NASA log under-reading massively

[Amulet]

My NASA log under-reads enormously. I have it set to +50%, and I suspect it's still under-reading by 25% or more.

Brand new paddle wheel and spindle installed last week, all spanking clean, and even brushed around the hole with a bent bottlebrush.

It is positioned aft, in the turn of the bilges. Is moving it likely to help?

Could it be that turbulence caused by the clinker planking simply makes it a no hoper?

Any other owners of clinker boats got this to work?

 

[oldharry]

Position is wrong - the paddle wheel needs to be up forward getting as clean a water flow as possible. The other possibility - unlikely being a new transducer - is that one of the tiny magnets embedded in the paddle wheel has dropped out. Usually happens after over enthusiastic cleaning with a power jet cleaner, running the log up to 100+knots! This is not unknown i n the yard where the 'lads' fooling around at clean off time try to see how high they can get a log to read....

On a clinker boat, you could put a shaped wooden pad, to lift it parallel to the plank above to give it a cleaner entry

 

[Twister_Ken]

It is positioned aft, in the turn of the bilges. Is moving it likely to help?

Yes. Move it as far forward as poss. At the moment it's operating in very turbulent water, not a clean linear flow.

 

[Seajet]

Agree, position wrong, should be forward.

The clinker hull might agravate things with more turbulence in the 'boundary layer' too.

Good luck if speaking to the manufacturers, in my experience you'll need it.

After having once bought a boat with such kit ( not NASA ), I don't understand why people bother with paddle wheel logs, a nasty hole in the hull ( not to mention lightning strikes sometimes blow out such skin fittings !) and a constant hassle clearing out wee beasties etc.

Not accurate for navigation over any distance even if sited properly.

I know my boat so can judge how well she's doing, there's the plotter for SOG if bothered, and on any decent trip I use a tried & proven trail log as back-up...

 

[Amulet]

Agree, position wrong, should be forward.

Sounds like a consensus.

....
Good luck if speaking to the manufacturers, in my experience you'll need it.

Actually my experience with NASA is very positive indeed in terms of technical advice.

...clearing out wee beasties etc.

Not accurate for navigation over any distance even if sited properly.

In truth it is just a toy, but I have a Clipper Duet depth and log, and it pisses me off to see it not working. All real navigation is done on the GPS data.

 

[Seajet]

You can't beat the thrum of a trail log on the pushpit !

Not only does it give speed and distance remarkably accurately ( even my 1980ish Wasp model - once in a brief moment of having money, I bought a proper ship's Walker log, boxed complete with lubricating oil, spares and instructions inside the lid, now on a cabinet )...

I was a bit surprised at the Verl thread saying 'Gosh ! 9 knots ! ' - my Anderson 22 regularly goes over the 10 knot stop, given a fair breeze in the right waves, we once averaged over 7 knots between St.Peter Port to Salcombe, obviously a perfect F4-5 fine reach.

We didn't believe the log ( no early sat for us, GPS was yet to come ) but had to believe it when the typical clouds outlining Salcombe entrance appeared.

Back to trail logs, the thrumming sound is actually very reassuring when as a skipper off-watch, one can be lulled to sleep but still sense what's going on.

 

[johnalison]

I think there is a lot of defeatism around. Though-hull logs can work perfectly well, though I haven't used the distance function in anger since the days of Decca. I used to have an excellent "electronic" log from Seafarer with no moving parts which worked impeccably, though it was mounted somewhere aft.

Anyone buying a log should be looking for one with the option to retract it past a flap valve for easy cleaning.

 

[Seajet]

I don't know about 'defeatism', but if one looks at hull mounted logs with an engineering mind, maybe with the thought of aeronautics in the back of one's mind, the things just cannot be accurate.

I know the Seafarer job you are referring to, I think it measured doppler effect between 2 sensors.

Nice idea, but it was still within the boundary layer, a thin area of limited turbulence which 'sticks' around a surface in a moving fluid ( until stall but that's a different subject ) ; be it low speed in high density water, or high speed in low density air ( high Mach speed eventually making the airflow dense ).

