Powering a 12-foot tinny Pt.1

For decades the humble 12-foot (3.66m) tinny has been the ideal size for a couple of anglers to get out on a lake or river. It doesn't require a lot of power and can either be car-topped or trailered depending on the size and type of vehicle. On a compact keel-roller and bilge-pad trailer - and powered by a two-stroke 15 - the towing weight with fuel and fishing tackle is around 200 to 250kg and easily handled by a car with an engine of one litre or more.

Back in the 1980s, tinnies such as the 3.66m Clark Clipper performed very well with a 9.9, and one I borrowed in 1987 with a 216cc Johnson 9.9 averaged 37.1kmh at WOT pushing a total of 280kg including two adults and tackle. But as hulls have grown in bulk, 15s are the favourite, because most weigh no more than their de-rated 9.9 counterparts.

When Mercury released its new 262cc crossflow US-built 15 in 1988, I needed a hull for controlled testing of this engine.

Fortunately, Andrew Short Marine of Taren Point, NSW, was building Sea Al tinnies at the time and supplied me with a Super Skua (below left), which measured 3.6 by 1.4 metres and was rated to 20hp. The hull was perfect for testing the Merc, which I borrowed for six months and used to survey Hunter Valley. I still have the Super Skua, which has since done more than 3600km afloat!

Spinning a 10.5-inch pitch alloy prop and pushing a total of 285kg, the Merc averaged 41.3kmh at WOT using 8.0lt/h. Over a 'loop' of cruising, trolling and including 10 per cent of WOT, the Merc used 3.3lt/h averaging 15kmh.

After returning the Merc, the Super Skua was used to test a range a engines up to 18hp through to 1996 when I borrowed a 246cc loopcharged Yamaha 15F, which I evaluated for eight years.

Spinning a standard 10.5-inch pitch prop and pushing a total of 295kg, the 15F averaged 46.2kmh and 6060rpm at WOT using 7.2lt/h and over the 15kmh loop used 2.7lt/h.

Tohatsu's 169cc loopcharged M9.8B is one of the corporation's most popular outboards because it outperforms eights but doesn't weigh any more, although it doesn't have the torque of 9.9s, most of which are de-rated 15s. However, on a narrower 12-footer like the Super Skua it returned significant cost savings over the 15F. For example, at WOT spinning a 9.5-inch prop and pushing 285kg (due to its 11kg lighter weight), the averages were 35.6kmh and 5700rpm using 5.3lt/h. So for 30 per cent faster WOT averages the 15F used 36 per cent more fuel.

But whereas the 15F planed the Super Skua at half-throttle, the M9.8B needed almost two-thirds and across the 15kmh loop, the M9.8B used 2.5lt/h, a saving of only seven per cent because the Tohatsu had to work harder around the loop to cover the distance in the allotted time. However, as the average speed was reduced, the M9.8B used less fuel due to its smaller displacement powerhead, and averaging 11kmh it used 1.7lt/h compared to 2.2 for the Yammie at the same average speed.

On the Super Skua, 8hp is about the smallest engine that can provide reasonable planing speeds. Less than this and the hull won't fully plane, using a lot of fuel in the process.

Tohatsu's M8B provided reasonable performance but on this hull had to be worked fairly hard, compared to when it was mounted on my Sea Jay 3.4 Punt (more here). Spinning an 8.3-inch pitch prop and pushing 285kg it averaged 28.5kmh and 5130rpm at WOT using 4.2lt/h. Compared to the M8B, on this hull the M9.8B was 25 per cent faster but used 26 per cent more fuel, less of a difference than the 15F compared to the M9.8B because above 40kmh 12-foot tinnies require substantially more power to overcome the hull-skin friction from weld lines.

The M9.8B cruised at 26.7kmh and 4660rpm at two-thirds throttle using 3.6lt/h but the M8B needed three-quarters throttle to maintain a cruising speed, averaging 24.8kmh and 4600rpm using 3.4lt/h. But across the 15kmh loop with 10 per cent WOT operation the M8B used 2.0lt/h, a big saving over the M9.8B and 15F.

Results of this testing showed that if you are re-powering an older tinny and intend travelling long distances, then opt for a 15. For shorter runs, the 9.8 is the best compromise because it returns speeds and fuel efficiency midway between an eight and 15, yet, because of its lesser transom weight than a 15, the hull will trim better afloat, important with these relatively narrow, shortshaft transom hulls that have limited buoyancy aft.