Working out a decent laminate for a rudder

[Zoro]

I don't know what prior experience you may have in making rudders so I may be assuming too little experience compared to reality.My choice of method would be to scarph a short piece of hard wood to the exposed ends of the underwater sections of the paulownia strips,as illustrated below (if the file appears).
The only way I would consider adding a groove for a tough insert strip would be with a router cutter of this style as the greater surface area of the face would give the router some stability.

I've made a few over the years, for smaller boats. Normally a clear cedar core with a couple of layers of glass.

That and restoring foam filled, female molded blades and boards that have ended up cracked and soaked. They have all been chopped strand, production boat stuff.

 

[Zoro]

I had a bit of free time and wanted to see if I could create a CAD model of the rudder in question.It's a bit rough and ready but shows the useful features'You may need some suitable software to open it-most 3D software should cope with step format or if you don't have it,download Freecad which costs exactly what you might hope from the name.

If you have it in FreeCAD format, I'd like to see how you created if.

This is a quick exercise I did in FreeCAD watching a tutorial, it's OK as far as it goes, but I have to learn how to plot the exact foil section I want. Just tweaking a bspline seems a bit hit-and-miss.

I had to call it a pdf to upload it.

 

[Tops]

FreeCAD should be able to read in airfoils in the *.dat format.
https://m-selig.ae.illinois.edu/ads/coord/n12.dat copy the text and save as a text file with the extension .dat

@ Zoro the foil you attached above looks nice, eventually you will want it not to taper to zero at the trailing edge.

 

[Zoro]

FreeCAD should be able to read in airfoils in the *.dat format.
https://m-selig.ae.illinois.edu/ads/coord/n12.dat copy the text and save as a text file with the extension .dat

View attachment 198445

@ Zoro the foil you attached above looks nice, eventually you will want it not to taper to zero at the trailing edge.

Thanks

Yes, that seems to have worked. I think I can generate .dat files on that NACA foil website. Now I just have to workout how to use the import properly. More vid tutorials!

 

[wet feet]

There is an add on for importing .dat but it gives the panelled surface that aerodynamicists regard as normal in their field.I find the two rail sweep rather weird in operation but it does work in it's own way.I find it a bit slower to use constrained points in construction mode on the Sketcher workbench and to locate a spline through them if I want a single surface.I have had luck with scaling and locating these sections along the outline and then lofting through them.For the tip fairing I have sometimes used a narrow extrusion of the profile-say 2mm and created a fairing between the bottom of the foil and the edge of the narrow strip with G2 or better continuity to the foil and G0 at the narrow strip.In hindsight I would have used an upswept arc at the bottom of the rudder head, using a rise of maybe 80mm across the chord.I have been learning Frecad for a while as the Path/Cam workbench is useful for generating toolpaths for my hobby CNC router and vastly cheaper than a seat of Mastercam.For surface blending,this video may be useful

 

[Zoro]

There is an add on for importing .dat but it gives the panelled surface that aerodynamicists regard as normal in their field.I find the two rail sweep rather weird in operation but it does work in it's own way.I find it a bit slower to use constrained points in construction mode on the Sketcher workbench and to locate a spline through them if I want a single surface.I have had luck with scaling and locating these sections along the outline and then lofting through them.For the tip fairing I have sometimes used a narrow extrusion of the profile-say 2mm and created a fairing between the bottom of the foil and the edge of the narrow strip with G2 or better continuity to the foil and G0 at the narrow strip.In hindsight I would have used an upswept arc at the bottom of the rudder head, using a rise of maybe 80mm across the chord.I have been learning Frecad for a while as the Path/Cam workbench is useful for generating toolpaths for my hobby CNC router and vastly cheaper than a seat of Mastercam.For surface blending,this video may be useful

Thanks, I have to get my head around all of this stuff. I'm about 12 months into learning FreeCAD, my son wanted a 3D printer so we jumped into the CAD world. It's been remarkably useful, the boat has a few functional 3d printed parts that seem to be working fine. We are considering an upgrade so we can print some of the stronger "engineering" plastics. So far we have been restricted to PETG, OK, but not great.

 

[Zoro]

Thinking more about this, is there merit to putting a dense hardwood under the rudder gudgeons to take the localised compression loads I assume occur?

 

[wet feet]

A lot would depend on the width of the straps,as far as compression loads are concerned.The clamping forces might be sufficient to resist the shear loads imposed by the bolts.In ordinary use the loads shouldn't be that great but if you hit a hefty bit of flotsam,who knows?I might be tempted to locate a Tufnol rod where the forward bolt of the gudgeon passes through the head of the rudder and drill the bolt hole after sheathing.

Incidentally,do you plan to vacuum bag the sheathing?

