Fuel consumption at low speed?

Lost Horizons said:

So, is it safe to say that the class of boats that includes Carvers and Sea Rays cannot be trusted with an autopilot maintaining a course in moderate conditions at 6-7 knots?

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I don't think that is a fair assessment. I do not have an autopilot. But I do know that the most current generation of autopilots do an exceptional job of maintaining course regardless of speed and hull configuration. So take that FWIW....obviously not much. But I don't think an autopilot would have much issue with holding a course on my boat.

Feel free to ask more direct questions because I feel your are kind of beating around the bush???

Better autopilots have a speed input or have manual selection for high and low speed. My boat would do 14 to 16 knots with the original motor and displacement cruise 6 to 8 knots. Parameters are very different for those 2 ranges. The Robertson (Simrad) AP20 has a high and low speed selector and 2 sets of programmable parameters. It took a little while, but my boat now tracks like it's on rails at 7 knots. Autopilots are one of those instruments where user fine tuning is everything.

Ted

Correction on turbo vs naturally assperrated engines

DonF said:

Lepke - I agree with you on the maintenance issues but not sure about your statements on fuel consumption. ("The high hp turbo version of the same engine burns up to 30 gallons an hour at full power and 2.5x hp. The 12v71 natural (non-turbo) burns about 12 gallons an hour at about the same hp as the turbo 671.") At http://boatdiesel.com/Engines/Engines.cfm?TZ=-4&SC=1366.768
I found that the 671M burns 6 gal/hr at 80hp and the 671TI burns 5 gal/hr at 80hp both at 1400 rpm and I took the data from the prop hp curves. So the turbo does burn about 20% more fuel than the non turbo but not twice the fuel.

I believe that in the case of 4 stroke diesels (at least some) the turbo charged version gets a little better fuel consumption. I'll check the data sheets and post when I get more time.

Happy cruising - -

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Looking more closely at the plots it appears that they both burn about the same amount of fuel for a given HP. If there is any difference it will probably be with in the error bands of the data. As said above I used data sheet from Boatdiesel.com.

I apologize for posting the incorrect info.

Don

Lost Horizons said:

So, is it safe to say that the class of boats that includes Carvers and Sea Rays cannot be trusted with an autopilot maintaining a course in moderate conditions at 6-7 knots?

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Not at all. We have a Meridian 459 which will cruise at 20 knots but 90% of the time we cruise at 7 knots. The Raymarine ST6001 autopilot does a sterling job in heading hold or tracking a course through the chart plotter at anything above 5 knots.

I have found the sweet spot for best economy for us with twin 6bta's is 1200 rpm on both engines equals 98 hp using the torque curve and that gives us 7 knots and 12 to 14 litres per hour depending on the sea conditions. Haven't try single engine cruise yet.

Baker said:

Feel free to ask more direct questions because I feel your are kind of beating around the bush???

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I will, thank you. Should probably start another thread.

Lost Horizons said:

So, is it safe to say that the class of boats that includes Carvers and Sea Rays cannot be trusted with an autopilot maintaining a course in moderate conditions at 6-7 knots?

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I don't think so. Our boat would be in that "class" and our autopilot (Furuno Navpilot 511) tracks well at 2.5 kts (our trolling speed), at 7-8 kts (our "trawler" speed) and anywhere between 15-24 kits (planing speeds).

There was a set-up and training regime, installer did most of that before we did sea trials and final "training" -- but otherwise it didn't seem like anything special was required.

-Chris

Lost Horizons said:

So, is it safe to say that the class of boats that includes Carvers and Sea Rays cannot be trusted with an autopilot maintaining a course in moderate conditions at 6-7 knots?

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No, not only is it not safe to assume, that's a totally erroneous assumption. Autopilots have adjustments and can be tuned for conditions. in fact, the autopilot, properly adjusted, will maintain the course far better than one can manually.

BandB said:

No, not only is it not safe to assume, that's a totally erroneous assumption. Autopilots have adjustments and can be tuned for conditions. in fact, the autopilot, properly adjusted, will maintain the course far better than one can manually.

