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What RPM should I be cruising at?


Herdwick

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Newbie question. Our new to us 35’ flat bottom narrowboat will only seem to go approx 2.5mph. We have only been out a few times, and previous owner said to cruise at around 1500rpm. Even if we go to around 1800rpm it is still only around 2.5mph - based  25 minutes to go from one mile marker to the next (and not having to slow for boats or anything).

 

I know our boat has a Nanni Diesel 3.75 engine. I have no idea about gearbox or prop pitch etc.

 

Is 1500 or even 1800 rpm babying it? Should I be cruising at higher revs? 
 

4mph seems like an unachievable speed with this boat. Am I doing it wrong?

 

side note: if 2.5mph is max speed, would I be able to do the Ribble Link?

 

Thanks

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Shape of hull and depth of water will restrict you speed, if by increasing your revs the speed doesnt increase then there is probably not enough water for you to go any faster.

A standard flat bottomed NB should be able to achieve up to about 6mph.

 

The other limiting factor is prop size, If the prop is too small then it can rev freely but you'll not move very fast, if the prop is too big the it is possible that you cannot get it to turn fast enough.

A bit like Goldilocks porridge, the prop must 'be just right' and matched to the boat hull, the engine and the gearbox.

 

One of our NBs had a 3:1 gearbox so the engine revs were much higher to achieve the same speed than a 2:1 gearbox. It was an ex-hire boat and the gearbox was chosen to make 'so much noise' that the hirers would not be able to 'break the speed limit'

 

What gearbox ratio do you have ?

 

When you increase revs but don't go any faster - do you get more smoke out if the exhaust ?

 

Cruising at 1300 - 1500rpm would be 'normal' for a modern engine, and could be as low as 800rpm for a vintage engine.

 

The 4mph is not a 'target' or an aspiration, you should cruise at a speed that does not cause waves to hit the bank - that could be 4mph, or more likely 2 or 3 mph.

 

Get onto a river or deep canal and try again and see what happens.

Presumably your boat isn't at Bispham.

Edited by Alan de Enfield
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Are you sure there is no debris caught on the prop? Have you asked the revious owner what speed he thought he was doing at 1500rpm? It does sound remarkable slow as a top speed - a half decent walking speed is 4 mph.

 

Tam

Edited by Tam & Di
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32 minutes ago, Herdwick said:

Our new to us 35’ flat bottom narrowboat will only seem to go approx 2.5mph. We have only been out a few times, and previous owner said to cruise at around 1500rpm. Even if we go to around 1800rpm it is still only around 2.5mph - based  25 minutes to go from one mile marker to the next (and not having to slow for boats or anything).

I think this illustrates that the issue you're experiencing is  shallow water effect. The more power you apply, the more the stern digs in and exacerbates the braking effect. Just accept that your comfortable cruising speed is 2.5 mph and enjoy the fuel economy benefits of maintaining that speed with the lowest revs you can - this may well be even lower than the 1500 you quote. My boat is 57' with a Beta 43 engine and 17" prop and I rarely ring on more than 1350 and frequently less and 2.5 mph is pretty much my usual rate of progress too - more and more revs make increasingly less difference. In deeper water all that changes - try it out for yourself and learn how your boat performs.

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What waterway are you cruising on? Have you taken it on a deeper one, or a river? If it is a shallow narrow canal, then 2.5mph may be as fast as you can go. The boat is having to push the water out of the way. More power will just squat the stern down in to the mud more.

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27 minutes ago, Sea Dog said:

I think this illustrates that the issue you're experiencing is  shallow water effect. The more power you apply, the more the stern digs in and exacerbates the braking effect. Just accept that your comfortable cruising speed is 2.5 mph and enjoy the fuel economy benefits of maintaining that speed with the lowest revs you can - this may well be even lower than the 1500 you quote. My boat is 57' with a Beta 43 engine and 17" prop and I rarely ring on more than 1350 and frequently less and 2.5 mph is pretty much my usual rate of progress too - more and more revs make increasingly less difference. In deeper water all that changes - try it out for yourself and learn how your boat performs.

