An interesting fact was brought up to me by the Technical engineer for Divinycell. Priming H80 foam takes 300gram of epoxy per sq metre... thats 300g x 8.5sq m for the hull's outside alone... 2.55kg! Another 2.55 kg for the inside as well...

Weight of hull calculation
cloth inside and outside (200g/sqm ) = 3.4kg
epoxy to saturate cloth = 3.4kg
Diviny cell H80 10mm thick = 6.8kg
Epoxy for priming both sides = 5.1kg
Reinforcement ( estimated ) 1.3kg

Total = 20kg

20kg before you put fittings, daggerboard well, bulkheads, deck possibly etc. into the hull?

Rolf,
I'm including the deck in the 20kg but not the daggerboard case and the fittings so maybe 22kg total. What are you expecting for your Blade? The priming and the amount of epoxy for fairing is what is adding on the kilos..

Hi pilgrim,

Ok, if you cant get lighter than 200g/sqm your original layout might be a good solution in fact. Didn't know you are limited by availability...

Cavitation depends on speed and load... most people think, that the rudders stall while sailing fast downwind, but actually going 17kts and keeping the boat on course let the boards cavitate (sometimes). If you see bubbles in the water it is cavitation. Some are from the tip vortex, you can´t avoid that, but if you see bubbles at the leading edge, than is cavitation. The cavitation triggers than the stall. A good foil will not cavitate, some designers reduce thickness to achieve that, but your rudder will be less strong or must be build heavier and it will have a lower maximum lift coefficient, hence needs to be larger.


Water has a higher viscosity than air, hence the Reynoldsnumbers are higher. That's why dedicated RC plane foils are not suited for boards and rudders. Classic airplane foils as the NACA 00xx works once you are fast enough without turbulator.

IMO boards and rudders will never have a laminar boundary layer, sinvce the surface water itself is already turbulent. Hence the foils which I designed are all turbulent foils.

Hope my answer wasn"t too nerdy...

Cheers,

Klaus

The main weight driver is the foam, because it is heavy and soaks resin. With honeycomb you could save a lot. But 20kg is already at the lower end. Are you vacuum bagging?

Cheers,

Klaus

Klaus,
Can you send me the .dat file for your modified NACA0012 section?

NO, I'm not vacuum baggin yet but I hope to do so soon.

Shane

Actually there are many possibilities depending on your target. To avoid confusion I selected one, which has about 4% more lift, slightly less drag and 4% less tendency for cavitation (although I think it is not necessary for a board).
You can either reduce the board area (best bz redicing chord and keeping span) by 4% or profit from the better manouverability at low speeds (e.g. at the start line).
The pictures shows you the difference between the NACA 0012 at the modified section. The new section has the thickness more forward. Gives you an impression how sensitive a foil is too surface quality.

Cheers,

Klaus

10mm is quite thick for your foam can you get something thinner? 6mm would take almost 3kg out of each hull.

Klaus,
Thanks, I will use it for the dagger board. I ran the NACA 0012 through Profili and the Coefficient of Lift is terrrible after the AoA gets more than 14 degrees... I wonder if thats the software limitation or the airfoil limitation. Going to NACA 0015 buys you another 2 degrees to 16 degrees before Cl goes south too. Change in Drag with AoA 1s linear but climbs steeply after about 8 degrees... shows how gentle you should be with the rudder.

Scarecrow,
I followed the LR2 project in the A cat class and this is what they used. I tried a sheet of 6mm and it is really floppy before the carbon is applied. The 10mm is much easier to use and gives a n1ce fair hull when you glue it up on the mold. I'm still on target to get under 23 kg and my big problem is laying up the cloth on the vertical sides inside the hull.... Its a major pain!