 

[Zoro]

A lot would depend on the width of the straps,as far as compression loads are concerned.The clamping forces might be sufficient to resist the shear loads imposed by the bolts.In ordinary use the loads shouldn't be that great but if you hit a hefty bit of flotsam,who knows?I might be tempted to locate a Tufnol rod where the forward bolt of the gudgeon passes through the head of the rudder and drill the bolt hole after sheathing.

Incidentally,do you plan to vacuum bag the sheathing?

Thanks.

I might bag it, I have to replace some equipment to do that, so maybe not... the budget is tightening at the moment!

I was looking at some 300gm uni carbon earlier, it was surprisingly cheap, a little over double glass. Not a great expense in the scheme of the project. Tempted to use some of that for the uni layers. Less bulk.

 

[wet feet]

If you do manage to cover your rudder in one operation,it may be possible to ignore the suggestion to use blunt trailing edge.I have found that adding a slightly flexible support inside the vac bag,but outside the release film will support the trailing edge laminate.With the thinnest sensible trailing edge you can reduce the possibility of a humming rudder.It isn't the easiest thing to get a rudder as large as you propose to use bagged within the working life of the resin,but with a well briefed helper it can be done.I hope the attached file explains sufficiently.

 

[Tops]

Letting in some harder wood into the softer wood under the hardware makes sense. Also consider the technique of over-drilling holes, filling those with resin and milled fibers, cure, then re-drill the holes the correct size for hardware so there is way less potential for water to reach the brand new wood core.

The thing I regret doing on my last set of vacuum-bagged plywood foils is NOT wrapping the leading and trailing edges with a strip lightweight plain weave at 45 degree bias to the edges.
Were I to make another, the edges would get wrapped, maybe once before bagging and possible again after the initial post-bagging clean-up. I'd probably apply the pre-wrap a few hours before bagging (within the resin's re-coat without sanding window) so I could fiddle with it a little without jeopardizing a larger batch of materials.



If one did a hybrid molding technique like Wet Feet shows in the PDF above, the trailing edge would not need a pre-wrap and maybe not even a post-wrap.

 

[Zoro]

Letting in some harder wood into the softer wood under the hardware makes sense. Also consider the technique of over-drilling holes, filling those with resin and milled fibers, cure, then re-drill the holes the correct size for hardware so there is way less potential for water to reach the brand new wood core.

Yes, I have done this with deck fittings in the past when a ply core was involved. It's a good idea IMO.

The thing I regret doing on my last set of vacuum-bagged plywood foils is NOT wrapping the leading and trailing edges with a strip lightweight plain weave at 45 degree bias to the edges.
Were I to make another, the edges would get wrapped, maybe once before bagging and possible again after the initial post-bagging clean-up. I'd probably apply the pre-wrap a few hours before bagging (within the resin's re-coat without sanding window) so I could fiddle with it a little without jeopardizing a larger batch of materials.

View attachment 198579

If one did a hybrid molding technique like Wet Feet shows in the PDF above, the trailing edge would not need a pre-wrap and maybe not even a post-wrap.

I think I might tape the edges first, under (and maybe over) the double bias and uni. I was focused on wrapping the double bias, but I am now thinking that structurally the uni is doing most of the work and wrapping that makes no sense so really I only need to be concerned about water proofing the edge joins.

It's looking like this won't be bagged and will be laid up in sections, one side @ a time.

Not throwing stones at the use of plywood, I've sailed a few boats with ply foils, but a few years ago I ran across an article explaining why they fail and that it wasn't the best choice of core. I can't find it now, found this instead...

Why Plywood breaks - Real wood for Centreboards and Rudders https://www.storerboatplans.com/foils/plywood-vs-timber-for-centreboards-leeboards-and-rudders/

Might be worth considering. That said, I know some classes down here have used it for decades.

 

[Zoro]

I've made some material enquires and as luck would have it I ended up talking to someone who has done a lot of rudders, including for my boat class.

The suggestion was...

1. Use Oregon at least until the end of the gudgeons with lighter laminate. Basically, the heavier core and lighter laminate end up being stronger for a similar weight @ less expense.

2. 600gm Biaxial with two 400gm Double Bias + epoxy was the recommendation.

3. Don't mix carbon and glass, the carbon will snap before the glass reaches its limits. + the core as suggested is supplying the stiffness.

I guess that's inline with what was suggested earlier. Probably gives me more wiggle room as the core will be a good amount of the strength.

 

[Zoro]

Any tricks for shaping the elliptical tip? I've only ever made straight foils before.

 

[BlueBell]

Any tricks for shaping the elliptical tip? I've only ever made straight foils before.

Ya, use a string loop and two fixed points and draw it.
Google

OR

A data program to CNC the shape.