new rudder construction

contessaman

contessaman

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Right, boat needs a new rudder this winter - the old one has more osmosis and blisters than could be possibly imagined and has even split due to water ingress.

The existing one is grp. its unbalanced on the back of a long keel quite low aspect ratio on a s/s stock with wheel steering. hinge tang to bottom of keel top tang attaches integral to rudder stock.
its also made completely flat and isnt a very hydrodynamic prospect. its about 40mm thick all over.

my plans for new one:

12mm top spec marine ply from robbins

tripled up in areas of top and bottom hinge tangs (with epoxy) = 36mm thick

rest of ply clad in 12mm foam sheet either side (total thickness again 36mm)

use sanding block to give it a round leading edge and a tapered trailing edge. (a slight aerofoil profile)

then laminate the whole thing in biaxial cloth/epoxy/ CSM alternating between layers.

finally drill mounting holes oversize then stuff epoxy/cloth into them then re-drill (so water cant get into core)

result should be light yet strong. ply rather than foam where the tangs go mean it shouldn't compress when the bolts are tightened. aerofoil shape might mean it will stall at a higher angle.

thoughts? previous experience? anything?

TVM
 
I'm building a rudder very similar to this construction at the moment but with a 6mm elite ply core with solid timber strong points and the rest foam and the lot glassed in a combination of biax/csm to build the right thickness for the size of rudder. I'm confident it will be light and strong compared to the original. Would recommend reading the west PDF on rudder construction
 
Alternative is to laminate with cedar strips, perhaps with mahogany in way of tang fastenings. You could build in more shape by using different widths strips as you go aft. easy to plane off then finish to final shape. Sheath in glass cloth and epoxy.
 
When laminating do not use CSM which is intended for polyester resins only.With epoxy resins you can use nothing but rovings.Make sure you don´t allow the resin to fully cure between layers.If you allow the epoxy to fully harden it'll have to be abraded before laminating further layers on.
 
Your plan will produce a rudder possibly a bit on the hefty side but that's no bad thing; I assume from your user name that it's for a Contessa, so unless you really want the experience of laminating your own a replacement could probably be ordered from the chap who built your boat;

http://www.jeremyrogers.co.uk/

30boat said:
When laminating do not use CSM which is intended for polyester resins only.With epoxy resins you can use nothing but rovings.Make sure you don´t allow the resin to fully cure between layers.If you allow the epoxy to fully harden it'll have to be abraded before laminating further layers on.
Click to expand...

You can get CSM that's compatible with epoxy resin; it's important to get this right when ordering as there are 2 different types. Having said that CSM will soak up large amounts of expensive epoxy; it's not really necessary to interleave the roving layers.
 
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contessaman said:
Right, boat needs a new rudder this winter.

The existing one is grp. its unbalanced on the back of a long keel quite low aspect ratio on a s/s stock with wheel steering. hinge tang to bottom of keel top tang attaches integral to rudder stock.
its also made completely flat and isn't a very hydrodynamic prospect. its about 40mm thick all over.

my plans for new one:

12mm top spec marine ply from Robbins

tripled up in areas of top and bottom hinge tangs (with epoxy) = 36mm thick

You need to add a couple of mm to allow a minimum of 1 mm of resin between the ply slabe, squeezing them too tight will weaken the joint. Adding some Micro-fibres to the epoxy will give tou a stronger joint.

rest of ply clad in 12mm foam sheet either side (total thickness again 36mm)

Ensure the foam is 'closed cell' foam or you have problems later.

use sanding block to give it a round leading edge and a tapered trailing edge. (a slight aerofoil profile)

You can balance the rudder at this point by having 15% of the total area in front of the shaft centreline.

then laminate the whole thing in biaxial cloth/epoxy/ CSM alternating between layers.

I would avoid the use of CSM with an epoxy, it used a lot more resin and has to be a special binder that will work with epoxy.

finally drill mounting holes oversize then stuff epoxy/cloth into them then re-drill (so water cant get into core)

Use micro fibres for this, if you use cloth, you will be cutting through strands and exposing raw cloth, a great recipe for disaster.

result should be light yet strong. ply rather than foam where the tangs go mean it shouldn't compress when the bolts are tightened. aerofoil shape might mean it will stall at a higher angle.

Consider rebating the ply to take the tangs and full with micro-fibre mix for a solid joint.

thoughts? previous experience? anything?

Also consider adding 'O' rings to the rudder stock to prevent water entry via the shaft due to different expansion and contraction rates of timber and steel.

Clean the area, slip on 2 or three 'O' rings with Vaseline, under and between them, then cover with rovings 3 layers of 150 gram will do, this gets fully glassed in when the rudder is fully built.

TVM
Click to expand...

