Okay, “performance” is kind-of a misnomer when dealing with a 15,000-pound cruiser; the Christina Rose is all about comfort. While inboard-outboard (I/O) stern drives are generally 10-15% more efficient than inboards, Christina’s low-speed handling with inboard mid-ship props and rudders is outstanding, completely un-matched by her I/O sister-ships. Pod-drives can be as much as 50% more efficient than inboards with similar low-speed dexterity, but knock just one pod off the bottom and wait for that $20,000 repair bill! Of course, the 2004 Cruisers Yachts 3372 and 3375 was never offered with pods and her classic inboards have stood the test of time…
Christina received professionally-built engines in 2013, to replace her tired (and failing) remanufactured MerCruiser 350 MAG MX 6.2L MPI Horizon Inboard twin small-block motors (model# 30620102S, catalog# 884718). Maryland Performance worked with us to optimize everything, solely with the purpose of efficiency and reliability in mind, not with a focus on performance or top-end speed per-se. New bearings, new lighter but stronger rods and valves, high efficiency roller rocker arms, plus high-efficiency oil pumps. We took the crappy 1-angle ground stock heads then ported, polished, and finished with a 3-angle exhaust-valve grind plus a 5-angle intake-valve grind to optimize air-flow and keep the valve-seats clean. We installed new springs to make sure those new pushrods and valves performed flawlessly. We splayed the crank-caps to reduce crank movement and installed external engine-oil coolers to further increase reliability and longevity of the Rose’s engines.
Those MX 6.2 MPI Horizon engines were originally specified as 320HP by MerCruiser, with a listed WOT of 5200RPM, though Christina never achieved over 4500RPM on either engine with both throttles firewalled. Christina runs a set of left-hand and right-hand Michigan 19×24 (19″-diameter, 24″-pitch) Quad-blade bronze props through a set of Borg Velvet Drives with 2.5:1 ratios:
When Maryland Performance finished with the engines, each was churning out over 370HP with no changes to compression or bore, the result of airflow efficiency improvements and reduction of rotational friction. With her new engines, Christina achieved 5100RPM at WOT on her shakedown cruise, fully-loaded with 4-crew, gear, fuel, and water… operating SIGNIFICANTLY quieter and with less vibration than the original engines. Cooler, too.
As part of their professional rebuilding services, Maryland Performance (http://www.marylandperformance.com) performed break-in of the new engines on their test-stand dynamometers for 4.5-hours, which is the absolute-best way to ensure proper break-in of new engines. Here’s a look at their detailed performance information for Christina’s MX 6.2L MPI engines now:
|STBD Engine||PORT Engine|
So, what do all those RPM-numbers mean in the real-world? Here are some of Christina’s performance measurements taken since rebuild (always populating more points, so stop-back often!):
|Dual RPM||Speed MPH||Fuel Burn Gal/hr||Econ MPG||Water PSI||Oil PSI|
A bunch of time was spent trying to gather data for economy verification whether running on 1-engine or 2-engines could save or cost fuel consumption, thanks to a disagreement with a fellow Cruisers Yachts boating friend, Ken. Ken said, “running on one engine is more economical” while Christina’s captain believed “running on two engines was more economical” because of a couple data-points seen during previous operation of the Rose. As it turns out, we’re both right (and both wrong)… here’s why…
Apparently, operating one engine at full-displacement speeds (no-wake) can see better fuel-economy than the same speed running both engines, but that difference is only for speeds much-below 6-knots. As speed increases above 6-knots (for the Christina Rose, anyway) the increased RPMs combined with what must be prop-slip works together to perform worse for a single than two slower-spinning twins. So, Ken was right below 6-knots and Captain Mike was correct above 6-knots. Here’s some data:
|Single Engine RPM||Dual Engine RPM||Speed Knots||Single Fuel Burn Gal/hr||Dual Fuel Burn Gal/hr||Econ Sing nm/gal||Econ Dual nm/gal|
The trends of single versus twin engine operation shows different slopes, and twin-engine economy catches up to single-engine operation around 7.5-knots.
There are five potential problems with intentionally peg-legging the boat though:
1) There may not be ample water flow to the shaft-bearings (which are spinning with the un-powered propeller)
2) The un-powered transmission (Borg Velvet Drive) is spinning, without cooling
3) The dash-gauges still require power for monitoring, which means the un-powered engine’s computer (key) should be on, which runs down that battery
4) The generator then has to be operated (consuming between 0.2 and 0.3gal/hr) to keep the un-powered engine’s battery charged
5) Running one engine requires that engine to work harder and at a higher RPM for the same speed (300-500RPMs on average)
but, fortunately all those issues are relatively minor, especially when peg-legging because you have to!
Happy Wakes! Feel free to drop me a note anytime, ChristinaRose@edickent.com