Proper Wing Keel Templates?

All,

I'm setting up to fair the wing keel on my Beneteau First 235 using templates from Computer Keels. Does anyone know what the proper (theoretical) shape should be for the wings? Should they be symmetrical NACA foils, like the keel itself? Should they be asymmetrical?

At the very least I can take a template off one of the wings to ensure that they're identical... but if there is a shape I should be optimizing for, I'd love to know what it is.

Thanks,
Michael

 

Anyone? Nobody knows anything about wing keels?

Sure... It's probably silly to put this much detail into an already performance-compromised keel design... But I need this thing to perform as well as possible, given it's constraints.

 

Might wait till the normal work week, it's a holiday afterall.

Bill H.

 

Michael,

The section shape of winglets can be any reasonable faired shape. They are there to control the induced drag vortex that would normally come off the tip of the vertical blade of the keel had the winglets not been there. That is to say, the winglets are streamlined shape for low drag, not necessarily for lift. Also, as the boat pitches in a seaway, the angle of attack on the winglets will sometimes be angled up, other times down, and on average you really cannot predict which way lift is being generated. If the winglets were asymetric, they would tend to favor lift in one direction over the other. When heeling, lift off the top of the winglets would tend to send you to up and to leeward (not good) and lift off the bottom of the winglets would tend to send you down and to windward (down thereby creating more drag). So you make the winglets as small as necessary and make them thin and symetrical in shape to keep form drag to a minimum while at the same time controlling the tip vortex of the vertical blade.

Eric

 

Eric,

Thank you very much for taking the time to respond. I appreciate it. I also checked out your website. Your BS Bulb design looks pretty interesting.

This is one of those shoal draft compromise keels. It's my understanding that the "wings" on these keels are there to produce lift to help make up for the shorter overall keel foil. Do the same rules apply here?

I've included a couple of pictures of the keel so you can see the design...

I really appreciate you taking the time to help me out!

 

Michael,

I am familiar with this type of winged keel. If you heard that the winglets create lift, it was probably marketing hype. They don't create very much lift at all. Shallow keels like this have a very large tip vortex coming off the tip and trailing edge. This is the signature of induced drag--the bigger the vortex, the more the induced drag. You control the tip vortex by adding winglets such as these to keep the flow on the keel blade itself. The more it stays on the keel, the smaller will be the tip vortex, and so the less drag. Of course, you bank on the increase in form drag (drag due to their volume and form) of the winglets will be less than that gained by the reduction of induced drag so that the net overall drag will be less. This is why the winglets are positioned on the aft part of the tip.

The best thing that you can do regarding refairing is to make the keel and winglet surfaces as smooth as possible, rather than try to change the shape of the winglets. This may mean taking off the anti-fouling to bare metal, filling and fairing to a really smooth surface, then repainting with hard coat anti-fouling.

Eric

 

That's certainly easy enough.

In fact, that's exactly the plan... The keel is getting sandblasted in the next week or so, ospho'd (it's cast iron) and primered with a good epoxy primer. After that I'll be fairing the body of the keel using foil templates from Computer Keels (to ensure the vertical part of the keel is symmetrical and true--I don't believe it currently is). I'll make the winglets as fair and smooth as possible and not worry about optimizing their shape.

Thanks again for taking the time to help!

p.s. I sort of follow what you're saying regarding the reduction in drag (not a naval architect--). Do those winglets contribute to reducing leeway at all? If not, why did the original designer bother putting them on the keel? Isn't that a common idiom--a designer sticks wings on a shoal draft keel in order to help make up for the fact that it has less depth available to provide lateral resistance?

In my simple model of the world, those winglets provide additional resistance to the boat being dragged sideways... I thought it was provided via the wings generating lift (you've explained otherwise). But at the very least, there's another perpendicular surface there to grip the water. Is this at all correct?

If it were intended to be a high-performance keel, rather than a shoal draft keel, I could see a designer bothering to include winglets to reduce drag. But I'm guessing that they're there to help make up for the fact that it's a shorter keel than the original, deep fin version.

I'm trying to get the most performance out of this keel that I can for the time being. I am considering doing a keel swap down the road, as I race this boat fairly regularly...

 

Michael,

Yes, the winglets do reduce leeway on account of the "barn-door" type of drag that they produce, not necessarily lift.

