Replacing stringers and floors

I have a 1979 17.5' Ebko ski boat that I've nearly completely disassembled (except for the motor) to try and replace the completely rotten "sole" (as West System calls it). The more I got into it, the more I discovered just how deep I've gotten (I'd like to think I'm not in over my head yet...this is all new to me). To use the West System jargon, I discovered that the sole was completely rotten and in many places, completely gone. Surprisingly, the stringers and floors (perpendicular to the boat's long axis) were for the most part in good shape, with a few exceptions. I discovered a "floor" was in really bad shape and decided to remove it for replacement. I discovered that it is was made of two pieces of plywood on end, one piece measures exactly 1-3/16", while the other is 11/16". Of course, no one sells 1-3/16" plywood (not to mention I don't need a whole 4'x8' sheet) so my biggest hangup now is what should I use to replace it? Solid wood? Furthermore, the whole original design is a bit suspect (even to a novice like me) because the plywood is on end, and screwing the plywood "sole" to the "floor" (and stringers) when that time comes seems strange because the plywood isn't as strong oriented that way. (oddly enough, the original sole shows no evidence of being screwed to the stringers/floors anyway...I found no screws when ripping the sole out). So it looks like the sole was glued or bonded or something to the stringers and floors exclusively.

Could all of this be why Ebko apparently no longer builds boats?

Any help would be greatly appreciated ... I just need to know what to replace the wood with for the rotten "floor".

 

Triton

Ebko was built in Hastings, Nebraska. Back in 1979 Ebko was a leader in the marine industry with stylish upholstery and interior design. Ebko is no longer being built, but Bill Phillips, the President of Ebko, still lives in the Hastings, NE area and may be able to give you much more detail information.

I know the boat quite well, spent a lot of time at the factory and would rate its overall quality as fair to good for boats built during that time period.

Your 17.5’ is probably a Tri-Hull design and is either the Capri (17’) model or the Executive (18’) model. , and it probably has a metal-flake/gelcoat finish.

I was in the boat manufacturing industry for thirty-five years and am familiar with the floor/stringer system problems with boats built in that period. The stringers were typical built with ¾” marine plywood and the floor were built with ½” or ¾” marine plywood. Ebko did purchase custom made sizes of plywood, but normally it was the length; i.e., 10’ & 12’ lengths, and not the thickness, so I’m not sure what you have.

Once water penetrates the plywood it will tend to “Wick” through the entire floor and stringer system, in my opinion if replacing part of the floor and stringer is necessary, one should replace the entire floor/stringer system with ¾” CCA Pressure Treated Plywood – it will never rot!

Oh, by the way the floor was bonded and stabled to the stringer. The stables were only used to hold the system together until the bonding material hardened.

 

The terms West System use are standard marine usages and though they may seem odd, rather common to us.

The floor that needs replacing only requires 1 7/8" total material for the structure to be as it was. It can be made of any combo of, preferably plywood, that will arrive at this dimension.

I'd strongly recommend you age any PT (pressure treated) plywood (or lumber) for a long time before you install it in your boat. PT comes quite wet from the treating facility, making it unsuitable for most adhesives until the moisture content is below 15%.

You're correct in the finding of no fasteners through the sole to the sub structure (floors and stringers mostly) it was tabbed. Tabbing is basically 'glassed in brackets holding on to the edges of the sole. Most manufactures use some sort of temporary fasteners (staples) to fix things as the goo sets up.

Try not to over analyze the structure until you have a better understanding of the stresses and forces involved in the craft while underway. What may seem a poor orientation of the plywood may be an answer to torsional loading, not easily seen by the "novice"

Ebko built an okay boat, not a fine example, but not a bad one either. Being a production manufacture, some things were left off the "build" list to contain costs, most of those would have lead to a longer life, but . . .

Typically boats are kept in cosmetically good shape for some time, but the structure is left to waste away. Neglect is the biggest cause of folks like me keeping a staff in work. People just don't want to know, when they do, it's sold and you end up walking around on deck or sole that is slightly firmer (or maybe not) then the mattress you sleep on. This isn't as big a deal as it may seem to you now. Bang away at it, a little at a time and you'll get it done, probably better then new (not a bad thing, when you're farther from shore then you'd care to swim back to)

Good Luck,

 

I must tell you a bit more about your stringer system and CCA Pressure Treated Plywood.

During the late 1970’s Ebko used marine plywood to manufacture their boats. This was an accepted, although poor method to build the floor and stringer systems. In areas Ebko would double the 3/4” plywood stringer/grid system and fiberglassed it in place. The stringer was not always fully encapsulated, which allowed moisture to penetrate the plywood quite a bit quicker than the screw and/or staple penetrated areas. A ¾” CCA Pressure Treated Plywood floor is sufficient if it is fiberglassed coated and tabbed in place!

A floor, 1 7/8” thick will add a significant amount of weight and is simple not needed. AWPA Standards for CCA Pressure Treated Plywood: Salt Water (Pilings, bulkheads, etc.) contact is 2.5 lbs per cubic foot, above ground (Decking, fences, etc.) is 0.25 lbs per cubic foot. (Note this is for pine, Fir, which is better, is a bit heavier) CCA pressure treated wood is available in a variety of retentions for various applications. Preservative retentions are in pounds of preservative per cubic foot of wood as per the American Wood Preservers' Association. Assuming you are talking about the pounds per cubic foot, or one sheet 4'x8'x3/8" sheet of plywood (which is one cubic foot); the weight would be approximately 36 pounds. You figure it out. Depending on which model you have it only weighted between 2600 pounds and 3000 pounds new.

Crownline Boats, which build an excellent product, web-site, http://crownlineboats.com/aboutcrownline/tour/tour5.php, illustrates an excellent stringer system built with ¾” CCA Pressure Treated Plywood. I have built many thousands of family runabouts with a similar system using CCA Pressure Treated Plywood and never had a single failure.

Unless the CCA Pressure Treated Plywood is left outside, the comment about PT being quite wet is simple not factual. Most Mills that manufacture CCA Pressure Treat Plywood kiln dry it. To my knowledge all CCA Pressure Treated Plywood is kiln dried to equalize moisture content. By equalizing the moisture content with the atmosphere, the plywood resists splitting and cracking. Every piece of plywood is dried to nearly exactly the same moisture content, every time. Kiln drying helps eliminates shrinkage, twisting and warping.

One of the most effective wood preservatives available is chromated-copper-arsenate (CCA). CCA Pressure Treated Wood is lumber or plywood that has been impregnated under high pressure with preservatives that protect the wood from termites and fungal decay.

What makes CCA so effective is that wood treated with this preservative will literally last for decades — even in harsh outdoor exposures. Independent tests conducted by the United States Department of Agriculture (USDA) confirm this: wood stakes treated with Osmose CCA preservative remained resistant to termite and decay damage for over 60 years. That's an anticipated life of 5 to 10 time

 

I have a 1974 EBKO Executive Jet

How's your restoration coming? I think I need to replace my stringers as well. Was it a tough job?