Choosing the right prop

Good vibrations...

The essence of powering your displacement hull is the efficient conversion of engine power into thrust. And the fact remains that, for displacement hulls, the humble propeller is still the simplest and most efficient way of doing this.

The subject of correctly propping boats, both planing and displacement, is at the very least involved. In fact, it is considered by many to be somewhat of a black art. (Ed: following is a simple outline of some of the factors concerned with propping of displacement craft. Consider the below information but as the saying goes, if pain persists see your doctor...)

Four main factors must be considered in choosing a suitable prop: rotational speed; pitch of the prop blades; the diameter; and the blade area.

Rotational speed of the prop is crucial because the greater the blade tip speed, the greater the power consumed in spinning the prop. For example, a 750mm diameter prop with a tip speed of 100kmh absorbs around 12hp in spinning effort alone - pure loss because it contributes nothing to the forward thrust generated by the prop.

Using the example of the same 750mm diameter prop, when it is spun at 1200 revs, the tip speed is around 170kmh - about as fast as you'd want the prop to spin. Should the tip speed exceed 200kmh cavitation will occur.

Cavitation is where water suction ahead of the prop reduces water pressure below its vapour pressure, resulting in vapour bubbles which interrupt the solid flow of water to the prop. (It can also occur where a flat-ended timber deadwood ahead of the prop interrupts water flow, thus the timber around the propshaft sterntube bearing should be faired to allow as smooth a flow of water as possible to the prop.)

Severe cavitation is not only inefficient, it can actually erode the material from which the propeller is made and necessitate blade or entire prop replacement.

Optimum prop rotational speeds are achieved by changing the gearing. The latter is why it's essential to use the correct reduction gearing when engines are used in displacement hull applications. As a rule of thumb, engines that develop maximum power at 2200-2500 revs should use a 2:1 reduction, while high-revving diesels (3000-3500 revs) should have reductions between 2.5:1 and 3:1.

FEVER PITCH
The correct pitch-to-diameter ratio of a prop is also essential for good thrust efficiency and varies according to the type of hull.

For example, planing hull props generally have pitch-to-diameter ratios of 0.9-1.2 where the pitch is up to 1.2 times that of the diameter. In comparison a medium displacement hull needs a ratio of 0.8-0.9 where the pitch is 80-90% of the diameter.

Heavy or full displacement cruisers may require a ratio of 0.6-0.7, while trawler and workboat props often have a ratio of 0.5-0.6.

Pitch is a theoretical measurement of the distance a prop will move forward through one revolution. Clearly, as water is fluid some prop slip occurs. Slip is the difference between the pitch and actual forward movement. With shaftdrive planing hulls figures of 5-10% are quoted while 30-50% is the average for displacement hulls.

However, as the hull moves through the water, hull resistance, wave formation and converging water at the stern result in stern wake. The added factor of wake reduces slip to what is known as apparent slip and adds to the forward speed because the water flowing along the hull accelerates as it is sucked into the prop.

While theoretically two-bladed props are best for thrust efficiency, aperture size and excessive vibration usually prevent their use in powered craft.

The best compromise between efficiency and vibration is a three-bladed prop but these days four (and in some cases five) bladed props are more popular because they further reduce vibration and can incorporate more blade area for greater prop thrust in a given diameter.

Subsequently they can also create higher thrust efficiency when repowering a displacement cruiser. That said, in terms of thrust created for engine power consumed, four-blade props are about 4% less efficient than comparable three-bladers and to prevent cavitation from the aperture the prop diameter should not exceed 94% that of the three-blader.

That is, for a suitable replacement four-blade prop for a three-blader, multiply the diameter by 0.94 and pitch by 0.914. For example, if the three-blader has a diameter of 750mm and a pitch of 525mm, the replacement four-blade prop should be 705mm x 480mm.

Remember that a larger diameter prop, or one having more blade area, will sometimes need a larger diameter propshaft which may not be possible to fit inside an existing deadwood keel. Also as propshaft size increases (perhaps to make the most of a change to a deeper gearbox reduction ratio) the additional torque created may require a stronger thrust bearing.

STRAIGHT LINE APPROACH
As a rule when propping a displacement craft (assuming the prop does not run in an aperture) fit the largest diameter prop practicable.

Consider, however, that a bigger prop may require a steeper propshaft angle which will reduce thrust efficiency. There should be no more than about 7-8? down angle while the boat is at rest. (To reduce forward engine mounting height, most diesel manufacturers offer down angle gearboxes which allow the engines to be mounted near horizontal.)

Should your boat have twin engines then counter-rotating props will overcome prop torque when underway.

Props can be either outboard turning or inboard turning. Outboard turning props are where going ahead and viewed from aft the port prop is a left-hander (counter-clockwise) and starboard a right-hander (clockwise), while inboard props turn in the opposite direction.

There's really no difference in manoeuvrability between the two systems, though outboard turning props are the more common.

If, however, you operate mainly in shallow water, inboard turning props will suck up less debris from the seabed.

When fitting a new prop make sure the prop overhang or distance from the sterntube deadwood flange is no more than the diameter of the propshaft and, for example, if the diameter is 40mm then the overhang must not exceed 40mm.

I've tested many an old displacement hull where the overhang is two to three times the propshaft diameter, resulting in excessive vibration from the prop and shortened life of the sterntube bearing.