Good luck and fair winds. :)
 
The rudder will basically be the aft part of one big aerofoil section. ie from front of keel to trailing edge of the rudder. So the rudder itself should be a simple taper from front to back. The front should be semicircular to match as close as possible to thre trailing edge of thee keel. A slight concave shape to the aft edge of the keel would give least water disruption as it moves from keel to rudder.
The rudder should be laminated with epoxy and best is cloth not CSM. However if you want to use polyester resin (cheaper and goes hard quicker in cold depending on quantity of hardener) you need to make the internals quite thin and have the skin of the rudder in excess of 6mm thick. This gives you a polyester GRP rudder with internal mold left in place.
A thin epoxy laminated rudder is really a wooden rudder with GRP cover. The epoxy will adhere better to the wood. The former style polyester adheres well to itself and is so thick the adhesion to the wood is not so much concern because the GRP has the strength in itself. Don' t try to use polyester in a thin layer as it will not stick to the wood and will simply peel off. good luck olewill
 
exellent replies as always, thanks guys!

I should have put a photo on here, I'll do it later - its not for a contessa its a beneteau evasion32. the stainless rudder shaft does not go into the rudder anywhere it terminates in a wrap around tang which grips and is bolted to the top of the rudder. there is then a stainless hinge bolted to bottom of keel which then has another tang that bolts and grips to the bottom.

so, no CSM, micro balloons to fill and re-drill holes, closed cell foam.

good thoughts on aerofoil of rudder only being part of the bigger picture - the keel.

Interesting and tempting idea of polyester resin. I could then flow coat over the top producing, as you say, a GRP rudder with the internal mould still there. I suppost this is what the original rudder is made from so there is some merit in this style of construction. Certainly a lot cheaper than all that epoxy. I also have far more experience laminating in polyester resin.

oldsaltoz, your thoughts on the latter???

TVM
 
(quote)Interesting and tempting idea of polyester resin. I could then flow coat over the top producing, as you say, a GRP rudder with the internal mould still there. I suppost this is what the original rudder is made from so there is some merit in this style of construction. Certainly a lot cheaper than all that epoxy. I also have far more experience laminating in polyester resin.

oldsaltoz, your thoughts on the latter???

TVM

One thought comes to mind on the pro's and con's of all epoxy or mixed construction.

If damaged in any way an all epoxy becomes a simple repair.
If a mixed resin construction the internals may have to be replaced as any moisture will penetrate and cause major damage, though possible not in the short term. The last thing you want is any moisture inside a rudder.

If you have done some DIY fibre-glassing with standard resins you will find using an epoxy in not a great leap.

Only two things to remember.

1. Standard resin and CSM has a ratio of 3 parts resin to one part mat by weight (or volume as the difference is tiny).

2. Epoxy resin and cloth has a ration of 1 part resin to one part cloth by weight (or volume), so a lot less resin and lot more cloth, this means more care and rolling to the cloth is properly wetted out.

Applying a vacuum bag as suggested above is also a good system. this works all resin types.

Good luck and fair winds. :)
 
contessaman said:
Right, boat needs a new rudder this winter - the old one has more osmosis and blisters than could be possibly imagined and has even split due to water ingress.

The existing one is grp. its unbalanced on the back of a long keel quite low aspect ratio on a s/s stock with wheel steering. hinge tang to bottom of keel top tang attaches integral to rudder stock.
its also made completely flat and isnt a very hydrodynamic prospect. its about 40mm thick all over.

my plans for new one:

12mm top spec marine ply from robbins

tripled up in areas of top and bottom hinge tangs (with epoxy) = 36mm thick

rest of ply clad in 12mm foam sheet either side (total thickness again 36mm)

use sanding block to give it a round leading edge and a tapered trailing edge. (a slight aerofoil profile)

then laminate the whole thing in biaxial cloth/epoxy/ CSM alternating between layers.

finally drill mounting holes oversize then stuff epoxy/cloth into them then re-drill (so water cant get into core)

result should be light yet strong. ply rather than foam where the tangs go mean it shouldn't compress when the bolts are tightened. aerofoil shape might mean it will stall at a higher angle.

thoughts? previous experience? anything?

TVM
Click to expand...

Yep done that:

Its dead simple, i did mine out of two pieces of 25 mm marine ply in a vacuum then shaped the blank with electric planer, 24grit disc and lots of swearing dont bother with chopped strand - waste of time as theres no strength to it, just sheath with three layers of 450g bi-ax cloth - if you want to get super posh whack a kevlar tape all the way around the edge of the blade to help protect it.

a new rudder for my Centaur for £60. (Already had the ply)

If no vacuum, lots of dumbbells or a fat bird, pictures below:

https://picasaweb.google.com/110182886418433827802/NewRudder02?authuser=0&feat=directlink

Did the kevlar thing on my keels, check these bad boys out:

https://picasaweb.google.com/110182886418433827802/GoldColouredLoveliness?authuser=0&feat=directlink

cheers roger

www.agentlemansyacht.com
 
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