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Old 19-03-2014, 13:07   #46
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Re: Repowering? Bigger is better?

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Originally Posted by marujo.sortudo View Post

..... I don't expect the change would effect fuel consumption much as diesel fuel use strongly correlates to hp delivered ....

5. Better engine longevity. Engines don't like low RPMs as much as high RPMs. Also, when running the engine just to charge the batteries it will be loaded closer to 50% which should all improve its lifetime.
Regarding the first point, I agree that it will not affect it MUCH, but it will certainly help fuel consumption, to go to a smaller prime mover, especially if it is also used at anchor in lieu of a genset.

The specific fuel consumption curve attached shows that

At 2700 rpm a Yanmar 2GM20-F burns 208 grams per hour (for each horsepower it develops)

At 1400 rpm, it will burn 235 grams per hour/hp, an increase of 13 percent in fuel consumption.

This is typical for marine diesels in general, and is significant but not serious.

But remember that this is for a new engine.

Once the bigger engine has been prematurely worn by constant running at such a low load, it can easily be imagined that the fuel consumption will tip further in favour of the smaller unit.

For another thing, the exhaust elbow will clog prematurely - this is particularly true on this particular engine (worse still if raw-water cooled), unless it's run at about 80% full load all the time, and it progressively causes a major hit to the fuel efficiency until it's rectified again and the cycle restarts.

I also agree strongly with m.s's second point

... and, if memory serves, much of that post, and in fact, much of the thread:

it seems to me that the trend to larger engines in sailing boats leads to some non-obvious penalties - especially if it takes you over a continually lowering threshold, into the larger, less hardy and reliable engines for any particular marine diesel range at present:

unnecessarily complicated, electronically controlled, expensive-to-fix, and in some respects allergic to the marine environment and to voyaging particularities.
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Old 19-03-2014, 13:20   #47
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Re: Repowering? Bigger is better?

Yes, both the 35 and 38 are the same block, same bore, same weight, so what makes them different? Maybe this is a dumb question but in what way are they different that they have different hp at max rpms? Again, engines not my area so sorry about that.

Also, to answer Ann T. Cate. the boat is a shoal draft fin keel with a detached balanced spade rudder.
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Old 19-03-2014, 14:44   #48
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Re: Repowering? Bigger is better?

Probably the only real difference between the two is the max RPM (which determines max horsepower developed) and some of the supporting parts, e.g. a smaller heat exchanger could maybe be used? I'm sure Beta could give a better answer, in detail.

Completely in concurrence with Andrew's notes on efficiency and everything else he said, too. I was too lazy to look up the numbers, but you can decide how important 13% or a similar number would be to your wallet.
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Old 19-03-2014, 14:50   #49
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Re: Repowering? Bigger is better?

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Originally Posted by marujo.sortudo View Post
Probably the only real difference between the two is the max RPM (which determines max horsepower developed) and some of the supporting parts, e.g. a smaller heat exchanger could maybe be used? I'm sure Beta could give a better answer, in detail.

Completely in concurrence with Andrew's notes on efficiency and everything else he said, too. I was too lazy to look up the numbers, but you can decide how important 13% or a similar number would be to your wallet.
I didn't want to say it before but are we talking different stops on the throttle? Is it that obvious? Or is different water pumps? Different heatx? I don't know.
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Old 19-03-2014, 15:04   #50
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Re: Repowering? Bigger is better?

A given engine running at a higher RPM produces more horsepower. To complicate things the formula is HP=(Torque X RPM)/5252 where torque is in lb.ft.

Look at the performance curves for the Beta 35 and 38. The 35 burns almost 6liters/hr to make 35HP at 2800RPM, while at the same RPM, making the same HP, the 38 burns under 4liters/hr. Seems like the 38 is more efficient? The resolution of the graphs is not great so hard to tell exactly.

Whatever engine you get you may need to repitch the prop. Stanley at Beta can help with that.
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Old 19-03-2014, 15:08   #51
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Re: Repowering? Bigger is better?

