Rudder Failure - builder error!

[simplesimon]

Anders wrote

So a few  questions: How thick should a centerboard be in order to be strong enough? And btw. how thick is the centerboard in other dinks same size like the Coot etc. I was thinking about something like 15 - 18mm

A Mirror 10 (actually 11ft long) had a 12mm daggerboard IIRC

Coypu (12ft long, jib & mainsail, very stable) centreboard is 15mm (and still children manage to break them if they're not glassed)

Wayfarer centreboard is 18mm ply. (Never bothered to glass them)

Simon

[Port-Na-Storm]

I had the same dilemma when deciding whether Katie Beardie should have a centre board or dagger board.
The advantages and disadvantages are well known; Polythene Pam has a dagger board which is a fairly robust affair so its helpful to be able to remove it before heaving it onto the car roof, but it doesn't kick up  when beaching so it has to be lifted out and then it clutters up the boat. Mirror Dinghy sailors will tell you how a raised dagger board will catch on kicking straps etc. at the most inconvenient moments.



So Katie is having a centreboard, its a low profile high aspect ratio kind of thing in an attempt to keep it low in the cockpit and still be fairly effective.  This is a photo before i started butchering it yesterday. The horn had to go as the forward edge of the case is up against a bulkhead.
The pivot point is low down and forward so that I get as much depth as possible as far forward as possible.



One thing you could think about; Some boats have centreboards which have a slot instead of a pivot hole. With a bit of wiggling they can be removed which might suit you for car topping.

Cheers Graham

[Forthsailoar]

For my Selway Fisher Waterman Open Canoe, rigged for sailing, I used a piece of pine plan I had lying around as the leeboard. Around 3/4inch I reckon. Plenty strong enough. And much lighter than plywood of same thickness.

All the best

Osbert

On Monday, 19 September 2011 at 19:55, alopenboat [via UK HBBR Forum] wrote:

On 17 Sep 2011 at 11:04, Port-Na-Storm [via UK HBBR Forum] wrote:

>
>
> I have to admit after your experience I'm wondering whether the 9mm
> centreboard I'm about to fit to Katie Beardie will be up to the job.
> She is a 16' sailing canoe which will have a 40sqft or so sail area. I
> don't plan to be out in strong winds, and if I am I'll probably drop
> the rig and run for it but you never know. The case is made as is the
> board, and i've started the surgery to fit it into the hull so if I'm
> going to change my mind this is the time. My main consideration for
> using 9mm was to keep the weight down as I want to car top.
>

When he was at school my elder brother built a 15ft Percy Blandford
canoe and fitted it with sails. It sailed perfectly well on lakes and
rivers but every time it was sailed on the sea the lee boards broke.
We came to the conclusion that it was not the leeway load that was
the problem but the surging due to the waves.

Strong winds may only be a part of the problem.

--
Hoping for calm nights

Alastair Law,      
Yeovil, England.
<http://www.little.jim.freeuk.com>          

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[Paul H (admin)]

Grim,

What is the depth below the hull?  Scaled up 25%-ish your case+board look about right for a Mirror ...more pics please!

-Paul

[Port-Na-Storm]

The trick with the Mirror would be to end up with the centreboard in the same position as the dagger board when its down. How much space have you got between the leading edge of the board and the mast? Oh and that pesky kicking strap will be in there too, better change the rig to a balanced lug while you're at it.
G

[Jeremy]

Forthsailoar wrote

For my Selway Fisher Waterman Open Canoe, rigged for sailing, I used a piece of pine plan I had lying around as the leeboard. Around 3/4inch I reckon. Plenty strong enough. And much lighter than plywood of same thickness.

Generally a wide plank like this will be significantly stronger and stiffer than plywood for this application, as all the grain is running in the direction you need to take the loads.

If you think about it, plywood is far from ideal for making centreboards or rudders, as these see pretty high bending loads and need pretty much all of their strength and stiffness in one axis.

Jeremy

[Anders]

James
Your description of the centerboard you made for your Coot sounds absolutely perfect and I would do so if I was going to use a trailer. But I´m going to cartop. so a centerboard like yours is to heavy. I like your idea of routing the edge and filling it with epoxy.

Graham
The Mirror has problems with its daggerboard because its sloop rigged. Cat rigged boats have a longer distance between mast and daggerboard case and so the daggerboard shouldnt interfere with the Kicker. (another vote down for the mirror rig)

If making a centerboard out of solid spruce (strong and light) and sheating it with a strong cloth. Would 12 - 15mm be strong enough? or should it be 18mm? I´m talking about laminating a lot of sticks to make the plank in order to avoid wobling.
I think the reason for using plywood for boards is mainly because its more stable and also easyer to work.

Btw, we are talking about plywood as one kind of board. I think there´s no doubt that stronger plywood like birch will make a stronger board than weaker plywood such as okume/gaboon

Well... still thinking.