Propeller Pitch
“Finally Chinese New Year”, the workers at the shipyard must have thought. The last days before the holidays appeared busy, at least there was a lot of work to be done on my boat.
One of the many things Seahorse Marine had been working on last days was getting the engine up to proper speed by adjusting the propeller to the correct pitch. Wait, adjusting pitch?
It all had started well back in the year 2016 when I met Bill, a lovely British chap whose 55ft Diesel Duck was already well under construction.
Before that day I hadn’t looked up the engine issue any closer. I always thought to go with some sort of standard Tier IV engine, displacement somewhere between 4 and 5 liter, producing roughly 150Hp. Maybe a John Deere or Iveco. But again, that was before I have met Bill:
Gardner 6LXB – mind blowing
Bill had sourced an engine with stunning 10.4 liter displacement while producing “only” 127Hp. During our visit at the shipyard he showed me the beautiful beast which had just been delivered from UK: a shiny, polished, huge engine block from Gardner. The Gardner 6LXB blow my mind. No electronic control box, no turbo charger. Pure mechanical control!
Back home continued homing in onto the Gardner 6LXB. I read reviews, watched – no actually listened – to the sound of that engine on YouTube videos.
Decision made, engine ordered. At least that was what I, as newbie, thought. The other Bill, this time the American pro-forma owner of the shipyard, wanted the engine as soon as possible to keep my project going. As I found out later, the project would be delayed for years on grounds of whatsoever reasons anyway, but that is a totally different story.
So, American Bill wanted the engine at his shipyard as soon as possible, and I wanted to know which gearbox I should consider to get attached to the engine which was on order with Gardner marine already. So I inquired which sort of propeller Seahorse Marine intended to install on the boat and which gearbox ratio to use. The only reply I got from Seahorse Marine: “We will get to that later”.
Studying boat design – missing support from shipyard
“We will get to this later”, this kept on going on for weeks: Seahorse Marine wanted the engine as soon as possible but did not suggest what sort of gear box ratio to use.
At this stage I was very inclined getting a controllable pitch propeller. Especially on this sort of boat, with sails to support nice downwind runs, a controllable pitch propeller would have been perfect: Able to run the engine at optimum RPM and the propeller pitch adjusted for ideal load. I contacted various companies producing these very useful but also very expensive pieces of metal. Unfortunately I was not able to find a matching gearbox – shaft – propeller combination fitting my needs. It set me back to square one.
At the end I bought a book recommended to me: “Preliminary Design of Boats and Ships” by Cyrus Hamlin. Chapter 12 is dedicated to “Powering”. This chapter gave me a crash course and rough idea about optimum propeller size and optimum propeller RPM within a few pages. With boat weight, dimensions and engine parameters on hand I consulted numerous tables. Conclusion was that a 31″ propeller running at 700RPM should do.
As the engine is rated for up to 1,500RPM a gear box ratio of 2:1 was required. So I asked Gardner to purchase and attach an appropriate gear box from ZF.
Avoid Vibration
In the meantime I had heard a few good as well as a few bad stories about the shipyard. One story was about a 55ft Diesel Duck which had immense vibration problems. All yachts which had been finished by the shipyard had common universal joints to absorb smallest misalignment between gearbox and prop shaft. Delving deeper into the topic of vibration problems on yachts I figured I might reduce the probability of me encountering similar troubles by a specifically designed gearbox linkage as well by having a propeller with five blades.
A gearbox linkage was ordered from Germany and built to specification: The Centaflex AGM coupling should be suitable to reduce transmission of vibrations occurring at the prop and prop shaft to the engine block and on to the hull.
Pitch Adjustment Bolts
These a Pitch Adjustments which are limiting the pitch of the propeller in operation. Standard delivery of the Max Prop Whisper is with three adjustment screws for forward gear and three screws for reverse gear. Changing the bolts limits the angle which in turn sets the propeller pitch. These screws can be changed while at anchor… provided you have dive gear on board
Regarding the five blade propeller I chose a Max Prop Whisper. This propeller also has the huge advantage as the pitch is not fixed: A screw of particular length presets the propeller blade angle which in combination with the propeller diameter gives you a particular propeller pitch. Sounds complicated? Well, it isn’t. But it comes in very handy if you do not have a shipyard supporting you with the details they should. The study material I had on hand taught me the optimum RPM of the propeller and the optimum size, but it did not come down to the exact propeller pitch.
This is where another book came in handy: “Voyaging under power” by Robert Beebe.
One discussion point in this book is about propeller slip. With an anticipated propeller slip and anticipated cruising speed I was able to “guesstimate” the propeller pitch.
Gardner 6LXB is a fine engine – although it won’t accelerate my 50 tons trawler to warp speed
But I made a mistake: I was being overly optimistic regarding expected cruising speed of the vessel: I was oblivious to the truth: a mere 127Hp won’t speed my 50 tons trawler up to the fairy tell warp speed of 9kts at 1,200 engine RPM. The Gardner 6LXB is a fine durable engine, but not a Star Trek warp drive.
So last weeks, when Seahorse Marine did the first sea trial, I was robbed of my illusions: The engine achieved only 1,100RPM, well short of the anticipated 1,500RPM.
I went back into my books – and came back down to earth regarding expected cruising speed.
Then I asked Seahorse Marine to reduce the propeller blade angle by 4 degrees. This in turn reduced the propeller pitch from 23.6″ to 19″.
And the next sea trial worked as planned: 1,400RPM and 7.8kts. That was definitely more realistic than warp speed.
On a site note: Vicprop has a brilliant propeller sizing calculator on their website. It does not go up to a five blade propeller, but it would have given me an realistic idea of what to expect. Just interpolate and extrapolate the numbers you receive out of the calculations.
Once we start cruising I will report back how my drive train and propeller setup with propeller pitch work under real life conditions.
Until then, wish you all a Happy and Prosperous Year of the Tiger 2022.