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Even a poorly adjusted autopilot can keep a better course than I can...something to do with A.D.D I assume...
Bruce

Bruce B said:

Even a poorly adjusted autopilot can keep a better course than I can...something to do with A.D.D I assume...
Bruce

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I don't know if you've ever watched auto racing, but looking at the drivers in the cars, you'll see one whose turning of the wheel is so smooth and another who seems to be moving it back and forth every second. I think that's how many of us are manually steering, as we'll overcompensate and try to be too fine and precise. Now, I've also known people to set their autopilots like that and work them to death.

Lost

You must be listening to some salesman. Boat brands are not that important. Not all Hatteras or KKs are good and not all Bayliners Are bad.
My Carver was heavily built and the AP worked well at all speeds even on one engine.

bayview said:

My Carver was heavily built and the AP worked well at all speeds even on one engine.

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I meant to mention that "one engine" thing, too, since that's our usual trolling mode. The AP usually works fine.

There are some times when wind, and sometimes current, can overpower the AP's ability to maintain a steady course on one engine, but that goes away when we turn on the other engine and re-adjust the trolling valves... and usually in those instances, we'd also need both engines to make the right speed directly into the wind anyway.

-Chris

ranger42c said:

I meant to mention that "one engine" thing, too, since that's our usual trolling mode. The AP usually works fine.

There are some times when wind, and sometimes current, can overpower the AP's ability to maintain a steady course on one engine, but that goes away when we turn on the other engine and re-adjust the trolling valves... and usually in those instances, we'd also need both engines to make the right speed directly into the wind anyway.

-Chris

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There was a guy next to my slip with some kind of a two engine express cruiser. Once in a while he would return on one engine, and he could not turn into his slip without being pulled with dock lines by helpers. I don't think a boat like that will steer well in moderate sea conditions at low speed regardless who or what does the steering. For those small rudders to provide enough transverse thrust, certain speed is needed particularly in confused sea conditions. Have you ever encountered conditions when you had to slow down and have trouble maintaining the course?

BandB said:

No, not only is it not safe to assume, that's a totally erroneous assumption. Autopilots have adjustments and can be tuned for conditions. in fact, the autopilot, properly adjusted, will maintain the course far better than one can manually.

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Can you turn your boat into a slip in a marina with a narrow fairway without using differential steering or thrusters?

bayview said:

Lost

You must be listening to some salesman.

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I am a scientist, so I don't listen to salesmen at all.

Lost Horizons said:

Can you turn your boat into a slip in a marina with a narrow fairway without using differential steering or thrusters?

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What does that have to do with the section of mine that you quoted, which was discussing the use of autopilots and maintaining course using them with Sea Rays and Carvers. I don't use an autopilot to put the boat in a slip in a marina with a narrow fairway.

BandB said:

What does that have to do with the section of mine that you quoted, which was discussing the use of autopilots and maintaining course using them with Sea Rays and Carvers. I don't use an autopilot to put the boat in a slip in a marina with a narrow fairway.

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If you cannot steer the boat with the helm at certain conditions, I suppose an autopilot will be ineffective at similar conditions. With my last sailboat, I needed more than a knot or a knot and a half of boat speed to turn into a slip. Incidentally, the same minimum speed was needed for an autopilot to function properly.

Lost Horizons said:

There was a guy next to my slip with some kind of a two engine express cruiser. Once in a while he would return on one engine, and he could not turn into his slip without being pulled with dock lines by helpers. I don't think a boat like that will steer well in moderate sea conditions at low speed regardless who or what does the steering. For those small rudders to provide enough transverse thrust, certain speed is needed particularly in confused sea conditions. Have you ever encountered conditions when you had to slow down and have trouble maintaining the course?

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Yes, in many twin engine boats -- with high winds in snotty seas and running on one engine -- the AP may have some problem. So would I, if steering manually. Moderate seas, probably no problem. Windage is our more common issue.

