Daggerboard catamarans

[Barra]

Well Im with 44cc and seem to raise my boards more than most - running, on a beam reach in liaght winds if wanting leeway to my target, over 10 knots boatspeed per the manual.

I reckon for my cat at least the drag from wetted surface of boards down is more than 0.1 knots but admit I havnt tested it like Phil suggests. Ill give it a crack.

In the harbour I use the board down if the winds are an issue but just the one on whichever hull im helming so i can keep an eye on taught anchor rodes on the neighbours boats etc.

[UNCIVILIZED]

Where does this (fairly irrational) fear of using one's foils stem from? It reminds me of some of the things that I had to deprogram in my mom when it came to driving techniques. As she had a lot of goofy ideas about proper clutch, engine, & brake usage. Incorrect ones, that it took my brother & I quite a bit to sort out the falicies behind them with her. Talk about a pain in the ass!

[44'cruisingcat]

No fear, and it's not irrational. Why use board for no gain?

[44'cruisingcat]

Originally Posted by Barra

Well Im with 44cc and seem to raise my boards more than most - running, on a beam reach in liaght winds if wanting leeway to my target, over 10 knots boatspeed per the manual.

I reckon for my cat at least the drag from wetted surface of boards down is more than 0.1 knots but admit I havnt tested it like Phil suggests. Ill give it a crack.
.

I tested mine. Motoring in glassed out conditions board up was 1/2 knot faster at 6 knots.

[44'cruisingcat]

You certainly don't need any board down to gybe.

The big advantage of daggerboards is that you're not pushing them through the water when you don't need them, unlike mini keels.

If you're going to have them down all the time, might as well have mini keels...

Surfing, running or deep reaching, the LAST thing you want is to add drag forward.

[yeloya]

Originally Posted by UNCIVILIZED

Where does this (fairly irrational) fear of using one's foils stem from? It reminds me of some of the things that I had to deprogram in my mom when it came to driving techniques. As she had a lot of goofy ideas about proper clutch, engine, & brake usage. Incorrect ones, that it took my brother & I quite a bit to sort out the falicies behind them with her. Talk about a pain in the ass!

Civilized , just by curiosity, what kind of cat sailing experience do you have ? with fixed keel or daggerboard ? I am coaching people on cat sailing and have 50.000 nm under belt with cats, did the Atlantic twice, Med several times from Gib to the South of Turkey.
I agree that the instructions are most of the time BS. I had posted that Outremer 51 book says to not to exceed 9 kts upwind. I did easily 12-13 with this boat and she wasn't stressed at all. FP reefing instructions also are BS, doesn't talke into account the sea state and the capability of the crew, etc..
Nonetheless, daggerboards are different. You see and you will still see from other cats owner with daggerboards that there isn't any consensus.
In addition to Outremer and Catana 47, I've also sailed shortly Gunboat 62, Aikane 56 and Catana 58. They all have different kind of boards (thinner, thicker, longer, shorter, vertical or inclined inwards), different hull configurations, thus behave differently.

All I can say is that when the boat reaches certain speed, there is a tremendous force acting on the board. Actually it's impossible to push down the board. (some cats have a winch both for pushing and pulling the boards, but Outremer and Catana 47 has jus the winch for retracting, they go down by gravity) That's why I started this thread to get the first hand experience from the owners.

Cheers

Yeloya

[Stumble]

Originally Posted by valhalla360

My experience is with a Gemini so not a dagger board but similar operation:

Docking, a total pig with both boards up. Slides sideways around turns. Drop a board even half way and it produces an easily controlled boat. Really doesn't matter much which board you drop.

Going up wind, you really want a board. The first time out, we forgot to deploy the boards and did 3 or 4 tacks making zero ground upwind. Dropped a board and suddenly she goes to windward. At high speeds, it's probably going to be more a question of hull design. If the hulls are designed such that they create a bit of lift, it might be reasonable that at high speeds, you can retract the boards and still make ground upwind though I'm betting you still lose a lot to leaway. Might make sense to only partially deploy the board.

Which and how many boards to use:
- If just messing about, we would use one board fully (didn't matter which) or both boards half way. It gave up performance but you didn't have to mess with them.
- If optimizing performance, the downwind board only. It's deeper in the water as the downwind hull is pressed down.
- If in storm conditions, the upwind board only. If you do get a gust that manages to lift the hull, the board breaks free and the boat can slide downwind bleeding off some of the excess overturning forces.

I wanted to highlight the bolded section because it raises a point I didn't address earlier. It is possible to design a hull to generate lift, but the idea you would see it in a daggerboard boat is incredibly unlikely. In order to have hulls that's generate sufficient lift you have to accept they will also generate a lot of drag, it's just the nature of a wing, low aspect foils (or in this case hulls) generate a larger amount of drag for each unit of lift as a high aspect foil.

In the case of a daggerboard hull the designer has already assumed the lift is coming from the daggerboards, otherwise why put them there, so the hulls are necessarily going to be designed for minimum drag at a given displacement. The right distribution of displacement, and the right rocker, with no consideration to generating lift. Because why consider lift from the hulls if you are already getting as much as you can from the daggerboards.

I haven't made an exhaustive study of cruising Catamaran hull shapes, but I would bet any of them with daggerboards have adopted a distinctly U shaped hull form, because they don't need to rely on the hulls for lift. They have the boards for that.

