Part 1 - Four-strokes from 15-20hp

Honda BF15
The current-model BF15 offers power tilt with electric start, but the power ram can't be used to trim the boat above 2500rpm. Features include an upfront gearshift, an automatic choke and an automatic decompression for easier hand starting.

A rev limiter and automatic rev reduction through to engine shutdown in the event of low oil level or engine overheat are also excellent features.

On a Quintrex 3.7 Traveller vee-nose punt, the demo electric-start BF15 started instantly and had the lowest vibration levels of any twin-cylinder four-stroke I've yet tested. Using the manual recoil starter, a firm two-hand pull was needed when cold but only a light one-hand tug when warm. At no time did any oil smoke appear.

Spinning a 10in pitch prop and pushing a total of 350kg including two adults, the demo BF15 trolled at 3.5kmh on 770rpm using 0.4lt/h; cruising at 4500rpm we averaged 24.7kmh consuming 3.4lt/h, while the WOT averages were 37.3kmh and 5.4lt/h on 5800rpm.

Accessing the BF15's powerhead is very tight and really the lower cowl must be removed. Recommended servicing intervals are 50 hours or six months after the first 20 hours, and the waterpump impeller should be replaced every 100 hours or once a year. A canister-type oil filter is fitted.

With regular freshwater flushing the BF15 should stand up well to frequent saltwater usage.

Johnson/Suzuki DF15
This manual-choke Suzuki-built engine was tested briefly on a 3.7m vee-nose punt. Spinning a 10in prop and pushing a total of 340kg including two adults, the demo 15 trolled at 3.7kmh using 0.4lt/h and averaged 45kmh at Wide Open Throttle using 6.0lt/h. No revs were picked up on my OMC/Bombardier Shoptach.

The manual-start demo motor fired up easily hot or cold and never blew any oil smoke. Across the entire rev range, vibration levels were pleasantly low.

Powerhead access is very good and recommended servicing intervals are every 50 hours or six months after the first 20 hours, and the waterpump impeller should be replaced every 100 hours or once a year. A canister-type oil filter is not fitted.

From my extensive testing of Suzuki outboards recently, the engines have very good saltwater corrosion resistance and the Johnson/Suzuki 15 should be no exception.

Mariner/Mercury F15 Bigfoot
The F15 is available in standard and "Bigfoot" lower-unit versions. Features include a manual choke and a low-oil-level warning system, but no automatic rev-reduction facility. The overhead recoil starter is also omitted on electric-start models.

On a 3.8m Savage Snipe runabout, spinning a 10.5in prop and pushing a total of 350kg including two adults, the demo Bigfoot 15 trolled at 3.5kmh on 760rpm using 0.4lt/h and planed us quickly, but lost out at WOT where the drag of the lower unit limited our average speed to only 33.4kmh on 5300rpm consuming 5.8lt/h. The best cruising revs were 4500 where we averaged 26.9kmh consuming 3.9lt/h.

The outboard started instantly and no oil smoke appeared at any time.

Powerhead access is good and recommended servicing intervals are every 50 hours or six months. The waterpump impeller should be replaced every 100 hours or once a year. A canister-type oil filter is fitted.

According to owner reports, the F15 stands up well to saltwater usage.

Tohatsu MFS 15B
The MFS 15B has an upfront gearshift and an automatic choke. The rev limiter cuts in at 6250rpm and should the oil pressure drop below a preset level the engine automatically reduces to 2000rpm.

On a 3.7m Stessl Edgetracker, pushing a total of 380kg including two adults and spinning a 9.8in prop, the demo 15B trolled at 4.2kmh on 900rpm using 0.4lt/h and cruised at 26.4kmh on 4500rpm using 3.6lt/h. The WOT averages were 39.9kmh and 5.2lt/h on 5900rpm, but vibration levels were higher than expected.

The demo 15B started easily cold with a firm two-hand pull but only one hand was needed when hot. No oil smoke appeared at any time.

Powerhead access is very good and recommended servicing intervals are every 50 hours or six months after the initial 10-hour service, and the waterpump impeller should be replaced every 100 hours or once a year. A canister-type oil filter is fitted.

Based on the performance of its two-stroke counterpart, the MFS 15B should survive well in saltwater.

Yamaha F15A
The F15A utilises the lower unit and transom brackets from the two-stroke 15F and has an upfront gearshift, a manual choke and a low-oil-level warning system, but no automatic rev reduction.

Tested briefly on a 3.66m aluminium Blue Fin vee-nose punt, pushing a total of 340kg including two adults and spinning a 10.5in prop, the demo F15A trolled us at 3.7kmh on 760rpm using 0.4lt/h and averaged 45kmh at WOT consuming 6.0lt/h. The demo engine started easily with a firm two-hand pull when cold and one hand hot, and never blew any oil smoke.

Powerhead access is very good and recommended servicing intervals are every 50 hours or six months for the first two years after the initial 10-hour service. The waterpump impeller should be replaced every 100 hours or once a year.

Based on my experience of evaluating a two-stroke 15F for eight years, the F15A should have excellent saltwater corrosion resistance.

Tohatsu MFS 18B
Uprated from its 15hp counterpart the 18 has the same features, and on the same 3.7m Stessl Edgetracker vee-nose punt test platform and pushing the same load, the demo 18B provided a real performance edge, although it was no more difficult to start hot or cold. Again it didn't blow any oil smoke.

Spinning an 11in prop, the demo motor trolled at 4.4kmh on 950rpm using 0.4lt/h and had comparable noise and vibration levels to the 15. Again, the best cruising revs were 4500, where the 18 averaged 29.1kmh using 3.6lt/h, while the WOT averages were 42.8kmh and 5.8lt/h on 5800rpm.

Powerhead access, servicing intervals and saltwater corrosion resistance are the same as its 15B counterpart.

Honda BF20
The BF20 has the same starting ease and very low vibration/noise levels as its BF15 sibling.

On a Sea Jay 3.9 Nomad DLX vee-nose punt spinning a 10in prop and pushing a total of 420kg including two adults, the demo BF20 trolled at 3.2kmh on 850rpm using 0.4lt/h and the WOT averages were 37.5kmh and 7.1lt/h on 5380 revs using 7.1lt/h.

Powerhead access and servicing intervals are the same as the BF15, and the BF20 should survive frequent saltwater operation very well.

CONCLUSION
For performance, user-friendliness and a comparatively high output voltage-regulated alternator (that won't "fry" the starting battery on long runs to and from a favourite fishing spot), my pick of the bunch is Honda's BF20 - although the Tohatsu MFS 18B comes a close second.

In my opinion it's probably better to opt for a higher-output engine as there's no weight penalty when upgrading from the Honda and Tohatsu 15hp models. Make sure your hull is rated to carry more than a 15hp engine, as insurance and liability issues might be affected should you exceed the limits.

All the motors profiled here offer comparable economy figures, clean and quiet running, although for my money Honda and Tohatsu offer a few extra telling outboard-protection features.

All the engines listed comply with US EPA 2006 exhaust-emission regulations but only Honda's BF15/BF20 meets California Air Resources Board (CARB) 2008 equirements.

Two-year warranties for recreational usage are standard, and in addition the Mariner/Mercury F15 also has three years against corrosion perforation and four for selected ignition components.