But then running on one engine in a twin boat is what I'd consider a special case, and it comes with other issues like transmission lube, shaft lube, etc. so speed usually must be controlled to deal with that other stuff.

Anyway, I'd consider all that just a fact o' physics, not a particular criticism of one style of boat, one brand/model of AP, etc.

OTOH, with both engines running, at any speed, I've never yet encountered a situation where our AP couldn't maintain course. Our "slow" in cases where we're actually trying to go somewhere would usually be in the area of 7-8 kts. When trolling, that's closer to 2-2.5 kts.

Discussing the AP and docking in the same sentence doesn't make much sense to me.

I may or may nor be able to dock on one engine. That partly depends on which engine, and the details about the slip itself. I just had a one-engine episode, actually, but the wrong engine was running to be able to get into our own slip. Had it been our starboard engine running, I could have put the boat into our slip which is on the left side of our fairway, assuming one crew aboard to handle the spring line (which is a pretty normal docking tool for us, anyway).

Given that I was running on the port engine, and single-handing... and given there is a useful usually-empty "bail-out" slip -- straight bow-to entry -- at the end of my own dock anyway, I just put the boat in there, instead. Could have anchored to address the real (engine) problem. Could have landed at the fuel dock or somewhere else conveniently staffed. So I had options. No big deal.

And an AP wouldn't have been a useful tool for any of that docking stuff, anyway.

-Chris

Regarding slow speed; I guess it's all relative. My prairie does around 8 max. Running 6.3 is my usual. When out in above 3' following seas the autopilot can't keep the course. It takes too much time to correct. And when it does it applies counter rudder too much, too late.

All the yawing and slewing around tends to make guests queezy.

I do a better job by hand in heavy following seas. (Thanks for the suicide knob).

I am a bit confused as to the real question. In my limited experience, APs do a fine job at low speed, better ay course keeping than I do myself. Low speed is relative however. In my sailboats, the AP had no problem holding a course with a boat speed of 1 kt or better. Those sailboats had a real rudder and and fin keel however. My AP on my NP43 has no problem holding a course at a boat speed of 4 knots, but we usually cruise at 6.5 to 7 knots.

As BandB mentioned earlier, if your AP is setup properly for the boat you have, it should be able to hold a course at a lower speed just fine. Keep in mind I have no experience with trolling speeds in a power boat.

cappy208 said:

Regarding slow speed; I guess it's all relative. My prairie does around 8 max. Running 6.3 is my usual. When out in above 3' following seas the autopilot can't keep the course. It takes too much time to correct. And when it does it applies counter rudder too much, too late.
.

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I would suggest you work with the adjustments and sensitivity of your autopilot as any I've ever used could be adjusted to do just as you want in those conditions. I don't know what AP you have so can't say exactly what capabilities it has. Ultimately you should be able to mimic what you'd do by hand with the AP, just more consistently and with less work using the AP.

DonF said:

Turbo Charged engines should get better fuel economy as they use exhaust energy to increase the intake pressure. Without a turbo charger the exhaust energy that the turbo extracts is just wasted. Of course you have to be running at a high enough RPM to get the turbo to "spin up". Depending on the engine you could increase your fuel economy by around 10%.

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Don,

I doubt the turbo would give better economy. There's a lot to a turbo and a lot of losses. The big advantage of a turbo is that it gives more power from the same sized engine. I see only a disadvantage with boats and cars and would never have one (again). In an airplane, where you get more power at altitude they will be faster but not usually more efficient. I've had them, too.

I could argue strongly for a the largest engine you can get, normally aspirated and operate is at lower power for economy, but have the option to go fast, if money was no object.

Im not a turbo fan this week.

Seevee said:

Don,

The big advantage of a turbo is that it gives more power from the same sized engine. I see only a disadvantage with boats and cars and would never have one (again).

Im not a turbo fan this week.

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Every car I've ever owned has been a turbo, I believe. Boats turbo too. Turbo was once new and unknown and problematic from lack of knowledge as well as some poorly built but now they're pretty routine.

