13/10/2012 21:07:45
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?

14/10/2012 08:16:36
Alex 3627 (ex3556)
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.

Based on that, the sea is the perfect place for trying out high aspect ratio foils. Of course there are still other issues.... Longer centreboard means longer centreboard case required. I think this is probably why it has not been played around with much before. But various people (Dave Winder/John Turner) are now starting to play with new higher aspect foil designs as this is an area of Merlin Rocket design that has not seen much development for a long time.

The latest winders have an option of a longer centreboard case with a high aspect ratio centreboard though I have yet to actually see what the new foils look like and how much the ratio has changed from the old ones...

Hope this helps.

14/10/2012 08:35:26
Chris M
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. 

The spitfire wing shape ticks all the boxes, its an efficiant shape and loses area quickly as you retract the board hence why its lasted so long.

I suspect we'll probably end up with a half way house, higher aspect foils of a not too dissimilar shape but im happy to let others develop that!!

14/10/2012 09:48:54
Andy Hay
But what have you got in your new ship, Chris?

14/10/2012 15:36:04
David Henshall
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!


14/10/2012 17:37:32
chris m
Standard board but I've had the long case.

15/10/2012 09:21:40
Here's the original plans for the Merlin's board
15/10/2012 13:22:56
Fat Pig
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.

15/10/2012 14:39:17
Peter Mason 847
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.
To give you some idea of the relative drag terms, I have estimated the following for Bambusa 847 at a reference speed of 2 m/s:
wave making drag = 9.2 N
hull friction drag = 23.1 N
C/B friction = 3.6 N
rudder friction = 3.2 N

Total induced drag
(C/B and rudder) = 1.13 N
(The rudder is a significant contributor to induced drag because the free surface effect means that it is 2.5 times "worse" that the C/B for a given load.)

There are other drag terms which are not easy to estimate. One which I think is interesting is the interference drag at the junction of the hull and the C/B. I would guess that minimising this would be worth a lot more than deeper foils.

15/10/2012 18:04:56
Barry D
What a shame we outlawed dagger plates!!

15/10/2012 20:33:19
Alex 3627 (ex3556)
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.

I would however have thought that the pressure drag created by flow seperation on the surface of the airfoil when lift creating (beating or close reaching) would contribute more to the overall drag than the profile drag and this flow seperation can be reduced by having a shorter chord length i.e. making the board higher aspect ratio.

I may be wrong here on some finer points, I would have to dust off my old lecture notes to check... think they are in the loft at my parents...?

15/10/2012 20:39:58
Alex 3627 (ex3556)
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...

15/10/2012 22:58:11
Peter Mason
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.
I agree that drag due to flow separation can be massive.

15/10/2012 23:00:18
Andrew M
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)

16/10/2012 07:25:30
Alex 3627 (ex3556)
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.

16/10/2012 14:03:28
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.  

So deep foils are theoretically faster in steady conditions, but they are also more sensitive to use and less practical. They will stall more easiliy, making starting and manoevring trickier. Deeper foils also mean more heeling moment, so you will be overpowered earlier and from then on will be no faster.

21/10/2012 15:25:03
David Henshall sent me this photo of 'Gently's centreboard. What a whopper!
21/10/2012 21:53:31
David Henshall
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!


22/10/2012 00:06:35
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.
By the way the image is actually Iska's board (which is white) but it is exactly the same as Gently's. If I remember correctly it gives a draft of 5'6" with the plate down. The 25'rig gave the main an aspect ratio of over 3:1 and the mainsail foot is only about 12" longer than the foot of the jib (Total area 100sq ft)


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