Increasing Centerboard lift

Is it likely that lift on my centerboard could be improved by increasing the clearance between the pivot hole in the board and it's supporting bolt. I believe there is a stall angle after which improvements will go downhill rapidly.

I am thinking of inserting a stainless sleeve pressed and bonded into the C.B. The sleeve, prior to assembly could be machined on a lathe with bellmouthing at either end. The bore could be opened out progressively to provide increasing angles of attack.

 

I'd think a rubber bushing would be easier. It wouldn't wear and would be cheap to replace, and it would return to neutral by itself..


Alan

 

What you're trying to achieve is a jibing centerboard. It may have an effect on performance, but not for the reason you think.

Allowing the board to rotate its leading edge to windward does not increase the lift on the board. That's because the lift on the board is determined by the sail trim, not the design of the board. If you think about the lift on the board when sailing dead downwind vs upwind, I think you'll see this is true. The angle of attack of the board will adjust until the lift just balances the loading from the sails.

Instead of increasing the lift, what will happen when you rotate your board to weather is the bow of the boat will end up rotated to leeward. The board will still be going through the water at the same angle, but the hull will be pointed more into the direction of travel, so the leeway angle measured relative to the hull will be reduced. The boat will turn through a larger angle from one tack to another.

What may be affected more than the board is the sail trim. You've effectively moved the forestay to leeward. This can be especially useful if you have a very narrow sheeting base and flat sails. Rotating the board will actually increase the angle of attack of the sails.

 

If adjusting the angle of attack on the boards is inneffective, why do they have 'adjustable' boards on some ocean racers ?

The concept has even been patented

http://www.wipo.int/pctdb/en/wo.jsp?wo=1989003339&IA=WO1989003339&DISPLAY=DESC

On my current project, I am going to have the leading edge sit snuggly in a rounded front part of the cb case, and have cams adjust the trailing edge up to 6 degrees either way.

 

This is another issue. What Tom said (I think), is that given a particular centerboard, jibing it will not increase its lift. It is still the same board and will only work with the same effect, whether jibed or not. If you think of only the sails and the keel while ignoring the hull, this becomes clear.

What you are suggesting is increasing it's coefficient of lift with trim tabs or other foil adjusting devices. That may allow reducing the area and drag which may result in more speed. Two different issues.

 

I didn't say it would be ineffective. Just that it doesn't act in the manner one would initially expect. There are two main ways that a jibing board could be beneficial.

By rotating the bow to leeward to point into the direction of travel, you're reducing the leeway angle of the hull. This can reduce the drag of the hull in a couple of ways. One is by reducing the form drag. The other is by optimizing the span loading between the hull and board to reduce induced drag.

I've already mentioned the other way jibing the board can help, and that's by opening up the foretriangle, in much the same manner as with a ballestron boom (think Aerorig). The main can be presented at any angle to the wind by adjusting the sheet and vang, but presenting the foretriangle at a bigger angle to the apparent wind requires rotating the hull. The jibing board does that.

Six degrees sounds about the right range, to me. If you could measure the leeway angle when sailing with the board fixed, that might give you some idea as to what angle to shoot for.

 

The patent depicts a Laser with a modified daggerboard. I think that would be an outstanding way to demonstrate the effects of a jibing board. You can compare two Lasers side by side, one with the jibing board, and one with a fixed board. Going upwind, you'll be able to compare the heading of one boat relative to the other when the board is deflected.

This is a question that comes up a lot on this forum, and it would be really cool to have some photos from a chase boat showing the difference. I think it would help a lot of people understand what's going on if they could see something concrete like a side-by-side comparison.

 

I am confused, I thought the concept being talked about was adjusting the CB angle to the centreline of the boat.

Whether you rotated it from the centre, or from the leading edge, surely the effect is the same.

Maybe someone can draw me a picture or somefing'

 

No, it's the coefficient of lift that stays the same!

Rotating the board is not quite the same as using a trim tab. The tab changes the zero-lift angle of attack, which is much like rotating the board. In both cases, the angle of attack will be the same when measured from the zero-lift line. The chord of the rotated, symmetrical board will be angled to windward and the zero lift line will coincide with the chord. For the tabbed board, the chord will be aligned with the centerline of the boat, but the zero-lift line will be angled to windward.

The difference between the tab and physical rotation has to do with the shape of the board's drag polar. The symmetrical board's drag polar, and most especially any laminar low-drag "bucket", will be centered on zero lift. Going to windward, the lift coefficient will most likely lie outside the drag bucket unless the bucket is unusually wide and shallow, or the board is oversized. The drag bucket moves to higher lift coefficients as the tab is deflfected, so it's possible to center the drag bucket about the operating lift coefficient. This makes using a tab/flap more effective than simply rotating the board.

