How to Choose the right hull material for your skills

Discussion in 'Boatbuilding' started by yachtwork, Apr 6, 2010.

  1. yachtwork
    Joined: Jun 2008
    Posts: 45
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    Location: Vava 'u Tonga

    yachtwork Junior Member

    This is an article on choosing hull material based on what material your willing to work with. The original and photos can be found here-

    Choose the right hull material for your skills

    Before purchasing a boat consider if you’re willing to take on the type of work needed to maintain and repair the hull.

    Repairing the hull considerations

    The ability to repair different hull materials is often a matter of the type of work one is willing to tackle. For some grinding a fiberglass hull is pure agony, while, For others cutting steel with a hot torch can be a torturous experience. In this article were going to discuss the basics owning and repairing three of the major hull types-
    • Fiberglass
    • Steel
    • Aluminum
    Let’s start by considering some repair advantages and disadvantages of each hull material.


    • Resin and fiberglass are available the world over
    • It’s a very basic technology known in the most outback locations
    • Repairs are relatively cheap
    • Very few power tools are needed
    • Often the DIY can make his own repairs


    • Repairs available in the most remote locations
    • Needs only the most basic of welding machines
    • The constant paint maintenance can overwhelm a yachting vacation
    • Steel boats have about a 30 year life span due to interior rust
    • Steel produces high “sweat equity” return on effort invested due to the low cost of materials


    • Difficult to find repair facilities in remote locations
    • Specialized welding equipment is needed
    • Low, low maintenance. No rust. No paint above the waterline or in the bilge.
    • Highly susceptible to electrical corrosion
    • The “right” aluminum must be used in each repair.
    • The hulls are loud and easily transmit engine and equipment noise along with waterline noise
    • Difficult of effectively insulate
    • Reacts poorly with wood

    Let’s consider an overview of repairing each type of hull material from an owner’s perspective.


    Fiberglass is a strong, and almost inert material. It’s possible to own a boat for years without ever having to repair the hull except for a hull polish.

    The four most common reasons for needed hull repair are-
    • Impact
    • Blisters
    • Poor workmanship
    • Fastener failure


    Impact from striking a reef, another boat, or even cosmetic damage from normal wear and tear are all repairs within the scope of most DIY boaties.

    The repair typically starts with the use of a grinder, followed by-
    • Gelcoat removal till fresh, clean fiberglass is reached
    • New glass cloth is laid over the damaged area
    • Resin is painted over and into the cloth
    • The repair is left to cure and then ground smooth
    • Paint or gelcoat is applied to provide a finished look

    This type repair is not typically considered difficult, but the grinding of fiberglass can be an itchy, uncomfortable job. The trick is to start the work by gearing up with proper protective gear such as-
    • A Tyvex suit
    • Gloves
    • Respirator
    • Safety glasses

    Tip-Too often we see the DIY grinding fiberglass in shorts, tee-shirt, thongs. If you find you started a job in this manner and now are paying the price late in the day by itching and scratching try a cold shower and vinegar rinse. The cold shower prevents your pours from opening allowing the ground glass to rinse away without further penetrating your skin while the vinegar can break the bond of the polyester or epoxy resin allowing the sticky remains to be rinsed off without the use of harsh chemicals.


    Often fiberglass boat owners will comment on how they can maintain much of their boat with a few simple tools. The list often starts with a small 100mm angle grinder coupled to a 36-40 grit grinding disk with a hard rubber back. This inexpensive grinder and interchangeable heads is available at hardware stores the world over and allow basic repairs to the hull.


    When making a repair the DIY will have a choice between polyester or epoxy resin. Polyester is less expensive and often what the boat was originally built from. Epoxy has a higher price tag and can only be over coated with more epoxy, but its adhesion properties are much higher. Short of other knowledge to make a decision on it’s often best to make a repair with epoxy.


    Osmosis, or the “pox” is considered a major downfall of fiberglass boats. Blisters often show up on haul out and can mean a high priced repair. The long, challenging fix often revolves around stripping the complete gelcoat from the bottom of the hull using an electric planer like device. The boat is left to dry while the moisture content of the remaining hull is checked. Once the hull is considered dry enough a new layer of epoxy and glass is laid over the complete bottom followed by gelcoat.

    Repairing the “pox” can tax the DIY. One method to make the job easier is to hire out the initial stripping of the bottom. This gives the DIY a starting point to start the labor intensive repairs.

    Hull flex

    Oil-caning, or the hull skin flexing between the ribs often happens in thin production glass boats. The owner might first notice gelcoat cracks running parallel and near to the ribs. Left to their own devices the hull will eventually begin to form a crack.

