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BOAT INTERIORS - READ THIS FIRST:
While the following was written for the benefit of those building metal boats,  much of the content applies to boats built of other materials.
Insulation. Lining materials. Cabin soles. Building bunks. Head and shower compartments. The galley. The galley stove (gas, diesel, alcohol, paraffin). Heating. Stowage for food and stores. Hot  and pressure water. Water tankage. Comfortable seating; measurements and suggestions. Chart table. Sail stowage. Refrigerators and ice boxes. Ventilation.

FOAM INSULATION:   Now is the time to consider how you are going to select and install the insulation that is so important in any boat. Even fiberglass boats need insulation as they 'sweat' in the same manner as other boats, including metal ones.

   If you are planning spray-in-place foam insulation then this should be installed after the deck and superstructure are in place, but before you start work in the interior joinery. An alternative is to install 'bats' or sheets of foam glued or held in place by the 'ceiling'. See below in the section covering lining materials for details on installing the ceiling planking.

   My choice for foam insulation is the type of urethane foam that has fire resistant and non-toxic properties and is sprayed in place. In steel boats, the interior of the hull should be first grit-blasted and primed, or built of pre-primed materials. The foam will now be sprayed to a depth between 1" / 25 mm and 2" / 50 mm; the thicker the better. It is a good idea to cover the stringers and they will most likely be about 1 1/4" to 2" / 30 to 50 mm deep. The foam will form a skin and should be of a type that is forms a skin on the surface that is impervious to water. Make sure you choose a variety that in the event of fire does not give off toxic fumes. You will also need to make sure that the foam is of the self-extinguishing type.

   The supplier of the foam usually has all the equipment needed to install the material and charges by the cubic foot / cubic metre or by the pound / kilo. Make sure you obtain an estimate of the cost for the boat to be sprayed to the desired thickness. Be very careful that you get what you pay for. Measure the depth of the foam and check that it is reasonably constant thickness throughout the boat.

LINING MATERIALS:   There are a variety of materials that can be used to line the interior of your metal boat. This is one area where the wrong choice can damn your boat and scream 'AMATEUR'. If you want to achieve a decent resale value, then you had best get it right.

   If you are building a 'traditional' style metal boat such as a Spray replica, or if you like the warmth of an all timber interior, then ceiling planking can go a long way to achieving the right effect. Ceiling material should be 1" x 5/8" / 25 x 20 mm light coloured fine grained timber that is rounded or arrased (bevelled) on the outer edges. Install the planks longitudinally and space them at about 1/4" / 6 mm apart. You can plank only those areas that are visible after the joinery is complete, or you can plank the entire accommodation area and use the material as the lining for the various lockers. If you take this latter course then you had better reduce the spacing to say 1/8" / 3 mm so you will not lose small items though the gaps in the planking. If you only use ceiling where there are no lockers then you may have a problem getting a fair curve where there is no frame to attach one end of the planks.

   The lockers can be lined with plywood.  Leave the bottoms loose with a neat fit but not tight; you will often want to get under and behind the locker to have access to cables and the inner hull. Do not forget to put finger holes in the lining.

   Lining the entire interior of the accommodation with light ,plain, painted or veneered fancy plywood can produce a pleasing result when combined with solid timber trim. Too much varnish or areas of paintwork that are too large, can spoil the interior appearance, so you will need a balance between the two finishes. If you are unsure of this balance, then use as much varnish as you like, you can always paint over it at a later stage. The reverse is obviously not so simple.

   For deck-heads and the interiors of the cabin top you can use plywood that is painted. Use some timber trim to relieve and break up the area. Timber planks, similar to that used for 'ceiling' but say 2 in / 50 mm wide can also be used to line deck- heads and cabin tops. There are several alternative vinyl type products that have a light foam backing and are ideally suited to deck-heads and certain bulkheads. Be careful using this material for bulkheads in the main living areas as it can cheapen the look of your interior. The foam-backed vinyl can look acceptable in some parts of sleeping cabins. The use of vinyl is fine for deck head and cabin top linings in any part of the boat. Use timber trim as needed to break up any large expanses of lining material. 

