Sideways Force on a Centreboard/Daggerboard

[BlunderBus]

Hi Everyone,

I have a question...
Can I assume that the "righting moment" of my crew and I hiking out is the same righting moment that I need to produce with the centreboard/daggerboard to prevent leeway?

If it's not equivalent to the force being produced by the sails sideways then how shall I go about the calculation? Is it the overall force that is produced by the sails that I need to match with the centerboard to counter the leeway?

I'm trying to figure out how much area I need for a centreboard for my Javelin (14" skiff) as I'm keen to build a new one which has better performance that the current narrow blankie looking one.

Any suggestions would be greatly appreciated.
Thanks,
Hamish

 

[Petros]

that is not what determines the size of the dagger board. A large dagger board can generate the same amount of sideways force as a small one, it would just have a different angle of attack for any given speed. The problem comes in really light air when there is very little movement, a small one will not generate enough force to keep it from wandering around. A large one, that does work well in light air, would have a lot of drag at higher speeds because of the higher wetted area and thickness.

so it is always a trade off between light air performance and pointing ability, and high speed drag.

High aspect ratio foils are more efficient (lower lift to drag ratio), but are easy to stall, low aspect ratio more stall resistant at low speeds and in turbulent conditions.

that is why in dingy design they generally give a range of about 4 to 5 percent of the sail air. this is just a general rule, ideally you would have a small high aspect ratio dagger board for when conditions are fast, and a larger one for light air. Most class rules do not allow for such changes, so if you do this it would be for your own enjoyment.

The smaller the better for lower drag, but it will do you little good when just ghosting along and it can not generate enough side force to keep you from drifting with the wisps of air.

Pick yours size and take your chances.

 

[DCockey]

Hi Everyone,

I have a question...
Can I assume that the "righting moment" of my crew and I hiking out is the same righting moment that I need to produce with the centreboard/daggerboard to prevent leeway?

If it's not equivalent to the force being produced by the sails sideways then how shall I go about the calculation? Is it the overall force that is produced by the sails that I need to match with the centerboard to counter the leeway?

I'm trying to figure out how much area I need for a centreboard for my Javelin (14" skiff) as I'm keen to build a new one which has better performance that the current narrow blankie looking one.

Any suggestions would be greatly appreciated.
Thanks,
Hamish

When a boat is sailing at a constant speed and direction the transverse force on the daggerboard is equal in magnitude and opposite in direction to the transverse force on the sail (plus the aerodynamic transverse forces on the hull, etc and less any transverse hydrodynamic force on the hull). It is not directly related to the righting moment contribution from a trapeze, etc. The trapeze does let you keep the boat upright with a larger transverse force from the sail.

Edit: See Mikko's response below, post #8, for how the righting moment of the crew directly relates to the transverse force on the board as long as the boat is sailed upright.

 

[BlunderBus]

Thanks Petros.

With the Javelin Class we are allowed a board up to 1.667mm long (that includes the bit in the centrecase.

A lot of the guys lift their boards say 100mm in strong winds to cut the drag down.

I'll calculate the sail area and see what I come up with.

The design mentioned in this article about a board for a B14, where lift and drag were optimized. As with all sail racing it's the attention to detail (and not making as many mistakes) that leads to race wins. So I figured I start with knowing the numbers I need - how much lift force do I need to produce?
For a set speed does this amount of force determine the angle of attack of the board?

I know so far...
Righting Moment
Skipper Gravity : 9.81
Weight (Kg) Hiking Distance (m) Righting Moment (Nm)
65 1.06 678
Crew
Weight (Kg) Hiking Distance (m) Righting Moment (Nm)
95 1.94 1811

Total : 2489

Min ~ 1800
Probable ~ 2500
Max ~ 2800

 

[BlunderBus]

Thanks David,
If the boat is traveling at a constant speed, could it be that the transverse force (sideways) of the sail is more than the opposing transverse force (sideways) of the board couldn't it, if the boards didn't produce enough lift because it wasn't big enough?
And this would result in a heading that is not as close to where the boat is pointing? More Leeway / greater angle of attack?
Have I got that right?

How would I go about figuring out how much force the sails are producing in the transversely?
Is it the sail area * Lift coefficient of it's profile? Or do I need to break this down into sections as the chord is different/varies over the height of the sail?

 

[DCockey]

Thanks David,
If the boat is traveling at a constant speed, could it be that the transverse force (sideways) of the sail is more than the opposing transverse force (sideways) of the board couldn't it, if the boards didn't produce enough lift because it wasn't big enough?
And this would result in a heading that is not as close to where the boat is pointing? More Leeway / greater angle of attack?
Have I got that right?

If the forces are not balanced, ie the sum of the forces does not equal zero, then the boat will be accelerating.
F = m * a

If the transverse force of the sail is larger than that of the board, the leeway will increase which results in the angle of attack of the board increasing which in turn results in the board generating larger transverse force. When the forces are balanced the leeway will be constant.

How would I go about figuring out how much force the sails are producing in the transversely?
Is it the sail area * Lift coefficient of it's profile? Or do I need to break this down into sections as the chord is different/varies over the height of the sail?

Far from a simple calculation. You can write various equations with coefficients but the problem is deciding what value to use for the coefficients.

 

[SukiSolo]

Here in the UK, the Merlin Rocket Class (14') has a 1350mm depth limit under the keel and almost all new boats go to this limit. Same with National 12s which have a 1200mm limit. So the trend has been to high aspect ratio foils. As Petros has said it is quite tricky to get the ratio perfect because in a dinghy the demands change so fast in a very short time.

