DIY Mast Construction

[foam-n-fibre]

I’m looking at building my first foil over the winter and trying to figure out materials. I know that the mast needs to be stiff, and plan to use unidirectional and 45 degree biaxial carbon. The question is, what is inside? I know some are made of wood with carbon over top, some are G10 with less carbon over top, and others are solid carbon, and others are aluminum, which I think are often hollow(?) Are any carbon ones hollow? Also, are the solid carbon ones made thinner than other constructions?

I know that stiffness is key, but would like to make it as light as possible. But, I don’t want to underbuild it, I want to do it right. I have lots of corecell around, and am considering using a foam core. I KNOW that the skin on a foam core mast would have to be thicker than the skin over wood. I’m also considering having maybe a 1cm x 1cm strip of wood at the thickest part of the foil, like a stringer the length of the mast, since I know that part of the flex problem is keeping the foil at maximum thickness and not allowing it to compress thinner under bending loads. In this case the leading and trailing parts of the foil would be the foam parts, with a good amount of carbon over them, and some wood in the middle..

Is this possible? I’m sure there have been stories of failed or flexible masts made with foam cores, but the question is, how much carbon was on the skin on these ones? If solid carbon works well, and masts with lots of wood and a much thinner carbon skin also work, is there a combination of carbon and foam core that should be good? Or do I need so much carbon that the extra weight saved with the foam core is not much compared to a wood construction?

Thanks,
Peter

[Bille]

I would NOT, use a foam in any high stress part ; most every part in
a hydrofoil , will be seeing a bunch of stress.


BIlle

[tahoedirk]

Peter,
No foam , no wood , just carbon . Shoot for 13 or 14 x 110 mm for starters. Good luck choosing a profile!

I now believe the mast is the most challenging and important part of the whole gizmo!

have fun , Dirk

[revhed]

First off, please let us call it what it is, on this an english forum!

Laughingman posted a while back;
"You guys can't just make up new names for parts just because you are using a foil with a kite, these things have been around for decades..."

strut.JPG (32.99 KiB) Viewed 3196 times

And I;
viewtopic.php?f=196&t=2388064
From P F;
The difficult thing is, in our native language, "Mast" is the same word and easy to understand, whereas "Strut" is a thing many dont know about or uses, so it wont work well around here

That should not influence what the right english term is of course.

As "Strut" is the correct term, and used for the exact same in aviation, supporting the main bodies and having a low drag profile.

I find that using terms from aviation makes perfect sense, as they are used in the very same way, so most will recognize the terms immediately

Board
Strut
Fuselage
Front wing
Rear wing
Rudder (if present)

PF

Anyway, there will still be guys using the wrong term for this

D I Y con...STRU... c...T ion, you even type it correct yourself!

As mentioned above, solid carbon layups have proven to be the best for D I Y.
But,
for a test just made a hard wood core profiled 1 cm thick X 11cm wide x 90 cm long.
Wrapped 3, 1,000 g/m2 QUADRI carbon layers the center being over L E, with ends meeting at T E, 6 quad total, 24 layers.
Vac sac of course and cooked properly, seems good for now. Stiff and very little twist.
You say you want to be light, but better a little more weight and,or thickness than to much twist, flex.
T D has a great point, the strut is probably the hardest part to get well made while being critical to overall performance!
May I caution that the strut to fuse AND strut to board interface (plate or tuttle) NEEDS to be VERY well made and strong!
You do not want this,

I have since fixed it and flew most of the summer with so seems ok now, I was very lucky to not lose it!
I,we also are using mono block, meaning 1 peice T bars strut, fuse, plate with a vac sac layup using 200g/m2 to finish.
This is good also,

300px-Mortise_and_tenon_joint.svg.png (5.75 KiB) Viewed 3196 times

I agree that uni and bi bias for the strut is best as strands perpindicular to the LE, TE seem useless other than when used in QUADRI
R H

[foam-n-fibre]

I'm perfectly good with calling it the strut. In fact I thought it makes more sense too, and would have called it the strut if I didn't think it was an uphill battle! How about we combine maST and STrut to get maSTrut?? That's sure to catch on, right?

I've looked at a number of DIY builds on the french forum (no thanks to my 12 years of french verb conjugation in school, and thanks to google translate). I had not seen any that were described as a solid carbon construction, or none that I noticed. I did see some guys with molds so had wondered about that, but it was never clear.

So RH, you say you have 3000g/m2 of total carbon weight on each side of the hardwood core, do I understand that correctly? (That would be 90 oz per side if I convert right) Any sense about how thick that carbon ends up being? How thick is the finished foil over the 1cm core? I'll have to look up some material properties, but would have thought if your layup is good, that solid carbon must be overkill. Maybe its easier if you have a mold though. Or is a solid carbon one a bit thinner?

As for the weakness at the base plate issue, is a Tuttle box a more secure option, or is the high stress point just moved more into the board then? I know the base plate can allow adjustment if using a track on the board, but if I plan to ride strapless at least to start, is adjustment at the board any real use?

Changing subjects slightly, that "peck fuselage" image looks like it would have quite a stress concentration where the cut out is for the front wing. I can see why you don't want that far out in front of the STRUT.

