Triple IPS vs. Twin Diesel Inboards

Last week we tested the new Cruisers Yachts 520 Sports Coupe in Lake Michigan near Green Bay, Wisconsin. As you read this
Captain Rob Smith is going over his notes and the videos our crew shot of the new
boat and is writing his Test Report. He just emailed the test results which we share
with you. This is the first time we have gotten an almost apples-to-apples
comparison of a twin diesel inboard drive installation versus triple IPS. The results
are surprising, but perhaps not in ways you would expect.

The IPS system is supposed to be 35% more efficient,
right? Right, if you are replacing twin 550-hp diesels with twin 435-hp IPS. But
what if you are replacing twin 715-hp diesels with three 435-hp diesel IPS drives?
In this case we have a total of 1430-hp and two props being replaced by three engines
totaling 1305-hp driving six forward-facing props. The boats weigh about 42,000-lbs.,
with the Sports Coupe's added weight counter balanced somewhat with the IPS engine
package which is about 1,000-lbs. lighter than the two D-12s they replaced.



We said “almost” apples-to-apples because there usually seems to be little differences
in conditions or weights that make lab-like comparisons impossible. In this case,
when we tested the 520 Express it was 92-degrees and there were 4 people aboard.
Capt. Smith tested the Sports Coupe in 76-degree weather and there were three people
aboard.

Data from last week’s test of the Cruisers Yachts 520 Sports Coupe with IPS.

Results at "Best Cruise"

Where is “best cruise” and what constitutes it? We usually think of this designation
in terms of most miles per gallon at planing speeds. In the case of the IPS-powered
520 Sports Coupe that would be going 22.5 mph, getting .78 mpg. When we look at the
520 Express, we see that a technical definition of “best cruise” has us going 32.9
mph, getting .70 mpg. How do you compare that?

Test data on the 520 Express with twin 715-hp diesel inboard engines.

Under that definition of “best cruise” the IPS
Coupe is 11% more fuel efficient but a lot slower. At a comparable speed to the
inboard diesel – 31.9 mph at 3000 rpm  vs. 32.9 at 2000 rpm– the IPS Coupe gets .76 mpg, a 9% improvement.
We’d say that each individual owner should take a look at the performance tables,
decide how he boats, and then select the applicable comparisons.


Big, heavy boats like the 520 simply take a lot of horses to move them around at
speed no matter what. In such boats we like traveling at about 18 mph (16 knots)
when making long runs. Using that speed as our benchmark, we would get .78 mpg in
the IPS 520 Coupe and .58 mpg in the conventionally powered 520 Express version
– a 34.5% increase in fuel economy!

Results at WOT and Slow


The Coupe has a WOT speed of 41.4 mph, getting .66 mpg. The Express version with
the inboards goes 40.8 mph, getting .58 mpg. The IPS version is slightly faster
and uses slightly less fuel.

New 520 Sports Coupe has pleasing lines and clever features.

Because of the recent spike in fuel prices some owners
are learning the joys of going slow and smelling the daisies. At 9.2 mph, you’ll
get 2.67 miles per gallon with IPS version, compared with 8.8 mph at 750 rpm and
1.86 mpg with the twin diesels inboards.




Where the IPS Shines



In the time-to-plane and 0-30 measurements, the IPS model won hands down, being
12.9 seconds faster from standing stop to plane. The IPS-powered 520 was 17.2 seconds
faster 0 to 30, than the conventionally powered sister. This certainly proves that
6 forward facing counter-rotating props on pods are faster than two conventional
inboard props.



Then there is the joystick aspect to the IPS drives which, of course, the conventional
drive does not have. Builders tell us that in the larger boats, nearly all of their
orders are for boats with joysticks no matter what the engine performance.

Capt. Smith’s full test of the 520 Sports Coupe coming soon with video.

More To Come



For people who are looking for their last hurrah in boating, we think this boat
deserves serious investigation. In the weeks ahead we look forward to what our Director
of Testing, Capt. Rob Smith, has to say about the boat. Also, we will be doing something
unusual for us -- getting a female’s perspective of the 520 Sports Coupe in a special
three-part video. So stayed tuned to BoatTEST.com as we carefully examine every
aspect of this boat from several points of view.

We will be giving you a female's perspective of the new 520 Sports Coupe with video coming soon.

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