[Just Another Sa]

Originally Posted by yeloya

Below the video that I've taken when sailing south of Micanos at 35 kts from 90 degrees. I had 2nd reef on the main and the genoa half furled, making 14 occasionally 15 kts. She could easily make a bit more but didn't want to push her too much as I was new with this boat. Both daggerbords were half way down and the sea about 3-4 meters from the beam. You can see on the chart plotter that there are some leeway that could possibly be avoided by better use of daggerboards. Or not ??

Cheers
Yeloya

The best way of improving performance on that video would have been by preventing the main from stalling low down. The boom was a lot closer to centerline than it should have been at 80+ apparent wind angle based on the angle of the mainsheet, closer to horizontal than vertical. It should have been at least 20 degrees closer to vertical for max performance. That sail trim increased leeway forces that increased the drag under water a lot more than unoptimum daggerboard usage, which might have been necessary anyway by structural reasons as already said by others. But if not, one board fully down and the other one up is always better than both half way down from performance point of view.

Please explain what you think can be seen see on the chart plotter?

[Just Another Sa]

Originally Posted by 44'cruisingcat

I tested mine. Motoring in glassed out conditions board up was 1/2 knot faster at 6 knots.

In that case there is something seriously wrong in the shape of your board.
0,05 knots might be O.K. for a well shaped board. Unless you had both down and they are not aligned, that could also easily explain it. Producing lift into opposite directions by 2 boards do create a lot of drag.

[Just Another Sa]

Originally Posted by yeloya

All I can say is that when the boat reaches certain speed, there is a tremendous force acting on the board. Actually it's impossible to push down the board.

Cheers
Yeloya

If that's the case while motoring in calm seas, the tremendous force is only between the board and the case due to being wedge shaped and the force between board and water is much less, perhaps just 2% of the force on the case.

[Stumble]

Originally Posted by 44'cruisingcat

I tested mine. Motoring in glassed out conditions board up was 1/2 knot faster at 6 knots.

I suspect you have toed in boards. Great on performance boats like yours, where the windward board is pulled up, but lots of drag under motor. It really helps reduce leeway while beating, but they act against each other when motoring. .5kn is a massive drag issue. I am sure it's what you are seeing, but aligned daggerboards just shouldn't create that much drag.

[yeloya]

Originally Posted by Just Another Sa

The best way of improving performance on that video would have been by preventing the main from stalling low down. The boom was a lot closer to centerline than it should have been at 80+ apparent wind angle based on the angle of the mainsheet, closer to horizontal than vertical. It should have been at least 20 degrees closer to vertical for max performance. That sail trim increased leeway forces that increased the drag under water a lot more than unoptimum daggerboard usage, which might have been necessary anyway by structural reasons as already said by others. But if not, one board fully down and the other one up is always better than both half way down from performance point of view.

Please explain what you think can be seen see on the chart plotter?

You may be right, but all of the tell tales of the main were in perfect form and the A/P was very stable, no visible weather helm. (actually the boat was so stable that when I put off the A/P for 4-5 seconds the helm didn't move at all..)
Obviously there many other factors; more genoa + one more reef on the main or vicea versa one less reef on the main (I was on the second reef on the main) but the genoa furled further. With fixed keels and my experience I can feel that what the boat is asking and trim accordingly. I need to practice more with the daggerboards..

Cheers

Yeloya

[44'cruisingcat]

Originally Posted by Just Another Sa

In that case there is something seriously wrong in the shape of your board.
0,05 knots might be O.K. for a well shaped board. Unless you had both down and they are not aligned, that could also easily explain it. Producing lift into opposite directions by 2 boards do create a lot of drag.

If the difference between board fully up and down was 0.05 kts, nobody would have daggerboards! What would be the point? Might as well have fixed keels.

The shape is naca 10 section. Single board, so no possibility of misalignment. Fully down it is very deep, increases draught by over two metres.

I suspect people who claim virtually no difference in speed between boards fully up and down haven't actually tried it.

[catsketcher]

The great thing about boards is that they go down when you need them and up when you don't. My rule of thumb about putting them down when you can't sounds glib but isn't.

Boards are designed to resist the sideways sail force vector with the lowest possible drag. When they are loaded up (and reducing hull drag) they will push against the case and be hard to move. This is when they (or some part of them) are needed.

To get them down (or up if they are just a little stuck) you grab the board uphaul and ask the helmsman to do a quick jerk to leeward - steer hard downwind for 2 seconds. This unloads the board enough for you to move it up or down. It works especially well from a reach to a run.

On my one and only trip on an ORMA 60 the crew wanted to push the boards down when going hard to windward. I waited whilst they winched on the downhaul and was going to let them know the above trick. The board just went down, at full load to windward. Amazing design and construction.

44 - we seem to have very different boards. If that were my case I would be pulling mine up a fair bit. I will test my boat at slower speeds.

I found this on Boatdesign net. Tom Speer is a foil guru and he talks about about foil choice in this thread. Smaller foils are better - if they stay in the drag bucket. As for leeway - even he doesn't really know.

formula 18 katamaran daggerboard - Boat Design Forums

cheers

Phil

[44'cruisingcat]

Why on earth would you want board down on a run?