stiffness question for engine cover

First time here, great info, but a question or two please. I am planning to make an engine cover for my 18' jet boat. The plug is just about done so.... when making the mold, how thick should it be for a one-time item? I think not too if I use a framework to strengthen it well. And on about a 4' X 4' flat panel, what materials are recommended for maximum stiffness Vs. weight? I am seriously considering 2 layers of 1/4" balsa coring with glass between them, but I dont necessarily want to overbuild. Perhaps a 1/4" core with a 2-3 "ribs" to reinforce and support it? Any thoughts or suggestions would be helpful. Thanks, Steve

 

Make sure you form in a small crown in the form. I would look at a good divinicell foam core or a honeycomb material, in lieu of balsacore, unless you already have it. Are you planning any sound insulation in the box/ Keep in mind, if you are building a female mould, allow for the finish size, including any stiffners and sound shield. Several ways you can save some labor is to layup a full sheet of material and glass one side, and then cut it in pieces to fit the panels, especially for the side that goes inside, and do your initial finish work off the mould.

 

Do people step or sit on the cover?

 

People will be sitting for sure and probably standing occasionally. I am resisting using foam core as near as I can research balsa will give me more stiffness. The crown is a good idea. One supplier is recommending carbon or bi-axial E glass. Is he correct as far as stiffness? He claims carbon is 3 times stiffer than E glass. Steve

 

You can build in a grid of solid battens into the core structure, the thickness of the core material, say middle ways and on the ends, and strengthen the lid and structure. Carbon Fiber is a little extreme, IMHO. Hardtops are made out of two layers of 1808 Biaxall and foam, and spanned across ten feet and more, in most cases. We walk on th top of them. Most have 1/4 inch per foot of crown in them. We coat the foam with a thin layer of thickened cabosil before glassing it.

 

I've built lots of hardtops like that. Usually they have aluminum tubing every three or four feet for reinforcement. I think for an engine box 3/4" core is sufficient.

 

The grid idea has legs. Will I lose any strength if I infill the grid with balsa coring, or does it all need to remain in one piece to retain its integrity? Steve

 

Gonzo, all of the Merritt sportfishermen, and many of the custom buiders of towers, now, like Pipe Welders use continuous open tops and just border the rim with the tubes. Many also, will reinforce the outside of the top with a doubler piece, to show and stiffen the top rim. This will give you a beefier appearance without the weight of a 1 1/2 top. Some will mount electronic boxes, recess rod holders, and spreader lights in molded in boxes, now, to keep the weather from affecting the ever self destructing quartz lights.

The answer to your question concerning the gird. yes you can fill between the pieces with your core material and glass it. For the ease of construction, just glass up your core material, and then glue 1/2 " grid stiffeners to the face of the glass, and tab into place. It will not affect the insulation that much, if your care to use it.

if you wish to have a clean surface, we mortise the ends of the grid pieces , and in the areas that they cross over each other, that will be glassed inside, to carry the load across continiously.

The easiest way to do this, is still do your glass work on the whole core material, one side only, and then cut out the core material where the grid will go inside of it. Install the grid while on the jig, or form that you will need to keep the component from twisting in the construction stage of it. I hope I have made myself clear enough. I have to go out of town, but will check in tommorrow and answer anything that may sound confusing from my description of this procedure.

 

Thanks Oyster. Is that 1/2" square, or thick or what? And would the grid pieces be balsa also? Steve

 

There is a book from Ken Hankinson , Glen-L boat designs, called Fiberglasss Boatbuilding for Amateurs, that shows pictures that can help you with this layup. It addressed hull stiffeners, but I use it for just what we have been discussing here. I thought I had seen it, in a book, and search for it, and walla, there it was, chapter 26. I use pieces that range from 1 inch wide, to 1 1/4 inch wide for the battens. It also shows how you cut out the pieces of core and this will allow for a stronger laminate. Hope this helps.

 

Thanks Oyster, big help. Ill see if our small library has it. Now, the mold. After laying in my initaial layers of glass can or should I use a less expensive material to bulk the mold? I thought of cloth or screen wire. Or can I glass on 2 X 4's to "box" the mold? Or would a few layers of glass woith a frame gicve me enough strength for one pull? Steve

 

Let me ask you several questions. You state now that you are going to "lay in" glass, and bulking up you mould? How is it that you are laying "In" glass, if you are just building the female mould? . Is your female mould done from the plug that you originally had stated that you were building? Most female mould are done from male plugs, faired for a good finish workon your component. If not ,stop at this point. I will give you a better idea, along with what I told you. Most of the discussion was for your flat panel, per your first post.

 

Actually a bad choice of words on my part.Maybe I should have said "laying on" glass. I was referring to the glass for the mold. It is a female mold from a male plug. The engine cover is not actually a flat panel, but sort of a raised cover thats mostly flat panel . It drops at about a 45 degree angle to the bottom. Im OK here I think. I intend to use 2 layers of gelcoat then maybe .75 oz CS for the first layer, then maybe 2 oz cloth for the second. As I read in past posts here It seems then that you build mold thickness with either many layers of cloth or woven roving, but most molds need to have many products pulled from them. i just need to pull one. Hence the using a cheaper alternative than glass. Sorry about the misunderstanding. Steve

 

If you are planning to layup wet glass inside the female mould, it is critical that the female mould is stiff enough, so the new part does not show distortion, during the curing time, either reinforced with 3 /4 plywood behind the few layes of glass, , or a lot of glass in your layup. We use a tooling gelcoat, specially designed for moulds, in the place of regular gelcoat. This happens from the heat is generated by lamination.

This is what I would do, if this was mine. I would forgo the making of the female mould. As long as the plug is stiff and sturdy, I would layup the raw laminate, as instructed, on a rigid place, like a sheet of plywood, with slick formica on it, glassing the one side. Then I would cut the raw material in pieces, fitting it to the plug mould, tacking it in place by tabs, and do all of my grid work and finish glass work from the outside. Yes you will fair the surface of the box, upon completion. You will need to use some packing tape on the edges of the plug, to keep any glue from sticking the parts to the plug, in the initial gluing process,

But you will have the amount of time and material in the female mould doing that, so why not save this step in the process, and just do the part. When building the plug, did you fit it exactly to the area that the engine cover will go? Also, put some radius on the edges, before you tab the pieces together. I hold the pieces together with tape in sections of the corners, and do it in a multiple step process. I hate building female moulds for a one off part. Is this a boxed cover, or a three sided cover, that is fitted to the stern area? In a boxed cover form, you must have the opening of the female mould, a little larger than the closed area, so the part will come out of it, easier. Check back later to see if this is all for naught.

 

Fear naught, Oyster, it will not be all for naught. I suspicioned the mold would need to be stiff, hence my numerous silly questions. The plug is indeed fit extremely close to what is needed, and the edges are radiused oh-so-nicely. Unfoortunately the plug is, I don't think so stiff and rigid. It is built of 3 layers of 2" pink foam insulation glued and bolted together with a bondo skin around it. I am kind of thinking of doing the female and storing it for a little while until closer to spring. We will see, thank you very much for your valuable input. I will continue to lurk and learn! Steve