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Proving a concept

Apr 18, 2017 New products ,Technology

John Macfarlane asks the question: In an age where boat design is usually spawned by computers, can a scale model of a concept have any validity?

Over his 51-year career Alan Wright’s designed more keelboats for New Zealander’s than any other designer. Several factors underpin his success – including an uncanny ability to spot gaps in the market and design boats to suit.

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The halcyon days of local boat production are long gone, but at 85 Wright’s still spotting gaps in the market and designing boats.

This one – a powerboat with a difference – began in his Whangaparaoa home overlooking Gulf Harbour Marina, from where Wright has observed the huge wakes generated by modern planing powerboats.

“Most waste a big percentage of their horsepower creating waves. That got me thinking.”

For years he’s believed boats have become too expensive for the average family. Was there a way to create a modestly-priced, efficient launch?

His solution is a concept 9.5m semi-displacement launch – the Wright Craft 950. Besides a strong emphasis on simplicity, this retro-classic styled launch, powered by a 75hp engine, is designed to cruise comfortably at 14 knots.

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Fundamental to the horsepower/performance equation is the launch’s relatively narrow waterline hull, U-shaped forward section merging with a flat, planing surface aft.

Narrow-hulled powerboat hulls aren’t new, but what is new on the Wright Craft 950 is the wide spray rails. These rails – up to 300mm wide – sit well above the static waterline, but rapidly increase buoyancy and stability as the launch heels or encounters a seaway.

All boats have weaknesses and a potential one of this hull form is a propensity to roll in a choppy anchorage. Wright’s addressed this by incorporating built-in flopper-stoppers either side. They are simple, two metre x 300mm flat alloy plates welded to square alloy struts.

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The struts slide through matching GRP sleeves inside the boat. Pulled up the plates are hard against the spray rail: down they extend slightly deeper than the shallow keel. Each is held in position with a
simple pin.

To minimise weight and expense, interior accessories will be simple: a two-burner cooker, electric fridge, modest electrics and a manual WC. The interior’s open plan; besides the three transverse bulkheads the only other hindrance is a curtain to screen the WC. Up forward is a double bunk, and as there are no side decks, the anchor can be worked through a large forward hatch by standing on the bunk.

The main cabin will have twin settee berths, with the engine box between covered by a dining table. The cabin ceiling is open from the windscreen aft, giving an airy feel to the interior. Windows in the topsides will allow good visibility when sitting at the table.

Two steps up from the main cabin, the cockpit can be screened off around the stern with canvas screens. It features 1.8m seats either side aft, with the galley and helm station forward to port and starboard respectively.

An opening door in the transom folds down to become a boarding platform, which also allows the all-important dinghy to be slid part way into the cockpit when underway.

On paper, the Wright Craft would make an ideal picnic and day boat, with more than adequate accommodation for a couple and two children cruising for a week or two. Her seaworthy hull and low centre of gravity should handle most Gulf conditions, while the shallow draft helps when exploring estuaries and rivers.

But paper plans are one thing – would it translate into reality?

MODEL TEST
Lacking the motivation and funds to build a full-sized version, Wright tested the concept by building a 10 percent scale model – 0.95m long. Towed on a boom alongside his little power trimaran, it performed extremely well.

Wright then built another 0.95m model, this time complete with deck, cabin, retro-classic paint job, battery-powered motor and remote controls. Construction is strip-planked two-millimetre balsa, sheathed on the outside with three-ounce fibreglass cloth and epoxy. Including battery pack and engine it weighs in just on three kilograms.

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BOATING NEW ZEALAND GOT TO TEST IT
Now before anyone starts questions the validity of testing models, consider that prior to computer-aided design, most boats were designed from or with the aid of models. Provided scientific fundamentals are understood and followed, there’s a proven, time-tested relationships between a scale model and its full-sized counterpart.

For example, multiplying the speed of a 10 percent scale model of a 10m boat by 3.1 gives the speed of the full-sized version. A model’s wave making, spray generation, turning ability and behaviour in cross winds – again providing allowance is made for the scale difference – will closely match that of a full-sized replica.

