Displacement Hulls

The semi-displacement (or semi-planing) hull is probably the least understood of all hull forms. Hulls of this type function relatively efficiently at displacement speeds but when fitted with enough power will achieve speeds not far below those of some planing hulls.

A real benefit of owning a semi-displacement cruiser is that correctly powered it has the power to cross bars in safety, whereas most pure displacement hulls are too slow. On average, waves running in across a bar travel at 12-13kt, so if you have a cruiser capable of at least 15-16kt you should be able to keep on the back of these waves and eliminate the possibility of broaching.

Planing hull purists may argue that if you're going to push a hull to that speed range, why not have a planing hull that will maintain around 20kt with some in reserve. But what about long-distance passagemaking, where planing hulls simply don't have enough volume to carry the fuel required?

This is another instance when semi-displacement hulls come into their own. They typically have deep bilges and skegs long enough for good directional stability or tracking and deep enough to take the ground without smashing the props and rudders in the process. Yet their transoms provide sufficient buoyancy aft to prevent excessive squatting under power without providing too much buoyancy in a following sea.

Semi-displacement hulls are unusual in that they normally have two distinct fuel efficiency zones - one at pure displacement speeds, and the other at roughly the same speed where a similar-sized planing hull would start to lift out of the hole. Every semi-displacement hull I've tested so far has this quirk.

Earlier this year Trade-A-Boat detailed basic information on how much horsepower is needed to repower displacement hulls. For semi-displacement hulls the basics are the same and you still need to supply your local naval architect with information such as the number of engines to be fitted, the required speed range, the hull displacement and block coefficient.

Typically, this is 0.40-0.45 for semi-displacement hulls.

Once the hull displacement has been found by using a combination of waterline length, waterline beam, hull depth (not including the skeg) and block coefficient, your architect can then ascertain the power required.

The opportunity to compare a range of powerplant installations and their respective efficiency is not something that comes along every day. However thanks to comprehensive seatrials conducted by Grand Banks builder American Marine (supplied to Trade-A-Boat by David Stiles at Bakers Marina, tel (02) 9979 4522), we can offer some examples hereabouts. They make interesting reading.

The Grand Banks 52 is a good example of a hull that's relatively efficient at displacement speeds yet with some 'coaxing' can be pushed to almost planing-hull speeds. It has a hard chine hull with a deep forefoot and reasonably flat run aft. The forward end of the chines starts just about the static waterline, with moderate topsides flare and an almost vertical stem to increase waterline length at displacement speeds.

From amidships the chines run almost parallel to the static waterline aft to the transom, while from midships aft the hull run is almost constant with a slight downward kick just aft of the twin semi-balanced spade rudders. This acts as a constant trim tab to reduce squatting when the hull is pushed to semi-planing speeds.

The GB52 has a waterline length of 15.8m (overall hull length is just 0.5m more), a waterline beam of 4.6m and a hull depth of 1.2m. Combining this information with the block coefficient in standard-engine form the GB52 displaces 33.6 tonnes.

Should the desired cruising speed be 10kt, a standard formula shows that twin engines of 143 shaft horsepower (shp) each will be required to reach this speed. But additional factors such as sea surface conditions, deckhouse and flybridge air resistance and antifouling drag can double this figure.

For example GB52 powered by twin 210hp Cats averaged 9.6kt on 2000 revs consuming 16.1lt/hr each.

Increasing this average to 12.4kt at 2800 revs and Wide Open Throttle (WOT) sucked down 45.0lt/hr each, reducing the maximum possible cruising range from 1330 to 630nm.

To reach 15kt requires 319hp each motor and fitted with twin 420hp Cats the GB52 averaged 14.2kt on 2400rpm consuming 49.3lt/hr each motor. As these Cats are working in a greater efficiency zone at 2400 revs than the 210hp models at 2800 revs, the fuel efficiency at this speed is better and the range increases to 650nm, a result of the hull entering its second efficiency zone. However, by the time it was averaging 18.5kt on 2800 revs the consumption was up to 82.6lt/hr each motor and the range down to 500nm.

A similar situation occurred with twin 435hp Cats, where at 2400 revs the GB52 averaged 15.5kt while the motors each consumed 50.8lt/hr, returning 670nm. Increasing the revs to 2800 averaged 19.5kt and the consumption per motor shot up to 90.7lt/hr, reducing the range to 480nm.

Using the standard formula yet again, to reach 20kt the GB52 theoretically needs 570hp from each motor. When fitted with twin 660hp 3196 Cats the GB52 averaged 16.6kt at 2000 revs using 56.9lt/hr per motor, returning 650nm from the standard tankage. One reason for this very reasonable consumption was the electronic engine management system, which maintains the correct injection timing across the entire rev range.

From here the GB52's hull started to 'lock-up' much the same way a pure displacement hull does past hull speed. At 19.1kt and 2200 revs these engines consumed 106.0lt/hr each, returning 410nm, while at 21.9kt and 2300 revs the consumption was 125.4lt/hr each and range only 380nm.

All models from 210-435hp were fitted with Twin Disc MG 5061A single-speed gearboxes having 2.47:1 reduction ratios. The 660hp 3196 model had single-speed Twin Disc MG 5091 boxes with 2.04:1 reduction ratios. This choice of gear ratios enabled coarse-pitched thrust-efficient props to be fitted, essential for efficiency from displacement to semi-planing speeds.

When repowering a semi-displacement hull, choose engines that are rated more towards commercial applications than recreational. Examples of this rating are Cat Marine's 'C' classification and Volvo Penta's Marine Commercial Rating 2, which allow four hours of WOT operation in every 12. And where possible, choose engines that develop maximum power at less than 3000 revs and preferably between 2000-2500 revs.

Ensure the engines will achieve maximum rated revs with a full cruising load aboard and always choose turbocharged and intercooled engines over straight turbocharged engines.