New Engine, The Proof is in The Voyage
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John, What do you see as the advantages/disadvantages of a high torque/low revving engine? It sounds as if one of your reasons was for fuel savings which you are pleased to have accomplished. To what degree do you attribute the fuel savings to the low rev/hi torque engine vs the new engine just being 25% smaller? I am also interested by what you mean when you describe the Perkins as an “industrial” engine? Thanks, Dick Stevenson, s/v Alchemy
Hi Dick,
The key to the fuel savings is, as I understand it (I’m not an engineer) the torque curve of these engines which matches the drag characteristics of displacement hulls better than that of higher revving lighter engines. More on that here and here. In addition a lower revving engine will generally, all other things being equal, be more efficient than a screamer. Here is a comment from Conny Harlin, who is head of the power-train division of a large engine maker (not Perkins) that explains more about this.
When I say “industrial engine” I referring the fact that the Perkins engines are based on engines designed for industrial use where they will be required to run at or near full load for their entire working lives driving something like a generator or big pump. Marine use, on a displacement boat with a properly sized gear and prop, is much more like industrial use than the intermittent use that many lighter high revving engines are designed for.
These industrial engines are designed to run at high load for at least 10,000 hours before replacement or rebuild, whereas many recreational high revving marine engines will be at the end of their lives in 2000 to 3000 hours.
Dear John, I would like to comment on your statement that parts are readily available by couriers like FedEx. Some parts of the world that may work well, but in many of the countries I have spent time, Customs waylays the delivery and puts complications/expenses in the mix that effectively undermines delivery. Many countries, even 1st world, the best advice, (as absurd as it sounds) for important items, is often to fly out yourself, get the part and return with it in luggage. “Yacht in Transit” usually makes no difference to officials as well. So couriers can and do get the part to the country with speed only to have actual delivery become infuriatingly elusive behind a web of rules, fees, forms and protocols, always in a different language, and usually in a different location from where you are. Dick Stevenson, s/v Alchemy
Hi Dick,
A really good point. What I was saying, not very clearly I might add, was that Perkins seem to be able to provide pretty much any part quickly via courier if the local distributor does not have it in stock. But, as you say, that does not solve the problem of uncooperative and even corrupt local customs officials in some countries. That is one of the reasons we carry a huge inventory of spare parts. But, when all else fails, going to get the part, as you suggest, may be the best option.
Nice to hear that you guys are happy with your “Perkins” with low rpm /high torque layout. You see what a change in fuel-consumption that makes….I would also like to add that a low rpm engine last longer before it’s time for overhaul due to wear of bearing,bushings,seals.
And the best is “No worries about Turbo”failures!!!
ZF is a German quality brand and have been around in the industry like Perkins.
I do like your engine room,,clean, organized,and a NO bilge “Crap” floating around.
I’m glad that you guys are happy with your investment of power ,,,,Conny, S/Y Crusader
Hi Conny,
Thanks very much for the kind words and good information of low revving engines and ZF.
(Conny knows what he is talking about because he is the head of the power-train division of a large manufacturing company—not Perkins/Cat, as well as a boat owner)
Hi John,
Your mention that your Perkins has a 24v energize-to-run solenoid switch certainly caught my attention! Let me describe the events of a delivery from hell, or more specifically from San Diego to Costa Rica. The boat was a 54′ sport fisherman powered two brand new MAN V-10’s. Upon arriving at the fuel dock in Cabo for our first 1200 gallon refill, we were unable to get the engines to restart. One engine would start, but as soon as you tried to restart the second the starter draw killed the first. The “yacht’s” tool kit consisted of a couple of rusty Craftsmen socket wrenches, and as it turned out the owner had run out on an outstanding bill in Cabo so no mechanic would touch it. Next morning everything was back to normal, so after scratching our heads we toddled on south. When we shut the engines down at our next stop we were again dead in the water. The background cause probably stemmed from needing to run the engines at too low a RPM for the regulator settings as we were trying to save fuel and make long runs. (the difference between 8 GPH and 80 GPH on a boat of this type is a mere twitch of the wrist!)
Here is the sequence of the problem: The engines will not run unless they are receiving the full 24 volts required to hold the fuel supply solenoids open. The boat had a generator that runs continuously producing 120v. This in turn was set up to charge the house 12 v battery bank through a charger unit and charge the 24v engine bank through a separate charger. Engine mounted alternators also charged the engine battery bank. There was no crossover switch between the house and the engine batteries, as this would have involved a voltage conversion.
In this set up either a deteriorated battery bank or regulator or 24 v charger failure creates a power boat with zero horsepower rather than 2400 hp! We finally ended up disconnecting the house bank, re-jumping it to produce 24 volts, jump starting the engines from it, and getting enough output from the alternators to bring the engine bank up to 24v.
The take away lesson:
If you have an energize to run propulsion engine system, you MUST have a back up energy source capable of independently producing the juice necessary to keep your engine running. If you depend upon an inverter I’d want two installed in parallel ready to switch on immediately, powered from two separate battery banks. If you are sailing to remote and challenging locations either install a generator as the independent energy source or choose an older engine design that can run independently of electricity.
Hi RDE,
Sounds like a horror story on a way too complicated boat!
