Cockpit Deck and Central Hull Cavity Overhaul

Uncovering a Fifteen-year-old Boston Whaler Boat's Inner Secrets

This article shows in detail many of the features of the central hull cavity and the laminated deck assembly that covers it that are common on most classic Boston Whaler boats. This construction is seen on Outrage, Guardian, and Revenge models, particularly boats in the 18 to 25 foot range made after c.1980. The central hull cavity contains the fuel tank which has been foamed into place. This area is covered by a large molded laminate deck which has been reinforced with plywood. On older boats this structure can become weak due to rotting of the plywood, causing the deck to flex under load. Repair of this deck is a laborious process.

The design of the central fuel tank cavity and the use of foam to hold the aluminum or polyethylene fuel tank in place have been two topics of much discussion. This article provides a close up look at how the tank was installed in a typical Boston Whaler boat, and how the various components survived over many years of use.

As far as is known, there are no drains at the bottom of the central hull cavity, and this has raised some concern about the potential for water to become entrapped there. At initial construction, the fuel tank is fitted to the central cavity with fairly close tolerances, leaving not much room for accumulation of water. The tank is then foamed in place, and the foam provides a water-tight seal. Any water that comes to lie in the cavity is kept on the tank top and surrounding foam. However, it is possible that over time the foam can lose its bond to either the tank or the gel coat surface of the cavity, and this may afford the opportunity for water to sink to the bottom of the cavity. This is undesireable for a number of reasons, including the potential to corrode the fuel tank. Water sitting on the tank top will only be removed by evaporation.

It has also been seen that the foam can absorb and hold water, despite its "closed-cell" nature. Experience has shown that the foam used for this purpose in Boston Whaler boats can degrade over time and may hold water in its cells. This article sheds some light on that topic as well.

The boat shown here is a 1988 GUARDIAN 20 owned by Jason Richer. He acquired it after it had served a long stint in commerical/government use in the Florida Everglades National Park. In 2003 he undertook the removal of the large and heavy central cockpit laminated cover and inspected the fuel tank and other equipment that lay beneath. He kindly took a series of excellent photographs of his work, which are presented below. His comments and experiences have thus afforded this wonderful opportunity to see what is lurking below the deck of a classic Boston Whaler boat fifteen years after it left the factory.

Jason writes:

"Thought you may find this interesting. This is what I encountered when I lifted the deck of a 1988 Guardian 20'.

"In the event of a failure of the caulk in the floor seam, water can funnel into the gas tank cavity. At least that is what appears to be the case at the moment. I'll know more as the foam is removed.

"The (aluminum) tank appears to be very solid. I see no external corrosion, even where the foam is against the tank. I removed some foam for inspection purposes. It looks very clean so far. It would not appear that the foam encapsulation had a detrimental effect on the aluminum.

"By the way, all lines to and from the gas tank had reached the end of their serviceable life. I found them all to be very dry and cracked. Fifteen years seems to be the life span, even if they are not leaking to the point it is noticeable.

"I would bet most people with this age hull are pushing their luck not replacing them. Also, the hose connections were all loose. The hoses had hardened and the hose clamps were all loose with the exception of the line to the motor (from the fuel filter).

"I have already found a peculiar drain tube from the tank area that in my opinion is detrimental to the integrity of the compartment. (I include a close-up.) It is a white tube passing through the hull and is angled down toward the tank."

Central Fuel Tank Cavity Details


Photo: Hull with center deck removed from cockpit
Cockpit Center Deck
The large laminated deck cover has been removed from the cockpit to reveal the hull central cavity and its foamed-in-place fuel tank. The circular foam area in the middle is the sprue hole used during the molding and foaming process. Liquid is introduced through this hole and quickly expands into foam. By some artifice in their construction, Boston Whaler maintains the foam at a considerable pressure as the hull, liner, and foam all simultaneously cure together, forming the Unibond hull construction that is the fundamental essence of the Whaler brand. This causes the foam density to be higher than foam allowed to free-rise during the expansion/cure process. The foam surrounding the tank has a lower density than the foam which forms the hull structure.

Photo Credit: Jason Richer


Photo: Hull with center deck removed from cockpit
Stern View of Deck
On Outrage/Guardian models like this one, the deck cover extends all the way forward to the bow center locker. On Revenge models the deck stops at the cross member forward of the fuel tank. Fuel lines are still intact in this view.

Photo Credit: Jason Richer


Photo: Hull with center deck removed from cockpit
Fuel Tank Details
At the rear of the tank dual fuel lines attach to fittings on a removable cover plate on the tank. The green 10-AWG electrical cable bonds all metallic elements of the fuel system. The old fuel lines have been cut away, and some test borings made into the foam surrounding the tank. Forward of the fuel lines the metal cross pieces hold the tank in place. They are secured to the hull by clamps which are fastened by screws which sink into embedded backing material in the hull.

Photo Credit: Jason Richer


Photo: Hull with center deck removed from cockpit
Fuel Line Pass Through
The starboard cockpit drain sump is at the top in this view, its overboard drain through the hull bottom just visible. Two holes with brass grommets lead the fuel lines and bonding conductor through the hull bulwark on the starboard side. If the cockpit is swamped and the sump allowed to fill to the brim, water could seep past the fuel lines and enter the central cavity via these holes. Test borings in the foam surround were made to check for entrapped water at the bottom of the cavity. The foam was substantially dry at the surface but a small amount of water was found (in the form of ice!) to have accumulated at the bottom of the holes. Read more about this in a Forum thread that discusses this particular situation.

