Twin Asymmetrical daggerboards

[mastreb]

Because I likely need a heavier daggerboard to counter aloft weight with a new rig I'm planning, I've been thinking about how to apply better hydrodynamics to the daggerboard to reduce heeling. The current daggerboard is symetrical so it works the same on both tacks: Either tack, it creates lateral drag that reduces leeward slip but is aerodynamic when moving forward so presents the lowest possible resistance to forward motion. It's shape is designed to reduce forward drag, but not to create lateral lift (it does create some lift anyway as any fin in a fluid will when a fluid moves across it at any direction other than straight on).

But if we could add a significant amount of "lift", which is generated side force on a daggerboard, it could significantly counter heel, especially as it got speed. The reason the current daggerboard isn't shaped like an airplane wing section is because you'd get lift resisting heel on one tack, but lift increasing heel on the other.

So it occurs to me that one could put two daggerboards in the same trunk, running the hoist lines to opposite sides of the cabin, and make each daggerboard shaped specifically to generate the correct anti-heeling lift for that side tack. Of course they'd both have to be thinner than the current daggerboard to fit in the trunk.

The effect would be that you have to hoist and drop daggerboards when you tack tack as well as moving the jib around. It does create the annoyance (but not safety) issue that if you fail to do this, the boat will heel over much faster and farther when the wrong daggerboard is in the water (although it will never generate enough force to overcome the boat's natural resistance to heel).

My "back of the napkin" math indicates that you could reduce heel by a solid ten degrees at six knots, which would improve both light air performance and pointing. Furthermore, it would dramatically reduce (perhaps eliminate) weather helm and the tendency to round-up in all conditions. It will not prevent true "blow down" round-ups, but it will resist them much more than the current daggerboard does.

It's my belief that the very easy round-up on the Mac is actually encouraged by the current daggerboard when a weather helm turns it such that it goes hydrodynamic on the wrong side and increases heel. You can kind of feel the boat rotating on it when it suddenly rounds up.

Anyway, not sure whether I'd bother actually doing it, but since I'm apparently the kook in residence, I thought I'd bring it up for discussion.

[Judy B]

You're barking up the wrong tree tree, by several orders of magnitude.

Righting moment is what keeps the boat upright. Righting moment can not be increased to any significant degree by improving the lift/drag of underwater foils. No matter how efficicent you make the foils, you wan't get any measurable improvement in heeling.

Righting moment is created by the horizontal distance between the boats center of bouyancy (where the upward forces push on the hull) and the boat's center of gravity, where gravity acts on the whole boat.

Don't add weight aloft if you can help it. For every pound you add aloft, you'll need to add several pounds of ballast to the boat.

Quote:
"It's my belief that the very easy round-up on the Mac is actually encouraged by the current daggerboard when a weather helm turns it such that it goes hydrodynamic on the wrong side and increases heel. You can kind of feel the boat rotating on it when it suddenly rounds up."
End Quote.

This is not correct. The suddeness of the roundup is caused by stalling the rudder -- caused by too great an angle of attack or possibly cavitation. The boat rounds up suddenly because the underwater CLR suddenly moved forward.

May I suggest you read up on the physics of sailing from a knowlegable source? You're obviously either talented or well-educated in mathematics and physics of some sort so you ccould understand it. I recommend The Symmetry of Sailing by Ross Garrett.

Fair winds,
Judy B

[Crikey]

Judy, I would agree wholeheartedly with you, except for the fact that what is being mentioned here is a keel that delivers symmetrical lift as well. Your math is for a non reactive element - I think. Adding some mass to its section, such as a bulb, is often used to increase the righting moment - but that's not Maclike.
Matt describes something here like a swiss-army macgregor. From my experience though, trying to even budge the daggerboard, at speed, becomes near impossible. Trying to find that crossover second, during a tack, would be pretty hard to do at the best of times. Which leaves, jack-screws or hydraulics.
Does anyone know how much slop there is in the system? My suspicion is that there's a lot - and controlling that, I think, could prove very beneficial. I hope to dig a pit in the yard, come spring, and check things out.
Ross

[Judy B]