Lost Horizons said:

If you cannot steer the boat with the helm at certain conditions, I suppose an autopilot will be ineffective at similar conditions. With my last sailboat, I needed more than a knot or a knot and a half of boat speed to turn into a slip. Incidentally, the same minimum speed was needed for an autopilot to function properly.

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Not sure what you are after.

Absolutely some twin engine boats are difficult to operate on one engine and going slow the autopilot would be ineffective ......and?

Even the worst handling boats I have driven in open water will respond on one engine and autopilot.

Coming into a marina is a different ballgame.

What REALLY are you looking for?

The only turbochargers my family, in all its generations, "had" was the B-17 my father flew. (It was no match for the flak over Germany, however.) ... Since a turbocharger allows a given engine to create more power, wouldn't it wear out quicker than a naturally-aspirated one?

Turbos have been around for along time. Yes, more power for smaller volume engines but also a good use of exhaust gas. Maintenance on charge coolers, etc, but overall a good thing, IMO.

BTW, My wife's Uncle flew B17s over Germany as well. On one sortie only 2 of them came back alive. The greatest generation.

markpierce said:

The only turbochargers my family, in all its generations, "had" was the B-17 my father flew. (It was no match for the flak over Germany, however.) ... Since a turbocharger allows a given engine to create more power, wouldn't it wear out quicker than a naturally-aspirated one?

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psneeld said:

What REALLY are you looking for?

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Something that I can live on full time, preferably outside of marinas for long stretches (3-6 months at a time). Seasonal migrations from Florida/Bahamas to New England with occasional trips to Puerto Rico and the islands, if possible. Most likely I will not have help operating the boat, i. e. will be singlehanding it all the time. Hence, will need to rely on an autopilot in the "thorny pass" conditions (5-6 knots of speed in 6-8 foot seas for 300 miles) including taking short naps on passages. A sailboat makes sense for this, but I don't want to live in a conch shell for a cabin, and having a draft under 4 feet would be nice.

markpierce said:

The only turbochargers my family, in all its generations, "had" was the B-17 my father flew. (It was no match for the flak over Germany, however.) ... Since a turbocharger allows a given engine to create more power, wouldn't it wear out quicker than a naturally-aspirated one?

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My Dodge pickup with a Cummins 6BT 220 is turbo charged. Have 418,000 miles (approximately 8,300 hours) on the engine with original turbo (not liquid cooled). As long as you don't try to get ridiculous amounts of HP out of an engine and keep the oil clean, modern turbos don't seem to shorten engine life.

You and I have the same JD 4045 engines other than mine is turbo. Think we both burn around 2 GPH. Doubt either one of us will ever rebuild those engines.

Ted

Lost Horizons said:

Most likely I will not have help operating the boat, i. e. will be singlehanding it all the time. Hence, will need to rely on an autopilot in the "thorny pass" conditions (5-6 knots of speed in 6-8 foot seas for 300 miles) including taking short naps on passages.

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Sometimes everything bad that can happen comes together all at once... and if that happens to you, you'd just have to bite the bullet, stay awake, and steer for 300 miles.

But more often, everything that can go wrong doesn't actually do all that at the same time. Good to consider planning for it, but... If you're in a boat with working twins and 6-8' seas, AP should handle 6-8' seas. If you lose an engine, seas states might be kinder at the time, so AP should handle that anyway. And so forth.

6-8' seas in the ocean with a kindly wave period are no big deal anyway. It'd be pretty grim around here, but then I'd only have to steer for a few miles.

300 miles? Lots of short hops available throughout most of the area you describe.

Probably not too hard to find temporary crew for longer runs, if necessary.

Several ways to skin a tiger...

-Chris

With the modern electronic engines, you will get a continuous read out of your fuel burn. This is data I recorded over two seasons. Our boat is a Helmsman 38. Cummins QSB 5.9 HO. We displace about 26,000 pounds depending upon fuel and water loads. We swing a 5 blade propeller, 24 x 16.75. YMMV

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