Twin keel research

[Greenseas2]

Twin keel research is interesting, but the focus should not be limited to hydrodynamic design of the keels. One area where a lot of testing is needed is in the effect of the spaces between the hull and the keels. While we all realize that fluids cannot be compressed, air within fluids can be. The acute angled area between the keels and the hull offer resistance to heeling underway. there's either compression or evacuation in these areas, but not sure which. I suspect that a sccientist rather than naval architect would be needed to explain the effect. Where there is no hull between keels, the same effect applies and decreases forward speed with rapidly changing heeling moments on gusty days. Interesting stuff.

[brian eiland]

Originally Posted by Greenseas2

While we all realize that fluids cannot be compressed, air within fluids can be. The acute angled area between the keels and the hull offer resistance to heeling underway. there's either compression or evacuation in these areas, but not sure which.

I think you answered your own question at the beginning, there is no 'compressable flow' in these areas unless these areas approach the surface of the water.

[mallo]

We quite often when hard pressed lift the windward keel out of the water, as she puts it back into the water it can be quite noisy, I think this is due to the air being compressed and the “harder water” getting trapped under the hull/keel transaction.

[Greenseas2]

Brian, I somewhat disagree about compression occuring just at the surface in that all water, at any depth, has even a modicum of air as part of it's composition. The effect at lower depths may be surprising. This would be most imortant to twin keels with reverse planes to hold the boat down, rather than foils that are designed to lift.

[Greenseas2]

I believe that you are right in this aspect. I also believe that there are a lot more circumstances where the twin keels are affected by compression. It's a subject of interest, but unfortunately, I'm ill equipped to research it, but do understand the problem in theory as well as practical observations.

[tspeer]

Even pure air at the speeds we're talking about is effectively incompressible - there's next to no change in fluid density as it flows around the hull or foils. So "compression or evacuation" is not the issue.

However, there is an additional drag due to the acute angle of inside the intersection between the keel and hull. There's also a reduction in drag due to the obtuse angle on the outside, but the acute angle has more additional drag so there's a net loss.

What's happening is the effect of the hull on the keel is to make it behave as though the hull were a mirror reflecting a phantom keel inside the hull. When the keel meets the hull at a right angle, the phantom keel lines up in a straight extension of the keel, and the keel acts as though it is longer than it really is. When the keel meets the hull at an acute angle, the keel and its reflection form a "V" intersection.

Here's why. Compare the flow around a real keel with a dihedral break to make it a "V" like the keel and its reflection, but with no hull. The keel pushes the water aside as it flows around it, and there would be a component of this pushed-aside flow that would cross the contour of where the hull would be. But the flow around the phantom keel pushes aside the flow in the opposite direction, so along the bisector of the V where the hull was, the two components at right angles to the hull cancel out. But the two components parallel to the hull add together. It's as though two jets come out from the side of the two keel panels, hit each other, and then push each other outward along the bisector. The flow picture from the hull contour out with the keel+image keel and no hull looks just like the flow picture of the keel+hull, so the forces on the keel are the same. But it's a lot easier to calculate the flow around the V-shaped foil alone.

Since the velocity components parallel to the hull of the keel + image add together, all the pressure changes in the intersection are exaggerated. This means the effect on the boundary layer is also exaggerated. Especially on the inside corner, where the displaced flows are directed toward each other, compared to the outside corner where they are spread apart.

The higher velocity in the corner increases the skin friction, since skin friction is proportional to the local velocity squared. It's the velocity squared effect that makes the increase in drag on the inside exceed the reduction in drag on the outside. And the flow has to slow down from a higher peak velocity to near freestream by the time it gets to the trailing edge, so there's a more abrupt deceleration that can lead to the boundary layer separating easier. Both of these effects cause the drag to increase at intersections, espeically when the keel and hull meet at other than a right angle.

According to experimental data in Hoerner's "Fluid Dynamic Drag", there's next to no additional drag at 10 degrees from the perpendicular, but it starts picking up by 15 to 20 degrees, and by 30 degrees from perpendicular the interference drag is doubled.

[yhxaig]

the bruce roberts 65',I will build it as a twin keel like the westerly except the keels will be about 30'long by 3'.
Purpose #1 shallow water capability

How will it perform ?? I"ll have a few naval architects figure it out.

[westlawn5554X]

Well I found this at Newwave website checking on prosurf and found unusual twin keel, I think it is design by one of the reputate forum member. Nice work.

[MikeJohns]

No it is neither a twin keel or particularly unusual these days.
These are anti-roll keels which may well double as effective landing skids, however there is no relation to this and twin keels for a sailing boat.

[barklymad]

Has this thread dried up ? ( or changed title ??) I followed it with considerable interest till around August 06 , then seems no more ......shame.
I am thinking of building a steel YAGO , but am rather concerned re. the windage effects of raised leeboards , especially in big seas with big winds ; I have sailed a triple - keeled westerly for 14 years , by triple keel i mean true bilge keeler with a weighty central keel/skeg protecting the prop. Wonderful in a blow. Barklymad

[Brent Swain]

I minimize interaction and compression of water between the keels by angling them out at the bottom 25 degrees. This also makes the leeward keel upright when heeled 25 deegrees, reducing leeway. More upright keels increase the interaction between themselves .
Fore and aft, I've built them assymeterical and symetrical, but there was no noticeable difference in performance.
I used to foul my anchor rode around the twin keels when I imitated the Westerly Centaur shapes. Now I make the trailing edge vertical , which is a huge improvement. Any twin keel shape you plan on cruising with should be tested on a model to make sure the anchor rode doesn't easily foul them ,and to make sure it pulls free when under tension from any direction. Otherwise you could find them dangerous.
Keeping both the keels deep enough on the hull drastically reduces the noise problem.
Brent

[barklymad]

Brent , I`ve never , yet ,seen a swain , except for my sweet wee origami model based on the website printout , and on which i put bilge keels at 23o ; but i`m still intrigued by why bilge keels are not more used on other larger yachts , the main reason always given being "too much wetted area " .

Well , the area of my present westerly keels ( at 20 deg ) is , very approx, 1m2 each , i.e. 2m2 ; a boat with a 6.5 foot fin , up to 1 m wide , laterally , also has 2m2 approx ; ;ok , there is a little extra on the under belly , but what is the fuss about ?
I also think that , following Marchaj`s work , but not all his conclusions , to have a less deep keel structure when in heavy , maybe breaking , seas , is safer , due to the easier slippage down the leeward slope , like a niagara barrel , but less churning !
Jaylo

[barklymad]

? or maybe i missed out both keel surfaces ?? Jay

[FAST FRED]

the main reason always given being "too much wetted area " .

About 30 years ago the AYRS proposed a more modern bilge keel. Hydrofoil shape for leading and trailing edges and a simple plate to connect the two , and allow taking the ground.

Great at lift , far less drag.

FF

[ericsyd]

Hi all
Thanks to all who have made comment here. I am interested in building a hull with twin keel. Did anyone come up with some real data? Did anyone come across Mr Ian Wright's contact details? Are there any n.architects that have had real life expperience with twin keel (cruising 24 mtr).