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Old 07-26-2012, 07:25 PM   #41
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So the answer to Skips problem is to just keep going slower until he just can't stand going so slow and speed up a tad.
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Old 07-26-2012, 07:30 PM   #42
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Or his engine temp drops into the too-cool zone.
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Old 07-27-2012, 10:46 AM   #43
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I go back to Scary's comment about watching the water at the stern if what he says is true I should be able to find a RPM to give me the best fuel burn.
All I am really after is to determine the best RPM to run at.
Trying to use all the instruments at my disposal being the Tac, GPS and the calculations of the square root of the water line times 1.34.
How fast I am going is not really the issue. It is how much fuel I will burn to get to a given destination regardless of tide and current.
When I go out there is never a time line to follow.
Just how much it will cost to get where I am going.
I just leave early and come back late.

SD
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Old 07-27-2012, 10:52 AM   #44
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Originally Posted by skipperdude View Post
I go back to Scary's comment about watching the water at the stern if what he says is true I should be able to find a RPM to give me the best fuel burn.
All I am really after is to determine the best RPM to run at.
Trying to use all the instruments at my disposal being the Tac, GPS and the calculations of the square root of the water line times 1.34.
How fast I am going is not really the issue. It is how much fuel I will burn to get to a given destination regardless of tide and current.
When I go out there is never a time line to follow.
Just how much it will cost to get where I am going.
I just leave early and come back late.

SD
you bet..other than chop and a few other small variables that might change most economical due to some difficult to measure inertia, etc based variables...run the best rpm once you figured out what RPM gives you the best NM/gal.
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Old 07-27-2012, 11:14 AM   #45
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I don't know of any place to get a measured mile where I boat but the idea sure has merit.

Gotcha Marin, with the cost of fuel being what it is I would like to run at hull speed regardless of what the speed over ground is.

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OFB is correct but the reason for using the measured mile hearkens back to pre GPS days. With your GPS you can measure the precise distance between two points (bouys are nice) and use the double back run between those two points to calculate true boat speed through the water at a given rpm. Then use the prop demand performance data graph in the operator's manual to determine fuel burn at that rpm. This assumes you are not over propped.

I also think you may find that the most practical balance between mileage and speed will not be achieved at hull speed but at some lesser % of that value. Hull form and power plant would determine that rpm but for Delfin, it is 1275 rpm at a little over 7 knots and a burn of around 3.2 gph. Increase to hullspeed of 9.7 knots and the burn goes to 9.5 gph.
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Old 07-27-2012, 12:06 PM   #46
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This may be obvious, but I don't think I saw anyone say it.

MPG goes down as RPM goes up on all boats. So if fuel economy is the only consideration, travel at idle speed.

Most of us want some compromise between fuel use and time of transit. Here the hull speed (or slightly less) is where fuel use starts to increase exponentially and thus most of us don't want to pay for it regardless of an earlier arrival.

So you get to pick the balance based on MPG and speed you would like to go.

Below is the data for the AT 34. I choose to run at about 1600 RPM, 3.3 GPH, and 9 MPH.
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Old 07-27-2012, 12:20 PM   #47
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Thanks Larry. It is not just economy but the best rate of travel or how fast can my boat move Thur the water before the fuel consumption starts to climb.
Hence my original question. Theoretically the hull speed can be reached Thur calculations. Every hull is different every boat has a different engine.
I was trying to use data from my boat to determine best RPM

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Old 07-27-2012, 12:32 PM   #48
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It's also a matter of how "happy" the boat sounds/feels: something to do with the boat's individual harmonics.
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Old 07-27-2012, 12:40 PM   #49
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Thanks Larry. It is not just economy but the best rate of travel or how fast can my boat move Thur the water before the fuel consumption starts to climb.
Hence my original question. Theoretically the hull speed can be reached Thur calculations. Every hull is different every boat has a different engine.
I was trying to use data from my boat to determine best RPM

sd
Not sure... but the last post sounds like you are chasing your tail.

You usually have a choice...absolute best NMPG or something else which involves a speed closer to hull speed which is just a term where you are going as fast as your boat will go without incurring a fast rate of NMPG increase.

If you want to go a 1/2 knot faster...great...but it may reduce NMPG by say from 4 to 3.5...is that a penalty you can live with?

I know when I cruise I am going to be adjusting that ratio all the time. There are some days when I just need to go 40 miles and I can maximize NMPG. Other days I might need to get 80 miles in 10 hours and sacrifice my best NMPG to get where I wanna go by adding a 1/2 or whole knot...such is life. If all you ever want to do is save gas...there will be an RPM that gives you your best NMPG (all else equal) and if you can live with where that takes you as you are travelling...so be it. But there will usually be another factor in there that make you want to adjust your schedule so a ball park number is usually good enough in the big scheme of things.
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Old 07-27-2012, 12:46 PM   #50
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Reading all the responses on this thread, it seems the OP asked what time it was and received instructions on how to build a watch.

