Senator RH 750

Symbiosis, my dictionary informs me, is ‘an interaction between two different organisms, in close association, to the advantage of both’.

This fairly accurately describes the relationship between boat-builders Senator Boats and boat and marine retailers Firmans Marine, both based in the Hawkes Bay port city of Napier.

For a decade now Wayne McKinley has built Senator hulls for Firman’s Marine, which handles the fit-up, sales and promotion, and has considerable influence on the designs and layouts. Two basic types of aluminium boat are made: large monohulls from 8.5 to over 10 metres, and pontoon-type boats ranging from 4.5 up to 7.5 metres.

When Wellington dive company Splash Gordon went looking for a replacement dive boat for their well-worn Bluefin, partners Dave Drane and Bill Keddy settled on the largest of the Senator pontoon designs, the 7.5m RH750. The rig had to be on the water and ready for a starring role in the mid-November sinking of the frigate Wellington off Splash Gordon’s Island Bay base, where it would become an artificial reef and dive site. The timing was tight.

I went down to Napier to check out the rig on the same day that Dave and Steve were sea trialing and taking possession of their new baby. They hit the road back to Wellington that night with the big rig in tow.

Construction

This RH750 has 5mm bottoms, 3mm pontoons, and 3mm and 4mm topsides. Decks are 4mm chequerplate. The deadrise at the transom is 18°, with the bottom edge of the pontoon forming a down-turned chine.

Lengthwise, the hull is supported by four stringers, a flat plate across the keel V, and the pontoon construction. Laterally there are: a collision bulkhead forming the anchor-well in the bows; a bulkhead at the helm position; two under-floor sub-bulkheads forming the holds in the cockpit; and the transom construction.

There is a massive amount of reserve buoyancy in this hull, a feature I particularly like in pontoon designs. In this case there are four separate, pressure-tested compartments: three in the pontoons and a fourth U-shaped area under the deck. Total reserve buoyancy is 2.15 tonnes, much of it high in the gunwales, which should ensure that the hull floats upright, were it ever to be swamped.

The sealed decks drain to a sump under the transom wall; water can be removed from here via a 3700gph bilge pump. There are also scuppers in each stern corner of the cockpit. The scuppers did leak a little water back into the boat when everyone was standing down in the stern quarters (such as when fishing), but given the craft’s primary role as a dive boat, it is anticipated that the divers will be bringing a fair amount of water on board with them anyway, and wet decks will be par for the course.

Power and performance

As a dive charter boat, Southern Comfort will be required to carry loads of eight or ten divers, boatman, 20 or more dive tanks, weight belts and sundry gear. It is powered by twin 150hp Yamaha four-stroke outboards. Given that the boat will be largely used in Cook Strait, the new owners’ priorities are to provide safety and comfort in marginal conditions.

The twin outboards supply power and the insurance of having two engines. They have separate fuel-filter systems and twin batteries with split charging. Although the standard fuel tank is 230 litres, the test boat was fitted with an extended 340-litre tank. This tank is mounted in a self-contained well, with a separate bung-closed drainage line in case of any future tank leakage.

On test day the engines were spinning 19” pitch counter-rotating props, but it is also intended to try some 17” models to hopefully increase torque and reduce the revs a little.

We were well loaded, with seven big adults and a full tank of fuel, as we headed out of Ahuriri on a fine spring day. The bay was running a nice open swell of a metre or so, with the usual little bit of slop around Napier breakwater.

Boasting 300hp on the back, the big Senator is a real rocket ship, topping out at 40 knots (75kph) at 5600rpm. Sea Star hydraulic steering handles the two engines with ease.

The boat is a comfortable, soft traveller too, with no planing strakes and the pontoons effectively forming a down-turned chine that catches an air pocket, cushioning the landing. The trim tabs proved useful in this situation, as with a big crew and no seats, people kept moving about, altering the loading.

I tried shutting down and lifting one engine, and from a standing start the big Senator began planing on the single leg in about 15 seconds, despite the substantial load. This is an important feature for a twin-engine installation – it can be a long trip home at displacement speeds if a single leg can’t get you on plane!

Anchoring

While the Senator has a flat-topped, anti-skid-finished walkway around the cabin sides, the boat is fitted with a Maxwell warp and chain capstan that will take a permanently mounted anchor. This should greatly reduce the need to get out on the bow, as the capstan and cleat can be accessed through the hatch in the cabin top, and the capstan controls are at the helm station.

