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Tempest

Rebuilding a Centreboard: A question for the architects

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So my centreboard has finally given up the ghost.  I knew the day I would have to rebuild it was coming but I was hoping I could put the job off until next year.  Unfortunately the lift cable pulled out of the centreboard for this job has been pushed up in the queue. 

Here's a guys blog with an E25.  Pretty much the same boat but with a bigger CB.  He measured his CB and made a duplicate which is as close to the original as possible.

http://www.ericson25.com/2013/01/centerboard-construction-part-1.html?showComment=1532472780127#c1882537458077530083

So my question is: How close does a new board have to match the original and can I get away with updating to a more modern NACA profile?  If I do this, I may also want to slightly reshape my rudder to a NACA profile and I'm concerned that screwing with these things may upset the performance of my boat.  Am I making a mountain out of a mole hill or should I be concerned and try to stick to the factory profiles?

The reason I ask is because if I can redesign the CB with a NACA profile I can do the whole thing in CAD and I don't even need to get my hands dirty until it's time to install the board.

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Always better to use a newer, better shape as long as it will fit in the CB trunk.

Old time CB's were seldom much more than a flat plate with the edges rounded off

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In that application you will get no benefit from anything more modern than the 4 digit NACA profiles from the '30s. The nose shape (or the first say 15% of chord) is the only thing to really pay much attention to. That's for stall characteristics, not lift or drag. 

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4 hours ago, SloopJonB said:

Always better to use a newer, better shape as long as it will fit in the CB trunk.

Old time CB's were seldom much more than a flat plate with the edges rounded off

Yep, that is definitely part of the design criteria.  This one actually looks like it has some shape to it though and isn't just a flat plate.

2 hours ago, DDW said:

In that application you will get no benefit from anything more modern than the 4 digit NACA profiles from the '30s. The nose shape (or the first say 15% of chord) is the only thing to really pay much attention to. That's for stall characteristics, not lift or drag. 

Well a 4 digit NACA profile was what I was thinking.  I didn't know the NACA profile was that old.  Should I just use a profile that fits the max width & length (depth) of the trunk (probably 4"-5" wide and 8" deep)?  I would of course match the profile to the water direction; I think the CB hangs at around a 45° angle.

At this point I just don't want the profile to have any negative impact on the sailing performance/stability.  Any improvement in performance is gravy.

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Yeah, that'll work fine. The 4 digit series is from the early '30s, all they did was come up with a 4th order polynomial that was a good match for successful airfoils being used at the time. They wanted a mathematical description so that they could vary the thickness predictably for wind tunnel testing. Laminar flow came later, but isn't going to help on most cruisers (or even racers). The 4 digit symmetric profiles have good lift and stall characteristics, that's what you want. 

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The NACA airfoils were developed long ago, early in aviation development. Using them for boat foils came along much later but that doesn't make them "old" - they are timeless.

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For what it may be worth: consider the ballasting effects

Lioness has a centerboard that is cast bronze in a foil shape. 

about 5' tall by 15" section depth by maybe 3"  thick and weighs in at 900 lbs and takes a lead screw to hoist it. 

Natural corrosion/fouling resistance, though in 50 years the strap that hoisted it which was a different alloy corroded away requiring a replacement. 

Got sideways once across a rock in the Piscataqua near a dock during an ebb and bent it, yard removed it over the winter, built a fire under it and flattened with a forklift...

 

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On 7/25/2018 at 8:12 AM, DDW said:

Yeah, that'll work fine. The 4 digit series is from the early '30s, all they did was come up with a 4th order polynomial that was a good match for successful airfoils being used at the time. They wanted a mathematical description so that they could vary the thickness predictably for wind tunnel testing. Laminar flow came later, but isn't going to help on most cruisers (or even racers). The 4 digit symmetric profiles have good lift and stall characteristics, that's what you want. 

 

On 7/25/2018 at 8:56 AM, SloopJonB said:

The NACA airfoils were developed long ago, early in aviation development. Using them for boat foils came along much later but that doesn't make them "old" - they are timeless.

I guess my concern is that I'm going to change the position of my centre of lateral resistance by changing to foils that generate more lift.  Moving it forward will increase my weather helm and moving it back will increase my lee helm, right?  I doubt that improving the CB profile will change much but my rudder is pretty big.  The rudder is also the one I worry about as it could potentially move the CLR rear because it's a large surface area and it has leverage on it's side.... but perhaps I'm making a mountain out of a mole hill...

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57 minutes ago, Tempest said:

 

I guess my concern is that I'm going to change the position of my centre of lateral resistance by changing to foils that generate more lift.  Moving it forward will increase my weather helm and moving it back will increase my lee helm, right?  I doubt that improving the CB profile will change much but my rudder is pretty big.  The rudder is also the one I worry about as it could potentially move the CLR rear because it's a large surface area and it has leverage on it's side.... but perhaps I'm making a mountain out of a mole hill...

If your centerboard pivots, then you can swing it back to cut weather helm. 

If your rudder generates more lift, you should be able to use smaller deflection angle and hence less drag to achieve same stable course. 

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1 hour ago, LionessRacing said:

If your centerboard pivots, then you can swing it back to cut weather helm. 

If your rudder generates more lift, you should be able to use smaller deflection angle and hence less drag to achieve same stable course. 

I'm not concerned with cutting weather helm I'm worried about increasing lee helm.

But otherwise, those seem like valid points.

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In classical aerodynamics, the aerodynamic center can be taken as 25% of the chord. This is true for everything from a flat plate to a modern low drag section (within a small error margin). So unless you are changing the planform, I'd not expect just a change in section to do much to balance. 

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On 8/3/2018 at 7:05 PM, DDW said:

In classical aerodynamics, the aerodynamic center can be taken as 25% of the chord. This is true for everything from a flat plate to a modern low drag section (within a small error margin). So unless you are changing the planform, I'd not expect just a change in section to do much to balance. 

That's a very good point and one I hadn't considered.  The CB will be VERY close to the same planform but I may change the rudder a bit.... or not... maybe I'll just try to fair it.

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When you say "the cable pulled out" that sounds like failure under load..... how much of the pieces do you have left? Enough to use as a template for the basic dimensions and profile?

The thickness of the foil is obviously limited by the trunk, so when you look at the width of the trunk minus 3~4 mm, and the profile that will fit up into the trunk, that gives the chord length and thickness. So you can't just choose a wizard aero section that will make you boat fly.......

The other consideration is that your CB needs bearing surface to hold itself in the boat's centerline plane. So, the semicircular (scribed around the pivot bolt) part that stays up in the trunk should be flat & as tight as practical to the trunk sides. You can have one of those wizard aero sections and if the board is wobbling between 2 degrees and say 7 degrees angle of attack, the boat will be slow no matter what else is going on.

Hope this helps

FB- Doug

 

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