Can One Engine Drive Two Props?
If you have ever wondered if a single engine could drive two propellers, you are not alone. The answer is yes. Have you ever wondered if a mid-size diesel with tremendous torque would beat twin, hopped-up gas engines at best cruise? Have you ever wondered what would happen to the range and ride of a center console if all that weight hanging off the stern were placed forward and low, where it ought to be? Several years ago, Mastry Engine Center in Tampa, Florida, a large distributor for Yanmar diesels, began experimenting with 90-degree gear boxes and made some fascinating discoveries that have been kept secret until now.
Mastry Engine Center in Tampa, Florida, quietly installed in 15 test boats of all types what could become a revolutionary new propulsion system for certain marine applications. In the process, Mastry has become the nation’s leading expert in the technology. In the beginning it was thought the best application would be for trawler-type boats between 35’ and 50’, but when Mastry plopped a Yanmar 6LY3 480-hp in a 27’ high performance design, it got people thinking about the advantages of the system in small boats too.
Superior Gear Company of Stockton, Missouri, has been making gears for agricultural and heavy-duty off-road equipment since 1975. For the “Geared Up” three-box system in marine applications they use cut, spiral beveled gears which are smoother, quieter and longer-lasting than conventional gear designs. One of the first applications was in an Angler 252 center console used by the Destin, Florida, Sheriff’s Department for marine patrol.
Testing a new application in the lab under controlled conditions is one thing, but it’s quite another to have it undergo actual use in the harsh marine environment. Mastry and Superior Gear needed to get many hours on their system fast, and the best way to do it was in a marine patrol boat. After a year of service, the companies got the report from the operators with all the pros and cons.
Test in an Angler 252 Center Console
The Angler could handle twin 150-hp outboard motors and the marine patrol was used to outboard power. The first thing that the officers noticed was that the “Geared Up” diesel-powered boat would not go as fast. Top speed with the Yanmar diesel was 33 mph, but the Angler could go about 10 mph faster with the twin 150-hp outboard engines.
At cruising speeds, it was a different story. Here, the Yanmar went 27.5 mph at best cruise, about the same as the outboard-powered version. More importantly, it got 3.74 mpg – about 30% better than with the twin outboards, we are told by Mastry. That means that with the fuel capacity of 140 gallons, the Yanmar-powered boat would be able to go 470 miles on 90% of capacity, while the twin outboards could travel only 360 miles on the same fuel – 110 miles less.
There was a second advantage of the inboard diesel version with twin props: Because the engine weight was moved forward, under the leaning post, and positioned lower in the boat, the center of gravity was moved forward and lower and the boat rode much better with the Yanmar/”Geared Up” installation, according to the marine police.
When both boats went offshore for an emergency, the sheriff reported that they would both arrive about the same time because the diesel-powered boat could go full throttle and the outboard-powered boat had to throttle back in the rough stuff.
Another Comparison
We matched the Yanmar/“Geared Up” best cruise numbers in the Angler against those we got in a 25’ center console of a different brand weighing about the same, powered by twin 150-hp Yamaha 4-stroke outboards and got a speed of 29.2 mph and 3.17 mpg. Again, the two boats went roughly the same speed at best cruise, and the diesel with twin props was 18% more fuel efficient.
The marine patrol also reported that they “could hold on station better” with the twin-propped “Geared Up” version of the boat than with twin outboards. This stands to reason because the props can be set farther apart with the “Geared Up” gear boxes.
Porsche Has an Engine Change Operation
A 27’ Porsche originally built in the 1980s had already been neutered by the time Mastry took possession: the 454 Magnum MerCruiser engines had been taken out. Mastry says that with those engines its WOT was 79 mph. Mastry installed a 480-hp Yanmar 6LY3 diesel on the centerline and the three “Geared Up” boxes weighing a total of 350 lbs. Total: 1,761 lb. The two 454 magnums had weighed about 2,260 lb. total, so the boat now weighed nearly 500 lb. less with the single diesel.
Mastry was able to turn the engine around with the Geared Up system, which meant that the pulleys and belts were now at the transom and the drive shaft was low going forward. This seemingly small change in the engine envelope allowed Mastry to put in a bench seat for three people abaft the helm and companion seat – something that was impossible in the original version.
According to Mastry, the top speed of the 7,200 lb. 27’ Porsche with the single diesel was 62 mph, with best cruise at 47 mph at 2600 rpm, getting 4.35 mpg. Not bad for a high-performance boat.
Two More Comparisons
Our tests of the Baja 278, weighing only 5,100 lb., powered by a single 425-hp 496 Mag HO engine, recorded a WOT of 63.0 mph and a best cruise of 35.5 mph, and 2.97 mpg. The 7,200-lb. Mastry Porsche with a single 480-hp diesel was 1 mph slower at WOT, but 12 mph faster at best cruise – and got 46% better fuel mileage.
Our tests of the Baja 30 Outlaw, weighing 6900 lbs., powered by twin 425-hp 496 Mag HO engines, registered a top speed of 70.8 mph and at 3000 rpm a best cruise of 42 mph getting 1.65 mpg. The Mastry Porsche was 8.8 mph slower at WOT, but 5 mph faster at best cruise and got 164% better fuel mileage.
Here’s the Breakdown
While the Yanmar diesels are more expensive than gas engines, when two are replaced by one, and existing transmissions or stern drive units are used, the price gap closes somewhat. But the real story here is fuel economy, range, and, in the case of a single engine, ease of maneuverability.