   In the past, some builders, both professional and amateur, used carpet and carpet like materials for lining the hull and deck heads. These materials are inexpensive and easy to install but can look cheap! We do not recommend this route, besides the interior would look 'dated'.

CABIN SOLES:  The sole framing will need considerable planning. Usually L- angle is used for the main framing and some timber can be used where it would be attached to vertical plywood surfaces, such as bunk fronts and dinette ends. The size of the angle will vary depending on the span and spacing of the framing. Your plans should give you a guide. As the builder sometimes changes accommodation layouts, you may have to use your judgement. There are few things more annoying than a springy or squeaky sole.

   Plywood is universally accepted as the material to use for this area. The thickness will vary between 1/2" / 12 mm and 3/4" / 20 mm, depending on the spacing of the under sole framing. Usually 5/8" / 20 mm is sufficient and the thickness can be less if you plan to add teak or another timber surface. This is such an important area that you should try a sample area, before deciding on an overall thickness. A spongy sole is most undesirable, but you will not want to add excessive or unnecessary weight.

   While you are still constructing the interior, you should only fit and lay the plywood in position, do not screw it down until the all of the joinery work is completed. It is advisable to arrange the sole so you can remove all parts of it including those areas that form the bottom of lockers and closets. This may mean additional under-sole framing to provide a base at the edge of the particular area of the sole. Under no account 'build in' areas of the sole so they are impossible to remove. The plywood can be screwed down using self-tapping stainless steel screws.

   Make sure you have carefully planned hatches to those areas of the sole  which will need frequent or even infrequent access. The hatches should be laid out in an orderly manner and have aluminum or similar trim around the edges and be provided with flush 'ring-pulls'. Hatches in unseen areas such as inside lockers may have finger holes in lieu of more expensive hardware. Where carpet is installed, aluminum edge trim is an important feature around the sole hatches.

   Carpet covered soles are fine in most powerboats and also in well-maintained and dry sailboats. If your cruising habits mean that you will be bringing a considerable amount of water into the cabin, then carpet is not a sensible option. I personally like carpet, it offers a good footing under most conditions, is warm and attractive and considering the small area of a boat interior, it can be replaced at very little cost.

  Do not finish a teak and holly sole to a high gloss.  Rather use a mat finish that will give you some chance of remaining upright in adverse conditions. A slippery surface can be deadly in anything of a seaway.

BUILDING BUNKS:   Your first consideration will be how many permanent berths to build into your interior. Less is better! My idea of the perfect number of berths is a spacious double for use in port and in suitable weather, two single berths suitable for watch keeping and for use in adverse weather conditions, plus additional single berths for the permanent crew. The number of berths should not exceed four plus those required for the permanent crew. Boats under say 38 ft / 11.60 M should not have more than one double and two single berths otherwise too much of the interior space is used up in this manner.

   All boats are a compromise. You may have to be creative to provide the necessary sleeping accommodation, without turning the boat into one large dormitory. Avoid berths that are too narrow. Singles should be a minimum of 2 ft / 610 mm wide and preferably just a few inches / centimetres wider. Doubles should not bear that title unless they have a minimum width of 4 ft / 1.22 M and preferably wider. You will find 4 ft 6 in / 1.37 M is ideal and up to 5 ft / 1.52 M width is fine in a larger boat. For designers and builders, achieving adequate berth length, is always a problem. It is a fact that as each generation becomes taller, they require longer berths for a good nights sleep. In new designs we use 6 ft 6 in / 1.98 M, as a reasonable length. It is hard to include longer berths than this, without encroaching into other areas of the accommodation.

