New propellor!

[ditchcrawler]

13 hours ago, dmr said:

 

More pitch than diameter ???

The BD3 is a slightly bigger engine than the JD3, I think its 3.1litre??? and also (I think) does not rev quite so high, so a slightly bigger prop than the JD3 probably makes sense.  But your prop is still smaller than what the OP proposes for an older design 2.8l litre engine that revs quite high.

Anyway, havn't you got a funny hydraulic drive so who knows what is going on? (or am I thinking of somebody else?).

Thats me. That is why I said about 2:1 I think its 1.95 something. There shouldn't be any slippage but I don't know how much power it absorbs 

[GUMPY]

32 minutes ago, IanD said:

The shape and size of the boat affects the speed through the water which has a *small* effect on the prop size -- of course it assumes a decent water entry to the prop, only an idiot would design a hull without this 😉

It's a shame that 99% of narrowboats built now have vertical sided swims so the water entry to the prop is compromised.

I haven't seen a new NB hull with a double curved rear swim for many years.

[magnetman]

Not a narrow but this is one way of doing it (my country estate boat when I bought it 14 yars ago)

 

9ft beam so one can afford to have the chines. I expect it would take out too much internal space on a narrow but it has been done I believe on certain craft. 

 

Does get a bit of water to the prop. 

 

 

[Bee]

My sander is 300 watts, it spins a 5"sanding disc adequately, I think I'd like a bit more power for a boat. It is not a simple thing though, There are very few times that i have ever pushed the engine to anything like its full power or its max revs. to obtain the max speed the boat can do, most of the time it is only running at a fast tickover - say 1500 rpm or maybe a bit more - its a Beta 43 and an 18 x 12 prop. I reckon that many narrowboats have not got the best sized prop and the majority of displacement boats have a reasonable compromise at best and if you can't get the 'ideal' size then something near will work OK and you might just have to use an extra 100 rpm. to get your desired speed. Oh and E Bay is my first choice for somewhere to look

[IanD]

45 minutes ago, magnetman said:

300 watts to push a 23 ton narrow boat at 1.9mph on the middle Thames?

 

I do know from experience that it takes less than you think to push heavy boats but I have doubts this is achievable with 300w (0.4hp). 

 

I'd be wary of these sorts of calculations if designing a narrow boat which is meant to be satisfactory. 

 

So long as you go slowly enough it doesn't take much power to push a boat. Power goes up with speed^3 (and rpm^3) and both hydrodynamic theory and measurements agree with this -- so 0.4hp to do 1.9mph means 3kW/4hp (typical *measured* cruising power for a narrowboat) would give 4.1mph in deep water, so around 3mph in a typical canal -- and again this is perfectly reasonable.

 

Which again agrees with actual measurements -- so I'm sorry, but your doubt isn't justified... 😉

 

Nobody's saying that 300W is enough to power a narrowboat on the Thames -- or on a canal, unless 1.5mph or less is fast enough for you, and you don't mind taking forever to stop -- but don't forget that 1hp used to be plenty... 😉

[IanD]

8 minutes ago, Bee said:

My sander is 300 watts, it spins a 5"sanding disc adequately, I think I'd like a bit more power for a boat. It is not a simple thing though, There are very few times that i have ever pushed the engine to anything like its full power or its max revs. to obtain the max speed the boat can do, most of the time it is only running at a fast tickover - say 1500 rpm or maybe a bit more - its a Beta 43 and an 18 x 12 prop. I reckon that many narrowboats have not got the best sized prop and the majority of displacement boats have a reasonable compromise at best and if you can't get the 'ideal' size then something near will work OK and you might just have to use an extra 100 rpm. to get your desired speed. Oh and E Bay is my first choice for somewhere to look

That size for the Beta 43 is not far off what Vicprop recommends, which is 17.4" x 11.1" (43bhp/2800rpm) -- drop to 42bhp/2600rpm (from Beta 43 data sheet curves) and you get 18.1" x 11.9", suggesting that this is what will happen at full throttle with the recommended 18" x 12" prop -- the engine won't quite reach absolute maximum power/rpm but only loses 1hp, and 200rpm lower means less noise.

 

That would then give about 4bhp at 1200rpm and maybe 3mph when cruising on a canal.

 

A small difference in speed means a *huge* difference in power because of the cube law -- it's why some of the electric boat vendors make extravagant claims about required power and being able to run on solar power alone, when in reality they're running at half the power just by going 20% slower...