Hence the reason all high speed aircraft - even wonder-jets like the F-22 etc, have pitot airspeed sensing probes stuck just off the nose side, ie outside the boundary layer.

Racing yachts would consider this drag, so don't bother; they don't need such critically accurate information !

And I still say the thrum of a trail log on a passage is best - but agree one can't really use one in the Solent without some smart-arse racer undoubtedly called 'Jamie' ( I've been among dinghy and cruiser racing lots, it seemed a universal name ) with highlights in his hair coming stupidly close and snagging the line...

 

[Trundlebug]

An interesting post!

I have exactly the same type of NASA paddle wheel log and the same problem.
Although mine is sited correctly, i.e. under the deepest part of the hull in clean water between the bilge keels, the readings have been getting progressivley slower over the last 4 years of ownership.

I've tried cleaning it and recalibrating it but it's now on maximum +50% adjustment and I reckon it's still 20% under-reading.

I have bought a new paddle wheel and spindle to fit this winter whilst it's out on the hard, I was rather hoping that would be the magic cure.

Although I don't use it for navigation as such, it is very useful in my cruising area to show the strength / speed of the current on rivers and tidal stretches, which can be as high as 5 knots.

Also useful for showing all is well underneath, in that a given engine rpm corresponds to a given speed through the water, regardless of current or tide. You can't get that from GPS alone.

 

[Plevier]

I've recently fitted a NASA Clipper duet, brand new, and it underreads similarly, compared with the old Stowe that was formerly in the same hole and compared with the GPS. Disappointed but not had time to investigate yet.
It's particularly bad at low speeds, it seems to take a lot to get it going. By about 6kts it's much better.

 

[oldharry]

I don't know about 'defeatism', but if one looks at hull mounted logs with an engineering mind, maybe with the thought of aeronautics in the back of one's mind, the things just cannot be accurate.

Nice idea, but it was still within the boundary layer, a thin area of limited turbulence which 'sticks' around a surface in a moving fluid ( until stall but that's a different subject ) ; be it low speed in high density water, or high speed in low density air ( high Mach speed eventually making the airflow dense ).

Hence the reason all high speed aircraft - even wonder-jets like the F-22 etc, have pitot airspeed sensing probes stuck just off the nose side, ie outside the boundary layer.

No hull mounted log is ever accurate across the speed range, and should be 'tuned' to the specific installation to be accurate at the otional ' normal' cruising speed. That is the speed you would normally expect to be travelling at cruising speed under power, or under sail the speed you expect to attain on a reach in 12 - 15kts of wind in smooth water.

The main use for a log nowadays is as part of the back up system against GPS failure.

 

[Jcorstorphine]

Another disatisfied NASA customer

My NASA log under-reads enormously. I have it set to +50%, and I suspect it's still under-reading by 25% or more.

Brand new paddle wheel and spindle installed last week, all spanking clean, and even brushed around the hole with a bent bottlebrush.

It is positioned aft, in the turn of the bilges. Is moving it likely to help?

Could it be that turbulence caused by the clinker planking simply makes it a no hoper?

Any other owners of clinker boats got this to work?

I have a NASA Target log which I bought for my project boat about 10 years ago. My boat is a GRP bilge keel motorsailor. The boat was launched for the first time last year and in the snagging list was the fact that my NASA log under read by about 25 %. Shortly after hauling my boat out for the winter, I found out that I had not set the calibration factor so upon launching the boat this year I set it up to 50 but the unit still reads low. When I was clocking 7 knots on the GPS (confirmed by distance made good) the log was only reading 5.5 knots.

At the time I checked where I had installed the unit and came to the conclusion that the unit may be in a turbulent stream behind one of the bilge keels. However when I had the boat hauled out last week I checked the position and found it to be in clear water approx. midway between the bilge keel and the central keel.

Might seem a bit of a cheek but I have sent a letter of complaint to NASA because as far as I am concerned, the unit has never worked. What it needs is either a paddle wheel with 4 magnets or a tweek in the control unit which will give a factor of 100.

Regards