    The standard repair is to add longitudinal stiffeners to the inside of the hull. The project can be as simple as ripping a piece of PVC pipe lengthwise to lay against the hull and use as a mold to add a few layers of fiberglass cloth. The difficult part of this job is removing the interior of the yacht to reach the hull inside of the hull.

    Note-Boats built pre 1970’s did not tend to oilcan as the hulls were made thicker.

    Clean lines

    Fiberglass hull tend to have nice clean edges, radiused curves, and fast water drains. This is because one of the big advantages of building a boat in a mold is these delicacies can be thought out and built into the mold ahead of time so each successive boat has some elegant features.


    Glass boats, especially older production boats can be a great bargain costing much less than a “one off” version. Production boats also tend to have a known history and value. A quick Internet search will show the basic selling price and the boats reputation such as typical speed, best point of sail and even know design flaws.

    Daily maintenance

    Glass boats are often thought of as a low maintenance boat owning experience. A simple polish and waxing is often all that is need. Use a buffing machine with rubbing compound followed by an application of wax during the yearly maintenance can keep an old boat looking new.

    Glass boat pitfalls

    Deck leaks, fitting maintenance and lightening strikes are often considered the big three pitfalls of a fiberglass hull.

    While not really a hull repair, fiberglass boats are notorious for deck leaks and dealing with them is part of living on a fiberglass boat. Water finds it way into one of the many bolt holes securing a hatch, sailing track, or deck fitting only to migrate through the overhead to drip in some inconvenient place.

    Some tricks used to find the source of deck leaks are-
    • On a hot sunny day blast water at individual deck fittings. The idea is to limit the places water could have entered the boat while testing.
    • Try to get to the underside of the leaking fittings. Dry the areas as much as possible then use toilet paper laid over the fittings to identify the “wetspot” showing the individual leaking bolt.


    Fiberglass boats are held together by fittings. These are the nuts and bolts that secure the chain plates to the hull, the bolts that secure sailing track to the deck etc. Each bolt is a source of corrosion and possible leak. Metal boats don’t have this problem and it’s often one of the prime advantages of a metal boat over glass.

    Lightening strikes

    Fiberglass boats are susceptible to lightening strikes. While New Zealand in itself is not often thought of as a lightening prone area, if the boat is taken cruising lightening zones will be encountered. Metal boats are almost completely immune from lightening strikes. A glass boat can suffer thousands of dollars in damage after a severe strike.

    Aluminum hull repair

    Repairing an aluminum hull is relatively simple. The work is clean and the welding easily learned. The challenge revolves around finding the right equipment. Let’s look at a few basics of aluminum repair.


    Cutting aluminum is most often done with a powerful circular saw and a carbide tip blade. Amazingly a good carbide blade will cut right through a sheet of aluminum leaving a near perfect cut. Be sure to wear hearing and eye protection. The flying aluminum chips are notorious for their sharpness.

    Tip-Blade noise can be reduced by first cutting into a block of beeswax followed by the aluminum cut.


    Welding aluminum is fast and easy, but it takes a lot of power to maintain the molten welding “puddle”. The power source must be clean and consistent thus a large generator or good shore power is needed.

    The welding kit

    Let’s look at a typical aluminum welding kit. In the welders hand is a spool gun. A spool gun is a welding lead that can be held in one hand with a small spool of aluminum contained in the handle. The spool gun is powered by a power source, most often a DC inverter welder powered by a 6KW or larger power lead. The welding arc itself must be covered with an inert gas, most often Argon. This means a metal cylinder of gas and a regulator must be included in the welding package.

    While all this gear has dramatically shrunk in size over the last few years the welding kit bulk is still outside what most boats would carry aboard. This means when a repair is needed the first step is the yacht must find a competent shop. Inside New Zealand this is relatively easy. On the cruising circuit finding a good welder willing to come to the boat can be a challenge.

    Aluminum boat factoides

    • The welding process for aluminum is faster than steel. This means it costs less labor time to lay the same amount of weld in aluminum.
    • Aluminum is less expensive to work than steel since the cuts are made with a circular saw rather than a “gas ax”. This means no buying expensive gasses and less fire hazard from flying cutting sparks.
    • Aluminum does not need to be treated when the weld is complete. This translates to considerable cost savings when compared to a steel weld. Steel requires cleaning (sandblasting) and five coats of paint to prevent rust.
    • Aluminum has no life span. A steel boat lasts about 30 years till the cost of maintenance begins to overwhelm the budget. An aluminum boat that has been taken care of will still be “good as new” in the same time period.
    • Copper and mercury are big killers of aluminum. One drop of mercury (from a broken bilge pump float switch or thermometer) will eat through the hull in just weeks. Copper dust from bottom paint tracked onto shoes from walking through boatyards can leave a groove in an aluminum deck.

