boat speed

[Jack B]

Hi
After completeing anavigation course i was wondering if there are any clever was of measuring boat speed without using a log or GPS?
Thanks
jack

 

[Laurie]

Yup, it involves knowing a set distance (boat length), dropping a small floating item, & timing it over the distance. Factoring up will give you boat speed?

 

[prv]

Or alternatively, timing yourself over a longer distance between accurate fixes. This is exactly what the "measured miles" set up in various places are for, using transits to provide an accurate (in one dimension, anyway) fix at each end.

Pete

 

[johnalison]

I used to use the Dutchman's Log a lot in the old days. The trick is to find a length that divides easily. Thus a mile is about 6,000 feet, so one knot is 100 feet in a minute, or 25 feet in 15 seconds. So, 25 feet in 5 seconds is three knots, & so on. Please don't use beer cans.

 

[Searush]

Spit over the side & estimate the speed by counting seconds for the spit to cover a known distance. With a bit of practice you will get a pretty good idea just by watching the speed of passing flotsam or waves.

Don't forget that speed thro the water is not the same as speed over ground. Your GPS & log will not necessarily agree, even if both are right.

 

[Seajet]

Jack B,

I don't know where your boat is based but there is a 'measured mile' with daymarks just to the West of Portsmouth.

In an ideal world you might have a trailed log and a gps, and compare the 2 while looking at a tidal atlas showing the predicted tidal currents for the time.

I see from your profile you were asking about a boat, did you buy her ? By description she's a 'triple keel', which usually means the ballast weight in the central keel, the outer bilge keels being for keeping her upright when drying out on a half-tide mooring.

This is usually the sign of a 1960's - early 1970's design, and does incurr a lot of drag so lower speeds than more modern boats should be expected.

These more modern designs are true 'twin keelers' - as opposed to 'bilge keelers' such as you describe, but in general speech among boat owners the terms get confused ! - without the central stub, instead carrying the ballast in the two ( hopefully but not always hydro-dynamically 'aerofoil' shaped ) keels; with grp boats due to the problems of moulding the keels and getting the thing out of the mould, most such boats had bolt-on iron keels, though a few slightly naff examples had vertical keels, easier to get out of the mould but not a recipe for sailing performance !

It would be interesting to see a pic of your boat, I'm sure I'm not alone when I say I'd love to see her...

 

[VicS]

Hi
After completeing anavigation course i was wondering if there are any clever was of measuring boat speed without using a log or GPS?
Thanks
jack

A pitot tube... but I guess only for fast craft.

Ultrasonics .... but perhaps you are regarding that as a log.

 

[sarabande]

knowing the distance between position A and position B, and the time difference, plus tidal current data.

When I was racing keenly I could get to within half a knot of the true speed by the sound of the wash and the pattern of the wake. Worth developing that skill in case the technology goes on holiday one day.

 

[Seajet]

A pitot tube... but I guess only for fast craft.

Ultrasonics .... but perhaps you are regarding that as a log.

VicS,

I've never heard of a pitot tube on boats, but of course it's standard on almost all aircraft; as water is much denser and more forceful on a sensor than air, I suppose there's reason to think it would work on lower, boat type speeds.

In the 1980's there were a couple of companies making ultrasonic boat logs, B & G was one; there were 2 sensor plates, and the doppler rate between them was measured.

Judging by my experience of in-hull paddlewheel logs, I always thought this doppler idea just had to be better !

Edit;

One day at Dunsfold Photographic, a chap came in with his secretary, and looking at the pic which I use as my avatar ( Chief Test Pilot John Farley going past at 30', 300 knots in a Sea Harrier FRS1 ) - he explained the purpose of the pitot tube; she replied " Oh, I thought that was just for styling ! "...

 

[sarabande]

VicS,

I've never heard of a pitot tube on boats, but of course it's standard on almost all aircraft; as water is much denser and more forceful on a sensor than air, I suppose there's reason to think it would work on lower, boat type speeds.

Ah, SJ, you never came across the old pitometers, then ? A pressure sensing unit about three feet below the hull when deployed. we had to do all kinds of clever calibrations to ensure that the readouts were linear.


And then there was the Chernikef impeller log and distance recorder which had a flashing light to count the revolutions. You counted the flashes in a 20 second period, and looked up the results on a metallic table on the witch box.

 

[VicS]

VicS,

I've never heard of a pitot tube on boats, but of course it's standard on almost all aircraft; as water is much denser and more forceful on a sensor than air, I suppose there's reason to think it would work on lower, boat type speeds.

In the 1980's there were a couple of companies making ultrasonic boat logs, B & G was one; there were 2 sensor plates, and the doppler rate between them was measured.

