Dagger Boards --- I don't quite get it

[Merlin Hawaii]

Originally Posted by Limpet

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.

Daggerboards are the most effective leeway preventers yet devised, but they do MUCH more than just keep the boat from sliding sideways through the water

They work on the same principle that allows an airplane (through it's wings) to fly - Aero/Hydro Dynamic lift. The same principle that allows your sails to sail into the wind

Because daggerboards are, in essence, vertical wings; they create horizontal lift which lifts (pulls) the boat FORWARD AND TO WINDWARD through the water

In addition to their much less significant job of preventing sliding SIDEWAYS through the water

So, when the boat is stationary, the daggerboard(s) only function can be in preventing the boat from sliding to leeward

But when the boat is MOVING, it(they) assume their much more exciting task of dynamically pulling the boat FORWARD AND TO WINDWARD

Just like the sails

[Tropic Cat]

Although I agree with the explanations here in principal, my observations are slightly different. This may be boat dependent.

To determine my leeway, I used a machinists protractor and a GPS. What I discovered is that hull speed has a dramatic affect on leeway on my boat. The faster I go, the better my hull design performs and I have less leeway. My boat not only does not have dagger boards, I don't even have keels. I have an older British boat with multichine hulls which have considerable amount of rocker, very similar to planing power boats. Hull photos are in my signature below.

When I'm hard on the wind, I can sail to 30 degrees apparent at which point the boat begins to pinch and speed falls off dramatically and instantly. At 6.5 knots, my 'crab angle' is 5 degrees. At 4.5 knots it's 10 degrees and at lower speeds can approach 15 degrees.

I guess all this means is that... it depends....

Incidently, in addition to dagger board considerations already covered, no one has mentioned the problem of accidently tripping over those boards.

All in all, I'd sacrifice the performance and would rather sail a cat without boards.

[BigCat]

But when the boat is MOVING, it(they) assume their much more exciting task of dynamically pulling the boat FORWARD AND TO WINDWARD

Just like the sails

See:
Spinnaker Sailing Online Sailing Course = Section 2

I'd say the lift is probably a bit forward of what they show, maybe as far forward as the wind direction. I also don't agree that full keels don't generate lift, though I agree that a skinny keel (daggerboard vs. keel in catamaran terms,) will develop more lift per square foot that a long one. The use of deep v hulls that can resist leeway to some extent has fallen by the wayside in yacht design because it isn't very good at it, and because it has a lot more wetted surface than the alternatives. By deep v, I mean the Wharram type configuration.

[Nordic cat]

Originally Posted by rickm505

Incidently, in addition to dagger board considerations already covered, no one has mentioned the problem of accidently tripping over those boards.

All in all, I'd sacrifice the performance and would rather sail a cat without boards.

Rick, I think that this situation will never arise, as the speed of the wave (at the heights that could maybe endanger a cat) is such, that it passes under the boat so fast, that "tripping" over the daggerboards can not occur.

I read an article on this, but can't remember where.

Secondly, the area of the daggerboards and rudders is probably less than 5% of the underwater area, so very limited, not enough to trip over in any case.

Thirdly, the daggerboard will probably break long before....

just my opinion

Alan

[BigCat]

Originally Posted by Nordic cat

Rick, I think that this situation will never arise, as the speed of the wave (at the heights that could maybe endanger a cat) is such, that it passes under the boat so fast, that "tripping" over the daggerboards can not occur.

I read an article on this, but can't remember where.

Secondly, the area of the daggerboards and rudders is probably less than 5% of the underwater area, so very limited, not enough to trip over in any case.

Thirdly, the daggerboard will probably break long before....

just my opinion

Alan

Derek Kelsall and John Shuttleworth have debated whether you can make enough leeway in a storm to avoid harm from a breaking or very steep wave, by having retracted your daggerboards versus any danger of being held into the breaking wave by (non-retractable) keels. Kelsall did the math and said that it was impossible for a boat to accelerate sideways enough for the lack of boards to have the effect claimed by Shuttleworth, based on an analysis of the speed of wave progression. I seem to recall that Shuttleworth did an apprenticeship under Kelsall when he was starting out as a yacht designer.

