Best foam for rudder

[Seasick Ian]

Hi all,

I am rebuilding the rudder for my Hurley 22 after getting a new rudderstock made up. I'm unsure on what foam to use, I've heard a lot of different things said. Does it have to be closed-cell? Does it have to be....expensive? Any recommendations for a foam that is readily available online or in the chandlery?

 

[William_H]

For solid foam polyurethane is the best. Polystyrene is terrible and will dissolve with solvents in polyester resin and tends to be lumpy and hard to get smooth. You may however want to go for mix em up and pour (or squirt) into place polyurethane foam. This can be good to get into gaps. It expands hugely as it sets so beware of large deep volumes of it. Perhaps a combination of board polyurethane and mix up foam. good luck olewill

 

[Neeves]

I think a bit more information is necessary. If you have the two halves of the rudder, as glassed sections and you are going to join them with the new rudder stock inside then you need some form of injectable foam. If you are building the rudder from scratch then you will need an very different foam, in the form of a sheet which you will fair and then glass over.

 

[oldharry]

You definitely need closed cell foam, otherwise it will act like a sponge. Screwfix Fire Retardant foam in an aerosol can is very good closed cell injectable and veryvreasonably priced, but as above take care as it does expand very powerfully and can distort or even split the casing. I used it to replace buoyacy foam in a dinghy and its still good now 6 years on.

 

[Seasick Ian]

Thanks chaps, sorry I should have specified- I'm gluing the two halves back together so I'll need an expanding foam. As long as I drill a hole at the top to allow​ foam to escape, the pressure won't build up, right?

 

[Daydream believer]

Be careful with this. I decided to make my own dan buoy using a light fishing rod & a plastic bottle as the float.
When I filled the bottle with foam it turned to a gooey mess & , presumably, needed a lot of air to help it set. So, may I suggest, that whatever foam you do use you try a quantity in a transparent plastic bottle first

 

[rogerthebodger]

Hi all,

I am rebuilding the rudder for my Hurley 22 after getting a new rudderstock made up. I'm unsure on what foam to use, I've heard a lot of different things said. Does it have to be closed-cell? Does it have to be....expensive? Any recommendations for a foam that is readily available online or in the chandlery?

I don't understand why the need to fill the inside of a GRP rudder molding with foam

Water will get inside and even closed cell foam will absorb water and there will be no way to drain the water out.

I made a rudder for my monitor self steering gear. below shows the inside framework. Note how the tangs are arranged. This gives the maximum strength to the tang.shaft joint and the strength of the tangs in the correct direction to resist the turning load on the rudder covering. It also gives the rudder a better hydrofoil shape.

in my case I covered the rudder in 1 mm thick stainless steel, but it could be just as easy to make a flat GRP cover.

The water tightness can also be checked by doing a pressure test to check for leaks. Fittings can also be included to allow any water that does get inside to be drained out pressure tested to find the leak and repaired.

Just my thinking

 

[Kukri]

I don't understand why the need to fill the inside of a GRP rudder molding with foam

Water will get inside and even closed cell foam will absorb water and there will be no way to drain the water out.

I made a rudder for my monitor self steering gear. below shows the inside framework. Note how the tangs are arranged. This gives the maximum strength to the tang.shaft joint and the strength of the tangs in the correct direction to resist the turning load on the rudder covering. It also gives the rudder a better hydrofoil shape.

in my case I covered the rudder in 1 mm thick stainless steel, but it could be just as easy to make a flat GRP cover.

The water tightness can also be checked by doing a pressure test to check for leaks. Fittings can also be included to allow any water that does get inside to be drained out pressure tested to find the leak and repaired.

Just my thinking

That got my attention, as I see "making a new rudder" in my future - existing one is sodden core and osmosis-riddled GRP. Suveyor's comment was " OK for now, but I won't have you crossing an ocean with that!" Thanks.