Thanks for all the help. I'm sure I'll have more questions. Where I live there is only one foil in town (LF) and the only time I've seen it on the water was when I tried it in light winds and tried to break my ankle! (that's why I plan to go strapless)

Peter

edit: when you say quadri carbon, does that mean fibres are at 0, 90 and also +45 and -45 degrees?

[TPink]

my $.02 is that solid carbon is not necessary and heavy.

let's peel the onion.

If you had just one uni strip, 10cm x 100cm, wetted out and cured, it would have literally zero stiffness in flex and torsion. That represents the center of the strut.

now add a layer on each side, minor improvement yet still very flexy and twisty, you are approx 4mm of total thickness now.

You can keep adding w/ marginal improvement till you get to around 8-9 mm's of thickness before each addl layer makes noticeable improvement to the last. But you now have a relatively heavy core of approx 8mm's that is doing virtually the same thing as a much lighter core could.

The proof in the pudding comes in when you get to the last (outer) layers of the onion.
for example on my build, I nearly cut in half the flex number, and 1/3 the twist number by adding just one more layer of uni to the outside skins.

Point being, All the work of the carbon to resist flex and twist is done on the vary last outside layers.

[foam-n-fibre]

From an engineering point of view that totally makes sense. The stiffness should be proportional to the thickness cubed. The middle strip is at the "neutral axis" where it is not in tension or compression. However, I think that with the tendency for the foil to want to flex and become thinner in cross section when bending, there is certainly need for good support so it cannot collapse and become flat (like a cardboard roll becomes flat when you bend it). This is why I was hoping to put something else in the middle. If you had a 6mm core of something in the middle and 4mm carbon either side would that be a problem? Sounds like that would be overkill over hardwood, so it might also be more than needed over more carbon (unless you go thinner overall)

Of course it all fine to talk theory, but to quote Yogi Berra, "in theory there is no difference between theory and practice. In practice there is." Or maybe those who have done this before also know more theory!

Thanks,
Peter

[revhed]

I'm perfectly good with calling it the strut. In fact I thought it makes more sense too, and would have called it the strut if I didn't think it was an uphill battle!

So RH, you say you have 3000g/m2 of total carbon weight on each side of the hardwood core, do I understand that correctly? (That would be 90 oz per side if I convert right) Any sense about how thick that carbon ends up being? How thick is the finished foil over the 1cm core? I'll have to look up some material properties, but would have thought if your layup is good, that solid carbon must be overkill. Maybe its easier if you have a mold though. Or is a solid carbon one a bit thinner?

I know the base plate can allow adjustment if using a track on the board, but if I plan to ride strapless at least to start, is adjustment at the board any real use?

Changing subjects slightly, that "peck fuselage" image looks like it would have quite a stress concentration where the cut out is for the front wing. I can see why you don't want that far out in front of the STRUT.

Thanks for all the help. I'm sure I'll have more questions. Where I live there is only one foil in town (LF) and the only time I've seen it on the water was when I tried it in light winds and tried to break my ankle! (that's why I plan to go strapless)

Peter

edit: when you say quadri carbon, does that mean fibres are at 0, 90 and also +45 and -45 degrees?

YES! Supr stuff if you can get it!
There is 3 layers of 1,000 g m/2 quadri on each side that refers to the weight of the material not how much one uses.
1 layer ends up a nice even numbered 1 mm when vac saced.
So a 1 cm core ends up 16mm wide with 3 layers of quadri on each side.
As said this is flying just fine for now?
Yes the "moment of inerita" concept that you and t pink know of is correct, but for most diyers getting a good bond between core and carbon can be a challenge as they flex and twist different causing delam.
Consider a front foot hook to learn. A half cut NSI in great, enough board control for waterstart and having your front foot always in the same place helps find where the rear foot flying trilm place is.
We always base plate with thru M6 bolts thru boards.
But I really like the track system for center of lift fine tuning.

R H

[foam-n-fibre]

OK, here is how the theory works out. This is based on some carbon properties I found online using a bending modulus somewhere between that of unidirectional loaded axially, and woven carbon cloth. This is based on a test method with a 5 kg weight hung 85 cm from the base. I have approximated the strut cross section as an ellipse 8 cm long, with varying thickness. I know 8 cm is too short, but this makes up for the extra thickness in the ellipse toward the trailing edge. Maybe a 1/4 ellipse at the front and triangle at the back is better for the next time I try this. Here is what "theory" tells us. The main thing this ignores is any flattening of the foil cross section, which would be a lot with the thin wall, and less as the wall gets thicker.

Strut flex.JPG (29.69 KiB) Viewed 2959 times

First of all, I'm not sure if this is exactly what is found in real world tests. Carbon properties may vary, along with my approximation of the foil, but the comparison of dimensions should still be valid.

Generally speaking, you can have a solid foil, or you can increase the overall thickness by 1 mm and have the same or less deflection with a 3mm wall thickness. At a 15mm thickness, the weight of the mast is about half of that of the solid one. But you probably want something to support the cross section, so the weight saving is not quite as much. This shows why wood in the middle does not reduce the strength much. BTW, I found specs for oak (even though i heard it is not a great wood to bond to). Its properties give about 10 times the flex of carbon, so what the wood adds in terms of stiffness is minimal, just like the thinner carbon core does not offer much.

Of course... this is only the theory.....

Peter

[Kamikuza]

Mast, fuselage, wing, stabilizer.

A strut would be between wing and mast, for support