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Old 05-08-2008, 12:50   #1
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Question Dagger Boards --- I don't quite get it

I can certainly understand that the Dagger Board approach yields greater speeds sailing down wind when the boards are up, but I don't quite understand the claim that the boards allow closer pointing to the wind.

With the Main sheeted all the way in, as you put the bow closer and closer into the wind, there is a smaller and smaller component of force pushing the Cat forward. The Main does not know if the Hull has a Keel Or Dagger boards and the forward forces are the same.

So how is it dagger boards can yield closer pointing? Is it simply the reduced resistance thus the component of forward force is more effective? If this is the case, then I don't see how Monohulls can point closer, as they have a heavy displacement will lots of resistance to motion.
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Old 05-08-2008, 13:16   #2
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Dagger boards are more efficient foils. Much higher aspect ratio.
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Old 05-08-2008, 14:18   #3
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Sailplanes (gliders with really long wings) take advantage of a feature of high aspect ratio wingspans: They provide more lift with less drag. Short stubby wings are good on an airplane where there is a lot of power available, the airplane flies faster, but doesn't generate enough lift to stay airborne at slow speeds.

Relatively speaking, boats are always flying slow. Keels that are very long and narrow are much more efficient; thats why newer all-out racing designs have keels that look like glider wings, with a big blob of lead at the bottom.

But those extravagantly long keels are a liability in shallower water, and those boats just can't go to many of the places sailors want to go. Cruising catamarans have evolved in the opposite direction. Long, shallow keels allow you to get in closer, and sail in thinner water. Designers took advantage of this, and accepted poorer windward performance as a compromise. While this compromise would never have sold in the 1950's, today's sailing public seems to have accepted this trade off. And some of us have not. Thank God there are some builders like Maine Cat, Catana, Outremer, Freydis, Fastcat, and others (predominantly in the Southern Hemisphere) who build for the performance multihull sailor, taking the extra time and expense to build strong dagger boards into their designs.

An aside: shoal draft keels present a lot of barn-door area to resist the sideways force of the wind, and they do that at zero to very low speeds, such as getting in and out of a slip. Very high aspect ratio foils do not, and must be moving to develope lift, so getting caught in irons is a bigger problem than with stubs. While daggerboards and trunks are very sturdy, they are still an appendage sticking down further in the water, where hard things lurk, offering you your own answer to the "irresistable force encounters unmoveable object" conundrum.
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Old 05-08-2008, 14:46   #4
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One thing I would like to see is a builder design is a dagger board that could carry the load of the boat when put on the hard. Why not design a chain plate system on each side of the daggerboard well, that you could fix a heavy strap to, that would run over the top of the dagger board. Should add very little weight and would be a great system for bottom cleaning in shallow water.
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Old 05-08-2008, 14:52   #5
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I think airbags are the answer for that. There was a bag that would lift a truck off the ground when attached to the engine's exhaust. An engine is just a complicated air pump, right?
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Old 05-08-2008, 14:57   #6
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Daggerboards reduce leeway. The next time you are on a small boat like a Laser, pull the daggerboard up and then try sailing to weather, if you can. If someone else is out there on a similar boat, have them leave their daggerboard down and leave your daggerboard up. Observe who gets to the weather mark first.

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Old 05-08-2008, 15:32   #7
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Quote:
Originally Posted by Abaco View Post
One thing I would like to see is a builder design is a dagger board that could carry the load of the boat when put on the hard. Why not design a chain plate system on each side of the daggerboard well, that you could fix a heavy strap to, that would run over the top of the dagger board. Should add very little weight and would be a great system for bottom cleaning in shallow water.

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Old 05-08-2008, 15:45   #8
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Quote:
Originally Posted by Abaco View Post
One thing I would like to see is a builder design is a dagger board that could carry the load of the boat when put on the hard. Why not design a chain plate system on each side of the daggerboard well, that you could fix a heavy strap to, that would run over the top of the dagger board. Should add very little weight and would be a great system for bottom cleaning in shallow water.

The boat I'm designing is a cat that has both small stubby keels (for beaching etc) and daggerboards to minimise leeway. Best of both worlds! Costs a bit more, but the LAR keels also protect the drives and rudders, and let me have higher Aspect Ratio rudders for better efficiency as well.

I have added 4 reinforced lifting points on the boat so any standard 10 ton crane can lift it out, and we can stand on the keels and rudders.

So, yes it can be done, al be a bit different to what you suggested.

