Stringer And Deck repair

OK, so you want to make one big trapazoidal beam that takes the place of the two stringers and fills in the space between the two stringers? More to the point, two fore and aft beams, one port and one starboard.

Well, that's certainly thinking outside the box (actually, I guess that you're proposing making a box of sorts, right?).

I have no idea on that. What I would recommend is waiting for a reply from PAR and the more experienced guys here. It just seems to me sitting here thinking about it, that you will use a lot of materials (foam and fiberglass/resin) that you don't really need to use. As far as the engineering aspects are concerned I'm not qualified to make a judgement. My gut tells me that I'd do a lot of research here though before I'd go forward.

MIA

 

I think what is causing the confusion is the stringer should be 2" wide, 12" high and 16' long. A 12" WIDE stringer will not work (unless it's that high also).

Wood is quite strong, you have to make a proper stringer out of glass to replace the wooden one. I don't like wood because it is difficult to seal it properly.

 

You have to fill the cavity between the stringers with a flotation foam with a CLOSED CELL STRUCTURE. If you slip up on the foam it will absorb water, become extremely heavy and you will have to do it over again.

 

Plexus is a specialty adhesive and really isn't the best way to go here (besides it'll cost 10 times as much and has a real short working time). In fact, it's not even as strong as the polyester resins used in your hull. It is good for difficult plastic bonds, but you don't have that here.

By the time you make the stringer stiff enough with just resin and fabric, you'll rival any wood cored version in weight. The only way around this is to control resin to 'glass content with vacuum bagging or infusion, neither of which seem likely on this job.

A big hollow box will likely buckle under load unless the box has an internal web structure or substantial core. It's size will require a lot of material.

With epoxy, you can seal wood up tight. I've worked on boats that I epoxied 30 years ago and the wood looks like the day it was entombed in goo.

I would strongly recommend not attempting to re-engineer the stringers system. Again, if you elect to go all laminate, you'll use a lot more goo and fabric. You will not save any weight, okay maybe an ounce or two if you're lucky, but you'd have to be careful about wetout and saturation.

Foam is fine, as long as the laminate stays attached to it or the laminate is thick enough so it doesn't matter if it stays attached.

Those are the usual options. Wood uses the least amount of goo and fabric and is probably the lightest. Laminate only will use the most goo and fabric and probably will be the heaviest, though it will not have wood in it. Foam cored will use some what less goo and fabric, but relies on staying attached to the core to be an effective load bearing member. It's best if these structures are slightly over built (read more goo and fabric) unless you can effectively control 'glass/resin ratios, at which point is will be the lightest of the bunch.

So, if working with goo and fabric is your thing, skip the wood and use a cored or solid laminate for the stringers. If working with goo isn't high up on you good times meter, then epoxy sealed wood and tabbing is the way to go, which will also be cheaper and faster then the other two.

 

I am going to bed the stinger in epoxy paste as recommended. I have a question regarding the transom. I was going to put a layer of 1708 on the transom shell the 1 -3/4", then 1708 then 1-3/4" then 2 layers of 1708 bonded to hull. Should I coat the plywood first with 2 coats of epoxy let dry then apply the layers as described above. I am using MAS Epoxy. thanks for all the information. Epoxy and fiberglass can become confusing.

 

Again, you don't need nor want the mat in you laminate. It adds tremendously to the weight and resin requirements. Biax or regular cloth will be all you need. I'm not sure what you're referring to with the double 1 -3/4" something or others, but I hope this isn't the thickness of the transom. Your transom is likely around 1 1/2" thick.

Epoxy is a type of resin. Fiberglass (what I call 'glass) is material, which can be a number of different products, such as 1708 combo mat or biax, etc.

You don't need fabric (1708 or other wise) between layers of plywood in the transom, just thickened resin. Good contact between the two layers of plywood is what you need and drywall screws can insure this occurs.

 

Par
The transom is 1 1/2" thick. The 1 is one piece of 3/4" . Can I put the 2 sheets of 3/4" together on the bench? or laminate wet at one time?
Thanks

 

Either will work. On the bench offers more control.

