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Rudder recore material: Foam alternative?

5K views 21 replies 9 participants last post by  colemj 
#1 ·
I have my rudder of and am about to open it up for a rebulid job.

The standard procedure is to use expanding 16 pound foam as a rudder fill. At least that is what I have found in my research.

Anyone have any advice for a different material that would not absorb water and not be too heavy?

thanks!!
 
#2 ·
If you want light, you can try using insulation foam boards. These are closed cell rigid foam boards you can find in Lowes and Home Depo. If you want strong new rudder, pressure treated board is hard to beat. Shape it to size and lay fiberglass over it.
 
#3 · (Edited)
You don't mention the type, size or foil design of the rudder, nor why it needs to be rebuilt, which can make a difference in recommendation.

16lb 2-part closed cell foam would be the minimum density that should be used. Most commercially manufactured rudders use 20lb foam. Of course, I'm assuming a spade rudder with internal weldment.

Insulation foam boards will crush if you have an internal weldment unless you can somehow tie that weldment to the outer skins.

I have heard about, but not experimented with, using microballoons with thin epoxy to make a syntactic foam that is relatively light, waterproof and strong. I have my reservations about this, and it isn't the most practical method, but the one report I know of seems satisfied. Mixing and timing the production of the epoxy foam would be a very critical thing.

Mark
 
#7 ·
You don't mention the type, size or foil design of the rudder, nor why it needs to be rebuilt, which can make a difference in recommendation.

16lb 2-part closed cell foam would be the minimum density that should be used. Most commercially manufactured rudders use 20lb foam. Of course, I'm assuming a spade rudder with internal weldment.

Insulation foam boards will crush if you have an internal weldment unless you can somehow tie that weldment to the outer skins.

I have heard about, but not experimented with, using microballoons with thin epoxy to make a syntactic foam that is relatively light, waterproof and strong. I have my reservations about this, and it isn't the most practical method, but the one report I know of seems satisfied. Mixing and timing the production of the epoxy foam would be a very critical thing.

Mark
Mark, thanks. 16 pound is what I see mostly. I know that Foss Foam uses 20 pound. I need to check for that availability.

The idea of epoxy and light weight balloons is what I was thinking. In sailing and cruisers fourm there is guy who says he did such a thing. His responses confuse me so I am asking around for other experiences.

Rudders are interesting. If too heavy they tend to cause lee helm, if too light weather helm. (or did I say that backwards) But we still want them to be as light as possible for mechanical reasons.

A solid epoxy rudder would be way too heavy, not too mention expensive. It does seem that foam is the best compromise we have available to us.

In my search for the answer I have come across many different approaches but most are a variant of the foam method.

frank
 
#5 ·
The rudder has been submerged continuously since 2001 with only 2 months out of the water when I hauled. I am guessing that it will need some attention. I have the rudder home but have not opened it yet. It seems pretty good but there is some rust leaking out the drain holes along with some water.

It is skeg mounted. I have only 2 pictures from the last rebuild, and they should be attached.
 

Attachments

#6 ·
You may find the armature heavily rusted. Whale away at it with a heavy mallet or maul, and if it comes apart...time for a whole new rudder. If it still is solid, time for blasting and coating to prevent more corrosion. The problem is that every rudder with a shiny modern new stainless steel rudder post, and a frp shell, is sealed between the two with caulking or epoxy. And since the rudder port and the shell have different expansion rates, they WILL break that seal apart and you will get water intrusion, even if there is no other damage (nearly invisible hairline crack for a grounding or a bump) to let water in.

Beneteau has tried to overcome this by using carbon fiber rudder posts on some models, I'm told.

So whatever you do...seal the hell out of the rudder post area when you are wrapping it up.
 
#9 ·
The goal is neutral buoyancy, more than simple mass. A light, buoyant rudder will impart forces in one way, a heavy, negatively buoyant rudder the opposite. Ideally the rudder should be neutral, or at least as close as you can realistically get. In this sense, the weight of the foam might be a function of the mass of the rest of the rudder.
 
#10 ·
This is the first I've ever heard that the WEIGHT of the rudder can affect the helm. The rudder is a foil, the shape of the foil and the position of the foil will affect whether the boar is balanced (weather or lee helm) but the WEIGHT of the rudder shouldn't matter. If you have a light rudder and hang a 100# anchor on the stern rail, same same. If you have a heavy rudder and you move the batteries and lazarette junk forward, again, same same.
Sounds like there's something wrong with that picture.
 
#11 ·
#13 ·
I used woodworking tools; saw, drill, planer, etc. If you can give ESS drawings they can machine it for you.
 
#14 ·
I will post a picture of two once I get it open.

The thinking behind the weight is that if the rudder is heavier then the water it will tend to sink and take the helm in that direction. If lighter, it goes the other way..
I had read that written by a designer, not sure who.
 
#16 ·
Even heeled over the weight of the rudder shouldn't make any difference in steering. The rudder is a foil, and the water flow pressure profile will have more bearing on steering characteristics than weight. Assuming the rudder is built with materials somewhere between helium and lead.

