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Guessing your current speed (Hire Boat)


Cruising Mike

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We've been hiring for 20+ years. First thing to say is every boat has been different. On subject of speed 'vs' tacho, we did hire a boat a few years ago - I won't say where from - but it did seem seriously under-propped. It might have been a deliberate attempt by the hireco to limit speed, but the biggest annoyance was the noise ! Compared to the many other boats we've had, it seemed that the engine was screaming all the time. It is also interesting to note thast when Alvechurch built those two (?) boats with the streamlined hulls in an effort to save fuel, they actually found no significant saving since the eco-hull enabled the boat to go faster without making so much wash, so they went faster and used more fuel negating any savings.

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Blimey, I go along at 900-1000 rpm but revs versus speed is non-determinable in a fluid. As many have said already, actual speed is irrelevant but simple maths can give you the answer. 4mph is 5.86 feet per second, 2mph is just under 3 feet per second, If you know or can measure your cabin roof length, then as the front of the roof passes into the shadow of say a bridge-hole, start counting. When the back of the roof hits the shadow, stop counting. My roof is 43ft so less than 7secs & I'm speeding, about 14secs it's 2mph. If it's not sunny just think a bit, 4mph will take you about 10secs to pass a 60ft boat etc.

Brilliant, Mr. Foot! Have a greeno for that.

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We use a Garmin 126 marine GPS system. When we sold our seagoing boat we kept it. Gives us constant speed reading,time number of satelites etc.Not much point in setting waypoints and the track page is a bit of a laugh. Very good for matching engine revs to best speed. Cheap secondhand as old tech. Our engine(Isuzu 42) is rated to 3,000rpm.This is the manufacturer max for generators etc.It is downrated to 2,000rpm for marine use. Cant imagine a diesel engine pulling 3,000rpm in a narrow boat.

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If you know the length of the boat (surely the hire brochure gives that info) you just get one of the crew to shout from the front just as the bow goes into a bridge hole and you start timing. Then as you at the stern enter the bridge hole, note the time. Simple division gives you feet per second, and convert to mph by taking just two-thirds of that figure (eg 3 fps is 2 mph). Also then look at the rpm and hey presto you have now calibrated the rev counter too.

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If you know the length of the boat (surely the hire brochure gives that info) you just get one of the crew to shout from the front just as the bow goes into a bridge hole and you start timing. Then as you at the stern enter the bridge hole, note the time. Simple division gives you feet per second, and convert to mph by taking just two-thirds of that figure (eg 3 fps is 2 mph). Also then look at the rpm and hey presto you have now calibrated the rev counter too.

don't forget to allow for the speed of sound ;)

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My boat slows down substantially on entering a bridge hole...

 

How do you know???

 

And since you do know how, apply this technique to multiply the speed obtained from the method in post #54 by the "substantially " factor.

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If you know the length of the boat (surely the hire brochure gives that info) you just get one of the crew to shout from the front just as the bow goes into a bridge hole and you start timing. Then as you at the stern enter the bridge hole, note the time. Simple division gives you feet per second, and convert to mph by taking just two-thirds of that figure (eg 3 fps is 2 mph). Also then look at the rpm and hey presto you have now calibrated the rev counter too.

Very good, keep up mate, see post48. But no, you can't calibrate revs to speed on a boat. You'll go slower into a headwind than with a tailwind for example, but your revs could stay the same. I have noticed the apparent slowing down effect that MtB mentions, but not on all boats or all bridges, I noticed it most in the Midlands on the narrow canals, maybe it depends on engineering/construction techniques of the canal or draft of boat?

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How do you know???

 

And since you do know how, apply this technique to multiply the speed obtained from the method in post #54 by the "substantially " factor.

 

Visual perception. I can see (and feel) the boat slow noticably (ok a tautology!) as it enters a a bridge hole on the southern Oxford. Same effect occurs on entering a lock.

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Just completed 270 slow noisy revvy miles on a rather underpropped boat (I think half the prop has been eaten by galvanic corrosion). It's downright frustrating the number of people who stuck their heads out of moored craft to shout "slow down" at a boat doing under 2mph just because the engine was revving!

 

I've also observed the slowing down effect in some bridge holes on narrow canals. I thought this was just because the canal was shallower and narrower under the bridge.

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I've also observed the slowing down effect in some bridge holes on narrow canals. I thought this was just because the canal was shallower and narrower under the bridge.

 

It is. It'll be exaggerated going 'upstream' on waters where there is a significant flow, such as the Llangollen.

 

As an extreme case, we were going upstream through Radcot bridge, on the Thames, on yellow boards, got most of the way through & then started going backwards as a head built up in front of us.

 

We backed away, had a bite of lunch, cleared a bit of weed from the prop and had another go. Exactly the same result, though this time with the cleared prop we managed to force a way through with some more power.

 

Tim

Edited by Timleech
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A friend has a boat with a hydraulic drive fitted with a trolling valve so his engine always does a constant speed driving an electrical generator regardless of whether he is topped, flat out or going backwards so moored boats don't hear him slow down.

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How do you know???

 

And since you do know how, apply this technique to multiply the speed obtained from the method in post #54 by the "substantially " factor.

 

Well being intrigued, this afternoon I downloaded the SpeedometerView app mentioned earlier in this thread.

