ABOUT LOW IMPACT BOATING...

Low Impact Boating

– USING LESS ENERGY – is the best way to reduce environmental impact
– Current designs are very energy inefficient and need high energy input
– Use LOW DRAG HULL designs: needing less energy for propulsion
– Efficient energy storage and less energy used onboard to allow electric only
– Better handling and lower hull pressure waves reduce damage to canals

Environmental concerns and how we might find an affordable transition away from fossil fuels are concerning many today. With all alternative sources of energy, energy storage quickly becomes an issue and so finding ways to reduce energy needed for propulsion and on-board use becomes imperative.

Cadal Craft designs address these environmental concerns and provide updated options for river and canal boats, with a focus on vessels under 24m. We have been working with design concepts to reduce the environmental impact of boating for some time and are working towards the Low Impact Boating Initiative (more information page 49). We understand that a sudden switch to electric only or other energy sources can be costly or difficult due to lack of supporting infrastructure and limitations of energy storage. Our aim is to lower the environmental impact of our boating whilst working towards a transition away from fossil fuels. One very good way to do this is simply to use less energy, especially for propulsion. So, we researched how this might be achieved for inland waterways vessels and in particular our much-loved narrowboats.

Our first investigation was to find out why narrowboats need so much horsepower to move these days, typically 40-60hp, when the old work boats with all their heavy loads only needed one horse-power (possibly called Dobbin). We found that most of the loss in efficiency appeared to be as a result of hull design.

The old Harland & Wolff 1930’s Town Class working narrowboats had double chine hulls, a long swim of 17ft at the stern and were trimmed level where possible. Current narrowboat designs have developed since the 1970’s to accommodate low water levels in the canals and to reduce build cost. The hull shape has been adapted so much that now, bow aside, it is in fact nearly rectangular. This allows lower draughts and cheap production, but has the drawback of requiring higher propulsion power and reducing steerage and control. High power consumption makes it very hard to transition these boats to alternate energy sources.

After eight years of research, development, and testing, we combined the ideas of the old working boat designs with modern naval architectural concepts to develop a much more efficient hull. This has been tested and proven to have higher hydrodynamic efficiencies and requires 35-50% lower energy to move. This result not only lowers fuel cost, but gives a viable transition to electric.

The new hull design on our boats not only requires less propulsive power for the same speeds, but also uses a slower turning, larger prop which reduces the propeller jet wake and tip vortex damage to canal bottoms and sides. The hull shape itself generates lower bottom pressure and bow waves with consequent reduced waterway impact. As a result, our boats also have improved steerage, control and enhanced stability in rough conditions, allowing us to design to RCR Category C for safe passage in tidal and estuarine waters.

Energy efficiency applies not just to propulsion but also to many of the onboard systems and ways of energy storage. Our extensive knowledge of such systems has also enabled us to include many other energy efficient concepts on board – we have solar panels and alternative energy sources ; an efficient energy storage system using lithium batteries and efficient charging systems and a hull integrated heat exchanger and pump system using heat from the canal water to heat the boat.

WHY DO WE NEED A LOW DRAG HULL?
The best way to reduce environmental impact obviously is to just use less energy. If the hull is more efficient and needs less propulsion energy to move, not only are you saving on fuel or energy consumption, but you also need to store less energy for future consumption. Hull drag can significantly slow a boat’s movement through the water, so to reduce propulsion needed for any boat, we therefore need to reduce the amount of hull drag.

Hull Drag – This is a function of the underwater shape of the boat. The basic component is the friction of the water moving against the hull and this is increased by the return flow. In addition, there are pressure wave induced drag
created by the shape of the hull, closely related to waterline length (the longer the waterline, the lower the wave form drag), and drag caused by turbulent flow at the bow and stern, related to shape and length of entry and exit – swim length in narrowboat terms.

Return Flow – When a boat proceeds through water, she pushes water ahead of her. In order to not leave a hole behind the boat, this volume of water returns down the sides and under the bottom of the boat, moving faster than the speed of the hull. In a narrow or shallow channel this flow speed increases as there is only a confined space for it to move in. The faster water of the return flow under a boat causes a drop in pressure resulting in squat.

Stern Drag and Squat – Pressure waves and therefore drag at the stern are increased by squarer stern sections because this creates more water turbulence at the back of the boat than a streamlined stern shape. Squat is where the boat hull is dragged down in the water reducing the under-keel clearance between the base plate and the canal bottom. It is caused by the drop in pressure from the return flow under the bottom of the boat. In general squat is at maximum at the end of a boat which is closest to the bottom, almost always at the stern for narrowboats and barges. The turbulent flow from a squarer stern combined with the return flow exacerbate this effect and stern squat of up to 250mm (10 inches) has been recorded in tests on narrowboats. These effects are accentuated by the width of the base plate in a canal boat and the blockage factor in restricted waterways.

Blockage Factor – In confined waterways such as canals, the ratio of the cross sectional area of the boat compared to the cross-sectional area of the waterway is the blockage factor – i.e. how much of the canal is taken up by the underwater
cross-section of the boat. As the blockage factor increases, drag increases. The return flow accelerates due to the reduction in usable canal cross-sectional area and squat is induced – especially at the stern. Reducing base plate width and blockage factor is key to reducing hull drag.

Trim – This is the static fore and aft angle of the hull in the water, pivoting around the centre of floatation. Trim angle in narrowboats is the angle between the bottom of the canal and the hull base plate, which in almost all narrowboats is
parallel to the gunnels. For efficiency this angle should be zero – the base plate should be parallel with the bottom of the canal. If it is not, the blockage factor will increase, especially at the stern and so the stern squat is amplified – to the extent that a theoretical draught giving a healthy under keel clearance can have be reduced to zero, dramatically increasing drag as you would expect with the stern on the bottom.

Our New Low Drag Hull Design

Our hulls are designed to be in level trim, both when moored and when underway, reducing beam below the waterline due to our double chine hull, with a narrower base plate to resist squat and to reduce blockage factors and the associated return flows. They feature long (long!) stern swims to reduce wave form drag and narrow bow entry angles to reduce wave heights. Our tankage is in the middle of the boat to resist the common bow up (and therefore stern down) trim with empty tanks. Our uxter plates are above waterline at the stern and slope forwards to resist stern down trim when under way with people at the back of the boat. Our deeper hull form permits a larger, slower turning, much more efficient prop and with our level trim our draught underway is below that of most modern narrowboats which boast a shallow draught, but exhibit extreme stern squat while underway.

We design to RCR Category C – winds of Force 6 and waves up to to 2m – for increased stability in the rougher conditions found in tidal and estuarine waters. With our tanks, batteries and all pipe work pretty much below floor boards our interiors provide more space for your personalised fitout.

Our longer hulls (widebeams over 15m and narrowboats from 57ft) permit installation of our proprietary hull integrated heat exchanger permitting fitting of heat pumps for energy efficient heating (and cooling!) of your boat.