The hydrodynamic qualities of Plasti-Dip!

[styge]

To briefly summarize:

I have an Aluminum CS17, I am finishing up my rudder (also aluminum) and recently posted a question about rudder spacing. The subject of foil profiles for the rudder came up from JeffM which I pondered long and hard. I finally decided it had merit and added a very nice curve to my otherwise flat plate rudder, using bondo-glas. Came out great. As a finishing touch, I was planning to spray a fair portion of the rudder using Plasti-dip. It is very durable and would protect the seat tops when I stowed the rudder.

Now, I tried a sample spray on a test piece and it came out okay, but sets up with a slightly rubbery finish that is also very grippy, rather like the sole of a running shoe! Now, it seems to me that this may not be the ideal coating for the part of the rudder that in the water! Does anyone have any insight into whether this type of coating will have a negative effect on the handling / performance. It seems to me, that whenever you see the racing yachts, all their submerged surfaces are shiney and slick! Am I barking up the wrong tree or does anyone have any suggestions as to what I should consider coating my rudder with to finish it off, perhaps some type of epoxy paint.

Any thoughts anyone?

Thanks in advance.

Styge

[Hirilonde]

Is "plasti-dip" the stuff you dip tool handles into, like pliers?

[Hirilonde]

At the boatyard where I work we repaint aluminum spars all the time. We use Awlgrip products on them.

After prep and a serious cleaning of the surfaces we apply several coats of Awlgrip 545 2-part epoxy primer. If applied at intervals of 1 to 9 hours between coats it can be "hot" recoated; that means no sanding between coats needed. After 3 to 10 coats of primer (we have some very picky and wealthy customers) the primer is sanded with 400g paper and sprayed with Awlgrip 2-part linear polymer polyurethane topcoat. Some have us sand again and dress with clear.

Awlgrip can also be brushed. The results, though not a car finish like sprayed, are still quite good.

I will be using these products on my Spindrift 9N this winter. I will be "rolling and tipping"

If you choose to use 2 part epoxies and polyurethanes make sure to read the MSDSs and DO NOT skimp on the safety equipment they call for.

[JeffM]

Styge, from the point of view of water molecules moving over a surface, there's almost no such thing as a perfectly smooth, non-sticky material. The only issue I can think of it whether, with water flowing around it, this stuff deforms at all. Seems very unlikely!

[styge]

yes Hirilonde, Plast-dip is the stuf you dip tool handle into. It also comes in spray can form now. The spray can is the type I'm referring to. As for the surface of the rudder, I have it very smooth indeed, but the actual feel of the test surface is like that of a rubber mat, you can't really slide your hand over it easily and it makes me wonder if it will cause undue drag through the water. I'm probably over thinking this.

[Hirilonde]

I would think the plast-dip would work fine. The tacky characteristic it has for other solid objects has little if any relationship to how the coated rudder will flow through water. It probably isn't as drag free as the new high-tech hard racing bottom paints. But unless you are entering an important race; who cares? :wink:

[Arnie Eaton]

What you need to do is run your hand over the surface while your hand or the rudder is very wet.

[Tom Lathrop]

Every explanation of hydronamic flow that I have ever read says the the fluid flow rate at the surface is zero. Therefore the resistance of a rigid shape in fluid flow is based on the shape and fairness of the surface and not what the surface is made of.

That is why the racing rules have no restriction on what is painted on a surface. All sorts of stuff has been tried from soap to artificial fish slime to wax with no measurable difference. Monkeying with the mechanical face of the surface with things like Dennis Conner's riblets have some limited effect in limited conditions but most come back to a fair surface as smooth as you can make it as the best.

Things like trip wires, edge slats, turbulators and other laminar enhancement devices are another matter.

[fatehunter1972]

The qualities of the surface over which water flows always matter. With my current command, a 200 ft ferry, we notice a decrease in speed and increase in fuel consumption between bottom cleanings as the hull gets weeded up. On my previous command, a 380 ft ferry, we had bottom cleaning done by divers in the dock between regular drydockings. Those bottom cleanings payed for themselves in a few weeks by decreasing fuel consumption. The flow of a fluid of a fluid, such as water, is always zero at the surface and increases to the the full hull speed at a certain distance away from the hull. This layer is called the boundary layer. It is the depth of this layer that is so important. The more water that you drag along, ie. the thicker the boundary layer, the greater the drag. Now the CS17 is quite small but even so a weeded up skiff will still be slower than a smooth one.

