FPB Series

A New Paradigm for Cruising

Design Objectives

FPB 64

FPB 83

FPB 83 On Deck

FPB 83 Interior

FPB 83 Systems

Passages

How Things Are Working Out

Hull Insulation

Insulating the hull has been a special conundrum for the past 25 years. The normal approach is to use sprayed polyurethane foam. This is relatively fast and provides efficient insulation during the first few years of use.

But there are some problems. To begin with, the PU foam absorbs moisture over time. There is no way around this, short of a total vapor barrier, such as we use with the fridge and freezer. However, it is almost impossible to have a total vapor barrier, given all the structure, systems, and installation of pipework and wiring that takes place.

In our case, there is a special problem. Linda is allergic to mold and mildew. Over time, as the PU foam becomes damp, it turns into a mildew colony (and it loses its insulating properties).

On Beowulf we solved this problem by eliminating insulation. Careful attention to head and hull liners, and locker ventilation, kept her mold free. For cruising in the tropics, and temperate climates in summer, this worked fine. We also saved half a half ton of weight.

But for the new boat with higher latitudes on the horizon, we need a different approach. We've been working on this decision since the day we decided to go forward. Our search eventually led us to a special insulating material called "Armaflex".

This has been used for years in air conditioning ducts and other industrial applications - both inside and outside. A variant is also used in nuclear submarines. It has excellent acoustic properties, is good at insulation, is a very efficient reducer of condensation, acceptable flame spread ratings, and does not produce the toxic fumes associated with PU foams in a closed environment. Most important, it does not readily absorb moisture over time.

We went through a series of calculations using different thickness' of Armaflex throughout the boat, then adding in the heat load/loss from hatches, doors, windows, and our bodies.

Assuming an inside ambient temperature of 68 degrees F, and an outside temperature of 32F, 1/2" (12.6mm) thick material allows us to go to a humidity of 72% before we'd start to see condensation. From our previous experience in the colder climates, this should work fine. In terms of heat load for the Kabola boiler, this same situation works out to between 30/35,000 BTU - half the boiler's capacity. In the tropics, the aluminum hull, deck, and bulkheads add up to just 11,000 BTU with a 14F temperature delta (inside to outside) to which need to be added windows - which is within the capacity of the air conditioning system we are using.

In terms of insulation and noise, we have material under all of the cabin soles (on top of the tanks), hull sides, bulkheads, and all interior structural framing.

In our case, Linda's environmental needs drove the selection of this material. However, there are some other advantages to offset the extra material and labor costs (probably three times that of a sprayed PU system). The first of these is the noise attenuation previously mentioned. This material has wonderful sound attenuation properties. By using it throughout the boat - on all furniture and cabin sole surfaces (as well as the hull structure), interior noise and vibration is reduced to a minimum. That is one of the reasons the boat is so quiet (58db inthe saloon at 11 knots, and the genset, when it is running, is dead silent). Another advantage is if repairs need to be made. It is relatively easy to remove the Armaflex if we have to do any welding or other metal repair. And we can always easily get at any hull surface to inspect it.

The first step in the insulating process is to degrease and clean the aluminum surface (this is also critical with spray foaming).

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We ordered the Armaflex cut to specific widths to fit between frames, on frame webs and flanges (with a bit in wide form for bulkheads and under soles). There is an adhesive layer on one side, and a mylar film on the other. The installation process is labor intensive as already mentioned, but not prohibitive.

In order to make certain we do not develop pockets of condensation (which eventually lead to mold), every surface must be covered.

This includes all frames, intercoastals, and even pipework (see below).

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The completed job is very neat. Of course this is all hidden by head and hull liner panels, but it is nice to know that behind those panels is something that is pleasing to look at.

As mentioned, the bulkheads get a lining of Armaflex, as well as under the cabin soles. The bulkheads are more for noise than heat or condensation. This is looking at the forward end of the dressing room (a watertight bulkhead).

In the engine room (below) we are using 2" (50mm) material on the watertight bulkhead, and 1" (25mm) everywhere else. This is for sound attenuation. The norm would be to use an open cell material, with an integral barrier of lead or barium. However, because open cell material breeds mold, it is not an option for us. Given the very low horsepower requirements of this design at cruise speed, the type of engines we are using, and their soft mounting, this approach with the Armaflex keeps noise to almost undetectable level.

The photo above is looking aft, at the transom in the engine room. The vertical structures are the air intake down pipes before they were insulated. That "table" in the foreground is the structure onto which the rudder top-bearing carriers and hydraulic rams are bolted.