Re-coring an engine hatch. Coremat?

Hi. This is a first time post and I am a beginner at Coring a deck


I have a large hatch 55 inches long by 38 inches wide. It covers my engine and is both incredibly heavy and rotting at the core from The previous owner installing unbreaded hardware.

After debating between Coosa, Nidacore, and air cell/divinycell Cost weight, strength ,heat resistant (engine hatch) and local availability in New York City. I have Decided to use divinycell

I asked cruisers yachts, what core I presently have and they sent me this figure of the actual lamination schedule for my boat. I am not sure how to decipher it.

I plan to cut out the old core. Keep the old skin, laminated, new core, and drop it in Resin Putty.

Given what I am posting here what is the lamination schedule used and should I do something different?

it appears to be

• 1.5 oz of something (?)
• 2mm coremat (for thickness ?)
• 3 oz CSM
• 1/2” core (I will use divinycell not balsa )
• I’m not sure the rest here. I see 1808
• And something about red and white cat.

I would like to get the same thickness as the old panel. As little weight as possible and this is on a very used and walked on part of the back deck.

what is the thickness (1/2” or 3/4” divinucell ) you would use and how would you laminate it and drop it in the old hatch skin. ?


This is a 1996 boat and I see no need to use 1996 materials to get what I need

any suggestions ?

<<see attached file>>

 

Welcome to the forum.

The laminating schedule is from the outside in.
1st the thickness of gelcoat
2nd 1.5 oz of csm
3rd another layer of 1.5oz of csm
4th 2mm of coremat
5th another 3oz of csm

You should not disturb these layers.

The core which is 1/2 or 3/4 inch thick balsa (both dimensions are listed)

An iner skin of "1808" fiberglass cloth. 1808 is not the most common weave of FG. You may have to special order it if you are purchasing from a small store. If your FG supplier does not know what 1808 is, get a new supplier.

The red/blue cat refers to the original builder's short cut for identifying two different resin catalyst. Use the catalyst recommended for your intended resin.

My approach
-Grind away inner skin and core.
- bond new core to original outer skin
-use 1808 for a new inner skin
-fair and paint.

Trying to reuse the original inner skin is a fool's error.

Good luck

 

I might deviate from Blueknarr very slightly,not in the level of preparation or the need to remove the old core entirely,that is the essential first step.I suspect the outer layer of coremat is to help resistance to puncturing if a sharp object is dropped as 2mm of coremat doesn't add a lot of stiffness to a hatch of that size.Balsa would have been a good bit cheaper than divinycell and still is for that matter.The high quality foam will outlast it by a long time but do be sure to buy the correct density (usually expressed in Kg/cu.m) I would guess 85Kg/cu.m but you might go lighter or thinner if you can add a foam beam or beams to reduce the unsupported span.It is normally considered good practice to have a balanced laminate on either side of the core.I wouldn't get anxious about simply sticking with CSM for the whole job as it is much more readily available and it wasn't the glass that deteriorated.

 

a friend at the marina who does fiberglass recommended CSM-1708-CSM on both sides of the core when we were talking about nidacore . The original thickness has to be replicated because this is one of several removable hatches on the deck

my suspicion is that coremat was added for adding thickness with less weight in the original layup (denting and print through are other core mat uses too )Rather building up than multiple layers of CSM…. As the balsa hatch is very large and very very heavy.


What does adding beams for support entail? I have not seen that on any DIY video. Is this something I run down the longest side (there is a latch in the center short side)

 

Will you be walking on this thing?

I’d really recommend coosa over divinycel. You are gonna do a lot of work and discover that 4 pound core is really bendy. I’d recommend the 20 pound density coosa core if you can find it, and a simple laminate of 1708 or 1808 each side with the outside setup for the finish you want; if gc you need an ester..

The lid at 1/2” thick is 0.6 cuft. The weight of a 4 pound core that will bend is 2.4 pounds. It will need at least two layers of glass which will add 10 pounds of mass for a final total of 12.4 pounds. The 20 pound core lid is 12 pounds plus 5 pounds glass or 17 pounds, but the 20 pound core is hardware ready and less glass and epoxy work for the 5 pound penalty AND it will barely bend.

 

Thank you for this detailed analysis of the weight. . It’s certainly easier to laminate and for hardware. Very well explained!

(on cost Additionally resin is not free! ). I was originally going to do for engine noise but I hear over time it can squeak.

the bad news:

• only bluewater26 is available for pickup for me on long island. So that’s $359 vs $170 for divinycell.

• And the blue water 26 by your calculation is 16 pounds and now 9 pounds heavier and $190 more.

• the weight of the door and quietness of the engine are real concerns. Of course nobody wants a hatch that flexes of this size!

that’s why i was shying away from coosa (which also flexes when walked on a bit )

unless there is a local source I don’t know about

• Are you saying core would need 2 layers of 1708 roving On each side ? Vs coosa with one plus maybe CSM?

 

55x38 is pretty large for a single skin of 1708 or 1808 on 4pound core

see if you can get a 20 pound foam from Noah’s in Toronto .. they have a warehouse in Buffalo, NY

another option would be a 12 pound core like Corecell M200; it is far more rigid for the span, but m200 is spendy

 

buffalo is 5 hours each way from NYC lol.

If I’m going to get a 20 pound foam i can try to source coosa blue water 20 perhaps. Locally I can only find 26.

The divinycell h-80 (80kg/m3 or 5lbs per ft3) I assume it would require 2 layers on each side which is how I’ve done nidacore

coosa is one layer per side from your comment I suppose


I would get it locally where they also have coosa board here: https://www.Divinycell H-80 Plain https://www.reinforcedplasticslab.net/View/Divinycell-H-80-Plain.net/View/Divinycell-H-80-Plain

 

Since the question was asked about beams,I will attempt to upload a screenshot.the principle is to divide the unsupported span of the hatch surface and use deeper sections of foam, on top of the inner laminate,to increase the depth and hence add local stiffness.It isn't always possible to find the space,but where it is,the effect is good for the modest extra weight.They can run in any convenient direction.On the topic of weight,that hatch will suddenly become much lighter when the soggy balsa is removed.When the beam is covered in glass and ideally with at least one ply of 45/45 orientation the effect is a bit like this.

 

Foam cores can be shipped 2’x4’ or maybe 2’x6’ UPS affordably.

Noah’s is based in Toronto; their US hub ships out of Buffalo, so cheap to ship

otherwise, you can also add stiffeners as wetfeet mentions