Contents and photographs
© 2009 Mark Kennett

Site designed by Lesley Partridge

Building a steel boat:
the Independence 48

This article is not meant to be an exhaustive, plate-by-plate guide to building a steek boat the Independence 48, but a suggested schedule of how to go about it. If you are an inexperienced shipwright refer to recognised boatbuilding textbooks to enlarge your knowledge of the basic principles of boatbuilding.

steel boat building_Mark Kennett grinding

The Independence 48 is a conventionally built steel hull with a wooden superstructure. It is built upright as it is too large and heavy to turn over.

Site preparation and set up

The boat is lofted by whatever method you – as boat builder – decide to use, and patterns are taken off. It is a good idea to start a log book at this stage, noting down all useful information and ideas for construction that you have. The article on ‘Lofting and pattern-making’, which accompanies the plans for this boat, gives more information. 

boat building_loft plan

After you have completed lofting and pattern-making, prepare the site for building.

Think carefully about site layout with regard to materials handling, working and removing the finished hull.

The original boat was built keel up on three large wooden blocks, equi- spaced along the keel, and a fourth smaller block under the skeg.

You can set up the height of the blocks to give about 2–3 ft clearance between the ground and the bottom plate: less than 2 ft makes plating the bottom difficult and more than 3 ft (about a metre) makes welding the bottom harder and makes the overall height problematic.

Set up the keel and assemble it on the blocks with the correct amount of drag (fore and aft downward slope) as taken from the plans.

steel boat building - stem frames

The stem and keel with two frames set up. Note temporary round bar spacers on frames

Make sure the blocks are firmly secured to the ground and the keel is firmly secured and braced to the blocks and the ground.

If the blocks are made of steel, tack weld the keel to them to stop slipping. Steel on steel is a very dangerous and slippy combination.

Keel and deadwood assembly

After the keel is set up satisfactorily, you can add the skeg, deadwood, rudder tube, horn plate and stem bar.

steel boat building - keel and deadwood

The complete keel and deadwood, with the first frames set up from the bow

Now is the time to build the rudder heel assembly. This is discussed in the article, ‘Rudder and stern bearing’, which accompanies the plans for the Independence 48.

Framing up

Framing up is next. There are two ways of going about this. The first method is to assemble the entire frame complete with its floor off the keel. The frame is then fitted to the keel. Frames forward of station 3 include the raised deck futtock. This becomes the bulwark stanchion aft of station 3, which is added after the deck is plated. It is best to use full-width deck beams even in the way of the cabin, as they can be cut out after the hull is plated, and they hold the framing rigid while the hull is being plated up. 

Entire frame assembled and ready for fitting to the hull. Note homemade crane, made from 100mm steel box, on industrial castors, with half ton chain hoist

steel boat building - full frame held in crane

The second method is to assemble the frame on the keel as in wooden boat building. The floors or bottom futtocks are welded in place perpendicular to the LWL, and the bottom and side futtocks are added complete with their deck beam.

In both methods brace the frames to the workshop floor and to each other using the hull longitudinals, where possible. This ensures the frames are square, correctly spaced to each other and well braced.

steel boat building - frames from aft

Framed up – view from aft. Note the rudder tube

When all the frames are up, including the transom, and well braced, check the hull for fairness using a long (at least half hull length) wooden batten.

When all is fair, you can bend on the chine bar and tack it in place. It will bend on cold.

After this you can add all of the hull longitudinals.

Then you can bend on the sheer tube. It will have to be heated and bent into place incrementally, especially in the forward third of the hull. Do not put on the raised deck/bulwark cap tube until after the main deck is plated and the bulwark stanchions are in place, in order to ensure fairness of line.

steel boat building - frames and sheer tube

Framing: note the chine bar and side and bottom longitudinals welded in place; the sheer tube is being fitted

The next stage is to complete the rest of the framing. Add all knees, longitudinals, pillars, bitts, gussets, tabs, doubler plates – in fact all of the ‘under plate’ framing. Only omit the lower hull diagonals (which must be added after the side plates are in place), the engine beds and the bulwark stanchions and cap tube, as discussed earlier. 

steel boat building - after deck framing with hatches

Flat bar deck frames. Note the hatch coamings


You can now begin to plate the hull. It is best to start with the bottom plating, as this leaves the maximum access, air and light for the later plating. As you add each plate, make sure it is well shored up from the workshop floor to prevent its weight from pulling the framing out of line. Full strength is achieved only when the hull is fully plated.

From midships aft to the transom the bottom plating goes on without any twist, so you can use full-size plates. Plating should only be tack welded to other plating at this stage.

steel boat building - bottom plate, aft

Bottom plate, aft

From just ahead of midships to the stem bar, the bottom plate takes on an increasing twist, so you have to cut the bottom plate into wide strips to make it easier to form: perhaps 3 strips per side from just ahead of midships to about station 11/2, and then 2 strips, finishing with a small closer plate. You may need to use some heat to form the strips, and you will definitely need heat to form the closer plate. Welding the closer plate inside the hull is tricky and, when the time comes, an extra fat outside weld increment will give peace of mind. 

steel boat building - bow forefoot plated

Forefoot plating.
You can clearly see the last two strips and the vertical closer plate

The side plating can now start. The order of applying the plates is not critical but it is best to start in the middle and work towards the ends, alternating from the port to the starboard side to keep the stresses in the hull even.

steel boat constrution - side plating

On the original boat the method I used for marking up the plates was to offer up, draw round, drop down, cut, fit and tack weld.

