GRP rudder assembly technology-advice

[Vic]

We are currently building the 30 ft performance cruiser with the potential to build more than one boat hence we are looking for some sort of the technology used in production yachts.

We have encountered the problems which we have overlooked at the beginning as the assumptions seemed quite simple.
We have made the plug of the rudder (high aspect spade type) and the respective two piece female mould.
Further we made the rudder stock of the typical construction which can be seen on the manufacturers pages like Lewmar or Jeffa (i.e. tapered shape with the maximum diameter at the bottom bearing of some 55 millimetres and the horizontal grid of V-shaped elements welded to the stock -all 316 stainless steel).

Our intention (without paying too much consideration at the beginning) was to put the rudder stock between the GRP shells of the rudder -still in the mould- cover the mating edges of the shells with the structural adhesive, close the mould and inject the PVC foam.
Sounds simple at the beginning, however after the detailed consideration we have found out that we have no assurance that this process will produce the expected result.

Firstly - we are not certain if the bonding of the surfaces just by the squeezing the excess of the adhesive from the surfaces which are not parallel and with the relatively small mating surface at the trailing edge will be sufficiently reliable.

Secondly - how can we be sure that the PVC foam will on one hand fill evenly the hollow space and on the other hand the mould will not explode.

We believe that there is some widely recognised industrial procedure to address it however we are unable to find any literature.

Any advice will help

<hr width=100% size=1>Victor

 

[boatless]

I know that some production boat rudders are foam filled, but I don't really like it. Another common method is to fill the blade halves with a mush of chopped mat and resin, then squeeze together. Effective but heavy.

A combination of the two might work, buyt only if you inject the foam through drilled holes after solid filling in the way of the tangs.

There are two important points. One is that welds must never be flexed, so the support for the tangs must be complete - not just at the outer ends. Second is that it is practically impossible to make it waterproof, it's bound to open at the top slightly - unless completely solid.

Not much help I'm afraid, rudder building from shells is not a precise science.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[Talbot]

When I talked to a builder about replacement rudders, he eventually recommended that the whole rudder be made out of stainless and then faired. He reckoned it would be abt the same weight, much stronger, and would last a lot longer - it would also be cheaper!

What does the panel think?

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[jerryat]

Hi Talbot!

I very much agree. I friend of mine, with what we think is a VERY early Najad, has a stainless steel rudder. It is not solid, but looks like a 'normal rudder, i.e. hydrofoil section, and has been made of a solid s/s steel stock and s/s plate welded to form the shape. It's excellent. Light, very strong and no chance of water leak damage.

I think it would be a great step forward(?) if the major yacht manufacturers adopted this format. Is there a representative of any of the biggie boat builders out there who would care to comment?

Good sailing!




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[mirabriani]

Agree with talbot I used this for a centre board on 15 foot double ender. Be warned stainless sheet is hard to cut, probably best to give the supplier a pattern and have them cut and weld.
I used marine ply cheeks through bolted and shaped, using an electric planer and sander. I used a paint finish, looked good too.
Good Luck Briani

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[janversteeg]

What about a construction of ordinary steel, rudder stocks and welded hollow profiled rudderblade(s >> have a Catalac to) and have it galvanised ?

regards Jan

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[Sunnyseeker]

Simple curiosity, why didn't you use Alloy? much lighter.I could carry my rudder on my own, it was 9' top to bottom with 5' blade Alloy subframe. this was on a production Bavaria.
If its performance why a heavy stainless rudder...

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[William_H]

From an interested but totally inexperienced point of view. I gather you are commited to the skins with the SS frame work inside. Suggest there be openings at either the top or bottom or both so that more resin can be run into the interior so that it puddles along the leading and trailing edge join line frrom the inside. Is it practical to run resin into the bottom then again at the top to cover the web? Mid span webs would be a problem. Unless you could fill up to near the web with foam then run resin in to cover the web possibly with a little filler (so it can still take up a level) Then more foam. Foam put in in small batches should not cause the explosion problem we see when large batches are put in small spaces. Just some ideas which you may not have thought of . Good luck let us know how it went. will

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[William_H]

More thinks on the subject Vic. If you can reasonably dimension and profile the inside of the skins you could build up a glass and resin block around the ss webbs. Thes could be covered on the mating edges with mating bog so that when the skins are pressed together the bog squeezes out giving good support. This method would leave cavities which could be left air filled or filled with foam via holes drilled and later patched up. regards will

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[oldsaltoz]

G'day Vic,

This is not the mass production method but as you will not be manufacturing hundreds of rudders, may I suggest a method I have used many times.

First I make a meld, and make the two sides up, then I add a 5 mm strip to the leading and trailing edges so that when the sides are made they have a rebate.

Next I place the stock inside the first half and glass in the top of the stock adding a couple of "0" rings and a smear of Vaseline, this prevents the stock from leaking at the steel / glass interface.

Depending on the rudder design I may do some glassing at the bottom of the stock to provide some extra support.

Tip: Make sure the support structure inside the rudder can not move, foils should be fully welded around the shaft, not just tabs off the shaft.
Also ensure the internal structure is well clear of the top, bottom and edges, so that the glass can break rather the structure in the event of a major mishap.
Most rudders need some balance, I use 12% on most rudders.

Next all but fill the first half with closed cell balloons, ['Q' cells are fine]. to within 20 mm of the foils. wipe away any stray filler with some Acetone.
The second half is then part filled with 'Q' cells and lowered onto the fist half, ply boards are then clamped over the whole rudder to squeeze them together, then released to wipe off the edges with Acetone and add some micro fibres mixed with resin around the edge; re-clamp and let it cure.

The rebate left at the edges is now glassed and faired, one rudder ready to go; well after a coat or three of resin, epoxy primer, and paint then antifoul that is.

An alternative for you may be to include the rebate for glassing the edges and filling with 'Q' cells from the bottom up, just two holes to fill/glass when completed, lay the rudder on the trailing edge and inject from the bottom of the stack, holes drilled in the leading edge are easy to repair.

I hope this helps, lets hope someone who mass produces rudders can give you some good advise.

Andavagoodweekend, Don't miss the world cup rugby rematch on Saturday...



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Vic]

Gentlemen,

Thanks for sharing your experience. The posts from Boatless, Will and the detailed know how from Old Salt Oz will be the basis of our final technology.

I still wonder however how the reputed production boats manufacturers are setting the process assuming that their yearly production of a given type of the rudder is of an order of 100-200 units. I have heard recently about the technology used by the two well known medium size manufacturers who appear not to pay too much attention to the problems discussed here.

Regards

<hr width=100% size=1>Victor