Judy, I would agree wholeheartedly with you, except for the fact that what is being mentioned here is a keel that delivers symmetrical lift as well. Your math is for a non reactive element - I think. Adding some mass to its section, such as a bulb, is often used to increase the righting moment - but that's not Maclike.
Matt describes something here like a swiss-army macgregor. From my experience though, trying to even budge the daggerboard, at speed, becomes near impossible. Trying to find that crossover second, during a tack, would be pretty hard to do at the best of times. Which leaves, jack-screws or hydraulics.
Does anyone know how much slop there is in the system? My suspicion is that there's a lot - and controlling that, I think, could prove very beneficial. I hope to dig a pit in the yard, come spring, and check things out.
Ross

When you show me the physics, I'll believe an asymmetric keel foils on keels would significantly reduce heel. Let's see the equations.

I suggest that the most reliable way to reduce heel is to reduce the power of the sails so you're not overpowering the ballast.

OTOH, if you're just having fun being a crackpot, don't let me spoil your fun!

Fair winds,
Judy

[Crikey]

OTOH, if you're just having fun being a crackpot, don't let me spoil your fun!
Fair winds,
Judy

DOH!!

[Johnacuda]

Slightly off-topic, but these are the type of threads I love to read. They really get you thinking about what might be possible. I don't have a napkin to do the math on right now, and this idea would render the inderior completely unusable, but how abou this?

If a starboard and port daggerboard trunk with asymetric daggerboards were installed, would the weight of the lowered daggerboard increase the righting moment, or be offset by the raised leeward daggerboard?

Thanks for the book suggestion Judy, I think that would be right up my alley.

[seahouse]

Hey John –

I assume you mean the lowered (not raised) leeward daggerboard, right? To answer that accurately you would have to have a properly dimensioned drawing of the positions, size, and weight of the daggerboards in their various postions, so you would end up with a sequence of stability curves for each, like the one in Judy’s post, above.

Industrial design is my vocational area of expertise, and I also find that a bit of background study to get a grasp on already-known basics can go a long way to saving time that might be spent “reinventing the wheel”. (I am in no way discouraging any creative thinking, of course- that’s how the wheel got invented in the first place).

There are actually a number of excellent technical books on sailing physics (great reading), I’ve not encountered a reference to one yet that was not available to anyone interested on Amazon.com.

Of course, there are always the “unknowable” things that can only be found out by a proof of concept execution of a new design.

- Brian.

[mastreb]

Hi Judy,

Thanks for the reading recommendation--I'm off to find it now.

Righting moment is the dominant force that keeps a boat from heeling of course. But it needn't be the only force. If, for example, I mount a small leeward facing rocket atop the mast and light it off, it will with certainty reduce heel. So we can now add rocket assisted righting moment to our list of forces impacting the boat. Of course that's so impractical as to to be pointless other than as an extreme example.

Its not really a question of whether or not an assymetrical foil designed to maximize lateral lift to oppose heel would work to some degree--it will certainly generate some amount of counter force. The math is actually not that hard so I'll do it and post it in a follow up. The question is whether or not the amount of countering force it would generate would be enough to even notice, and then enough to be worth the hassle of swapping foils on a tack, is the question.

Given Ross's reminder that hauling up the daggerboard at speed is nearly impossible, as is getting it to drop at speed, and we have the answer: it's impractical even if there is enough force to bother with and even if you could get both daggerboards in the same trunk.

And so we're on to the next kooky possibility: A hinged daggerboard with a trailing edge flap and a "tiller" coming up out of the trunk that will allow you to make assymetrical lift to any degree you want.

Matt

[seahouse]

Hey Mastreb (and Crikey)-

I don’t think the friction problem with raising and lowering the DB is technically insurmountable. Rollers located along the lower inside of the trunk wall would reduce that friction considerably.

The friction under load on the stock arrangement is considerable because the DB presses against the side of the trunk, and both surfaces are rough, and because the large amount of clearance (I reduced that somewhat on my boat when it arrived) in all directions makes for a leveraged wedging action when the DB is more fully extended.

