This story is kind of cool. Nordhavn posted on their site an article about one of their salesman’s sons building his own Nordhavn 68, out of Legos!
And.. .on a different topic:
Yesterday afternoon, two of the other boats from our Siberian Sushi Run, Seabird and Grey Pearl, met with our “Siberian Connection”. We’re still working out the details of our arrival, touring and fueling, in Siberia. I’m in Cabo, so I had to miss the meeting. This morning, in my email, were these photos showing the terminal in Kamchatka. Reading between the lines I’d guess that our guys had questions about the terminal we’ll be arriving at.
My original plan for today’s blog had been to write about my hydraulic system. It’s the part of my boat I know the least about, and I figured that writing about it would give me the momentum to dig in and study. What follows will be dull to all but the most techie of you.. so, unless you have a high tolerance for pain, I’d suggest you stop reading here.
OK, still with me? Well, then let’s take a look at my hydraulic system, which controls a few things on my boat:
– Stern Thruster
– Bow Thruster
– Anchor Windlass
– Anchor Wash pump
– Hydraulic Bilge pump
– Hydraulic alternators
Under normal operation, while underway, there is only one item controlled by the hydraulic system: the stabilizers. The thrusters, and anchoring windlasses are only used when dropping anchor or entering a port.
There is, however, one other device, or pair of devices, that might be in use at all times: the Hydraulic Alternators. My boat has two hydraulically powered alternators which generate a combined 12kw of electricity. On my prior boat, an N62, I relied heavily on the hydraulic alternators while under way, and almost never ran my generator. On this boat, I did some early testing of the hydraulic alternators, and decided they consumed more fuel than just running the generator.
I’m now thinking I should take a new look at running the hydraulic alternators. It may be that even if the fuel burn is slightly more than the generator, that the alternators are still preferable. Here’s why: Generators need a lot of love and attention. Every 200 hours, you need to replace the oil. I tend to run a generator around the clock, which means I am changing the oil about every 7-8 days when cruising. This means LOTS of dirty oil to dispose of, and dirty filters, and lots of clean oil I have to keep around. And, of course generators also need new impellers from time to time, and new belts. The great thing about the hydraulic alternators is that you don’t need to maintain them. They just pump out electricity, as long as your main engines are running. Also, since I have two, I can decide if I want to run at full, or half capacity (run just one, or both alternators).
Nothing is free though. The hydraulic system is powered by a pump that sits on the back of my main engines, just behind the transmissions. This pump creates a drag on the engine, and it creates heat in the drive train. With a little creativity, I can compute the drag on the engine, caused by the hydraulic pump. There is a formula that converts kilowatts to horsepower. Each kilowatt is equivalent to about 1.34 horsepower. In other words, to generate 12kw puts about a 16 horsepower drag on my motor. Of course, that would be true if the alternators were 100% efficient. My guess is that their efficiency is closer to 80 to 90%, and gets less efficient at lower rpm. And, this is only the efficiency at the pump. There is also some inefficiency in moving the oil around the boat, and in converting that power to electricity within the hydraulic alternator. And, to really complete the story, the hydraulic alternators are 24v dc current. If I want 240v ac current, then the power needs converted by my inverters, which, you guessed it, adds another layer of inefficiency. My guess would be that to get anywhere near 12kw, I am actually creating a 20 or more horsepower drag on my main engine(s).
Where is the missing power you might ask? If I consume 20hp but only use 16hp of it for conversion to electricity, where did the rest go? The answer is: HEAT. Heat is an unfortunate side-product of all this converting, and is somewhat annoying. For instance, I monitor the temperatures on my transmission while underway. The transmission running the hydraulic pump tends to be at least 10-15 degrees warmer than the other transmission.
Adding 20hp in drag to one of my main engines is fairly material. My normal cruise speed is around 9 knots, at 1,350 rpm. At this speed each of my engines is putting out around 125hp, and burning around 7 gallons per hour. I have the option of running either or both hydraulic pumps, and have always run a single pump. In other words, one of my engines is putting out 125hp, to run the boat, plus some additional horsepower, to run the hydraulic pump. Assuming the hydraulic alternator were the only incremental drag, then I would be adding 20 horsepower. However, we’re forgetting the stabilizers. The stabilizers require some amount of horsepower, and I have no idea what that is. I scanned through all the literature for my stabilizers and wasn’t able to find it. Nor do I know if it is a constant drag on the engine, or if the drag goes up as the stabilizers get more active.
One would think that moving those giant fins through the water would take a fair amount of energy, but how much? I did a bit of detective work, and by looking at my records on fuel consumption, I can see that the engine running the hydraulic pumps tends to use about three quarters of a gallon more in fuel than the engine that isn’t running the pump. Interesting. Were I just a bit smarter, I bet I could back into how many horsepower the stabilizers are costing me. I do have a little data, so I can ballpark it. I know that my 16kw generator eats around 1.25 gallons per hour, and I’ve seen that my 12kw of hydraulic alternator uses approximately the same ( which is what convinced me that my 16kw generator was more efficient). So … my math is not accurate, but it feels like the stabilizers eat roughly the same fuel a 8kw generator might .. or, around 12 horsepower. In other words, my ballpark guesstimate, is that if I am running with both the hydraulic alternators, AND the stabilizers, I am consuming somewhere around an incremental 32 horsepower, which translates to somewhere around an additional 2 gallons per hour.
Why does this matter? Well, it may not, or it may. I don’t know. I’m still trying to puzzle these things out. I remember that when I took delivery of the boat, I was told to run both hydraulic pumps if I am running both hydraulic alternators. It seems to me that I’d rather have the load balanced between the two engines. Historically, I haven’t been running the hydraulic alternators. I never run both pumps, except when running the thrusters. Perhaps I should run both pumps all of the time? I won’t bore you with the details, but I researched the spec sheet for my hydraulic pump and computed the maximum horsepower output. By my math, the hydraulic pump is capable of putting out as much as 117 horsepower, at 1,800 rpm. At the lower rpms I run, the pump maxes out much lower; probably around 80 horsepower. If I only need 32 horsepower, then I shouldn’t need both pumps. So, why Nordhavn’s recommendation?
Here’s my theory: Perhaps it is because of the impact the hydraulic pump might be having on my main engines. Let’s say that I’m cruising at 1,350rpm, and running a hydraulic pump only on my port engine. I would be asking my starboard engine for 125hp, and my port engine for 157hp (125, plus the 32 for the stabilizers and hydraulic alternators). According to the charts, my port engine SHOULD be running at 1,500 rpm if I want that much horsepower out of it, but obviously it can’t. The two engines need to be in sync at 1,350 rpm. So, how does it work? What happens when two engines try to run at the same rpm, but with dramatically dissimilar loads? Hmmm… My guess is that this is acceptable within a few percent, but that if you have materially different loads, you overload one of the engines, which I assume has negative consequences somewhere along the line.
Remember I said that hydraulic pumps are less efficient at lower rpms? My guess is that with just the stabilizers the difference in horsepower requirement on the two engines is immaterial, and I am better with one pump running inefficiently, than I would be with two. When the extra drag of the hydraulic alternators occurs, I must run both pumps, or I’ll ultimately have engine problems. That’s my thinking anyhow.
That’s it for today. My brain is hurting thinking about all of this, but I do think I am learning a bit with all this research! This feels like a worthy line of enquiry….