[Merlin Hawaii]

NACA foil shapes have been established for many years now

And the difference between an accurately foiled daggerboard blade, and an unfoiled slab of wood or composite hanging down below the hull(s), is the difference between night and day as far as sailing to windward

Any longitudinal structure below the waterline (whether it's a keel or the hulls themselves) will produce a certain amount of resistance to leeway.

But foiled daggerboards (and the physics behind them) are in an entirely separate league

[BigCat]

Originally Posted by Merlin Hawaii

NACA foil shapes have been established for many years now

And the difference between an accurately foiled daggerboard blade, and an unfoiled slab of wood or composite hanging down below the hull(s), is the difference between night and day as far as sailing to windward

Any longitudinal structure below the waterline (whether it's a keel or the hulls themselves) will produce a certain amount of resistance to leeway.

But foiled daggerboards (and the physics behind them) are in an entirely separate league

This might be taken to imply that keels aren't usually NACA foil sections, and that isn't the case at all. We are making the plug for my BigCat 65 keel right now, and it certainly has NACA foil sections. Pixilation in this photo makes it look uneven, but is as smooth as an airplane wing. The attached drawing shows 3 waterlines from my keels.

[Merlin Hawaii]

Originally Posted by BigCat

This might be taken to imply that keels aren't usually NACA foil sections, and that isn't the case at all. We are making the plug for my BigCat 65 keel right now, and it certainly has NACA foil sections. Pixilation in this photo makes it look uneven, but is as smooth as an airplane wing. The attached drawing shows 3 waterlines from my keels.

I didn't mean to imply that all keels are not efficient foil shapes, but many are not (especially low aspect ratio keels)

Low aspect ratio keels, especially, must have a longer length to be effective

And the longer length necessitates a greater chord (width ) dimension, which is not feasible on many boats (especially cats)

The foil shape for the daggerboard on a Maxi Racing Tri capable of 40 knots is going to be far different from the foil shape of a keel on a boat capable of 18 knots ( I'm not referring to your boat )

So, as the speeds come down, the chord must go up, and that is just not practical, in many cases, on a long low aspect keel

So, the only other alternative is to reduce the length of the foil to accommodate the chord, and increase the depth below waterline (carried further, eventually becoming a daggerboard)

[Evan]

Originally Posted by BigCat

This might be taken to imply that keels aren't usually NACA foil sections, and that isn't the case at all. We are making the plug for my BigCat 65 keel right now, and it certainly has NACA foil sections. Pixilation in this photo makes it look uneven, but is as smooth as an airplane wing. The attached drawing shows 3 waterlines from my keels.

That looks horrible. Those flat trailing edges will be like dragging a bucket through the water. Why doesn't the keel taper to a proper point?

[Nordic cat]

Big Cat

I agree with Evan.
Those trailing edges will be nice Karmann vortex generators If you don't want them tapering to a point, then at least cut them off at an angle alot less than the 90 degrees shown.

Cheers

Alan

[BigCat]

Originally Posted by Merlin Hawaii

I didn't mean to imply that all keels are not efficient foil shapes, but many are not (especially low aspect ratio keels)

Low aspect ratio keels, especially, must have a longer length to be effective

And the longer length necessitates a greater chord (width ) dimension, which is not feasible on many boats (especially cats)

The foil shape for the daggerboard on a Maxi Racing Tri capable of 40 knots is going to be far different from the foil shape of a keel on a boat capable of 18 knots ( I'm not referring to your boat )

So, as the speeds come down, the chord must go up, and that is just not practical, in many cases, on a long low aspect keel

So, the only other alternative is to reduce the length of the foil to accommodate the chord, and increase the depth below waterline (carried further, eventually becoming a daggerboard)

The drawback to a wide chord is that it takes more effort to push a wide keel through the water. IMHO, you are best off dimensioning a keel as you prefer for other considerations, keeping it no wider that structural considerations suggest, and just make sure that you have a NACA shape proportioned to what is customary, with the widest point at about 40% aft of the start of the keel waterline at each keel waterline. Long keels just aren't that sensitive to issues such as sweep of the leading edge, so you can design around other considerations. My keels are positioned to fasten to the hulls at major watertight bulkheads, so they are very strong, and they are no wider than I think necessary to make them well braced against lateral loads. The leading and trailing edges are designed according to where I want the center of effort for balancing the helm. This is all quite easily done with CAD software, once you have any NACA shape in it to start with.