Photo Credit: Jason Richer


Photo: Hull with center deck removed from cockpit
Fuel Line Fill and Vent Hoses
At the forward end of the fuel tank the large fuel filler hose and smaller tank vent line attach. The T-shaped channel molded into the hull is interesting. Its precise purpose has not been deduced. Also notice the small white drain linking the central cavity with this port side channel.

Photo Credit: Jason Richer


Photo: Hull with center deck removed from cockpit
This small white drain apparently allows water in the outboard hull channel to drain into the central fuel tank cavity. Some small plywood remnants of the cover are in view. Notice the condition of the fuel filler hose at the outboard end; it shows signs of significant deterioration

Photo Credit: Jason Richer

Views of the underlying hull structures have also been seen in a recent article in Cetacea. It is interesting to compare the two views. In the Guardian model seen here, the port side of the central cavity seems to have lost a molded channel running fore and aft that was seen in an earlier Revenge hull. Compare this boat to the one seen in the photographs on Cetacea Page 70.

Repairing the Deck

Repairing the deck and replacing the rotted plywood proved to be a laborious undertaking. As Jason was in the process of making this repair, he again took excellent photographs, as well as mailing these updates:

"I have approximately five hours of scraping into the project. And it is safe to say that there is at least another six hours to go. The remaining half is in the same condition as the previous half, maybe a bit better. Maybe 8 hours... It depends upon the level of delaminating. Solid areas are like cement. Plywood is not an easy thing to separate. Especially when it has been impregnated with fiberglass resin. Each piece comes off with nothing short of extreme patience and determination. One square inch at a time no more than 1/8-inch thick is what gets removed with each pass of the chisel. With 3/4-inch of total thickness to remove, It's slooooooow!

"Well, I have reached the half way point. Close enough anyway. Far enough along that I can tell you it is not a job I would care to repeat. Unless, I am being paid handsomely that is...

"Yee-haaa! Can't tell you how ecstatic I am that this is done. In a nick of time too. It is supposed to rain for 3-4 days. I am hoping to get it indoors to do the epoxy work.

"Cut the pieces to fit this morning. What look like larger than necessary holes actually (conform to) the original plywood's cut-out's. I managed to obtain some real nice sign material--3/8-inch craft faced plywood--guaranteed for 5-year exposure. As it turned out, two layers of 3/8-inch is what Whaler had used originally. The pieces I am using are 2-foot by 4-foot. It looks like one pint of epoxy per (2 X 4) sheet will work out very well. It is coming out rather well."


Photo: Deck on saw horses in backyard.
Backyard Project
The deck cover was inverted and placed on saw horses in the backyard. Removal of the rotted plywood backing has begun. Jason's comments convey the amount of labor involved and the slow pace of progress. The plywood was laminated to the fiberglass molded deck using polyester resins in the original construction.

Photo Credit: Jason Richer


Photo: Close up view of plywood and a section where removal has been completed.
Removing the Plywood Backing
Using a hammer and chisel, the original plywood backing of the center deck cover is carefully removed an inch at a time.

Photo Credit: Jason Richer


Photo: Close up view of plywood and a section where removal has been completed.
Wading Into the Project
This is where Lady Macbeth's would say:
"I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er..."

Photo Credit: Jason Richer


Photo: Tools used.
Tools of the Trade
In time, the obstacles are overcome. Special care is needed around the several molded openings in the deck where hatch access plates are installed.

Photo Credit: Jason Richer


Photo: New plywood epoxied to molded deck.
New Plywood Backing
Using 2 X 4 foot pieces, the deck backing is replaced in segments.

Photo Credit: Jason Richer


Photo: New plywood epoxied to molded deck.
Epoxied In Place
Bonding the new backing to the original laminate is done with the encouragement of gravity and additional weights, and the excellent adhesion of WEST System epoxy.

Photo Credit: Jason Richer


Photo: Finished deck with new backing plywood installed.
Almost Done
The first layer of the deck plywood is now in place. The second 3/8-th-inch layer will be added with the seams oriented perpendicular to the first. The gaps between the plywood sections will thus be overlapped. The entire assembly will be given a top coat of epoxy to ensure water cannot penetrate into the wood again.

Photo Credit: Jason Richer

Final Installation

The ultimate re-installation of the repaired deck will have to wait for spring weather. At that time, and with the continued cooperation and fine work of Jason Richer, we'll expand this article to show the refurbished tank, new fuel lines, and repaired deck on his fine GUARDIAN 20.


Many thanks to Jason Richer for undertaking this project, for having the ability to take excellent photographs as he worked, and for graciously sharing them with his fellow classic Boston Whaler owners via this article.

Comments or Questions

If you have a question about this article, please post it to the linked message thread in the REPAIRS/MODS forum.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Portions Copyright © 2003 by James W. Hebert. Unauthorized reproduction prohibited!

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Author:James W. Hebert from photographs and notes supplied by Jason Richer.
This article first appeared January 24, 2004.