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Gmotion yachts 'Dual Speed' hull design reduces resistance by 45%

by Stuart Friezer on 5 Dec 2012
Gmotion 25 Aft terrace - Gmotion Yachts global launch Stuart Friezer
Motor Yacht designs tend to be polarised to either low speed trawler or high speed planing hull forms. In modern sailing skiffs and yachts we operate right across the speed range and need for performance has led to highly refined designs with efficiency far exceeding that of conventional motor yachts.

The internet is full of motor yacht designers' pipedreams; most of a magnitude and budget that the potential for realising such fantasies rarely becomes a reality.

Approaching the current changing market from an unusual approach has enabled 2 designers to develop designs that are not based on their own preferences but those expressed directly as briefs from two qualified potential buyers.

In all cases the buyers expressed clearly what they wanted included and from the briefs emerged a clear message for riding the GFC and getting clients to contract new builds.

Both clients' briefs were almost identical; the expedition yacht design must be pragmatic in a style which enabled owner friendly operation including launch and retrieval of watercraft and when necessary, drive the boat themselves. Both the potential clients tabled built yachts designs they admired but wanted improved. They both wanted a more spontaneous vessel which could react to weather conditions and move fast, or similarly cruise with optimized seakeeping. The most important point in the clients brief was always the need for efficiency in the hull design. The most unusual item of both clients brief was that they wanted their expedition yachts to move away from the trawler style and take on a more aggressive and sexy external styling. One thing the shipbuilding industry knows is there are no sexy expedition yachts.

Naval architect Stuart Friezer took this challenge head on and rather than creating a design to match the standards of performance speed and economy of existing expedition yachts he used them as a starting benchmark from which to improve. Applying his vast experience and training in hull form design and development for wave piercing catamaran designs for Incat Tasmania and high speed sailing yachts Stuart visualised a hull form which had never previously been built. The Gmotion design uses design elements from sailing skiffs and wave piercing catamarans in order to achieve an efficient 'dual speed' hull shape. After exhaustive design development Stuart modelled his creation, the Gmotion hull form, and the existing yacht, and ran both through comparative resistance testing.

The results proved Stuart's theory that the longer hull form with a fine bow and skiff like shape was far superior to traditional trawler type hull forms predominant on yachts of this style.

The Gmotion hull shape offers more efficient cruise speeds both below hull speed and above. Gmotion yachts will push through their hull speed and find a performance 'sweet spot' at around 1.25 x hull speed [where Hull speed (knots) = 3.04 x sq.rt. (L.W.L. in metres)].

So, Gmotion Fast Cruise Speed (knots) = 3.80 x sqrt(LWL in m)
GM20 17 knots
GM25 19 knots
GM34 22 knots
GM45 25 knots
GM 60 29 knots
GM 76 33 knots

Compared to high speed motor yachts with V-bottomed prismatic hull forms the Gmotion hull is far superior at this speed and it is not until speed is over 1.7 x hull speed that they can be less efficient. For the GM34 this is above 30 knots so for an owner wanting higher speeds the Gmotion hull form is a very efficient alternative. The Gmotion hull seakeeping testing proved to be far superior to both the low speed trawler and high speed planning hull forms. The long fine Gmotion bow slices through smaller waves and gently lifts the hull over bigger swells to smooth the motion in the ocean. At the same time Gmotion hulls don’t suffer the bow broaching effect that axe & bulbous bow designs experience in following seas.

When the results had validated the idea of a dual speed expedition yacht Stuart asked Ricky Smith who had been in attendance at both client meetings to undertake the challenge of styling the G Motion hull form into a sexy motor yacht with an interior driven by the same pragmatic approach applied to the hull. The result is a distinctive exterior styling which retains a powerful masculine, hard edged feel without unnecessary knuckles, styling complications or adornment. Sweeping stairs wrap around 180 degree full height glass slide away doors, and while the exterior styling has been simplified and references contempory minimalism it has an understated elegance. On the GM34 model the traditional flybridge has been replaced by an oversized crow's nest deck offering both retreat privacy and a great vantage point lookout. This is due to having the oversized decks, mid terrace deck, open transom terracing available as well as the dining banquette offered on the foredeck. The inclusion of the pod drive units resulted in enlarged accommodation spaces on the lower deck. The designers added a unique full beam lower deck guest's Media Room or casual saloon facility at the bottom of the guests stairs which can also be sectioned off the hallway and create an addition 2 rooms if needed. Arriving on the lower deck into an open full beam space rather than a hallway is typical to the designs of Gmotion Yachts range where light and panorama are major considerations for those on board.

The confluence of naval architecture and interior designer is not common; naval architecture being an engineering science and interior design an aesthetic driven practice. Stuart proposed designing the interior bulkhead structure to the interior concept thereby effectively cutting out material, weight and build time. The end result is an interior design which presents the facilities expected in such a vessel as well as having an underlying cost benefit ethic of reducing unnecessary time, materials and cost. When the interior and exterior styling were first completed they decided to test the market with a last minute visit to the 2012 Monaco Yacht Show where they presented the design to selected top brokers and shipyards present. Overwhelmed by the positive responses from the industry the pair of designers were immediately approached by a European yard to design a 25m Gmotion version to align with their current market and sales demographic. was formed and the first design requested returned for consideration in just under four weeks.

The leading brokers and shipyards could see immediately the GMotion designs range now including the 34 and 25metre expedition models, a 13metre cabin cruiser plus a 6 metre custom matching tender would be easy to build with little to no tumblehome and almost vertical hull plating for the majority of the shape. Including IPS pods for ease of installation and efficiency and removing the need for bow thrusters in the build schedule compound the overall appeal of this design. Many yachts have a build program of minimum 3 years however the 34 metre GMotion Yacht design will reduce that dramatically allowing owners to be back on water in a much shorter timeframe.

Despite the markets fascination with the current spread of motor yachts designed well over 50 metres it is good for the market to have designers who are concentrating on designing for a market demographic which has the potential to help many new build projects be realised.

The Gmotion Yachts Expedition Series models 34 and 25 both feature large open aft decks with a unique feature where the tender and jet ski is housed on a mid terrace platform between the aft deck and the aft stairs. This lowered terrace design feature ensures the aft view from the interior is not impeded while cruising. It also, then creates a mid terrace beach deck open to the transom with masses of locker storage for sun lounges, barbeques and fender stowage in the most perfect location for access.

• Gmotion Yachts Range ; 34 and 25 metre Expedition models plus 13 metre cabin cruiser.
• Gmotion Yachts Tenders ; 6 metre and 4 metre custom

DRAFT MAX : 1.7m
SPEED Max/Cruise/Cruise : 25 / 20 / 13
RANGE : 5000 nm
MAIN ENGINES : 4 x Volvo Penta D11/13
PROPULSION : 4 x Volvo Penta IPS
GENERATORS : 2 x 50 kW
FUEL CAPACITY : 27000 litres
BALLAST SW : 8000 litres
FRESH WATER : 6000 litres
SULLAGE/GRAY WATER : 2000 litres
CONSTRUCTION : Aluminium / Composite
CREW : 5

Zhik ZKG 660x82Colligo Marine 660x82Ancasta Ker 33 660x82

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