Centerboard / Daggerboard Shape

[Eric O]

I found this description of a LB/DB experiment in a book by Thomas Firth Jones that I thought was interesting.

After the first season, I cut Monkeys' bottom and built a daggerboard trunk. To make sure the experiment was equable (if you change more than one thing at a time, you don't know which change caused the improvement), I first tried the leeboard in the daggerboard trunk, with no change in the leeboard's shape or depth. The improvement was unbelievable. She steered much easier, and made much less leeway. I'm certain that no one who has ever tried a good sailboat, first with a leeboard and then with a daggerboard, would ever go back to a leeboard. There simply is no comparison.

Other sources talk about having something across the end of CB/DB or fin keel as an improvement as well suggesting that a bulb at the end of a DB may have more advantage than just putting the weight down low. Perhaps this is part of the idea behind wing keels.

[Eric O]

adm,

You seem to have a bit of technical knowledge on the subject of foil shapes. I don't. So maybe you can help me out a bit more.

- How much effect does thickness of a CB or DB have on performance? Is there an optimum ratio (length/width to thickness)? Would it be different for a foil shaped board vs. more of a flat plate with rounded edges?

- When you talked about leaving the edge blunt, not rounded, you were referring to the trailing edge only or both edges?

- What do you think of the Rhodes 22 approach of almost reversing the foil and putting the thin edge forward and the thick edge aft with their "diamond board"? The web page that describes the concept is here. http://www.rhodes22.com/keel_diamond_small.html

[adm]

Eric,

- How much effect does thickness of a CB or DB have on performance? Is there an optimum ratio (length/width to thickness)? Would it be different for a foil shaped board vs. more of a flat plate with rounded edges?

In general for laminar foils thicker ones have potential of generating more lift and stall at higher angles then thin ones On down side they generate more drag. Lift to drag ratio also depend on aspect ratio - where higher aspects ( long and narrow) are more efficient - and lateral shape of the appendage. For CB/DB you want to be on less drag side and use relatively thin profiles - no more then NACA0024. Keep in mind that there are many important factors involved. Let me explain. DB/CB must have some minimal thickness to be sufficiently stiff to resist bending forces. let say that 1 inch plywood is stiff enouch. Also you have limit how wide is can be )let say 12 inches. Those dimentions linits thickness and lengh of the foil chord. From this point you can calculate that you need to use NACA0016 as the foil shape. This foil will have maximum thickness only at 30 percent of the chord and the rest will be thinner so finished board may not be stiff enouch and you will have to go either with thicker profile and use some flat "extensions" as explained in mu previous foil or use thicker foil altogether. Without going to too much technical analysis I would recommend to use 0024 in this case and some flat sides and "fudge" pivot mout. Difference will be negligible for your application on Mac19. On competitive racer this is another story. In general there will be a lot of modeling and testing to get to optimal design. And key word is optimal - it all depends what are design limits and requirements. There is no one clear cut answer.
Zeno's Arrow center board design is not too good but as you can see the owner is very happy and a lot of people want to use it anyway. It shows that the longer board with higher aspect ratio and with improvment over the bottom part induced drag (small horizontal wing) - can perform better than original.

- When you talked about leaving the edge blunt, not rounded, you were referring to the trailing edge only or both edges?

Trailing edge is the one. It should have as thin as practical but sharp and square edges. It is slightly better that way then leaving them rounded and it is easy to achieve. Desidners make the foil chord slightly longer by few percent and cut it to lenght at the trailing end.

- What do you think of the Rhodes 22 approach of almost reversing the foil and putting the thin edge forward and the thick edge aft with their "diamond board"?

If you look at profile of wing of supersonic planes going faster then speed of sound it uses simmilar principles.
It have some merit for really high speeds but it is rather sales gimmick for me. I do not see any real advantage for average sailor. As you can see is not popular and there are reason for it. There are many very good foils performing better then diamond at low speeds.

[Don T]

Hello:
The boat has leeward movement through the water thus giving an angle of attack to the centerboard. Just like a wing this produces lift to windward and reduces the leeward movement. This is one of those balancing acts found in sailing. Designers try to balance the size/lift of the board, balast and sail area to give the best performance.

[Frank C]

- What do you think of the Rhodes 22 "diamond board"? ...

... merit for really high speeds but it is rather sales gimmick for me. I do not see any real advantage for average sailor.

Kaz,
After choosing the best length and shape for the Mac's centerboard performance, then it's useful to address the question of "trunk drag." I've never found "proof" of it, but the opinion is widespread that the Mac's centerboard trunk is to blame for a considerable portion of the Mac's poor sailing performance.

One of the biggest advantages to the diamond board is its primary emphasis on solving the problem of trunk-drag. The Gemini catamarans use the same design (pretty successfully, if I understand correctly).

[adm]

Frank,

Mac's long, relatively wide and deep slot is in probably in the worst possible location in respect to smooth flow of water over the bottom of the hull. The culprit lies in induced eddies.
You may try to close it by some sort of overlapping rubber strips which may still allow to raise and park CB.

