Hybrid of catamaran and sailboard

Hybrid of catamaran and sailboard:
could this project make any sense?


Hybrid “beach” catamaran, 2-seater.



The mast and two struts form a tripod that can rotate a full turn on a circular rail.
In the “operational mode” the mast (and the sail) are inclined windward to produce some vertical lift component.
There is one more degree of freedom – the mast (or just the boom) can swivel +/-90 in regards to the tripod.


The sail is represented by a rectangle, otherwise it’s difficult to see the angles.
The centre mass of the crew is sketched as a ball.
Dagger boards / rudders are inclined inward to produce some vertical lift. Hydrodynamic scheme is “canard” to increase yaw direction stability. The mast position is moved aft compared to “classical” catamarans.
Minor details (the trampoline, seats for the crew etc.) are not drawn.
There are no shrouds or other ropes in the air in this scheme or rigging.

The following picture represents tacking (the moment when catamaran is headed into the wind).




Kinematics of the rigging.
1) The "forward" rotation of the tripod is limited by an adjustable limit stop.
Classical rigging analogue: the mainsheet limits the rotation of the boom.
2) When tacking, the tripod rotates “backward”, the swivel of the mast is in interlocked motion with the rotation of the tripod.
Classical rigging analogue: the mainsheet sags when tacking.
In other words, this inclined sail could be controlled absolutely in the same manner like the classical “Bermuda rig”.

 

It would be simpler to use a ball joint at the apex of the tripod and control the wing with a couple of vangs or sheets.

 

I'we chosen a more complicated design in attempt to improve the aerodynamics of the sail.

1) One strut of the tripod is the leading edge of the sail, so there is no interference of this strut with the sail.
In case of a fixed tripod there could be a strut parallel to the sail (wing) at a close distance to the sail (wing) surface. It could result in a poor aerodynamics.

2) The other two struts can be made from the tubes with an elliptical cross-section. Since the tripod is rotating, the airflow around these struts would be in "correct" direction. Thus, the aerodynamic drag of these struts could be reduced.

However, you could be right - the simplicity of a fixed tripod (or a 4-strut "pyramide") can overwhelm its aerodynamic disadvantages.

 

that circular track will be heavy if it is stiff enough to not distort with sailing loads, also the center of pressure on the sail would move fore and aft relative to the hull. it would change weather/lee helm of the rudder, and could make some sailing modes almost uncontrollable if nearly at the stall point of the rudder or keel. Seems to me there would be high risk of that track getting stuck or jambed, and likely result is the crew goes for a swim.

If you are concerned with aerodynamic efficiency go with a fully cantilevered mast, this would allow the sail to swing 360 deg, and not have the loss of the angle on the sail. It would also likely be lighter and much simpler to build and make work, even with the high cantilever loads at the base of the mast.

 

Not obvious.
We need some kind of frame to connect the hulls. The track can be a part of this frame.
There are tracks for the mainsheet on large yachts, and these traks work reasonable.
I've checked what is offered for sliding doors. I understand that these mechanisms are not suitable for maritime applications, but I was trying to get the idea. There are lots of devices for "garage sliding doors" with a load limit 200 kgf. That's the same order of magnitude we need for a small catamaran.

A little bit. Aprroximately the same distance like in the standard rigging. In this particular picture the "upper" and the "bottom" parts of the sail are symmetrical and the centre of mast doesn't move when the tripod is rotating.

Yes, you're absolutely correct. If the tripod get stuck it would be a disaster.

You're correct. I have a free-standing mast on my dinghy and it works well.
But I think it's interesting to try something new

 

You seem to have reinvented the proa without the simplicity of it. Do not switch the mast from side to side, switch the ends of the boat.

 

There are no "inventions" in this post at all.
Just a proposal. The kinematics when tacking (the motions of the sail and the motion of the crew in front of the mast) was copied from a sailboard. Nothing new.

Yes, you are absolutely correct - a proa is more simple. On the other hand, if one needs to make a lot of tacking, a proa has some disadvantages.

 

Assuming you can get the mechanics of rotating to work without being very heavy, you have no way to control the boom as you tack.
Even when just going from a tack to a run you will need a great deal of main sheet line and will not have a good purchase.
How will you gybe?

There is going to be a great deal of inertia in the tripod which will make normal sail control very slow - possibly dangerous in a high wind.

I'd like to see your track "cars" which would support both down thrust of the mast and up thrust of the two short tripod supports.
Any large circular tubes will be high drag.

OBTW, that centerboard position will not work at all. It needs to be located a little behind the center of effort of the mainsail. Not a criticism of the tripod arrangement

 

The principle is taken from the mechanism called "maltese cross drive"

1) After some angle the boom is fixed relative to the tripod. The angle between the hull and the boom = angle of tripod rotation.

2) Before some angle the boom is always parallel to the hull regardles of the rotation of the tripod.

3) Small transition area between modes 1) and 2).

