Is there a consensus of opinion about what the best type or design of rudder / centreboard currently available for open water/sea on a Winder Merlin. Some interesting insights from Alex on the 'other' post which have gone now. I've heard good things about high aspect foils - are they optimised for a particular type of sailing? |
I'm glad you found my previous post useful. High aspect ratio foils are going to be more efficient in all scenarios. The only problem with them is practicality. To get them high aspect ratio, means making them longer which makes going aground more likely. Therefore the only places where a high aspect ratio board is practical, is somewhere where going aground while racing is not an issue. |
The problem with the high aspect foils currently (Adressed by JT's hatchet gadget) is that when you lift the board you don't lose any surface area and don't lose power. |
But what have you got in your new ship, Chris? |
I wish we had the ability to add pictures.... as then I'd post of picture of the c'b on Gently. High Aspect indeed..... you need a depth sounder(back then a lead line) anywhere inside of the continental shelf! They are certainly deep, plus of course they had a great big lump of metal in the bottom. Of course the other downside back then was the fact that the board was on rollers and had to come aft before dropping down into position! |
Standard board but I've had the long case. |
Here's the original plans for the Merlin's board http://www.merlinrocket.co.uk/gallery/view_photo.asp?folder=gallery/vintage/original_plans&file=original_plans_hull_side.jpg |
And to think that everyone laughed at Fat Pig's extra long rudder back in 1990. As usual, Mr Dalby (snr) was way ahead of his time! We did have to chop six inches off it when we tried to use it on the river. |
Not sure that ultra high aspect ratios for foils are that beneficial. True the drag due to carrying lateral load (induced drag) will come down and the leeway will be a tad reduced, with also a consequential reduction in hull drag. However, the overturning moment will go up a bit and, more importantly, the frictional drag will be increased for a given area. If you calculate the shear stress in the flow round a NACA type foil, you find that about half the frictional drag occurs in the first 15% of the chord length. Hence, a deep narrow foil with a long leading edge will have more frictional drag. And the frictional drag is bigger than the induced drag. So just chasing low induced drag might not be a good idea. |
What a shame we outlawed dagger plates!! |
These are some good points you make Peter though I think you meant the profile drag would be increased which is correct due to the larger frontal area of the board when it is longer, the frictional drag would be the same for a given surface area no matter what shape. |
And yes, dagger boards are more efficient however they are also annoying, inpractical and hopeless for boats with a raking rig without a fancy John-Turner-esque forward and aft adjustment system of some kind... |
Alex, that is interesting. I hadnt thought that chord length would be an important contributing factor in causing separation and so increasing pressure drag. Please do dig out your lecture notes if you can. |
Looking at the drawings of Merlin though makes you realise how completely radical and left-field Jack Holt's thinking was - high aspect rotating rig, high aspect raking centreboard that was lifted back into the hull through a small slot so the case didn't hold more water, decked-in hull to add stiffness like a monocoque structure, we could go on. Some of it didn't work, but some of that was due to shortcomings in the materials that would only be sorted out 40+ years later (e.g. carbon spars) |
I take it back Peter. Having now checked, the flow seperation in the absence of devices to keep it attached, actually increases as the chord length/thickness ratio increases. So if the high aspect ratio board was really thin compared to a thicker low aspect ratio, it would be the same. This is an unlikely scenario though as it is easier to make a thin low aspect ratio than a thin high aspect ratio board for obvious structural reasons. |
Deep (long span) foils have less drag than short ones. Then it is a case of reducing the chord length to whatever area you need to get the desired amount of lift. |
David Henshall sent me this photo of 'Gently's centreboard. What a whopper! http://www.merlinrocket.co.uk/gallery/view_photo.asp?folder=gallery/vintage&file=gently_centreboard.jpg |
Thanks Mags for putting the picture up! Not only is the board deep, it is also damn heavy, witness the 40ld of lead in the tip. Worse, the early Holt boats had the Holt roller system, so the board was stowed in the case forward of the mast. To get the board down, the crew had to undo a line from around a Tee cleat and then ease the board aft. At some point it would reach the point of no return and rotate downwards - and drop! Having sailed Gently, I'd characterise sailing a boat of that vintage of a board that wouldn't stay up and a rudder that wouldn't stay down!! I've more pictures of some classic merlin c'boards - I'll pass these on! |
David, The centreboard of the c.1946 Holt design Int 14 I have is quite abit longer and more slender that the original merlins' board. It also uses the rolling rather than pivoted method. Certainly something Holt believed in at the time. |