Stainless rudder stock needs replacing in foam core rudder?

If you hot wire styrofoam between the ply templates make sure they are very tight or you will get a small ammount of hollow emerging later. It does not always behave in a 'perfect' way. Better to cut very slightly proud and fair in with a board. I've seen too many 'closed cell' foams that have become sponges!
When fridges are blown with the insulation they are in massively solid holders. Yes I know they are thin metal skin, but post blowing of foam exerts massive force even with an exit point. Which most likely rules out another option!.

Rather with Waikikin on the construction. A couple of decent templates to check symmetry and section is all you'd need to get a very fair section. Fairly quick for a one off.

 

OK guys. I think the plan will be to use a NACA 0014 (rather than the 63 which it had) and stick to a similar planform as original just for simplicity. I may even not bother with the elliptical shape as it will be much harder to make fair. It probably wont make much difference?

Now the question is the tabbing. The old one had a hollow V tab with thin wall material. Which is strong and light but seems to work best only with the pour foam method which we will not be using. We will just build it up with regular 130kg boat building foam. This will work better with a conventional flat plate tab.

I will make the tab around the same area as the old one. But just weld the flat plate to the tube. What thickness should this tab material be?

Also I had the rudder stock tubing checked to ABS ocean racing rules by a NA (thanks for that Mate!) and the old one was marginal but not terrible. The new one will be very slightly thicker wall.

 

Same as the wall thickness...

 

Cheers 7-8mm. Ok can I play devils advovcate here. The last rudder did not use stainless for any of the rudder parts besides the large stock and none of the mild steel was rusted. The lower stock and tabbing was just mild steel. Now I will be using stainless for both tubes, but can I get away with mild steel for the tabbing? That is a lot of flat plate to order in stainless.

Click for pic.

https://www.dropbox.com/s/h68qjv83yemtb5p/IMG_2906 (Copy).JPG

 

Make your own in glass if you like... Get some hpr epoxy adhesive from ATL composites and wrap your shaft with glass so it lays out and forms a tab at the same time. Sandwich this in your foam, and when you glass around the outside of the blade, make the outer laminate contact with the glass you wrapped the around the shaft. This way the torque is transmitted into the outer laminate and the core via the tab.

 

I would love to build it this way, but I am told this is not a good idea with a stainless shaft as the bond between epoxy and stainless can not relied upon?

I just found this. Seems this is specifically for bonding to metal. Would this meet official structural requirements if I done it this way? Maybe a mini tab of say 10mm or some welds around the shaft would prevent the shaft from turning inside the fiberglass?

https://www.google.com.au/url?sa=t&...3ndNtSDaO1iUFUUnQ&sig2=tugT_F4HZkPxrqu5J98HbA

 

Is it possible to 'crush' the lower part of the tube? If you can put it into a roller and create an oval section it will grip better and/or allow glass/epoxy to become a secondary larger tab. Similar to that which Groper suggests above. Just perhaps a thought, but I prefer to have a mechanical as well as a chemical key. Mostly that combination is vastly superior to just pure adhesive connection. Even local indents in the tube would work well.

 

Apparently worked fine last time, One of the rationales that applied to the cat rudders we used to do similar was that if the rudder was impacted/damaged that the metal armature inside may bend but would still be there to act as a half rudder.
Jeff.

 

Welding a hunk of plate along the aft edge of the shaft will improve both the mechanical aspects and increase the bond surface area. This is a common ploy and usually the plate has a few holes in it, if only to lighten it, provides more "keying" surface. There's nothing unusual about that linked epoxy, that appropriate fillers in a quality marine epoxy couldn't do. The key is the surface prep, which is easy enough with a DA and some crocus or emery cloth of a moderately coarse grit.

 

If you have absolute assurance that no moisture will ever reach the tabs, carbon steel would be ok. However, carbon steel welded to stainless steel can rust faster than if it were all carbon steel (the old sacrificial anode kind of thing). Ask yourself, with all of the work involved in what you are doing, is the savings worth it. A middle ground in terms of material is chromemoly.

 

You might want to check out the videos on "boatworks today" on you tube. He has a series titled "rebuilding a foam core rudder", in which he repairs a rudder from a Freedom 35. This rudder has no tabbing or any structure but the rudder post (hollow tube). The designer did not even spec sealing the top of the post to keep water out,that's how it failed.
I hope my Pearson is designed better

 

Things move slowly sometimes. I have built a new armature with a new stock, all 316 (stock sized specified by a NA) and I am ready to turn it into a new rudder.

I managed to keep one side of the original rudder so I cant see the point in making a whole new one. My plan is to cut out 4-5 half ribs of plywood to mirror the foil shape then block between these with foam and fair down to them. The will end up staying inside the rudder (I have 6mm marine ply for this).

Question is what is the quickest and most accurate way of getting my ribs to match the foil shape of the existing rudder half? Sure the first one will be easy as I can just trace around the edge but what about the rest?

 

You could do worse than use a profile guage. Here in the UK I can only get 300mm long ones but even this can be used for greater lengths if you can get a datum 'face' such as a flat plank. Then you can scribe onto card or scrap thin ply and trace. then cut and fair until it matches the 'good' rudder face as a female. If you then cut ply to that and reduce by skin thickness you have the inside face. Me, I'd get the skin thickness and hold pencil inboard by that thickness and scribe using finger along ply edge or you could use a proper scribe or marker guage.

You can then also use the card templates as a guide when fairing the new half as they will be external face accurate. Alow a smidgen for two way diagonally sanding when you fair the section. Never sand straight along the section unless roughing btw, but I'm sure you know that.

If the 63 series section is not far away from the preferred 0014 you could always pack it up slightly. Depends on the exact differences in construction as to how to do that ie lay on more glass and feather or micro balloon, and resin. The 63 series is still pretty reasonable IMHO.
Either way getting a good gelcoat finish will be a fair bit of work sanding wise but doable. Make sure you make full use of the 'cutting' window when the gel is set enough to make it easy.

 

All you need is the chord lengths at a series of stations along the depth of the rudder.

Or you can draw the plan form in autocad and get the chord lengths from the drawn out plan form. Once you have the chord lengths, you can use the online foil section calculator which will plot the foil curvature based on the chord lengths for whatever foil section you want.

Once you have your section profile for each chord length, get them down to the local CNC router, and have each section machined out of your chosen material.

Once you have these, set them up at the correct vertical spacing or station seperation distance.

If you fair down to these, it will come out perfect, fully tapered foil section matching your plan form.

This way it doesn't matter how accurate your old one was, you will know your new one is perfect.

 

Seems like a false economy to save some glass & resin, maybe better to do as I thinks Groper has indicated & just make it.... both sides to a recommended profile
However if you want to take off the shape you need to establish the centerline as a datum & fwd or back edge as datums. To get the shape off you can "spile" it, google up "joggle stick" or some such modern description..... some blocks hot melt glued along the lines of sections you want will help to position the "spiling" batten/loosely fitting template then you can draw from spiled info the shape of the OUTSIDE of the existing half.... but then you gotta deduct the predicted thickness of glass/laminate/filler your going to apply.... this is the "bearding line" that your plywood shapes will be... that's where the system falls down some as on that leading edge it's nice to wrap your laminate around............ & of course the old half may not offer up in an ideal fashion to your new armature...............
Jeff