 

[Muddy Boat]

When I repaired mine, I took one side of the rudder off and filled with the 2-part expanding foam and then faired and relayed the grp. I did however try two types of expanding foam. Firstly ('cos I wanted to do it 'properly') I used epoxy closed cell. This, like epoxy in general, has a significant price premium. I found that it did not expand as much as I expected (or specified) and set as hard as stone, so by volume was heavier than the 2-part polyester I then continued the job with, which I used to use in canoes. It expands like a wild thing and is much easier to fair.

If you are using the squirty stuff, there are less aggressive 'expanding' varieties (fixing foam?) which may be better to use as the expanding stuff can bend door frames (ask how I know!!).

A single hole at the top of your rudder is unlikely to allow enough of a pressure relief, as the foam expands as it sets, so will not continue to flow out of the one hole.

As mentioned previously, it is still important to make sure the rudder is sealed.

 

[lw395]

There is info out there about making a dinghy rudder from foam. They can be either profiled foam, with FRP laid up after, or foam filled mouldings.
It occurs to me that there is less benefit in making a yacht rudder particularly light. Yes it's at the end of the boat, but it is also ballast, or if light, negative ballast!

My concerns would be the bend-resistance of the rudder, and the problem of joining the two halves strongly enough, if it's moulded in two halves.
The mechanical properties of the foam are important.
There is also the question of how does the torque on the rudder stock get transferred to the skins of the rudder? Poor design of this detail will result in stresses which will cause cracks and leaks.
As well as sheet polyurethane foam, 1-pack (aerosol) polyurethane (builders) foam, 2 pack poured foam, you can also use epoxy resin bulked with glass bubbles. You could also have some of the core made of wood.

 

[macd]

The usual foam for use as a GRP core material is closed-cell PVC. It's usually sold in sheet form. I've no idea if it's available as an expanding foam, but I daresay Mr Google does.

Be careful with this. I decided to make my own dan buoy using a light fishing rod & a plastic bottle as the float.
When I filled the bottle with foam it turned to a gooey mess & , presumably, needed a lot of air to help it set. So, may I suggest, that whatever foam you do use you try a quantity in a transparent plastic bottle first

It's probably not air it was short of, but water. Many polyurethane compounds, such as foaming wood glue, are actually two-pack. The second pack, which comes gratis, is atmospheric moisture. In very dry conditions, the materials being bonded are dampened to improve the cure.

 

[Tranona]

Think I would favour using solid foam sheet cut to fit the voids around the tangs laid in one half. Then profile the top to fit the other half of the shell. Then bond the two together and cover the join with epoxy glass tape. don't forget to insert an o ring around the top (and if necessary the bottom) of the stock where it enters the rudder.

This is essentially the same way I built my rudder except both the core and the outer were wood then sheathed in epoxy glass cloth. Still sound 25 years on.

 

[Neeves]

I'd go with Tranona. If its already joined they are usually joined with tape, and its pretty thin which is only one reason they leak so easily. Its an easy task, usually to slit the tape and take the 2 half off. You can then do as Tranona suggests, fill will solid foam which you would epoxy in, then fair, then epoxy the other half on, then tape epoxy the tape, a shim of filler fair and paint. Te foam will give it strength, which you will not get from two mouldings of glass unless they are heavy.

You might get a comment but rudders hit things, why they can leak, it might be worth while using a heavier duty tape to join the 2 halves it might last longer.

 

[Daydream believer]

I suspect that putting tape around the outside edge is an awkward operation as the weave will want it to come unstuck, unlike doing a tape joint to an internal angle. This will leave air bubbles under the tape.
I have found this to be a problem in bending quite thin cloths around external angles in model making. It will be much harder with heavy weight cloth with an acute angle. I have found that in small sections it can be "clamped" with adhesive tape but this is not practical with something large like a rudder blade.