So, rotation of the tripod results in following.
1) The angle between boom and hull decreases untill the boom becomes approx. parallel to the hull.
2) Then the bottom of the mast goes to the other side of the boat with boom parallel to the hull.
3) After it, the angle between boom and mast increases to the other side until the whole mechanism takes symmetrical position on the other side.

Yes, it is a problem. Actually, it depends on the scale, and the system could work on a small vessel.

Yes, the kinematics in the "normal mode" (as described above) wouldn't cope with the wind gusts. The moment of inertia is too high. I was thinking on some kind of emergency release, so the boom can start rotate freely regards to the tripod. There is a lot of space (i.e. clearance to the struts) in this scheme to do so.

I've made some hand-writed sketches of this part of the mechanism. Yes, the forces are in the different directions. So:
- The cross section of the circular rail must be optimized to cope with these forces.
- The rail must be made from a standard aluminium profile.
- The rail must cope with the standard industrial bending technology to make it circular.
If you need to, I can convert my sketches to a better looking CAD pictures and post it.

Actually, I was thinking on struts with ~ elliptical cross section. In this scheme the struts work like a proa - the leading edge and the trailing edge are reversed when tacking.
That's discussible, whether the aerodynamics of these struts would be worse or better than the aerodynamics of the wires (in case of the standard rigging). You are absolutey correct - a cylinder produces lots of drag. Especially - vibrating cylinder (standard rigging).

If you need to, we can discuss it.
Despite I've studied aerodynamics, I'm a physicist, not a naval engineer. There could be a mistake in my evaluations. But once again - the canard scheme arized from the positions of the aerodynamic / hydrodynamic centres of efforts and the requirements on stability . Not because I've tried to make a nice-looking picture from an aesthetic point of view.

 

Nick,

I'm a mechanical engineer - also not a naval engineer.
Most of the issues raised result in lots of complexity, more weight, and difficulty in high wind control.
As a "use to be" catamaran sailor, I am allergic to anything that prevents rapid gust response.
I'm going to let you work out the issues, there is too much to invent to succeed for my interest. I'm just glad you recognize the issues.

About the canard. Even in an aircraft (Long easy) the center of lift has to match the COG. If your idea is to have the rudder be a significant producer of lift then the configuration could work, but the rudder should probably be a lot larger - I realize this is a sketch.
Bill Roberts had a catamaran design that had a similar configuration - I heard it worked but nothing more- it did not become very popular. But his boats were never in very large scale production.

 

The standard rigging with shrouds (not a free-standing) has a serious drawback - it is impossible to rotate the boom forward. Catamarans usually capsize diagonally over the nose.
One can made a free-standing mast.
What else? That's why I draw this sketch. In case of emergency the boom can be instantly turned forward without rotating of the whole tripod. It could be important when sailing downwind.

Absolutely correct. The rudder is supposed to produce lift. Something like that is already done on "classical" yachts when the keel (which is in front) can rotate a little bit.


Let's talk about airplanes with tandem wings / classical scheme / canard.
We will use terms "front wing" and "rear wing"

Lets introduce the term "geometrical angle of attack"
For the front wing
geometrical angle of attack = angle of attack
For the rear wing
geometrical angle of attack = angle of attack if there were no disturbance from the front wing

Let's suppose our airplane is in horizontal flight and all rotational moments are compensated.

Lemma. The equilibrium is stable provided that
|front wing geometrical angle of attack| > |rear wing geometrical angle of attack|

When talking about catamarans it is reasonable to have a ~ zero angle of attack for the hulls. It gives some additional restriction on the angles.

What we get. It is reasonable to rotate the front lifting surface. And, since our physical strength is limited, it makes sense to make this surface small. To keep the balance, we need to move this surface forward.

Well, in some sense the SailRocket is a canard.
They built the first variant by a "classical" scheme and it lost stability and crashed in the very first test run.
The second variant has the cabin (and center of mass) in front of the sail/ This variant was successful.

When I get "canard" from my evaluations, I was a little bit surprised and googled whether such as scheme was applied to yacht designs before. Yes, there were vessels with good sailing results.

 

Catamarans don't usually capsize diagonally when running down wind.
They capsize on a reach.
Running down wind there is much less wind pressure since the relative wind speed is less and both bows contribute to capsize resistance.
Actually on a run, one way to reduce sail pressure is to sheet in the boom, giving less projected area. Not done much because if you then turn to a reach you are instantly increasing the wind pressure, tending to turn in to a diagonal capsize, where as when you turn into a run with the boom full out the pressure is reduced since the sail does not have enough angle of attack.

Aircraft with tandem wings don't operate at equal angle of attack to the actual wind due to downwash of the front "wing".

Fwd steering has been tried before and generally not accepted due to the overcontrol issue.

I have never seen anyone who has achieved a zero angle of attack on the hull in practice. I know there have been some attempts.

It was my impression that Sail Rocket was first crashed due to total lift off of the boat. Not really anything to do with a canard or not.

 

Update.

Interesting comments by CT249 on disadvantages of my design
http://www.boatdesign.net/forums/bo...ng-rig-small-light-race-cruise-tri-51821.html
#9 and #15