If you look at the orientation of all the fins, keel and winglets, when the boat is sailing at, say, 20 degrees of heel, the lift from the keel points 20 degrees above horizontal. But the lift from the winglets, if there is any, points downward 20 degrees off vertical. So the lion's share of the lift in the correct direction (horizontal to windward) comes from the keel blade. The deeper the keel and the higher its aspect ratio (depth of span divided by chord length) the more lift and the less induced drag. As the keel gets shallower and longer, lift diminishes and induced drag goes up. The more induced drag, and the less lift, the more leeway the boat has. With winglets, there is some reduction of induced drag, but also the addition of more drag from the winglets' "barn door" effect, preventing the boat from slipping so much to leeward.

Eric

 

I note that PHRF gives an allowance of 3 to 6 sec/mile for the wing keel over a deep fin for most designs. Possibly more sometimes, but not a lot. I have never heard any justification, and wonder if there is any guidence from a VPP. One of the truths of PHRF is that compromise boat (e.g. racer/cruiser, or shallow-draft version) is more likely to have a disadvantageous rating than a full-bore raceboat.

I wouldn't give up the shallow draft of my Capri 22WK for a slight improvement in rating advantage, but I don't race in a very competitive fleet. Someone, somewhere, probably a broker, said that the wings on the Capri were really just there for the ballast effect. I do expect that they wings are a bit better upwind than a bulb would be.

 

A calculated VPP would not give you any information about a winged keel because they calculate results based only on basic design parameters of the boat's hull and rig dimensions. There is no factor figured into the calculations that would account for winglets, other than the fact that the center of gravity might be different (could be higher or lower, depending on what it is compared to).

PHRF ratings are generally figured on actual sailing performance, which, in some ways, is a more sure way of judging performance. It takes into account the experience of the crew, and ratings can change over time depending on the boat's/crew's success.

Eric

 

I have believed that a typical sailboat of today will dip it's nose or lift the stern when it heels over...and that would make the winglets point a little downwards.
And if the boat is run rather fast under engine, will the stern "set" down, so the winglets will angle up?
Is this tha case?

 

Raggi Thor,

Ideally, a sailboat will stay nearly level as it heels over, and that is why we check heeled waterlines and the center of the submerged volume (longitudinal center of buoyancy) at heeled waterlines to minimize the trim at heel. If a boat trims down by the bow as she heels, then she will tend to have more leeway because the local angle of attack will create lift on the leeward side of the keel and the boat tends to go to leeward. If the boat trims up by the bow as she heels, then the leeway will be minimized, or in radical designs the boat will have "windway" because the lift is on the windward side of the keel.

If the boat were to have large winglets, depending on how they were shaped (section and planform) then they can generate some lift due to angle of attack caused by trim. It can get pretty complicated. If the winglets are of symmetrical foil section, then stern-down trim will give some upward lift, and bow-down trim will give some downward lift. One has to be careful where the point of lift is generated, because if it is too far aft of the center of flotation (center of the floating waterplane) then just the opposite effects could happen.

One can also make the winglets of asymmetrical shape such that they always generate upward lift at zero trim. In addition, a certain amount of attack can also be built into the keel. I have seen this built-in angle of attack done in both directions, some down, some up, depending on the hull and keel design, and the location and size of the wings.

One also has to be aware that if winglets are too big, then the boat could be slower in light air due to the added wetted surface friction of the winglets. You can get yourself into a host of problems if they are not done right. I think this is why on most keel and bulb designs, the winglets are kept as small as possible to control just the trailing vortex off the keel tip, and so they have minimal lift effect on the hull as a whole.

Eric

 

New Member Needs Help Navigating This Website

Sorry no help with your question. But I need some help on how to post a question to all that may be able to help on this website. The only way I could figure out a way to make a contact was to reply to a recent submittal.

I am looking for a power prediciton for a semi-displacement 36' craft. Something similar to the Savitsky Brown Paper from the issue of Marine Technology Oct. 1976.

Thanks,

Seajack

 

Seajack,

If you go to the top of any of the forums, there is an icon near the top to the left that says "New Thread". Click on that, and it opens a new window in which to post a new title and a message.

As you are probably aware, a Savitsky power prediction is not a simple method, and it really applies to planing craft, not well to semi-displacement craft. Complex computer programs have been written to determine horsepower requirements for planing hulls.

For simple calculations and fairly reliable power predictions and propeller specifications, I have found Dave Gerr's Propeller Handbook to be quite useful. He gives powering curves for displacement, semi-displacement, and planing vessels. I have used Crouch's formula quite successfully, and I have found Dave Gerr's displacement powering formula to work pretty well. I always use the Bp-delta method for determining propeller specifications for a first pass, and then I confirm this with the NavCad software for a final powering prediction. The NavCad software is expensive, about $6,000 new, I believe, and a lesser amount for periodic updates. But I think it is very reliable, and very scientifically based.

Eric