You will need to make a pattern for the side closer plate at the stem.

steel boat construction - bowplate pattern being made steel boat construction - bowplate being fitted

Making a pattern for the bowplate

Fitting the plate

steel boat construction - bowplate tacked on steel boat construction - hull plating complete

Tacking the plate on. Note adjustable screw dog, weld-on adjustable plate lever and G clamps, all used to press the plate against the framing

Hull plating complete

Deck plating

You can now plate the main deck. The plate here is lighter and all the welding is in the flat position, so this is usually straightforward.   

steel boat construction - aft deck plated

Then you can make up all of the bulwark framing and weld it to the main deck plate. Install the after bitts as they form part of the bulwark aft.

steel boat building - bulwark frame and bitts

Bend the bulwark cap tube into place, and plate the sides of the raised deck, the deck itself and the transom bulwark. 

steel boat building - foredeck and bulwark framing and Mark painting

steel boat building - plating the raised deck forward

Plating the raised deck, or bulwark, forward

steel boat construction - foredeck plated

Plating the foredeck

You now have a basic hull.

Finish welding

Finish welding can now start. Proper sequence is important here.

First of all complete the framing welds, if you haven’t done so already, working from amidships towards the ends and from side to side.

Frame-to-plate welds are next, working in the same sequence. In steel boat construction frames are not continuously welded to the plate except in the way of built-in fuel tanks that utilise the hull plate as an oil-tight component. Instead, frame-to-plate welds are in ‘chain’ or ‘staggered’ form. This provides ample strength with minimum structural deformation and makes for a stronger hull with less locked-in stress.

Plate to frame welding. 

Note staggered weld increments on the side frames (about 2–3 inches long – 50–75 mm). The sheer tube is eventually welded continuously inside and out

steel boat building - interior welds

Then go on to plate-to-plate and plate-to-keel welding.

Steel boat shell plate is always welded both inside and out, and these welds are of course continuous. It is best to start inside the hull as there are more access problems here. Welding should start amidships. Keep weld increments relatively short to minimise heat-induced distortion. Short means using at most no more than half an electrode at a time, and then skipping a space before recommencing welding. ‘Backstep’ welding is a good idea on long increments.

Complete athwartship seams first, starting amidships and working alternately from side to side and forward and aft. Then make the longitudinal welds, working from the keel up. Make sure that all welds are chipped and inspected before you move on.

When all inside welding is complete you can start outside welding. Open the weld seam slightly with an angle grinder and then start welding in a similar sequence to that used inside.

You can finish the interior metalwork now. Add all internal framing such as diagonals and cabin sole beams if you haven’t done so already. Now is the time to fit non-structural clips and brackets that will support the hull lining and aid the fit out of the hull.

Make up and install engine beds, rudder and fuel tanks.

steel boat construction - engine beds

Engine beds. Note shaft tube with 20 mm flange fitted


The best method for making a bolt-in fuel tank secure is to fabricate the tank base first, usually from substantial angle steel, and to make sure it bears properly on all supporting members. Bolt holes should be drilled and the base test bolted to ensure accurate line up. Then build the tank on the base. This makes the final installation fairly straightforward. Add heavy top braces after the tank is installed. Secure all tanks top and bottom in a substantial manner.

boat construction – fuel tanks

Fuel tanks. These have two baffles. Note cleanout doors in the front

If direct welding to the tank is not possible build a support frame. Water tanks, often made from lightweight stainless steel, are usually installed in this way. You can see the support frame in the picture below.

steel boat building - water tanks in place

Painting and ballast

Paint the hull inside and out with its anti-corrosive scheme. You can leave the final finish coats until the fit out is over.

If you are adding ballast, now is the best time to do it.

See the articles on 'Fitting out’ and ‘Ballasting’, which are in the plans package of the Independence 48.


As mentioned at the start of this article the Independence 48 has a wooden superstructure for lightness, and now is the time to fit it. Bolt timber framing to the steel flange as per plans, using galvanised bolts and plenty of bedding compound.

boat building - wood framing for the cabin structure

The timber framing facilitates the conversion to timber and you can build the bulkheads, cabins and wheelhouse in the normal manner. Add the bulwark planking at this time and the basic hull is complete and ready for fitting out.

boat construction - the basic hull – out of the shed

The basic hull. The shed was too low for the wheelhouse which was added later

See the article ‘Fitting out’, in the plans package of the Independence 48, for more information.

Where now?

To get an overview of the site, go to the home page Mark Kennett Boat Designs.

If you are interested in buying the plans for the Independence 48 click on boat plans to find more information about this and other boats. If you want to see the study pack for more information about the Independence 48, click on 'buy boat plans' below. Or click on 'technical articles' to find more general information about boats and boat building. If you are building, rebuilding or repairing a boat and want some help or advice, click on 'marine consultancy'.