Two-stroke props for yachts

Although there's no substitute for using a portable tachometer to determine the revs a yacht's auxiliary outboard is reaching, it's still important to have a few hull facts on hand to get an idea of the approximate prop pitch required.

In the guide following, we have worked out approximately the right pitch needed for your motor. These calculations are based on a series of prop slip formulae and the hull speed of a yacht in knots, depending on the WOT (Wide Open Throttle) rev range of the motor.

Hull speed is normally 1.4 to 1.6 times the square root of the waterline length in feet. For example, a 6.7m Bluebird has a waterline length of 18ft. A Bluebird has a relatively narrow transom, so the square root is multiplied by 1.4, giving a hull speed of 5.9kt. This also applies to older keelboats, such as the Folkboat 25 and Tophat 25. But lighter displacement racing and shallow draft hulls (eg, J24s) have broader transoms, less hull bottom rocker and are 1.5 to 1.6.

The two-stroke models below suit keelboats from 6.5-9m. As Bombardier Recreational Products Australia had not yet re-released a range of Johnson two-strokes under 25hp when this column was written, its motors were omitted from the guide.

MARINER AND MERCURY
Mercury Marine offers a comprehensive range of two-stroke models from 4-15hp, but none are available with high reverse thrust props.

The smallest model suitable for keelboats is the single-cylinder 4hp, which is available with a 6in high-thrust prop and lower unit ports, which divert exhaust gases above the anti-ventilation plate in reverse. Spinning this prop, the WOT rev range of 4500-5500 should average 5.7-6.9kt.

The uprated 5hp counterpart can also be fitted with this prop, but misses out on the exhaust ports. It should average the same speeds at the same revs, but provides significantly more torque for heavier hulls.

The 9.75 x 6.5in high-thrust prop suits models from 6-15hp. The 6hp has a WOT range of only 4000-5000rpm and should average 6.4-8.0kt.

The same prop suits the 8hp model, which, in the Sailmate version, also has the exhaust ports of the 4hp. At 4500-5500rpm the 8hp - and its 9.9hp and Marathon/Sea Pro 10hp counterparts - should average 7.2-8.8kt, so they may well not be able to operate at WOT.

The 15hp model has a WOT range of 5000-6000rpm and spinning the same prop should average 8.0-9.6kt. For this motor a Solas 10 x 7 (pictured at right), re-pitched to 6in, may be the answer.

TOHATSU
Tohatsu has 10 models from 3.5 to 18hp, but none of the props are high-thrust.

Correctly propped, the direct-drive M3.5A is a brilliant little motor. The WOT range is 4200-5300rpm and the optional 7.4 x 4.3in weedless prop is the answer.

At WOT on my timber Bluebird, pushing a total of 1.8 tonnes, a M3.5A averaged 5.9kt at 5100rpm, consuming 2.2lt/h and at two-thirds throttle it averaged 5.8kt on 5000rpm using 1.9lt/h. At one-third throttle it still averaged 4.8kt on 4000rpm, consuming just 1.4lt/h.

If using the clutch-equipped M3.5B, I recommend the 7.4 x 5.7in 'F6' prop, which tends to 'flatten' its blades under load, reducing prop pitch. At 4200-5300rpm it should average 5.0-6.4kt.

As the smallest prop for the single-cylinder 4/5hp model is a semi-weedless 7.875 x 7-incher, expect 6.0-7.4kt at the WOT rev range of 4500-5500. Of course, the 6in high-thrust prop from the Mercury Marine Sailmate 4hp can always be fitted to the 4/5.

The smallest prop for the 6-9.8hp models has a pitch of 6.5in. Its raked design is unsuitable for yachts, but for the 6/8 models the WOT range of 4500-5500rpm should average 6.4-7.8kt, which is adequate for lighter displacement hulls to 8m. But as the 9.8hp has a WOT range of 5000-6000rpm and should average 7.1-8.5kt, it may pay to re-pitch the prop down to 6in.

Although a 6in pitch prop is available for the 9.9-18hp models, this also has raked blades. But because the 9.9hp has a WOT range of only 4500-5300rpm, it should average 6.6-7.7kt, whereas the 15hp and 18hp, with their range of 5200-5800rpm, should average 7.6-8.5kt and only suit lighter yachts to 9m. Alternatively, Solas offers 10 x 7 alloy or 9.25 x 7 stainless steel props that provide much better thrust efficiency.

SUPER 15
No high-thrust props are available for the Tohatsu-built Super 15, but Solas makes a four-bladed 10 x 7 alloy prop that can be re-pitched to 6in. At the WOT range of 5000-6000rpm, the 15 should average 8.0-9.6kt.

YAMAHA
Six models are available in the 4-15hp range and four of these are available with fat-bladed, high reverse thrust or 'Dual Thrust' props.

The four and 5hp models are available with the same narrow-blade 7.4 x 6.5in semi-weedless prop that should average 5.8-7.1kt at the WOT range of 4500-5500rpm. Although power astern is dismal with this prop, both motors can be swung 180? for full power astern.

The 6/8 model is available with either 9.75 x 5 or 9.75 x 7 Dual Thrust props. The 6hp should average 4.7-5.9kt with the 5in model and 6.6-8.3 with the seven-incher at the WOT range of 4000-5000rpm.

Tested on my Bluebird with the 7in prop, an 8hp averaged 6.5kt at WOT on 3900rpm, consuming 3.9lt/h, and stopped the boat dead from 5kt in 20 seconds. But unless your hull has a light displacement, I recommend the 5in prop, which should average 5.3-6.5kt at the WOT range of 4500-5500rpm.

The 9.9F and 15F are available with either a 9.75 x 6.5in or 9.75 x 8in Dual Thrust prop and on my 5.5m semi-displacement LFB the eight-incher on the 15 stopped a one-tonne load dead in only five seconds from 7kt. But for yachts I recommend the 6.5-incher, which, on both motors, should average 6.9-8.5kt at the WOT range of 4500-5500rpm.