Daggarboard Lift - ?

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How does a boat's (H18 or Tiger, etc..) boards produce "lift" if they are symmetrical and interchangable?

Of course the boards provide lateral resistance (when the boat is flat)... but lift?

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I am not an engineer but the reality is the narrow long boards do create lift. The H18 boards may lift but it is very limited. Having raced my H18 with the F18s it became very apparent. I could match them on boat speed going to "A" but they would pull toward the mark while running the same lay angle as me. The boats move windward without adjusting angle of attack, they lift to weather. Any of the newer designs has the ability. I believe the width of the H18 boards is the major difference. H18 boards were designed to stop the boat from sliding, evident if you try to sail with them up, but not to lift the boat. I assume the concept didn't exist when the H18 was designed. F16s, F17s, F18s and the 20s all run long narrow boards, some are even canted and can move in the slot. When I sailed the Fox I could really feel the boards, the power on them is extreme. I broke 3 daggers on that boat while double trapped in the ocean. I was considering reworking my H18 dagger wells for a while but have given up racing all together at this point.

It is real, the boards do lift! Perhaps the narrow shape creates the lift. The boards are not strong like the H18, I have never seen a H18 dagger snap but I have seen many "F" boards break.

I do understand what you're saying, and have seen it myself in little races in the bay.... same pointing angle as boats with deep narrow boards... but they "lift" to the mark...

I just don't see how it's the same "lift" from a wing or a sail...

If the boards are the same on both sides, they don't know which way they should be lifting.

I'm beginning to think the term "lift" WRT H18 or tiger boards is not entirely accurate?

So what is this phenomenon we see?

I'm no engineer either... I suppose if I was I wouldn't be asking! Thanks!

Lift refers to the component of force generated by a fluid flowing across a surface which is perpendicular to the fluid flow. (Whereas drag refers to the component of force which is parallel to the fluid flow and in the direction of the fluid flow).

When someone is refering to the lift generated by the daggerboards, what they are talking about is the lateral resistance which opposes the sails force. Note that as the boat heels, a component of that force will be directed vertically, but this force is generally negligable since the more the boat heels, the more it slows and so total lift generation is reduced.
The only time a significant amount of lift would be directed upward would be when vertically lifting hydrofoils are used (think Moth or Trifoiler T-foils), or when curved banana boards are used (C-Class cats, New Nacra 20, A-cats).

The reason the Hobie 18 daggerboards make lift in spite of the fact that they're symmetrical is because the boat travels through the water at an angle of attack such that the board is presented to the water flow at an angle. This angle is only a few degrees so it is basically imperceptible to the sailor. The angle of attack creates a high pressure on the leeward side of the board and a low pressure on the windward side of the board which creates lift. Assymetric foils are more efficient, but they still work on the same concept and require an angle of attack in order to produce lift.

sm

so for general conversation about daggerboard lift...

we're talking mostly about lateral resistance...

...and to a small, imperceptible degree a 'pressure differential' between the windward/leeward sides...

cool.

Correct.



Not exactly. The "imperceptible degree" we're talking about is the angular degree that the boat travels through the water. This angle varies due to several factors, but in general it would be 5 degrees or less. Very hard for the sailor to notice.

The force generated by the pressure differential is not imperceptible. Pull up your dagger boards and watch as your boat slips sideways. The force generated by the daggerboards is roughly equal to the sideways force generated by the sails which, when sailing double trapped close hauled, is quite significant.

sm

[quote="srm"][quote="RobPatt"]so for general conversation about daggerboard lift...

we're talking mostly about lateral resistance...[/quote]

Correct.

[quote]...and to a small, imperceptible degree a 'pressure differential' between the windward/leeward sides...[/quote]

Not exactly. The "imperceptible degree" we're talking about is the angular degree that the boat travels through the water. This angle varies due to several factors, but in general it would be 5 degrees or less. Very hard for the sailor to notice.

The force generated by the pressure differential is not imperceptible. Pull up your dagger boards and watch as your boat slips sideways. The force generated by the daggerboards is roughly equal to the sideways force generated by the sails which, when sailing double trapped close hauled, is quite significant.

sm[/quote]

We refer to pitchpoling as the boat kind of tripping over it's bows.

Well when the boat heels is it in a sustained 'tripping' sidways over it's boards? (because they are resisting the boat's slippage sideways)

Would raising the boards would let the boat slip sideways more... and 'trip' sideways, or heel over... LESS? (could be good for me since I sail at light crew weight?) I'd make less headway to windward, but maybe less chance of flipping?

Thanks for the patience and lessons.

Could you please elaborate? Im not up on all the correct terminology, thanks!

Yes. Imagine looking at the back of a boat sailing upwind on starboard tack. The port hull acts as the fulcrum, the sails and daggerboards are acting to flip the boat in a counterclockwise direction, the crew on the trap act to rotate the boat in a clockwise direction. When these forces are in balance, the boat moves forward at a steady angle of heel.



