Mercury F90

Mercury's DOHC F90 is quieter across the entire rev range than its Honda BF90 and Yamaha F80A/F100A competition. It also has incredible midrange torque for a 90, as I found out in November when testing one on a Signature 520R.

The 520R is a versatile 5.2m runabout that's equally at home with an Optimax 135 or the F90. When I first tested a 520R in December 1998, it was powered by an Optimax 135 spinning a 19-inch pitch stainless steel Laser II prop. Pushing a total of 950kg (including two adults), it cruised at 56.4kmh on 4000 revs consuming 23.5lt/hr and at Wide Open Throttle (WOT) averaged 80.5kmh on 5400rpm using 48lt/hr.

To test the F90's torque we tried four different Quicksilver Vengeance props, the standard stainless steel prop sold with the F90.

Pushing a total of 920kg (including two adults) and spinning the 18-inch Vengeance, according to my GPS the F90 achieved a clean plane at 26.6kmh on only 2500 revs and cruised at 55.0kmh on 4000 revs. But as the motor locked up at 4500 revs (61.9kmh) I wouldn't recommend using this prop unless the motor was fully run-in and only lightly loaded.

The 16-inch prop also allowed a clean plane on 2500 revs but at a slightly slower 24kmh. At 4000 revs we averaged 54.3kmh and at WOT (4900 revs) 64kmh, though again the motor didn't reach the bottom of Mercury Marine's recommended WOT rev range.

The 14-incher proved the best all-rounder and allowed for a low Dead Slow Troll (DST) average of 3.9kmh on 700 revs - ideal for freshwater lures. At 1000 revs we averaged 5.5kmh, 1500 revs (8.8kmh), 2000 revs (11.3kmh) and 2500 revs (a semi-planing 17.5kmh).

Increasing the revs another 400 planed us at 22.2kmh and by 3000 revs we were averaging 29.6kmh. At 3500 revs we averaged 40.3kmh and at 4000 revs and the best cruising speed 45.5kmh consuming 10.5lt/hr. Another 500 revs returned 51.1kmh and 5000 revs 59.9kmh.

The WOT average was 66kmh on 5300 revs using 28lt/hr and even at these revs we could hold a normal conversation at the helm. But as the motor had locked up at these revs we also tried a 12-incher to raise the revs to above midway of the WOT rev range where all motors should operate.

Unfortunately the F90 again locked up at 5300 revs and 58.4kmh and trimming the motor right out raised the revs to 5400 but dropped the speed to 57kmh due to prop blow-out.

Hamilton's Marine, which supplied the demo F90, also found 5300 revs to be the maximum for the de-rated F75.

At least spinning the 12-incher the F90 had amazing acceleration for a four-stroke 90 and when the throttle was 'floored' reached 4000 revs in a couple of seconds. If you wanted to deepwater-start a slalom skier this prop would be my choice.

Through tight figures of eight at 4000 revs none of the props suffered ventilation.

The demo F90 started instantly hot or cold, with no oil smoke appearing at any time, and was so quiet at idle you could barely hear it running. Interestingly for a carburetted four-stroke there was minimal drop in revs when forward or reverse was engaged.

Providing the antiventilation plate was kept at least three-quarters immersed no cooling water starvation occurred and when the plate was fully immersed power astern was very good.

Servicing the F90 appears straightforward and recommended intervals are every 100 operating hours or once a year after the first 20 hours or three months. Hamilton's said it also recommends a service every 50 hours or six months and with all its services checks the valve clearances, bolt tension and waterpump impeller to ensure maximum engine performance and reliability.

The F90 is smoother running than its direct Honda and Yamaha competition and with its minimal intrusion aboard would make a day afloat a real pleasure!