sea trial results

[jim rosenthal]

I have been thinking about new propellers for Blue Note, as the ones currently fitted are what was put on the boat after her Caterpillar repower in the early to mid 90s. Because the power ratings and weight of the Cummins 370s were similar, the existing wheels were never changed out, but lately I had begun to wonder if a better set of wheels would perform better- as in: higher cruise for a given rpm level, better fuel economy, smoother running. As a first step General Propeller in FL asked for some information on what size are currently on the boat, and what the performance levels were.

The current wheels, which are rather old but have been tuned up by PropScan shops a few times, including about two years ago, are 21.25" diameter Michigan DynaQuads, mod cup, pitch 21.50". (when we redid the PSS seal bellows, the shafts were taken out and checked for straightness as well as tuning up the wheels)

Performance today was as follows (we did two runs back and forth to average out wind and current, although there wasn't much of either; we did the runs at slack peak tide, just by luck)

1) Speed 2125 revs Port boost 8 lbs, EGT940, Stbd 7 lbs EGT 890 Speed 18.8 knots
2) " 2275 " " 11 lbs " 925 " 10 lbs " 880 Speed 21.9 knots
3) " 2450 " " 13 lbs " 920 " 13 lbs " 875 Speed 24.2 knots
4) " 2600 " " 16 lbs " 910 " 15 lbs " 880 Speed 25.7 knots
5) " 2800 " " 14 lbs " 910 " 14 lbs " 880 Speed 27.2 knots*
6) " 2950 " " 17 lbs " 910 " 16 lbs " 880 Speed 28.5 knots
7) " 3150 " " 24 lbs " 910 " 26 lbs " 880 Speed 30.2 knots

We also did a return run, over the same track, immediately, and figures were essentially the same within a few decimal points.
* not sure I am right about this as I would not expect boost to drop two pounds per side with the boat going even faster. there may have been enough vibration that I misread both gauges. I wonder.

This is pretty creditable performance, I would think, for a nearly fifty year old hull, albeit with modern Cummins diesels. I am interested to find out whether it can be improved upon- my definition of an improvement would be a higher cruise speed for a given set of rpms. I don't run the boat at WOT (I can't recall the last time I did before today, although it's nice to see she matched her prior performance on her sea trials when the Cummins engines were installed)

Comments? Someone will, I suspect, tell me to leave well enough alone, and in fact I may do that, but I am curious to see what's out there.

As far as the difference between the Turbocator readings port to starboard, I am not sure they are significant. One engine has always run a little warmer than the other on EGT, but since (I think) they are in the normal operating range, I haven't worried about it.

[Walter P]

Hi Jim,

Most can only dream of those performance numbers. You seem to have a little extra time on your hands
and a few extra dollars burning a hole in your pocket. Remember the old saying...."If it ain't broke, don't
fix it..


Walt

[Sonic Boom]

I could only dream of those numbers!

Out of curiosity, does the engine with the warmer EGT's have an extra bend or two in the exhaust routing since these are inline 6's and have asymmetrical exhaust?

[jim rosenthal]

You know, I am going to have to look when it cools down in there. That never occurred to me. Maybe there's more back pressure on that side. I think you may be right. How about that!

[Finalee]

Nice numbers.
My Tiara with 330 Cummins weighs 13000 lbs.
Cruise 2400 rpm - 26 knots.
Max 2950 rpm - 33 knots.

Egt p- 880. Stbd- 925