   Berths can be framed up in 2" x 1 1/2" / 50 x 30 mm timber or 2" x 1" x 1/4" L- angle. Depending on which metal you use to build your hull, framing in the same material may give satisfactory results. In steel boats and in the interests of saving weight, you may prefer to use timber framing throughout the interior. Plywood of 5/8" / 20 mm thickness will be adequate for all berths and if you have adequate framing then 1/2" / 12 mm may be sufficient. Berths should have hatches in the top to allow access for stowage and inspection of the boxed in areas of the hull. The plywood tops of the berth should also have a few 1 in / 25 mm diameter holes bored at random to allow the air to circulate in the area under the mattress.

   Face the berths with a timber board of around 6 in x 1 in / 200 x 25 mm and round off the top and bottom of this face plank to remove any sharp corners. This facing will hold the mattress in position and give a finished look to the berth.

   The mattresses should be of good quality foam, between 4 in and 6 in / 100 to 200 mm in thickness and covered with a light cotton or other suitable fabric. Hooray for the Duvet! Duvets make the best bedding arrangement; they are easy to make up, especially where a berth does not have access from all sides. A fitted under-sheet combined with a duvet that is equipped with a slip on cover makes for perfect sleeping. This arrangement also provides easy bed making in the morning. Duvets, when they are used on settee berths, have the additional advantage of being easy to stow.

HEAD AND SHOWER COMPARTMENTS:  If space permits, a separate shower compartment is very desirable, especially on any boat intended to be used as a live-aboard. The shower can be totally separate with its own entrance, or simply a shower stall entered through the main head. On boats where the regular crew is four, I would prefer two medium sized heads and one separate shower compartment. Choices in this area are a personal matter so you should discuss the options with your partner and those family members, who will be crewing regularly on the boat.

   The floor of the shower/head can be formed from fiberglass and laminated as a one-piece unit. Do not forget to include a non-skid surface. The actual shower pan can be slightly lowered and fitted with teak slats. This arrangement may allow a wider sole, even if some areas include a slight slope. Separate shower stalls can have a tiled sole, a nice touch if your boat can accommodate the additional weight. The walls or bulkheads in the shower/head area should all be lined with a plastic finish such as Laminex (tm) or Formica (tm). The entire floor area as well as the bulkheads and cabinets, should be designed for easy cleaning.

   If you are building or own a small or minimalist boat, then you may be happy with a 'shower bag'. These plastic bag devices are wonderfully efficient. Provided you have a reasonable amount of sunshine each day, one bag can provide hot showers for two. Shower bags are great water savers and recommended on vessels where replenishment of water tanks is infrequent. Shower bags can be a great back up device and even if used only infrequently, they will pay handsomely for the small amount of stowage space they require.

   Drainage for the shower can take many forms. In our sailboat the 28 ft / 8.53 M steel Spray K*I*S*S, the builder had fitted a fiberglass shower tray in the head and drained this into the main sump in the keel. The sump was emptied by way of a manual bilge pump located in the cockpit. This pump also served as the emergency bilge pump. This arrangement was fine except that you had to pump out the water soon after your shower, otherwise the sump would generate sufficient gas to set off the alarm intended to service the gas stove rather than the shower sump. A better arrangement would have been to have a separate 'grey water' tank to temporally store the shower water, or an automatic pump. In our present boat we have a plastic sump about 1ft / 305 mm square, fitted with a lid and an automatic Rule shower/bilge pump. The 800 gallon / 3,028 Litre per hour unit, is the same size as those of smaller capacity and this unit provides a backup to the regular bilge pumping arrangements. Shower pumps need regular cleaning, usually about once every week or more often if the facility is used by more than two persons.