 

27 minutes ago, Jen-in-Wellies said:

What waterway are you cruising on? Have you taken it on a deeper one, or a river? If it is a shallow narrow canal, then 2.5mph may be as fast as you can go. The boat is having to push the water out of the way. More power will just squat the stern down in to the mud more.

 

Both the above. If the OP wants a more informed answer we need to know which canal and where.  I found parts of the Ashby and Chesterfield bad, bt almost any narrow canal has places that severely limt the top  speed.

 

 

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You don’t say where you were trying the boat out. On a typical narrow canal 2.5 mph would be a reasonable max speed, but on a deep river you could expect to be able to go faster.  As more revs make little difference it is the Hull going through the water that is limiting the speed, usually due to there being nor much water under 5he boat.

 

All the above of course assumes the prop is clear.

 

In terms of the Ribble link, you need to know what the boat will do in deep water before being able to decide.  Ours will only do 5mph in deep still water (the G&S is a good test ground, deep canal with a 6mph speed limit), without starting to push the engine revs up, we had no problem on the Ribble link.

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A few years ago we went out of Teddington with a several other boats. One man was very new to boating but an ex high ranking military man who naturally took control. Leaving the lock he shouted to us and the other boats "increase revs to 1500, increase revs to 1500". I didn't have the heart to tell him that we were only doing 900 and out our engine only revs to 1200.

 

............Dave

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2 hours ago, john6767 said:

You don’t say where you were trying the boat out. On a typical narrow canal 2.5 mph would be a reasonable max speed, but on a deep river you could expect to be able to go faster.  As more revs make little difference it is the Hull going through the water that is limiting the speed, usually due to there being nor much water under 5he boat.

 

All the above of course assumes the prop is clear. 

He does mention mile markers so it could be the GU or T&M he should be able to manage more than 2MPH, I was doing a little over 3 on the Ashby most of the time and actually got to 4 on a bit of the N Oxford,(only a little bit) Today I have done 12 miles in 4¼ hours from north of Ansty to just north of Hillmorton 

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We have quite a deep drafted boat but rarely cannot achieve 3mph, more like 3.5mph unless it's a very shallow canal. And this is at 1300rpm without any breaking wash, or even much in the way of wash. Seems likely to me the OP has something on the prop, or just a very small prop.

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A genuine question, or two -

 

How much influence does underwater shape have?

How much influence does standard of maintenance have e.g. over-tightened alternator belt(s) and/or stern gland??

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16 minutes ago, Col_T said:

A genuine question, or two -

 

How much influence does underwater shape have?

How much influence does standard of maintenance have e.g. over-tightened alternator belt(s) and/or stern gland??

If the engine can reach the revs then if nothing is slipping then the prop is turning at the gearbox ratio to engines revolutions. 

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Ah, okay. So dodgy maintenance - over-tightened belts and / or stern gland - is irrelevant as they will have already pulled down revs. But what about underwater hull shape - the fabled skip vs a boat with long swims fire and aft?

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23 minutes ago, Col_T said:

Ah, okay. So dodgy maintenance - over-tightened belts and / or stern gland - is irrelevant as they will have already pulled down revs. But what about underwater hull shape - the fabled skip vs a boat with long swims fire and aft?

Most of the skips manage over 3MPH some with more wash than others.

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We are on the Lancashire Canal. How shallow is a shallow canal? I think the Lancashire is mostly 5 feet deep ish? In the middle?

 

yesterday I timed again, going bang on at 1500rpm, took exactly 25 mins to go one mile (in between Galgate and Garstang).

 

I should have measured our prop when it was out of the water, and this is the only pic I have. I looked at the other narrowboats that were out of the water and thought they all looked about the same size. But when cruising we are constantly having to let other boats pass us - so others are able to go faster, just not us. That’s the biggest problem really. If everyone else were as slow as us I probably wouldn’t mind. But then there’s that Ribble Link that we want to cross to get to the rest of the network!?

 

 

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That is a shorter boat and in general the shorter the boat the shorter the swims are so the boat is as stable as possible within its size. Short stubby swims make it harder to get water into the prop and also tend to push more water in front of it than long swims will.

 

I very much doubt the Lancaster is that deep nowadays, not that I have been on it. Just like most other canals I would expect abut 2ft of water and then a couple of foot of mud slurry that increases its viscosity as you  go down. The pole may look a sf its gone down 5 ft but a lot of that will be in mud.

