Cat Centerboards Single vs Twin.

[rayaldridge]

Originally Posted by Zed

I want to avoid cases exiting into the water on a moored boat.

Why is that?

[Zed]

Growth... I want clean cases and clean boards, all the time, no antifoul, no nuthin.

[bill broome]

there is turbulence on a center mount board, which decays pointing and speed. inescapable, but an anti-cavitation plate will help.

i think it's worth it, to escape from jamming and paint problems. only a class racer would notice the difference, assuming you get the size of the board right.

[Zed]

What about Doug's suggestion of a forward angle? Say 7 degrees or so.

[sabahcat]

Originally Posted by Zed

Growth... I want clean cases and clean boards, all the time, no antifoul, no nuthin.

My last cat was moored all the time

No growth on inside of cases or boards, ever.

Did have some on the very ends of the board where they exited the hull though.

[Zed]

Why no growth? I have a small fish sanctuary going at the moment, what was so special about your cases?

[bad dog]

Don't know about Sabah's cases, but on a Crowther 48 I helped look after for 10 years, we never had growth inside either. I expected it, but it just didn't happen. I think that if you get the clearance right, there isn't enough light getting in to support the growth? Usually there will be 100mm or so of board left protruding, but this is antifouled easily - we used to a/fould the whole bottom of the board, but this probably wasn't needed.

[sabahcat]

Originally Posted by bad dog

Don't know about Sabah's cases, but on a Crowther 48 I helped look after for 10 years, we never had growth inside either. I expected it, but it just didn't happen. I think that if you get the clearance right, there isn't enough light getting in to support the growth? Usually there will be 100mm or so of board left protruding, but this is antifouled easily - we used to a/fould the whole bottom of the board, but this probably wasn't needed.

Same story

Built board
Wrapped in lino
strip planked case

Leaves a gap of about 3mm/side

Antifouled the board as well, once in 7 years and only had crud grow where the antifoul had rubbed off from running through sand.

[isvflorin]

Very interesting conversation.
I think there are a few more aspects that have to be considered:

1. Shouldn't a single foil make the cat tack better ? A twin foil cat pivots on 2 foils which must have different angles in order to rotate properly. Same thing goes to the rudders, where the outward rudder must have a lower attack angle compared to the inward rudder (toe in), other wise there is excessive drag to one of the foils. But if you toe in the foils while going straight there is obvious unnecessary drag. So in order to make a twin foil tack more efficiently the design should allow for one of the boards to achieve toe in at the moment of the tack. Needless to say if the platform isn't stiff enough , tiny toe in and tuning adjustments are probably useless as they are not consistent as the platform twists and flexes.

2. Kick up !!! A single central foil can be designed to kick up if it hits something. Daggerboards and centerboard cases need to add extra reinforcement to take a hit, that means aditional weight, monohull sailors are happy with that, but why strip the cat out of its advantages (weight).

3. Redundancy. A twin foil setup offers that, but if you brake your single foil setup you're in trouble.

4. Bridge deck or open deck. Easier to setup a single foil on a bridge deck due to extra support, but a single foil on an open deck cat induces lots of twist in the platform that has to be properly supported, other wise who knows what angle the tip of the board will have due to twist...

5. A single foil will need to have a larger area compared to a twin setup, means it is either going to be really long or have an aspect ratio penalty.

So why doesn't it catch on a lot of designs ? The single foil is Highly dependable on platform design, while you can always slap some cases in any kind of hulls ensuring enough rigidity for the foils. Single foils will take higher point loads which are hard to deal with, means expensive.

I would say that the most important thing for off shore sailing, regarding the foil setup is the ability to kick up. That means you don't need to overbuilt the boat in order to take a hit, kick up will ensure hull integrity, while the performance aspects are roughly the same, the differences are tiny, sailor skill is what makes the boat sail fast (if you want that).

So if you think of inshore sailing, maybe all this doesn't matter and the concern should be practicality, than who gives a crap, you can clean the dboard wells with pressurized water once in a while.

