Diesel Sterndrive performance

[Fr. Frank]

I got to wondering, so I went to a local Mercruiser re-power specialist, and asked about diesel sterndrive performance and fuel efficiency. he is the only one I could find who has re-powered with diesel sterndrives. Here's what I got:

To date, he has only re-powered 3 boats with Mercruiser diesel sterndrives

1. Most recently, a 1978 26' Stamas Express (Flybridge, 10' beam), replaced twin 165hp I6 250ci Mercruisers/Alpha 1 drives, with CR twin 2.8L 4 cyl. 200hp Cummins/Mercruiser sterdrives, Bravo 1's @ 1.81:1, 23P props.
Performance at WOT is 39 mph, 3850 rpm, at 20.2 gph combined fuel flow.
Cruise is 28 mph at 2800 rpm, 12.7 gph combined fuel flow. He says they picked up 10 mph in top speed, using half the fuel.

#2. Early this year, a 1983 SeaRay 26' Amberjack, replaced a single 454ci 330hp Mercruiser/TRS 2 drive, with single 4.2L 350hp Cummins/Mercruiser diesel, Bravo 3.
WOT 3900 rpms, 21.8 gph, 38mph.
Cruise 3000 rpm, 10.7 gph, 31 mph.

3. Three years ago, a 1981 Robalo 21' Center Console. replaced a single jackshafted 200 hp 5.0L V8 Mercruiser/Alpha 1, with a single jackshafted 2.8L 4 cyl. 230hp Yanmar/Mercruiser turbo-diesel, Bravo 1, unknown gear ratio, 23P.
WOT is 3800 rpm, 44 mph, 14.9 gph.
Cruise is 3200 rpm, 35-36, 7 gph. (Yes, he said only 7 gph at 35 mph)
The top speed is the same as before, but fuel burn is less than half as before. He also said the jackshaft is no longer an option on the 2.8L engines since Mercruiser switched to Cummins engines.

BTW, the Stamas has already blown 2 Bravo 1 drives in just 3 months, although the owner claims he never blew an Alpha drive with the previous motors.

The only other specs I could find on diesel stern drive performance were all on OEM installs.

[GradySailfish]

That 83' SeaRay Amberjack is a TANK. It doesn't slice through the waves as nicely as a Sea Craft...it just shamses the waves out of its way ! My brother had one a few years ago. We did a near total resotore including some pretty trick upholstery. Only problem was the boat was kinda a dog with a single 350.

Here's some more stats on Cummins Merc diesels.
My 25 Grady Sailfish was originally powered with Mercruiser 470 I/O's. She cruised at 18 knots at 3200 RPM buring arounnd 12-14 GPH. Top speed was about 31 knots.

I repowered with Cummins Mercruiser 1.7 DTI Stern Drives and my cruise went up to about 20-21.5 knots at 3300 RPM.(I can cruise faster if I wanted to) At this speed I burn 5-7 GPH Combined. Wideopen throttles I run 4400 RPM at top out at the same speed as before, 30-31 knots.

My fuel efficiency doubled while giving a higher cruise speed.



Instruments 1



Instruments 2



Cummins Mercs