Board - Mast attachment

[Florian Scharscher]

Hi everyone,

I'm seeking a good solution on mounting a Moses mast (flat plate, 4 bolts) to a self build board.

It's my first board building project and I am currently at a stage where I have carefully foiled out the board shape from a 50mm polystyrene foam blank.
I haven't used a stringer so far and I am at a point where I have to build in the reinforcement for the mast attachment.

I came up with this idea of gluing 3 stringers (6mm plywood) in the back of the board (file below/ top view) and fill 4 rectangular sections (for bolts) with epoxy resin to create a light jet strong cage:



I would appreciate your input on wether or not this could work aswell as other ideas on how to tackle this.

On a last note: I'll be glassing in epoxy using a vacuum bag and I have purchased a few lengths of 5oz e-glass. I believe that extra glass in the mast attachment region is required and I'm very open to suggestions on the glass schedule.



Thank you!
Flo

[Hawaiis]

I have never made a fiberglass board, only wooden ones.
If it was me, I would use carbon fiber in place of fiberglass, especially the bottom of the board.

[zfennell]

some questions:
extruded or expanded PS foam?
density of foam?
board thickness?


do you have access to any higher density structural foam like divinicel or corecell?

[ahmthai]

The EPS is not strong enough to secure the inserts. I know from personal experience because I had inserts pull out from EPS and lost my mast.

You will need to set your inserts into PVC foam (divincel/corecell) and laminate over the top. I would make a plate slightly larger than the Moses base plate, over drill the holes for the inserts, epoxy the inserts in place, epoxy the plate in the board and then laminate over the top. Be sure to add plenty of re-enforcement between the back of the plate to about 3/4 to the nose. There is a lot of force on the nose of the board when you crash forward (much more than with a standard kiteboard). I cracked several of the first boards I modified for the foil.

Good luck

[ahmthai]

One more thing: The back holes of the base plate should be just in front of your back foot/footstrap location.

[Florian Scharscher]

Hawaiis, I wanted to use S-glass for my first board, unfortunately I couldn't buy it at my local shop.
When I purchased the 5oz E-glass I had a look at carbon but couldn't justify the expense on the first board.
Now I'm somewhere in between, really don't want to mess up and rather reinforce respectively.

zfennell, I'm using XPS extruded polystyrene board (blue color) with a density of 35kg/m3 and a thickness of 50mm.
The tail section has no rocker or concave, a full 50mm thickness to attach the foil base plate.
I looked up divinicel and airex and I think can get it in Perth...

ahmthai, thanks also for your advice and sorry to hear you lost your mast, sounds horrible.
I assume tribble safety is the way to go especially in doubt.

* I now understand that the board - mast attachment area has to be made of a higher density foam to act against compression.
* clearance holes (25mm diameter) to be placed and epoxy filled in high density foam. Can these be drilled to accommodate a M6 bolts afterwards?
* several layers to be added to the top and bottom attachment area (larger then mast foot). 2x 5oz top + 2x 5oz bottom mast foot?
* I believe adding a center stringer all the way through the board would make quite an impact on the stability?
* what should the final glass schedule look like to prevent from any nose cracks like ahmtai mentioned? 2x 5oz top + 3x 5oz bottom?

Thanks everyone!
Flo

[ozchrisb]

Skip the stringer and add glass or carbon. Stringers don't help you very much.

[Peter_Frank]

Hmmm, actually, stringers will work great if you use a square piece of plywood for your "baseplate".

This way you will distribute the load, and you wont get a "weak" point, like if you had to reinforce with what seems like endless layers of glass or carbon

I have mounted a plywood plate countersunk into my surfboard, and then added glass/epoxy on top, to distribute the load (and dont even have a stringer eventhough it would be much better) - works fine.

PF

[Florian Scharscher]

Thanks for your great suggestions.
After thinking it through I believe the best way is to make it rather too strong.

I'm aiming for:
* a ply wood base plate with a stringer glued on each side.
* drill 20mm holes into wood base plate and fill them with resin.
Once dry, drill 7mm holes into resin inserts.
* attach base plate to the board and bond it with 2x 5oz top and bottom in the stress area.
* glass the hole board 2x 5oz top and 3x 5oz bottom.

(Just putting it down this way in case anybody decides to look it up)

Will come back with some photos during next week.

Thanks,
Flo

[zfennell]

Peter has pretty good point regarding the need for stringers.
particularly if you are using low density foam without a sandwich skin.

normally the bottom skin is better supported by the water and bending forces tend to put the bottom in a more stable tensile state.

once the board lifts out of the water, bending forces in the board could easliy change direction.

i would think that there is a significant reaction force between your feet and the mast attachment plate that may tend to buckle the bottom skin of your board (bottom skin becomes compressive).

in line with your desire for over-kill in the structure until the details get sorted.
you may also consider:
-a top plate as well as the bottom plate to better distribute stresss to both skins.
- "crush " sleeves (ss or fiberglass tubing) instead of just resin in the holes to keep you from damaging the core by overtightening the screws.
-xtra patch or 2 of glass cloth extending well past the plywood to minimise any stress 'risers'
- roughen up the XPS to the point of being extreme because of historically poor bond strength and tendancy to delaminate.

imo.
-bill