In my personal opinion, I would aim for a deep but moderate width foil which will work when raised. In heavy air and dagger board boats you can halve the area and be fast. A symptom of too little area is very light air up to top end Force 2 where the boat points fine it just makes light leeway. As little as less than 100sq mm extra area can cure this depending on the design. Been there and cured it .

There are boats out there with foils that are too narrow, you can see them cavitate and stall far too early. At the scale of a Javelin I would guesstimate that a c/board would need to be anywhere between 26 and 30mm thick for real world use. This assumes glass/epoxy sheathing and a light timber (WR Cedar) core perhaps with a bit of spruce down the centre chord. Rudder thickness 24-28mm with similar construction but thicker if really long, but this is based on my experience in 12-14' classes and building a few foils + what I have seen work and fail. Stiffness by the cube so a couple of mm thicker is significant. Don't hold me to those sizes on the Javelin but in my experience that should be a workable range, see what the quick guys are using.

Do not forget if you rake a board that the chord shape changes - it moves aft. Again a personal thing, but I've ended up making boards for slightly slower craft than the Javelin somewhere along the 6000 laminar flow series NASA profiles. I have experimented with max chord from leading edge as well as leading edge profile and have found my prefered position(s). It is a trade off too, some shapes are better from stall and others a bit better at higher speed. At the scale of dinghies, if you race against the same guys week in week out you can empirically test your solution. That is real world results not theory, and I changed the chord position several times before finding a preference.
I am currently over 4% different to one of the class I sail in's leading makers but they work fine. The foils alone when I tested them made the difference of several boat lengths over a 800m beat.

This post may not help your calculations but hopefully it helps you think about things, that will give you good results. Oh and BTW don't forget to taper your foils each side so they flex a little like a mast especially the board. Rudder should stay stiff especially near the hull.

 

[Mikko Brummer]

Hi Everyone,

I have a question...
Can I assume that the "righting moment" of my crew and I hiking out is the same righting moment that I need to produce with the centreboard/daggerboard to prevent leeway?

Hamish

Yes you may. We use this all the time to predict sail or board loading. See attached sketch. Also attached the definite guides to (505) board design, by Bransford Eck. Ancient, but still the best papers on the subject and quite valid.

 

[SukiSolo]

Nice one Mikko. Those papers confirm a lot of stuff I've picked up over the years. Like max board case (in some classes) certainly max forward and the leading edge far forward. Also thinning towards the tip as I advocate but keeping the chord % similar as it runs down the board. I've only sailed 505s' never built stuff for them so have very limited in depth knowledge of them.

Personally I don't like the 0009 series, it always seems to stall and not reattach fast in the real world. Also agree on thickness percentage, though I have leant to the thinner end recently. What is really encouraging is the test it and see attitude, ie measure or two boat test, put a load cell on it etc. Good stuff and just as valid today as when it was written. My own guess is that the '32 section or even fractionally further aft would be a good section today.

There is a lot of stuff about gaskets, fairing in and around as well so a complex subject. I hope to rig up some prototype fairing in at slot area stuff in the winter to learn a bit more about minimising drag there. To improve my far from sloppy efforts so far, even if I do say it myself.

 

[DCockey]

Yes you may. We use this all the time to predict sail or board loading. See attached sketch. Also attached the definite guides to (505) board design, by Bransford Eck. Ancient, but still the best papers on the subject and quite valid.

Mikko, thanks. In my reply above I hadn't thought about resolving the forces with the assumption the boat would stay upright so that the righting moment would equal the moment from the crew on the trapeze.

 

[BlunderBus]

Hey thanks guys, all the input is really good.
I'll mull on it a bit, but it looks like the best approach is to make an educated guess and try something out.

Thanks again.
Hamish

 

[SukiSolo]

Well done BlunderBus. The Bethwaite book High Performance Sailing gives a loose idea of what is possible. The 505 pdfs imply laminar flow is not possible but I think it is(within reason), and Bethwaite illustrates work which started at 50% laminar improved to 90%. Of course the speed the boat travels does matter but even something at 3-4kn should achieve it with a good section.

The old diagrams for the bucket window of laminar are correct in perfect conditions. The 3D vortices are harder to predict (thanks Mikko for the sail ones) but you will never eliminate them with 3D motion of the hull, only minimise.
Marchaj implies less laminar flow but does not use more modern chord shapes and I have yet to see a section with max thickness at 50% or further aft!.

Also of interest is the gybing board stuff. Do you use it in the Javelin? I can in the 'One Design' (haha) I sail and others I have sailed but have yet to see it work better in them. I have been toying with building an experimental gybing board but with a very small angle of attack. I understand it works in the Hornet class and with very wide angles up to 7°.

Note carefully Bethwaite was unable to measure leeway in his tests, he does not dismiss it, just reports a measurement 'difficulty'!. I tend to agree as most boats with good foils make almost no perceptible leeway. You do notice underboarded boats. An example would be the junior class the International Cadet. If you stuck a board in 200mm deeper it would be quicker upwind than all the others period.

If you can, check out what Phil Milanes has, as sections for fastish boats like the 505. Good guy with lots of experience and very helpful when I have repaired some of his foils which were later damaged by owners. They are very high quality and I needed to maintain that with the repairs.

If you go to Moth T foils there is a very good extract from full size tank testing in the US available on the web. Thanks to the guys who published that great work.