Speed thru water or GPS speed over ground

What cracks me up is the big deal being made about it...one is easily derived from the other or compensated for.

It's a small math matter and you are where you want to be...both are important or irrelavent...but one (SOG) can be measured pretty accurately and the curent CALCULATED accurately...but speed through the water...well I have never trusted those paddlewheels so I would calculate speed through water before I ever wanted to bet on a direct measurement of speed through water.

But if you are confident in your measurement...then do it your way...whichever one you want...

While it may not be an issue in other places, calculating the currents around here is difficult to impossible. The current velocity varies all over the place due to the islands, meanderings of channels, width of passes, and so on. True, there are current tables available that tell you what the average current will be in the more major channels and bodies of water. But the actual current your boat is in at any given time varies all over the place, sometimes several times in a couple of boat lengths.

This becomes very obvious when trying to hold a course. The boat can get shoved around to the point where you can often feel the current sheers as the boat suddenly slows down or speeds up or is pushed sideways by a current change of up to a couple of knots sometimes. I'm sure it is the same in Maine and the Maritimes, too, anywhere where the tidal range gets up in the 10, 15, 20 or more foot-range.

In fact, if the objective is to calculate what SD is after and you use the through-the-water speed, you have to make sure you're doing this in an area of relatively constant current velocity because the current sheers can even mess up a knotmeter momentarily.

I agree about the paddlewheel accuracy. Unless you're able to compare the knotmeter against a measured distance like you can between the markers on one of the Lake Washington floating bridges that were put there for that purpose, it's kind of a crap shoot as to their accuracy. But assuming one's knotmeter is at least consistent in its inaccuracy, it will work fine for what SDs trying to find out.

Or he can do his calculations using the GPS at slack or near-slack current .....if he can find some somewhere.

reverse course ...average the speeds...pretty quick and I'm guessing more accurate than most paddlewheels.

That would certainly work if the rate of current change was low. Sometimes around here its not. Don't know how long it would take to make the runs and calculate the fuel burn and get the rest of the data SD wants, but it's conceivable you could start your tests in a 2 knot current and complete them in a 3 knot or faster current. You'd want to pick your place and your test time pretty carefully.

Gee, it isn't that complicated to determine current in most/many circumstances. After preparing the RPM/speed table, one can easily calculate the current. For example, if at 1600 RPM one should be making 6.0 knots (for our theoretical boat), so if one is going 5 knots over the bottom (per the GPS SOG reading) directly against the current, there is a 1.0 knot current. If one is moving at an angle to the current, then some trigonometry is needed to calculate rather than simple arithmetic when doing directly into or with the current.

Where there are significant currents, fuel efficiency (GPM) is increased if RPM are reduced somewhat when going with the current and for going against the current the RPM is somewhat increased compared to the target RPM/GPM under no-current conditions. This point was made earlier.

How do you know your knotmeter is accurate?

The point is that if SD uses speed through the water for his calculations the whole current thing becomes moot and uneccesary.

While we regularly get 2-3 knots of current around here too..there is ample opportunity to do it when the current is less or find a variation in the bodies of water where the current is much less.

Again...doesn't matter if the current is 50 knots (I've done the same calculations in a helo long before loran c, gps....) reverse direction (takes at most 30 seconds) and average.

Marin said:

How do you know your knotmeter is accurate?

The point is that if SD uses speed through the water for his calculations the whole current thing becomes moot and uneccesary.

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assuming he can accurately measure it....

based on his original post we are all talking about nothing....

if he is worried about economy, as long as he isn't in a chop where momentum might have some effect...then rpm and accepting whatever speed over ground he gets...he gets...that will be a repeatable economy based on RPM...nothing else....unless you get tricky and probably need some kind of fuel flow meter to show actual MPG based on flow and SOG.

Marin said:

That would certainly work if the rate of current change was low. Sometimes around here its not. Don't know how long it would take to make the runs and calculate the fuel burn and get the rest of the data SD wants, but it's conceivable you could start your tests in a 2 knot current and complete them in a 3 knot or faster current. You'd want to pick your place and your test time pretty carefully.

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It takes only a couple of minutes or less in each direction if using GPS SOG readings.

Well, since SD is just trying to find an rpm at which his boat's performance is at it's most economical all he needs is something indicatiing speed so he can include it in his calculations. Doesnt really matter how accurate the speed is as long as it's consistent for the rpm setting. So I'd do it with speed through the water and not bother with the whole current compensation, GPS, etc. thing.

I don't know about his boat but we always get the same indicated speed through the water for every rpm (except when a current sheer throws the paddlewheel off). The only thing we use the GPS speed for is to see how we're being affected by the current as Mark described above.

And given the current strengths around here, half the time we don't want to know how we're being affected by the current.

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.

Or his engine temp drops into the too-cool zone.

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

skipperdude said:

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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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.

skipperdude said:

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.

SD

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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.

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.

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

It's also a matter of how "happy" the boat sounds/feels: something to do with the boat's individual harmonics.

skipperdude said:

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

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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.

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.

SD...should have just caled a pro before you start messing around with all the huge math problems we threw at you...

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.

rwidman said:

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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That gave me a good laugh. But you could be right on.

SD

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.

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.

markpierce said:

Isn't that what fuel efficiency is all about? For a slow boat, fuel costs are almost irrelevant.

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That pretty much sums it up for me and it only took you two sentences to do it.

"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.

FF

markpierce said:

Isn't that what fuel efficiency is all about? For a slow boat, fuel costs are almost irrelevant.

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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."

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.

Marin said:

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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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.