Cheers

alan
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Old 05-08-2008, 16:31   #9
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The implication is that dagger boards just reduce leeway. This still is not making sense with regard to pointing.
With a Cat with Keels, the boat simply slows when attempting to point closely to the wind. It's not that the energy is going to a leeward slide. The twin Long Keels seem to do a very good job at leeway.
I have read that DBs can provide 1-2 knots more speed, and allows for 5-7 degrees closer pointing to the wind. But it is still not clear as to why.
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Old 05-08-2008, 17:19   #10
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Long keels do NOT do a very good job of resisting leeway. They do a so-so to marginally adequate job of that. In a typical south-easterly breeze, I have to tack my PDQ 36 with shoal draft keels 9 times going up the South River to get to my slip. A high performance cat, the Catana 39, does it in 7 tacks. THATS embarrassing.
With the debatable exception of a cat with jibing boards, (that is, their leading edges are free to pitch up to windward while their trailing edges are restrained) or asymmetrical airfoils that are raised or lowered depending on which side the wind is from, ALL SAILBOATS SKID SIDEWAYS. Some a lot, some very little. Shoal draft keels have to sail at a higher angle of attack to achieve the same resistance to skidding that a high aspect ratio keel can accomplish, with less drag.
This is a subtle thing, accomplishing a few degrees of windward performance, and its not all that obvious to the human eye, but its crystal clear to the stop watches on the committee boat at the finish line. And a quick check of my pocket book at the end of the day reveals I lost more bets than I won!
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Old 05-08-2008, 17:19   #11
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Keels vs. daggerboards.

Quote:
Originally Posted by Limpet View Post
The implication is that dagger boards just reduce leeway. This still is not making sense with regard to pointing. With a Cat with Keels, the boat simply slows when attempting to point closely to the wind. It's not that the energy is going to a leeward slide. The twin Long Keels seem to do a very good job at leeway. I have read that DBs can provide 1-2 knots more speed, and allows for 5-7 degrees closer pointing to the wind. But it is still not clear as to why.
I'd say that the phrase 'closer pointing' is misleading you. It is rather that a high aspect ratio daggerboard makes less leeway if the boat is making good speed, and so make good a higher course (- low aspect foils, such as keels, are better at low speeds.) Daggerboards and keels develop lift, just like sails or wings. Also, they can be retracted when off the wind, thus improving light airs performance by reducing wetted surface, which creates drag through friction against the water.

Keels are stronger, can make it possible to dry out the boat between tides without hurting the boat, and help protect the prop, prop shaft, and rudder. They don't take up interior room in the already rather cramped hulls of catamarans. Also, they can be used as tanks. I am using keels in my BigCat 65 as sewage tanks, which gets the sewage tank far enough below the toilets that the boat can use gravity drained heads like those used in motor homes and travel trailers. (aka caravans in the UK.) If septic tanks are holed, you don't have the problem of losing precious drinking water (precious if you are offshore,) or diesel, which can hurt the environment and get you in trouble if leaked.

The problem with strong daggerboards for beaching is that many designers deliberately make them a bit weak, so that they will break off rather than damage the boat if they hit, for example, a floating ship container or coral head.
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Old 05-08-2008, 22:38   #12
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Hi Limpet
A boat without centre boards when pointing too high will slip sideways as there is nothing to resist this sideways motion. This sideways (leeward) movement reduces the gains of higher pointing. To reduce this a lower angle of attack into the wind is optimal. Centre boards reduce this sideways slip allowing a higher angle to be sailed.

Helen
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Old 05-08-2008, 22:48   #13
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I have sailed a Dolphin 46' with Daggerboards, but I was not paying much attention to the pointing and/or leeway issues....was just glad to be sailing. I'm very curious to give it a try again.
Likewise, I have sailed many Cats with Keels and have not noticed the leeway issues.
As I think back to my days on Mono's, they really heeled close hauled into the wind, I'm wondering if a large part of the heel was related to the leeway force against the keel?
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Old 05-08-2008, 23:59   #14
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Leverage

Quote:
Originally Posted by Limpet View Post
I have sailed a Dolphin 46' with Daggerboards, but I was not paying much attention to the pointing and/or leeway issues....was just glad to be sailing. I'm very curious to give it a try again.
Likewise, I have sailed many Cats with Keels and have not noticed the leeway issues.
As I think back to my days on Mono's, they really heeled close hauled into the wind, I'm wondering if a large part of the heel was related to the leeway force against the keel?
The keel is a short lever and the mast is a long lever.
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Old 06-08-2008, 09:55   #15
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Now you have it, Limpet. And in the same situation, a catamaran's windward hull comes up a bit, while the downwind hull sinks a bit, but rarely more than 8 to 10 degrees. The next time you go out, take a hand bearing compass and sight back along the wake. If its nice and straight, you have a good helmsman. Add or subtract 180 degrees (whichever gives you a positive number), and it will not be the same as the boat's heading. The difference is the crab angle (crabs walk sideways!) at speed, a dagger board cat will have a very small crab angle, and will be sailing 5 degrees closer to the apparent wind. A shoal draft cat will have as much as a 4 degree crab angle, and will be sailing more than 50 degrees from the apparent wind.
Both cats will get to their destinations faster if they are not sailed hard on the wind. This is because they can exceed the traditional hull speed limit of mono hulls. Cats will be making faster progress upwind if they fall off a bit and go faster. Mono hulls won't gain as much speed that way, so they stay hard on the wind [with the sails pulled in close to the boat] to make their best progress upwind, or VMG [velocity made good to weather.] Sometimes you have to draw it out to see what's happening, but many GPSs will calculate that for you. On a racing boat, a Tactician keeps track of that information, and knows whether its better to tack when the wind changes direction, or to continue to sail to the other side of the course where the winds might be more favorable when they get there.
I apologize if it sounds like I'm talking down to you, but I've been trying to explain a subject that has lots of nuances as simply as possible. Polysyllabicy is the last pretense of the armchair sophisticate
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