Centerboard ply core


Camtron0@...
 

Thanks again for the advice. I can learn some tricks and have a router :) and some family members around whom have build houses and sailing dinghies. We should be able to do it. 
 
I'll keep you guys posted as I go
 
I also have started another thread for the other big boat project of the winter which is repairing my Wingnet attachment holes. Is greatly appreciate any advice anyone has for that job. 
https://fct.groups.io/g/TrailerTris/topic/tt_720_wingnet_attachment_rot/76951147?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,0,0,76951147


Camtron0@...
 
Edited

Hi everyone
I've cut out the core foam for the new centerboard.   I had enough foam to make 2 so we've got a spare if need be.  We're going to hand shape the boards with an electric planer.  You mentioned that you made your centerboard out of foam core and carbon fiber.  Would you recommend going carbon fiber or fiberglass unidirectional weave or a mixture?  I'm also wondering if it is adviseable run a scarf down the sides of the board to fill in with fiberglass and or carbon fiber?  Does anyone have a layup schedule for either? or where to find one.  Thanks

Also the foam core I'm using is an unknown here.  I got it from a family member who told me it came from a windmill blade factory and was used there as core material.  After a little research I believe it to be PVC closed cell foam.  There are pictures of it in the Pulling my Centerboard thread https://fct.groups.io/g/TrailerTris/message/4218?p=,,,20,0,0,0::Created,,Centerboard,20,2,0,81192046
Does anyone have experience working with this?  

I've never done any fiberglass or carbon work before.  

Thanks for any information
Thanks!


Try Flying
 

I'll prefix everything with I'm not an engineer nor composite construction guru.

That said here are some tips/opinions
  • Carbon has a greater tensile strength to weight than glass and this does not necessarily mean its always the best option (stiffness can cause higher loading). Furthermore mixing glass and carbon in a laminate can (not always) mean that the glass does nothing as the carbon takes the load until it fails and then the glass is already past it yield strength.
  • laminated structures hate rapid transitions hence its best to stagger and taper successive layers
  • the idea is putting fibers orientated in the direction of the expected forces... uni directional is very good along the length of the board at its widest chord (deminishing returns as you get to the extremities fore aft)
  • Fibers need to be held firmly in place in the desired orientation so one sandwiches uni in between layers of 45 degree weave ( uni is vertical an inside and outside laminates are +-45)  you can do this by getting +-45 degree weave or rotating 0-90.
  • the encompassing biax give "hoop" strength
  • Biax is very good at following complex curves compared to woven laminates
  • better to use several thinner laminates than one heavy one (also depends but in general)
  • glass is far easier to "wet out" and easier to spot dry spots. With carbon (especially uni) I wet out on a piece of glass (or something else non stick) and fully saturate the piece before squeegee out the excess. Good to have an ecess of resin below the laminate that is forces up thru the fibers than the other way around
  • get lots of throw away gloves and use white vinegar for cleanup
  • Peelply is worth the cost and a cheap alternative to vacuum setup (but vacuuming is the bomb :)
  • rebate for the uni down the spine I think I used 100mm wide slot for the uni on the C/B but will check
  • max compression forces are at the widest chord of the foil and at the exit of the case .... I have high density foam here but a hardwood spine would be even better and not require to be wrapped in carbon to increase stiffness. (if you look closely at the pics of the three foils earlier in the thread you can see the different colour foam
  • Use a scale to measure out your epoxy
  • get a good pair of scissors
  • plastic milk containers are the ideal mixing pot as the epoxy will pop straight out when dried
  • more epoxy does not equate to higher strength ..actually opposite and is just there to hold the fibers in place. by the same token dry laminate has zero strength... 
  • you can tell if you have too much as the laminates will squirm around everywhere... get a decent squeegee and use peelply . (good tip is put a small amount of epoxy on the top of the peelply to allow the squeegee to slide (ie lube) and stops it sucking out too much epoxy from the laminate.
  • set everything up and ready to go before starting to mix epoxy. you need to work methodically and quickly (not rushed but faster on warmer days)
  • thinner epoxy is easier than thicker (I like Gurit for laminating but its pricey)this is particularly true with carbon
  •  practice on a bit of ply before you tackle your foil


