Integral Diesel Tank

Why We BUILT AN ADDITIONAL DIESEL TANK

Whilst cruising the Med, Caribbean and the coasts of USA, fuel was always readily available and in the few areas of Alaska where the wind was non existent and fuel resupply unreliable we had our jerry cans as well as the boats 200 liter tank. As a long term solution for cruising to remote areas this was not a good solution. The only place we could store the cans was in the lazaret as far aft as it was possible to be and also well above the waterline, the worst possible place to be putting 200 kgs! With our planned trip to the South Pacific in mind, and Peter’s aversion to diesel prices in French Polynesia and the South Pacific in general, getting rid of the jerry cans and  also having enough fuel to get all the way from San Diego to New Zealand was attractive. Having removed the aft water tank, the opportunity was available to put in another tank under the port bunk. Making the tank integral meant the weight of the fuel could be moved both forward and lower. Instead of a tank sitting on top of the hull grid we would be using the considerable volume between the girders of the hull liner. 

Construction DETAILS

We wanted about 200 liters of fuel in this tank so the first job was to measure the space and determine the position of the tank sides. These would be made of 9mm plywood. Because we did not need to maximize the volume relative to the tanks footprint, it was possible for the flange for the tank top to be external, this greatly simplifies the glassing job. While the tank extends beyond the fore and aft girder on the out board side the curve of the hull at this point meant no appreciable gain in volume would have been made by hugely complicating the project to include it. Hence the step in the bottom of the tank.

Once the gelcoat on the liner had been ground off and the tank sides glued in place, a 40mm x 45mm lip was added to form the top flange. As part of the tank was going to be under our wet-weather and winter-clothes storage the top was made in two sections. The larger piece under the berth top would be completely removable but the narrower section would be permanently glued and glassed in place. This piece would also have an aluminium plate through which all the tanks fittings bar the filler pipe would be run. I milled this plate with an “O” ring groove and blind holes, threaded for machine screws so that it could be installed into the pre glassed top section before the top was glued in place. I make these pieces oval shaped so it is possible to remove them, although in this case the need to do so would be very unlikely since the entire tank can be accessed from the large section of the top.

All the corners, side to side, bottom to sides, and from the fore and aft girder to the plywood outboard bottom were glassed with 2 layers of 450gsm double bias cloth laid wet on wet. As I always do, this glass was wet out off the boat and sandwiched between sheets of waxed paper. Doing this means working at a nice bench, properly wetting out and squeegeeing the the resin to ensure complete saturation but using no more resin than required. Once on the boat all that needs to be done is to brush a thin coat of resin onto the areas to be glassed and then peel off one side of the paper, apply the glass, smooth it into place and then remove the paper from the other side. About as difficult as using double sided tape and not much messier. Once all the corners were done an extra coat of resin, slightly thickened with colloidal silica was brushed on. The point of this is to give enough material to be able to sand with a coarse grit to make a good bonding surface without removing any of the glass reinforcement.

For the glass to be applied without risking air bubbles behind it as it laid over the small radius turn between the sides and the top flange and also have a smooth flat surface on the flange, a simple “L” shaped mould was made and “dry” fitted to the tank flange. The corners were carefully miter cut to an exact fit. Next the glass for the sides and the bottom of the tank was cut and, again wet out on the bench, before being laid into the tank. The flange mould was then tacked in place with a few panel pins. These could be placed on the outer edges outside the area that would have the gasket. Lead weights held it down. This could also have been done with a  vacuum bag but would have been a difficult operation that would not have made much difference, if any, to the finish of the top flange. Furthermore, unlike any other structure for the boat, a higher resin-to-glass ratio is not a concern with tank building. In fact a higher ratio is better as the point of the glass is mainly to fuel proof the plywood. For this reason when the resin on the glass was cured to the point that when touched with a finger it felt slightly sticky, but the glass was immovable, several extra coats of resin were brushed over the entire tank. 

While the bunk above the tank is only used by the dog and even then not during ocean passages, (she sleeps huddled next to one of us) good baffles would both reduce noise and reduce the chance of sucking air, in rough water, if the level was low. These were completely fabricated off the boat and then glued and glassed into place. The top edges of the baffles also have a flange to allow more lid fastenings. Next, the final flange for the two sections of the top was glued in place with the narrow section of the top containing all the fittings. 

The top was cut and dry fitted before being sheathed with 450gsm glass, this was done on the vacuum table with the glass laid directly on the polyethylene surface and the ply on top. It was done like this as the most important thing was to have a smooth shiny unblemished finish that would be a perfect mating surface for the expanded PTFE Tape that would make the gasket.

After fitting the fuel gauge sender unit, skin fitting for the filler and the filling vent line which runs from the filler inside the fill hose and into the tank the lid was installed with the gasket. The tank was pressure tested with the dinghy pump to about 3 psi, The plywood and glass top bulged up about 3 millimeters between the fastenings on the sides and those through the baffles!! But no leaks! About 700lbs was pressing up on the lid.

I do tank venting with two hoses. One is open only when filling the tank. For small tanks like this the venting hose can be run from the highest point in the tank, then into the filler hose and then terminate right at lower part of the filler cap threads.It is then closed off when the filler is capped. On large tanks a separate large diameter hose is used to allow faster filling. This hose would have its own cap and be closed after filling. Another small vent is fitted to the tank but this one is only to allow air in. Instead of a cap it has a non return valve. As fuel is removed, air replaces it but changes in temperature can’t allow the tank to expel and ingest fresh, moisture laden air with every day night cycle. This is a serious problem in the tropics unless the tank is completely full. On my last boat I had 1400 liter tanks and after 10 years in the tropics had not a drop of water in them, nor did the water separator filters ever show any, despite one or other invariably being near empty and in any case only getting replenished every year or two when somewhere fuel was relatively inexpensive.