Most of today’s boats are built using fiberglass as the main material and molding as the method of building.
You may also want to read the Fiberglass Guide before you start working on your mold or find an appropriate boat plan to build mold form. To prevent your mold from sticking to the plug, the plug must be coated with plastic film known as “PVA”. When the gelcoat has cured so that it cannot be scratched off with the fingernail at the edge of the mold, which takes from 2 to 4 hours to overnight in cool or humid weather, you are ready for the “skin coat.” This is a layer of ? or 1oz.
Depending upon the condition of the mold surface, it may have to be sanded with 220 grit working up to 600 grit wet or dry. My understanding is that the bottom of the wood should be glassed up with resin and cloth prior to install to seal the wood. A lot of this is on the internet already but I can't seem to find anything about what type of cloth or mat to use, how heavy and how many layers.
It depends,there are a lot of choices on how large of an area you want to do,and what you want to spend. You can use Polyester resin,you can use Epoxy resin,you have a choice of material ranging from mat,to stitch mat,and various weights.You can use plywood,or you can use balsa or foamcore.
I have done a few,and work in the marine fiberglass and resin supply business.Send me a pm with the specs on what kind of boat and what you want to replace,and I'll be glad to help if I can.
When doing the floor in my Sea Ray, I applied a thin (8oz?) layer of porous fiberglass cloth as the first layer over the cured, resin covered wood. Never Done - I don't know exactly how many hours I have into the floor; I had to replace the transom in my boat and had a lot of other hurtles to overcome not related to the floor after the old floor was out.
Well,if you do a proper job with the glass work the repair should last as long as you do.In almost all cases,it's not the resin or core that fails,it always because of poor glass work,or holes that were never sealed when mounting things like seats,rod racks,wiring,pumps,engine mounts etc. As for the core,you're going to get all kinds of opinions on what to use,as you can already see from the replies so far. Most of the decks,and restorations I've done,were with marine fir,or composites,but they really aren't necessary.The only difference between marine plywood,and the better grades of regular fir plywood,is that the marine ply has no voids in any of the plies,and often contains more plies,making it a stronger product. I also would not use Epoxy resin for your repair,(for the top part anyway) and I say that not because it's not better than polyester(it is),but because it's not necessary to use it in this repair.
It is much more expensive than a high quality polyester resin,and is overkill for what you are doing.Also epoxy resin will not wet out fiberglass mat,as it can't dissolve the binders in the mat that holds the fibers together. There isn't a production boat builder on the planet that uses epoxy on their decks,and that includes all the high priced ones.
I would use a high quality Polyester resin,not the junk that is sold in places like Home Depot,Lowes etc.They carry low quality resins,and some of it can be old and unstable. Good high quality polyester Resins do have a shelf life,and if stored properly,out of the sun and heat,you will have no problem with them.I routinely use resins that are more than six or eight months old with no problem. We carry resins from Polyesters,Vinylesters,Dions,and Epoxies in quantities from 55 gallon drums to 5 gallon,one gallon and quart sizes. I think the best way for you to start is to cut out a small piece of the glass from the deck so that you can expose the core.Now you can assess the damage. It may only be a small area of the core that is bad,you can tell by gradually removing the core until you hit dry wood. In many cases on a lot of boats,you'll find a core of plywood under the areas like seats where they had to install screws etc,and end grain balsa core under the rest of the deck. Many times you'll find that if you chisel out the rotted (peat moss)core,you'll hit a bottom layer of fiberglass,and often times you can bed in a new piece of core with some resin thickened with cabosil(thickening powder)or some hull and deck putty and then finish up by laying up some layers of mat or 1708 stitch mat(mat and double bias cloth)over it. If you find that the whole deck is shot,then you'll have to remove it all and install a new one.Every boat is different so different procedures will apply. If you have a good solid stringer system that previously supported the deck,then it makes things a bit easier. You would need to cut your core(plywood or whatever)to size and prepare the bottom,before installing it back to the stringers or other attachment points.
There are lot's of ways to do it,you could waterproof the bottom by applying a prime coat of no wax polyester resin to the plywood,and after its kicked(cured a bit)lay up a layer of 6ox mat over it lay the mat dry on it then pour some resin on it and wet it out by using a roller to saturate all the material.
Be especially careful to completely saturate the edges of the plywood with the epoxy,as plywood absorbs water on the edges like a sponge.