When I do my foils I lay all the oversized laminates down dry on my working table including the peelply (mark the centerline of the leading edge). Then place the core blank on this with the trailing edge facing me. I wet out the core with a short cheap brush (I cut the bristles down) and fold the 1st layer over on top of this  putting excessive epoxy on the leading edge. Working the epoxy thru this with the brush in a stipple fashion. this is then repeated for each successive layer (epoxy coat fold over dry laminate stipple thru). The uni is wetted out on a glass table and transferred over to the job as it is particularly a pain to get properly wetted out and requires a bit of a forceful squeegeeing. once all the layers and peelply have been applied to the 1st side the core is gently rolled over (away from you) to rinse and repeat for the 2nd. Again add extra at the leading edge and use the brush to keep everything nice and tight and wet not soaked. You want to keep compressing each layer with the next not have a floppy wet sponge affair on the last layer that you are then trying to squeeze the excess out of. Use the brush to only sweep in one direction (away from leading edge) and along the weave direction (not across the bias)
Methodical work will allow you to tease the laminated to follow the shape. Ends are tricky and the last bit to do once both sides are laminated

The way I do the tip is try and get the biax to follow the leading edge (knee?) as far as possible then trim it with darts slits etc to have side one wrap over and be laid flat under side two and the the opposite. So its all wet and gooey and you need to gently lift side two in that area but it is doable.. less is more so try and not stretch or have the biax turn to a mess of loose strands.
For me at this stage I'm also closing the vacuum bag etc but we will skip this step.

You want to have a string already attached to your core (whoops .. this is a "but first" instruction) as you then want to hang the whole mess up so it can set while vertical. Make sure it is out of the direct sunlight or sources of heat on one side = warped = a lot of swearing


I'll dig up what the laminating schedule I used on mine and post later.

Hope this helps and if anyone has an alternative method or sees some issues with this one... please post.

Cheers

Mark


Try Flying
 

On my C/B I use 7 layers of 300gm uni sandwiched between two layers of 200gm biax with an additional layer on the head and a strip down the leading edge.  The uni was 100mm wide and staggered by 120mm steps from the longest to the shortest. I used a similar layup from the plans for a F85sr rudder but substituted the 5 layer s of 400gm uni with 7 of 300gm as I found it easier to source.

If I was to do it again I would substitute the carbon wrapped high density foam spine with hardwood for added strength, ease of build an cost (HD foam is very pricey)

Hope that helps as a starting point

Cheers


Camtron0@...
 

Thanks so much Mark.  It really helps.  I'll be taking your advice.  I can post pictures as we go to make it easier for the next person.  


Try Flying
 

FYI all those laminates listed are Carbon.


Try Flying
 

If you are careful you should end up with a fairly fair board once you remove the peelply however its quite likely that there will be areas around the Uni and leading edge tape that need some fairing compound. I use micro balloons at this stage and fill and sand this to get a nice shape without removing any Carbon if at all possible. Its worth running a template gauge over the board at this point to ensure symmetry (been bitten before). Once your happy then I would put a light weave glass over the board to protect the carbon and make a base for fine fairing with micro light and high build primer.

A good trick to see you have a nice shape without flats and hollows is to wet the board down with water and look at the reflection of a straight edge (distant gutter, fluorescent  etc)  at a shallow  angle. Tilting and sweeping your vantage point will let you find local imperfections easily. Trust your sense of touch also as you can pickup very small imperfections when you don't use your eyes...just clean up the board with acetone to remove any oils before painting.

And the number one tip.... make sure that you dont end up with a beautiful board that is just a bit too fat to fit in the case... been there done that....massive swearing usually ensues.


Camtron0@...
 

Materials on the way!  I'll take pictures as we go and post them.  

I was wondering about the hardwood spine.  How wide/ thick / long should it be roughly?  I saw on another forum a suggestion to use hardwood 75mm to 100mm wide and full thickness of the centerboard.  So that the laminations adhere directly to the spine.  Does it need to be that wide or can it be like 20mm?  
Thanks


Try Flying
 

If you are doing 100mm wide uni CF laminates like I described then the hardwood spine only needs to be say 50mm wide 

I'll take a pick of a rudder cut in cross section to illustrate this arvo.



Basically the 7 layers of CF uni (in red) is sandwiched between the dbl bias (in black) and laid over the spar shown in brown. Because of the multiple layers of uni one needs a rebate to accommodate and maintain the exterior form and minimize the amount of fairing compound.