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We tested the model on Onepoto Lake, a picturesque saltwater lagoon on Auckland’s North Shore, which allowed us to closely observe and photograph its performance from the walkway around its perimeter.

First speed. Wright has two different battery-packs and fitted with the smaller NiMH one, the model achieved 4.5 knots true, equating to 14 knots cruising speed in a full-size version. This would require a 75hp engine.

With the gruntier Lithium battery-pack fitted, the model’s speed jumped to nine knots, equating to around 28 knots in a full-size version, well above the design criteria and into planing speeds. This would require an engine of around 125/150hp.

The Wright Craft has been designed as a semi-displacement hull, but the fact that it can be driven to planing speeds over 20 knots says a lot about its low resistance.

At all speeds, the model left a noticeably clean wake and, more importantly, there was no planing hump in the transition between displacement and semi-displacement speeds.

The radio-controlled steering on the model was super sensitive, a function of the gearing ratio between the control unit and the rudder shaft. While this could be de-sensitised by lengthening the lever arm on the rudder, it’s an irrelevant issue for a full-sized version.

At full speed – nine knots – the model’s wide spray rails discharged an impressive stream of water either side. But it remained low, indicating the full-sized version would remain dry when travelling at speed into a head sea. Turning at low speed the model stayed flat, but it banked inwards during sharp high speed turns – as one would expect.

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I was seriously impressed at the model’s performance, lack of wake and easily-driven hull. Yes, it’s a model, and yes, words, theories and predictions on paper are no substitute for the real thing.

Wright hasn’t yet prepared proper construction drawings nor consulted boatbuilders; he’ll first see if there’s any demand. A one-off could be built with strip-planked cedar, sheathed in fibreglass and epoxy, while a limited production version could be built with a solid GRP bottom and GRP foam topsides, decks and cabin.

Simple systems and a modest engine means low maintenance and running costs.

While the Wright Craft 950 could suit many different people, it’s likely to appeal strongly to yachties looking to make the transition from sail to power. Many ex-yachties become frustrated by what’s available on the second-hand launch market, which tends to be dominated by planing launches driven by large horsepower engines and consequent high fuel consumption.

At 14 knots the Wright Craft 950’s fuel consumption would be somewhere around 12 to 14 litres per hour. And while a planing launch can be quicker in flat water, most ex-yachties will find the prospect of cruising at 14 knots more than fast enough. And for those wanting more space, Wright’s also designed a 12m version.

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While the top end of the planing launch market has plenty of choice, along with eye watering prices, this launch is aimed at those with more modest aspirations, the classic Kiwi family. For those brave enough to walk a road less travelled, who value simplicity, economy and leaving a more modest footprint on the  planet, a Wright Craft 950 could be ideal.

Common-sense design
Two things have driven recent powerboat design. First, the fixed length of marina berths encourages boats to be built bigger through depth, beam and height, rather than length, leading to high-resistance hull shapes.

Second, lighter and more powerful engines means these higher resistance hull shapes can be made to perform well with more horsepower.

These factors have shaped the design of the V-bottomed flybridge planing launch, which due to its increased beam, height, windage and weight, often requires high horsepower engines.

Consider this. Take a given quantity of boatbuilding material, labour and engineering, for the same price one could build either a 10m V-bottomed, flybridge, planing launch, or a 12m, low-profile, non-flybridge, semi-displacement launch.

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While both launches will have similar volume and accommodation, compared to the 10m planing launch, the 12m semi-displacement launch will be more easily driven with higher stability. She will be more comfortable underway, a better seaboat, have less wind resistance, be easier to drive, produce less wake, need less than half the horsepower and burn one third the fuel.

The only downside is the increased cost of owning/renting a 12m-marina berth instead of a 10m – and being satisfied with a little less speed.

The fact that such common-sense powerboat designs are not more popular only underscores Voltaire’s observation: “Common sense is not so common.”

EXPRESSION OF INTEREST: Anyone interested in exploring this design should call John on + 64 21 1634 280.

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