Whenever making boat modifications, it’s a good idea to write down as much as possible about the idea.

Sometimes this is called a mission statement.

Let’s take a typical situation and use it as an example of how to achieve a change in the cabin.

The yacht is a 40ft/12.5m cruiser with a couple living on board.

Plenty of time is spent in harbour and as these two are competent, they do not need friends on board to help when at sea.

In harbour, they sleep on the two forward berths which are made into a double by a triangular plywood insert at the aft end of the berths.

At sea, one person is on watch at night and the other sleeps in the quarter berth where it is impossible to be thrown out in rough weather.

All this means that the two settees in the saloon are never needed for sleeping.

The trouble with these settees is that they are uncomfortable to sit on while watching TV.

The set is secured to the top of the forward bulkhead in the saloon. The settees face inboard whereas the crew want comfortable seats facing more or less forward towards the TV set.

If the aft ends of the saloon settees are turned into correctly angled armchairs, the forward ends are still available for meals.

All that has to be done is for the settees to be chopped in half and the cut-away aft lengths replaced by forward-facing seats.

Naturally, these new chairs have to be extra comfortable with thick, soft cushions and armrests.

Boat modifications: Get the costings right

The first requirements are a set of plans and a specification of the work and materials.

These are essential for pricing and ordering everything needed to complete the job.

There is the plywood and ‘tree’ wood, the fastenings, glue, sandpaper, paint and varnish, thinners and brush cleaning fluids, paint brushes and Jenny brushes, and finally the cushions.

This is a chance to make sure that the whole project is affordable.

These days, shipwrights planning a job of this sort allow 10% on material costs and labour charges to deal with inflation.

In addition, they add a further 5% or more for unexpected problems.

For instance, if there’s a fortnight of freezing fog, glues should not be used till the weather warms up and that upsets the time scale.

Often, the first setback comes when the owner finds it impossible to get hold of a set of the original construction drawings of their yacht.

It is sometimes easier to get the drawings of an old wooden yacht than a much younger fibreglass one.

The plans of some famous yacht designers are kept in archives such as The Maritime Museum at Greenwich.

Many boat owners’ associations also have records of plans and technical details.

Making a set of design drawings is not hard, especially if there is someone available to help.

These days many school children and university undergraduates are involved in drawing plans as part of their courses.

Working single-handed to gather dimensions is not easy.

It’s true, the end of a measuring tape can be secured in place using duct tape, but having a helper makes things a lot quicker and easier.

A laser device for taking measurements needs only one person.

Making plans Figure 1 (below) shows some of the dimensions needed.

At this stage, an accuracy of say half an inch or 12mm is allowable.

However, it is best to try and ensure all the measurements are spot on.

The first job is to mark the yacht’s centreline in the cabin, at the deckhead, on the bulkheads and down the cabin sole.

A bright coloured self-adhesive tape is good, but the working edge of the tape must be marked on the tape with arrows.

A failure to do this may result in the wrong edge of the tape being used when measuring at right angles to this tape.

Large sheets of white paper or tracing paper are needed to transfer the measurements into a plan.

Continues below…

If broad sheets of paper are not available, some people Sellotape standard A4 sheets together, then turn the attached pages over and draw on the side which has no tape on it.

Occasionally, some types of smooth wallpaper can be used.

The new armchairs are drawn in elevation, plan and section view, that is looking athwartships, downwards and forward or aft.

Some designers do not always draw section views.

That is taking a risk and at every stage, good shipwrights avoid risks.

Section views are important in this case as ideally the chairs must not obtrude much onto the cabin sole.

When the drawings are complete, a clever idea is to stick them high up on a wall and leave them there for easy viewing over a few days or weeks.

Changes will come to mind.

Perhaps the width of the new chairs is going to be too narrow for old people who get little exercise? Is there space for a reading light behind one or both chairs? Would footrests enhance the chairs?

A period of contemplation is often well worth the delay.

Boat modifications: Required materials

The drawings should show not only the shapes of the new chairs but also their construction.

Ideally, marine-quality ply is used, but exterior grade is usually a lot cheaper.