As far as the aft ends of the keels go, various studies have recommended 90 degree trailing edges, cut off at 90 percent of the foil length. Those of us who have actually worked with fiberglass will tell you that you want good access to roll out the glass inside of the keel, and a flat trailing end will give you that. Check a few books on yacht design, and you will find that I am in the mainstream on this. If these were extremly high aspect ratio daggerboards, I would make a complete foil shape, but this is neither necessary nor desirable on a LAR keel. Those readers who have studdied daggerboards should understand that LAR keels follow somewhat different guidelines.

[Merlin Hawaii]

Originally Posted by BigCat

The drawback to a wide chord is that it takes more effort to push a wide keel through the water.

If we're talking about NACA foils:

A low speed NACA foil (whether it be an airplane wing, a daggerboard, or a keel) is, by definition, going to have a greater chord to length ratio than a high speed foil of the same area. Think of the wings of a fighter plane compared to the wings of a crop duster

So, in actuality a low speed NACA foil (with its greater chord) is going to be EASIER to push through the water (at low speeds), as the pyhsics involved are assisting you.

As the speed increases the optimum foil shape would have a reduced chord to length ratio, and a low speed foil will not only be harder to push through the water, it will also have reduced windward and leeway performance

With most low speed cats, we're talking about relatively low speed foils, as the narrow blade on a 40 knot tri will not be as efficient

The physics of a NACA foil doesn't care about any "structural consideration" constraints. It's physics fit into a very small envelope (for optimum efficiency) and allows for little deviation from such.

So you can't modify the foil (NACA) to fit the structural application, you have to modify the application to fit the foil (once again, for optimum efficiency)

[Merlin Hawaii]

Originally Posted by Evan

That looks horrible. Those flat trailing edges will be like dragging a bucket through the water. Why doesn't the keel taper to a proper point?

NACA determined that the most efficient foils have a slight "squared off" trailing edge, as opposed to tapering to a point

So, most racing foils will have a flat trailing edge

Big Cat's are on the extreme side, however

It is usually a much smaller proportion of the chord dimension

[mikereed100]

Originally Posted by BigCat

If these were extremly high aspect ratio daggerboards, I would make a complete foil shape, but this is neither necessary nor desirable on a LAR keel. Those readers who have studdied daggerboards should understand that LAR keels follow somewhat different guidelines.

This amazes me and I would love to learn more. Intuitively I would think that as fine a trailing edge as practical would be desireable for LAR foils as well. Do you have any online references for these studies?

Mike

[BigCat]

Well, let's see. There is the trusty Marchaj, who says "The trailing edge is of relatively little importance, as it lies in the area of turbulent wake. There is no scientific foundation for the popular belief that the centerboard or fin must taper as sharply as a razor. Experiments have proved that cutting the trailing edge off blunt for up to 5% of the profile length does not influence resistance, and it is undoubtedly more practical." page 284, 'Sailing Theory and Practice."

I think the eye wants to judge the cut off as a percentage of the width of the keel, but scientists all describe it as a percentage of the length of the keel - and this is probably why my cutoff looks wide. However, the aspect ratio of my keels are quite low, less than 6 widths to 1 length. The trailing edge which looks so wide in my diagram is 5.8" wide on the highest waterline, which is 16' 8" long, or 5.8/200=.029 of the length. The othe keel waterlines are the exact same shape and proportions, and so also .029 of the length, well within Marchaj's guideline of 5%. Consider that the keel will be about .6" of thickness in the laminate, and you get a trailing edge on the inside of 4.6" at the top of the keel and a bit less than 1.5" at the bottom. Less is not practical to make in hand rolled fiberglass. These keels are tanks, remember. Since I am well within the turbulent zone at the aft end of my keel design, I have no qualms about the width of the trailing edge's flat surface. As far as vibration goes, just the right 'v' aft has a bit less vibration, but if it is slightly off, it would have 3 or 4 times as much vibration, per Larsson and Eliasson, pages 127 and 128 (Principles of Yacht Design.)