Vespera’s Stringers, Girders, Floor Timbers and Motor Mounts

Many boat manufactures imbed marine plywood in fiberglass laminate to form structural members. These structural members include the “grid” that surrounds the keel and supports the floor. The “grid “ includes semi circular shaped members that run port to starboard (floors or floor timbers) and rectangular shaped members running forward/aft (girders) that connect the sides of a square box surrounding the keel. Similar members are used to support the motor mounts. Imbedded plywood may also be used for stringers that run the length of the hull, above and below the waterline. Stringers run the same direction as girders but the girders are taller. In a raised salon, the girders and floor timbers support the floor as well as form the structural grid around the keel. Finally plywood may be imbedded in the chain plate supports and transom.

Much of Vespera’s structural plywood below the water line wicked water and rotted. If the plywood was below the waterline in the bilge (100% submerged or frequently submerged) and had even one small hole drilled into it, the wicking problem occurred. For Vespera this problem was curable because all of the structural members were easily accessed. The floor and furniture could be unbolted. The repair cost was 320 hours (2 solid man months) and about $1000 in materials and consumables.

When Vespera was built holes were drilled into the fiberglass/plywood structural members for plumbing, wiring, refrigeration, and drainage. The holes were sealed at the factory using a standard method of grinding out about a ¼ inch cylinder of wood and filling the void with epoxy. Why the seals failed can only be guessed. Ice is one probable cause since Vespera spent many seasons in Michigan. But ice alone does not explain all of the seal failure. Because fiberglass laminate and plywood expand and contract at different rates, Allan Vaitses ( an expert with marine wood and fiberglass) believes that fiberglass and plywood will eventually shear. If a plywood core is moving inside a fiberglass casing, the seals covering the holes will crack especially if the seals are made with resin alone having little structural strength. One small cracked seal can wick water 5 or 10 feet away from the crack. Here a rot factory is created. Once moisture gets into the wood the freeze/thaw cycle promotes more expansion/contraction and delamination occurs. This hastens the migration of water. Plywood, saturated in water with no air, makes an ideal environment for rot.

How do you determine which structural members are rotted? The old fashioned way is to sound them out with a mallet. Solid members ring like a bell. Rotted members will sound with a thud. But this method will not find wet, un-rotten, members unless you are very skilled. Use a good moisture meter and you will find the problem areas immediately. The moisture meter never lied on the extensive work that was done on Vespera.

How do your cure the problem? The traditional cure is gruesome. You slice out old structural members and rebuild them using marine plywood as they were made originally. I contacted Vespera’s original designer who recommended this cure. Also the boat yards that I contacted recommended the same. Two big questions concerned me. First, cutting out the original structural members was very destructive. The laminate covering the structural members was sound and the lay-up was an integral part of the hull. New structural members simply cannot be bonded to the hull as well as the original? But more important, if I put in new plywood cores back into the boat, what would stop the wicking problem from occurring again? The boat yard answer that I was given was most troubling. The problem probably will occur again about every 15 years. Think of it as a real nasty oil change. It’s just what you should expect from a wet, flat bottomed boat with plywood structural cores.

What I desired was a permanent repair that would last the life of the boat. The solution was to find a core material that lives in water (reinforced concrete) instead of a core material that dies in water (plywood). And instead of removing the structural members, the wooden cores would be ground out of the existing fiberglass structure and the original fiberglass laminate would be left alone. Reinforced concrete would be poured to replace the wooden core. Concrete not only survives in water it thrives in water! Concrete is used to build billion dollar bridges that stand up in salt water for decades.

Water friendly concrete mixtures are clearly described in manuals for building ferro-concrete hulls. Most of these books were written in the 70’s and are out of print but are available from a large library. The basic recipe is simple. Use as little water as possible when mixing the concrete and make sure that the concrete cures very slowly. The basic recipe, % by weight: 12.5%water. 35% concrete, 17.5% sand and 35% ¼ inch (buck shot) rocks. For reinforcement use 3/8 inch epoxy coated rebar available at commercial contractor supplies (not Home Depot). The details of working with concrete are described in the ferro-concrete boat building manuals.

There are some unique problems in attempting to marry fiberglass laminate to concrete. Fiberglass and the concrete expand and contract at different rates so they cannot be bonded. The solution is to not attempt a bond. Instead use mechanical fasteners such as hot dipped galvanized nuts and bolts. (This is the same principal that Alan Vaitses used to marry wood to fiberglass. He used mechanical fasteners and purposely avoided a bond between the two dissimilar materials.) Be sure to put the fasteners in before the concrete is poured. The fasteners can do double duty. Before the concrete sets up the fasteners can be used to support the rebar. Another problem with concrete is that it is much stronger than the hull and could over power the laminate when it expands. The solution here is to bed the hull/concrete joint with ¼ inch of polyurethane caulk. Use Vulkem 116 available from Home Depot for $3.95 per 10 oz cartridge. It is designed for use with concrete. The caulk between the concrete core and the hull well not only work as an expansion joint it will prevent water from getting into the joint. The water itself would not be as harmful as the ice that would result from winter storage.

Replacing a plywood core with concrete is not a quick cure! It is a permanent cure and, though very time consuming, a cure can be made by a boat owner for a small cost. It would be much quicker to cut out the original structure and replace it but this cure would be temporary and require a lot more skill and money. Also replacing the structural members by rebuilding them with wood would actually be inferior to original structure because the new laminate would be a secondary bond. We want a cure that is a step up not a step backwards.

It is very time consuming to remove wooded cores without damaging the laminate cover or hull. Though it takes only a few minutes to use a Sawsall to slice off the top edge of the laminate that covers a structural member, removing the core is a slow job. Where the wood is rotted you can remove it with a spoon, that’s easy. But where the wet wood is sound or ½ rotten you will have a fight. Also there may be big obstacles that make your work awkward. The wooden cores beneath Vespera’s motor mounts were bad. I had to remove the cores with the 85 HP Perkins in place. And the structural members can be deep. The deeper they are the harder it is to dig out the wood. Vespera had six structural members that were from 12-17 inches top to bottom, 2.5 inches wide and 8-10 feet long. To remove the wooden cores I used a rented ¾ inch Milwaukee drill motor (great machine), several 16 inch augers, a chain saw and a homebuilt chisel made from a 2 foot tool steel bar.

Before pouring the concrete prepare the hollowed out cavities carefully. Cut the epoxy coated rebar and seal the ends with polyurethane caulk. Position the rebar 4 inches apart by hanging the rebar on the bolts that will mechanically attach the sides of the fiberglass laminate to the concrete core. 24 hours before pouring the concrete apply ¼ inch of polyurethane caulk to the hull where it will meet the concrete. When pouring the concrete vibrate the structure. Remove the blade from your Sawsall and it will make a great vibrator.

If several adjacent structural members need to be gutted, fix 1 member at a time and wait 3 weeks before gutting the next member. This will give a structural member 3 weeks to cure so that it can support weight before you go on to the next member.

What about the additional weight from the concrete. The concrete cores put into Vespera added about 1000 pounds of weight to a 33,000 pound boat. This is 1000 lbs of weight added in the bilge. Considering that Vespera carries up to 2400 lbs in liquids, a 1000 lb addition in the bilge is small.

After giving your concrete cores a month to cure wrap them with a mallet and she well ring like a bell!


Stan Sroga



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