Carbon tiller design

Discussion in 'Sailboats' started by mitchel, Sep 20, 2011.

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mitchelNew Member

Hey all,

I have a 40' cruiser and I'm thinking about changing my wood tiller with a carbon one, I already done some carbon works. I'm thinking about an hollow tiller

I had a look on book/web to find more information about tiller design, I have engineering skills but I should have made a mistake because tiller looks very strong with only few carbon layers :

With ABS I found a torque of 1000N.m on rudder stock.
Projected length of my tiller is 1m long,
Corresponding transverse force 1000N.
Section will be around 100mm x 70mm

I found on net that for carbon with epoxy under vacuum have those properties : Unidirectional carbon have tensile/compresive strength around 1100MPa and Bibiax have shear strenght around 400MPa.

Way of design

UNIDIRECTIONAL FIBERS
Bending moment : 1000N.m
Section width : 100mm
Corresponding tensile strength on side face : 1000/0,1 = 10000N
One side of tiller works in tensile, other one in compression
Stress : Force / Area

minimum area : Force / ultimate stress = 10000/1100 = 9mm2
with tiller heigh of 70mm, minimum thickness 0,12mm what is not a lot !

If I laminate 1mm of UNI on each face, security factor is 7,7 (quite a lot with only 1mm thick laminate)

BIBIAIX
I read the best way is to laminate inside skin with BBX, then UNI, then outside skin with BBX.
BBX have to deal with shear between UNI.
Shear force is 1000N (transverse force)
minimum shear area : 1000/400MPa = 2,5mm2
minimum total thickness (width=100mm) : 2,5/100 = 0,025mm
minimum thickness by face (top+bottom) : 0,0125mm what is not a lot too!

If I laminate 1mm of BB on each face, security factor is 80 (quite a lot with only 1mm thick laminate)

Did I make a mistake in torque calculation (but 1000N=100kg as maximum force on tiller look's ok)

Did I make a mistake in composite calculation?

Does my calculation ok but as hollow tiller other stress exist (buckling, wrikling?)

Thank all for your help

Mitch

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2. HussongPrevious Member

Where is that hyper-qualified engineer guy, Ad Hoc, when you really need him?

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PARYacht Designer/Builder

What advantage are you looking get with this tiller? Weight on a 40' yacht can't be an issue for such a small piece. A 39" tiller seems a wee bit short for a boat of that size too. Lastly, a rectangular section? How comfortable do you expect this to be?

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The moment you derive above is not the only load. There are practical considerations. Buckling, attachment point stresses, durability need to be considered (ex. suppose a 200lb man falls on the tiller during a tack). Also, I wonder about the moment you have. The moment you say on the rudder is 1000N-m (1kN-m) and the tiller is 1m long? Then the force required at the end of the tiller to hold it in place would be 1kN, or about 225 lbs. That is a heck of a lot of force to hold in one's hand while driving the boat. How long is the wood tiller that is already there? How heavy is it? How heavy is the boat? You might save more weight by cutting down your beer consumption...
Kidding aside, if you have a really high performance boat it might be worth the effort. But for a 40 foot cruiser, not so sure.

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mitchelNew Member

Who told you about my beer consumption?

I know that is not a big weight saving deal to fit a carbon tiller, I do it more for fun and beautiful finish.

I had an other look today, I'm pretty sure of my calculation but a think that bending is not dimensionning on a tiller. (Torque on rudder stock come from ABS ORY calcultation, that's maximum torque)

Dimensionning is more to have tiller that look's stiff, that can support my falling fellow on tack.

Do you have an idea of pratical load case for tiller?

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luff tensionJunior Member

Probably the most load it will get will be when you slip over pissed and land on top of it!!
Rather than hollow it would be easier to make it from foam and then skin it with uni carbon and DB. If you can find it use woven carbon sock instead of DB.

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BobBillSenior Member

Look, I am late on this, but think about using one or two graphite or Ti golf driver shafts.

Light, low cost, easily designed and fitted to your needs...I have made pool cues, hiking sticks (walking and tiller) and might do for full tiller, with some glass work and planned approach.

These sticks are very strong and very light...and very inexpensive. You can find old Ti shafts (usually) with head attached and easily removed.

Most are 44 inches long, but extending or shortening is saw/tape easy...really practical move even for just an extension. I am fashion one for next season to use on my little Kite dinghy...way better than the ash club on it now no reason cannot work on keelboat...

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Part of it is load prediction, the other part is stress concentrations.
For a dynamic load, like a 200lb person falling on it, F = m*dv/dt
But- you need to look at the attachment point stresses, which are bolt holes? Difficult to predict in a carbon laminate. Think about how you can attach the tiller that would alleviate this type of stress. Maybe fit the tube into a metal sleeve so you don't have to drill through it, or locally beef up the plies, etc. The devil is in the details as always.

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