From Two to Four
  |  First Published: December 2008

When deciding on buying a new engine for your boat people are full of advice, especially when changing from a two-stroke to a four-stroke. Some warn of the perils of four-strokes, such as overpricing, heavy weight and expensive to fix, whilst others swear that it was the best move they’ve ever made.

With this in mind, I decided to find out from Don Eccleston how his experience of swapping from a two-stroke to four-stroke has been.

Don owns a 5.2m Kevlacat named Secret Agent and for some time he had been toying with the idea of repowering his much loved cat with four strokes. He recently bit the bullet and, for about a $10,000 changeover, has gone from twin 90hp Yamaha two-strokes to two extra long shaft 60hp Yamaha four-strokes. Don settled on the twin 60hp four strokes because that was the heaviest motor he could get without having to upgrade the pods and, at $4000 a pair from Kevlacat, the price was prohibitive.

As expected, there were plenty of detractors along the way who have told him that he would be disappointingly underpowered, but he has now had the new motors for about two months and couldn’t be happier. Petrol consumption has dropped by nearly two thirds!

With the twin 90hp it was an expensive day out for Don trolling for his favourite quarry out wide of Cairns. A typical trip would use around 100L per motor.

At the 2007 Mission Beach Challenge, where Don won Champion Boat, Secret Agent used 102L per motor per day with the old twin two-stroke 90hp. This year, fishing exactly the same grounds and length of time with the new 60hp four-stroke Yamahas, the consumption dropped to an incredible 38L per motor per day. At a round figure of $1.50 a litre for fuel, Don’s fuel cost from $306 per day to $114 – a saving of an unbelievable $192 a day!

Even though the total horsepower has dropped from 180hp to 120hp, there has been no drop in performance. With the twin 90hp, Don cruised at 3800rpm doing 37km/h, while the new 60hp four-strokes purr along at 4500rpm doing 36k/ph. Four-strokes prefer to run at slightly higher revs than two-strokes so the difference in revs is not an issue, but the difference in fuel consumption and noise level is well worth crowing about.

It took Don quite a lot of trial and error to get the propping on the four-strokes just right and he certainly went about it in a very scientific manner. Four and three blade Solas stainless steel props, along with the standard three blade alloy factory props were trialled at various heights before Don settled on the three blade stainless steel Solas props on the second bottom hole as the optimum combination.

The load throughout the water tests was three adults and 240L of fuel. The best performance was produced by the 13.75 x 13 Solas stainless steel props bolted at the third hole from the bottom (one hole down from maximum engine height). The Solas props cost $1100 landed in Cairns, and Solas had offered to continue exchanging props until Don was happy with the performance. If they don’t do the job, Solas offered a full refund on return of the props in good order.

Don is absolutely wrapped in the new motors and obviously sees the fuel savings as the biggest advantage, but he is also stoked about the lack of noise. In fact, he accidentally keeps restarting the motor that is already running because he can’t hear it, so he’s now trying to train him-self to use the taco instead of his ears. The lack of fumes when trolling downwind is also a big plus in Don’s books.

From my calculations by the time Don has done 40 trips to the reef he will have totally paid for the repowering and the props with savings in fuel.

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