[magnetman]

12 minutes ago, IanD said:

 

So long as you go slowly enough it doesn't take much power to push a boat. Power goes up with speed^3 (and rpm^3) and both hydrodynamic theory and measurements agree with this -- so 0.4hp to do 1.9mph means 3kW/4hp (typical *measured* cruising power for a narrowboat) would give 4.1mph in deep water, so around 3mph in a typical canal -- and again this is perfectly reasonable.

 

Which again agrees with actual measurements -- so I'm sorry, but your doubt isn't justified... 😉

 

Nobody's saying that 300W is enough to power a narrowboat on the Thames -- or on a canal, unless 1.5mph or less is fast enough for you, and you don't mind taking forever to stop -- but don't forget that 1hp used to be plenty... 😉

 

I must try it as have a 300w motor on the canoe. To be fair it will move the mothership but put too large a prop on it and you overload the motor.

 

I suppose it probably would work but I think the 1.9mph is too optimistic. 

[magnetman]

 

 

ETA I also think the vicprop may be making some assumptions around longer boats having a higher hull speed. 

 

I'm not all that convinced this applies for narrow boats because they don't generate a long bow wave.

 

A "boat shaped boat" will create a bow wave which will gradually move down the side of the hull which eventually limits the speed unless you go into semi displacement or planing mode. 

 

I don't think a narrow boat does this so I feel that the 1.34 equation may not apply. Just a gut feeling. 

 

[IanD]

2 minutes ago, magnetman said:

 

 

ETA I also think the vicprop may be making some assumptions around longer boats having a higher hull speed. 

 

I'm not all that convinced this applies for narrow boats because they don't generate a long bow wave.

 

A "boat shaped boat" will create a bow wave which will gradually move down the side of the hull which eventually limits the speed unless you go into semi displacement or planing mode. 

 

I don't think a narrow boat does this so I feel that the 1.34 equation may not apply. Just a gut feeling. 

 

The Vicprop numbers are for a boat *in deep water*, in a canal the boat will go slower -- as I keep on saying...

 

However the only time a narrowboater is *ever* going to need full power is on a river (Thames, Severn, Avon, Trent, Ribble link...) and probably even then only when going against the current -- and this *is* deep water, on the scale of a narrowboat hull.

 

If you take the Beta 43 case this predicts 8.3kts in deep water, the displacement hull speed (60' boat) is 10.4kts which needs 87hp. Few narrowboats are ever going to get anywhere near this, they're so far below this that the normal hull drag numbers apply, the hull shape won't make that much difference. Longer boats do have a higher displacement limit and physics says this is true whatever shape the hull is, narrowboats included.

[magnetman]

I was not referring the water depth. 

 

i was concerned by the behaviour of a displacement hull. 

 

As I said there is a well known bow wave phenomenon which dictates hull speed. I don't think -typical- narrow boats behave in the same way as normal displacement craft. The bow wave does not get longer as you go faster. 

 

What I was getting at is that the calculations used by the vicprop program will be making certain assumptions and I believe narrow boats will be outside of the scope. 

 

Not just about maximum speed available for a given power installation.

Edited February 22, 2023 by magnetman

[IanD]

37 minutes ago, magnetman said:

I was not referring the water depth. 

 

i was concerned by the behaviour of a displacement hull. 

 

As I said there is a well known bow wave phenomenon which dictates hull speed. I don't think -typical- narrow boats behave in the same way as normal displacement craft. The bow wave does not get longer as you go faster. 

 

What I was getting at is that the calculations used by the vicprop program will be making certain assumptions and I believe narrow boats will be outside of the scope. 

 

Not just about maximum speed available for a given power installation.

 

If you think that narrowboats don't obey the same laws as other hulls, I've got bad news for you... 😉

 

Look at the shape of a narrowboat hull and both proportions and shape (including bow/stern) are actually pretty similar to a large container ship -- which are also almost rectangular because they have to go through the Suez canal -- and nobody thinks the equations (wake and hull drag) don't apply to these, Froude scaling *** also suggests the two are quite similar. AFAIK there's no evidence to back up your suggestion and a lot to say that it's not correct.

 

As I keep saying (but you keep ignoring) any differences between Vicprop and "real narrowboats" are far more likely to be speed -- which as I said, has been measured as considerably lower in canals, as expected -- and not the power/torque vs. rpm curves for a propeller, which are well known and very little influenced by the hull.

 

*** Froude number = speed/sqrt(length), two hulls with the same number have the same hydrodynamic properties, which is why scale models work. Evergreen G-class is 1314' x 192' x 53' (235,000tons, 80000hp) cruising at 23kts, divide speed by 5 and length by 25 gives 53' x 7.7' x 2.1' (15 tons, 5hp) at 4.6kts which is very close to a narrowboat -- which is a pretty accurate 1/25th scale model of the Evergreen 🙂

 

(but a lot easier to shift if you get it wedged diagonally across a canal...)

