Powering a 12-foot tinny Pt.2

Since the eighties, the humble 12-foot tinny has grown in size to where 3.5m hulls have larger internal volume than (3.66m) hulls did back then. Currently, there are no real 12-footers available and most are 3.75m with a greater beam and depth.

For example, in the '80s the car-toppable 3.66m Clark Clipper weighed 57kg but now even a 'lightweight' 12ft dinghy such as Horizon's 3.75 Dinghy has a 1.65m beam, weighs 85kg and is rated to 25hp. This removes it from the car-toppable class to needing a compact trailer and at least 15hp to perform well.

Powered by a 284cc loopcharged Suzuki DT15 the demo hull performed very well. Spinning a 10in prop and pushing 330kg, the Suzy gave us a clean plane at 18.6kmh and 3500rpm, and at two-thirds throttle averaged 28.6kmh and 4500rpm using 4.7lt/h. WOT was 40.8kmh and 5600rpm using 7.2lt/h, compared to 46.2kmh on my 3.6m Sea Al Super Skua with a Yamaha 15F (more here) pushing 295kg.

Similarly, the Bermuda 375 Dolphin dinghy, with a beam of 1.63m and rated to 20hp, needs a 15 to perform well. Powered by a 294cc loopcharged Mercury Super 15 (a de-rated Tohatsu 18) and spinning an 8.8in prop pushing 320kg, a clean plane was achieved at 18.7kmh and 3500rpm, while cruising at 4000rpm the averages were 25.0kmh and 4.1lt/h. The WOT averages were 37.7kmh and 5590rpm using 7.6lt/h.

Note, these engines are at the upper end of piston displacement for two-stroke 15s. I believe the 262cc crossflow Mercury 15 and 246cc loopcharged Yamaha 15F would have performed nowhere near as well and are suited to smaller hulls such as my Super Skua.

Of course, these figures are for hulls with a basic fitout and not a flat floor, tons of fishing tackle or an esky of cold ones!

Providing occupant weight is limited to no more than two adults, a two-stroke 9.9 can perform adequately and Suzuki's DT9.9, de-rated from the 15, is a good choice. Its piston displacement relative to output ensures plenty of mid-range torque. On a Blue Fin 3.8 Dinghy (below left) which has a hull weight of 95kg, spinning an 8in prop and a load of 330kg, the DT9.9 performed adequately. It planed us at 17.3kmh and 3600rpm, and cruising at 4700rpm averaged 23.8kmh and 4.1lt/h. WOT averages were 33.1kmh and 5700rpm using 6.3lt/h allowing a slightly heavier load to be carried without overtaxing the motor.

But, had a slightly less powerful engine such as Tohatsu's 169cc M9.8B been fitted, the hull simply wouldn't have performed, because, unlike the DT9.9, the M9.8B is uprated from its M8B counterpart and has nowhere near the torque.

The M8B has a WOT rpm range of 4500 to 5500 and should reach 5200 to deliver full power, while the M9.8B's range is 5000 to 6000rpm and should reach 5700rpm.

The Suzuki DT9.9 has a WOT rpm range of 4500 to 5500 and was significantly under-propped on the 3.8 Blue Fin. It could have another inch of prop pitch but, being at the upper limit of its capability, the 10in may have been too big. However, the DT15 has a WOT range of 4800 to 5600rpm and the standard 10in was spot-on for the Horizon 3.75.

The Mercury 15 has a WOT range of 5000 to 6000rpm and should reach 5700rpm, while the Super 15 has a 4750 to 5500rpm range and needs to reach 5300 to 5400rpm. The Yamaha 15F has a relatively low WOT range of 4500 to 5500rpm but performs best around 5500rpm; on the Super Skua it was significantly under-propped, but fitting the optional 12in prop would have overloaded it.

You may wonder why the engines mentioned are two-strokes. The reason is simple.

On the Super Skua, the com-bination of narrow beam and limited buoyancy aft prevents mounting a more powerful four-stroke than Tohatsu's 9.8, which, although light at 37kg, is a kilo heavier than Yamaha's 15F. In the late '80s, I tested a Tohatsu 18 on this hull and found the 42kg seriously affected trim and reduced transom freeboard to less than acceptable.

Suzuki's DT15 weighs 38.5kg compared to 43 for the DF15 and, while the Super 15 weighs 42kg, the four-stroke Merc 15 is 50kg. Both these four-strokes are more suited to a V-nose punt of at least 3.8 to 4.0m which has a shallower deadrise aft and subsequently, more buoyancy.