 

Setting my engine at 400rpm my boat was achieving a reasonably consistent 3mph on the southern Oxford this afternoon. On entering a lift bridge hole SpeedometerView records the speed of my boat as falling from 3mph to about 1.8mph. I observed this happening on every lift bridge hole I passed through.

 

The fall is progressive and only ceases once the stern clears the bridge hole, when the boat gradually speeds up to 3mph again.

 

It was not possible to do the same experiment with masonry bridges as the proximity of the masonry seemed to stop SpeedometerView from working, making it return a speed of 0mph near and under the bridge.

Edited by Mike the Boilerman
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That's quite a speed drop, I think on the K&A I notice it more in the swing bridge holes than the ordinary ones. How accurate are these app things at boat speeds? Feet per sec is accurate enough I reckon and using shadow lines on the rooftop is gloriously low tech.

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That's quite a speed drop, I think on the K&A I notice it more in the swing bridge holes than the ordinary ones. How accurate are these app things at boat speeds? Feet per sec is accurate enough I reckon and using shadow lines on the rooftop is gloriously low tech.

 

Well the lift bridge 'oles are only about 8ft wide and dead shallow. K&A swing bridge 'oles are by definition 15ft wide so the effect I describe will be massively diluted.

 

I agree about the shadow line arithmetic but the values quoted earlier are obviously wrong!

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That's quite a speed drop, I think on the K&A I notice it more in the swing bridge holes than the ordinary ones. How accurate are these app things at boat speeds? Feet per sec is accurate enough I reckon and using shadow lines on the rooftop is gloriously low tech.

It's not unusual on the Macc and Peak forest for a deep draughted boat to come to a complete stand until sufficient water has been moved from in front of the boat to behind it to allow progress.

 

George ex nb Alton retired

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Just completed 270 slow noisy revvy miles on a rather underpropped boat (I think half the prop has been eaten by galvanic corrosion). It's downright frustrating the number of people who stuck their heads out of moored craft to shout "slow down" at a boat doing under 2mph just because the engine was revving!

 

 

There's no pleasing some people, pass moored boats with a low revving vintage twin at idle and they still tell you to slow down!

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That's quite a speed drop, I think on the K&A I notice it more in the swing bridge holes than the ordinary ones. How accurate are these app things at boat speeds? Feet per sec is accurate enough I reckon and using shadow lines on the rooftop is gloriously low tech.

The Ullyse app I use has a "slow speed" setting, so gives speed to 0.1 mph, it seems pretty accurate for seeing the speed of the boat. As MtB notes, it shows speed dropping through bridgeholes on narrow canals, although I don't drop as much speed in my clonecraft as MtB does in his historic boats, and mine works even when the masonry blocks the view to the satellites :)

 

They are an interesting toy to get a feel for your speed, rather than something to use all the time.

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Oxford Thames (down stream), today 1500 rpm = 4 mph.

 

No idea of the speed of the flow. wink.png

 

Interestingly, I studied the speed of the flow quite closely a few autumns ago when I was bringing Aldebaran up to Oxford from Reading.

 

Each hop I would walk next to the river on yellow or red boards to estimate the speed of the flow. Even under the worst red board conditions I could always keep up with the speed of the flow without needing to break into a jog. My fast walking pace is about 4mph and matched the speed of the flow much of the time.

 

In benign conditions like now I'd estimate the flow speed to be about 1 mph.

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Yes you're right about the Thames, it's never all that fast especially in summer. Mind you, some of the bridge holes on that are a different matter. There's one near Windsor, Victoria possibly, came through that on yellows last year & it was fierce. And then there's Sodding Bridge....

Yes the bridge hole speed check is obviously flawed cos it's unrepeatable on different waterways & boats. It was working quite well the other week when I tried it on the Long Pound but I was also cross checking against other stuff with a decent shadow line. Mind you, I'm lucky to get more than 2 an' a half to 3 on there.

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Hmmm, 1mph is definitely 1.466 fps.

 

I withdraw my outrageous assertion your figures were wrong. Having looked again I can't imagine why I suggested this. They look fine to me now.

 

I think there must have been something wrong with the alcohol...

 

My apologies.

 

MtB

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New to Android 'phones I am fascinated by the 'apps' (and dismayed by the battery and data consumption). My Moto E 2nd generation finds 18/28 satellites in <6s inside my steel boat. The position error is 10 to 30 feet which would make an average speed calculation inaccurate at low speeds over short distances. Apparently, recent GPS speedometers calculate speed from the Doppler effect on the UHF signals from many satellites and accuracy can approach 0.01 knots at any speed using high sampling rates. A resolution of 0.1mph seems possible if sufficient satellites are available and there are not many reflected signals.

 

Alan

 

 

.

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It's not unusual on the Macc and Peak forest for a deep draughted boat to come to a complete stand until sufficient water has been moved from in front of the boat to behind it to allow progress.

 

George ex nb Alton retired

 

^^^^^^ This ^^^^^^

 

and on quite a few of the narrow canals further South, as well.

 

Quite a few bridge holes we have taken "Sickle" through where you just have to wind the power down, (using more is counter-productive), and wait until the boat sorts itself out.

 

The disused stop lock at Kings Norton used to be similar, but I think it is possible the works there have improved the situation since.

 

"Sickle" will regularly come to a temporary near standstill at places where "Chalice" hardly used to slow at all. Goes with the territory of being deeper than other boats, (and also somewhat wider, so less water can pass easily down the gaps at each side, which I guess may also affect it in cases where bugger all is passing underneath!).

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