"Boat Speed" vs. SOG calibration challenge

[jr_spyder]

Hello all,

Comparing "Boat speed", provided by my paddle-wheel sensor in the hull, to SOG, provided by GPS, is an ongoing mystery on my boat. I very carefully calibrate boat speed as required in no current, no wind, etc., and get it to match SOG very reliably while motoring. Also while motoring in a known current the difference in the two makes sense. It's all good.

I have confirmed with other independent GPS devices that my SOG reading is always correct (not surprisingly).

Then I get out in the wind, get the boat heeled over and the two readings no longer makes sense. The Boat Speed always seems to read higher than it should, as much as 1.5 kts. It doesn't matter which tack I'm on, or what the known current is, the Boat Speed tells me I'm going through the water faster than reality.

It's not a huge deal since SOG tells the truth, but my B&G gear does some calculations based on this input, such as current and set & drift speed/direction, which don't seem right. I'd like it all to work properly, because I'm a geek.

My hull sensor is an AirMar DST800 feeding an all B&G Zeus/Triton network. I'm usually pretty good with this stuff but I'm stumped as to why I can't get this right. My only wonder now is if the location of the sensor is in a spot where the fluid flow accelerates under that part of the hull when the boat heels. I'm not sure that's even possible, but I'm reaching here.

Ideas?

Thanks,

JR

[Suijin]

If the boat is heeling you're probably going to windward. If you're going to windward, the wind is probably probably creating wind-driven current. Does the discrepancy increase the more close hauled you are and the higher the wind speed? In a stiff breeze it's easy to see 1.5 knots of wind driven current.

[charliehows]

The short answer is; it took humans 3000 years to find a workable answer to this question, and another 300 years to refine it to the present level of sophistication. You are basically asking “how do I reconcile the difference between dead reckoning and calculated positions.”

[jr_spyder]

Suijin, I agree with that possibility, but I was just on a beam reach for five days from Bermuda to Antigua with the true wind pretty much right on the beam and the apparent just forward of course. So the wind driven current would have been more on the beam, nonetheless I had a constant ~1 kt discrepancy in the boat speed. Winds were 13 - 16 kts most of the time (after 2 days of dead calm motoring). Will pay more attention to the close hauled question next time.

I'll also mention my crew, all experienced sailors with their own boats, thought my numbers were off more then they should be.

[StuM]

Where is the paddle wheel in relation to the keel and propellor? in a location where the propellor when motoring creates some sort of eddy effect, slowing down the water flow at that point?

As described, it suggest that you are getting flow over the paddlewheel impeded when motoring, and you are calibrating based on that water flow. When you re not motoring, the flow over the paddlewheel is not impeded.

[a64pilot]

I’m with Stu, I would calibrate under sail. Who cares about it when motoring.
I’m betting due to position error that you can have either sailing accurate or motoring, but not both.

[jr_spyder]

The paddle wheel is approx 16 - 18' forward of the propeller, just at the leading edge and off to starboard of the full keel. So I wouldn't think there is any prop effect on the paddle.

Interesting thought that the paddle wheel is incorrect under motor so therefore calibrated wrong from the start.

[a64pilot]

A prop of course pulls water, there is a vacuum in front of the prop. In fact I believe a prop more pulls a boat than pushes it, but perhaps not.
It also does so in a corkscrew fashion due to prop rotation, being on the starboard side, I think it’s being blocked.
Just reread your post, mentally I thought 16 to 18 inches, but 16 feet is a long way away, it ought not be affected by the prop, but apparently is, or how do you explain it being accurate under power, but not sail?

[smac999]

how do you know the currents?.... from a tide chart produced years ahead?

maybe your wheel is right and your chart is wrong...

because using the wheel is the only way to actually know the current...

[deblen]

I may be all wet here but:

During slack tide (no water current) & motoring straight
PaddleWheel Speed = GPS SOG = VMG towards a waypoint straight ahead.

During cross current & motoring straight
PW speed = true speed thru the water.
SOG = true speed over bottom.
The two MAY agree.
VMG will be lower than both PW & SOG,because you are "crabbing"with the current.

During slack tide & sailing straight ahead
PW will be low,because boat is "crabbing" due to leeward set caused by wind,which pushes PW at an angle thru the water.
SOG = true speed over bottom.
VMG will be low due to "crabbing"

During cross current & sailing straight ahead
SOG = true speed over bottom.
PW & VMG will both be different due to "crabbing" caused by wind (leeward set)
PW & VMG will both be different due to current direction & current speed.

I used the term "crabbing" ,rather than set/drift,to avoid confusion-they are essentially the same,but there is "by current" & "by wind".
I used "straight ahead" to infer that no rudder course changes were made.

IMHO-the PW will spin at same speed whether boat is leaning or upright.
However,IMHO,the PW will run slower if the water hits it at an angle,though I believe in normal conditions,any difference would be miniscule.
I assumed there was no technical fault with OP's PW or GPS.

I don't have an answer on how to make OP's PW & SOG agree & I personally don't believe they should.
My experience has been that they rarely do.
Both PW,SOG & VMG have their uses.

Len

[donradcliffe]

On the B&G race instrumentation, there are fudge factors for the paddle wheel calibration You can adjust for heel angle to port and starboard, and you can make the paddle wheel knots vs pulses/sec nonlinear with speed. I'm wondering whether these calibration factors exist on your boat and whether some PO has tweaked them.

[44'cruisingcat]

My boatspeed reads low at low speed and high at high speed. Ie, if I calibrate it to agree with the GPS at 8.5 knots, it will read maybe 1/2 knot low at 6, and 1/2 knot high at 11.

I just put it down to Airmar transducers being crap.

[fxykty]

Originally Posted by 44'cruisingcat

My boatspeed reads low at low speed and high at high speed. Ie, if I calibrate it to agree with the GPS at 8.5 knots, it will read maybe 1/2 knot low at 6, and 1/2 knot high at 11.

I just put it down to Airmar transducers being crap.

I have Raymarine on my boat and exactly the same figures as you, callibrated equal at 8.5 knots as well. Though the lows and highs are up to 1 knot respectively at the extremes. Go figure!

Is it a paddle wheel issue? Do folks with ultrasonic speed have the same issue?

[CatNewBee]

Well your bottom is curved - like a wing, you have the water stream along this shape and it creates laminar or turbulent flows, what you measure at a specific point on your hull and calibrate to a given SOG.

When you heel, the curve radius / shape of the "wing" in the water changes and the pressure / stream / speed of the water around your sensor changes too due to the changed shape. It is normal, that you will get different pressure / velocity / type of stream through the water then.

This is the reason why some racing instruments allow non-linear calibrations for different angles and speeds.

[Suijin]

One more point that may bear on your initial calibration. In some bodies of water high or low tide does not translate into no current. In some estuaries, for example, there can be significant current under the boat at high tide as the tidal surge moves through. You're literally sitting on top of the "bulge" of water that is moving through under you. So if you assumed no current based on a tide table vs. a current table, your assumption may have been wrong. In addition, most current tables are approximate, with the current changing significantly based on lunar cycle.

I'm of the mind that your initial calibration is off, for whatever reason.