Shane

Be carefull when comparing 2D with 3D aerodynamic data. The angle of attack will be higher in 3D (over 20°) than on 2D, but the cLmax is smaller, because the slope is lower. And take the right Reynoldsnumber (e.g. 500000) and turbulence level (n=3)as it has a huge effect. Thicker airfoils have higher cLmax, but more drag and are more prone to cavitation, which is more important for a rudder than cLmax. Hence I modified a 16% thick Naca section for the T to get the cavitation right, of course we had higher drag... Can send this you as well, if you want

seems that I didn't manage to attach the .dat file.
So here is 12% thick board section:

p122510
1.000000 0.001278
0.993619 0.002110
0.982901 0.003491
0.970676 0.005040
0.956886 0.006757
0.941596 0.008624
0.925004 0.010608
0.907383 0.012669
0.889016 0.014769
0.870139 0.016879
0.850929 0.018979
0.831505 0.021054
0.811942 0.023098
0.792289 0.025106
0.772578 0.027076
0.752827 0.029006
0.733049 0.030897
0.713254 0.032747
0.693449 0.034557
0.673639 0.036325
0.653830 0.038051
0.634027 0.039734
0.614233 0.041373
0.594456 0.042967
0.574699 0.044514
0.554967 0.046011
0.535267 0.047458
0.515606 0.048849
0.495989 0.050184
0.476425 0.051458
0.456922 0.052667
0.437490 0.053806
0.418139 0.054872
0.398883 0.055857
0.379734 0.056757
0.360712 0.057565
0.341835 0.058272
0.323128 0.058872
0.304621 0.059355
0.286349 0.059713
0.268359 0.059937
0.250707 0.060017
0.233461 0.059945
0.216696 0.059709
0.200480 0.059296
0.184876 0.058701
0.169948 0.057921
0.155758 0.056960
0.142359 0.055829
0.129798 0.054544
0.118103 0.053128
0.107284 0.051605
0.097330 0.050001
0.088210 0.048340
0.079877 0.046643
0.072275 0.044926
0.065339 0.043201
0.059008 0.041478
0.053220 0.039761
0.047918 0.038053
0.043050 0.036355
0.038571 0.034665
0.034441 0.032982
0.030626 0.031303
0.027095 0.029624
0.023823 0.027940
0.020790 0.026247
0.017978 0.024540
0.015373 0.022812
0.012966 0.021058
0.010750 0.019271
0.008722 0.017445
0.006881 0.015574
0.005237 0.013650
0.003784 0.011677
0.002568 0.009636
0.001540 0.007555
0.000799 0.005422
0.000291 0.003251
0.000022 0.001085
0.000022 -0.001085
0.000291 -0.003251
0.000799 -0.005422
0.001540 -0.007555
0.002568 -0.009636
0.003784 -0.011677
0.005237 -0.013650
0.006881 -0.015575
0.008722 -0.017445
0.010750 -0.019271
0.012966 -0.021058
0.015373 -0.022812
0.017978 -0.024540
0.020790 -0.026247
0.023823 -0.027940
0.027095 -0.029624
0.030626 -0.031303
0.034441 -0.032982
0.038571 -0.034665
0.043050 -0.036355
0.047918 -0.038053
0.053220 -0.039761
0.059008 -0.041478
0.065339 -0.043201
0.072275 -0.044926
0.079877 -0.046643
0.088210 -0.048341
0.097330 -0.050001
0.107284 -0.051605
0.118103 -0.053128
0.129798 -0.054544
0.142360 -0.055829
0.155758 -0.056960
0.169949 -0.057921
0.184876 -0.058701
0.200480 -0.059296
0.216697 -0.059709
0.233462 -0.059945
0.250707 -0.060017
0.268359 -0.059937
0.286349 -0.059713
0.304621 -0.059355
0.323128 -0.058872
0.341835 -0.058272
0.360712 -0.057565
0.379734 -0.056757
0.398882 -0.055857
0.418139 -0.054872
0.437490 -0.053806
0.456922 -0.052667
0.476425 -0.051458
0.495989 -0.050184
0.515606 -0.048849
0.535267 -0.047458
0.554967 -0.046011
0.574698 -0.044514
0.594456 -0.042967
0.614233 -0.041373
0.634026 -0.039734
0.653830 -0.038051
0.673639 -0.036325
0.693449 -0.034557
0.713254 -0.032747
0.733049 -0.030897
0.752827 -0.029006
0.772578 -0.027076
0.792289 -0.025106
0.811942 -0.023098
0.831504 -0.021054
0.850929 -0.018979
0.870139 -0.016879
0.889016 -0.014769
0.907383 -0.012669
0.925004 -0.010608
0.941596 -0.008624
0.956886 -0.006757
0.970676 -0.005040
0.982901 -0.003491
0.993619 -0.002110
1.000000 -0.001278

Cheers,

Klaus

Pilgrim,

I think this is very cool project and am glad you are sharing it with us.