It is usual to varnish yacht furniture after it has been stained to match the existing parts on board.

Normally what is wanted is ply with high- quality exterior laminates made of timber that matches the existing furniture.

Plywood 12mm or ½in thick is widely available and makes strong furniture.

If a thinner ply is used, it’s likely to need some reinforcing with stiffeners glued and bolted across the major dimensions to add strength.

Figure 2 (above) shows the front of the port chair.

There is a widespread agreement going back many decades among boatbuilders that ply edges are covered.

This is partly to prevent the end grain from soaking up moisture and in time rotting.

It is also to give a good appearance.

In addition, screws should not be driven into the edges of ply panels as the end grain gives a poor grip, even when the screws are long.

The battens on ply edges give a better join for adjacent parts.

Slow and steady

The aft ends of the settees have to be cut away to make space for the new chairs.

Using the drawings as a guide, lines are marked on the bases and fronts of the settees.

Before making the first cut through the wood, the marks are rechecked.

It’s better to be slow and safe than make a mistake at this or any stage.

The cuts should be made with a tenon saw or jigsaw with a ‘fine’ blade.

The aim here is to get a neat ‘smooth’ cut, not one that tears the edges of the existing furniture.

The cut edges of the settee base and front need stiffening with battens, typically 40 x 40mm or 1.5 x 1.5in in section.

These are glued and screwed or bolted along the edges.

By using bolts few, if any, clamps are needed, and if the glueing is less than 100% perfect the bolts will ensure the stiffeners work well.

We have to remember the saying: “Frost frightens glues!”

Parts like the top of the chair backs are made of ply with its edges ‘sealed’ by hardwood edge pieces (Figure 3, below).

The backrest cushion is held in place by a row of bolts so that when the yacht is laid up, the cushions are easy to take off.

At this stage, a professional shipwright or carpenter will start to make the new armchairs.

The components will usually partly be fabricated in the workshop and then completed in the saloon.

Professionals rig up mains electricity in the cabin with a multi-socket set for several lights and for the electric tools used on the job.

In practice, cordless drills are now the most favoured, but they need recharging, so suitable sockets in the cabin are still needed.

No one wants to be nipping up and down the boat-side ladder too often.

Avoid pitfalls with prototypes

For an amateur, a good case can be made for building a mock-up.

This is a crude construction of the chair, fabricated from scraps of plywood or hardboard or chipboard.

Sometimes even stiff cardboard can used but this is seldom totally satisfactory.

Ideally, the mock-up is so strong and well supported that it can be tried out.

The crew sits down on it and checks that the TV set is comfortably seen, as this is the object of the whole exercise.

For this test, the heaviest of the assembled team flops down into the chair… with care!

Using pillows or borrowed cushions in place of the final seat cushions ensures the dimensions are correct.

Questions are asked. Is the chair the correct height and at the best angle? Does it obtrude too much across the cabin sole?

Professional finish

When my wife and I were building a 42ft/13m family cruiser, we were lucky enough to have a friend who ran a small factory making furniture.

We sent him drawings of the pieces of furniture we needed and he agreed to make these parts when the factory was quiet, so he quoted a low price.

Each of those parts was narrow enough to fit through the main hatch.

Once in the cabin, adjacent parts were glued and bolted together, then bolted in place and made secure with edge fibreglassing.

For some people altering a cabin, this is a good procedure to follow even though it costs more than a wholly DIY job.

It ensures the new furniture looks professional and fits in with the unaltered parts.

When getting a furniture firm to do part of the work, it’s important to insist that no ferrous fastenings, hinges or door locks are used because they will soon start to rust when the boat is afloat.

Care is needed to avoid using steel parts thinly coated to make them look as if they are fabricated from bronze.

A magnet is used to test the hinges and door furniture like handles and cabin door hooks.

Using the mock-up, the furniture is easy to fabricate as the exact sizes of each part are known.

Every part should be made just a bit stronger than seems right because the sea does stress everything on board.

