Our old engine was a Cummins six cylinder naturally aspirated (no turbo-charger) 5.9 litre diesel that theoretically put out 120 horse power (HP) at 2800 RPM. (I say “theoretically”, because we suspect that it never did develop its specified horse power or torque due to the blow-by problem.) It swung a 21 inch MaxProp through a Velvet Drive 72 transmission with 1.88:1 reduction gear.
I’m sharing all this mind-numbing detail because it represents an essential starting point for what turned out to be a complex multi-factor decision that many other boat owners will eventually be faced with.
John: this was an informative and well thought out post— thank you. I agree that there seems an irrational trend that more HP is better and I agree with your conclusions. One thing I would add is that while the MaxProp on our Hinckley SW 42 was acceptable, we found that the AUTOprop self pitching propeller (imported from Bruntons) gives much more power and speed for a given RPM since it pitches according to load. We have had lots of friends who tried it to good effectiveness on their vessels also.
-PRW.
Hi Pete,
A really good point on the AutoProp. I have also heard that they are particularly efficient when motor-sailing since their pitch automatically gets coarser under light load. I would also wonder if they are less susceptible to the tip speed problem than the MaxProp.
On the down side, am I not right in thinking that they require a shaft lock to properly feather, that is unless you have a mechanically activated transmission that will lock the shaft in reverse? In addition I have heard at least one story of an AutoProp shedding a blade, whereas I have been using MaxProps for some 30 years without any problems at all.
Having said that, I think that after we recover from the present project we will have a closer look at the AutoProp.
John:
You are right that they do require you to stop the shaft from turning. On PATIENCE, we have a mechanical transmission which does that when we put it into gear. Other friends of ours use a separate shaft lock.
We too heard some stories some years ago about a blade issue on some props. In speaking with Steve Armitage at AB Marine in Newport, RI (the source for all things AutoProp!), we learned that there may have been some manufacturing issues at Brunton’s —now solved. We have used our Autoprop for 11 years on PATIENCE with no issues; our old MaxProp is in a box in the forepeak!
Best,
-Pete.
Ending your note with: “Next post we will look at the options in engines that were available to us and the one we chose,” is like the hook at the end of an episode of the weekly serials shown at the movie theaters where, as a kid, I spent my Saturday mornings. I can’t wait for the next episode when (hopefully) I’ll find out the winner in the engine sweepstakes.
Hi Westbrook,
It is all part of my evil plot (to borrow more from Saturday serial movies) bwaa ha ha ha ha!
Great thinking on a complex decision making subject. My feeling is that since marine diesels like to be worked hard, there is no real need for more than a minimal ‘power reserve’. You have not touched upon simplicity. Nowadays marine diesels have become far too complex and way too much reliance is placed on fatally flawed ‘cpu’ technology that simply cannot be repaired at sea. I eagerly await the next episode. My guess is a mid range 4045 John Deere.
Hi David,
The complexity issue with many of today’s engines is a really good point.
More on that in the next post.