The answer to his question is in there, he just has to find it.
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Old 07-27-2012, 12:48 PM   #51
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SD...should have just caled a pro before you start messing around with all the huge math problems we threw at you...
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Old 07-27-2012, 12:56 PM   #52
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Skip .... True ... Every boat is different and the hull speed calculation only tells so much. For example a boat w a bulbous bow has the same WLL with or without the bulb but the "effective" WLL is considerably greater. A little submerged transom and the right shaped aft section can also lengthen the "effective" WLL. A reshaped stern or a bulb would give Willy a very noticeable increase in top speed. Added wetted surface and other considerations could make cruise speed suffer or some other aspect of the hull design could suffer ... Like stern sea directional stability. Top speed on a slow pleasure boat is not the greatest priority. Lots of kayaks are so skinny at both ends hull speed means practically nothing. Great laker is right ... The slower you go the less you'll burn. No rocket science .... Just go slow. Or find a good FULL DISP boat.

I think what SD was looking for was a speed/fuel burn curve like for a planing hull where ther'e IS a best speed for MPG ... Just "over the hump". Not GPH but MPG. Slower boats march to a different set of rules.
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Old 07-27-2012, 01:00 PM   #53
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Reading all the responses on this thread, it seems the OP asked what time it was and received instructions on how to build a watch.

The answer to his question is in there, he just has to find it.
That gave me a good laugh. But you could be right on.

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Old 07-27-2012, 06:52 PM   #54
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In thinking about what SD is trying to figure out, I've come to the conclusion that in the overall scheme of things, it probably doesn't matter all that much. Obviously you want to know what a power setting is that a) doesn't wear the engine unnecessarily, b) doesn't run it so slow that the combustion temperatures are too low, c) doesn't gobble fuel like it's going out of style, and d) moves the boat at a decent enough clip that you don't forget where you're going in the time it takes you to get there.

For us in our boat, it's 1600-1700 rpm. Adding another 100-200 rpm to buck a current isn't going to increase the fuel burn enough to care about, and reducing it 100-200 rpm when we are going down-current isn't going to save us enough fuel to care about. So we just leave it at 1600-1700 rpm all the time.

Fuel, even at today's higher prices, is such an small part of the overall ownership cost of a boat like ours that we rarely even consider it. When we start to run low we buy more.

There are situations where finding the fuel use sweet spot is worthwhile---- long range cruising or cruising an a year round, always-on-the-move basis.

But for the kind of coastal cruising we do fuel is an irrrelevant cost. Which is not the same thing as saying it's an insignificant cost. But we're not going to change what we do with the boat if our fuel costs go up or go down so figuring out how to burn 0.6354679832666 gallons an hour less is not going to pay for the effort of figuring out that number in the first place.
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Old 07-27-2012, 07:22 PM   #55
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Marin, as far as this subject goes, we're on the same wavelength. My fuel costs (about $8 dollars or less an hour) are about one-fourth the annual berthing fees and one-twelth of annual maintenance and improvements. And we're not even counting property taxes and insurance.

Isn't that what fuel efficiency is all about? For a slow boat, fuel costs are almost irrelevant.
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Old 07-27-2012, 07:34 PM   #56
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Isn't that what fuel efficiency is all about? For a slow boat, fuel costs are almost irrelevant.
That pretty much sums it up for me and it only took you two sentences to do it.
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Old 07-28-2012, 06:49 AM   #57
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"For a slow boat, fuel costs are almost irrelevant".

At a few hours a year .perhaps.

For folks that will be going somewhere or even just running the loop , once you start to think in 1000 hours years , rather than a few hundred , efficiency is a concern.

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Old 07-28-2012, 10:15 AM   #58
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Isn't that what fuel efficiency is all about? For a slow boat, fuel costs are almost irrelevant.
I don't think so. Travelling at 2K RPM as opposed to 3K RMP, I'm using roughly 1/2 the fuel. A recent cruise cost $1K in fuel. The difference between that and $2K is not irrelevant.

"Your mileage may vary."
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Old 07-28-2012, 02:58 PM   #59
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Probably depends on the engine. 3K rpm would make our engines fly to little bits and 2K would wear them out on short order. The engines we have are happiest in the range from 1500 to 1800 rpm. Below 1500 and they can start to run too cool. So for us, the difference between 1700 and 1600 or 1500 is not going to make a difference in fuel consumption worth even thinking about. Not saying there isn't a difference, but compared to the other costs in boating, whatever the difference is it's pocket change.
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Old 07-28-2012, 03:39 PM   #60
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Probably depends on the engine. 3K rpm would make our engines fly to little bits and 2K would wear them out on short order. The engines we have are happiest in the range from 1500 to 1800 rpm. Below 1500 and they can start to run too cool. So for us, the difference between 1700 and 1600 or 1500 is not going to make a difference in fuel consumption worth even thinking about. Not saying there isn't a difference, but compared to the other costs in boating, whatever the difference is it's pocket change.
The actual RPM is not the point. Divide my figures by two, three, or whatever fits your boat.

There's a speed where your boat operates efficiently and the amount of fuel required to exceed that speed increases much more than the actual speed gain. That's my point.
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