Sponsored By

A Single Engine, Twin Screws?

Revolutionary drive system uses a

single engine to drive two propellers

When it’s time to retire old twin gas or diesel engines and the replacement cost is higher than the boat’s resale value, owners face a dilemma. If the boat is valuable or money isn’t a problem, the decision is easy. When the owner can’t justify the cost of new engines, but doesn’t want to sell the boat for peanuts, the decision becomes more difficult.
One option is an innovative gearing system that relies on a single engine to drive twin propellers. Called “Geared Up,” it consists of a series of shafts and gearboxes that attach to a single engine bolted (in most cases) between the two innermost hull stringers. Often the existing transmissions, shafts, struts and propellers can be used with little or no modifications. The benefits include comparable performance and speed characteristics (depending on the size of the replacement engine) and improved fuel economy for about 75 percent of the cost of new twins.

How it Works

The gearboxes are produced by Superior Gear Company of Stockton, Missouri, which has built more than one million heavy-duty gearboxes within the past 25 years. They teamed up with Mastry Marine of Saint Petersburg, Florida, to develop the “Geared Up” system for boats. Mastry developed a range of applications for the system and now provides sales, technical support, custom design and fitting, plus installation. To date, about 40 of the units—which retail for about $7,500 each—have been put into boats ranging in length from 25 to 45 feet. Mastry also distributes the system to boatyards and boat dealers around the country where the system can be installed by most skilled mechanics.
At the heart of the system are three, bevelled-gear, aluminium 1:1 ratio gearboxes. A primary splitter gearbox attaches to the engine with a simple bobtail adapter. That gearbox splits the engine output to two counter-rotating shafts (50 percent of the horsepower to each side). Using universal joints, each of those shafts connects to a 90 degree gearbox, then to the boat’s transmission. The gears are designed so there is no real horsepower loss between the engine and the transmissions. All the components are soft mounted so that engine vibration and torque are isolated from the drive.
The Geared Up system can be mounted with the engine facing forward, aft, or even off center. It can also be connected directly to stern drive units. The primary requirement is that there be a minimum of 50 inches between the shafts for regular inboard installations. Because of the use of universal joints, the system allows for greater flexibility in prop shaft angle and alignment than a traditional installation.

How Much Horsepower?

The system is rated for use with engines of up to 500 horsepower. The general rule in determining the amount of horsepower required to provide equivalent performance is to double the horsepower of the original twins. However, some owners will want a bigger engine for increased speed; some will be happy with a smaller engine, losing a few knots and saving on capital and operating costs.

The Installation

When switching from twins to a single, modifications have to be made to the boat’s systems. The extent of those modifications will depend largely on how well the existing drive system will fit with the new engine, and whether the existing transmissions, shafts, struts and props can be reused. If the stringers are in good shape, the new engine can straddle the two centre stringers and no structural modifications are necessary. The new engine has to be rewired and plumbed into the fuel system. The exhaust has to be modified and cooling provided for the new gearboxes. The props may have to be re-pitched or replaced depending on the torque and the operating specs for the new engine. Engine gauges need to be replaced and the shift levers at the helm station(s) need to be changed to accommodate a single throttle control.

Operating the System

Driving a boat with the Geared Up system is only slightly different than with twin engines. When manoeuvring at idle speeds, the gears can be operated as normal. However, because there is only one engine and a single throttle control, the two props can’t be operated at different speeds (unless one is in neutral and one is in gear). Otherwise, the system offers all the advantages of a twin engine vessel.

Benefits

The biggest benefit of the Geared Up system is cost; one engine and one exhaust instead of two. A single engine means fuel savings from 20 to 40 percent (again, depending on engine choice) over those old twin diesels, especially with today’s fuel efficient, clean burning engines. Twin gas conversions typically see savings of up to 50 percent. Maintenance costs will also be reduced. Both props turn at the same speed, and this permanent synchronization means smoother tracking and less vibration. A single engine also means less noise. The only significant downside is that there isn’t that second engine for backup, with the lesser issues of not being able to use asynchronous engine speeds for maneuvering. A low-speed, joystick-type maneuvering system has yet to be devised. While replacing a boat’s existing twin engines with a single simply isn’t an option for some owners, the advantages of reduced fuel consumption and carbon emissions and the increased engine compartment space offered by Geared Up might prove a good alternative.