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Old 07-25-2012, 02:10 PM   #1
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Speed thru water or GPS speed over ground

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
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Old 07-25-2012, 02:21 PM   #2
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The only thing the boat cares about is its speed through the water. That is the speed the engines are working to achieve and it is this work that affects the fuel burn. What the whole body of water happens to be doing and which direction it's going has no bearing whatsoever on how hard the engines are working.

Now if you are going with the current, you will obviously get there faster so you'll use less fuel during the trip at any given rpm simply because the boat is running for a shorter period of time. If you are going against the current the opposite will be true. But the boat will be burning fuel at the same rate either way.

One way to think of it (the same thing applies to planes, by the way) is if you are on a train going to Chicago at 60 mph, how much more do you have to work to walk to the front of the train as to the back of the train?

Of course, the energy you expend to walk either direction in the train is the same. The fact the whole train is heading to Chicago at 60 mph has no effect whatsover on your walking speed or the energy you expend to do it.

The relationship between speed through the water (or air) and speed over the ground is the same.

So your speed over the ground will tell you how long it will take you to get there, and your speed through the water is telling you how hard your engines have to work.
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Old 07-25-2012, 02:33 PM   #3
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Right, but to calculate the proper RPM
to acheive the true hull speed would it not be better to use a speed thru water than a speed over ground?

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Old 07-25-2012, 02:41 PM   #4
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Yep. Unless, of course, you did it in water that had no current movement whatsoever. Then the speed through the water and the speed over the ground will be the same.
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Old 07-25-2012, 02:49 PM   #5
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Right, What I need to do is travel at high slack tide and record the RPM for that water and just ignore the GPS speed over ground.
That will give me MY optimum fuel burn at Hull speed.

This came about when playing with my garmin that has an ap for speed sensors and water temp.
My transducer does not provide those readings. I could opt for a new transducer with all the bells and whistles or do as I said and just do my own calculations at high or low slack tide.
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Old 07-25-2012, 03:01 PM   #6
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If you do this at a true slack tide, then (assuming no effects from wind) your boat's speed through the water will be the same as your boat's speed over the bottom. If you're really lucky your boat's knotmeter will agree with the GPS. But usually they don't, the GPS being the more accurate.

But the advantage of using the knotmeter for your fuel burn@x-speed calculations is that, even if it's off, it will always be off by the same amount. So regardless of what the current and wind are doing to your speed over the bottom, your speed through the water, and therefore your fuel burn, will always be the same at x-rpm.

Or put another way, if that's the fuel burn to speed ratio you want, using the same rpm will always get you the same fuel burn at the same speed through the water regardless what the current and wind are doing to your boat's speed over the bottom.
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Old 07-25-2012, 03:06 PM   #7
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From my predicted log race days.

measured mile. Do a timed run one way turn around and do it the other. I prefered to do this twice in each direction.

Do this using a couple of power settings ( rpm ) to get the actual speed you are doing per each setting.
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Old 07-25-2012, 03:26 PM   #8
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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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Old 07-25-2012, 03:39 PM   #9
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GPS can set you a Mile mark.
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Old 07-25-2012, 04:00 PM   #10
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I suppose I could drop a bouy and then another and make the run.

Now that I think about it all I really want is to know the proper RPM to acheive hull speed thru slack water.
The distance doesn't really matter that much.
what I have been doing is to try and keep a speed of 7.4 knts regardless of hull speed. By doing this I sometimes run at a higher RPM to maintain that speed which is not hull speed it is speed over ground.

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Old 07-25-2012, 05:09 PM   #11
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Not having a full disp hull you don't need to know what the hull speed of your boat is. It appears to me that you are under the impression that running your boat at hull speed is going to net you increased efficiency. Not so. If ther'e is an advantage to running at hull speed it would manifest itself with full disp hulls only. Their submerged aft sections ride on the face of the boat's following wave and the theory is that the FD boat surfs a bit at hull speed (when the wave is there). It is dependent on the Quarter Beam Buttock Line's angle. With steep QBBLs the surfing will act a bit like the Bart rail system that propels the cars with the motors going up a hill. Going down the circuits switch and the motor is used as a brake and performs as a generator reclaiming some of the energy lost going up the hill. Our boats make a wave at the bow and the water heaps up at the stern at hull speed and to some degree pushes the boat ahead. This is the theory but I've never seen any concrete evidence that it actually works. It should show up as a "blip" on the graph line for a fuel v/s speed graph. Never seen it.
But w a wide, flat and submerged transom any chance of doing some surfing on the boats 1st following wave is lost as the transom is even deeper in the water than at rest. There may be a tiny bit of surfing energy reclaimed but it's more than lost as the stern creates even more drag because it's stern is deeper in the water and the straight run aft produces a QBBL that is not at all ideal for the surfing advantage.
Bottom line is just go slower. Simple as that. That's all you need to know about what speed to go for best efficiency.
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Old 07-25-2012, 05:19 PM   #12
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I should add that if you want to know how fast you're going through the water find a body of water where ther'e is a minimum of eddys and swirls and cross currents and assume a course parallel to the long part of the body of water. Stabilize your speed for a minute or so, take some bearings and come about to 180 degrees of your original course and after the speed stabilizes note your speed and average it w the speed noted going the other way.
This is not accurate but it's close.
I'd like to run in a lake and observe all my speed/rpm numbers but I haven't had the chance.
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Old 07-25-2012, 05:27 PM   #13
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I know that when my boat gets above 7 knts or so she squats deeper into the water.
I have been told this by observers on other boats. Also above the 7 knts my fuel consumption increases as evedent by the floscan.
I have read where a full displacement hull does get a lift in efficency moving thru the water but it is more from the Brounelli effect. Same as holding the back of a teaspoon under a stream of running water. The water sucks the spoon in as the water follows the curve of the spoon or hull of a full displacement boat in this case.
It would seem to me that there has to be a place where even a semi displacement boat will turn the curve and attain a better fuel to RPM ratio.