If going down this route I suggest the OP tries a small section around the edge of a 3ft length of plywood first to see how successful he is at doing it.
The leading edges of the tape will need to be faired in some way after application involving more work

I suspect that most GRP rudders are not joined using tape but are bonded between the lip of the two skins. This is far less labour intensive & makes for a smoother outer surface. I would also suggest that the majority of modern blades are filled with foam that is expanded after the 2 skins have been bonded around the shaft. One side first being bonded to the tangs if the tangs are just simple rods.
The foam can be poured as the 2 halves are being mated or poured after they have been bonded
Jefa of Denmark, who are large manufacturers of rudders, do have a section of their website explaining how their rudders are made & bonded which is quite interesting http://www.jefa.com/blade/blade.htm. It also explains reasons for selection of aluminium for the stock instead of stainless which is another bone of contention between yachtsmen

 

[Neeves]

You may well be correct. I had a new rudder after I bent the stock of the original, which was stainless. The old rudder was tape bonded (2 sides taped together). It might have been faired after taping - no idea. but it had a solid foam interior, not injected foam. The new one was complete and without cutting it in half I cannot find out how it was made. Except the new one had a carbon fibre stock. Cannot recall the reason for the change from staleness to carbon. I was told it was also made from foam panels.

 

[lw395]

......
Jefa of Denmark, who are large manufacturers of rudders, do have a section of their website explaining how their rudders are made & bonded which is quite interesting http://www.jefa.com/blade/blade.htm. It also explains reasons for selection of aluminium for the stock instead of stainless which is another bone of contention between yachtsmen

Interesting web page. They are using serious metal moulds designed to take a lot of pressure from the foam expanding. That is not a DIY method!

 

[Daydream believer]

Interesting web page. They are using serious metal moulds designed to take a lot of pressure from the foam expanding. That is not a DIY method!

Visited a dinghy foil manufacturer ( Dave Marchant) yesterday in Burnham on Crouch & they use aluminium moulds in the same way, but without the stock of course

 

[Dipper]

I don't understand why the need to fill the inside of a GRP rudder molding with foam

My rudder originally had a hollow space and no foam. It developed a leak and ended up full of water. Because there was slight play in the spacers allowing limited up and down movement, the rudder would develop quite a lot of momentum in heavy seas due to the mass of water inside which was a little bit worrying. It thumped up and down quite violently. As a temporary measure I drilled some holes and squirted some foam inside which reduced the mass and improved the situation.

Several years later the tang welds failed. When I cut the rudder open to repair it the foam was wet but certainly not saturated as has been suggested by others. It still felt quite light. It wasn't two part closed cell foam either.

 

[rogerthebodger]

My rudder originally had a hollow space and no foam. It developed a leak and ended up full of water. Because there was slight play in the spacers allowing limited up and down movement, the rudder would develop quite a lot of momentum in heavy seas due to the mass of water inside which was a little bit worrying. It thumped up and down quite violently. As a temporary measure I drilled some holes and squirted some foam inside which reduced the mass and improved the situation.

Several years later the tang welds failed. When I cut the rudder open to repair it the foam was wet but certainly not saturated as has been suggested by others. It still felt quite light. It wasn't two part closed cell foam either.

Thats interesting.

The main rudder on my boat is mild steel with a stainless shaft and tangs. It is hollow with a 1/4 inch BSP plug in the top.

After manufacture I pressure tested it then filled it with old engine oil to stop internal rusting. If it did ever get holled I would see by having an oil slick follow my boat.

To me having a foam filled cavity with welded stainless steel fittings inside is a recipe for crevice corrosion and tang failure if ever water gets into the foam around the tang area.

 

[lw395]

My rudder originally had a hollow space and no foam. It developed a leak and ended up full of water. Because there was slight play in the spacers allowing limited up and down movement, the rudder would develop quite a lot of momentum in heavy seas due to the mass of water inside which was a little bit worrying. It thumped up and down quite violently. As a temporary measure I drilled some holes and squirted some foam inside which reduced the mass and improved the situation.

Several years later the tang welds failed. When I cut the rudder open to repair it the foam was wet but certainly not saturated as has been suggested by others. It still felt quite light. It wasn't two part closed cell foam either.

I'd guess that full of water it was close to neutral buoyancy, filled with foam it would be buoyant and held against the thrust washers?
Even ultralight rudders bang around badly when the bearing s are slack, the inertia of the boat and the water around the blade is huge.
I've found builders' foam to be 'fairly' closed cell, having tried some in a mooring buoy.