Keel Profile

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clonlara

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what is accepted to be the best profile for the leading and indeed the trailing edge of a keel. I have just dropped my swing keel and had it shotblasted with a view to doing some remedial work. The keel is just 10mm approx mild steel plate with flat edges. I am actually looking at reshapeing the keel using 1.2mm sheet steel to give it a more aerodynamic shape. So should the leading edge be bullnose or have a sharper profile? Its over to the experts.
Colin
 
A rounded leading edge will help but shaping the trailing edge is probably more important: a long, shallow point will shed vortices with far less drag than a blunt or flat end. Look no further than a standard aerofoil (or hydrofoil) shape for guidance.
 
You need a front end that encourages 'laminar flow', ie water to pass round the keel without detaching, or creating vortices.... this is usually accepted to be a rounded profile like that on a wing, but without the profiling differences designed to create lift in one direction only... on the trailing edge, the generally accepted profile is thin, but without a razor sharp trailing edge, ie a narrow flat section of a few mm.... this is designed to allow the water to detach cleanly without creating turbulence, and hence drag.

Tons of research online on this subject... try googling NACA... which is more about wings than keels, hence assymmetric, but will give you a lot of insight
 
Both of the above are true. NACA will also provide you a series of standarised aerofoil sections. You can just slice this in halve and then copy to create a symmetrical section. Ideally go for one that has a low lift coefficient, as you want to minimise the amount of force being created. Even a symmtrical section will produce "lift". This is because as you move through the water you will have a small angle created between the chord line of your keel (Very leading edge centre to very trailing edge centre) and the relative fluid flow (in this case the water). This is due to the fact you have wind trying its hardest to push your boat sideways. Clearly any of this "lift" that you create will do two things, first of all it will try to pull your boat sideways, which is not what you are looking for and more importantly it will create drag, and the more lift you create the more drag you create. Therefore go for an aerofoil shape that produces as little force as possible.

This will be one that has a minimal amount or curvature, and also the ration between width and length has as little change as possible. For this read that you don't want something really fat at the front then really thin at the back, over a short length of keel (from leading edge to trailing edge) What you would want is one that is thin at the front and back, spead over a longer keel (again by long i mean from bow to stern rather than depth) The true panacea would be a perfectly flat plate, however this clearly is impracticle so you do still want some aerofoil shape to the keel.

There you go a nice short reply for you! :-) *insert sarcasm here*
 
Cariadco said:
Funnily enough, my (lead) keel's trailing edge is chamferred to one side. Can't remember which side at the mo' though.

Often wondered why that is??
Click to expand...
Depends which side you dress.......;<))

Probably to avoid rapid alternate separation that cause vibration.

For the OP, just look at the foils of any decent dinghy or class catamaran. There are some sites explaining 'how to' and the NACA sections. If you can, an 8% up to 12% section would be good. ( The thickness at max, compared to the 'cord' or fore and aft measurement)

Having built a swing keel out of sheet, I am intrigued by your idea of modifying your existing keel with thin sheet. In my case I used 4mm, over a frame.Galvanized then ballasted up to the required wt. with lead.
DW
 
It's funny how often things come up on this forum just as I'm about to tackle a similar job.

My main project this winter is to replace the swing keel on my boat. Like the OP's the existing keel is about 10mm thck.

I was planning to make a new one, perhaps a little thicker if possible and to then get it galvanised. (The reason I'm replacing it is that the hole for the uphaul has been redrilled once before but there's really not enough metal left to redrill yet again)

I intend to radius the leading edge and to taper the trailing edge to get it as near to an aerofoil section as possible. The leading edge is easy enough with an angle grinder, but there will be a lot of metal to remove on the trailing edge. Any ideas on how best to do it? I really don't fancy spending hour after hour with an angle grinder.
 
Rigger Mortice said:
I
I intend to radius the leading edge and to taper the trailing edge to get it as near to an aerofoil section as possible. The leading edge is easy enough with an angle grinder, but there will be a lot of metal to remove on the trailing edge. Any ideas on how best to do it? I really don't fancy spending hour after hour with an angle grinder.
Click to expand...

Hour after hour of your time might compare favourably with the rates of a machinist outfit.

Just curious.... Can't you repair the cable attachment area?.
DW
 
DownWest said:
.. Can't you repair the cable attachment area?.
DW
Click to expand...

I thought about that but it really has rusted away quite a lot. Add to that the hinge pin needs to be checked and, as the keel has to come off I may just as well renew it all now. I'ld hate to do the pin then have to do the keel a couple of years later....it's quite a big job!
 
clonlara said:
what is accepted to be the best profile for the leading and indeed the trailing edge of a keel. I have just dropped my swing keel and had it shotblasted with a view to doing some remedial work. The keel is just 10mm approx mild steel plate with flat edges. I am actually looking at reshapeing the keel using 1.2mm sheet steel to give it a more aerodynamic shape. So should the leading edge be bullnose or have a sharper profile? Its over to the experts.
Colin
Click to expand...

What is the chord, the fore and aft dimension of your keel when lowered ?

