Xflir5 is telling me as chord length reduces and the reynold number drops drag goes up.

[tropostudio]

A boat 7m long that makes ~10km/hr or 6.2 mph, with or without foils attached. Was the boat foil-borne with them attached? You didn't mention it.
You are analyzing 2D foils at Re at Re 100,000. So that means you are using a foil chord on order of 2-3 cm? This sure sounds like a project to foil a low speed, human powered boat.

Your 3D printed foils are breaking, so you may switch to a cambered plate steel foil. We have no idea what the finish is on the printed foil.

Are you juggling too many variables here? The qualities of strength and finish for similar craft - say kite foilboards - is what you should be aiming for. You can't make informed comparisons amongst foils, in practice, without getting your fabrication and structures sorted out. I'd venture that if you can do that, correlating results to software will be a lot more meaningful. You'll at least have some evidence to back up assumptions. Don't mean to be rude - just practical.

 

[Urepedese]

I do realise I'm dealing with some clever people here You display that clearly.

No I won't get offended, you are incredibly generous with your time and why would I not be grateful of that?

On the counter side I've had experience of publishing my ideas too early and someone else taking off with it. I am also used to people not reading long diatribes when I do include all the detail. A bit like I shot from the hip before fully digesting your post.

Yes I've been getting it on foil regularly no problem. Early foils were marginal and were discarded. So I'm quite a way down the discovery track. and chord length I've been using slighter bigger than you say except for the box foil. One foil that worked was 90mm at 2:1 AR and worked as well as anything else. It wasn't in the test though. Interestingly even though I'm not getting the speed benefit yet the foiling has other benefits. I just had plans of using the foil in an event in October but I'm not likely to do that now unless I get a miracle result soon. So I'll keep chipping away at it and the next opportunity to debut it will be April.

I found comment online about surfboard foils that 70mm was the bottom limit for chord length, they go faster but it reflects what I've found.

Yes quite true about fabrication. I was thinking polycarbonate printed as the final product if 3D printed. A bit of chicken and egg scenario between test product and final product. Connection also. Foils are breaking along the faring line for the bolt and I believe fatigue from flexing is an issue. A steel plate will eliminate that.

I've printed with different filaments and found PLA is actually the best so far. I'm going a few percent oversize and sanding it back to 400 W/D (I know I can go finer yet, I'm impatient sometimes). Some of the foils I've painted, first white, then a layer of black as a guide coat. With printing I have found problems with the trailing edge holding together but I'm getting smarter at how I orientate the print. I had trouble with one modified PLA filament, modified to make it tougher and more like ABS, but it softened a foil I left on the dashboard and ruined one. Didn't help it was a black filament and painting the next one white helped.

I'm working with what is easy and what I have available for now. Printed plastic or steel. I've skinned one foil with fibreglass by printing a clamp together mold, it still broke. Laser cut steel a possibility I'm considering if a prototype cambered foil works, that allows me to align mounting holes to the mast, and in fact cut the mast also. I can see that carbon fibre isn't necessarily a good choice when it is so small and dimensional accuracy is needed. Investment casting aluminium a possibility.

 

[tropostudio]

If you already have the 3D printer going, I'd recommend looking into using it to make molds for composite foils. If you could iterate and test rapidly along with analysis, I bet you'd make faster and more deliberate progress.

Easycomposites has a wealth of information on making copression molded parts using 3D printed molds:
Materials, equipment and training for advanced composites with next-day shipping and expert technical advice. - Easy Composites https://www.easycomposites.co.uk/

Here is video of a guy designing and making CF pump foils that looks very relevant to your project:
I Built a Carbon Hydrofoil With a 3D Printed Mold
The sanding and finoshing of the mold surface would be the most time consuming part of the process, but the mold surfaces are pretty accessible.

The process works well. I've done it several times Screenshots below summarize a recent project to add accessory M5 bosses to a CF fat bike fork for racks, fenders, water bottles. I photographed the fork, reverse engineered it in 3D CAD, printed test parts until I had a good fit, and then made 3D printed piece-molds to cast finished parts using Uni-D and chopped CF directly onto the fork.

 

[wet feet]

I may perhaps be stating the obvious,but your foils will have to support the weight of the boat plus crew.Can you rig up some kind of test arrangement to see if they can actually do this before committing the vessel to water?
Carbon foils would be more certain to maintain the designed shape than 3D printed versions and are not unduly difficult to make,as has been suggested.The downside is the time taken to make the moulds and prepare them to a high class finish.You may not know that foiling Moths are beginning to use metal for
the upper portion of their daggerboards,which they seem to be describing as a strut.Direct machining of the metal plus some time spent polishing out the machining marks might be the fastest way to make a durable foil,depending on the shop rates of your chosen machine shop.For a small foil the weight penalty won't be too severe.