I've carefully measured fuel consumption at different RPM's for my engine (a nominal 30hp at 3000 RPM.) I've also logged speed under no wind, no current conditions. All this is useful to have on your boat should range or other fuel planning be needed. Here's my chart:

RPM GPH HP Kts Rng nmpg
900 0.10 2 3.4 1495 33.2
1200 0.28 5 4.1 654 14.5
1500 0.46 8 4.6 457 10.2
1800 0.63 11 5.1 367 8.1
2100 0.81 15 5.6 314 7.0
2400 0.98 18 6.1 278 6.2
2700 1.16 21 6.5 253 5.6
3000 1.34 24 6.9 233 5.2

GPH = gallons per hour, HP = approximate HP based on a rule of thumb of 18 hp per gallon of diesel burned, Kts = speed in calm conditions, Rng = range in nautical miles based on my 45 gallon tank, nmpg = nautical miles per gallon

As you can see, my mileage at idle is astronomical and goes down quickly as RPM goes up. You can see the effect this has on range. The effect on the wallet is less obvious if you assume that low RPM's are harder on the engine. I generally cruise between 1500 and 1800 RPM to get good fuel efficiency while not beating the engine up too much. If I had a smaller engine, I might run it even slower sometimes as we really like cruising at that speed and I wouldn't be so worried about beating the engine up.

A smaller engine could actually save your bacon if dismasted at sea and you needed to motor a long way to port with limited fuel supplies. All this, because you could run at a lower effective HP--more efficiently even--and use less fuel than a bigger engine would be capable of. On the other hand, if a 6 knot current is pushing you onto a reef, you'll be wishing you had the biggest engine and prop available. Trade offs, as usual.
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Old 19-03-2014, 15:08   #52
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Re: Repowering? Bigger is better?

Well to start with different governor settings in the fuel control to allow the higher RPM, possibly different injectors to flow more fuel with a shorter injection interval due to higher RPM, although in a perfect world it would have a different heat exchanger built just for it, almost always that is not the case, and we consumers don't want that either.
Example back in the day it didn't matter which Chevrolet motor you had a little 4 cyl or a V8 454, they all had the exact same valve lifter

Often engines HP etc are classified that way in order to meet an OEM's spec, it's not the actual engine HP. Big boat manufacturer comes up to you as an engine manufacturer and says, I want a 35 HP engine. You don't have one, so you just de-rate your 38 to 35 HP by turning the RPM down
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Old 19-03-2014, 15:42   #53
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Re: Repowering? Bigger is better?

Quote:
Originally Posted by marujo.sortudo View Post
I've carefully measured fuel consumption at different RPM's for my engine (a nominal 30hp at 3000 RPM.) I've also logged speed under no wind, no current conditions. All this is useful to have on your boat should range or other fuel planning be needed. Here's my chart:

RPM GPH HP Kts Rng nmpg
900 0.10 2 3.4 1495 33.2
1200 0.28 5 4.1 654 14.5
1500 0.46 8 4.6 457 10.2
1800 0.63 11 5.1 367 8.1
2100 0.81 15 5.6 314 7.0
2400 0.98 18 6.1 278 6.2
2700 1.16 21 6.5 253 5.6
3000 1.34 24 6.9 233 5.2

GPH = gallons per hour, HP = approximate HP based on a rule of thumb of 18 hp per gallon of diesel burned, Kts = speed in calm conditions, Rng = range in nautical miles based on my 45 gallon tank, nmpg = nautical miles per gallon

As you can see, my mileage at idle is astronomical and goes down quickly as RPM goes up. You can see the effect this has on range. The effect on the wallet is less obvious if you assume that low RPM's are harder on the engine. I generally cruise between 1500 and 1800 RPM to get good fuel efficiency while not beating the engine up too much. If I had a smaller engine, I might run it even slower sometimes as we really like cruising at that speed and I wouldn't be so worried about beating the engine up.

A smaller engine could actually save your bacon if dismasted at sea and you needed to motor a long way to port with limited fuel supplies. All this, because you could run at a lower effective HP--more efficiently even--and use less fuel than a bigger engine would be capable of. On the other hand, if a 6 knot current is pushing you onto a reef, you'll be wishing you had the biggest engine and prop available. Trade offs, as usual.
"Mileage at idle is astronomical"

Imagine how much more true this would be if the propellor was doing the same revs as when your engine is idling, but a smaller engine than yours, running at 80% of full revs, was driving it through a deeper reduction.

- - - - -

I'm planning (for a 7500kg boat, 10m waterline) a 10hp and 20hp diesel, each driving a hydraulic pump, so that by running either or both engines, it will be possible to choose between (nominally) 10, 20 or 30 hp at the prop without degrading efficiency and service intervals.

Naturally for long distances in a calm, I would run the 10hp engine (which realistically produces 8hp at the crank, maybe 7 at the prop if I'm lucky)

And for short runs, I should be able to run at intermediate, more finely graduated power outputs at the prop, while running the engine(s) at optimum load, by running the surplus hydraulic power into an accumulator.

