Rudder shape ?

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Rupert
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Re: Rudder shape ?

Post by Rupert » Thu Dec 17, 2015 12:03 pm

Alan, Rooster tail with old rudder, none with new. Maybe!

Trouble with the Minisail is the inability to get the stock clear of the water.
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alan williams
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Re: Rudder shape ?

Post by alan williams » Thu Dec 17, 2015 12:19 pm

Fair point Rupert. I had forgotten how low the rear end of a Minisail sits in the water. With the need to produce a stock strong enough to take the load and very little space to do it in. Has any one tried a dagger board type rudders, with a carbon reinforced stock, as it could be made smaller and lighter? Or is carbon prohibited?
Happy Christmas
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Re: Rudder shape ?

Post by JimC » Thu Dec 17, 2015 1:33 pm

alan williams wrote:Has any one tried a dagger board type rudders, with a carbon reinforced stock, as it could be made smaller and lighter? Or is carbon prohibited?
Although I tend to use carbon just for convenience (fewer layers) I see no reason why a glass daggerboard stock shouldn't be perfectly adequate on a minisail. A predominantly glass dagger rudder stock on my old Cherub was still going last time I saw any photos. The main thing is to give it enough width.

Have a read of this. http://www.uk-cherub.org/doku.php/tech/rudderstock Making one for a minisail out of just glass I would increase the width of the bottom plate a bit. Also make sure you use proper woven glass, not chopped strand mat. CSM adds nothing really except bulk.

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trebor
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Re: Rudder shape ?

Post by trebor » Thu Dec 17, 2015 6:44 pm

I like your disclaimer Jim :D

I had identified issue with Rudder stock, just changing this would remove a fair amount of drag.
I intended to use something similar to Aquabat Stock, which is a cast aluminium frame, I now like the idea of making one. 8)
http://s1149.photobucket.com/user/aquab ... o.jpg.html
Back of Rudder stock is well below water line in this shot.
http://s1149.photobucket.com/user/aquab ... ?sort=3&o=
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trebor
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Re: Rudder shape ?

Post by trebor » Sat Jan 02, 2016 6:55 pm

I have sailed boat today with original Rudder, which was swept back, I have altered it to a near vertical position, this has eliminated weather helm, but when travelling at speed blade osillates from side to side, (it is turning almost has though you are propelling) not a lot but enough to be annoying, any idea what is generating force to do this?
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realnutter
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Re: Rudder shape ?

Post by realnutter » Sat Jan 02, 2016 7:08 pm

Have you moved it so far forwards that the tip is actually forward of the pintle line?

That might cause it to flutter...
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Re: Rudder shape ?

Post by Nigel » Sat Jan 02, 2016 8:47 pm

my guess would be turbulence amplified by play in the pivot. When the blade was angled backward, the flow of water acted to hold it in a stable position. Now you have removed the drag from behind the pivot point, it is showing up the play.

I would check for wear in the pivot hole and also the clearance between the blade and the sides of the stock

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trebor
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Re: Rudder shape ?

Post by trebor » Sat Jan 02, 2016 11:26 pm

Matt,
I don't know, didn't notice, could be, I will check tomorrow.
Nigel,
Their is definitely no play in pintle, all items related to stock, pintle and mounting bracket are new, only blade is old, but it is not loose in stock.
This is like a wobbly wheel on a shopping trolley.
The leading edge of blade was swept back about 75 degrees, it is now almost 90 degrees, the bottom front edge is probably 6" further forward.
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Re: Rudder shape ?

Post by Nigel » Sun Jan 03, 2016 10:00 am

Hi Robert,

I meant the pivot hole on the blade itself - but if you grasp the blade and there is no movement of it within the stock then possibly something akin to a "speed wobble" on a motorbike is going on. It may be fixable by moving the pivot hole in the blade/stock forward to change the balance but that becomes a trial and error process so not an ideal approach.

Can you post a photo of the blade etc?

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trebor
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Re: Rudder shape ?

Post by trebor » Sun Jan 03, 2016 1:08 pm

I removed material from blade just below stock, this allowed the blade to come forward, the weather helm which was vicious in any decent wind, now gone, brilliant !
The blade was then refinished, the pivot hole is not worn, their is a brass bush, I fitted this, plus a 8mm stainless allen bolt.
The tiller and stock were re-built after imploding at nationals, it was an incredibly poor design.
http://s1149.photobucket.com/user/aquab ... g.html?o=2
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Rupert
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Re: Rudder shape ?

Post by Rupert » Sun Jan 03, 2016 1:30 pm

One possible is that it is simply because the blade is flat sided with bevelled edges, rather than an aerofoil shape. By putting it more vertical, the angle of attack has changed, and the flat sides are now vibrating. Can you tell I'm not a physicist?

I too would have thought pintle wobble, as those pins going through the plates are not ideal, but if you have solved that, it must be to do with hydrodynamics, I guess.
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Max McCarthy
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Re: Rudder shape ?

Post by Max McCarthy » Sun Jan 03, 2016 2:09 pm

trebor wrote:I have sailed boat today with original Rudder, which was swept back, I have altered it to a near vertical position, this has eliminated weather helm, but when travelling at speed blade osillates from side to side, (it is turning almost has though you are propelling) not a lot but enough to be annoying, any idea what is generating force to do this?
Do you think this may be more to do with the dagger board instead of the rudder itself?

If the daggerboard is not a flush fit it could be moving about within its box, causing turbulent flow (such as this) and at a speed which is great enough, this turbulent flow could be affecting the rudder. Before, when the rudder was at an angle, it may well have been able to dissipate the effect of this by being at an incline to the flow, but bringing it up to more closely vertical, could well increase the effect of this at lower speeds.

If I were you I'd check the dagger board and how flush it is to the box before doing anything that requires work on the rudder stock.

Cheers,

Max
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ian60
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Re: Rudder shape ?

Post by ian60 » Sun Jan 03, 2016 6:22 pm

Although I'm not familiar with this type of stock the pivot bolt seems a long way back in the stock, most of the ones I've come across are more central. I wonder if being so far back you get an effect a bit like on those big, flat B & Q trolleys which have the steering wheels at the back. They are a s*d to push, much easier to pull. I know you have said there is no play of the rudder within the stock but any play or flex in the blade might be exacerbated with the pivot so far back.
As others have said, when the rudder was angled back such effects might have been lessened. What's it like if you angle this set up back the way it was? Does the flutter disappear or reduce?. The weather helm will certainly come back but it would be a useful check.

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trebor
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Re: Rudder shape ?

Post by trebor » Sun Jan 03, 2016 6:43 pm

Max,
Sprint is fitted with centreboard, no play in this.
Rupert,
Perfectly understandable, I had thought about this, but dismissed it, now you have mentioned has well, could be, board is very wide compared to Blade on Aquabat, (Aquabat blade is aerofoil shaped though).
Ian,
Their is no play, but blade could possibly flex.
Nigel,
Here are a couple of photos, I have shown how blade used to be angled and how it is now. Just scroll from side to side to see both images.
Apologies for using photobucket, I know ads can be a pain if you do not use a ad-blocker, I am currently setting up my website to host photos.

http://s1149.photobucket.com/user/aquab ... o.jpg.html
http://s1149.photobucket.com/user/aquab ... g.html?o=0
Last edited by trebor on Sun Jan 03, 2016 7:17 pm, edited 1 time in total.
Robert
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Re: Rudder shape ?

Post by Nigel » Sun Jan 03, 2016 7:03 pm

Thanks Robert,

my money would be on it being the pivot hole being so far back on the blade.......but no guarantees :)

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