Edited February 22, 2023 by IanD

[cuthound]

23 hours ago, David Mack said:

When we bought former Grand Union Canal Carrying Co Ltd narrow boat Belfast No. 115, I assembled the following facts:

 

• Belfast 115 was built by Harland and Wolff at North Woolwich. HMS Belfast was built by Harland and Wolff in Belfast.
• Belfast 115 was registered on 9 December 1936. HMS Belfast’s keel was laid down on 10 December 1936.
• Belfast 115, after the end of its carrying life, was sold off by Government-owned BWB to the DNBP charity in 1971. HMS Belfast, after withdrawal from naval service in 1963, was sold by the Government to a preservation trust charity in 1971.
• On the other hand, HMS Belfast is 8.6 times as long, 9 times as wide and draws 6.6 times as much as Belfast 115.
• And HMS Belfast was built with 48 guns and 6 torpedo tubes, and we have none of either!

 

 

On our first shareboat I interrupted a holiday to attend the annual owners meeting. At the owners meeting I suggested fitting the boat with torpedo tubes, having had a boat pull out in front of me just before a long lock flight and who proceeded to faff about for 20 minutes at each lock, delaying me considerably. Surprisingly none of other owners agreed with this suggestion. 😞

 

 

 

Edited February 22, 2023 by cuthound
Missing worm

[IanD]

23 minutes ago, cuthound said:

 

On our first shareboat I interrupted a holiday to attend the annual owners meeting. At the owners meeting I suggested fitting the boat with torpedo tubes, having had a boat pull out in front of me just before a long lock flight and who proceeded to faff about for 20 minutes at each lock, delaying me considerably. Surprisingly none of other owners agreed with this suggestion. 😞

 

I think I've got a torpedo tube, but it fires sideways which is strange...

[cuthound]

9 minutes ago, IanD said:

I think I've got a torpedo tube, but it fires sideways which is strange...

It should come in handy for dealing with those unwanted overtaking attempts... 😂😅

[IanD]

2 minutes ago, cuthound said:

It should come in handy for dealing with those unwanted overtaking attempts... 😂😅

Thing is, the rudder is the most vulnerable target -- but if I wait until then and they do a Bismarck they could take me out as well...

[magnetman]

MRL is the thing to have. 

[David Mack]

9 hours ago, DShK said:

 

I've been told that it's possible to swap props in the water - if you're careful (given brass/bronze isn't magnetic!).

Probably only if you have a weedhatch!

[Alan de Enfield]

4 minutes ago, David Mack said:

Probably only if you have a weedhatch!

 

 

[Tony Brooks]

18 hours ago, magnetman said:

As I said there is a well known bow wave phenomenon which dictates hull speed. I don't think -typical- narrow boats behave in the same way as normal displacement craft. The bow wave does not get longer as you go faster. 

 

Maybe not, but the stern certainly drops in the water, so the prop is trying to push the boat uphill, the faster you go, the more it drops. This seems analogous to the stern falling into the trough of the bow wave on a boat shaped boat.

 

I am not getting into the rest because I feel decent prop sizing is only one removed from witchcraft - especially on canal boats in use on canals.

Edited February 23, 2023 by Tony Brooks

[MtB]

1 hour ago, Tony Brooks said:

I am not getting into the rest because I feel decent prop sizing is only one removed from witchcraft - especially on canal boats in use on canals.

 

i agree, specifically with canal boats. 

 

I've messed around with different prop sizes on a number of my boats and I've concluded that any prop size spat out by a prop calc feels underpropped once fitted to a narrowboat with a vintage engine.

 

And conversely, any blade that works well and feels good on a NB with a vintage lump tends to actually be overpropped. I think this is because vintage engines feel good cruising at slow revs, and nothing like at full chat. But it is mostly about the perceptions and opinions of the steerer, rather than the results of some calculations.

[DShK]

So my I'm both under propped and over propped 🤪 with regard to the noise I think I am going to (with careful help from friends) take the floor above the prop shaft up, run the engine while still roped up. And see if the prop shaft is moving in a way it shouldn't. I do at times get vibrations in my steps through the floor, so I'm not sure if that is coming through the shaft or the hull. Might be enlightening?

[IanD]

12 minutes ago, MtB said:

 

i agree, specifically with canal boats. 

 

I've messed around with different prop sizes on a number of my boats and I've concluded that any prop size spat out by a prop calc feels underpropped once fitted to a narrowboat with a vintage engine.