 

We're talking about the same thing - rotation of the board about its spanwise axis.

The difference between rotating about the leading edge, quarter-chord, mid-chord, or trailing edge has to do with the moments about the pivot axis. The moment about the leading edge is stable, so you have apply a continuous torque to the board to deflect it away from feathering into the the direction of travel. Pivoting about the quarter-chord will result in very low moments about the pivot and it will be very easy to put the board at any angle.

Pivoting about mid chord is unstable - the board will tend to increase the angle by itself. This has the advantage that the board will tack itself. As soon as the load reverses sign on the board, it will flip to the opposite stop. The sailor doesn't have to do anything. Pivoting about the trailing edge is even more unstable, and will do the same thing.

 

Yes, I understand your points there. I anticipated that effect caused by different axis', and I also thought that adjustment by moving the trailing edge was the lesser of two weevils. It might require more effort to re-position the trailing edge, but it means that the centreboard case only has to be enlarged at the rear, and not at the front of the case as required by a more central pivoting axis.

My next major think is how to design a 'cover plate' that will cope with any angle of the board, and still prevent cavitation or turbulence.

The Tornadoes have some flexible rubber 'skirts', but these can often get jammed in the centrecase after a while, and really are not totally satisfactory from a streamline point of view.

One crazy scheme involves having an inflatable bags either side of the traiing edge (green) that not only position the c'board (brown) when inflated, but also close the gap in the cb case (black). The downside might be having to have a source of compressed air on board, or a foot pump perhaps?

 

centre board

When I built my last model sailing barge I took the oportunity to fit a steerable fin. I know that real sailing barges do not have fins but models to 1/24 scale need about 55 square inches and a keel weight of about 9 pounds.

The fin on my model moves through 6 degrees in response to a three position switch on the transmitter. When beating in a steady wind it is easy to see the change in course to go closer to the wind when the fin is set to windward.

One day there will be a crew member on big monohulls who operates a joystick to swing the keel to windward to keep the hull upright and twists it to keep the hull in line with the course.

In passing the model has a mizzen that functions normally as part of the whole rig but can be moved independently if required. It is very useful to balance the rig and unload the rudder.

 

I doubt this.

Are you saying the VMG is higher with the fin set to positive AoA in relation to the boat? That the actual course over ground is closer to the wind?

The lift from the fin = side force from the sails.
The lift from the fin is proportional to the angle of the fin to the course through the water.
When you change the angle of the foil to the boat, you do not change the course of the boat, you only change the heading. The lift requirement of the foil does not change, and the course is dependent on the required AoA of the foil, how can the course change? The difference in angle between course and heading is the 'leeway angle' or more correctly the foil attack angle.

There is some reduction in drag, by making the boat's heading equal to the course. However this reduction is very small, the form drag of the hull is something like 3-5% of the total hydro drag, the induced drag from the foils is 97-95%.

Reducing the angle of the hull to the course, does not reduce the hull drag to zero, it only reduces the hull drag caused by the leeway angle to zero. How much hull drag is due to leeway? 20%? 33%? 5%?

The net gain from canting the foil to the hull then is some percentage of the 3-5% hull's total drag. Less than 1%.

A 1% change in hull drag, is not going to change the course the boat sails enough to be "easy to see", what is "easy to see" is the hulls heading change when the foil angle to the hull centreline is changed. It will look like the boat is heading farther off the wind with the foil set off centre to windward, it will look like the boat is pointing higher with the foil set off centre to leeward. The boat's course has not changed, only the heading has changed.

The attached diagram shows some of the forces involved. The large blue arrow is the course sailed. The centre hull of the boat shows the heading. If you think of the centre hull as the foil, you can see that changing the angle of the foil to the centre line of the boat has no effect on the course.

The white arrows are the total of aero and hydro forces. They are equal and opposite.

Note that the true wind is not shown. The boat has no idea where the true wind is. The boat sails in apparent wind. On a fast boat that can sail faster than the true wind, the apparent wind is always forward of the beam. Any time the AW is forward the foils must lift and there will be a 'leeway angle'.

For a course change to be "easy to see", the hull drag would have to *very* high at the leeway angle. Heading changes are definitely easy to see.

 

What if any are the benefits to installing 'a more efficient' set of sails without looking at a 'new and improved' replacement for the stock CB.....?

 

If the board is fair ... smoother than a baby's butt ... and the hull is clean and fair.

New sails, proper rig tune and time in the boat will yield big bangs for the bucks.

You didn't say what boat this is for. But if it lives in the water, cleaning the bottom and fairing the foils will have a great effect, possibly as great as new sails if the sails you have are only a couple of years old. (assuming the sails were fast when new)

If this is your Mac 26, and the sails are the ones that came with the boat, there is great room for improvement.