Here are a couple of the issues I see in this process
Have you fully weighed your materials?
• H80 core will vary 25% in actual density. Acceptable density for this range will span the full difference between the H60 and H100
• In theory it takes 300 g of resin to seal the face of the core. We have found the H80 typically takes 20% more as its surface roughness is very high. With hand lam you can under coat the core, but this may cause long term life issues. Cores with a finer surface texture like Rohacell or certain grades of corecell will take significantly less resin but are more expensive.
• Non spec fibers have a weight tolerance of 10%
• Hand lamination and maintaining a fiber/glass ratio is a crap shoot unless you have done it A LOT
• Do not underestimate the weight of the finish – Fairing compound and paint. Paint will be about 2 kg alone. (half gel coat weight) The weight of the fairing depends on how good you want the thing to look and how much you get tired of sanding.
It matters not at the end of the day, but weight is not that easy to control no matter the process. Some of the builders have enough materials they can mix and match lots to keep things tight, or have a relationship where they can not accept certain batches of materials that a home builder might have to.

On the laminate, is this a sloop or Uni rigged boat? Sloop rigged configurations will have significantly more loads and if you are using the LR A class as your model and A has almost no load compared to a 2 up sloop rigged F16. Adding plenty of material to inside hull face between the bridal and the front beam is critical. Hull failures are almost always in compression. Compression resistance is mostly a function of the thickness. Carbon is stiffer than glass but you can get into problems as builders cut to single layers for the carbon and it is difficult to keep it in column.
Single skin also may be great for the design while sailing but will be very fragile, especially with H80 core. Heel dents and damage from shore/rollers and trailers will be an issue. Take very good care of this boat. An A has single skin but at 75 kg is a lot easier to move around.

Best of Luck

You got my curiosity going to the point that I had to get up and go downstairs to my shop to look at the Divinicell I have in my possession, to see if these claims of a "rough surface" have any merit. After using boxes and boxes of 1-1/2" H130, ¾” H100, ¾” & ¼” H80, ¾” H60, and ¾” H45 I have to ask if we are talking about the same Divinicell? Are you dealing with seconds? I have never seen density variances of 25% on any of their products...

Looking at the fine cell structure of H80 in front of me it would seem that attempting to "fill the open cells" with bog prior to glassing may be something that makes sense academically, but does it translate to any appreciable weight savings in actual construction?

After bogging you have to go back and impregnate the cloth using enough resin to get a complete bond to the bogged surface...and your resin system better have a very long pot life if you have aspirations of bogging the entire hull surface and then laminating the cloth while the bog is still wet.
1) Who is this technical engineer for Divinicell that made this claim and where on their web site can this information be verified?
2) What is the ratio of resin to glass bubbles/micro balloons that is being used?
3) If it takes 300 grams of pure resin to fill in the cut cells of a sq/meter of material how many grams of bog does it take to fill in the cut cells? What is the resulting proposed saving of weight?
4) Does the 300 grams /per/sq/m consider only one side of the foam or both the front and back surfaces of a sandwich?
5) Is this something that only works at the desk with a calculator? Or will it be reproducible in the work shop?
6) Will a carefully prepared small test panel (even up to a full sq/meter) give you an accurate sample, taking into consideration the difficulty of reproducing this on the large skin surface area of a hull unless you employ a small army of expert laminators working in unison?
7) Your bond is only as strong as its weakest link. Resin/Filler/Foam. Is the bog stronger or weaker than the H80?
8) Could the very thing you are trying to avoid (filling in the cut cells of the foam with resin) actually work against you? Could having the resin filling the cells actually give a better mechanical bond, and possibly increase stiffness a very slight amount? When using West System resin on wood do they call for bog over the wood to prevent the resin from soaking into the grain? No, just the opposite. Almost any adhesive will perform better when the bonding surfaces are roughed up, why should this scenario be any different? In the commercial use of Divinicell that I have participated in the past, we never used bog between two structural components for concerns of delamination.(This last comment is contingent on the bog being weaker than the particular PVC cross linked structural foam being used).

