Good boats w/ shoal draft keels?

[John Stevens]

What is the downside to adding a wing?

John

[water addict]

extra drag, especially at non-optimum speeds and changing angles of attack

[John Stevens]

I have to make up for the 8 inches I'm taking off the original rudder. Wouldn't the drag on the wing be about the same as the 8 inches I cut off the rudder?

John

[sharpii2]

Originally Posted by John Stevens

What is the downside to adding a wing?

John

I would imagine many of the same downsides as adding a bulb, but to a lesser degree.

The first being that you are adding a lot of wieght low. That is good by a stability/sail carrying standpoint but bad and sometimes very bad by an engineering standpoint. Imagine your dreamship beating into a ferocious channel chop in a race. The boat is pushed over on its side by maybe as much as 30-40deg. The keel is now no longer hanging down as it would be on paper but now projecting out to the side. Now throw in a sudden gust plus a sudden drop from a wave top.

Now I challenge you to calculate the bending loads on the keel where it joins the hull. Its kinda like a diving board bolted too a wall with just an angle iron on the top and bottom of where it joins the wall and a very heavy diver bouncing up and down on it as he prepares to leap.

The second being that you now have a more complicated structure that is more vulnerable to to damage. Shoal draft boats have to live with the reality of groundings just as deep water boats have to live with the reality of large breaking waves. I have nick named the torpedo like bulb keel boats 'kelp catcher specials' (they are also good for fishing for lobster pots). With a winged keel, there is a very strong possibility that the wing part will get bent as the boat rolls over while aground and the tide running out. While no one grounds their boat on purpose, it can be counted on to happen by accident from time to time.

The third being that, unlike a bulb keel, a winged keel needs to be carefully designed to insure the wings actually inprove performance rather than just add drag.

EDITORIAL:

I have often wondered why deep keels don't have wings. I have entertained the idea of having a much lighter keel with a wing on the bottom. The wing would be designed to produce downward lift. That way once the boat was moving it would create 'virtual ballast', or ballast just when the boat needs it most. Not only that, but it would also create windward 'lift' to some degree as the boat heels over. The only problem with this scheme is that it would be nice to be able to 'kill' the downward lift when going down wind. To do that, the wing would have to be either adjustable in pitch or have a sizeable flap on the trailing edge. More engineering than suits my tastes, but perhaps less than that needed for a 'canting keel'. Such a keel, by virtue of its lighter weight, would be a better candidate as a lifting keel.

Bob

"that's my story. And I'm sticking to it."

[SeaDrive]

Originally Posted by John Stevens

I took 8 inches off and I'm adding a wing in an effort to make up for the 8 inches.

Designer Phil Bolger puts wings on rudders all the time. He says that, to him, a rudder now looks naked without one. If the pivot axis of the rudder is near vertical, it will probably work fine.

Bob, that is one of the possible, though as yet untested, applications for the kFOIL. A low aspect shoal draft fixed keel could be designed with retractable wings near the bottom of the keel. They could be deployed when needed and retracted otherwise. They would be designed to automaticaly retract if they encountered any kind of object.
If a boat was designed from the get go with this system it might be able to get away with less area than a normal fixed keel or even reduce draft more than if the system was added to an existing boat.

[John Stevens]

Bob,

I'm talking about putting the wing on my rudder not keel.

John

John, as long as the pivot axis is vertical and you put the wing on parallel to the waterline( span athwhartship; line of 0° lift thru the foil chord parallel to the wl) I can't see it hurting the boat and it might make up a little for the reduced span that you require for shallow draft.The lower the aspect of the rudder wing the less effect it will have.
I believe that Garry Hoyt has used endplate type wings to improve the performance of shallow draft catboat rudders. And numerous other boats have used rudder wings to IMPROVE displacement upwind performance on boats that are gennerally sailed flat such as I-14's and pre full flying Moths.
-----------------------------------
Depending on the size of your boat and the size of the foil you add to it you might want to have the whole thing looked at by a Naval Architect /Marine Engineer because in certain conditions there will be loads imparted to the rudder attachment points that they were not designed for.It probably wouldn't cost too much and might save you a lot in the long run.

[water addict]

Originally Posted by John Stevens

I have to make up for the 8 inches I'm taking off the original rudder. Wouldn't the drag on the wing be about the same as the 8 inches I cut off the rudder?

John

If the area of your wings is about the same as the area of the 8" you cut off, then the drag would be close to the same, yes. However, you will be getting marginal increase in control IF the wings are properly designed. The problem is that the wings generally are not good if they are out of their design window, so they just add drag with little to no improved lift.

It sure can't hurt to try your rudder with the 8" snipped off and see how it performs. If, when you try it in a range of conditions it seems to be performing poorly, then it would be good to look at the conditions under which it aint workin. Then figure what to do about it.

[John Stevens]

On the last prototype I just built a triangle on the bottom of the rudder, 8 inches across aft to a point in the front. Is there a "best" shape to build it? I can't find any pictures on the internet, nor can I find any information about what different shaped wings do.

I believe that the 8 inches I'm taking off has got to hurt the performance somehow. Jim Taylor disigned the Precision 18 and must have had a reason for making the rudder the size he made it.

Do you have any thought as to how you'd build the wing and why?

Thanks,

John

[FAST FRED]

You may find a plate on the TOP of the rudder to stop air entrainment down the rudder & shaft from the surface , does more than a wing on the bottom.

FAST FRED

[water addict]

John,
Actually, I own a Precision 15 and was thinking about reducing the size of the rudder. The 15 has a bit of lee helm as designed. I'm guessing Taylor designed it so that when the jib is furled, the weather helm won't be too much- actually the boat balances nicely when the jib is furled. I have a couple options I will try this summer before I mess with chopping the rudder though. I'm going to try raking the mast a bit. If that doesn't help, I'll move the mast base back a little as well as rake it.

I'm not sure how the 18 balances compared to the 15. The point I want to relay is that boat balance, foil size, and lots of other parameters are not exact science. Designers have rule of thumb ranges that they use when laying out the design. How the boat performs on the water usually presents some surprises. Of course slap whatever you want on your rudder, it's your boat. I'm just saying, if it were mine, I'd try it first and see if it's necessary.

[John Stevens]

Fred,

Where on the top would the plate go? How many inches down into the water?



Water Addict- I'm not sure if I was clear but I'm making a prototype (from cheap wood) not cutting my Precision rudder. Once I find the right mix I'll build it out of good wood.

[FAST FRED]

The TOP of the rudder (esp if it is almost at the surface ) or the shaft an inch or so below the sailing waterline will help prevent airation.

About an all around inch is plenty.

FAST FRED

[SuperPiper]

Originally Posted by Lorsail

. . .as long as the pivot axis is vertical and you put the wing on parallel to the waterline( span athwhartship; line of 0° lift thru the foil chord parallel to the wl) . . .

Doug:

If the boat had significant rocker, I would be thinking that the wing should be parallel to the water flow. On my little cruiser, the hull meets the transom at 17degrees above horizontal. Therefore, the wing should point upward (downward ?) for the least amount of drag. Also, I wonder if a kick-up rudder was allowed to float if it would find its own neutral position.