Fiberglass and Foam Core Tips

I am considering building a hard dodger using ¾” Divinycell covered with biaxial cloth and polyester resin. I have shaped and glassed a large number of surfboards so I am familiar with laminating in this regard. However, I am not sure what the best process is for finishing off the squared edges of the sandwich core such as the aft edge of the dodger top and areas around the window openings. I am assuming these edges get de-cored but I don’t know what filler to use with the polyester resin and if additional cloth around the edges is required.

Can anyone provide tips on finishing these edges?
- How deep to de-core
- What is the best filler
- Do the edges need to be glassed over after filling – if so, what cloth weight etc

Any help would be great!
Thanks
Frank

 

Think carefully about this.

Often edges such as the aft edge you mentioned, will need stiffening or the whole thing can end uo too flexible and wobbly. So to improve the stiffneds, you can double the core around these edges and use a router with a round over bit to create a rolled edge profile and simply wrap your laminate completely around it. Using a router gives you a perfect profile that needs little finishing work before painting.

If you simply want to decore and backfill, i use a mixture of micro balloons in the resin until it has a non sag consistency. I decore to depth of 8-10mm , again using a router with another type of bit simply because it's very fast and easy. In theory the depth isn't important, it only serves to create a solid edge which can be painted or otherwise keeps an aesthetic finish.

 

Thanks Groper
I was planning to double up at the aft perimeter as you suggested.

I need to get down to local shop to buy some biaxial cloth to understand how it lays and wets-out as I am used to working with standard weave 4 oz cloth. if the edges are wrapped, do I use the biaxial with the CSM on back similar to 1708 or just straight biaxial and what weight do you suggest?

Thanks
Frank

 

Hi Frank,
Groper is right on in this area, depending on headroom under a generous radius to the aft lower edge can be much kinder, also a tapered beam is sometime incorperated to top edge... rain is directed outwards & stiffens the edge & makes a more secure hand hold, also think if you are going to have covers/awning attached here & what the screws might get a bite into.
Even a WRCedar beam capping the back might be ok glassed in but more suitable with epoxy resin or if using some foam as beam upstand a uni glass cap to top & bottom will help... or just double/triple your fine cloth. I'd suggest you use your familiar 4-6oz cloth around edges & use the plain biaxial on the broad expanses, 20mm foam indicates a max radii of 10mm or 20 if you double it, biax will go but wont like it so much, epoxy might be a way better choice & leave out the combi fabric with the chop, big resin & therefore weight saving will pay for the extra cost of epoxy, only reason not to do that is if you are contact molding the surface off a melamine mold or similar & using gelcoat, last hard top I did just that with a molded downstand for the front and sides & an athwartships "pocket" that molded the aft edge beam.

Jeff.

 

I wouldn't use biax as we call it here - which is 0 and 90deg fibers stitched glass. I would use double bias - which is +-45deg fibers stitched glass. Reason is to wrap around an edge it's much easier when the fibers are not perpendicular to that edge which half of them will be if using 0-90 fabric. How much depends on the strength and toughness requirements ... But to wrap a tight radius I wouldn't use anything over 600gsm cloth weight, 450gsm is nicer. A combi cloth like 1708 won't be as easy to wrap...

The weight of cloth depends on how big this thing is and what kind of unsupported panel areas we are talking about. Does it need to support a dozen people standing on it , I don't know... This is an engineering question not so much a practical " how to " question.

 

True,

It does become an engineering question. My assumtion would indicate hard dodger = shade, maybe with occasional access, we dont know. The DB will be better to wrap the edges but "might" be a poor choice for the span particularily the aft edge with continuity of fiber, in the web of a beam might be good combined with uni caps wrapped under it. Kinda depends on the size and support given to perimeter and amount of overhang past bulkhead... if there's a bulkhead or other support, maybe it's on posts? Frank hasn't said.

All the best from Jeff

 

Thanks Guys, great input. This would be replacing the existing canvas dodger. I would use the existing SS dodger frame for the basic shape and to support the mock-up. The new hard dodger would have some tweaks so I would cover old SS the frame with door skins or mica to the new shape (slightly longer, window placement and aesthetics). Once I have the mock-up looking right, I will cover it with Divinycell and laminate the outside. Remove it from the mock-up and laminate the inside. That’s the general plan and I am sure there are a lot of details that still need to be worked out for instance I am not sure how stable the dodger will be with only one side laminated etc.
For size reference, the boat is 40’. The athwart width of the top is right at 8’ and the fore and aft length at the centerline is 3’ 6”. The top has a natural camber of 3-3/4”. There is no need to stand on top but my gut tells that if it can’t support one person it won’t be strong enough for normal wear and tear when leaning on it in a seaway etc (shade tree composite engineering at its best!). Note that the hard dodger will be self supporting, the old canvas dodger frame will be removed.

I am flying blind on the lamination schedule, however I just purchased a 2’ x 3’ of Divinycell to experiment with. I plan to cut the panel and re-join it a 110 degree angle to simulate the side panel to top intersection. Nicelly round the outside corner and fillet the inside corner with polyester and microballoons. Then laminate both sides (and overlap the corners) with 2 layers of 1708 biaxial inside and out. The 1708 is 45/45 and has ¾ oz CSM on one side. 2 layers of 1708 yields a theoretical thickness of .090” Then give her some heave ho testing to see how it all feels as far as joint strength core compression.

 

2 layers inside and out of 1708 is overkill. A single layer would be fine. The global engineing of the panel will be stiffness driven and this is most efficiently achieved via the thickness of th core. The entire perimeter would ideally need doubling or use vertical edges which also gives the roof the appearance of depth etc. you can also incorporate a stiffening grid on the top which can double as solar panel mounting rails etc... Just a few ideas for you.

If it only has to take occasional light foot traffic, then a single layer of 1708 will suffice... Toughness can be built up with laminate thickness, global strength and stiffness should ideally be achieved with core thickness....

 

Thanks Groper, that will save on cost and weight. I'll experiment with my sample of core material. Unfortunately it won't be for a week or so due work.

Thanks again!
Frank