You could put one layer of lightweight cloth over the epoxy,but it isn't necessary although it would be a bit of extra insurance to keep the plywood face from checking. If you thicken the gel coat to a ketchup consistency with some cabosil it will make a beautiful non skid surface.
He's the type of guy who won't sell you what you don't need,and he can give you great advice with no bs.
Actually, there is very little rot in my boat except for some small localized areas where screws when through the floor to hold the helm chair and a cabinet just behind it. Anyway, I'm just going to cut out an area right behind the helm seats about 5' x 5' and replace it with new marine plywood and glass it in - bottom and top side.
3) Also, in the 'finished' photo, is the white color up around the sides of the floor area paint or gelcoat? I would say that a possibility for the warpage in the floor was from wet and dry cycles with the high heat and humidity of the Florida environment. Just for ha ha's make sure you check every single area that you can see or feel under the deck and in the bilge to make sure nothing else is brewing under there.
What you have going on is very common,and I think that your making the right decision to replace only the small part that,s rotted.
What you see in that photo is the finished results after I built the new engine beds and transom and tied the stringers back in.
I'm going to pull the engines (twin 350's) then I'll be able to get right down in there for a better inspection. Just curiously, why did you decide on gelcoat in the bilge and floor instead of two-part paint for everything?
Sadly, the backyard pool was as far as the boat actually went, because of a couple building errors on my part.
This was not a small boat by any stretch, and in the end it measured a full 16 feet long, and 2 feet wide in the middle. This project only cost me about $100, and making the changes I describe above to the design will make it a worthy investment. Scott and John sail their Normsboats in the recent Florida 120, shown here off Navarre Beach.
In the past few Bobsboat episodes we have figured out the shapes of all the panels and then all the required bevels. One guiding rule to keep in mind is that structural elements in a boat are almost always defined by stiffness and not strength. With that weight in mind we might add 120 pounds for a motor larger than AF4's 10 or 15 hp and a bit of fuel. Usualy there is a huge negative mold for every boat part that used when the boat is put together and this negative mold is used to make positive molds. You must either build a part from scratch using wood, plaster, polyester putty, sheet metal, etc.
So if you have some plans to head out to the Pacific Northwest and would like to do some fishing while your out there go to the STS site and get all the info you need. Apply several layers of glass in alternating directions so the 'grain' of the glass is different between layers and seams are staggered.
Second guiding rule is that if you double the thickness of a structural element you will usually increase it bending stiffness by a factor of 8! One of their functions is to keep the boat in shape but that is easily done since decked boats in particular form a rigid box. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans.
Perfect for most structural work, such as tabbing bulkheads, bilge stringers, knees, and most any other structural fiberglass hull repairs). Then Duckworks sends me an email about the order and then I send the plans right from me to you. The idea is that the solid framing carries the load and will be secured with glue, bolts and screws to the side of the boat. You can see in this photo how little water the boat needs even though it is over 20' long - maybe 4" will float it.

Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. Molds are also made from fiberglass and are covered with a perfect layer of molding gelcoat.
The plug is generally a male model (a positive model) exactly like the item you wish to build in all the details. So you can't just pile it on like you can with a sail boat that will be trailered (any cartopped boat has to be light to be successful). If there is a problem with this it will be with twisting under the power and weight of the motor.
With a plywood boat, if you keep the panel thicknesses the same as the original, you would expect the hull weight to also increase by that amount.
Not only that but the volume of water the boat displaces can be in the same proportion which means it can be heavier by 73% and look the same in the water. The chine corners need to be armored with fiberglass set in epoxy and inner seams given a fillet of thickened epoxy to keep water from creeping into the seams. Boat builder than applies some molding wax over the glecoat and polishes it so it will later on be easier to remove the part form the mold. But I've seen the very same design done with thicker ply which weighed 150 pounds and was no joy to cartop. Bulkheads also introduce loads into the hull shell and the obvious thought here is the bulkhead you hang your mast on in a sailboat. Traditionally this was dealt with with a "knee" that ran like a triangle from the motor area to the keel of the boat. In general I think they can be pretty light if the loads are introduced into the edges of the bulkhead and not into the center.
My own favorite solution is to put a slop well right in front of the transom that boxes it all in. There seems to be a general rule of thumb for powerboats that it takes a horse for every 50 pounds of weight to plane and then you need 28 hp for this boat. Not only does it reduce twisting but it stiffens the entire stern of the boat in every way.
The Coast Guard gives advice about powering boats and this one would max at 35 hp based on length, width, and the fact that it has a flat bottom which can be prone to handling problems if overpowered.
It was nothing but trouble both from the standpoint of flexing and leaking (through knots and such) but has never failed. And if you are sloppy like me the "slop" qualities, where all the slop drains out through the stern and not into the bottom of the boat, are well appreciated. On the other hand, this boat used solo and stripped might weigh in at 1000 pounds total and do quite well with 20 hp. As with the old Jinni it can be frightening to watch in rough water and guaranteed to make you slow down.
In this caseit isn't quite so simple and I'm guessing the bare AF4G hull would weigh about 700 pounds since the larger boat has built in seats and another bulkhead and some thicker framing. In my experience with AF4 these hulls will plane with less than a horse for each 50 pounds. It is strong enough and stiff enough and anything extra you add will only detract from the experience.
By the way, I would expect the cost of the project both in labor and materials to increase in the same proportion as weight. His is like iron although his boat is about 150 pounds total heavier, not all of it in the bottom.
Using the same number for the 1400 pound AF4G the 28 hp motor might plane the loaded boat at 60% throttle.

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