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Old 07-25-2012, 05:31 PM   #14
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I'd like to run in a lake and observe all my speed/rpm numbers but I haven't had the chance.
Take your boat to Lake Washington. There is, or at least there used to be, a measured mile marked on the north side of the 520 floating bridge. It was put there specifically for the boat builders, yards, and brokers from Lake Union and the Ship Canal to use to determine various speed and power settings.
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Old 07-25-2012, 06:24 PM   #15
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I use engine RPM to estimate speed through the water. With 200 RPM intervals (1200, 1400, 1600, etcetera), I measure the speed over ground on a calm day using the GPS SOG display, going with and opposite the current and average the results. (Slack tides around here are illusive and momentary.) Make a table or chart of the results for future reference.
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Old 07-25-2012, 06:42 PM   #16
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So regardless of what the current and wind are doing to your speed over the bottom, your speed through the water, and therefore your fuel burn, will always be the same at x-rpm.
Ah come on Marin ...

If are cruising along through the water at around half throttle and just under 8 knots your boat will require X horsepower and burn Y fuel. That is kind of a no-brainer and no one can find anything there to argue about ... probably.

But after jotting down your fuel burn as displayed on the hyper accurate Burn-O-Matic Marine Fuel Management Computer that was just installed, the breeze came up, right on the bow at 20 knots. You keep moving through the water at exactly the same speed according to the optional Nav-O-Matic Hull Speed Module you shelled out to have installed along with the Burn-O-Matic, what do you think the fuel flow will show on the Flow-O-Matic module?

If you are driving a typical trawler style boat with about a 12 foot beam and a house about 10 feet high above the waterline, what do you think the horsepower readout will show on the Pow-O-Matic Module?

A. The same

B. 5 HP less

C. 6 HP more

D. The RPM will decrease

The answer is:
A
B
C
D
B and D,
A and D
B and C
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Old 07-25-2012, 08:28 PM   #17
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Right, What I need to do is travel at high slack tide and record the RPM for that water and just ignore the GPS speed over ground.
That will give me MY optimum fuel burn at Hull speed...........
It needs to be slack current. Slack tide and slack current can be at different times in many places.

Find the most efficient RPM at slack tide, then just run that RPM for the most efficient cruising, provided you don't go insane travelling that slowly.

2K RPM gets me 7 knots at slack current so I cruise at 2K RPM unless there's a reason to speed up or slow down. The absolute most efficient speed for my boat is probably slower than that, perhaps 6 knots.

Note: On the Atlantic ICW, that 2K RPM will get me anywhere from below 5 knots to nearly 9 knots depending on the current which can reverse every few miles.
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Old 07-25-2012, 09:17 PM   #18
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I'm curious why mph (or kph is you're metric)? Personally, I base all my fuel burn statistics on Gal./Hr. I suppose I shouldn't assume you have an hour meter on your motor, but I do. I figure that over the long-haul, it's a better indicator of the AVERAGE burn. There has been a learning curve with sight tubes and fuel cell capacity, but now I can get pretty close.

Your mileage may vary!
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Old 07-25-2012, 09:28 PM   #19
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If the boat has to buck a wind and if the requirement is to maintain the same speed through the water it will take more power to do it and will therefore burn more fuel. Unlike the current the boat is riding in, the wind produces drag on the boat so you need to either add power to overcome the drag and maintain the same speed through the water or you can leave the power alone and accept a slower speed through the water.
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Old 07-25-2012, 09:28 PM   #20
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Determining hull speed is very simple.

This requires no mumbo jumbo BS. Here's the tried and true simple way of determining hull speed. Watch the water behind your stern, when the water begins to pull away and leave a depression aft of the stern you have reached hull speed. It's just that simple. The harder you push your boat the bigger the depression until you use enough power to begin to plane on the surface. The squat your talking about is your boat falling into that depression. Many of us don't have enough hp to plane. Even fairly round shapes will plane given enough HP. Using this method you don't need to know what the current is doing. Your most efficient speed is slightly below hull speed.
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