   Manual or vacuum toilets are a must. The electric varieties, while they do reduce the solids to a fine mist, are incredibly noisy. Unless you can find a quiet version do not consider these devices. To those traditionalists who wonder what is this fellow doing talking about electric toilets; please remember that not all metal boats are built as basic cruisers. Even some of the smaller variety, are built as finely fitted yachts. The 'Lavac' toilet is one of several proven units and is available as a manual, (recommended) or more recently as an electric unit  (unknown quantity). Make sure, especially in a sailboat, that the plumbing for your toilet is such that it cannot back- siphon and sink the ship.

   You should fit a reasonably sized hand basin. If fresh water is at a premium then the smaller variety saves water. About 9 in / 228 mm diameter is the smallest that can be called a basin. On smaller boats and boats with medium -sized head/shower combinations, a convenient arrangement is to have a water -saving type shower rose, serve as both the tap for the basin and as a shower head. The flexible supply hose leads through the counter top and self stows under the basin. The rose is drawn out when required as a shower. When using this system, and if hot water is available then a two-tap mixer can serve both shower and basin. On all boats, the basin or sink should have a shut off valve or sea- cock fitted and its location should be familiar to all crewmembers. This advice applies to all inlets, outlets and sea-cocks.

THE GALLEY: This is an important area of your boat and if you want to keep the cook happy, then you had better get it right! In my books 'Choosing For Cruising' and 'Choosing a Cruising Powerboat (see appendix 1), I have covered the subject of designing a galley in considerable detail. These books have much more information on all aspects of design than space allows here.

   You can arrange the galley benches to suit the available space. It has been found that a 'U' or 'L' shaped arrangement usually works the best. In a sailboat, the stove is best placed outboard, facing inboard. For easiest drainage, the sink on a sailboat should be as close to the centre line as practical. Galley benches can be built from 3/4" / 20 mm plywood and covered with Formica, Laminex or other laminated plastic. If weight saving is not critical then tiled bench tops add a nice touch to any galley. Make sure that you round off any corners, otherwise the cook will soon be covered in bruises.

   Framing for the galley can be 2 in x 1 1/2" timber. The framework will generally be arranged to accommodate the size doors and drawers that are available from the available standard sizes. Unless you are fitting out a Dutch barge or other vessel that is mainly intended for use in inland waters, you are not advised to install the standard kitchen units that are available at the local DIY store or lumber yard.

   There are many suppliers of ready-made teak and mahogany door and drawer fronts that you can incorporate into your galley and elsewhere. These items may be more expensive than those you construct yourself, but unless you can produce fine cabinet and joinery work ,you are advised to at least investigate this option. Resale value of your boat will be considerably affected by the quality of the interior finish of your vessel. Three-ply sliding galley doors with finger holes as openers may be inexpensive and easy to construct but they will add little to the resale value of your boat.

THE GALLEY STOVE:   Generally known in Europe as a 'Cooker' and elsewhere as a 'Stove' this appliance will get considerable use in your boat. First you must decide which fuel you will use. The choices include bottled petroleum gas, bottled natural gas (only available in certain areas), diesel fuel oil, alcohol or kerosene (paraffin). In times past, galley stoves were always 'gimballed' and today most, when installed in sailboats, are still arranged in that manner. Powerboats and stiff sailboats like the ‘Spray types’ may not require gimballed stoves. Discuss this option with the manufacturer of your chosen appliance.

   Today, many if not most, new sail and powerboats are fitted with bottled gas that is commonly known as LPG (liquefied petroleum gas). This gas is available in two main types called, butane and propane. The two types are similar in usage, and each has its advocates. If you choose gas, then you will probably choose the one that is more readily available in your area. Most appliances will burn either type; some may need minor adjustments to the burners to get the best results. If you choose gas as a cooking and/or heating fuel, you must have a certified technician install the system. Also check it over on a regular basis. LPG is a wonderful aid on any boat, but it is heavier than air and can lie in the bilge. Even a small amount of stray of gas, when ignited in the confines of a boat, can cause a catastrophic explosion. You must locate the bottles in their own self-draining locker; fortunately this is easily arranged in most metal boats. Usually two bottles are carried, thus ensuring continuity of supply, when one bottle runs out you switch to the next and refill the other at the first available opportunity. We have had gas in our own boats and I have taken my own advice regarding installation and servicing of the installation and the individual appliances. If you have gas aboard, then a reliable gas detector with one or more sensors must be installed. One sensor is required for each gas appliance that you have aboard. If your gas appliances are grouped in one area then a sensor may be arranged to suit the group.

