Rudder Construction Questions

I am looking for some advice on the construction of a rudder for my 46’ motorsailer project. When I purchased the unfinished hull, I also obtained the rudder mold. It consists of two mating female halves. The rudder itself is a high aspect spade. Two questions relate to the construction, and one is a bit more “design” orientated.

1. I see two different approaches to the construction of the rudder blade itself. (Assuming the shaft and related metal structure are complete). A) I can laminate each of the two half forms separately, and then position the two completed laminate shells around the shaft. I would next laminate the edges together and then inject foam in the voids around the metal structure. Or B) I could clamp the two mold halves around the shaft, inject foam and then laminate over the completed male foam shape? Or perhaps there is another method that I am completely overlooking???
2. I want to be able to remove the rudder while the boat is on the hard, resting on its keel. The only way I have seen to accomplish this is through the use of flanges located on the shaft, just above the top of the rudder. This seems straight forward enough, but I don’t like the additional drag created by the flanges. Is there a better way to do this?
3. Now to the design issue…the rudder is designed to hang with a slight aft rake. I would like to orientate the shaft in the vertical postion to facilitate the attachment of rack and pinion steering and emergency tiller, as well as for the cockpit layout. I believe rudders are raked aft in order to shed kelp, lines and other similar debris, but am not sure if there are other issues involved? Anyone care to comment?

Best Regards,

Sea Jay

 

Having experience only on a much smaller rudder project, I would go for laminating over the finished shape if the rudder is asymmetrical (both sides are different) as yours seems to be. If the rudder halves were symetrical, then I might consider building a mould.

My thinking is that that the amount of work to build a set of two precise half moulds, would be the same as shaping a complete rudder. But if you end up with a rudder after all that work, not just moulds - then you are ahead.

It also removes the risk of voids when pouring foam down the completed hollow rudder, or worse, deforming the fibreglass as the foam expands. The voids are especially likely if you have any kind of protrusions coming off the shaft, as I would expect.

 

Thanks RWatson. I'm with you on the potential problem of voids under a "pre-laminated" skin. However, I was doing a little research and came across a great web page, detailing the laminate-then-foam method.

http://newrudders.com/?page_id=12

I'm sure there is a trick to getting the right amount of foam in the mold. It appears that they pour the foam and then clamp. I was thinking the other way around.

SeaJay

 

Aha - that is a very interesting article that confirms my qualms about foam being poured into a hollow rudder.

It appears that the problems of distortion and voids require a special foam blend and strong external metal re-inforcing to solve.

I suspect that the reason they pressurise the foam, is to make it really dense and compact. This would add quite a bit of strength to the ruddder too, I suspect.

It would need an engineer to compute whether you need to go to as much trouble, or whether your boat would be quite as happy with some normal air pressure foam carved to shape with sufficient glass around it.

It might even be worth getting a quote from NewRudders to do your rudder. Paying the extra for an experienced and specialised job on such a critical component could be good value. I bet they could provide a lot of useful advice about re-shaping as well.

Ah the decisions .... will they never end!

 

Another thing to consider is the liquid foam you are using (if you go that route). I have been told that a number of those two part foam formulations, are not very much "closed cell", though they are advertised as such. If you are concerned about water absorption in your core material, shaping closed cell airex may be better.

So I have been told... ;-)

 

I understand the importance of the closed cell foam, but I seem to recall a post somewhere around her that was commenting on the quality of some of the newer foams. I guess what really has me wondering about this is just how the buider of the forms intended to finish them. As far as I can see, you would only make this type of mold if you intended to use a pourable foam. Otherwise, why go through all of the trouble?

 

Yeah, I understand. It all depends on how "obsessed" one is about water intrusion. lol. Local yard guys have said they often find foam cored rudders to be wet. Either through osmosis or damage that was not addressed promptly. But never really a strength issue with the rudder.

One datapoint: our cored rudder on our 28 footer is wet. Purely through osmosis we think. Never had any damage, or blisters. Again, not really an issue, but it is wet nonethe less.

 

Make the two halves. Fill with multiple layers of structural foam (and bog). being carefull to minimise bog quantities so it doesn't cook.

Dry fit stock to both halves and remove material as required.

Bond and glass stock into one half.

Glue other half together with excess glue.

Clean extra glue of edges

Glass tape over join.

This is one of the worst ideas I've ever seen on the net. Your proposed join is at the most highly stressed part of the stock. You'll only remove the rudder a couple of times in the life of the boat. Don't compromise its design and your safety to save time on those rare occations.

Your rudder stock is almost certainly design perpendicular to the local hull allowing it to rotate freely. If you make it vertical you'll need to build a small skeg to fill the gap.

 

Chris,

Thanks for the details on that construction method. I thought that might be how it was done, but wasn't sure if the intent was to abandon the shaping of foam to the stock, in favor of a pourable foam.

With respect to the rudder stock flange...I guess I left out a bit of info in my description. The detail which I described is one I had on a 44' trawler and I have also seen it on commercial fishing vessels. Here is an example:

http://books.google.com/books?id=gj...=X&oi=book_result&resnum=8&ct=result#PPA61,M1

However, the rudders in question are not spade rudders, but rather have various methods of support from skegs and keels, and typically done to facilatate prop shaft removal. I have the mold under discussion but am not not committed to it at this point...still looking at other options.

You are correct about the modifications necessary if the rudder is orientated in a vertical postition. This is another reason why I may opt for a completely different approach.

Thanks for the feedback.

 

Here are two links to Eric Sponberg's designs that use a retractable (removable) rudder.

http://boatbuilding.com/article.php/Globetrotter42

http://www.proboat-digital.com/proboat/200508/?pg=5

I like the simplicity of this concept but I suspect the execution could be problematic (expensive), but maybe not.