The issue with that is that you need to get both sectional shape and roughess correct at the first try.

Huh? In a wood laminate?

I see it as just the opposite.

Am I missing something? We're talking smoothness as in like glass is smooth, sandpaper is not right?

I think technique is the answer. Once the shape is fair at 220 grit then it is more a case of polishing.

You will get both smoothness and shape right as homebuilder as long as you have enough sanding paper, filler and patients as thousands of homebuilder and model aircraft builder have already shown.
To get the section shape right you can work with a negative template gauge (don't know the right english word for it, a plank where you cut out the sape of section). With that you can easily see where to remove or add material. You have to sand and fill until you have the shape. The section is more sensible to errors at the nose than at the end and more sensitive for errors in streamwise direction than along the span. I hate sanding but I go down to 1000 and then start polishing, but if you after a nice look as well you may even uses finer sanding paper.
The real problem is that a catamaran has 4 foils... but after sanding to hulls you are already into the zen of sanding.

Cheers,

Klaus

"...negative template gauge" as always check Gougeon Brothers, lots of stuff there.

I like to spray the work area with a light dusting of black lacquer, it shows the contours nicely with only a little sanding.

If you think the foil is fair spray it with a high build primer and then wet sand with a cork block and 1200 grit until it looks flat and shiny with no dimples. Look at the reflections. They must all have clean straight lines. Then spray a top coat and wet sand again. I went 800 and then 1200 (don't press too hard with the 800 or you will have probs removing the scratches it makes) and then cutting compound. I would have liked to go over with 2,000grit but couldn't source it at the time. I was making the plug for the rudder on my Tri. There is a pic in a thread on Farrier tris. I started off by getting the reflection of the trees to look straight and eventually got the extremely fine lines on the diffuser of a hand held fluro light to look spot on. Then I felt that I was in the ball park.
Someone mentioned the word ZEN. Forget about the time it takes because it is all about getting it right.Suspend time and get into the zone to get it right. If you don't think you can do that just buy something off the shelf.

So that is what is called "optical quality"

Even if Zen is a goal in itself, I wonder what the real world difference on performance between optical quality and non-optical quality is.

I'm not likely to find out, surfaces with that level of finish are all but impossible to maintain.

I think primer sanded to 320, then painted followed by 600,800,1000 grit wet sanding plus polish. Even then, normal usage will mar that finish so it will be maintained with soap and water.

What Phill gave us was a good method to determine fairness by looking at the surface. I think it was a very good post in its clarity.

With "perfect" fairness and surface finish the performance will still be good as the surface detoriates. If fairness and finish is inferior to begin with the performance will decrease faster while maintenance will be harder.


I have a hard time trying to think of an easy to follow procedure to produce that level of fairness within a reasonable amount of time, by hand. CNC have its advantages.

Next question could be what level of fairness and surface finish an "off the shelf" product has, and how they are produced?

From "back then" I think Phills foils used a homebrew CNC mill to produce cores and moulds. He would finish the cores and moulds by some hand sanding. It has been some time since I read this so please dont quote me and take this as gospel.

During the development of foiling Moths they hand built the foils to "good enough" performance to make them fly efficiently. It is surely still possible if one has the Zen

Imo, the first place to start is selecting a foil shape. How does one go about choosing? Btw, all I've seen are simple line drawings are there more accurate descriptions somewhere?

Pete,
You could buy Profili. Not sure these days but it used to be around 40 euros. You could buy it online. It has a huge database of foils and it will not only give you the offsetts to create the foil shapes but also all the foil characteristics including lift and drag.

Thanks Phil. And I like your slogan, seems we're tuned into the same station.

http://www.profili2.com/

Looks a little complicated. All I have is a simple Epson printer. Has anyone used this with a home pc?

Pete,
It has been quite a while but I have used it on my home PC and personal laptop. There is no problem with processing. If the problem is just printing why not store the file on a thumb drive and take the thumb drive to a plan print office. The last time I got something printed this way it cost me $4 per file.

Regards,
Phill

There is a trial version that can be downloaded and tested. The software itself is relatively lightweight in normal use. It runs fine on my 8 years old laptop.

The software is complicated becouse the topic is non-trivial (at least I find it a complicated subject when trying to do the math by hand)

I liked the new functionality in Profili 2 Pro for saving 3D design files and even producing G-Code for male or female moulds. Take the file to a CNC shop and have the mould or core machined with very little overhead for them ($$$ to set up CAM and produce G-code)

Ok now I'm intrigued. When generating STL files from Profili 2 Pro, can you specify the wing washout at each span-wise station? I have non-linear washout requirements...

XFLR5 is free and worth every penny. Pete, this is a non-trivial subject matter; I have a 4 year degree in Aero Engineering and I still feel like a newbie when it comes to foil design, and an even lesser man when it comes to hydrofoils. Guys make entire careers out of this subject (Tom Speers for example, Boeing engineer and BMW Oracle foil specialist). My #1 suggestion is to call or email Matt and get a quote on the long boards, I think you'll be pleasantly surprised and quickly realize you can't do this for less. My #2 suggestion is start with a NACA 0010. My #3 suggestion is simply extend your existing Blade foils to the desired length.

I'm fearful this will quickly become an exercise in futility for all the reasons you cite.

#3 has occurred to me and will be the first step in any case.

I'm beginning to suspect that using aero foils (if I may use that term)for hydro foils may be a mistake. I cited an article from Vesta sail rocket to that effect.

Bernard Smith may well have the last word in the matter.

Sam,

I belive the "standard advice" for cat foils is NACA0007 for daggers and NACA0012 for rudders.

Then the customization and differences in philosophies starts. From there follows a big debate where I quickly am out of my depth. Some of these discussions seems more like religious debates than fact based ones (you can choose which facts to select and debate from). In the end it comes down to who to trust.

Well, I'm going to start with some cheap materials to make a model. Then whittle away to suite my fancy.

If I can make any sense out of the data I'll try to apply it.