Unique Design Concept.
We always have the same requirements when designing small, lightweight, narrow-beam boats:
Materials: how to make them strong enough while keeping weight and cost as low as possible
Stability: how to stop them rolling over and achieve a high degree of stability
Proportions: how to offer full interior headroom and yet prevent them looking too tall for their length
Style: how to give them a bit of style and prevent them looking like a shed in a bath tub.
Our design brief to ourselves has always been to come up with something which is light, strong and not too expensive. With 15 years experience and having reviewed all the alternatives, we have concluded that, for us, glass-reinforced epoxy/ply composite is the way to go.
We could provide a hull which is relatively inexpensive by using conventional GRP. But if we made it strong enough, it would be too heavy and would still be prone to problems of osmosis and difficulties of repair. Furthermore, owners would be limited to the shape that came out of the mould.
We could provide a hull which is immensely strong and light by the use of structural PVC foam, carbon fibre, kevlar and epoxy resin systems. This is available as an option where weight is critical, but, for all normal purposes, it is ruled out on grounds of cost since it adds around £5,000 to the price of a 28ft shell.
We could have had the shell made out of aluminium. But, aluminium has become very expensive, is not particularly light if strength is to be maintained and, contrary to popular myth, is not entirely resistant to corrosion. Repairs, especially to welds, while out cruising, would be difficult to achieve.
We could build in plywood, which is relatively inexpensive and easy to repair. Actually, good quality (marine) plywood is an excellent boat building material. World War II motor torpedo boats, some of which are still around, were build from plywood. So, for that matter, were the De Havilland Mosquito bombers, which, powered by two Rolls-Royce Merlin engines, achieved speeds over 400 mph and easily outran the Me-109s sent up to intercept them.
Wood fibres, weight for weight, are stronger than steel and, weight for weight, plywood is considerably stiffer than conventional GRP. Plywood panels can be joined together in such a way that the joints are stronger that the material around them. The only problem is that, when it gets wet, plywood will eventually rot if left untreated.
Apart from that, plywood does meet all three of our design criteria. So, if plywood could be used as part of a composite material that exploited its inherent strength and stiffness without exposing it to water and without adding too much expense, we might have a material that would do the trick.
Indeed, wood/epoxy composite is the direction we have taken, by developing a method of using plywood as a core material sandwiched between two skins of glass-reinforced epoxy - although we sometimes use structural foam, end-grain balsa or polypropylene honeycomb as core material for specialist applications.
Actually, we’ve been building boats this way for over 15 years. Our first one, a 50ft river cruiser, was bought by a Belgian dentist who kept it moored on the River Seine in Paris, near the Bois de Boulogne, where he used it as a weekend base for extra-curricular activities, apparently, with his dental nurses – thus giving rise to a rich vein of puns involving the investigation of cavities and so forth – which I tried to resist but couldn’t.
More recently we have used it in the construction of our 50ft “Dutch barges’ the Bon Viveur 1500s (see photo, below) some of them clocking up over 700 cruising hours a season bumping and scraping their way round the continental waterways with little more than a bit of body filler and a lick of paint by way of annual hull maintenance.
'Why not use normal GRP for the skins?' you might wonder. The answer is that normal GRP involves polyester resin that is not totally non-porous (hence osmosis problems) and doesn't stick to anything except itself very easily. It doesn't really stick all that well to glass fibres either, relying instead on a sort of bird's nest effect combined with bulk to build up strength and a degree of impact resistance.
Epoxy resin, on the other hand, is 500 times less porous than polyester and many times stronger and stiffer. It is also a fierce adhesive, especially to glass fibre. We use only glass fibre fabric - with continuous strands of fibre woven or stitched together which is very strong and abrasion-resistant - even without the addition of epoxy resin - as opposed to chopped strand glass fibre often found in GRP which has almost no inherent strength.
So for us, most of the time, it is simply a matter of selecting the desired proportion of plywood to epoxy/glass:
the cheapest option uses 12mm thick plywood and around 3.6kgs/sq m of epoxy/glass, the middle option uses 9mm of plywood and 5.4kgs/sq m of epoxy/glass and the most expensive alternative uses 6mm plywood with 7.2kgs/sq m of epoxy/glass. The more epoxy/glass used, the greater the protection of the plywood, but the higher the cost. The weight is about the same. To reduce weight, it is necessary to replace the plywood with a lighter core - generally more expensive than plywood and requiring additional glass/epoxy to keep the strength up.
All narrow craft have a tendency to roll over. The longer they are and the more you raise their centre of gravity, by adding superstructures for example, the more pronounced this tendency becomes. Overcoming the problem would normally be a relatively simple matter of adding weight (ballast) to the keel to lower the centre of gravity.
However, since our boat has to be light enough to be trailable, we can't simply add weight - unless the ballast is water that can be jettisoned before the boat is pulled up a slipway.
In our case, we would like the water draft to be about 400 mm and that would require 1.8 tonnes of water ballast- which is clearly not practical.
However, apart from adding weight, it is also possible to pull a boat down into the water by removing buoyancy/volume (remember Archimedes?).
We actually do both. The Gems all have an elongated tear-drop shaped 'box keel' below the hull bottom, about 400mm deep, 1110mm wide at the widest point and tapering towards the stern. This eliminates a great deal of buoyancy relative to a conventional 'V' bottom and simultaneously provides a hydrodynamically efficient shape.