Judging by my experience of in-hull paddlewheel logs, I always thought this doppler idea just had to be better !

Some (larger) outboard engines have a pitot tube built into them.

When I was fitting out my boat in the late 1970s Seafarer made an ultrasonic log to match their popular Seafarer 3 echosounder. A bit unreliable IIRC.

 

[FWB]

When I was fitting out my boat in the late 1970s Seafarer made an ultrasonic log to match their popular Seafarer 3 echosounder. A bit unreliable IIRC.

In my boat, I have a Raymarine ultrasonic log transducer that has worked perfectly for the past 10 years.

 

[Seajet]

VicS,

yes Seafarer were the other lot I was trying to remember; in my opinion paddlewheel transducers are so inaccurate they're no use to man nor beast, I suppose they only exist now as a novelty while everone looks at their gps, which of course is a very different reading.

Sarabande,

no, off the top of my head I've never heard of it, though I'm into history and instruments, used to be a part of 'instrument calibration & development' on Harriers & Hawks, though I was a technical photographer not instrument bod per se.

Also I've added my favourite photo of John Farley doing his bit - he did that 4 times for me as my Hasselblad camera only gave 1 shot per pass - so please pander to my much diminished ego and JF's skill by having a look !

 

[prv]

An old RIB that a mate used to borrow had a kind of pitot tube for its speedo. At least, not technically a pitot as I believe that requires both a dynamic and a static orifice; this was just a plastic moulding on the transom with clear tubing running into the back of the gauge on the console. The plastic moulding had a forward-facing hole which I guess pushed water up the tube, increasing the pressure of the air above it and turning the dial. I was rather surprised that it worked, but it did so tolerably well.

Ultrasonic logs appear to be reliable these days, as Dylan says, and would certainly be my choice if fitting from new. No more clogging up with seaweed and shells.

You can also get ultrasonic wind sensors if you're fed up with your NASA whirlybits coming off.

Pete

 

[philip_stevens]

VicS,

I've never heard of a pitot tube on boats, but of course it's standard on almost all aircraft; as water is much denser and more forceful on a sensor than air, I suppose there's reason to think it would work on lower, boat type speeds.

The containerships and fruitships I worked on, all had pitôt tube speed log installations.

The pitôt tube was about 8 to 10 feet long (depending on the ship type) and had to penetrate the tank spaces below the engine room.

I had to change a few over the years, that got bent on obstructions when the Nav. department on the bridge forgot to lift them in shallow waters. They were raised and lowered by either pneumatic or motor chain drive.

They had to be forced out the bottom of the outer tube by forcing the new pitôt tube in from the top. A bit hairy, but there were two gate valves on the outer tube.

 

[Lakesailor]

Hi
After completeing anavigation course i was wondering if there are any clever was of measuring boat speed without using a log or GPS?
Thanks
jack

Why?

 

[Bosun Higgs]

Hi
After completeing anavigation course i was wondering if there are any clever was of measuring boat speed without using a log or GPS?
Thanks
jack

As Einstein showed, speed is relative. What matters is if you are going faster than the boat alongside you. If you are its your skill. If you arent its because he's got his engine on / you arent really trying / he's got a bigger boat / you need new sails if only SWMBO would allow it.

Navigation? Forget it. If God had meant us to navigate he wouldnt have given us GPS.

 

[Laurie]

As Einstein showed, speed is relative. What matters is if you are going faster than the boat alongside you. If you are its your skill. If you arent its because he's got his engine on / you arent really trying / he's got a bigger boat / you need new sails if only SWMBO would allow it.

Navigation? Forget it. If God had meant us to navigate he wouldnt have given us GPS.

The only trouble with using logs is that they can become a hazard to navigation...

 

[Oldhairy]

My log is an old Seafarer. Big and ugly but works a treat as long as I don't anti-foul it. And my Seafarer echo sounder can be read in dirty rivers where no digital read out makes sense.

 

[Searush]

The other traditonal technique is to use a traingular bit of wood fastened to a long rope with knots in it at regular intervals. Use a 3 strand rope with a knot near the end & then fasten one strand to each corner, with one secured by means of a wooden peg. You will also require an hourglass type timer. chuc the log in the water, set the sand running & count the knots as they pass thro your hand until the sand has all gone. Then a sharp tug on the line will release the pegged strand & the whole lot can be recovered,


If you calculate the correct knot distance for the timer glass, each knot = 1 nautical mph, hence the use of knots as a speed unit for boats. Fast clipper ships were sometimes said to be doing "10kts & a Chinaman" as the speed could mean that the pull on the line could be strong enough to carry a small person overboard when they tried to dislodge the peg.

Bloody electronics, a complete waste of time. A rope & a bit of wood works fine.