While I wish that the Perkins had a energize to stop solenoid running on 12 volts, I don’t really feel that the way the Perkins is set up is that big a threat to reliability. We have a bunch of different ways to make or get 12 volts including the house bank, an independent engine battery, three separate chargers running off the generator and the generator start battery itself. Also if we did not have 12 volts the solenoid would be the least of our problems since we would not be able to turn the starter motor and start the engine—a limitation that applies to most engines out there since the demise of hand cranks. We do carry a spare 12 to 24 volt converter (and a spare solenoid), but I would not wire it in parallel since both the service unit and the spare could be damaged in, say, a lightning strike, and changing the unit is a matter of five minutes (two screws, one plug).
In summary, while I think Perkins made a design error in this area, few things are perfect and I don’t feel that it is a problem that disqualifies the engine.
Hi John,
Sounds like you have adequately thought thru the redundancy problem. Why am I not surprised!
One thing I did find surprising on the MAN installation was how little voltage drop from full charge was required to shut the solenoid.
Cheers,
Richard
ps; If you think the sport fisherman was overly complicated you’d love the 112′ Sparkman & Stephens twin engined, active stabilized, triple gen set sailboat I once managed the build for! In fact all boats that try to become floating houses are too complicated, with the possible exception of Steve Davis ‘s designs.
Dear John,
Thanks for the clarifications which, for me, inexorably leads to more questions. Beta and Nanni engines are based on Kubota blocks, some Westerbeke on Mitsubishi blocks. Would you consider them “industrial” engines? The only “built for marine use from scratch” engine that I am aware of is Yanmar (unsure about Volvo, Vetus etc.).
I could see a high revving engine only getting 2-3000 hrs on a boat where there is only 50-150 hrs a year on the clock. Many (or most) sailboats likely fall into this category and they would likely figure they got their money’s worth at 20 to 30 years of use. Those of us who live aboard and go significant distances often clock up 500 and more in hrs. per year. Many (again likely most) have higher revving engines and I do not know any of my cruising friends who believe they will get so few hours out of their engines.
As for fuel economy, I would be very curious as to the actual figures comparing a well set up high revving engine and well set up low revving engine. I suspect it is not all that great for the average displacement sailboat. A conundrum is the fact that basically, lower speeds save fuel dramatically (as measured miles per gal.), but the engine “likes” higher speeds. (My Westerbeke 42B uses approx 0.55 gph to push me 5.0 knots and 0.95 gph gets me 6.0 knots—4500 hrs on eng). I almost never go optimal speed/rpm for the engine (70-80 % of power available if memory serves which would push me approx 6.6 knots) as the fuel usage just shoots up off the charts (mpg drops comenserately). I am not sure how to get around this we all want “reserve” power for emergencies and those times when it is important to pound our way up-current, up-wind and up-swell/wave.
Thanks as always for your thoughts, Dick
I’m really not enough of an expert to get into this level of detail with any authority. But I think the key issue here is just the one you identify: With a high revving engine it is very difficult to load it properly in a sailboat and still have enough power for all conditions. The result is the insanity we often see with say a 75 HP 3600 RPM engine in a 45 foot boat where no more than 40 HP at about 2400 RPM will every be used and the same boat would do a lot better with a 40 HP low revving high torque engine, assuming you could find such a thing these days. (My first draft suggested a Beta, but then I found that their 38 HP revved at 3600 RPM—woops.)
I guess the real bottom line for me is how many commercial vessels or industrial generators do you see driven by high revving screamers? Precious few. In fact Yanmar themselves will not even sell an engine to a commercial lobster boat in Maine. So, yes the weekend sailor that does 150 hours a year will be fine with a screamer, but those of us that put commercial equivalent numbers of hours on our engines might be better served to consider the engines that the pros use.
As for fuel efficiency comparison of two well set up engines, one a screamer the other high torque, I guess I would just defer to Conny Harlin on that one (see the link on my last post).
The bottom line for us is that the Perkins has given us really good fuel consumption at low revs (we can go down to less than a gallon a hour at 6 knots if we must) but will still push MC into a 25 knot breeze with a big chop, it just works for us, and way better than the higher revving Cummins ever did.
Hi John,
I’ve often seen identical block marine engines (including Yanmars) with two different power ratings and service recommendations available from the factory. Often the only difference is the governor settings for RPM, and perhaps the injector pump delivery mapping. So I’m not sure how much difference there really is between a “low speed” and a “high speed” engine in these smaller sizes.
That “yacht rated” 3600 rpm 75 hp engine you mention will in fact produce about 40hp at 2400 rpm. If it were coupled to a controllable pitch propeller like we sometimes use in large sailing vessels it might well run quite comfortably loaded up to work at 2400 rpm and last as long as the same engine rated at 40hp, assuming it has electronic sensor controlled fuel mapping. In order to use the available 75 hp. you need a way to change the propeller loading on the fly.
Choosing a broad torque curve and a simple, non-turbo design like the Perkins gives a long engine life and the widest usable power band that can be absorbed by a fixed pitch prop, even though it might theoretically be outperformed by the set-up I just described.
Hi RDE,
Great comment, as you say that’s the bottom line. Sure there are other ways to deal with the intrinsic inefficiency of many sailboat power trains—variable pitch props, diesel electric (in theory)—but a low revving engine with a flat torque curve like the Perkins is the simplest and least expensive. And it has the added advantage that low revving engines, all other things being equal, last longer too.