Hello all,

To bring you up to speed, I'm done glassing my floats and am ready to paint them. A few days ago, I got to talking to a guy that works alot with fiberglass and high speed boats (real boats). But anyway, he was telling me that if I left the bottom of the floats "orange pealed" or slightly textured other than polished or "glass", that the floats would actually be faster and would release from the water easier. I'm just wondering if any of you know anything about this or would comment about it.

Thanks

I don't think so.

A coating of Rain-x might be the ticket.

Maybe like dimples on a golf ball... but I doubt it would make a noticeable difference in our models as they spend so little time in the water at speed.

Now, if you had an overweight, underpowered beast..................................

Nubbly surface texture is one of the things in the "bad" list of features of the gray, blow-molded floats we all love to hate. On a golf ball they help the flow stay attached. You don't want that on the bottom of a float: you want the flow to separate at the step.

So why do they polish surfboards? You don't want any drag on the floats, i've owned and sold real boats part time for a dealer and I can tell you...smooth bottoms!!

The guy who mentioned the benefits of orange peel surface to reduce friction is ALMOST right, but not quite. America's Cup boats - those that you see on ESPN every 4 years - have been using wnat is called 'Riblets' for the past 20 years ago or so on their hulls - below the water line. These riblets are very small grooves that run lengthwise to the boat and reduce drag. The grooves are not actually part of the hull, but are applied to the surface of the hull much as a decal is applied. The depth, width and spacing of the grooves was developed with millions of $ in research and developmental testing.

I doubt that there would be any positive or negative impact on the operation of a RC float plane, and the down side is that if the exact right groove depth, width, spacing and orientation to the floats of the grooves wasn't used the floats would be less able to get up on step and the speed could be reduced.

In my humble opinion a textured surface would be superior in that it creates a turbulant flow and thus promotes boundary layer seperation. The golf ball analogy is probably correct in that by seperating the boundary layer you don't get an irregular seperation pattern, A slick surface will cause the fluid to stick to the surface while a rough one will not. This is why we have a step on the floats; to force seperation. A rough surface is like having thousands of mini steps. The big step, though, is the one that fluids cannot ignore.
A smooth, shiny surface sure looks better though. Put enough power on the plane and even Gee Bee floats will work.
Speaking of Gee Bee floats their textured surface is probably the only good thing about them. The round corners kill them; unless of course, you hang on lots of power.
Peter

The fastest boats on the water are the unlimited hydroplanes. At 200 MPH they are fast and stable, without using the orange peel effect. What they use is a flat running surface or a series of mini-steps along the bottom of the sponsons. They also use SHARP EDGES on the running surfaces to prevent the hull from being sucked down by the water. I think their design would be better for aircraft floats than the dimpled surface of a slow moving sail boat

Keep in mind that the riblets I mentioned are definitely not dimples. I agree with the Unlimited Hydro analogy rather than the golf ball one. As one reason why, consider that the golf ball is spinning, so the dimples are being used for lift as well as stability - quite different than what we are looking for. In this age of the airlines looking for any fuel economy that they can get and the military looking for any performance benefits, I truly believe that if dimples were the way to go that we would be seeing dimples on the wings of these planes. Air is a fluid just like water, only thinner!

Unlimited Hydros have so little of their hull in the water at speed...often only the bottom of the prop and the skeg...probably not the best analagy.....

We are just talking theory here...unless someone makes two pairs of floats - one with texture, and one smooth, and tests them on the same aircraft (under controlled, measured, and repeatable conditions) we will never know for sure.

I still believe it will not make a bit of difference one way or the other..............

If you think about it a golf ball is spinning so the textured surface (dimples) present an equal roughness no matter which way it sees the air. A hydroplane with mini steps is only going in one direction so the texture can be refined for a unidirectional flow. As Nony says there is not much of the boat in the water anyway.
I still cast my vote with the textured surface. If we had the patience to create the mini steps that would be best but secondly, and not far behind, is a random texture.
Peter

You're forgetting something. A hydroplane starts out standing still, where the whole boat is in the water. The mini-steps are used to make the right sponson get up on plane easier so it can get up onto the last 12" of the sponson. They also come into play in the corners. The Skid fin slams the right sponson into the water, making the mini-steps again functional. Can't speak for all the boats, but the Oh Boy! Oberto is set up with a series of plates with a sloped leading edge about 1" high and a flat area about 6" long running the rear third of the right sponson. These plates are designed to break up the water surface tension and free up the sponson to give the boat more speed. Since Driver Steve David took home the Driver's High Points Championship for this year, there must be something to it

Send me your favorite set of floats. I will stab at them with an icepick for a while, and pour gravel on them from my hayloft, and maybe dribble little bitty gobs of epoxy on them. Then you can have them back and tell me how much better they are.

Sorry bro but you don't polish surfboards. You wax the top so that when you stand on it you don't just slip off.

A rough surface generally increases the thickness of the boundary layer -- more drag. That being said, a very fine-finished, but not glassy surface, can create a thin (relatively) boundary layer hydrophylic surface effect (as opposed to the hydrophobic effect of a glassy surface). This will help keep the surface "wetted" & allow the water to flow around curves & corners without detached (turbulent) flow. This is the idea applied to displacement-hull boats. Similarly, golf ball dimples are designed to keep the flow attached as the air flows around the "corner", delaying separation as long as possible.

This is NOT what you want with planing hulls & aircraft floats, where a minimum-thickness boundary layer and a sharp separation at the step (and trailing edge) is required. The smoothest possible finish will give you the best float performance.

Ok, you've got my attention. What in the world are you doing with gravel in your hay loft?

I've heard of bats in ones belfry.......

I think a number of us aare talking about different things here. Agreed, a smooth shiny surface will have less surface drag as it moves over whatever, but the objective here is not to reduce friction but to promote seperation. It has long been known that even a paint stripe on the wing of a Mustang can cause unwanted seperation on the surface of a laminar flow wing. At speed a turbulant flow does indeed cause unwanted drag but at model and light aircraft speeds (slow) the air will tend to stick to the surface anyway. We had an incident at our float fly where one of the fellows built the tip floats of his North Star with an upswept rear section. That plane would NOT leave the water until a couple of strips of tape were placed across the float at the lowest point. All the step really is is a big turbulator. Try building a seaplane without one!
Build your floats with sharp edges and a decent size step and they will work, rough or smooth.
Peter

Peter -- you are quite right about the sharp edge at the step & trailing edge. However (sorry), you are quite wrong about the surface texture. For a planing hull it should be glassy smooth if possible. A rough surface texture will require substantially more power to push, because of the macro-irregularities, and the greatly increased thickness of the boundary layer. In the case of separation, we are not speaking of flow separation in the sense of turbulent vs laminar flow in a single-phase environment, we are speaking of actual air-water interface separation, as planing hulls operate in a two-phase environment. The smooth surface minimizes the boundary layer thickness & parasitic form drag (from irregularities), while the sharp step ensures the transfer of air to the underside of the hull (a round edge delays, or prevents the water seperation)

I guess the analogy I was trying to create was that of an Air Hockey table vs a croquet table. I think you will have to agree that the coefficiet of friction of an air hockey table is about as close to zero as can be attained. Since turbulent flow creates bubbles of air in the water in contact with the surface it should have less friction.
What the hell, I'm too lazy to create a glass surface so I'm going to stick to my argument to justify my less than perfect finishes.

How's that saying go, "As long as it works, it's all good"