The bow rails are fixed, but there is plenty of space to fit a large anchor under the rails. Internally, the anchor-well can be got to through a hatch in the forward bulkhead, should there be any minor tangles to sort out.

Layout

As a day dive boat, there is little space devoted to live-aboard comforts. The fully lined (except the chequerplate deck, which runs back a little under the wheelhouse deck) forecabin would sleep two adults if required. There is a toilet situated under the berths, a cabin light, and two small side shelves. The cabin can be made secure with a sliding, locking, wheelhouse door.

Out in the wheelhouse is a large carpeted dash with a back lip and grab rail, where the compass binnacle and sounder are fitted. Other instrumentation is flush-mounted in the helm console. In fitting with its utilitarian design, helm seats have been left out altogether, as the helmsman travels standing anyway (in the often-rough waters of Cook Strait) and more passengers can fit under the shelter of the big hard top without seating taking up the space. To give these people something to hang onto, two full-length rails are fitted under the cabin roof and a side-to-side rail is at the rear edge of the hardtop. In addition, there are two (removable) vertical poles designed to take canopy drops and make winter trips a little more comfortable.

The windscreen is 6mm toughened glass, with a sliding side window on the helmsman’s side. Visibility is good, and further aided during rough weather by two sweep wipers.

The cockpit is large, with a chequerplate deck and various dive fittings that we will get to in a moment. Mooring cleats are welded to the bottom of the rear hardtop face and on the stern corners. Twin batteries are mounted in a locker up in the transom wall, which is also fitted with a walk-through gate adjacent to the boarding platform, grab rails and ladder.

Mounting plates on the stern allow fittings like transducers and trim tabs to be added without drilling into the hull.

Fishability

lthough intended for some fishing charters as well, perhaps the sub heading here should be ‘Diveability’. The pontoon construction of the RH750 gives huge load-carrying and excellent stability – both pre-requisites for its role. The chequerplate deck gives good footing, and dive gear stowage is well taken care of.

Centrally in the cockpit is a removable vertical tank rack, which will take 10 dive bottles. (This does obstruct access to one of the transom lockers and the underdeck hold, but is easily removed when necessary.) Full-length, double-level side shelves will take a total of a further 16 tanks under the gunwales.

The transom walk-through has an angled drop-gate that allows divers easy access into the boat, via a T-dive ladder and chequerplate boarding platform.

With the central tank rack removed, there is a heap of fishing room. The wide gunwales with deck-tread panels are comfortable to sit on, and although the side shelves protrude a little, an angler can get some mid-thigh support by leaning on the gunwale faces. On the hardtop is a nine-position rocket launcher, which also mounts a cockpit spotlight.

While we were drifting around out in Hawkes Bay and I was taking notes, some of the crew used the opportunity to drop a line. With a drop-in baitboard in one of the four through-gunwale rodholders, four anglers fishing was workable, even with the tank rack in place. Unfortunately, the boat seemed to sense it was heading to Wellington, as all the lads could come up with were red cod and barracouta. (Just kidding guys!)

Trailering

This is a big rig, but still easily within the over-width regulations. The trailer supplied is a DMW Premier Series, tandem-axle (14” galvanised rims, eight-ply tires) cradle A-frame design with an entry inlet. It has zinc-treated leaf-spring suspension, and the brake system is hydraulic, acting on one axle. Stainless steel callipers work on bronze discs, and a brake-flushing kit is also fitted to keep the system as trouble-free as possible.

Twelve pairs of wobble rollers per side support the boat, and the fit-out includes submersible LCD lights, wind-down jockey wheel, dual-ratio manual winch and dual coupling. Tow weight (with full fuel load) is approximately 2.3 tonnes.

All in all

This big pontoon hull should be ideal for its allotted task, transporting big dive parties (up to ten divers) in speed and comfort around the sometimes inhospitable Cook Strait, and especially to dive the frigate Wellington.

I called Dave Drane of Splash Gordon not long after the boat test, and in two weeks they had put 60 divers down on the wreck. He was very happy with the boat’s performance and fuel economy.

The RH750 is an ideal multi-purpose workboat that can be customised to meet a wide variety of tasks.

Specifications

Length over all 7.5m
Hull length 6.9m
Beam external 2.5m
Beam internal 1.95m
Bottoms 6mm
Pontoons 3mm
Deadrise 18°
Horse power rating 150–300hp
Key turn packages from $76,814
As tested $125,000

	This article is reproduced with permission of New Zealand Fishing News  by Sam Mossman Re-publishing elsewhere is prohibited