 

And conversely, any blade that works well and feels good on a NB with a vintage lump tends to actually be overpropped. I think this is because vintage engines feel good cruising at slow revs, and nothing like at full chat. But it is mostly about the perceptions and opinions of the steerer, rather than the results of some calculations.

 

That last bit is also what I said -- particularly with a trad engine people may prefer to be overpropped to cruise at slow revs, or underpropped because otherwise the boat is too fast at idle and badly-moored boaters will stick their heads out and shout "slow down!" 🙂

 

However if you do want to get the most power out of the engine for use on rivers -- probably the only time this will ever actually be needed, when going against a strong flow -- which can be needed even more with a lower-powered trad engine than a modern one, then a matched prop (or one close to one, like the Beta 43 case mentioned above) is a good idea. That's when the "some calculations" are useful, because that's what they're based on doing.

1 minute ago, DShK said:

So my I'm both under propped and over propped 🤪 with regard to the noise I think I am going to (with careful help from friends) take the floor above the prop shaft up, run the engine while still roped up. And see if the prop shaft is moving in a way it shouldn't. I do at times get vibrations in my steps through the floor, so I'm not sure if that is coming through the shaft or the hull. Might be enlightening?

Do you only get the noise if you're in gear, or also if you rev the engine with the gearbox in neutral?

[Bee]

Changing props through a weedhatch is kind of possible with luck and grim patience but the last time I changed a prop (Out of water) it was not easy and more to the point the key needed a bit filing off of the top as it was a bit too high and the prop would not slide nicely on the shaft taper. Might not make a very big difference at canal revolutions but it needed doing properly. As for dropping the key into 4 feet of murky water well I'd rather not even think about that!

 

[magnetman]

The do vary. Sometimes a prop will be completely stuck. I once had a prop and shaft off one of my boats (on the hard) and the man at the boatyard said I would need oxyacetylene to get it off.  Shaft was being replaced anyway so I zipped it off just below the prop with a slitting disc, took the nut off rested the prop on two pieces of wood and whacked the end of the shaft with a sledgehammer. It dropped out of the taper. So much for needing heat...

 

I you did get a prop properly stuck on the taper I don't see how you could apply the needed force with it in the water, working through a weed hatch.

 

 

 

 

15 minutes ago, Bee said:

Changing props through a weedhatch is kind of possible with luck and grim patience but the last time I changed a prop (Out of water) it was not easy and more to the point the key needed a bit filing off of the top as it was a bit too high and the prop would not slide nicely on the shaft taper. Might not make a very big difference at canal revolutions but it needed doing properly. As for dropping the key into 4 feet of murky water well I'd rather not even think about that!

 

When I was a teenager we had a speedboat with a 20hp outboard. Garden by the River. I had to change the prop once leaning over the outboard which was tilted out of water. Dropped the non magnetic dome nut in the River. Went in to find it and never located it. Bought another one. Did same thing.

 

Moral is if doing this sort of job try to have something beneath the area to capture dropped items. Not all that easy underwater but I think one of those big flexible rubber buckets with a brick or two in it might work maybe with a rope on each handle and somehow secured to the boat in the right place.

 

Could be done?

Edited February 23, 2023 by magnetman

[IanD]

8 minutes ago, magnetman said:

The do vary. Sometimes a prop will be completely stuck. I once had a prop and shaft off one of my boats (on the hard) and the man at the boatyard said I would need oxyacetylene to get it off.  Shaft was being replaced anyway so I zipped it off just below the prop with a slitting disc, took the nut off rested the prop on two pieces of wood and whacked the end of the shaft with a sledgehammer. It dropped out of the taper. So much for needing heat...

 

I you did get a prop properly stuck on the taper I don't see how you could apply the needed force with it in the water, working through a weed hatch.

 

 

Using a sledgehammer (or a slide hammer, or any hammer...) to remove a prop in situ with the shaft still in place is a recipe for buggering up the gearbox bearings -- don't do it, the damage could only show up (expensively) later... 😞

 

https://marinehowto.com/a-new-prop-shaft/

Slide-Hammers = NO!

Before you engage a boat yard to do this job, I will type this so it hopefully makes sense; NEVER use a “slide-hammer” to remove a shaft from a coupling if the coupling is attached to the gear box!

 

Slide hammers can cause brinneling of the bearings or races in the gear box. The shock loads imparted on the static bearings, by the “slide-hammer”, actually create flat-spots in the races or bearings themselves. Your gear box may appear to work for some time after the slide-hammer event but eventually, the damage rears its ugly head and it’s next to impossible to lay blame on the slide-hammer user as they rarely fail instantaneously.

Edited February 23, 2023 by IanD