Propping yacht outboards - Part Three

Choosing the right prop for a four-stroke auxiliary outboard is even more critical than with two-strokes. With the exception of Yamaha's F4A, all other four-strokes up to 18hp perform better at the upper end of their Wide Open Throttle (WOT) rev range.

We have omitted models from Bombardier Recreational Products Australia because at the time of writing no indication had been given by the company as to the availability of Evinrude models under 25hp.

HONDA
Four models are available in the 5-15hp range. Don't be tempted to use the BF2D (which is available in a longshaft version) on even small keelboats such as the Endeavour 19, as its tiny air-cooled powerhead will rapidly overheat under heavy loads.

The BF5 is available with only one prop size, a 7.875 x 7in semi-weedless model, severely limiting its suitability for pushing yachts. At 4500-5500rpm the motor should average 6.2-7.5kt, so you may not be able to operate this motor at WOT on heavier hulls.

The BF8 Power Thrust model is available with a 10 x 6.5in four-blader, which at 4500-5500rpm should average 6.7-7.6kt - perfect for most keelboats in the 7-8m range. At 5000-6000rpm, the BF10 should average 6.9-8.2kt.

The old BF9.9 and current BF15 are available with the same Power Thrust prop as the BF8/10 model and, with the broad WOT rev range of 5000-6200, should average 7.6-9.5kt. But I recommend a buying 10 x 7in alloy Solas prop and re-pitching it down to 6in.

MARINER AND MERCURY
Five models are available in the 4-15hp range, but none have high-thrust props suitable for yachts.

The 4-6hp models are all single-cylinder units and the smallest prop available is the 7.875 x 7in semi-weedless unit that will slip under load. At the WOT range of 4500-5500rpm the 4hp and 5hp should average 6.0-7.4kt, which is acceptable for keelboats up to 7.5m. The 6hp model has a range of 5000-6000rpm and 6.7-8.0kt, so you may not be able to operate this model at WOT.

Alternatively, you can fit the 6in high-thrust prop from the two-stroke Sailmate 4, which on the four-stroke 4/5 should average 5.7-6.9kt at 4500-5500rpm. Fitted to the four-stroke 6hp, this prop should average 6.3-7.5kt at the WOT rev range of 5000-6000. Unfortunately, even with this prop, power astern will be restricted as all of these fours-strokes can only be turned 75° to either port or starboard.

The smallest prop for the 9.9/15 Bigfoot has a 9.5in pitch and suits semi-displacement workboats better than keelboats, but Solas makes both an 8in alloy or 7in pitch stainless steel prop. The latter would be my choice and at the WOT range of 4500-5500rpm expect 6.4-7.9kt, which is adequate for keelboats up to 9m.

TOHATSU
Six models are available in the range, from 4-18hp, but as with the two-stroke range, no high-thrust props are available.

As Tohatsu builds the 4-6hp range for Mercury Marine, the comments for the comparable Mariner and Mercury models also apply for Tohatsu, including fitting the 6in high-thrust prop from the Mariner/Mercury Sailmate 4.

There's only one prop available for the 9.9-18hp models: a 6in pitch model with raked blades, which loses a lot of thrust under load. This prop should average 5.7-6.9kt at the 9.9hp WOT rev range of 4500-5500 and 6.3-7.5kt for the 15/18 WOT rev range of 5000-6000. But as the stopping power would be so poor, Solas makes 10 x 7in alloy or 9.25 x 7in stainless steel props that should take advantage of the torque available from these motors, particularly the 18hp.

YAMAHA
Five models are available in the range from 4-15hp, and again these suit keelboats from 7-9m. High reverse or 'Dual Thrust' props are available for 6-15hp models.

Although the F4A has the same gear ratio as its two-stroke 4A counterpart, it develops a lot more torque and, on lighter hulls, can handle the 7.5 x 7in high-thrust prop. At the WOT range of 4000-5000rpm the 7.4 x 6.5in semi-weedless prop should enable the F4A to average 5.1-6.4kt, while the larger prop should average 6.6-8.3kt.

As the 6/8 model revs higher than its two-stroke counterpart I recommend the 9 x 5in Dual Thrust prop. On the 6hp, with its WOT rev range of 4500-5500, this should average 5.3-6.5kt and the 8hp at 5000-6000rpm should be in the 5.9-7.1kt range.

Yamaha is the only company to aim its 9.9hp and 15hp at completely different markets. With its massive 2.92:1 gear ratio, the F9.9D can easily handle an 8in pitch Dual Thrust prop and at the WOT rev range of 4500-5500, should average 6.2-7.4kt, which is ideal for keelboats up to 9m. On a Cole 23, the 9.9hp had no trouble reaching 7kt at half throttle.

But as the 15hp is designed more for planing hulls, I recommend the 9.75 x 6.5in Dual Thrust prop, which should average 6.9-8.5kt, which is adequate for similarly-sized yachts, but is better for lighter displacement hulls.

DON'T FORGET REGULAR MAINTENANCE!
Correctly propped, a four-stroke outboard is the nearest thing to having an inboard diesel, but without the initial expense or servicing costs down the track. But four-stroke outboards require more regular maintenance than two-strokes.

The motor should be taken ashore occasionally and freshwater-flushed for at least ten minutes, or failing this, take it for a run once a week to clear aluminium chloride (commonly mistaken as salt) crystals from the cooling water passages.

And even if you only use the motor a few times per year, change the oil (and filter where fitted) every six months. This is particularly important if you only idle from the mooring or marina berth out to where the wind is, as oil deteriorates far faster under these conditions than when the engine is worked hard.

Finally, don't forget to check the powerhead zinc anode (where fitted), which reduces corrosion in the cooling water passages. As with the zinc anode under or above the antiventilation plate, this should be replaced when 50% eroded.

For more information on a suitable Solas prop for your motor, contact Steve Evans tel (07) 5479 4898 or email sales@solas.com.au