When I’ve designed any part of a yacht, I often pause to consider if it is totally reliable offshore in severe conditions, and have sometimes added 10% or 15% extra thickness to be safe.

Of course, when designing racing craft the tendency is to go the other way and skim off thicknesses to save weight.

Each new part, and some of the existing furniture too, should have all edges and corners well rounded.

This is to avoid injuries when the boat is jumping about a lot.

It is also to ensure that varnish and paint will adhere well and not wear off quickly.

On some cheaply built craft, instead of being rounded, the edges are bevelled.

This may reduce the cost of the work a tiny bit but does not go far enough or look professional.

Likewise, some boat factories save a little money by fitting low fiddles which do not hold cups and plates in place in bad weather.

It is widely agreed that a ‘safe fiddle is 75mm or 3in high (See Figure 4 above).

Protect your furniture

It’s a good idea to stain and varnish components before they’re fixed in place, then apply the last varnish coats once the new furniture is finally secured in the cabin.

Care is needed not to apply varnish to edges which will later be glassed onto the hull or bulkheads.

The new edge glassing should extend at least 100mm or 4in onto the furniture part and the hull structure, but 150mm or 6in is my preference. (Figure 5, below)

Many boat factories use just one layer of glass, but three make sense.

Each strip of glass should be wider than the one under it so that if one line of glassing is less than perfectly bonded, the join will still be tight and secure.

For the cushions, a good way forward is to invite an upholsterer on board to measure everything up.

However, money can be saved by making patterns from stiff, uncreased brown paper and handing these to an upholstery firm.

On the new armchairs, each cushion has to be secured so that when the yacht heels or rolls, the cushions will stay put.

On factory-built boats, the cushions are secured onto the settees and backrests by strips of Velcro.

These work for a few years then start to fail.

Sometimes, one layer peels off, often due to the millions of tiny ‘hooks’ getting clogged; sometimes pieces are lost.

Secure the cushions

A more reliable method is to have lengthy tabs on each cushion screwed or better still, bolted in place.

When the cushions have to be taken into dry warm storage for the winter, the fastenings are undone.

In time, unwinding then later replacing a row of screws means that larger screws are needed and the holes get damaged.

This is why bolts are preferred; they are designed to be easily reused when springtime comes and the cushions are secured in place back on board.

One place where the furniture can usually be improved is the navigator’s seat by the chart table.

It’s normally a simple perch with no armrest or even a footrest chock on the cabin sole, inboard of the seat.

As a result, when the yacht heels, the navigator either slides off the seat or ends up clutching the table edge and working with difficulty.

An armchair here makes life a lot more comfortable, but the inboard armrest must be made to fold out of the way, otherwise the navigator will have difficulty getting onto the seat.

This part, which hinges or slides out of the way, must be ultra-strong.

It has to take the whole weight of a heavy person when an awkward wave hits the yacht.

More useful reading

The Boat Data Book by Ian Nicolson and Richard Nicolson (7th edition, Adlard Coles), £25. All the dimensions needed for onboard furniture are shown in this book, as well as data about fastenings, etc.

Buy The Boat Data Book at Amazon (UK)

Buy The Boat Data Book at Waterstones (UK)

Buy The Boat Data Book at Amazon (US)

Build Your Own Boat by Ian Nicolson (Allen & Unwin), RRP £17.99. No longer in print, but available second-hand. Plenty of practical advice and many illustrations.

Improve Your Own Boat by Ian Nicolson (WW Norton and Company, Inc), £17.99. A sequel to the above. with lots of practical projects and illustrations.

Buy Improve Your Own Boat at Amazon (UK)

Buy Improve Your Own Boat at Amazon (US)

Introduction to Yacht Design: For Boat Buyers, Owners, Students & Novice Designers by Ian Nicolson (Fernhurst) £15.99. A good starting point for novices.

Buy Introduction to Yacht Design at Amazon (UK)

Buy Introduction to Yacht Design at Waterstones (UK)

Buy Introduction to Yacht Design at Amazon (US)

Note: We may earn a commission when you buy through links on our site, at no extra cost to you. This doesn’t affect our editorial independence.

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