I think the main boondoggle is going to be making sure that changes in the daggerboard angle don’t significantly increase drag, which would counter your goal. Given that any more than 3 degrees (from longitudinal axis) on the rudder is bad, I would guess that the DB angle would need to be kept less than this too, and to work in synchrony with it.

-Brian.

[Judy B]

A keel with a flap isn't kooky. If I recall something I read years ago, it has been done before and it did indeed increase lift and improve pointing by measurably reducing leeway. I'm sure you could do it, given enough money to invest or enough DIY engineering/fabrication skills.

I"m not sure why it hasn't become popular on expensive boats, but I'd guess perhaps it wasn't cost effective compared to other solution or it's hard to built it so it's reliable.

.... I don't recall reading that keel with a flap contributed noticably to reducing heeling...

Judy

[mastreb]

A keel with a flap isn't kooky. If I recall something I read years ago, it has been done before and it did indeed increase lift and improve pointing by measurably reducing leeway. I'm sure you could do it, given enough money to invest or enough DIY engineering/fabrication skills.

I"m not sure why it hasn't become popular on expensive boats, but I'd guess perhaps it wasn't cost effective compared to other solution or it's hard to built it so it's reliable.

.... I don't recall reading that keel with a flap contributed noticably to reducing heeling...

Judy

Hi Judy,

So this twin daggerboard concept would have exactly the same effect as a dagger with a flap, except that the flap position is fixed, and rather than moving it side-to-side on a tack, you'd switch boards.

It's very possible that the reduction in heel is so slight as to be unnoticeable--I haven't yet done the force calculations, and as anyone who works in aerodynamics will tell you, nothing about how a foil will perform is obvious.

It's also very difficult to compare heel reduction when testing, because you don't know how the boat was going to heel otherwise. Slight improvements would definitely be unnoticeable.

Anyway, as you have correctly surmised, my ideas come from thought experiments that lack the grounding of experience, so your comments and wisdom are always truly appreciated.

You should look over my thread on the wing sail rig, because I'm going to ask you where to buy bulk Dacron to make that wing.

Matt

[mastreb]

Brian--perhaps spraying down the daggerboard and trunk with "liquid rollers" would to the job

Kidding aside, just greasing the trunk would probably handle it.

[Crikey]

Here's another take on some of these aspects:
Whatever dagger-board gets used, it has to be oriented underwater no different than a hydrofoil providing lift for a racing boat, just inverted.
Using a jack-screw type mechanism on the blade(s) could be made no more difficult than furling a Genoa.
Augumenting any kind of foil with a flap is going to be very challenging, particularly if it needs to be expendable, in an emergency.
Any deployed system must have to be constantly under control. As a foil (and sorry, I don't do Reynolds numbers) it would be generating a very substantial amount of force. If this was not countered, or reversed during a tack, the boat would capsize.
It's possible, due to the push of the water against the existing ' sloppy' dagger-board, that performance is affected negatively.

What if the stock DB could be oriented sufficiently, at any point of its travel?
Similarly, what if the DB was somehow seized at this point so that it didn't wobble inside the slot?
As Judy says, that would be one upright and fast puppy.

Ross

[Calin]

A keel with a flap isn't kooky. If I recall something I read years ago, it has been done before and it did indeed increase lift and improve pointing by measurably reducing leeway. I'm sure you could do it, given enough money to invest or enough DIY engineering/fabrication skills.

I"m not sure why it hasn't become popular on expensive boats, but I'd guess perhaps it wasn't cost effective compared to other solution or it's hard to built it so it's reliable.

.... I don't recall reading that keel with a flap contributed noticably to reducing heeling...

Judy

I assume the reason why it has not been popular is that there are major disadvantages that come with it. If the flap is adjusted to reduce the leeway it will increase heeling and if adjusted to decrease heeling it will increase the leeway.May be a good option on cat where the design is more heeling resilient but on our tender MACs I do not see a benefit.

[Judy B]

America's Cup 1982 - I think two boats had keels wit flap for negative leeway and pointed like crazy. It was in San Diego, where light winds prevail. the engineering is intimidating, due to the large forces on the flap.

But again, with wasn't for control of heeling, it was for better pointing.