 

It's not just prop diameter, the pitch is important as well. As a general rule you want to swing the largest diameter prop that you can allowing enough tip clearance so may boats with a similar stern post height will look as if they have the same diameter prop but if the pitch is not enough you will need high revs for a given speed but I would expect more revs to give more speed until the maximum hull speed is reached or the engine runs out of revs. The maximum hull speed will be reduced in confined waters.

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3 hours ago, Herdwick said:

We are on the Lancashire Canal. How shallow is a shallow canal? I think the Lancashire is mostly 5 feet deep ish? In the middle?

 

yesterday I timed again, going bang on at 1500rpm, took exactly 25 mins to go one mile (in between Galgate and Garstang).

 

I should have measured our prop when it was out of the water, and this is the only pic I have. I looked at the other narrowboats that were out of the water and thought they all looked about the same size. But when cruising we are constantly having to let other boats pass us - so others are able to go faster, just not us. That’s the biggest problem really. If everyone else were as slow as us I probably wouldn’t mind. But then there’s that Ribble Link that we want to cross to get to the rest of the network!?

 

 

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The Lancaster is a very shallow canal and you are not going to be going very fast at all.  The are plenty of people on there that like to try going fast, and that canal is a bit of a pain for speeding boats.  If you want to try the boat in deep water nip down to Glasson docks and have a blast there and see what it can do.

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The canal width & depth will make a huge difference to how fast your boat can go.

for ours (45 foot cruiser stern with fairly long swims) the max on the ashby canal was 3mph (13-1400 rpm), pushing the revs higher actually slowed us down.

for contrast the same boat on deep water (thames going upstream) the same 13-1400 rpm gave 4.5mph and pushing the revs up to 2900 gave 9.5mph (we couldn't push higher on the revs as that was as fast as the engine could turn the prop, trying to push faster didnt increase revs but gave black smoke). as the flow was low on the thames at the time you could probably add an extra 1 - 1.5 mph onto the speed for speed through the water

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I recall having read somewhere (I don't recall where) that the optimum speed is the speed of a wave in the canal.  Once your boat has reached that speed, you effectively get carried along with your bow wave. As the speed of a wave is a function of the water depth, it makes speed adjustment for water depth simple. I have never made any scientific experiments myself, but as a rule of thumb  it usually works for me. 

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So many things affect boat speed. You can have something with a very square fore end (Many of the Humber and Northern boats are pretty much flat fronted - as are many European barges) but they nearly always have a well thought out back end, as much taper as possible to get water to the rudder and propeller. Even lumbering old sailing ships had a lot of shape in the stern.  Short narrowboats often suffer from the sort of boxiness that means the poor old propeller is using far too much energy accelerating  water around the short swim and there's not enough water being shoved out the back. This is not a scientific explanation! I think I can see quite a wide flat 'sternpost' as well, this will not help.  Seagoing yachty vessels and suchlike often have the shaft a bit longer and supported by a 'P' or 'A' bracket to get the propeller clear of the hull, this might be a practical solution, the rudder might have to have a bit of work too. Not a hard thing to do and somebody who understands these things could advise but probably not a canal boatyard. I'm really saying this as a possibility if all else fails so don't despair.

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Thanks for all the replies and advice. At least I know I wasn’t giving it not enough rpm’s. I over did it if anything. Without spending money on a new prop or gearbox or stern modification, I can live with it have a max speed of 2.5mph. It is what it is.

 

But can I still manage the Ribble Link?

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1 hour ago, Herdwick said:

But can I still manage the Ribble Link?

 

As advised by someone else. Go to Glasson docks and see what it will do on wide deeper water. Even a car satnav will give you a decent indication of speed.

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From fluidics lectures at college many years ago, I learned that, counterintuitively, the streamline configuration for lowest flow resistance, for sub-sonic flow at least, is a round nose facing the flow and a tapered tail downstream.  I noticed that that is how the footings of the piers of the railway viaduct that crosses the (non-tidal) River Chelmer in Chelmsford are constructed.  

Edited by Ronaldo47
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