Comparing apples with apples, these two designs share the same general arrangement, structural layout, equipment and machinery. The designs are by Transformarine Naval Architects and are a first round, unrefined look at the structure and weights. Both vessels exhibit near-level trim throughout the standard loading ranges. One thing to note is that these boats are not your run-of-the-mill 8m trailer boats. Their structures are to Lloyds SSC scantlings, they feature a self-draining deck, inboard diesels and they could be trailed on a COF trailer, over-width restrictions will apply.
Layout
The layout (general arrangement or GA) chosen for these vessels is a lock-up hardtop cabin. The cockpit is designed with sports/ game fishing in mind, with the bulwarks wrapping around the transom. A transom door sits to starboard of the live-bait tank and tuna tubes. The central raised island engine-box can double as seating or run as a tackle rigging station on top. A double seat against the cabin bulkhead provides shelter for passengers and can house a large ice-box or freezer. A side boarding gate is optional. Moving forward, the saloon has seating for four and features a small galley to starboard, aft of the helm seat. Drawer refrigeration is below the aft twin seats. A table can fold up and out from under the side deck, with this seating able to convert to a single berth. There is an option to move the galley forward under the forward saloon seat. The aft facing windows can be opened in a number of ways allowing indoor/outdoor flow from cabin to cockpit. An enclosed head with approximately 1.83m (6ft) headroom is situated forward and below the large helm station, which has plenty of room for two large plotters and standard instruments. The forward cabin features a large V-berth, with bow-thruster compartment below.
Hull Shapes
Both 8-metres feature the same radiused transom, with a large, chamfered counter to help with backing up on gamefish. The chamfer is designed to create lift in reverse, whilst the radius (in plan view) allows the water to sweep around the sides more easily. The T8 has a flared bow created by it having two topside plates at different angles to help facilitate a dryer ride and turn the spray away. The result of this, as evident in the renderings, is a large deck area forward. As you can imagine, extra deck equals extra structure and weight. The foredeck is not the best part of a vessel to carry extra weight, we’ll touch on this a bit later in ‘Some interesting points’. The V8, with its vertical bow, has very upright side plates forward, this being dictated by the stem angle and bow cone. Due to the bottom plate shape, the running strakes and the chines, this hull shape runs surprisingly dry. It is possible to run flared topsides with developed plate on these hulls and we previously created a design with this. Both vessels from approximately midships aft have relatively vertical topsides; good stability is all about the chine beam and length of chine in the water.
Structure
Designed and built no differently to superyachts with their structural layouts, we run a relatively constant frame spacing, longitudinal stringers and girders. The marine grade 5083 aluminium vessels have 8mm keel, 6mm bottoms, chines, keel-strip and transom, with 5mm sides and ring frames, 4mm plate for the decks and cabin. For your information 8mm bottom plates are not required on a vessel this size, you’ll only be carrying around extra weight and as a consequence suffer an increase in fuel consumption. There is no ‘glued’ structure to eliminate the side plate weld distortion; using glue is bad practice due to the glue’s relatively short service life, typically around the length of a new boat’s warranty period. If painted, a small skim of fairing compound will fair any weld distortion.
Weight
Some handy things to know about a boat’s weight; the lightships displacement is the weight of everything on board except fluids (fuel and water), persons, personal effects (clothing, day or overnight bag, etc.) and stores (food, drinks, etc.); these are all called deadweights. Included in your lightships is everything that normally stays on the boat, from fishing gear, boat-hook, fenders and right down to your cutlery. So when it comes to a vessel’s weight on a trailer you add up the lightship, fluids and the trailer weight itself for your towing mass. Be careful not to overload your trailer or vehicle past their limits as this could have ramifications on insurance in a road accident.
Some Interesting Points
- Due to the T8 (red) having a beamy, flared bow, the associated extra weight of this causes a trim issue. To rectify this we have approximately 50mm rise of keel aft, decreasing the buoyancy aft to ‘balance’ the vessel correctly so it sits and runs right. The more mass at the extremes of a vessel (bow & stern), can create a pendulum effect and increase pitching in a seaway.
- Mass – The difference between the aluminium weights in the two boats is less than 60kg and the difference in the lightships (dry weight) is less than 100kg. These two differences are not the same because, amongst other things, the size and hence weight of the windows, interior fitout and of the squabs, are different for the two boats. It should be noted that the V8 fully loaded (6 pax., fuel & water) pretty much sits on its design waterline while the T8 sits 21mm deep. A vessel with more waterplane area is a better load carrier.
- Moment to Change Trim (MCTC) and Vertical Centre of Gravity (VCG) – The V8 has a lower centre of gravity and approximately 27% greater MCTC; this will equate to less pitching in a seaway. The MCTC is derived from the waterplane area and the ability to pitch the boat longitudinally.
- TPC (tonnes per centimetre of immersion) is an interesting hydrostatic measurement in Naval Architecture, as it tells us how much weight it will take to sink the vessel 1cm further into the water. At the design waterline the V8 requires 159kg to sink the boat 1cm while the T8 requires 138kg. This shows that the V8 is a slightly better load carrier.
- Displacement Hull Speed – This is a simple calculation based on a Froude number of 0.4 and the waterline length. This is a good rule-of-thumb number for efficient trolling speed before the pre-planing hump. The V8, with its vertical bow giving it a longer waterline length, has nearly 1/2 a knot over the T8!
Advantages of a vertical bow boat:
- Softer ride due to the finer forward bottom sections.
- Less pitching and better stability.
- Increased interior volume forward. Largest possible volume for a trailer boat (monohull) of a given length. It is ever more important to maximise your boat’s volume as you pay per metre for storing at marinas, boat-stackers and other storage facilities.
- Nicer, faster and more efficient ride at displacement speeds, which is great for trolling or traversing speed-limited zones.
Advantages of a conventional bow boat:
- When retrieving the anchor in a seaway you are less likely to get bow-strike as the anchor exits the water. However, a longer bowsprit on a vertical bow boat negates this advantage.
- Cheaper and easier to build.
- Larger foredeck area.
- Many prefer the styling.
Naval Architect Blair Lewis (of Transformarine Naval Architects) has 20 years’ experience in designing developable plate aluminium boats and structures. “Developable” means that all the parts ultimately forming the outside surfaces of the hull can be cut out of flat sheets and curved to form the hull shape without needing to be distorted by being either stretched or compressed in any plate’s own plane.
