Twin Asymmetrical daggerboards

[Fred Rio]

Ok ... I am a little lost in this one ...

If I am on port tack the boat heels to starboard right?

So if I wanted the apply a force on the dagger board in order to straighten the boat I would apply the force on the port side of the dagger board right?

But if I designed a foil that would apply a force on the port side then I would be pushing the boat to starboard which is exactly the opposite of what we want isn't it??????

Did I miss something?

Fred

[Johnacuda]

OK, light-bulb moment, I'm starting to get this now. To reduce heel, and point better, we'd need to use a foil/flap to exert a little pressure and cause some slippage to leeward. Too much, and we'd end up with boats that erformed like drift cars.

[Ixneigh]

My instinct on this, with no particular qualifications except for decades on small sailing craft, says no. Dont waste your time with it.
Roger has fine tuned the powersailors to be all around great boats in a manageble length and beam. It has its limits. more sail up high on this particular hull is a case in diminishing returns. The only thing that will keep her upright is more ballast, more weight. Less speed under power. Less speed in light airs. I put 350 lbs of inside lead ballast in mine. And shes still tender, in the conditions I often sail in. and no fatser then a stock M. Just more comfortable.
my 2C is stick with the original sailplan, go for super good sails and rigging. This is coming from the originator of the Unicorn Lug rig. So I aint afraid to do crazy stuff.

In order for the daggerboard to have any of the desired anti-heeling effect, the curved surface would have to be to leeward. My instincts say this is just not going to be effective. Maybe the reverse, on a good heavy keel-centerboarder, emphasis on heavy. The daggerboard could drop down below the (veryvery heavy) deep fin keel for a bit more "lift" to wndward. Yes it would actually ADD to the heel, slightly, but the whole boat would be so heavy it wouldnt matter.
Ix

[Calin]

Ok ... I am a little lost in this one ...

If I am on port tack the boat heels to starboard right?

So if I wanted the apply a force on the dagger board in order to straighten the boat I would apply the force on the port side of the dagger board right?

But if I designed a foil that would apply a force on the port side then I would be pushing the boat to starboard which is exactly the opposite of what we want isn't it??????

Did I miss something?

Fred

You haven't missed anything. Your physics are right. Trying to reduce the heeling using dagger board foil would push the boat leeway. If one wants to have a lift on the dagger board pushing the boat leeway, it would increase heeling. In light wind, or in the case of a stable boat (cat) the heeling may be insignificant and the boat may point better. For our MACs a foil on the dagger board is likely not worthy.

This been said, I think the best benefit from sailing performance improvement, at least for me, is from improving my sails adjustment in all wind conditions and pointing (attached a link for a good advice I am trying to put in practice). There are enough dimensions to it already. Adding an additional one (adjusting the foil on the dagger board) would be overkill for me.
http://www.macgregorsailors.com/forum/v ... 72#p253395

[Crikey]

Ok ... I am a little lost in this one ...
If I am on port tack the boat heels to starboard right?
So if I wanted the apply a force on the dagger board in order to straighten the boat I would apply the force on the port side of the dagger board right?
But if I designed a foil that would apply a force on the port side then I would be pushing the boat to starboard which is exactly the opposite of what we want isn't it??????
Did I miss something?
Fred

You haven't missed anything. Your physics are right. Trying to reduce the heeling using dagger board foil would push the boat leeway. If one wants to have a lift on the dagger board pushing the boat leeway, it would increase heeling. In light wind, or in the case of a stable boat (cat) the heeling may be insignificant and the boat may point better. For our MACs a foil on the dagger board is likely not worthy.
This been said, I think the best benefit from sailing performance improvement, at least for me, is from improving my sails adjustment in all wind conditions and pointing (attached a link for a good advice I am trying to put in practice). There are enough dimensions to it already. Adding an additional one (adjusting the foil on the dagger board) would be overkill for me.

Calin, I'm not quite workin with this. Clearly, an extra righting force applied in (from) that direction is going to add to the existing leeway already in place. Like crabbing a plane; that can be adjusted for somewhat. The question is whether the gains obtained from righting the rig that drives the boat, exceeds the complexity required to achieve it. And it certainly does all add up!

The M is not supposed to have its dagger-board fully deployed and is installed with a rope limiter to indicate that point. Considering how much it moves around while retracted, I suspect it's not a glove fit when it's in the partially deployed position either. My hunch, which I can't test until spring, is that I can deflect it to at least 5 degrees of either side of straight down. Collectively, that would be equal to free movement of the mast and hull heel within half of the prime 20 degree sailing sweet spot. That is not dealt with on fixed keel boats.
Ross

[Highlander]

Ross

I agree the D/B can deflect 5-9 degree's "Canting" depending on it's length of deployment . The D/B only needs to be deployed anywhere from 1/4 to 2/3 for most sailing angles, directly down wind requires no deployment the D/B can be completely retracted , full D/B deployment is only required for pointing up-wind , although because on my cutter rig I can make exceptions & I really should record them for future & further recolection to see what r my sweet spots & mark the D/B line for these best sailing aproaches I used to have them in the back of my head somewhere & kept telling myself to record these measurements for future reference !! now I cannot remember ! , I really need a tactiction with me to record & calculate all this stuff while its all happening & going down LOL to get a true reading on all tactical adjustments & sailing conditions

J
PS U can always slide the D/B in the up-ward possition above the cabin roof sole to see how much it deflects "cants" from side to side for a rough comparison

[Crikey]

John, you may be right with the 'topside' obsevation, but the baby's buttomed up tighter than a fish's (deleted) right now so that will have to wait until the spring. If anyone else...? I also think the DB has a bulkier top, compared to the bottom - so would need to be reversed to indicate properly. Someone with a good pair of lungs, in warmer waters, could easily put this argument to rest.
Overall, any movement would probably be as detrimental as excessive slop in the rudders.