If it is a typical foam filled with a reasonable internal weldment, then the blade will likely be lighter than water, and the entire blade/post likely just slightly lighter or neutral (but that extra post weight will be almost entirely inside the boat).

Mark
 
#17 ·
"if the rudder is heavier then the water it will tend to sink "
Nice concept, assuming that the rudder is uncontrolled and thus *able* to rise or sink on its own. As opposed to a real life rudder, which is being held in position by the steering gear, and by hydrodynamic forces as the water flows over it.

IOW: Nice theory, but similar to Schrodinger's Cat, it really doesn't matter in the bigger picture.
 
#18 ·
Rudders, at least on a sailboat, are generally designed to be neutral, for obvious reasons. So, when repairing a rudder, the idea is to maintain that design characteristic and ideal trim setup. Unless the design is deficient, I'm not seeing where there is any advantage to be had in adding or removing buoyancy to either end of a boat. Even to the small degree a rudder represents.

In short, why half-ass it? Do it right. Or at least do the best you can.

It's not theory, it's well understood practice.

When you add or remove buoyancy, you change the moment of inertia at the stern. Even small changes can be amplified in a complex system of hydrodynamics and moments. Plus, a neutral rudder only has to move against the forces of the water across the foil, as opposed to working against it's own buoyancy or lack thereof, which of course is always in flux as the boat rolls. This is not to say it's the end of the world if the rudder is a bit heavy or light, which is why it's an easy corner to cut, and you often see rudders that are quite buoyant. I removed the rudder from my first boat, a Catalina 22 (an outboard rudder, of course), and it would float on the water like a piece of Styrofoam. My next boat, a C&C, had a much better designed rudder that truly was nearly neutral.
 
#19 ·
If the rudder is rebuilt in any normal manner - meaning not filled with lead or helium - then any weight difference from original will be inconsequential. Actually, I think you could fill it with lead or helium and still be inconsequential in practice. But the reality is that he will likely use foam and glass, which will be perfectly fine no matter how heavy or light it is relative to the original.

I'm sure there are many old wooden rudders that have been replaced with foam and glass, changing the weight and buoyancy significantly, with no practical issues. Same with changes in internal weldment structure/weight, etc.

It would be a sensitive boat indeed that changed its trim to any extent based on a few pounds of rudder weight or buoyancy difference one way or the other. Particularly the OP's design.

Most rudders are not neutrally buoyant. While they may be so when sat free in the water, the blade is generally highly buoyant - and it is the blade that is mostly in the water. The heavy stock is generally well inside the boat itself - often above the water line.

I actually installed a spade rudder in the water the other day, so got a refresher on how they behave in the water (blade floats high, post sinks low, overall buoyancy just bobbing at surface - quite a pain getting it lined up under the boat and into and up the bearings).

Mark
 
#20 ·
It varies from rudder to rudder. Some rudders need to be ballasted to get the blade end to sink for installation, others need the reverse, and will sink. My C&C rudder for example just barely wanted to sink...which made it very easy to get back in the boat. Relatively easy, anyways.

It doesn't matter that the blade is more buoyant than the stock, because the sum resultant buoyancy is attached to the boat. The stern will be that much more or less buoyant overall, regardless, because the blade is the part actually displacing water. Its the net buoyancy that matters.

Using foam and glass is fine. Most rudders are built that way to begin with. What's concerning to me is cheap rudder webbing construction, where there not enough steel, and it seems proper to strengthen the rudder with additional stainless. I'm mostly commenting in this thread on the idea of weight as it relates to rudders instead of buoyancy. A "heavy" rudder with positive buoyancy doesn't actually weigh anything (as far as the boat is concerned), as it displaces more than it's own weight. It will obvious effect inertia, but the steering feel won't be all that bad unless the rudder is seriously heavy or light.

I think we're arguing about a fine difference...but I'm kinda anal about structural issues. I'd actually built a 3D model of my rudder in CAD, in order to specifically determine its volume and displacement and dimensions, in order to make sure I would retain the same characteristics after we re-cored it.
 
#21 ·
Thanks everyone. I remember reading about the buoyancy thing and rudders. The author was a designer. It did not make that much sense to me either....

I have decided to us 16 pound expanding urethane foam. Most likely will use US Composites foam. I just need to find the best price now.

I will fill do some reinforcing and then fill with foam. The spokes are plain carbon steel as far as I can tell. One spoke was a little rusty, but not bad. Since I opened it and it was outside in the night air (no rain) the spokes have a bit or rust. That is amazing that they were submerged all that time and did not rust. I am beginning to love this rudder. I have considered replacing that spokes with SS, and I will talk to a welder this week.

Tartan made a good rudder.

I will post pictures...
 
#22 ·
It is common for the internal structure to be mild steel on a stainless post. The thought is that it would never be in contact with water because it is encased in foam and glass. That's the working theory, anyhow - we all know how it works in practice.

If yours was free of corrosion, indicating no water intrusion or slop play on the post, what about the rudder made you want to rebuild it?

Mark
 
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