Un­der the Hood

Passage Maker - - News & Notes -

The Belize 54 I tested was fit­ted stan­dard with twin Volvo Penta IPS 800 diesel en­gines and pod drives (each max rated at 600 horse­power). When I ar­rived at the dock, Riviera/ Belize In­ter­na­tional Sales Direc­tor, Chris McCaf­ferty, had the yacht warm­ing up with the quar­ter­deck en­gine room hatch open. (Note that, be­ing at one time a rag-bag­ger, I out­right refuse to call a flush aft deck a “cock­pit.”)

Big diesel iron is pretty noisy, even at idle. And gen­er­ally, the big­ger it is, the nois­ier it is. So as I dropped down the lad­der into the Belize’s com­pact but work­able en­gine room, I won­dered whether I should have brought my ear plugs. But as my head de­scended be­low the hatch coam­ing, I im­me­di­ately no­ticed how quiet the diesels were. And any lin­ger­ing doubt about my need for ear plugs was dis­pelled en­tirely when I later ran the yacht at wide open throt­tle. But more on that later, when we talk about per­for­mance.

In the mean­time, just to en­sure we’re all on the same page, let me re­mind you that the Volvo Penta IPS in­cor­po­rates an un­der­the-hull drive leg that car­ries two for­ward-fac­ing, counter-ro­tat­ing pro­pel­lers that ro­tate through a full 360 de­grees as di­rected by a fully elec­tronic con­trol box. The drive leg is not “hard” or “close-cou­pled” to the propul­sion en­gine, but in­stead is at­tached via a jack shaft that has a uni­ver­sal joint at each end. This setup al­lows the en­gine to be “soft” (re­siliently) mounted, which min­i­mizes trans­mis­sion of noise and vi­bra­tion to the hull struc­ture. Soft-mount­ing the en­gine is pos­si­ble be­cause, with IPS, pro­pel­ler thrust is trans­mit­ted to the hull through the drive-leg mount­ing flange, not through the en­gine mounts as would be the case with a tra­di­tional setup (prop­shaft to close-cou­pled ma­rine trans­mis­sion).

The Volvo Penta IPS drive leg (called a “pod drive”) also in­cor­po­rates a for­ward-fac­ing pro­pel­ler ar­range­ment. This puts its props in clean, undis­turbed wa­ter, in con­trast to what hap­pens when they are placed in a trail­ing po­si­tion rel­a­tive to the drive leg. The re­sult is a sig­nif­i­cant in­crease in pro­pel­ler ef­fi­ciency. And that’s not all. The ef­fi­ciency of the drive is fur­ther en­hanced by Volvo Penta’s sig­na­ture twin counter-ro­tat­ing pro­pel­ler ar­range­ment.

I need to point out that the im­proved ef­fi­ciency of twin counter-ro­tat­ing pro­pel­lers is not just empty mar­ket­ing hype. When I first started test­ing Volvo Penta DuoProp drives for a num­ber of boat­ing mag­a­zines in the 1990s, I was of­ten able to ar­range to test a non-DuoProp ver­sion of the same boat at the same time. And I can say with­out hes­i­ta­tion that the dif­fer­ences in terms of pro­pel­ler bite in fast, hard turns and dur­ing ac­cel­er­a­tion to top speed from a dead stop were sig­nif­i­cant. Dur­ing my re­cent test of the Belize 54 Day­bridge, I ob­served that Volvo Penta IPS takes th­ese im­prove­ments to the next level.

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