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.
If you can find a BSFC map for your engine, you can find exactly where it (the engine, not the hull) will be most efficient. Often around 2200 rpm.
Boats need quite a bit of extra power for when there is a major storm – wind load adds substantial drag that has to be overcome. If that will never happen to you then yes, size it to cruise exactly at the optimal point on the BSFC map.
Hi Jon,
Thanks for the comment. I have to admit that I had to look up what a BSFC map is. For those who are equally challenged, here is the link http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption
We found that, even when it was blowing hard, the extra power of the Cummins over the Perkins just was not much use to us. We could give her more throttle, but it did not make a lot of difference.
I guess if we had had a larger ratio on the transmission and a bigger prop than we could have used more of the Cummins’ power.
In summary we are pretty happy with the 87 hp Perkins and have no regrets about going down in size. We also think there are a lot of sailboats out there that have way more power in their engines than they are using.
Times change though and now John has in his latest post refined his choice of engine and criteria but before the result of the HYMAR project was now out? 41 pages, but the wealth of data collected on both engines and props is invaluable. Nigel Calder kicking it of on a Malo. It makes interessting reading and may re-inform John on his current choices/criteria for the Adventure 40? http://www.autoprop.info/Latest_News/pdfs/HYMAR.PDF
I find this document rather confusing, looks like they didn’ t really qualify requirements (sailing boats or motor-boats, blue-water type or day-cruisers etc…), they have done something of a mess between theorical points (engine efficiency, propeller efficiency, electric motors/generators/converters/batteries efficiency, computer control & supervision, series hybrids, paralell hybrids….) “classic” solutions or implemetations for/of thoses points, and, a few non-classic offerings, or prototypes, by 3 small established providers : Steyr, Mastervolt and Brunton in a way that is really difficult to read because of this mess.
They conclude by saying that mastervolt prototype series DC to DC system, including DC generator, DC converter, DC motor and acid-lead batteries, isn’t very efficient (a “new prototype” of a T-Model Ford the way they did them in 1908 wouldn’t be either…) and that the Steyr parallel hybrid offering (fully non-classic and I guess rather expensive) is cheaper and more efficient than a really modern “classic” (AC!…) series system they didn’t test and they don’t even speak about, although this kind of system in very common, or the rule today, in large commercial cruising-boats and cargos and can be downsised without much technical problems.
Looks like what we call “Noyer le poisson” down there (in English: “Drowning the fish”)
Laurent
I just found this and calculated my numbers based on our 19″ prop (a four-bladed VariProp which we’ve found superior for maneuvering at low speeds) and a 1.88: 1 ZF25M hydraulic transmission. Fuel efficiency RPMs and hull speed seem quite accurate and while I have assumed I was lugging a bit, now I’m not so sure as the numbers hew closely at a pitch of 15 to what we are seeing in flat water. Thanks, John…maybe I’ll hold off on flattening the pitch a notch!
Hi, John: I am about ready to replace my generally satisfactory Perkins 4-108. Water pumps are beginning to fail and I fear interior parts may be next. It is 43 years and thousands of hours old. It always starts and runs well, although is beginning to leave lots of soot on the transom. But your series makes me fear getting a new engine and ruining it. Anyway, I have two questions. 1. Can you open your prop tip speed spread sheet so I can use it? It denies me access. 2. What is your view of the Yanmar 45 hp common rail, computer controlled engines. I hesitate to have that complexity on the boat, but a friend just installed one and it nearly doubled his fuel efficiency. And would the new technology help with the loading problem? Thanks, as always.
Hi Terence,
If it were me, I would replace the 4-108 with a Beta marine. Colin writes about just that here: https://www.morganscloud.com/2020/05/03/planning-and-budgeting-a-refit-the-engine/
That’s exactly what I did and I’m very happy with our Beta 43!