A rudder is a foil, not a hull, so some other factors come into play. On a foil, at some point aft of the leading edge, the nice smooth flow of fluid over the surface (called a laminar flow) will break up into a turbulent flow. On most foils this will occur a little distance back from the point of greatest foil thickness. There are various different foils shapes that have different characteristics but they all have turbulent flow at some point. You use a foil, the rudder, to generate lift, such as turning the rudder to create a force to turn the boat. When this happers the flow over the top surface, (the face of the rudder away from the boat) will try to separate from the surface. When this flow separates completely, when all the laminar flow becomes turbulent, that is when your rudder, or any foil, becomes stalled and loses all effectiveness. Surface qualities matter greatly in this case by affecting the angle at which this separation will occur. That's why airplanes can't fly very well with frost on the wings. Even a 1 mm layer of frost on a wing will dramatically reduce the capacity of the wing to generate lift. Your rudder will be similarly affected.

Now for the surface area of a rudder, likely you would never be able to measure the difference in speed through the water. However when you use the rudder to turn the boat your rudder may lose lift and effectiveness at a smaller angle than a nice smooth one. This may not be noticeable at slower speeds. I don't think that it mattered much that the rudders on big old sailing ships were made of rough boards and were of a flate plate construction. however if you would be surfing down the face of a wave at ten knots and you turned the rudder to the same angle that you would normally turn it at slower speeds, you may very well stall the rudder, due to the large increase in forces involved. I encourage you to to experiment with this. Also pick a up a book on flying. Almost any book on planes will give very good descriptions of laminar flow, stalls, how lift is generated etc.

Sorry for getting such a case of typing diarhea, but I think it always helps to know the science behind the stuff we do.

Michael

Dreaming of a CS 17

[Barry Pyeatt]

Interestingly, Plastic-dip is commonly used as a finish for heat shrink Dacron because it doesn't disturb the rip-stop characteristics of the Dacron like paint will. There are many small craft that use heat shrink Dacron for the surface skin on them. Plastic-coat helps the strength factor and impact resistance but doesn't dramatically effect the flow over it. If applied correctly, it can be smooth and the "boundry layer" is quite thin and managable in tank studies. If you were into racing it wouldn't be a good choice but for your application you shoudn't have adverse effects in using it.

If you were to run your hand over the surface of a sub hull you would find that it isn't all that slick either. But hydrodynamically it is slick in the water.

[JeffM]

Welcome aboard, Michael! As you may have noticed, the folks here are generally appreciative of professional knowledge. Do you really think the difference between, say, a varnished rudder and the same rudder with a smooth rubber coating would make a practical difference in the thickness of the boundary layer? If so, I might be refinishing my own rudder and centerboard this winter!

[Tom Lathrop]

Michael is correct in everything he said but I think there was a misunderstanding.

Certainly any disturbance to the smooth fair shape of a laminar foil or hull will have an effect on its resistance, usually bad. I thought the issue here was what the foil surface was made of, not its shape or roughness. That being the case, how would the water know whether the surface was metal, fiberglass, painted wood or Plasti-dip? If the final shape was unchanged and mechanical characteristics were the same (meaning the surface would not deform to hydro forces), the performance should remain unchanged.

[fatehunter1972]

Jeff and Tom,

There are many different factors that can affect the boundary layer. I don't profess to be an expert on this, I just like to read. Look at a golf ball. It has dimples on its' surface that enhances its' directional stability. Does it affect the drag? I have no idea. The fact it is a sphere though makes it a very draggy shape allready. And the submarine with a non-smooth surface. I think that that surface is more likely that way for its acoustic proerties; being silent in the water, than it's hydrodynamic properties. These types of subs probably value their silence more than their efficiency. Many airplanes have vortex generators on the top lifting surface. These vortex generators re-energize the air layer and allow it to stick better to the wing at high angles of attack. You don't notice these devices on the bottom surface though. NASA conducted experiments where they actually had a dedicated jet engine sucking air off the wing aft of the point of maximum thickness in an effort to reduce the depth of the boundary layer. While very effective in reducing drag and increasing lift (less turbulent flow) it was inefficient on the whole.

As for your rudder and centerboard. I really don't think it matters. On my first 40 ft steel tug there was a flat plate rudder with barnacles, rough paint, and zinc anodes hanging off it. It still worked. Mind you there was an extra trim tab on the back (a becker rudder) and a horizontal plate on the bottom to reduce vortices peeling off the bottom of the rudder. I would not go to the trouble of resurfacing your foils, unless you have something to compare them to and they are obviously deficient. Just get out there and sail and have a blast. As my father says: Do what you can while you can, before you know it you will be an old man.

when I get some time I'll desribe a rudder that I had on a cargo ship that was very obviously inefficient for drag purposes but excellent as a rudder.

I love this site and forum. I have learned so much about the CS series and all the adventures people have had with them. I still find myself going back through all the old posts.

cheers,

Michael

Dreaming of a CS 17