I've found it very hard to even raise the DB when you're running, or lower it while you're reaching. It will work if you swear hard enough! Got any tricks?
Ross

[raycarlson]

with a surface are of only 4-5 sq feet, and needing say 1000lbs of lifting force to reasonably resist the heeling force of a 32 ft mast you would need on such a thin air foil speeds approaching 2-3 hundred miles per hour to create your 1000 pounds of lifting force.I guess you need a bigger motor for that.you also would have to seriously strengthen your dagger board tunnel inside the cabin to take any serious increase in structural loading.sounds at least like a weekend project huh!!

[mastreb]

with a surface are of only 4-5 sq feet, and needing say 1000lbs of lifting force to reasonably resist the heeling force of a 32 ft mast you would need on such a thin air foil speeds approaching 2-3 hundred miles per hour to create your 1000 pounds of lifting force.I guess you need a bigger motor for that.you also would have to seriously strengthen your dagger board tunnel inside the cabin to take any serious increase in structural loading.sounds at least like a weekend project huh!!

The math:

1 sq. M of surface area
1025 density of seawater
6 knots
1.5 coefficient of lift

=1555 lbs. force of side thrust. The daggerboard could potentially generate more lift than the sails.

Density of seawater is so much higher than that of the atmosphere that even small foils generate extraordinary amounts of lift. That's why questions like these are hard to intuit. But the forces involved show that its a question worth asking.

Ross's question of whether or not rotating the board in the trunk to change the angle of attack and therefore generate lift is actually quite relevant. an 8 degree side-to-side lateral movement of the board could change the pointing characteristics of the boat dramatically. It should be investigated, and could be done with a simple long stick with a wedge end that you jam into the back of the trunk and change the position on tacks.

I can do this next time I'm out with my dinghy oar and test the results with my anemometer and inclinometer. Much easier than swimming under the boat

Matt

[Crikey]

Correct me if I'm wrong; The existing dagger-board is not shaped like a curved foil, it is completely symmetrical and designed to track straight. However, turned even slightly it would still generate a lifting factor. I found some interesting information from the manual -
(M) Draft: 12 inches, Draft: board down:5 feet, 9 inches
meaning the board is only extending 4ft, 9 inches! Really?
Matt, that would be less than half of a 1.5 coef (I'll try and work that out) and if you were to put a number on the energy generated by a deflected dagger-board would you come up with a livable amount, so long as you weren't stalling it? I think the 1000lbs Ray mentions is a considerable force - perhaps too large a handicap. Surely a foil of this type would generate so much lift, that if the boat suddenly went into any kind of lull, and still retained leeway, it would capsize towards upwind. Probably take no more than two boat lengths.
Strangely, there is a range of foil patterns, adopted by the P51 wing design in WWII for one, that resemble the MacGregor mold profile, that respond well, within small angles of attack. I actually only think a couple of hundred pounds of lift is going to be sufficient to work as a displaced lever, that far away from the center of buoyancy. Forget trim tabs - turn the dagger-board! One of these might work for gripping the trailing edge:https://www.spinlock.co.uk/en-US/hardwa ... uff-feeder
R.

[Crikey]

Much easier than swimming under the boat

[Hamin' X]

Do a Google on "gybing centerboard". The sideways pressure automatically changes their angle of attack. The X centerboard does this and I assumed that the M daggerboard did as well. This is the clunking sound at anchor that folks try to eliminate. Get rid of it an you are limiting "performance" quite a bit.

~Rich

[mastreb]

Ross,

That's all correct. I did a little more looking around and have decided that the NACA 0012 foil fits the shape of the daggerboard best. That foil with a reynolds number matching water at 6 knots yields a cL of just about 1.0 and a stall angle of 10 degrees.

My earlier estimate of 1 sq. m. of surface area is also high--0.5 sq. m is about right.

Given these more accurate numbers, the expected pound force on the dagger when rotated would be 777 lbs. Considerable, and more righting force than a weighted keel.

Matt

[mastreb]

Hi Rich,

Thanks for the pointer! So that does explain why rotating the entire foil won't really work: The foil generates so much lift that it actually rotates the entire boat so the foil can remain "straight" through the water.

Matt

[Ixneigh]

This boat was designed with computers like modern aircraft are. I have my doubts that any underwater tinkering is going to improve it. Even my set of skegs only improves things for my particular use and has slight detriment under wot power.
However, rigging the boat with simple lee boards will allow you to experiment all you want, cheaply. Make yourself a few foils out of plywood, glue on a support rail near the waterline with silicon glue, and lash the top of the leeboard to the deck.
I can all but promise you the foil curve to leeward flat surface to windward will barely sail. The flat surface to leeward curved surface to weather will improve pointing in light winds but will be insanIty in stronger winds.
That jibing centerboard bit is just us boatbuilders way of saying there needs to be some slop in the fit, else you ain't gonna be able to get the thing up. This was especially true of wood centerboards. Happy coincidence that it has marginal benefit. Roger could have stuck a huge motor on the 26 d and maybe had a better sailor but I wouldn't have bought one. I wanted the room. I did not want the pop top. I expect most people who buy these boats wanted that too.