Dec/24 DP Question: Thruster Curves

Introduction:  There were some disagreements about thruster curves a couple months ago.  Someone asked what they thought was a simple question, but was confused by the “wrong” answers, because thrusters were more variable than he was aware.  I recently answered a related question and it made me think.  Now, it’s my turn to bow to reality.  I want to explain why it is possible to be simultaneously right and wrong, the need to test, and the fundamental need for vessel specific thrust/power charts on the DP desk.

But first an important seasonal detour:

Merry Christmas and Happy New Year to you all, and my thanks to those out on the ships during the holidays.  This also applies to those working shutdowns and keeping the world running and safe in other industries and services.

Anchoring:  Anchoring is an appropriate error for the marine industry.  Anchoring is the error of applying hard fought and proven solutions in places where they don’t apply.  They are assumptions that are more dangerous, because you know they are true, but are applying them where they don’t belong.  As Mark Twain said “It’s the things you know that ain’t so that get you.”  It’s no good being right in theory, when we need to be right in practice, and it’s no good being right in practice on the wrong ship.  Crew need time to know the characteristics of their particular vessel.  This usually requires time to play with and test it, but increased demands for efficiency often mean there is little time for this vital process.  For example, every DPO & DPE should know the relationship between the thruster power feedback and thrust control (pitch, speed, or combo) feedback.  This is where our story starts, and where it goes wrong on all sides.

Question:  Our fixed speed (FS), controlled pitch propeller (CPP) thrusters in our 2 split design are only running at 30% load at 60% pitch, but the customers have us losing redundancy at 50% load.  Please help.

30% Blind:  I anchored on my hands-on knowledge of old FS CPP thrusters and misunderstood his question.  Really old CPP fixed speed thrusters that I have worked with had 40% power at 0 pitch/thrust and more modern ones had 30% base load at zero pitch.  The last I heard, the most efficient modern FS CPPs still had 20-25% base load with no thrust, just from mixing the ocean.  So, I’m hung up on 30% load at 60% pitch - how is that possible?  Even a VSD, FPP would be at 46% power at 60% thrust (for VSD FPP thrust is proportional to the square of rpm and power to the cube, 77% rpm= 60% thrust = 46% power).  That’s ignoring the possible 5-10% base load for the VSD/transformer electrical losses depending on where the load is measured.  30% doesn’t look possible.  He can’t be looking at motor load, so maybe he is looking at generator load.

Linear/Quadratic:  My other assumption is the flat relationship between thrust and power on FS CPP thruster.  For example, if pitch is 0 then power could be 30%, 50% would be 65% load, and 100% pitch would be 100% load.  How do I know?  The power was taken at every 20% on various ships and used to check the pitch calibration.  I’ve seen in on a number of ships, so I assumed that was normal.  I’ve had the charts to prove it.  I’m wrong and have reason to think that I’m right.  Well, using my linear assumption, with a typical 30% based load, 50% base load is 20% of the remaining 70% power which is 2/7=28% pitch.  It looks like the charterer is being conservative, but I need enough spare power to support the dynamic margin and said so.

Misinterpretation:  If I had just listened to what he had said instead of jumping to validation.  I would have answered his question better.  His question was about losing redundancy because he’s at 60% pitch and far beyond the expected redundant pitch limits of a maximum of 40% pitch, but still not exceeding the power limits in the ASOG.  Being beyond the 40% pitch was critical (50% for 2 split, minus 10% for 20% dynamic margin, possibly minus other factors), because CPP pitch and thrust are generally linear, but I failed to understand the question because I had a power value that looked wrong.  Based on my experience, I didn’t believe the 30% load at 60% pitch, despite being sent blurry screen shots that I couldn’t read, and misinterpreted his question because the thrust relationship looked wrong.  I recommended taking some points on the pitch/power curve (something the DPEs should already have) and using that for his argument.  We talked past each other and politely left it hanging.

Itch:  I wasn’t happy with where it ended, and started looking for where I might have gone wrong.  Recording the real vessel specific values was the real next step, and should have already been on the DP desk.  It’s hard to detect pitch feedback faults developing, if you don’t have a handy reference chart.  So, that was good advice for any ship without them, but the question was unresolved.  I started checking my assumptions and found the following in my search:

Power Relationship:  Long ago someone asked me about the FS CPP pitch/power relationship and I told him my standard base load plus linear pitch/power relationship.  I was surprised when he didn’t use it later, but never followed up.  IMCA M140 3.6 Rev 1 talks about thruster power/thrust relationships, but wasn’t a credible source, as it didn’t differentiate between the very different characteristics of variable (low base load, VSD) and fixed speed (high base load, direct motor) CPP and provided a thrust/pitch relationship that would be subject to change depending on the pitch/speed application.  The 1.7 power relationship between pitch and thrust might apply to some VSD CPPs (if low base was ignored), but FS CPP would certainly have a different relationship.  In fixed speed, the relationship between thrust and pitch is mostly linear.  The power/thrust section actually failed to address that relationship.  M140 is currently being revised.  Searching through the MTS DP Conference papers revealed the charts above.  So, my experience of a linear pitch/power relationship wasn’t universal.  Could I be remembering a torque/power relationship?  That would tend to be more linear, but disagrees with specific details (pitch vs. power for calibration).

Unsolved:  Assuming a 1.73 power relationship between pitch and power, then expected loaded is 41%.  That still doesn’t match the reported 30%.  Worse, adding in a 20% base load (as far down as I dare go) drives it back up to 61% load.  With a 20% base load, running at 30% load at 60% pitch FS would require a relationship to the 4.5th power.  That’s unreasonable.  Variable speed, CPP is possible, but not FS CPP at 30% load and 60% pitch.  Real readings trump theory, but theory catches errors.

Application:  Do you know the relationship between the thrust feedback and the power feedback for each thruster on your ship?  It is vital to know and understand.  Feedback can lie, but power feedback and thrust feedback (pitch and/or speed) are independent feedbacks with known relationships, so incorrect feedback can and must be detected when the thrust and power feedbacks don’t match.  This requires knowing the relationship, so the problem can be seen developing, and the thruster fault safely managed.  If there isn’t a convenient table on the DP desk, then get it from the DPEs.  If they don’t have it, then generate a table during trials.

Conclusion:  I handled the question poorly, but I have been reminded not to fixate on bad data.  60% pitch isn’t redundant for fixed speed.  It might be redundant for variable speed CPP, depending on what the speed/pitch relationship is.  I also learned that my assumption of FS CPP linear pitch/power relationship was wrong.  What I really hope, is that you questioned how well you know the thrust/power feedback relationships for your ship.  If you don’t have them nailed down, then nail them down, learn them, and add a quick reminder at the DP desk.  They are really useful for finding feedback errors that endanger DP.

Request:

This is the last DP article of the month, so next week will be the Dec/24 DP Incident article.  Please message me with recent lessons that you think are worth sharing, or incidents that you would like looked at.  Don’t use the article comments, as they aren’t private.  I’m looking for general lessons based on real life incidents or observations, not company secrets. Thank you to those who provide some.