Much like a hybrid car, which achieves significant efficiency gains by always running the IC engine at optimum load.

A CF poster recently pointed out that Peugeot propose using exactly this setup (except with only one IC engine) for a hybrid car, ie a hydraulic pump, motor, and accumulator.

For marketing reasons, they are portraying it as 'Air' (sigh)
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Old 19-03-2014, 16:07   #54
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Re: Repowering? Bigger is better?

I just spoke to my mechanic who is the local distributor of the Beta range. He has no idea why they have two engines so close together. I'm sure there's a story there.

The block, pistons, etc. are the same. He said that there are subtle differences to the timing and camshaft profile, etc. A bit more than just the governor screw being turned a bit, but not a lot more. The peak torque and hp curves get moved up one or two hundred rpm as a result of the small changes.

He recommended the 35 for me because there's not much difference between them and I'll end up running at slightly lower rpm so it'll be a little quieter. The differences are small, though.

Also, I'll be adjusting my prop for max rpm, so if I cruise a bit closer to the max rpm, my prop will be closer to the optimum pitch for that rpm. Again, a small difference in this case.
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Old 19-03-2014, 16:25   #55
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Re: Repowering? Bigger is better?

So if the 35 and the 38 are running the same RPM, are they producing the same HP? Why not just turn the 38 the same RPM, but if you need it for whatever reason the extra 3 HP is there?
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Old 19-03-2014, 16:32   #56
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Re: Repowering? Bigger is better?

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So if the 35 and the 38 are running the same RPM, are they producing the same HP? Why not just turn the 38 the same RPM, but if you need it for whatever reason the extra 3 HP is there?
No, they aren't. But the differences are small. The peak torque seems to be the thing that is the most different (by maybe 100-200 rpm, hard to tell as it is pretty flat). Peak rpm is where you're getting the most "bang for the buck" in his words.
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Old 19-03-2014, 16:34   #57
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Re: Repowering? Bigger is better?

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So if the 35 and the 38 are running the same RPM, are they producing the same HP? Why not just turn the 38 the same RPM, but if you need it for whatever reason the extra 3 HP is there?
My thoughts exactly. It's hard to tell looking at the performance graphs, but you would think that if you had the option of say a bigger heatx or whatever why wouldn't you get it? It's not like cost is a factor, the 35 costs $29.33 per pound and the 38 cost $29.97 per pound, so what? Why even offer the 35?
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Old 19-03-2014, 16:36   #58
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Re: Repowering? Bigger is better?

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No, they aren't. But the differences are small. The peak torque seems to be the thing that is the most different (by maybe 100-200 rpm, hard to tell as it is pretty flat). Peak rpm is where you're getting the most "bang for the buck" in his words.
It's a tractor engine to begin with, are there tractor applications where the torque difference matters? However small? And if it does matter, does it matter to the boat and prop combo? Where the proper pairing can make a bigger difference than might be noticed without looking for it?
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Old 19-03-2014, 16:43   #59
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Re: Repowering? Bigger is better?

Marujo--good post!

Quote:
Originally Posted by Andrew Troup View Post
...I'm planning (for a 7500kg boat, 10m waterline) a 10hp and 20hp diesel, each driving a hydraulic pump, so that by running either or both engines, it will be possible to choose between (nominally) 10, 20 or 30 hp at the prop without degrading efficiency and service intervals...
ot Andrew--even though the hydraulics reduce efficiency, as compared to a straight drive, this is a wonderfully interesting idea! Technically, we should expect this system to be somewhat less reliable, more troublesome, and more maintenance, because of the increased number of engines. But the redundancy and convenience of so many options, and the ability to tailor your setup to match existing conditions make this very compelling.

Adding a DC powered hydraulic pump would multiply the options. And all this could easily lead to hydraulic windlass(s) and winch(s).
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Old 19-03-2014, 16:45   #60
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Re: Repowering? Bigger is better?

So if you prop for max rpm, you're going to have a larger prop on the 35, I think (not a prop expert). You're propping for 2800 rpm which is also closer to my likely cruising rpm since the 35 is actually a bit big for my boat, so I will be running it a bit slow.

Peak torque (best bang for buck) point on the 35 is at 2000 rpm which is at 26.5 hp and is 800 rpm below the peak.

Peak torque (best bang for buck) point on the 38 is at 2300 rpm which is at 30.5 hp and is 1300 rpm below the peak.

So if I accept that it would be nice to cruise at max torque, then I will be sucking less gas (due to lower hp) and quieter (due to lower rpm) and my prop will be better matched if I go with the 35.

I think.
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