    Insulating an aluminum hull

    Aluminum hulls are difficult to heat and cool. Often we hear “just insulate it”, but this is not really an appropriate answer. Our personal sailing yacht for over twenty years has a steel hull and aluminum cabins. This means I live in a world comprised of half aluminum and half steel. The steel has one inch of laid in insulation while the aluminum has two inches of blown in insulation with a half inch of wood over the top.

    Standing inside the boat on a cold day it is readily apparent the cabin tops are “sucking” the heat from my body. In cold climates the effect is even more dramatic. Screws into aluminum with exposed heads form icicles inside the boat while exposed steel simply condensates.

    In Alaska areas of exposed aluminum built up ice over a half inch thick with the diesel heater is set to maximum. Meanwhile the steel may be chilly, but it has yet to form ice on the inside of the cabin. This might explain why aluminum makes such good cookware.

    Repairing steel

    Steel is often considered “easy” to repair, but it should also be considered “dirty” to repair. Every aspect of the job leaves a mess. Weather its grinding dust left on the deck, burned ash in your lungs, or steel dust in the air that covers surrounding boats with rust streaks, steel messy to work with.

    Most often steel hull repairs are needed after an impact, or a rust hole has opened up under the water line. Typically the repair steps are-
    • Decide on the amount of plate to be changed by use of ultrasound or visual inspection
    • Mark the edges of the cut
    • Cut the straight sections with a cutting disk or plasma cutter
    • Cut the curved corners with a hole saw, gas ax, or plasma cutter
    • Cut a new piece of plate and hold it into location
    • Tack the plate to hold its position.
    • Weld the outside of the plate back-stepping to prevent warping
    • Grind the weld from the inside and re-weld. Finish with another weld from the outside.
    • Wire brush or sandblast and paint on five coats of epoxy.

    Some tricks to making steel repairs-

    • Use high quality electrodes. Low quality electrodes produce low quality welds that tend to crack on cooling or worse during subsequent impacts.
    • Pay particular attention to the grinding out of bad spots on a weld. Much time is saved by grinding deeper to clean out the impurity and thus produce a good second weld.
    • Don’t rush the welds. Be sure to take time to let the weld cool. Distortion, weld cracks, and rusting welds often come from overheated welds.


    Look at a piece of raw steel and you’ll notice the surface will either shiny, lightly rusted, or black. Black is the left over mill scale remaining from the production process. Mill scale MUST be removed prior to welding or painting. This is because mill scale is not really bonded to the steel so it will eventually lift off ruining the best paint job.

    Worse, mill scale is located on the galvanic action chart slightly higher than steel, thus the mill scale will cause the steel to rust in a marine environment. Ever see steel repair that has simply rotted away in a matter of years, or in some cases months? This is most often caused by the thin layer of mil scale that was not removed during the repair process.

    Removing mill scale

    Mill scale is difficult to remove. It is harder than steel and thus difficult to grind. To remove mill scale before a repair try-
    • Buying steel plate that specifically had the mill scale removed
    • Leave the plate exposed to the elements for a short time to allow the surface rust to form thus weakening the mill scale
    • Grind or sandblast the surface of the plate
    • Acid soak the steel plate to lift the mill scale

    Steel welding gear needed


    Welding gear for steel is cheap and getting cheaper. Modern welding machines are solid state, weighing about three kilos and can be fit under a single arm. The basic machine can be purchased as stick welder, or for a few hundred extra dollars the tig option can be added allowing welding of special metals such as stainless steel, bronze, brass etc. A modern welder can easily be carried on the smallest of vessels for repairs in remote locations. Steel does not require the high current of aluminum to weld, thus a much smaller generator can be used.

    Plasma cutter

    A plasma cutter is a special machine that cuts metal by using compressed air and electricity. This makes them specially suited for boats where tanks of special gas can be difficult to come by. The compressed air can be supplied by a small onboard compressor, or even the low pressure side of a SCUBA regulator. Expect to use at least a 4KW genset to power the newer smaller machines.

    Note-In New Zealand machines can be purchased that are a four in one. Stick welder/Tig/Mig (power source)/plasma cutter. While a little larger than a single welder these new welding machines give almost any boat complete shop facilities in one package.


    While there is no perfect hull material choosing “your” material is often a matter of forethought and what “devil” your willing to live with. By taking the time now to decide on the work you’re willing to perform you can narrow you search in purchasing a new boat.
  2. Brasstom
    Joined: Jun 2005
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    Location: Alexandria, VA

    Brasstom Dedicated Boat Dreamer

    Wow, the article doesn't even bother mentioning Wood as a possible hull material? haha, that makes me sad...
  3. troy2000
    Joined: Nov 2009
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    Location: California

    troy2000 Senior Member

    I noticed that, too. But it didn't make me sad; I'm too busy designing my next boat to waste time on sorrow.:)
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