 

I know this is an old thread but I have been reading extensively here and have a question. First, I'm a DIY boat owner so forgive me if some of my questions are basic.
In reading this thread, I see references to the importance or lack of importance of core materials used in stringer construction. My question, if you have glassed over wood core stringers that are wet/rotting but have not changed shape, could you simply grind them and lay up more glass with epoxy resin to reinforce them and forget about the wood inside? Toatally removing old stingers and core and then rebuilding from a bare hull seems like reinventing the wheel so to speak. If the shape is there and you can build the thickness of glass with repeating layers of biaxial fabric and epoxy would that yield the strength need for proper support? I know I asked that question twice, just trying to clarify what I mean.

If this idea is not feasible, what would be better for engine bed stringes, dimensional lumber such as white/red oak or a laminate of marine plywood? BTW the stringers I'm talking about are about 2" thick and about 10" tall. I have no idea what was used originally.
I'm not currently doing a project, but I may have to in a year or two, so I'm trying to do my homework ahead of time. If I do have to rip and tear, I certainly don't want to have to do it twice.
Thanks

 

Leaving a soaking wet/rotten core in the boat has 2 major problems that I can see right away.

One, the added rotten/soaked core is going to add lots of weight compared to a dry wooden core, nevermind the extra fiberglass and resin to make the grid strong enough to support the loads in the boat. As much work as it is, grinding out the core and replacing it is not that much more work than preping and adding more fiberglass to your grid. I also prefer to make sure I have done things right the first time. You wouldnt want to attempt a fix, and end up just replacing it all over again.

Two, stringers/bulkheads offer more than just hull support and a level surface to mount the sole. Depending on where and what accesories you have, the wooden core could be relied on for holding the fasteners of whatever is mounted to it. For example an engine mount. Rotten cores do not hold fasteners, and from my expirience, straight laminate doesnt hold very much better.

As for the lumber to replace it with, that all depends on what you have available and your budget. I like plywood for its uniformity, and you can make just about any thickness required by laminating pieces together. There are several types of marine plywood, some more exspensive than others. I was told that douglass fir is not the best choice for the sole because it requires a minimum of 6 oz cloth to properly seal, where as the okume mahogany (which is more exspensive) is lighter and requires only several coats of epoxy to be watertight. I used fir for my stringers/bulkheads because they are completely laminated in 4 layers of 17 oz biaxial and generously coated in epoxy. The sole I used okume mahogany because it wasnt feasible to fiberglass the underside of the sole.

 

What wood does as a stringer or core element is withstand compression as the stringer (laminate and all) takes on a load. If the wood core is mush, then the laminate attached to it will likely buckle or crush. You can increase laminate thickness to compensate for a mushy core, but there's a fairly large weight penalty, not to mention lots of working with goo and itchy materials. If the wood has a fairly high moisture content (probable) then cold weather can freeze it, possibly busting open the laminate.

So, as you can see it's usually best to remove and replace a spent core. Wood is used for several reasons: weight, strength, fastener holding, durability, etc. You can eliminate wood if you like, but you'll pay dearly for these products and they will not have all the benefits of using a wood core. For example you could use foam, but this doesn't hold a fastener worth a damn. You could use a honeycomb product and have the same problem, with both of these materials costing 10 times as much as plain old wood. Why do you think we still use it?

Douglas fir can make a fine sole, but as mentioned it does need a fairly heavy 'glass sheathing, but you'd want one anyway to prevent damage, improve waterproofing and for abrasion resistance. Okoume isn't a mahogany, though is a very nice, light weight hardwood. I much prefer it in many application, because of color and weight. It's not especially rot resistant so it need consider protection and it's not as strong as other species. Meranti is another common plywood species (often called Philippine mahogany, but it's not a mahogany either). It's a dark hardwood, that is fairly coarse, but finishes well, is about 20% heavier the Okoume and also stronger then Okoume. Sapele is a very nice hardwood plywood and is a real mahogany (African). It's strong durable and pretty, but also getting fairly rare and costly as a result.

For most repairs, Douglas fir will do, mostly because it's going to be coated and covered in goo and fabrics anyway, so who cares what's under all the plastic and paint.