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Old 07-25-2012, 11:27 PM   #21
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The last trip through GA i had both operating the GPS speed and the water speed from the Trandsucer. Heading into a hard current or up some of the rivers then back down made Huge differences in overland speed measured against water speed. But in the end of the day i think what i lost heading into the current and heading up the rivers was returned in running with the current and down the rivers, at some points we were down to 5.6 mph over land other times we hit 14 MPH overland and the water speed stayed at 10 MPH rpms stayed the same 85% of WOT.
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Old 07-26-2012, 07:04 AM   #22
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Great discussion despite all the "almost info" floating around efficiency/hull speed.

Back to SD's OP....the trouble with many of the speed through water transducers...is they just aren't all that accurate. You can play and play with them but from one day to the next...even one run to the next they give some fairly erroneous info. Sailors are always fiddling with them but many who want REALY accurate speed through water measurements buy the really expensive stand alone models.

I think for most engine issues....using GPS is just fine for determining speed measurement if you can correct for any current (by running up/down and averaging, running cross current or in open water with no current).
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Old 07-26-2012, 07:16 AM   #23
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Hull speed as noted is far from the optimum speed for the lowest fuel cost.

The only folks that frequently operate near "hull speed" are blow boats with a good wind from the after quarter , or a spinnaker.

Since wind is free , why not.the extra hp is free.

For a marine motorist the concept of a hull speed cruise might gain a K or so but at double or triple the fuel cost.

Attempting to climb out of ones wake up and over the bow wave is not cheap.

The simple answer is to measure the LWL of the boat and find the square root of the LWL.

That is known a SL.

.9 to 1.15 x the SL will give most fat boats their cruise speed thru the water.

A really skinny commuter style or CAT or TRI can go a bit faster with the same fuel use..

50 ft boat at best 49 ft LWL the SL is 7 .

SL .9 is 6.3 SL 1.15 is 8.05 somewhere between will be the sweet spot .

The "hull speed is 9.38K , so that extra K or 2 will double to triple the hourly burn, so few actually ever run at hull speed.

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Old 07-26-2012, 07:41 AM   #24
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Hull speed as noted is far from the optimum speed for the lowest fuel cost.

The only folks that frequently operate near "hull speed" are blow boats with a good wind from the after quarter , or a spinnaker.

Since wind is free , why not.the extra hp is free.

For a marine motorist the concept of a hull speed cruise might gain a K or so but at double or triple the fuel cost.

Attempting to climb out of ones wake up and over the bow wave is not cheap.

The simple answer is to measure the LWL of the boat and find the square root of the LWL.

That is known a SL.

.9 to 1.15 x the SL will give most fat boats their cruise speed thru the water.

A really skinny commuter style or CAT or TRI can go a bit faster with the same fuel use..

50 ft boat at best 49 ft LWL the SL is 7 .

SL .9 is 6.3 SL 1.15 is 8.05 somewhere between will be the sweet spot .

The "hull speed is 9.38K , so that extra K or 2 will double to triple the hourly burn, so few actually ever run at hull speed.

FF
For my Albin...definitely agree with that 0.9-1.1 number. On my trip up from FT Lauderdale, based on what the PO told me I was using 2-2.5X the fuel I expected trying to get 8 knots. The Last part of the trip I slowed to 1600-1700rpm which brought me down below 7 knots and I dropped back into the 2gph or less area. Trying to get 8 knots and I was probably burning 4gph or better on my 135 Lehman.
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Old 07-26-2012, 12:46 PM   #25
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Originally Posted by FF View Post
Hull speed as noted is far from the optimum speed for the lowest fuel cost.

The only folks that frequently operate near "hull speed" are blow boats with a good wind from the after quarter , or a spinnaker.

Since wind is free , why not.the extra hp is free.

For a marine motorist the concept of a hull speed cruise might gain a K or so but at double or triple the fuel cost.

Attempting to climb out of ones wake up and over the bow wave is not cheap.

The simple answer is to measure the LWL of the boat and find the square root of the LWL.

That is known a SL.

.9 to 1.15 x the SL will give most fat boats their cruise speed thru the water.

A really skinny commuter style or CAT or TRI can go a bit faster with the same fuel use..

50 ft boat at best 49 ft LWL the SL is 7 .

SL .9 is 6.3 SL 1.15 is 8.05 somewhere between will be the sweet spot .

The "hull speed is 9.38K , so that extra K or 2 will double to triple the hourly burn, so few actually ever run at hull speed.

FF
The basic hull speed formula is a good starting point, however there variations as noted due to hull shape and beam. Since you brought up sail boats, I spent years racing them and my little trick of watching my stern wave wasn't a trick I discovered but a standard method used by most racing skippers optimizing trim and tacking angle. this is the simplest way of establishing hull speed. As you may have noticed the max range numbers quoted by Nordhavn or Kadie Krogan the are all at very slow speeds "6knts or less.
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Old 07-26-2012, 01:20 PM   #26
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The GPS shows speed over ground. A transducer shows speed thru water.
When calculating fuel burn and associated RPM and speed.
would you get better indication of fuel burn using a speed thru water than a gps speed.

