canting keels and all that

hi all,

I am really impressed by the potential of canting keels - all that righting moment, means you can design rather more slender hulls, less resistance, both wave and friction, hence lighter and more efficient.

My question is, at what point and for what reasons does the keel become inefficient as a leeway preventer?

At maximum cant, say 45 degrees, is it just a matter of loss of projected area, and an adverse vertical component, or is there some kind of loss in terms of lift and drag of the foill itself as well?

cheers guys, dionysis.

 

Dionysis,

This is a tricky question. The horizontal lift vector (ie. normal to the keel when the boat is stationary with keel at 0 degrees) will change with the Cosine of the angle (measured from the vertical), but it also introduces a cross-flow. This is similar to flying a wing in air sideslipped. ie. The flow is no longer normal to the leading edge. This will cause a change in Cl and Cd of the keel.

However, I would think that the benefit of reducing the hull wetted area (and possibly wave drag) would far outweigh in increase in drag, and thus, even if you had to point slightly lower, you would still get better VMG than before. If you can, build a model and stick bits of wool over it, then blow air past it, at different angles of heel and keel cant. Watgh the pieces of wool, and this will give an idea of the flow in the boundary layer. A small handle with wire and wool tufts would show stream-lines around the keel and hull.

Good Luck,

Tim B.

 

Thought as much!

Tim,

I thought as much. The loss of 40% horizontal side force, and an adverse vertical lift component adding to heel, together with loss of efficiency due to slip, and its induced drag penalty is a high price to pay. No wonder the open 60 yachts have separate centerboards to counter leeway and minimise any contribution from the canting keel.

But then again, they have proportionately larger wetted surface than the kind of vessel I had in mind. After all, maximum righing moment is only needed beating into strong winds.

I agree VMG may be increased for a very slim hull, and your idea of modelling the flow is well worth looking into.

Where is a sensitive VPP when you need one!!

Thanks and cheers, dionysis.

 

Tim,
I'm kinda curious as to where your crossflow comes from when the keel is canted. I've looked, but can't find any ;-)
The effectiveness of the keel certainly drops off dramatically with cant and heel, but due to regular stuff lke angle, not crossflow.
The Black Sea 40 used a canting fin (not designed for lift, just low drag) with a pair of asymm daggerboards for leeway. It _feels_ as though she crabs to windward, but I think that's just wishful thinking.
Steve

 

The canting keels I've seen are either a regular IOR style or struts with a bulb. How about a wing instead of a bulb? It should produce more lift than a normal foil keel. I think it would eliminate some of the problems with fixed wing keels. For example optimal angle of the wings to the keel.

 

I've just reasoned through this question, and I was right about the leeway. Note that righting moment is just F*D, leeward force is F*D*Cos(theta). As the boat heels, the bow will tend to sink faster than the stern, and thus the boat will be put at a pitch. The pitch and heel, combine to put the keel at an angle of attack, but also an effective angle of sideslip, causing spanwise (correct term for crossflow) flow towards the tip. What is complicated is that as the keel is canted to windward, the sideslip angle changes, as does the maximum pressure coefficient on the keel.

Gonzo,
May I suggest that we add the wings on later, when we actuallly know what's going on on the keel itself. I like the idea though, and we will address it.

I think this question is going to take most of our fluid dynamics and hydrostatics skills!! As well as Cad and VPP, possibly even with some models examined. Anyone got a 1metre class model yacht I can borrow?

Cheers,

Tim B.

 

Check this link out, might be some good stuff:
ftp://ftp.fh-kiel.de/maschinenwesen/schiffbau/pub/Graf/Publications/

Cheers,

Tim B.

 

Tim,
I still have to take issue on the spanwise flow. Why "will" the bow sink faster than the stern? If you are designing a boat to have a canting keel, then this is the kind of thing you "design out" of it. Since it doesn't have to fit a restrictive rating rule that favours fat sterns, you can make the boat heel "flat".

Gonzo - Let me think about the wings. I'm not saying they shouldn't work, but they _feel_ wrong ;-)
Mind you, some things feel wrong until you realise that it's because they do work so well.

Steve

 

Tim,
While hitting the send button on the above, I realised where you are coming from. The extra leeway from having an inefficient keel in "leeway" mode _would_ create a spanwise flow.
Steve - begging the question of why you would _ever_ want a boat that made that much leeway.........

 

no added spanwise flow

Tom Speer in an email suggests there is no added loss due to spanwise flow or 'slip'. He also suggested incorporating 45 degree anhedral wings to the bottom of the keel to gain some more side force, vis:




What made me start thinking of keeping the canting keel as the hydrodynamic lift device, was the idea of using a multiple foil keel: a la the condor keel; very efficient upwind.

Check out Philip Carter's site:

Innovations in IACC Keel Design


Cheers, dionysis.

 

woops, try again

 

yehaaa

 

Hi All,

I beforehand am sorry for the fact that has come in discussion of a problem. Your discussion very much has interested me with certain the parties. On my sight, the problems of a flow around a hull - are difficult enough in solution. The problem of a flow of any superposition of bodies - is even more difficult. To some extent of accuracy these problems can be decided with the help of the complex and expensive software. But purchase and operation of such software can to itself allow not all. The exactest answers can be given only by a model experiment. But such luxury can afford too not all.
Question. Whether there are any methods of solution of similar problems, which can give acceptable accuracy at the acceptable price?
I understand, that there are many empirical datas. But what to do if you on any design stage understand, that they cannot be applied to your construction?

Thanks, Dim.

 

what to do?

Hi Dim,

I agree with you it is a problem. Even if you had the money for the expensive software, you have to learn to use the software confidently and this can take a long time.

One way out is to go back to what Tim B says up the top, about making smaller models and looking at the flow from a qualitative point of view using wool tufts etc.

Sometimes you got no choice.

Cheers, dionysis

 

Thanks for your answer Dionysis.

I with you completely agree. I was decided so many to write only because the problems of a flow's calculation are very interesting to me. I wanted know your judgement on existence simple (in practical calculations), but reliable (from the view point of outcome's quality) of methods. I think, that them do not exist for today. Sorry, for bad translation.

Thanks, Dim.