Oscillating centreboard problem

[Chris Lee]

We have just finished and launched hull #541.
We have a problem with the centreboard and I am wondering if anyone else has had similar experiences?
Everything is fine until we reach 15-16 kts then a violent oscillation happens. The centreboard is obviously 'tacking' its way through the water in a very violent fashion. I can see that a board with less ability to twist would help this sutuation but I want to know the cause before I do any major surgery. We only have uni carbon going along the lenth of the board which does allow it to twist a bit/
I am wondering if the front end of the bulb is setting up a high level of resistance to the water at high speed and getting the thing started??
Any thoughts??
Apart from that the boat is great, she wants to go faster and we can't, very frustrating.
Chris

[jray]

I had a vibration around 8 to 10 kts. I made a adjustment to the trailing edge of the strut, angled it to around 30 deg, got away from it being a 90° and the vibration went away. Don't have any advice on torsional/twist of the strut. I can't believe it would be a good thing.

[i550sailor@aol.com]

What are the tolerances between the Keel and Keel trunk on the boats experiencing vibration, I am guessing they are loose, I have seen this on several different sport boats, I an working on my keel today and trying to decide, how tight I want to make it, but not too tight.... I would like some input from the finished boats on what they think?.

[jray]

I kept the sleeve/strut fit fairly tight, probably .030 to .050 thousands. It's a chore getting it aligned to install when under the boat on a lift but happy I kept the tolerances where they are. Unsure what others have done with theirs.

[micah202]

This.,,, https://www.flickr.com/photos/59103003@N05/7505862536/
My boat came with a snug fitting sleeve trunk. It was a pita to raise and lower the board without jamups. I also like the idea of having some protection in the case of hitting bottom.

......the plastic blocks bolt to the top of the keel., the extra space is filled with pour-foam in bags for perfect fill.


Chris, those oscillations sound pretty wild..it sounds like your theory is likely on track. Why not build a 2nd board?...that way you can keep sailing while you develop the penultimate

[Chris Lee]

Yes they shake the whole boat violently. The case is lined with industrial carpet type stuff, very hard and dense which was vacuumed onto the board under layers of biaxial glass to form a sleeve. Very good fit, no movement there.
I think I am going to stiffen the board by adding carbon uni in trenches. We live in a place where the summer breeze is normally 20-25kts so its hard to keep below the critical speed if the kite is up.
I would like to know more about the shape of the bulb, what are its different speed behaviours?? I am also considering some little vertical winglets on the very back end of the bulb. If the bulb is causing drag we should know that and have an alternative shape which behaves better at high speeds.
Cheers

[Chad]

A chamfer/bevel on the trailing edge addresses humming issues, but it sounds like you've got big oscillations going on.

The carpet is one possible source- if the foil can rotate a little in the case, it will when it's neutrally loaded, going downhill. You can probably see if it's tight enough by diving under and grabbing the foil and twisting it to see if you can physically get it to move in the case. Removing some carpet and replacing/inserting a couple of plastic wedges down from the top when the board is lowered should cure the oscillation if it is from the carpet being too soft. Basically just remove the slop with something that locks the blade's head firmly in the case.

If that's not it, then you've probably built a foil that is weak in torsion. I've heard of two solutions: build little bulb fins like you mentioned, and/or adding a layer of +/-45 biax to the foil. With your carpeted case, you might be able to get away with sanding your paint off, adding the biax and re-fairing, without getting it to tight in the case. 0 degree unis won't do much- your problem comes from keel twist, not bending.

[Tim Ford]

We live in a place where the summer breeze is normally 20-25kts

Holy crap! Are you accepting immigrants at this point?

[noemar]

No need for a passport in MA where the afternoon breeze in Buzzards Bay runs 15-20 + everyday