack for the bruce foils. When the hulls become totally airborn the angle of attack can only increase trying to lift the boat even higher out of the water which in turn would increase the angle of attack and so on. The only two things that can stop this run away proce is to lift the rear of the boat to but it doesn't look like the rear foils are designed to do that. They look to horizontal to me. The other thing that can stop this runaway proces is the reduction of wetted foiling area. downside of this approach is cavitation as the all the weight of the craft then needs to be carried by an ever decreasing foil area. This can only be done by increasing the pressure difference over the foil which is mostly done by increasingly lowering pressure on the top side of the foil bringing is quickly closer the vapour pressure of the water. Once this limit is reached cavitation sets in.

There is no additions information available of the catri 26 so lets look at it's closest brother the catri 27



If you look closely you can just at the rear of the amas see the little arrow tail foils , in the drawing they are above the waterline. These foils will not lift the back of teh boat out of the water. At max they will lift the stern of the ama level with the waterline. Only the little horizontal foil on the rudder could lift the back of the boat free of the waterline but it is awfully close to the water surface which is not good for a hydrofoil and besides if the mainhull is lifted free of the watersurface the rudder foil would be lifted free of the water too. See how it is set higher than the keel of the mainhull. Also in the explanations of Eglais the rudder foil is intended for smoothing out the sternwave, I'm unsure to what extent it is also intended as a lifting foil.

In short my take on things is that the rear of the boat can be lifted no further than just in contact with the watersurface. The front of the boat is not limited in this respect and therefor any increase in lift in the front will be accompanied by tilting teh craft on its rear which will have the side effect of increasing the angle of attack on the main foils which in turn will cause them to produce more lift and trying to lift the front even further which lead again to even bigger angle of attacks .... Ergo the runaway system. Ofcourse the reduction of area will try to stop this but at any rate the increase in angle of attack will not make it likely the foil produces lift at its drag minimized setting.

This is just the reason why full foiling craft have activily controled attitude foils. Eglais system is so neat because it doesn't have any of those. Downside is ofcourse that acts based on certain garanteed conditions and being level with the watersurface looks to be one of them.

Now the catri 26 maybe designed differently, which I don't know, I can think of a few way in which to do that. But given the catri 27 I would like to see a picture of the full foiling catri tri, meaning with all of it's hulls noticably clear of the watersurface.

>>>The trick is that the boat is designed for a bigger crew and the displacement is lower when singlehanding. This enables the boat to be 100% supported by the foils.

from a weight perspective yes, but not from the perspective of the intended way the foiling system is designed acts to become a stabil, controllable and low drag system.


>>He also said that, contrary to your expectations, the self-stabilizing system (rear outrigger foils) continues to work under this conditions and the boat remains remarkably stable and easy to sail.


That may be so, but than I would like to see where the control system is. It maybe a unpowered passive system but I can't recognize it in the catri 27 design from the detail supplied on the websites. So please explain to me why it does stay stabil.


>>The reasons appear to be:
1) there is far more lift in the forward bruce foil then in the rear foils - just look at their sizes.


This is no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.


>>>2) the position of the bruce foils in front of the mast give better leverage. (with the added benefit of the "shared lift" - the same concept Bill Roberts uses in his latest ARC 21)


This is no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.



>>3) The reefed sails in heavy weather generate the same power - but in a lower position up the mast. This configuration requires less righting moment (reduced vertical load on the foils) and is less prone to pitchpole (due to the smaller leverage arm of the sail's horizontal thrust).

Yes this is again no garantee for controllability nor efficient = low drag foiling when full foiling. Nor a garantee for a smaller drag to driving force ration than say a Tornado.

When I look at the claim made on http://www.multihull.de/catrimarine/

About the Catri 26 c (the prototype you refer too) having 50 square mtr sailarea per ton weight all I can say is :

Tornado = 135 sq.mtr. / ton = 72 sq.mtr / ton when including crew
Hobie 16 = 130 sq.mtr / ton = 65 sq. mtr. / ton when incl crew
F16 = 168 sq. mtr. / ton = 71 sq. mtr. / ton when incl crew

The catri 26c therefor needs to have 50/72 = 70 % of the drag to power ratio than the Tornado. In this respect the extra width of the catri will definately help by increasing the power in this quotient. But the drag of the catri may not increase accordingly when going from the ligher tornado to the heavier and bulkier catri.

Lets discuss this further but first :

>>I think this is enough to understand why the design features of the Catri help keep the boat controlable and with a bow up attitude all the time.


Well it was not pitchpoling that I was talking about. I'm talking about maintaining a low drag to power ratio. I do see the power thing in this but I have my doubts about the amount of drag. I do do see it is reduced with respect to comparable tris, but I'm not convinced about it being small enough to achieve 30 + speeds. The other thing was full foiling with the catri system.

But now lets get to the drag.

Once again because I have no further info on teh catri 26 I refer to catri 27. This baby weights in at 1700 lbs without crew. 1700 lbs = 770 kg's. This weight would normally be translated into displacement which in turn causes froms of drag like wetted surface drag, wave making drag, suction on hull etc. Roughly speaking. The whole idea of foiling is to reduce the displacement and thus reduce these forms of drag. The cost of this is of course the same drag forms that are being created by the foils itself and the induced drag of the foil itself. This is a new form of drag.