[rayaldridge]

The interior of my case is anti-fouled, and when the boat is in her slip, the daggerboard is out of the water-- I have a pin position on the board that when used holds the board above the waterline. Anyway, no fouling problems, and the slip is in a canal in a suburban neighborhood where fouling occurs with amazing speed.

I don't think a kick-up board is a vital necessity, even for a cruising boat. The case must be strong, of course, which adds a little weight, but not as much as a kick-up centerboard case will, since the later must be a lot bigger. Breaking a daggerboard is not a terribly big deal, really, since most multis will go to windward a little even without boards. Furthermore, it's lateral pressure, such as drifting sideways onto a sandbank, that's most likely to break a board, and that may well break a centerboard too.

[Doug Lord]

Originally Posted by isvflorin

Very interesting conversation.
I think there are a few more aspects that have to be considered:

1. Shouldn't a single foil make the cat tack better ? A twin foil cat pivots on 2 foils which must have different angles in order to rotate properly. Same thing goes to the rudders, where the outward rudder must have a lower attack angle compared to the inward rudder (toe in), other wise there is excessive drag to one of the foils.


5. A single foil will need to have a larger area compared to a twin setup, means it is either going to be really long or have an aspect ratio penalty.

.

==================
1.- I don't think that's necessarily true-most twin board cats aren't sailed with both boards down all the time-at least the relatively small under 25 footers. And if you're going with a single center board(dagger or centerboard), why not a single rudder like Happy Feet? The Rave, Hobie Trifoiler,Moths, Mirabaud, and numerous other foilers have proved that surface piercing vertical foils can work very well.
--
5. Why? Again, most cats sailing with retractable daggerboards sail with only one down at a time therefore the area of each must be sufficient for windward work. In other words, those boats have twice the foil area that they need. So a boat with a single foil would have the same area in the water as a twin setup + a few inches more for the area right at the surface.
Again, a single board angled forward about 7 degrees will be very resistant to ventilation caused by the proximity of the loaded foil to the waters surface.

[catsketcher]

The single centre mounted board has been done. I met a Jarcat owner at the top of Moreton Bay who liked the big board he put in the Jarcat. He said it made a big difference to upwind ability. Albert Sedlemeyer used pivoting boards under the bridgedecks of some of his cats. I didn't like the engineering of the one I saw - it looked very clunky and innefficient.

The little Waller 700 trailer cats have a single centre mounted daggerboard with a case that goes through the cabin. My little 6 metre cat has a single board in one hull. I like it that way.

One other thing about boards is size. My 38 ft cat has two boards that I can just handle. If I were to go to one it would be more of a hassle to get them out and in to paint but I could live with that. Maybe it is aesthetic - Some of Oram's cats with one huge board on one side look a little askew to me.

As for growth - I agree with the others - don't build a square box case. Build a moulded case and the antifoul the board. I have very little need to scrape my large cats case. The square case of my old tri was often scraped. As for strength - it is not hard to make a case very strong with 2 kg of unis. Most fast cats sail downwind with their boards up anyway so the need to worry is lessened.

Modern composites mean you really have to whack a modern case hard before it will do anything nasty.

cheers

Phil

[Zed]

I was toying with the idea of a Woods Sango (put your hands over your ears Richard!) with a few mods. One would be to replace the pop down floor with a flying hull shaped floor and then maybe fitting a swinging centerboard in the bridge deck structure making it easy to keep the board totally clear of the water when not in use.

[bad dog]

I had a moored day-sailing Stingray for some years back in the 90s, with no cases in hulls, and a single centreboard off an 18' skiff. It was hinged for kick up, but this meant an open backed case, which is not an inherently strong design. It sailed fine for a cruiser, and the extra weight (5-10kg??) in the case was not an issue. But the foil's leading edge was always just kissing the water on the mooring, so it got dirty quickly in spite of being antifouled.

Tacking should be a function of the balance of the rig, not one board vs two.

[Zed]

I fear that a Sango with the central hull might suffer the same. It will probably end up too close to the water.