Yes, this is a particularly useful technique on small dinghys, like a Laser. High wind, you can pull the board up to heel less. Doing this moves the center of lateral resistance closer to the hull and reduces heeling moment. Also since you're travelling faster and your apparant wind is more off to the side, you need less board anyway. On a boat like the H18, the boards really aren't that deep or efficient, so it won't make a huge difference, but if you're really overpowered, you could pull them up a little (maybe 8").

sm

Translation:
We had equal speed on a close hauled course (going to "A"), but they didn't slip to leeward as much as I did, so they seemed to lift to windward.

Let me try to answer some of the questions posed in this thread (and I am an engineer, a Naval Architect to be precise):

How does a symmetrical foil create lift?
srm got this right. A sailboat sailing upwind side slips to leeward. This angle is alpha (a) - the angle of attack. On a symmetrical foil, lift (force perpendicular to the centerline of the foil) and drag (force parallel to the centerline of the foil) increase proportionately with a until the foil stalls, when flow separates from the low pressure side. At stall, the drag continues to increase with α, however, lift actually decreases. Typically, a < 4°. Stall occurs at approximately a ~ 6°

Why are Hobie 18 daggerboards so different from an F18's or other "modern" catamaran?
The reason has more to do with aesthetics and industrial design than anything else. Early Hobies have always had more of an "organic" design that other catamarans, which comes from their surfboard shapers' heritage. Consider that Phil Edwards carved the Hobie 16 and Hobie 18 hull shapes by hand from blocks of foam before any molds were made. No lines drawings, no technical drawings - he just did what "looked right". So the 18 has these rounded, parallelogram-shaped boards.

The 18's boards are low-aspect ratio - they are wide for their length. Consequently, they're thick and structurally easy to make, without the need for exotic materials (like carbon - very exotic in the late '70s). They also provide the needed lift without a lot of draft (the boat can sail in shallower water before running aground).

High-aspect ratio boards are difficult to build because they're thin and long. Almost all of them have carbon fiber reinforcement.

So why high-aspect ratio boards? Efficiency. A high-aspect ratio board can provide the same lift with less drag than a comparable low-aspect board.

Can I pull my boards up to keep the boat from "tripping"?
Well, sort of. By pulling up the boards and decreasing their underwater area, you decrease the amount of lift they generate. You'll side slip more, and the boards may stall, increasing drag dramatically. You won't reduce the heeling force, which is caused by the sails. There are other and better ways to balance the boat - flatten the sail with the downhaul and outhaul, hike harder, travel out the main some - there are a lot of ways to do it. (And again, srm got it right)

Thanks for that! I think I really understand this now.

One question though - when under spin downwind it is common practice to lift the boards, is this really counter productive? creating drag while limiting heel?

I have found myself overpowered under spin many times and driving the power off requires some serious manuevering on the race course. This element of racing has always scared me, afraid I will run over someone trying to keep on my feet. Is there a way to depower the spin? My H18 traveler barely moves and releasing the main sheet is mast suicide. Other than driving it off how do I get the spin under control?

Really good thread!

Agreed and THANK YOU to many for sharing so much about daggarboards.

I think I'm starting to get it... a bit.

Maybe I should try and make a new set of H18 boards, but 4 or 6 inches longer, and maybe even with one side flat.

It'd be fun to see how it sails differently. They'd probably snap!

I could get a trashed board, cut it and see what's inside.

Wonder if old surfboard style w/ (maybe oak) wood strips, foam, and STRONG glass (kevlar?) would be strong enough to make the board a few inches longer w/one side flat, other curved.

When both boards are down, they canx eachother like the H14 or H16 hulls... but w/one board, maybe could get 'lifted' more....

Makes you (well me 'cause I'm still working this around) go 'humm.....'

My prediction...a lot of work for very little return.

You'd be much better off getting a used set of high-aspect daggerboards (from say a Hobie 20 or an F-18) and re-working the dagerboard trunk to accept the new boards. Extending a Hobie 18 daggerboard by 4 to 6 inches is not likely to have any noticable effect, you'd be starting off with a pretty inefficient shape right from the onset.



H18 daggerboards are simply a foam core with an external layer of fiberglass and gelcoat. The only reinforcement is the outside skin of fiberglass.

sm

This is not a true statement. I think you missed the part about "angle of attack."

Both boards see the same angle of attack and therefore create "lift" in the same direction.

Jim

If the wind is light we leave the boards down to get the hull up sooner. When it start to heat up we lift the boards to allow slip so we can drive the boat harder. Down wind slip is your friend and once up to speed the hulls will drive a good line so the boards can come up. Drive it up to get speed and some height in the hull then drive down to gain VMG. try to keep the hull up while driving as deep as you can with good speed.

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Rich Vilvens
F-18 5150
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http://www.sailblogs.com/member/f-185150sailing/