Induced drag can be explained in the following way. When a foil is place in streaming gas or liquid than two main forces are created. One is lift which is the result of the increase in pressure on the the foil and a larger reduction in pressure above the foil. The second is a force parallel to the current (often called induced) drag; which is also the result of the named pressure differences. Picture this. When a horizontal plate is absolutely flat an parallel to the current then any pressure difference that may exist over the plat will only experience a vertical force (lift) there will hardly be any horizontal force. Now tilt the plate relative to the current, ergo incrase the angle of attack. Now the pressure difference will cause both a vertical force as a horizontal one. Increase the angle even more and the horizontal force will grow because its projected area will grow and the vertical force will decrease because its projected area will decrease. For a given pressure difference that is. Ergo, For a given angle of attack the ratio between the produced lift and the produced drag is a constant. Mostly this constant decrease when increasing the angle of attack.

very good high aspect foils have ratios of about 30 ; Slender gilder plane wings and such.

Low aspect foils and curved plates = sails will have ratios between 5 and 10.

Medium aspect foils (or low aspect onces with end plates) will have ratios between 10 and 30. Mostly around 20 or so.

For now I wouldn't place the Catri foils into the efficient high aspect group but rather in the medium group. Very high rations are not easy to achieve and older glider planes used to have rations of 20 themselfs.

With other effects like the nearness of the watersurface distrupting ideal operation of the foils I would not place the catri foil past a ratio of 20.

So a craft of 1700 lbs will have a induced drag of say 1700/20 = 85 lbs. when full foiling just as a result of producing the lift needed to lift the hulls.

What am I trying to say here ? That foiling may reduce drag with respect to a comparable craft liek the farrier F27 but that this is not the same a having low or no drag. The total drag may still be substantial. Now foiling may well be more efficient in carrying weight than displacement when looking at weight to drag ratios but that may well be a long way from making a 1700 lbs foiling craft less draggy than a 375 lbs Tornado when not even looking at lightweight boats like the M20 of just 242 lbs. So how can it be that a tornado is claimed to be limited to 23 knots when a much much heavier boat is easily capable of blasting past 30 knots = no less than 30 % faster.

This can only happen when foiling is so increadibally much more efficient than displacement; it is this assumption that I have trouble with. Not that it is more efficient but that it is that much more efficient.


>>About the square tops, the same guy ordered a new set of sails - with a square top main. I don't think the square top will make that much difference in speed - I expect the difference to be relevant only in match racing conditions.


Well the biggest advantages of squaretops are their improved gust responses and their improvements in controlling twist.


>>The designer says that the eliptical top is better suited for heavy weather/high speed conditions, while the square top is better in light weather.


The designer is overlooking the gust response of squaretops. Also squaretops twist off better in strong winds. Pin head often displays the behaviour of twisting of the middle part while bringing back in the top of the sail. Not something you want.


>>He has doubts about the behaviour of of square tops under heavy weather and for him this weights more then the light weather/match racing advantages (for safety reasons). This is a minor issue for him, though.

Well gusts responses and the ability to power down the top of the sail are very much inprovements in contrability = safety too.


>>The 2 ft length difference between a Tornado and the Catri 22 is almost irrelevant.

That is why I compared the two.

>>The Catri is a lot wider, so the righting moment is bigger and it can carry a lot more sail then the Tornado.

And it is alot more heavy too. I mean the catri 23 C is 750 kg's = 1655 lbs. with crew 900 kg = 1986 lbs = 2,81 times heavier than a Tornado with crew. Increase in sailarea should be about 1,6 times mostly as the result of the jib. ratio mainsails = 1.33


>>Also, it is relatively less draggy in top speed conditions.

Is that so ? 90 % foiling means that still 90 kg's is in displacement compared to 320 kg's of the tornado. Alot less I admit but then the Tornado doesn't have the induced drag of that 810 kg's that is being carried by the foils. Not to meantion the aerodynamic drag of the cabin, extra beams and extra ama. And at 30 knots these aerodynamic effects are very noticable, ever stuck your hand out of the car while driving at 30 knots ?

For now I don't see strong indications for the claim :"is relatively less draggy" when comparing the catris to tornado's.

Currently it all revolves around a claim that a swedisch team achieved 32 knots as indicated by the GPS. This comes close to a cicular reasoning where these 32 knots must mean that it is alot less draggy which in turn must mean that is can achieve 32 knots. Besides how many claims did we see that people achieved 30 knots on cats by virtue of looking on their GPS.

I mean if cats can only achieve 30 knots by having help from a way maybe that catri was boosted to 30 knots by the same phenomenon ?

>>It seems more appropriate to compare the Catri potential with totally airborne hydrofoil boats, like the Rave, for example.

This I don't agree with fundamentally. The systems used on the rave are significantly different from the catri system. for example the catri system links sideways resistance force directly to lift produced. While the rave system can largely produce any amount of lift independent of the side resistance needed.

>>Then the 32 knots can be seen as a conservative figure.

Well, then I'm looking forward to seeing the catri enter the weymouth speed trails and set a record.


>>I see the Catri as a conventional tri in light winds that automatically converts into a hydrofoil tri in stronger winds. A very smart combination!


Well that is it beyond a doubt.

Ofcourse my sceptis in the much faster than a tornado discussion must be viewed seperately from my admiration of Eglais designs. As you know Luiz when we discussed the Catri before you purchased the plans, I expect a great deal from this design and it definately rates high on my list.

And really, if I ever go for a cruising platform the choice will be between a Catri and a conventional monohull capable of making long journeys.

I'm looking forward to more innovation from Eglais and his design team.

with kind regards,


wouter