So-called Bigfoot gearcases still confuse many boaties Put simply these units incorporate larger reduction ratios (hence bigger gears and the need for a bigger lower unit) and in some cases allow the use of a larger diameter propeller.

It is the former characteristic that is of most importance, however, for running a larger reduction ratio effectively multiplies the torque available at the propeller. This is a consideration never more important than when considering Bigfoot gearcases' effect in the two versus four-stroke debate.

With this in mind I'm frequently concerned by the 'misinformation' published in some quarters regarding the relative performance of two and four-stroke outboards. And indeed the 'logic' espoused about why some perform better than others.

What follows are a few thoughts on the subject of Bigfoots and four-strokes...

Recently one magazine published a test of four outboards on the same 4.6m hull. The tester wrote that one four-stroke performed better than another simply because it had a significantly greater piston displacement. While this is indeed a factor, it's only part of the story the real reason that the engine in question outperformed the other was that it had a 2.3:1 gear ratio 'Bigfoot' lower unit (from the maker's two-stroke 75/90 model), whereas the other was limited to a conventional 2.09:1 ratio.

In my opinion, Bigfoot gearcases play an important role in getting the best from outboards especially four-strokes. Indeed, I'd state from about 30hp upwards and on boats over 4.2m which are starting to get quite bulky (and heavy) for their length, four-strokes need deep gear reductions (ie: Bigfoots) to effectively convert engine torque to thrust.

The reason for this is simple many of these engines develop maximum torque above 4000rpm with 'peaky' curves that start over 3000 revs. Subsequently they have nowhere near the bottom-end torque of comparable two-strokes.

However, this is not always the case. The four-stroke Suzuki DF70 and Evinrude 70 utilise a SOHC motor that produces maximum torque at around 3500 revs. Running a deep 2.42:1 ratio lower unit from the Johnson 70, this 'flat' torque curve allows the Evinrude/Suzuki fours to swing coarse pitch and thrust-efficient props.

Both these motors produce a similar amount of maximum torque as the two-stroke Yamaha 90, which has 12% less piston displacement. On a 4.6m hull, the Suzuki 70 was almost as quick as the Yammie, again showing the advantage of fitting the deep ratio lower unit compared to the Yammie's 2:1 ratio.

Yamaha's four-stroke F50B with its 2.33:1 gear ratio is a prime example of Bigfoot efficiency, even though maximum torque is developed at 4600 revs. Like the four-stroke Merc 50, it has no trouble pushing planing hulls to 6.0m, which is why it's so popular with pro fishing guides in North Queensland and the Northern Territory.

It's my assertion that the absence of a deep gear ratio is one reason why Honda's BF115/130, though well-designed and engineered, has lacklustre bottom-end performance in comparison to its direct injection two-stroke competition.

The big Honda's 2:1 gear ratio in the same lower unit as fitted to the BF75/90 (which has a 2.3:1 ratio) is simply not deep enough to take full advantage of the engine's torque curve, especially in the sub-70kmh speed range.

Interestingly, above about 3500rpm and on similarly-loaded 5.5m hulls, a demo BF115 returned comparable performance to a Ficht 115 despite the latter running a 2.25:1 ratio. Being a de-rated 130, the Honda had more torque than the Ficht, which allowed it to reach the same Wide Open Throttle average of 64kmh.

In other words, where a motor is operating above 70kmh, the drag of a Bigfoot lower unit can become excessive negating gains.

Under 30hp, Bigfoot lower units only work well with hulls that would normally be fitted with a more powerful two-stroke. For example, an Evinrude 15 four-stroke with its 2.42:1 box returned a comparable WOT speed to the comparable Johno two-stroke on a 4.3m Stacer Proline vee-nose punt. Similarly, an Evinrude 8 (four-stroke) with the same box did a great job of powering a hefty trailerable Ally Craft 3.75 'Cody' dinghy, which normally would have been fitted with a two-stroke 15.

However on lightweight planing hulls, deep ratio lower units can actually hamper planing hull performance. For example, tested on a Savage 380 Snipe Sportsman, a four-stroke Mariner Bigfoot 15 (2.42:1) planed our two-person load almost as quickly as a two-stroke Suzuki DT15 on a comparable hull but 'died' in its upper rev range. The large lower unit and broad-bladed prop simply created too much drag at planing speeds, limiting the WOT average to 33kmh compared to 40 for the Suzy (which had a 1.92:1 ratio).

In comparison, the four-stroke Suzuki DF15 with the same ratio box performed just about as well as its DT15 counterpart.

And though it took a while longer to reach a clean plane (due to its substantially greater weight) a four-stroke Merc 30 with the standard 2:1 lower unit returned comparable WOT performance to a two-stroke 30 on a Bermuda 4.10 runabout...

The lesson in all of this? Before you buy a four-stroke outboard have a long, hard think about the hull you'll mount it on, the speeds you expect to operate it at and then choose the model that best suits your needs!