Volvo D4 300-hp diesel sterndrive
The diesel sterndrive is the way to go if you’re looking for the ultimate in range (more than 300 nautical miles at cruise with a 10 percent reserve, and that’s with a mere 100-gallon fuel capacity), economy and responsive handling, both dockside and at speed. The inboard and waterjet just can’t compete with the Volvo Duoprop’s efficiency, acceleration and directional control.
This boat readily backs downwind at idle in either direction, turns in about two boat lengths at high speed, and accelerates strongly. The power steering allowed fingertip control, and it was very responsive at just three-and-a-third turns from lock-to-lock. The only annoying thing was the distinct clunk when shifting into gear.
If range is your first priority, then the diesel sterndrive is the best pick, at least at high cruise speeds, since the diesel inboard offers better economy at speeds up to 20 knots. At 7 knots, the diesel inboard gets a whopping 6.5 nautical miles per gallon compared to 5.3 nmpg for the diesel I/O and 2.6 nmpg for the gas I/O and the diesel waterjet. At 20 knots, the diesel inboard still gets the best economy, while at 25 knots and above, the diesel sterndrive gets the nod, with the sterndrive’s lower drag (compared to the inboard) coming into play. The waterjet doesn’t compete efficiency-wise with either of the other diesels, though it beats the gas I/O hands down the faster you go above 23 knots.
Another thing to love about the diesel sterndrive is the electronic controls; they work so smoothly and easily that you have to be careful at first if you’re used to mechanical controls. Neutral detent is just firm and noticeable enough to get your attention, and during our divisional tactics maneuvering, with the four boats in close formation, these controls were a real pleasure to operate.
Read the other stories in this package: On Powerboats – Propulsion test: inboard, sterndrive, jet The Sea Trials – Volvo 375-hp gas sterndrive The Sea Trials – Volvo D4 300-hp diesel inboard The Sea Trials – Yanmar 315/Ultrajet
Unlike the diesel inboard, the sterndrive’s rpm dropped from 3,000 to 2,500 rpm in a hard turn. A full turn to port and starboard took essentially the same time: 10.3 seconds. That’s fast, in case you haven’t tried this at 25 to 30 knots, and a little more than half the time it took the inboard. Unlike the D4 inboard, the D4 sterndrive started to accelerate hard right from 1,200 rpm, taking off like the DeLorean in the film “Back to the Future.” Acceleration to plane was 5.7 seconds, 3 seconds faster than the inboard, and time to 20 knots was 7.3 seconds, almost 4 seconds faster than the inboard with the same engine.
The smaller-diameter sterndrive props have less inertia (like a smaller flywheel) for the diesel to overcome when spinning up, which, as Willard points out, is part of the reason for the stronger acceleration. And the props are more efficient, taking the twist out of the discharge race and directing all of the thrust aft rather than in a spiral.
Both Volvo D4 common-rail diesels ran essentially smoke-free, including when starting up cold at the dock and upon hard acceleration out on the water. They also are remarkably smooth vibration-wise, with sequential common-rail fuel injection and excellent mounts taking the credit.