As for diamod board solving the trunk drag. I not familiar how exactly those trunks are configured so I can only say if they are long slots (simmilar to Mac's) I need better explanation how it can be done. Peharps they can cover it in more intelligent way utilizing diamond foil shape. But shape itself is less efficient then rounded ones. It is actually quite bad for laminar flow.

[Eric O]

Frank,

If trunk drag is a significant performance factor, then a DB will have an advantage over a CB because the board fills the opening in a DB. The M should have less drag than the X and perform better as a consequence.

[Frank C]

If trunk drag is a significant performance factor, then a DB will have an advantage over a CB because the board fills the opening in a DB. The M should have less drag than the X and perform better as a consequence.

Do I recall correctly that this is precisely the 26M advantage that Roger trumpets over the 26X ???

And, I don't recall about the Rhodes, but the Gemini actually has Pie-shaped boards, and similarly shaped trunks, one each hull. When the CB is dropped, it's profile looks like a triangle, with upper segment still remaining inside to fill the trunk slot.

[Eric O]

It is not clear to me which would be of the greater benefit: a flat thin board with a narrow trunk opeing to minimize trunk drag at the expense of a less efficient board shape, or an air foil shape for better board performance but with a larger trunk opening for with more trunk drag. Anyone know the answer to this one?

[Frank C]

IMO, neither ... I'd choose the foil shape, PLUS use a vinyl or rubber gasket to close the opening as the board drops from the trunk. The gasket is adhered to the flat hull bottom, covering the slot, then is split lengthwise so the board can move past. If the gasket is too thick (1/4" ?) it will impede deploying & stowing the board, but if too flexible (1/8" ?) it won't be effective.

Another idea was cleverly depicted by some member here, using an animated gif. His idea was a hard shutter that would be pulled down to cover the slot as the board rotates downwards. Unfortunately, since the trunk narrows considerably up top (where a shutter needs to reside) it would be problematic to create a shutter that can fill the slot. I think I'd toy with this concept ...

Adapt the "giffed" idea as a length of PVC pipe that rests above the board, and is "pulled down" to the trunk slot as the board is lowered. Also use a fairly thin, (1/8") vinyl gasket. The PVC pipe is drawn down to rest above the gasket, reinforcing the two halves so they can better serve as barrier to water churning. The PVC need not be as long as the aft-slot, maybe only half that length (Note: we're just trying just to fair the bottom & reduce turbulence, NOT trying to exclude water from the trunk)

BTW, I have the 1/8" vinyl sitting on the shelf, waiting for that "future day" when I R&R the centerboard. (The stuff comes on big rolls about 12" wide, and is used as "refrigerator doors.") Not quite sure how to implement the PVC pipe, but I'll prolly just go with some 3M 4200 to adhere the gasket, for starters.

[baldbaby2000]

Is the daggerboard on the M supposed to stay in line with the boat or is it designed to shift it's angle to windward slightly (gybing)? I wondering about the effect of adding rubber bumper material on the angle of the board.

BB

[Moe]

If the upper part of the daggerboard is like the lower part, i.e. fatter forward, that will become the point of pivot, and the board will gybe backwards, "weathervaning" and reducing the angle of attack, like the X's centerboard, rather than increasing it like a proper gybing board.

[baldbaby2000]

If the upper part of the daggerboard is like the lower part, i.e. fatter forward, that will become the point of pivot, and the board will gybe backwards, "weathervaning" and reducing the angle of attack, like the X's centerboard, rather than increasing it like a proper gybing board.

Bummer. I see what you mean. I wonder if the upper part of the board can be reshaped.

BB

[Moe]

I would hope that the M's daggerboard profile at the top would be the same both fore and aft as the trunk dimension OR if the daggerboard profile is the same top to bottom, that the trunk would also be shaped in that profile to prevent backwards gybing.

One of the things that would concern me with improving the lift of the board would be its increased contribution to heeling. And with increased heeling and thus angle of the board, would come reduced effectiveness. Which in turn would reduce its contribution to heeling until a balance, probably at a greater heel, would be established.

While adding ballast to the board might help offset the increased heeling, aren't we already at the point that we consider heeling excessive and are looking for weight to decrease that?

[baldbaby2000]

I thought the effectiveness of the gybing board came from the fact that for a given board angle of attack, the boat hull would be travelling straight instead of crabbing slightly. Maybe I'm missing something but I don't see that increasing heel, just decreasing drag.

Having said that I probably won't mess with the gybing effect because I don't really know much about it. I am still interested in adding weight but not quite as much as before after considering other options. The order of my priorities now are:

1) Add a Cunningham. I've noticed how much easier I can control the sail shape when reefed. Probably because the luff rope can slide as would be the case with a tight Cunningham.

2) Add running backstays. Should let me get a little mast bend and keep the headsail luff from sagging.

3) Add weight to the board. Will probably wait for more feedback.

BB