You actually stand in this keel in those areas where full headroom is needed, notably galley and shower compartment. The keel is at its widest amidships where the galley and the shower compartment are. However, as full headroom is not required when you are sitting down and as the seating area is at the front of the cabin, we have sealed off the front end of the box keel and turned it into a variable water ballast tank.
As the box keel tapers aft, buoyancy is reduced progressively towards the stern where a certain amount of weight will also be concentrated - engine, batteries, fuel, cases of liquid refreshment, people - so the craft will have a natural tendency to trim by the stern.
However, when the Gem is launched, the sealed forward end of the box keel can be allowed to fill with water (by letting out air) until the boat sits in trim. The amount of water required will be determined by load, number of people aboard, choice and position of engine and so forth. When the boat is in trim, the air valve is closed to prevent any more water entering. When recovering the boat up a slipway, the air valve is opened and the water ballast floods out.
In this way, the centre of gravity is reduced and the weight of the boat is concentrated down the middle leading to a degree of stability normally only found on heavily ballasted steel boats. Sitting lower in the water also has the advantage of improving steering response and reducing the 'sail area' of the boat and thus her susceptibility to the effect of crosswinds.
As you will see in the photos above, the water draft at the chines (edges of the hull) is only a couple of inches. This means that you can moor much closer to a sloping bank -especially where there is a risk of underwater debris - than would normally be the case.
Given that internal headroom is fairly standard at around 6'-2", a short boat will always have a tendency to look too tall for its length. Happily, the measures we take to ensure stability which pull the boat down into the water also ensure that her proportions visible above the water-line are those of a much bigger boat.
The smaller the boat the more effective we have to be in our use of the available volume and, although it would not be all that pleasing to the eye, the most effective and practical would be an elongated floating cube, or - to use a more disparaging term - a shed in a bath tub.
Because we are able to build our boats lighter without compromising strength, we can afford to make them a bit longer and a bit more spacious without compromising towability. Then we have tried to give them a bit of style by injecting them with something of the flavour of a Dutch barge, in miniature. The basis for this is the sheerline, the swept-up line running up to the bow, which some may think is also reminiscent of a tug.
Some may also think we have made the Gem 26, with its open aft deck, longer than it needs to be in order to get the proportions right. Actually, the idea was to provide an eating/sitting out area with enough space to be converted to a sleeping area for two under an awning - mainly with continental cruising in mind.
Thanks to our construction methods which do not involve conventional moulds, we can make the boat a bit shorter or we can elongate the cabin if an owner wants us to and has no need of such a relatively large aft deck. Neither modification has much effect on the styling or the cost of a shell.
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What shall we call our Little Gem Boats, then, these lightweight boats that can be loaded onto a trailer, towed behind a 4 x 4 and launched down a slipway? Towable boats? Trailable boats? Trailerable boats? trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boats? Trailboats? Trailer boats? These Little Gem Boats are described as mini barges to distinguish them from narrow boats sincve their style is loosely based on a Dutch Barge.
I don’t suppose it really matters so long as we call them Gem Boats or Little Gem boats, since we all know what we mean by any of the expressions ‘towable’ boats, ‘trailable boats’, 'trailerable boats',‘trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boats’, trailer boats or trailboats, but technically we shouldn’t call them towable boats, trailer boats, trailable boats, trailerable boat', trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boatsor trailboats because it is the trailer that is towed and not the boat. Trailable boats is a universally accepted expression, so is trailerable boats but I suspect we shall all come to call then trailboats or trailer boats by way of shorthand. I suppose trailer boats is as good as anything since the boats do go on a trailer.
In our case, whatever you want to call the
Little Gem Boats - towable boats, trailable boats, trailerable boats, trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boats
or trailboats - the construction technique is the same and involves the pre-fabrication
under vacuum of foam- or ply-cored composite (sandwich) panels with outer
skins of epoxy/multi directional glassfibre fabric – which is immensely
strong, despite its light weight, important in a trailable boat. It won’t
rot or suffer from osmosis and will require almost no maintenance.
Weight for weight, wood fibres are stronger than steel and our epoxy/glass materials are commonly used in the construction of racing catamarans, other trailer boats, towable boats, trailable boats, trailerable boats, trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boats’or trailboats and similar highly stressed craft. We use the same composite construction in our trailer boats, towable boats, trailable boats, trailerable boats, trail-boat, boatshare, boat-share, boat share, shared ownership, syndicate boats or trailboats as in the construction of our 12 tonne 15m "Dutch" barges – the Bon Viveur 1500 – which notch up over 700 cruising hours on the French canals every year and require only a bit of patching up with body filler and a lick of paint at the end of the season.
It is probably also worth mentioning that the Little Gem 26 - a 26ft mini barge - forms the basis for a boat-share (boat-share, boatshare) co-ownership syndicate on the French inland waterways. Boat-share (boat-share, boatshare) co-ownership syndicates in France work a bit like time share except that, in boat-share (boat-share, boatshare) co-ownership syndicates, shareholders actually own part of the asset and members of boat-share (boat-share, boatshare) co-ownership syndicates receive their share of the proceeds when the asset (boat) is sold. Our boat-share (boat-share, boatshare) co-ownership syndicates run for a minimum of 10 years and at the end of this period members have the option of dissolving the boat-share (boat-share, boatshare) co-ownership syndicate and selling the boat and distributing the proceeds among themselves according to the number of shares they hold or keeping the boat-share (boat-share, boatshare) co-ownership syndicate running.