A swing keel made by a plate 10mm thick does not allow much in terms of hydro fairing I am afraid...
A small 50cm chord keel with a symmetrical 0010 profile (to take a simple one, not necessarily the best) would need a 50mm maximum thickness, that is a lot thicker than the plate you have..

FWIW, you can approximate most 4-digit NACA profiles leading edge with a parabolic shape without losing much, the problem being that the required thickness quickly becomes a lot more than you have available.

If you cannot increase thickness, the second best is to concentrate in fairing and smoothing the whole surface as much as you can imho





re trailing edge shape, there is a paper about the differences between a sharp one, a square one, an asymmetric cut, a wedge shaped edge, etc, I'll try to find it; the asymmetrical one is sometimes used as a cure for a vibrating keel or especially rudders, but hardly chosen when the keel is first built
 
I know a little about fluid dynamics .

Does the lifting mechanism or slot preclude thickening the keel?

If it does, then grinding a profile on the leading edge will help. If you can extend the trailing edge and taper it that will help too.

I can give you more ideas when you reply.
 
The information I've got is '60 degrees of a circle' for the leading edge,trailing edge left square rather than being feathered to a point.These are what I've used making a dagger board,rudder,and now building another dagger using NACA foil coordinates.As stated,10mm gives you little to play with.
Cheers
 
Swing keel

As said the thickness will be the limiting factor in your modifications unless you are able to increse the width of the slot.
Unfortunately increasing the width of the slot may also increase drag by leaving a wide hole in the hull for turbulence.
Generally a foil will be max thickness at about 1/3 distance from the front. The trailing edge will taper from there. The leading edge can be semicircular or sharper in profile. A semicircular profile tends to give better lift at high angles of attack. (high angle of attack meaning large leeway at low speed which I don't think will be a concern. Likewise a thicker keel can give more lift at low speeds high angle of attack but a thinner foil should be OK.
I have always attributed good performance from my trailer sailer to the keel. This is a vertically dropping style so wastes a lot of cabin space. However it means the foil profile can be ideal. It is about 7cms thick at the thickest (contains some lead) and has a leading edge profile somewhere between sharp and semicircular. It also means there is no gap when the keel is down so min turbulence.
Theoretically there is advantage in having a semicircular tip (like a spitfire wing tip in plan) however mine is pretty square across the end to give max lead at max depth.
I have seen a lot of fin keel boats that have a very flat bottom of keel to make them easy to balance the boat on both sideways and fore and aft. I think this is a short coming in hydrodynamic efficiency. I may be wrong here. But it seemed to me that the bottom of the keel should have a smooth profile (semi circular or at least no sharp edges) as viewed from the front so that water flowing from high pressure (leeward) side to low pressure side should do so smoothly without turbulence.
Anyway inpractice you can see what is best now see what you can acheive. good luck olewill
 
further info

Thank you for the info supplied so far. The boat in question is a Gibsea 76. Although the keel is only about 10 mm the slot is appros 40 mm which gives me some room for modification. My origional plan was just to add some weight as she is quite tippy but after dropping the keel I thought I would look at some further improvements. so what I`m thinking is to weld a rod approx 8mm the length of the keel (allowing it to taper at the bottom) about 1/3 from the leading edge. I will then cover the keel with 1.2mm sheet steel thus forming an aerodynamic shape, this would also allow me to fill the void created with molten lead givivg me more weight below. Hope this explains things. Am I mad to be even attempting this?
 
Conventional wisdom says 'don't muck around with ballast ratios'. But I can't see too much wrong with your idea. Couple of points: Can the pivot and hoist cope with the extra weight? And, like I did, galvanizing the whole thing before pouring the lead, to avoid the obvious corrosion probs. Without working it out, at a guess, your keel is going to be a LOT heavier. Best do some numbers first... Twice as heavy comes to mind, even at 30mm max thickness. Prob a bit more, as the lead infill is 30% heavier.
DW
 
markjc said:
Both of the above are true. NACA will also provide you a series of standarised aerofoil sections. You can just slice this in halve and then copy to create a symmetrical section. Ideally go for one that has a low lift coefficient, as you want to minimise the amount of force being created. Even a symmtrical section will produce "lift". This is because as you move through the water you will have a small angle created between the chord line of your keel (Very leading edge centre to very trailing edge centre) and the relative fluid flow (in this case the water). This is due to the fact you have wind trying its hardest to push your boat sideways. Clearly any of this "lift" that you create will do two things, first of all it will try to pull your boat sideways, which is not what you are looking for and more importantly it will create drag, and the more lift you create the more drag you create. Therefore go for an aerofoil shape that produces as little force as possible.
Click to expand...

Surely you are not trying to minimise lift, because that lift is what is stopping the boat being blown sideways? You want to maximise it. If you are beating on starboard tack the sails will be trying to push the boat forwards and to port. You want the forward motion, but as little as possible of the sideways to port movement. As the boat is moving not directly forwards but slightly to port, this will generate lift pulling the keel to starboard. The more lift to starboard, the less the leeway, and the better course you are making good to windward.
 
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