If one is using open cell Styrofoam bogging is imperative, but the closed cell???

Robert

mini,
No, I didnt weigh the foam but I am keeping track of the epoxy I use. So far I've used 3.75kg for the outside of the hull alone

1.5kg to wet the cloth ( 7.5 sq m x 0.2 kg/m )
2.25 kg to fill the foam ( 7.5 sq m x 0.3 kg/m )

Yes, I am fully expecting to add weight during fairing but you know, building a boat is not about perfection, good enough will do. Purists will throw up their hands with my philosophy but working alone and at home, part time, the greatest mistake is to get so caught up with perfection that you get burned out and dont finish the project. I'll try to keep the weight down but a sound structure is more important.
Its a sloop rig and I'll be reinforcing 'midships with an extra layer of carbon. Compression failure is something I am very conscious of so the deck and the inside of the hulls will have extra reinforcement. This extra reinforcement is something the LR2 didnt have so this cat will be at least twice as strong as the A cat. In order to keep the weight down I will have only one layer on the outside of the hulls so if I get T boned I will have to do repairs for sure...everything is a compromise.

Shane

Seeker
Question 3) IF you use a lightweight filler, the density of the epoxy filler mix is less so you have less weight for the same volume and you might have 250g/ sq m per side added weight.
4) The 300g/sq m is per side so 600g/sq m for both sides.

7) I dont know what will happen if you dont prime the faom but when I removed a section of foam to add a piece of plywood for the chainplate I found the epoxy soaked foam really tough and the bond between foam and carbon reassuringly tough too.. I didnt use vacuum so I was pleased at that...

Shane

PS from the Divinycell Tech Sheet..

Primer coverage rates
H60 - 360g/sq m
H80 - 300g/sq m
H100 - 275g
H200 - 200g/sq m

Looking at the numbers I see that getting the H60 doesnt save you a lot of weight because you have to put much more epoxy to prime it.

1 sq m of H60 primed both sides = 600g + 360g +360g = 1320g
1 sqm of H80 primed both sides = 800g + 300 +300 = 1400g
1 sq m of H100 primed both sides = 1000g + 275g +275g = 1550g

H80 seems to be a more optimal foam than H60.

Pilgrim,

50% fiber content and about 1 oz/ sf (300g/m2) is still pretty typical for hand lamination.

You do not save weight, but by wetting the foam and then working to press the fiber through the resin rather than the other way it works a bit easier, especially with carbon as the carbon is a lot more work to get properly wet out. With some practice you can get 60% fiber content, 70 will require bagging.

On the material weights I have to agree with mini.

They make the core in a huge bun with the heavy density material at the bottom and the light at the top. It gets sliced in ranges and we have seen variance easily as high as 25% from order to order. Their spec sheet says 10% typical. There used to be a Diab University or some such name. The H80 has a 1 oz per sf value and adding cuts, perfs, or the groove core for infusion can be more than double that in practice. We have how to sheets that all use these same numbers from them, but I do not know if it is on the web site. This is not something to brag about. There are a lot of other choices in core, some take even more resin. Some cores have a much finer cell structure and take less. We have proven these numbers in test lams and in practice. Even the very fine cell structure on the expensive Rohacell stuff only save maybe 25% in resin for the wet out.

By the way, running test samples of your parts is a very good idea. This will tell you the real weights and usually point out trouble spots like you had bunching the fiber on the sides.

Good luck

I am picturing the giant foam bun being sliced like a loaf of bread.... the foam structure is like bread too...epoxy spread like butter on bread.

When I got the foam they told me a few sheets had minor imperfections but the sheet was within spec... I wasnt happy because at that price I should get perfect sheets! Anyway, the foam was on the way so I let them placate me with a 5% discount.

I think you can easily find brazil white wood. It is also good for sail. I also want to sail but never try in my life. Because i scared. But one day i will do. Hop that dy will come soon in my life.