The Sea Trials – Volvo D4 300-hp diesel inboard

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Volvo D4 300-hp diesel inboard

This boat stands out for its smooth drivetrain, with low vibrations from a fast idle on up and none of the gear chatter or whine of the sterndrive. It’s a very quiet package; the sound of the waves slapping the hull at speed are as noticeable as the engine exhaust. The builder has done a good job isolating radiated noise through the engine box and deck, with exhaust noise — as low as it is — predominating.

 

Steering at speed was crisp and responsive at just four turns from lock-to-lock. The boat didn’t have power steering, but the rudder is well-balanced, minimizing steering effort. A 360-degree, 30-knot turn took 20 seconds to port and 18.5 seconds to starboard — very good time for an inboard, which deflects, rather than redirects, prop thrust.

The Volvo D4 inboard accelerates strongly once the turbo kicks in at 2,200 rpm. We measured time to plane (defined here as when the bow starts to drop back down) at 8.7 seconds, with or without tabs. Time to 20 knots was 11 seconds. Once the turbo boost kicks in, the boat really takes off, but the gas version will beat any diesel off the line, at least for the first few yards.

Keep in mind that this model has an inline drivetrain, so the engine is in the same location as the sterndrive or waterjet models. There’s less weight at the transom, though, with no lower unit or entrained waterjet, so center of gravity is a little farther forward on this boat.

Combined with the shaft angle’s tendency to push the bow down at high speed, the boat runs at just 2 or 3 degrees of trim, and there’s no lower unit to trim the bow up to increase speed. This slows the boat a tad with the increased wetted bottom area, which accounts to some degree for the inboard’s lower top end compared to the D4-powered sterndrive. The sterndrive is also more slippery under water, and its counter-rotating props are more efficient, which accounts for the rest of the difference. That said, the 3.6-knot top end difference is less than one might expect, and the 3,000 rpm cruise difference drops to 2.6 knots, though the sterndrive’s efficiency is markedly better, as we’ll discuss.

Dockside, the inboard’s turning circle is much wider than the sterndrive’s, though it really doesn’t matter as much with the bow thruster option. The rudder is big enough to steer astern fairly effectively. The trick is to gun it to gain sternway, then back off on the throttle to an idle, lessening the prop’s side force and allowing the rudder to dig in. This boat will back and fill as well as any high-speed inboard of its size when working in close quarters. When put in gear, response is instantaneous; in about 3 seconds, the boat is doing 5 knots, with the big ZF 5-blade prop quickly gaining traction.

So why buy the inboard diesel? It’s the ultimate in simplicity and reliability, and it’s noticeably quieter than the sterndrives — including, believe it or not, the gas sterndrive — when running at speed. And fuel economy is much better than the gas sterndrive or diesel waterjet. Also, with the electronically controlled Volvo diesel, rpm doesn’t drop off in a turn. I was impressed with the inboard’s performance overall, especially its low noise (though it clunks when shifting like the diesel sterndrive), vibe levels, and responsiveness to the throttle and wheel at speed. Just don’t expect the

maneuverability or high-speed efficiency of the sterndrive.

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