Centre board and rudder spec

[northernlad]

Hi

I am looking at building up a centre board and rudder for 733.

What's the nest options for this laminated timber and glass sheath
Or just grp?

Also which is the preferred naca profile

[colin]

Hi,

Unless you plan to go into production I would stick with timber core/sheath construction. I use Red Cedar in strip form - it is of great help if you have a friendly woodworking shop that can run the glued core through a thicknesser to provide you with an accurate blank. I laminate with two layers of 280gm 2x2 twill woven glass (I use Easy Composites; www.easycomposites.co.uk ) which drapes very well. You should also use a laminating epoxy. I vacuum bag both layers in one go wrapping the glass around the leading edge - you can trim off the excess trailing edge cloth post cure. If you cannot vacuum bag then I would layup one side at a time.

I use NACA four digit series. For the centreboard 0010 and 0009 for the rudder. Note that as the centreboard is normally raked aft, the true chord of the foil is much wider as water is flowing across a longer 'chord' . You should also note that as the limiting factor is the case width this will determine the chord width. So, for say a 25mm case width, a 00010 section foil will equate to a chord of 250mm. In the same way the internal width of the rudder stock determines the rudder chord. Again for example, a 20mm rudder stock internal width with a 0009 section would result in a chord of 180mm. If you are planning to use a tapered or elliptical foil profile you will need to recalculate for the tip and intermediate sections (for elliptical sections every 100mm is ok). I use a spreadsheet to calculate the foil thickness at each station to give real numbers to work to and don't forget to allow for the sheath thickness!

One critical area is the leading edge profile. If it is too sharp the foil will be prone to cavitate - especially the rudder at extreme angles. Too blunt and you are just adding unnecessary drag. I use a constant radius leading edge that blends into the NACA parabola - a rounding if you like. As the centreboard operates at say 3 degrees angle of attack you can use a smallish radius say 1% of the chord, whereas for the rudder that will be required to operate at large angles of attack something like 1.5% would be required. Again, for elliptical foils this radius will change with chord.

Below is my current elliptical centreboard during vacuumed bagging on the kitchen table...

 
Best of luck,
Colin

[northernlad]

Many thanks for that.

This could get complicated.

What size strips of cedar would you recommend?

[stephane29]

Hi,
I am looking as well building up a centre board and a rudder. Is there overall drawings availlable somewhere ?

[bertfatal]

Not many rules on centreboards or rudders, see section 7&8 of the class rules. The CB must fit inside the CB box when retracted and both the CB and rudder must be made of either wood, grp or a combination of the two.

RK

[stephane29]

Thank you RK for your answer. I am rescuing the 721. I found her in south France and brought back to Brittany. I miss her centerboard and rudder. As there is no British Moth in my area, I can't easly figure out what the centerboard and rudder should look like.
At least some pictures would help to avoid reinventing the wheel.
Stephane.

[bertfatal]

Stephane, if you send me your email address by clicking on my user name, bertfatal, I will send you a photo of my rudder and CB.

[stephane29]

Hi, I sent you my email, I am waiting the photo.