   Paraffin or kerosene stoves, once the mainstay of any galley, have largely given way to LPG gas. If you can stand the smell (although some fuels are now supposedly odour free) and you don't mind the fiddly lighting procedure, then you may find this an ideal fuel for the galley stove.

   Galley stoves fired with diesel fuel would seem the obvious alternative. Diesel fuel does work well in a properly set up appliance. The drawback is that it takes these stoves some time to reach operating temperature. Perhaps diesel fuel stoves are more suited for use in colder climates. To be practical, you should be able to leave the stove on low heat between meal times.

STOWAGE FOR FOOD AND STORES:  This is another area where the person who will actually be working in the galley, should be consulted at the planning stage. The storage lockers or galley cupboards should be arranged to make best use of the available space. The plates, mugs and eating utensils, should be always to hand. Regularly used food such as condiments that do not need refrigeration, should be all, more or less, at eye level. Pots and pans will normally be stored in lower cupboards and cleaning and dish washing fluids may be under the sink or in nearby lockers. If you have steps nearby, these make excellent stowage areas when combined with hinged tops. If you lay out your galley in a similar manner to a regular house kitchen, taking into account the obvious space restrictions and with the necessary changes to suit the marine environment, then you will go a long way towards keeping the cook happy.

   Under- sole stowage for canned goods and other non-perishable foods is a great idea. Try to arrange these lockers so that the cook is able to access them without having to disrupt the area around the galley. Properly fitted hatches in the sole and 'drop in' plastic bins are most useful. Most general foodstuffs, except items that need refrigerating, can be stored in these under-sole lockers. All hatches in the sole should be equipped with ring-pulls or finger holes.

   Lockers in the galley, and elsewhere, will need to be ventilated. Louvered doors or doors with built in vents or other arrangement will be required to allow air to circulate freely in these areas. You will need a 'trash bin' locker so this can often be arranged on the end of one of the lower galley benches. The bin can be hinged at the bottom and designed to accept a standard medium size garbage bag. Most cruising folks and live-aboards use supermarket sacks as garbage or trash bin liners.

REFRIGERATION:    This is another subject too complex to be covered in detail in this text, however there are a few comments that should be made at this time. Firstly, avoid most ready built refrigerators that can only be powered by 12 volt DC power. They can consume large quantities of your valuable battery storage capacity. In powerboats you can use gas refrigeration and this is generally very efficient. It does generate a considerable amount of heat, so make sure it is properly ventilated. For more information on this important and complex subject, please see recommended reading in appendix 1

ICE BOXES:    These can useful but need to be very well insulated to be of any value. The most practical arrangement is an icebox that can be used to take one of the freezer conversion kits. A freezer compressor that is powered by a take off from the main engine can be a good alternative in cruising or live-aboard vessels.

HEATING THE CABIN:   For cabin heating, the diesel-fuelled heater has no peer. A drip feed version is ideal for installation in any boat, one per cabin, if you have a large cruiser. These heaters are trouble free and throw out great quantities of heat for a miserly usage of diesel fuel. Dickinson (Canada and USA) and Taylor (UK) are two popular makes but there are several others available in all parts of the world. All fuel-consuming appliances require good   ventilation, so keep this in mind when installing your heaters and similar items.

Diesel powered, forced air heating can be a troublesome partner aboard any boat. The Eberspacher or similar units need lots of TLC to keep to them operating. This type of forced air heating with multiple outlets is a great convenience, but it can be difficult to maintain.