Not sure how to phrase my question.

I try to travel at hull speed but that means changing the throttle as tides and currents alter movement thru the water.

Would I be better off just using the GPS?

Or would a true speed thru water work better and just ignore the speed over ground.

SD
Ignore speed over ground. It is irrelevant to hull speed. Hull speed relates only to speed through the water. Perhaps a simpler way to manage fuel burn in your individual vessel would be to create an old fashioned graph relating engine rpm to fuel burn. By adding a third parameter, speed through the water, you would have in a single graph your speed and fuel consumption versus your engine rpm's. Cheers!
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Old 07-26-2012, 01:48 PM   #27
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Scary,
You're best speed for efficiency is not "just" below hull speed but about one whole knot below HS. And when the water begins to pull away from the stern you're way above hull speed.
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Old 07-26-2012, 01:57 PM   #28
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As I read SD's original question The thing that jumps out to me at least, is speed thru the water vs. speed over the ground? Speed thru the water matters not one wit. Unless your voyaging to another vessel, in the same water, speed over the ground is what matters. If your voyaging from one land feature to another, the land doesn't move a bit in the current. What matters is that speed over the ground (miles per hour) divided by gallons per hour = miles per gal.
In my case, in very round numbers:
20 mph = 20 gph = 1 mpg
9 mph = 5 gph = 1.8 mpg
6 mph = 2 gph = 3 mpg (apprx. hull speed)

I boat on a river, current never goes away or changes dierection. If I am pushing a 6mph flood current running at the very economical hull speed nets me 0 mph over the ground, 0 mpg or infinite gals per mile. If I run 9 mph thru water, I net 3 mph over ground and 0.6 mpg. If I run 20 mph, I net 14 mph over ground and 0.7 mpg. Now, with said 20 mph with the associated driftwood would be DUMB, but cheaper, if your lucky. Fortunately, normal current is 1.5 to 2 mph.

If I want to be economical going down stream, I would shut the motors down and drift. I love infinite mpg, unfortunatly that doesn't work with wind and 1200' long barge tows. If I was operating in tidal area and an infinite schedule, I would play the currents to the hilt. Unfortunately, if I'm on the boat now, I have a destination to get to, It's time to go. knowing my gph burn rates for various rpms, and SOG from the GPS is all I care about.
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Old 07-26-2012, 02:46 PM   #29
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Speed through the water matters not one whit.
It does to what SD is trying to determine.

Speed over the bottom matters with regards to when you're going to get somewhere. But it doesn't matter to the boat or the engines. For any given rpm they work just as hard going downstream as they do going upstream if current is the only factor. Wind has a different effect.

SD is trying to determine his optimum engine setting for efficiency through the water, which means coming up with the best combination of rpm, fuel consumption, and boat speed. Speed over the ground is useless for calculating this because it will vary, and perhaps vary widely with the current. But the boat doesn't see this variation--- it just knows what it has to do to move through the water. Which way or how fast the whole body of water it's moving in is going is irrelevant to the boat.

If SD can run the tests at slack current or use timed runs over a measured distance in both directions in a current and then take the average that will work, of course. Or as psneeld said, if the current is slow enough the GPS measurement of speed over the bottom will be close enough for SD's calculations.

But where he boats, I'm guessing currents can perhaps get up into the double digits in speed. They can around here and he's much farther north with bigger tide changes. Slack current sometimes only lasts 15 minutes or so, which is usually not enough time to make the different runs that are needed to get the results he's after. So under these conditions, speed through the water is what you want to use because then it doesn't matter what the current is doing as far as gathering the data SD needs to gather.

Once he's come up wth the optimum power setting for his boat, it's up to him if he wants to exceed this going up-current so he can get where he wants to go by x-time, or back off the power going down-current to burn even less fuel. But he needs a baseline, and for that he needs to know what the boat is doing going through the water, not over the bottom.
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Old 07-26-2012, 03:05 PM   #30
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Marin is right on . it is not the speed over ground that is the issue. It is speed thru water.
using speed over ground with all the currents and tides. I am never operating at the same RPM to retain the at or near hull speed or the optimum speed to HP ratio. for the best fuel consumption.

It has been noted that the speed transducers are never acurate.
I would prefer to just set the throttle at the best RPM and forget it and go as best as can. Regardless or what the speed over ground is.
It is all about the best fuel consumption.

SD
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Old 07-26-2012, 04:54 PM   #31
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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...
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Old 07-26-2012, 05:39 PM   #32
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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.
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Old 07-26-2012, 05:47 PM   #33
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reverse course ...average the speeds...pretty quick and I'm guessing more accurate than most paddlewheels.
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Old 07-26-2012, 06:08 PM   #34
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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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Old 07-26-2012, 06:16 PM   #35
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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.
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Old 07-26-2012, 06:19 PM   #36
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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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Old 07-26-2012, 06:27 PM   #37
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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.
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Old 07-26-2012, 06:32 PM   #38
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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.
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.
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Old 07-26-2012, 06:42 PM   #39
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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.
It takes only a couple of minutes or less in each direction if using GPS SOG readings.
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Old 07-26-2012, 07:07 PM   #40
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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.
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