Jump to content

Centerboards


Howard

Recommended Posts

Have been looking at various cruising boat designs, keeping with the thought of shoal draft. That comes with a centerboard. Two designs seem to present themselves. I'm describing them as High and Low aspect boards.

The high aspect board is probably similar to most daggerboards, and would give you the best performance to weather. But this is to be a cruising boat. It needs to perform....but that's not the only criteria.

The following sketch generally depicts what I think the differences are. Are there positives and negatives I'm not aware of?

To me, the low aspect board is the best choice.....with knowledge that you are going to give up some performance to weather....which would be outweighed by a lot of other benefits to the low aspect design....unless it's a complete dog and won't go anywhere.

Any thoughts?

post-613-1294976451_thumb.jpg

Link to comment
Share on other sites


Howard,

What do you consider to be the advantages of a low aspect CB?

Unless the board is to be ballasted, the lifting moment is not very great for the high aspect kind. Other than sailing performance, the major advantage of the high aspect CB is that it interferes much less with the boat interior. For a small cruising boat this can be a major concern.

Link to comment
Share on other sites

I guess you start off with the understanding that the CB's purpose is as a lifting foil. It's there to counter leeway and to get you going to weather. From that standpoint, it's probably been decided long ago the high aspect keel does that job best. If I understand correctly, the most exotic of the racing sleds all use high aspect foils....with the America's Cup boats having one 8 or 10 feet long with several thousand pounds of lead bulb on the end.

That being said, the CB has to operate in a hostile environment. Not only does the leeway create a large moment, but hanging down like it does, it's subject to being smacked by things it might hit....not only going forward, but if you have a bit of leeway going, or more likely...drifting sideways...the force may be from the side as much as from forward. It looks to me like the high aspect board like that could easily be damaged.

On a racing sled, that's an acceptable risk and you go with it. But on a cruising boat...especially one intended for shallow water, you are going to be in contact with the bottom all the time. It just seems like you are asking for trouble with it. That long lever has a lot of leverage and holds potential for a great deal of damage. You could either snap the board...or worse...pry the pin right out of the CB trunk...which is below the waterline.

With the low aspect board, all the lever arms are shortened, so most of the torque and leverage on the high stress points are reduced. You can build a board that is going to survive an awfull lot of abuse.

The other benefit of the low aspect board, as I drew it, is that the pivot pin is above the waterline. In the event you had a problem with the CB....you could remove the trunk cap and/or pull the pin and either lift the CB up or drop it out the bottom...with the boat still in the water...without risking the boat.

It later occured to me that a wooden CB would actually float...and unless weight is applied, isn't even going to sink. If just enough weight is applied to get it to sink...the lifting moment to get it back up in the trunk is not all that great. So I guess that argument is out.

So perhaps it does all boil down to the trade off of performance vs. risk of damage. If the low aspect board doesn't help much and you get 20 degrees of leeway....you might as well not even have a board. If it's only a few degrees....say 5 or less....difference....then you are back to the pros and cons game again.

Link to comment
Share on other sites

I have hit stuff with a high aspect CB and know of many others who have done so. The only broken board that I know of is when a guy ran aground in a soft bottom with a CB that had a broken lift cable. In having the boat towed off, they twisted the boat around and broke the poor thing off.

It is not difficult to make a small cruiser CB sufficently strong to handle any reasonable incident. The superior performance and extra space in the cabin makes the choice simple for me.

How much extra performance? I have no idea and that would not be the deciding factor for me in a cruiser.

Link to comment
Share on other sites

Tom:

Thanks for the advice. It occurs to me that if you were to retract the high aspect board up into the trunk as far as the low aspect board hangs down....you essentially have the same thing....and could do that when you are bouncing along in really shallow water..... if you were concerned about it getting damaged. And we are really only talking about sailing on a dead beat. Off the wind....it's safely tucked up anyway. It could be I've got a solution in search of a problem. I tend to do that sometimes.

As for the performance....that too presents a simple solution. You can retract the high aspect board up into the trunk to the point it's hanging down the same as the low aspect board. If it performs better down than up....that should be telling you something.

I had not considered the issue of the trunk in the cabin. My boat has a trunk...but it's under the sole in a partial keel and also tucked under the cockpit. It's like I don't even have a trunk. On the other hand, I once sailed on a San Juan 21, with swing keel and didn't have a useable interior because of the trunk. I can see why you wouldn't want that.

Thanks again.

Howard

Link to comment
Share on other sites

Howard, looking at your high aspect design, a few desirable things come to mind.

Try to locate the pivot pin at or just above the water line, it removes potential leak problems with the CB trunk.

I would also round the top forward corner of the board to avoid wasted trunk room forward of the pivot point. Also the top aft corner, to lower the trunk height.

Then, have you considered a board that's deployed somewhat swept back rather then with a vertical leading edge? This gives a huge advantage in seaweed choked waters and with lobster pot ropes. I just sail straight over the ropes with my swept back board, the rope gets pushed down and resurfaces AFT of the rudder blade :wink: .

Another advantage of swept CB's is that the lifting tackle can be fully within the CB trunk, eliminating hum and drag there.

Do round the lower edges of the board, especially the forward one.

Klaus

Link to comment
Share on other sites

  • 1 month later...

Check out Paul Fisher's centerboard design. This is the design of the centerboard that's going in my 2006 project--Paul's Sandgrouse Yawl.

S_F_Centerboard_Design.jpg

There are a couple of things I like about this. Most importantly, this centerboard will provide most of my ballast without requiring any trunk in the cabin. I'll have a small trunk in the cockpit for the board's "shoe", but my small cabin will benefit from the that extra room. Another thing thing I like is having the bolt readily accessible. The bolt will be easily reached while on the trailer for maintenance.

Now, I have no experience in sailing the boat, so I don't know what trimming it will be like. With that extra weight at the end, it might give you some interesting trimming options as you move it through it's movement range.

(credit to Selway Fisher Design at www.selway-fisher.com)

Link to comment
Share on other sites

  • 3 months later...

I have a high aspect CB on a coresound 17. Pull the thing up halfway and three things happen:

1. boat speed drops dramatically

2. leeway goes through the roof

3. staying on course becomes difficult

The only time I pull the board at all is running straight down wind in light air.

I built the board out of fiberglass/epoxy with a foam core, naca 012 foil. I have grounded it (oops) and sat on the end of it (in the shop). If I break that board I am going to want to be wearing a seat belt :-)

Link to comment
Share on other sites

Just to toss another iron on the fire...

I just finished sailing a leeboarder for a week and there was no interior space lost to a centerboard trunk.

I accidently managed to drift sideways onto a coral head and based on the sudden leeward roll of the boat I think at least that boat would have gone over before breaking the board (fortunately we didn't go over OR lose the board).

I didn't find the extra work of changing boards while tacking to be too disagreeable.

There's also the possibility of locating the centerboard off center enough to make the trunk into a seat in the cabin. Doesn't Graham's P22 or Belhaven have that feature?

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.