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A large part of my business for the past 20 years has been the total replacement of bottoms of runabouts . When doing the job I like to do what in my opinion is best for the boat and what’s best for owner . I thought that I’d like to share a few of my techniques here to help inform the boat owner contemplating a new bottom , and give some pointers to the do it your self’er . I would like to add a disclaimer that a little bit of knowledge can be a dangerous thing , and what I am sharing here is only a brief overview .  Over the years I’ve noticed that there is some controversy about what the best way to replace a bottom in a boat . There is the “Epoxy” debate , but people’s understanding of what an epoxy bottom is can range from having every piece of wood replaced and coated in Epoxy as it is being replaced , to just slathering on some magical goo to the existing rotten wood . Another concern that I encounter is that the wood will rot from the inside out because it cannot “breathe” .  The whole premise of the “Epoxy bottom is that since wood moves dimensionally with changes in moisture content ,  that if it is sealed from moisture it will remain inert and not move . The concept is that if the wood is dimensionally stable it can be bonded with a relatively rigid adhesive like the epoxy itself.  This premise holds true until somehow somewhere  moisture is given a chance to get into the first piece of wood  . This can be an abrasion , or perhaps somebody screwing a bilge pump or some other  fitting into that perfectly sealed piece of wood , somewhere down the road after the boat has left the shop . Once this happens the water molecules will find their way along those screw threads , abrasion , or any other spot that the coating has been compromised and very slowly allow moisture to get into the wood . Depending on the size of the piece of wood it can swell with a lot of force . The bigger the piece of wood the more force that it will exert when it moves .  If a piece of wood is large enough , when that piece of wood moves it will fracture the rigid epoxy coating and allow a bit more moisture into the wood . Epoxy is a very strong material , but it is not strong enough to overcome the forces created by the  expansion of larger pieces of wood .  Therefore Epoxy is best suited to laminations of smaller pieces of wood and laminations .  Epoxy does not work well for bonding Thick pieces of wood that are going to be used under the water . If you bond a few chunks of 2 inch oak for a deadwood or other underwater member it is probably going to have problems .

            On the other hand the common conception is  that a traditional bottom is going to leak every spring and will require a great deal of maintenance and can not be kept on a boat lift out of the water and beyond the thrashing of the savage wake from those wakeboard people . This is because over the years as wood planking  is allowed to expand and tighten up , the bottom the wood becomes compressed and when allowed to dry out is smaller than the size when the plank was new . The degree of this change depends a lot on the grain orientation of the plank stock selected when the boat was planked. Quarter sawn or vertical grain material is far superior to flat sawn stock when it comes to dimensional stability . Quarter sawn or vertical  grain lumber carries a premium , and most lumber yards these days do not know what Quarter saw lumber is.   I use vertical grain lumber whenever possible , but in order to get it I have to hand select all of my lumber at the lumberyard. One of the innovations  that wooden boat builders came up with in the thirties was the double planked bottom . This consisted of a framework planked diagonally with a thin layer ( ¼ “ -3/8” ) thick followed by a layer of canvas soaked in paint followed by a thicker layer (1/2″ –  5/8 ” )of fore and aft traditional planking . The advantage of this method  of construction was that when the boat was new the canvas membrane would prevent the boat from leaking until the planking swelled tight  . The canvas was usually bedded in paint , but over a few years the canvas was just as leaky as the planking after the boat dried out , and really serves little purpose other than to hold water between the plank layers . The old greavette guys I used to work with called it the “linger wet ”  The other shortfall of the canvas and paint was that it did not take advantage of the extra strength that could have been realized if the two layers had been bonded together . Imagine that stiff piece of plywood with no glue between the veneers , It would be quite flexible ,like a deck of cards ,  but once those layers are bonded together it has a great deal of strength .

            The one advantage of working in restoration and repair of wooden boats for the past twenty five years is that I’ve had a chance to see what holds up , what works and what doesn’t .  When replacing a hard chine bottom I use a method that has held up very well over the years .  When I put a new bottom on a hard chine boat , such as a Chris Craft or a Greavette  the method that I use is one that I refer to as a “5200 “ bottom . This method is not suited to a displacement launch , or round bottom boat but works very well on a hard chined boat .  So what is a 5200 bottom  ?  A5200 bottom is essentially a double planked bottom that uses a product called 3M 5200 between the layers to form a tough flexible membrane between the two layers . The product has a very tenacious bond that is stronger than the wood , while at the same time being slightly flexible to allow some movement without breaking the bond between the wood and the 5200.  With this method there are two layers of planking with alternating grain orientations , bonded to minimize movement . The boat can live on a trailer , or boat lift without having to worry about it drying out . The construction is essentially the same as used by the original builder , but takes advantage of a superior product to the canvas skin that was not available when the boat was new .

With epoxy there is very little give in the bond . Once the limit is passed the bond simply fails  5200 has some give and flex .When using any adhesive proper application is important .  Too many times I’ve taken something apart to discover the “squiggle ” method of adhesive application was used . You know who you are . I’ve  even see Mike Holmes do it .The adhesive is squiggled on and the two pieces are stuck together without spreading out the adhesive onto a uniform film . The result is a few spots that are stuck together and a lot of voids that fill up with water and eventual decay .  When spreading adhesive I like to use a notched spreader to ensure that there is 100 percent contact to the adhesive whenever two pieces of wood are joined .

     Another important detail often overlooked is to make sure that there are no sharp corners on any framework on the inside of the structural work . It may seem like an unnecessary job , but I feel that it is important to chamfer any sharp edges , even we’re they will not be seen . Wherever there is a sharp edge is the first part that a Finnish will break down , and I consider a good Finnish on every piece of wood to be an important component of the integrity of the job . For finishing a bilge I like to use a product called S1 Made by industrial formulators  . It is a fairly thin two part Epoxy sealer that penetrates the wood much better than any of the more common gluing resins that are often used and is much better at sealing wood from moisture penetration that conventional paints or varnishes . Another similar penetrating epoxy is Smiths , sold primarily in the United states . The penetrating epoxy is used as an undercoat , and when top coated with an enamel looks just like the boat did when originally built . The primary difference between a penetration epoxy sealer and the regular epoxy used primarily for bonding is that the viscosity of the epoxy sealer is much closer to that of water , while a more common epoxy resin is closer to corn syrup in thickness . A sealing epoxy is quite thin and will take usually three coats before it stops soaking into the wood . A laminating or bonding epoxy is quite thick and doesn’t penetrate that far into the wood . 

There are several other considerations to be taken into account when replacing a bottom , far too many to cover here . But one last one that I’d like to add is that no matter how good or magical any coating is it will not turn a species of wood with no decay resistance into a decay resistant species . I also avoid the use of sap wood . The sapwood is the whitish layer closest to the outside of the tree . Sap wood in a boat can start to rot in a very short period of time .