Rotating Dagger boards on a cruising catamaran

[rayaldridge]

This probably doesn't apply to a big boat, where control lines and a purchase are used to raise and lower daggers, but if you tip the board out rather than in, you have the problem that you can't find a place to stand to get the board up. On Slider, her board lies along the inside of the hull, and if it gets jammed, at least I can get my feet under me to heave it up. If necessary I could attach a tackle to the mast-- not as easy if the board tips out. One advantage to the outboard dagger is that it can sometimes be one-piece, but it isn't hard to have a folding handle on the dagger so that the boom doesn't hit it when up.

I know there's a lot of strongly held opinion on the matter, but i general I think a daggerboard is a more efficient solution that a centerboard. While it's true that the centerboard is less vulnerable to damage, it isn't completely free from the possibility. It will retract if the impact comes from ahead, but it's likely that at some point the boat may drift sideways onto a mud flat, and that sideways pressure is as likely to snap off a centerboard as a daggerboard. Or worse, jam it so that it can't be raised.

While it is true that hitting something at speed with a dagger means a broken board... I think the trick is to be sure that the case is much stronger than the board, so that the dagger breaks and not the case. Crush boxes are an option, and I've seen daggers that are spring loaded so that they kick back off of a detent and pop right up.

I have to admit that in my next boat, I'm really wanting to try a single rudder and dagger mounted in a slender central hull that comes down few inches below the waterline of the two main hulls. (This is because of various problems associated with a folding cat design.) This would increase skin friction modestly, but prevent ventilation at speed-- so you'd be giving up a very small amount of low-end speed.

Derek Kelsall designed a series of small cruising cats with a central board and rudder, and from the reports of those that sailed them, they did fine.

[Becaris]

Another question... if I was going to rake the dagger boards forward to reduce ventilation, how much of an angle are we talking about? Anyone know?

[truecougarblue]

Minimal forward rake, 3 degrees would probably do nicely. You are just trying to make sure that any air entrained has it's path of least resistance back to the surface and not along the foil.

[CatBuilder]

Agreed with Truecougarblue. Also, if you rake them, make sure you keep the center of lateral resistance the same.

[Becaris]

A this point I haven't heard a good reason why I shouldn't have the boards exit just above waterline, but rake them forward around 3 degrees to cut down on ventilation. I get the advantages of low marine growth issues in the cases, the room inside the hulls and out of the walking area on top of the deck, and no resistance from the slots below waterline. I'm consulting with the navel architect and builder, but unless someone convinces me with new information, I don't see why I shouldn't do it this way.

[redreuben]

By the time you take into account a full cruising load and heel angle I would be surprised if you could do this and still have a board that in its raised position wasn't a nuisance, meaning sticking out deck too much.
Just put it far enough inboard of the centreline so when boat is beached you can easily get at slot if need be and it doesn't fill with pebbles/sand.
Put it inboard so it can be safely raised without going over the side !
Also inboard, you can use a halyard from mast if need be.
HNY
RR

[upchurchmr]

If you ventilate the board you will need to make it a little bigger to account for the loss of lateral area.
What the heck (PC Reigns!), its only a $50 - $100K boat????, Right? I probably underestimate. Somebody will buy it, eventually.
Sorry for being snotty, but that is the biggest reason for not doing it that I know.
I personally would make it exit at half the depth of the hull, that gets it off centerline, avoids a lot of trouble with jamming, but keeps the boat sailing better. That's what its for right - sailing?
Guess I am not sorry enough about the snotty thing.
Anyone who is building any boat has my admiration.

Good luck, happy new year.

Marc

[Doug Lord]

I think I understand from reading the whole thread(unless I missed something)
that Becaris' boat is mostly done. Even so I don't get any reason why a single centerboard from a center nacelle is not considered on more cruising cats:
1) you can angle it forward to reduce ventilation(Moth angle =7 degrees),
2) it will pivot up out of the water if it hits something,
3) the trunk is always 100% free of marine growth,
4) you can carry a spare board.
See this thread and Len Surtees design-you could even contact him and ask how its going. Many cats have been designed with a center, centerboard and while I don't think it makes a lot of sense on a race boat(except a multimode cat like Hydroptere.ch,see below) it seems to me to make a whole lotta sense on a cruising boat.
Surtees Centre Pod Cat

Picture below- Hydroptere.ch-almost square cat-this is a multimode design: for high speed it uses surface piercing lifting main foils and rudder t-foils, for non foiling conditions it uses a daggerboard from the center nacelle which can be clearly seen here.It also uses variable geometry, retractable, planing steps. I haven't seen any arguments against a center,centerboard that can't be blown away by this example used by a team with an awesome research budget-it works for a team going after speed records-it would surely work in a centerboard version for a cruising cat:

click on image-

[redreuben]

Further to Dougs points above, you maintain integrity of the hulls, no holes, slots etc.
I believe the Kelsall solution to ventilation was to simply add a bit more depth.
But if you set it up as a centreboard between the hulls you can angle it forward at will, kick it back if it fouls, many many pluses. And unlike many of the examples shown I don't believe you need to run the pod full length, just enough to house the board is enough.

[Becaris]

Originally Posted by upchurchmr

If you ventilate the board you will need to make it a little bigger to account for the loss of lateral area.
What the heck (PC Reigns!), its only a $50 - $100K boat????, Right? I probably underestimate. Somebody will buy it, eventually.
Sorry for being snotty, but that is the biggest reason for not doing it that I know.
I personally would make it exit at half the depth of the hull, that gets it off centerline, avoids a lot of trouble with jamming, but keeps the boat sailing better. That's what its for right - sailing?
Guess I am not sorry enough about the snotty thing.
Anyone who is building any boat has my admiration.

Good luck, happy new year.

Marc

50,000? Hell, the mast cost $50,000 (carbon fiber). Multiply your estimates times ten, and add a bunch more. However, unlike some people, I do not make decisions about what I design based on resale value, since I have no intention of selling the boat, only using it for my enjoyment. That, and I am not financially challenged. I made the money for this boat by designing innovative products, not by doing things the way everyone else did them. I see no reason not to continue doing the same thing now. If someone shows me a reason why something I'm considering won't work... that's fine. However, telling me that it will hurt my resale value... that means absolutely nothing to me. I've been told to do things the same as everyone else before, and had I followed that advice, I would not have millions to burn on designing my own boat.

[Gary Baigent]

Becaris, okay, have them exit high but rake them a little more than 3 degrees - because your large boat is still going to pitch and wave movement is going to cause board ventilation, and 3 degrees is a very slight angle indeed. I'd suggest 5. The Moths are 7 ... and they do that for a special reason.
ps: like your attitude about resale.

[Becaris]

Originally Posted by Gary Baigent

Becaris, okay, have them exit high but rake them a little more than 3 degrees - because your large boat is still going to pitch and wave movement is going to cause board ventilation, and 3 degrees is a very slight angle indeed. I'd suggest 5. The Moths are 7 ... and they do that for a special reason.
ps: like your attitude about resale.

Well, that's no problem, even 7 degrees isn't an issue. But I'll let the architect decide any details like that, I'm just a concept designer, I bow to the guys who have to do the actual math.