 

[Skip Johnson]

For prototype foils you might look at FRP (G10) it's almost as strong and stiff as aluminum and far easier to work. The parallel laminations make it easy to get a specified shape. For me it hasn't created the fiberglass itch common to sanding into regular fiberglass.

A few years ago a client was really interested in doing a foil borne human powered racing craft after seeing an early video of a fellow walking around on a foil only contraption with no buoyancy involved. I did some research and quite a bit of calculation and preliminary design study and declined going down that path (FWIW he was happy with the craft I did design for him and he did well with it). That's not to say it can't or won't be done but it was beyond what I thought my capabilities were for this particular project.

Best wishes on the deal, hope to see some interesting stuff come April.

 

[tropostudio]

@Urepedese - the thread has moved away from your original question(s) into talk about materials and fabrication, which is applicable because you are trying to correlate analysis with prototyping. We still don't know your specific goals and parameters, but we have hints. What is not clear is your approach to design and testing. Some suggestions, many of which have been brought up [Anyone withknowledge or experience - please feel free to debunk anything that follows]:

Prior to any 2D or 3D wing analysis:

1. Know your displacement, as pointed out by @wet feet. Is the boat fully foiling or foil assisted? You need to know how much you are lifting and how far you can get it up out of the water to clear waves, minimize strut drag, etc.
2. Be certain of your design speed. Sounds like you have that pretty well sorted, as you mention test runs of ~10 km/hr with or without your current foil prototypes
3. Know how much power you have available. Sounds like that's known based on your tests. You are currently focusing on the horizontal wing, but there is drag to contend with on struts, junctures, hardware.
4. Be aware of the limitations imposed by materials, budget, and fabrication options. If you are 3D printing test foils and they break, you either have to sort out the printing process or switch to a method that gets you a better structure. Personally, I'd not allow a particular fabrication method to define my 2D and 3D foil design. But I'd also be thinking about ways I could build (or afford to have someone else build) an optimal design, and what concessions I might make if I couldn't quite pull that off. To wit: If an optimal 3D foil ends up being very high aspect ratio, highly loaded, and a very thin, can I pull that off? If not, would going with a thicker section be a deal breaker for performance? (Note I am not suggesting high AR, thin wing is the answer - it's an example).
5. Analysis paralaysis. Software is a model. Wind tunnels tests are a model. For example, Ncrit as a metric for turbulence in freestream flow may be 10-12 in a clean wind tunnel, or 1-3 in water. Another example: Michael Selig has documented variance across identically-numbered foils in different wind tunnels. Results across several Re are markedly differnet. Optimization is relative to the measuring tool and test environment. Analyzing minutae may not matter in the 'big pond.'

No onto analysis using XFLR5. I use XFLOW, because it's more current and XFLR is no longer updated. I'd bet output will be close if not identical - no guarantees.

From previous posts you have looked at SG6043 (Selig low-Reynolds foil often used in model aircraft or small-scale UAV's) alongside a NACA 6505 (noted as a flat-plate version, so its a modification?).

1. Rather than get hung up where and whether flow separates and re-attaches, run your analyses across a range of Re. Pick a starting point for Ncrit and Xtr that seem reasonable 'based on those knowledgeable in the field.' You can run analyses on that range of Re whiole varying Ncrit or Xtr later if merited. You haven't established a planform, so you don't know your representative length - chord length - yet. You have a design velocity, but there will be variations in practice. Re in water will vary based on temperature a lot more than it does in air. Importantly, you'll get a better general sense of how changing Re affects CL/CD and other metrics by looking at a range
2. You may already know this, but lift slope is pretty similar for most foil sections. Stall conditions and AOA vary, and drag changes a lot.
3. Check out the XFLOW run below. Re 100,000-400,000. Mach 0.344 (trivial might as well note that its 4.5x what it is in air). Ncrit of 3 and Xtr of 25% chord (looked at what others suggest and seems right).
4. SG6043 seems like a good choice, and better than NACA6505. Higher CL/CD, wider drag bucket, and a more gradual stall. What other low Re, low-drag foils have you considered? Have you compared them across a range of Re to select 2 or 3 candidates for further review and testing via 3D wing analysis and prototypes?
5. After you settle on the 2D foil section or sections, you run those across a very large range of Re so that XFLOW can do a 3D wing analysis. As noted in post #12, XFLR/FLOW5 need the wide range in order to estimate viscous drag fro a 3D wing. Once you have a 3D analysis working for one wing, you can investigate changes to planform, aspect ratio, and affects of geometric or aerodynmaic twist (chnaging foil section from root-to-tip).
6. Then you make structurally-sound prototypes using consistent fabrication methods and finishes, to see if they prove out anywhere near your analysis!