More outboard alternatives

Back in the June issue of Trade-A-Boat, we covered DFI two-stroke and four-stroke outboards as alternatives to sterndrives in moored cruisers.

With its flat torque curve and excellent fuel efficiency at low to midrange revs, a four-stroke outboard can also be a very cost-effective alternative to a diesel inboard when correctly matched to hull length and displacement.

If you've ever hired a houseboat powered by even a single four-stroke outboard, you would have seen how well it can push the hull &151 and on a well-designed displacement cruiser the performance is much better!

Recently, a mate of mine was quoted $12,000 for a twin-cylinder Yanmar 2GM20 to replace the ageing single-cylinder BMW diesel in his displacement hull. In contrast, a Mercury F25 or carburetted F30 with power trim and tilt retail for around $5700 and $7100 respectively. This is a huge saving for owners who might clock up only 50 running hours per year.

While some fibreglass hulls such as the Roberts Longboat 21 have been designed for either inboard diesel or four-stroke outboard power, mounting a hefty four-stroke outboard on the transom of a timber cruiser not intended for the weight or thrust of an outboard can create some headaches. To prevent hull trim problems caused by relocating the engine's weight from amidships to mostly outboard of the transom, the outboard must weigh substantially less than the inboard diesel it would replace. Also, the fuel tank should be moved from aft of the engine mounts (the normal location) to amidships, so that when either full or near empty it won't affect hull trim.

Mounting an outboard on a full-height transom with an unbroken crown requires either a custom-built outboard bracket or a recess cut in the transom &151 the latter a major structural alteration. The motor should be mounted at a height that places the anti-ventilation plate slightly below the transom bottom to allow turbulence-free water to reach the prop. At low speeds in rough seas some thrust will be lost due to the hull pitching, but at normal cruising speeds the hull squat should eliminate this problem.

With an inboard, prop thrust is normally taken at the shaft log or gearbox output flange, while the transom on timber hulls would need to be beefed up using additional knees to distribute the thrust load to the keelson and/or hull stringers and transverse frames.

RE-POWERING EXAMPLES
To assist Trade-A-Boat readers in choosing a suitable four-stroke outboard, my three examples are 6m, 7.5m and 9m cruisers, all having full displacement hulls with minimal transom immersion at rest.

Based on standard naval architecture formulae, the 6m hull displaces one tonne, has a hull speed of 5.6kt and requires 3.7 shaft horsepower (shp) for calm water operation. But as even a slight sea and headwind would require double this amount, 7.4shp is my minimum requirement.

As the shp formula is based on using a diesel that might rev to only 3500rpm, compared to 5500 for an outboard, it's also important to consider engine torque.

A good example is Yanmar's single-cylinder 1GM10 diesel, which develops 8.5shp at 3600rpm, 18 Newton metres (Nm) of torque at 3100rpm and weighs 76kg. A comparable four-stroke outboard is the Mercury F15, which develops 15.0shp at 5000rpm and approximately 20Nm of torque at about 4000rpm.

For displacement hulls the Bigfoot model is the way to go, as it has a 2.42:1 gear ratio that effectively converts engine torque to prop thrust. Complete with electric starting, the longshaft model weighs 60kg and the extralong 62.5kg.

Based on a standard prop power curve, at 3100rpm the 1GM10 consumes 1.3lt/h, whereas the F15 uses 3.9lt/h. The Wide Open Throttle (WOT) fuel consumptions are 2.3 and 5.8lt/h.

The next example is 7.5m, which displaces 2.6 tonnes and has a hull speed of 6.1kt. The recommended minimum is 14.8shp and a twin-cylinder Yanmar 2GM20 would do an adequate job; it develops 17.1shp at 3600rpm, 37Nm at 3000 and weighs 106kg.

When opting for an outboard, I recommend two Mercury models, the F25 and F30, which with power trim and tilt weigh 88 and 96kg. The F25 develops 25.1shp at 5750rpm and approximately 32Nm at about 4000.

While the F25 has a usefully low 2.42:1 gear ratio, in my opinion it doesn't develop quite enough torque for the job, so the F30 may be the better choice. This develops 30.1shp at 5750rpm and approximately 40Nm at about 4000rpm. Although at 2:1 the gear ratio is much taller and loses out on some thrust efficiency, being a de-rated 40 the F30 is a completely understressed motor.

At 3000rpm the 2GM20 uses 2.5lt/h and at WOT 4.6lt/h, whereas at 4000rpm and WOT the F25 consumes 5.0 and 9.5lt/h and the F30 5.0 and 10.8lt/h.

The 9m example worked out at 5.8 tonnes with a hull speed of 7.0kt and required a minimum of 24.8shp. Yanmar's three-cylinder 3GM30 could do the job, but far better would be the three-cylinder 3JH2E diesel, which develops 35.5shp at 3600rpm, 78Nm at 3000 and weighs 178kg. The 3JH2E consumes 4.3lt/h at 3000rpm and 9.3 at WOT.

To provide a similar level of performance, the carburetted Mercury F50 or F60 are needed. Both Bigfoot versions have 2.3:1 gear ratios and weigh 114kg with power trim and tilt. The F50 develops 50.0shp at 5750rpm and approximately 80Nm at 4000, while the F60 develops 60.0shp at 5750rpm and about 90Nm at the same revs.

At 4000rpm and WOT the F50 consumes 7.4 and 18.0lt/h, whereas the F60 uses approximately eight and 19lt/h.

Last year Mercury Marine released EFI versions of the F30, F50 and F60, but for cruisers operated at sea level the additional purchase price over their carburetted counterparts is not cost-effective. Mercury's carburettors rarely go out of tune and should contaminated fuel picked up from a marina pump get past the water-separating fuel filter, carbies are much easier to disassemble and clean than fuel injection systems.