Engine Size

[Maverick]

Anyone tell me if thers a (rule of thumb) size for an engine for a 30ft narrowboat? reason I'm asking is mine is currently 30hp and its obviously to big for the boat, wheras some engines ive seen advertised on small narrowboats appear to be no larger than an outboard that a fibreglass would use at 10 & 11 hp

[Tony Brooks]

Anyone tell me if thers a (rule of thumb) size for an engine for a 30ft narrowboat? reason I'm asking is mine is currently 30hp and its obviously to big for the boat, wheras some engines ive seen advertised on small narrowboats appear to be no larger than an outboard that a fibreglass would use at 10 & 11 hp

 

 

The first thing to say is that the answer is "it all depends upon what you are going to do with it"

 

The second is that most people measure engine size in hp, but for driving a prop torque and rpm are more important. A 72ft fully laden working boat with traditional engine may only have had perhaps 5 to 10 hp, but it had massive torque so (when matched to the correct prop - a very important issue) it would probably pull most of our modern 35 to 45 hp engined boats backwards.

 

On a narrow canal where pushing the water in front of you is probably the most speed limiting factor I would suggest 5hp would be more than ample (might not stop too well) - Waterways World published an article years ago that said a 60ft boat only needs abort 2hp at canal speed.

 

On deep, wide waters where wave making (in the absence of a speed limit) is the limiting factor you would need a far larger engine to attain maximum speed which may be about 8 or 9 knots in theory.

 

Anther factor to consider is smoothness/lack of vibration. On flexibly mounted engines this would typically indicate a 4 cylinder (or 3 with internal balance weights) and the lower hp engines tend to have less than four cylinders. On the other hand very small NBs tend to use one or two cylinder engines simply to save on space.

 

If I was going to spend a lot of time on tidal rivers (which are also subject to floods) I would probably specify something around 50hp for up to about 58 ft, for mixed use with summer visits to rivers and very occasional tidal use I would go for something a little above 30hp. I suspect most people would find 35 hp with a well matched prop adequate.

 

 

But they would all still only be delivering that 2 or 3 hp at canal speed!

 

Tony Brooks

[bottle]

Hi Maverick

 

Have a look here Beta Marine Rough guide to engine sizing/prop size. This is only a guide please read the first paragraph.

[gary4lw]

Hi Maverick

 

Have a look here Beta Marine Rough guide to engine sizing/prop size. This is only a guide please read the first paragraph.

Doesnt it depend how big your willy is ?

[MasterDabber]

Maverick. Thanks for asking the question I've been thinking about.

 

Some useful replies.

 

I'm looking at a secondhand nb purchase but looking at advertised details (even the pretty comprehensive ABNB ones I've received) there is not always information on tonnage and none that I have seen anywhere on prop specification. I guess these are questions to ask if interested in proceeding further.

[Timleech]

Always worth looking at the max prop size your boat, or planned boat, will take, & try to take full advantage of that space (you really need a couple of inches tip clearance or thereabouts). That might mean specifying a 3:1 gearbox, if available, rather than the more standard 2:1. The initial expense may be a bit higher, but it will be more efficient overall (especially starting & stopping), also if your decision on engine size is hovering between two different sizes then it'll probably mean you will be happy with the smaller of the two.

 

Tim

[Tony Brooks]

Maverick. Thanks for asking the question I've been thinking about.

 

Some useful replies.

 

I'm looking at a secondhand nb purchase but looking at advertised details (even the pretty comprehensive ABNB ones I've received) there is not always information on tonnage and none that I have seen anywhere on prop specification. I guess these are questions to ask if interested in proceeding further.

 

 

I think weight or displacement would be a better term than Tonnage. There is at least one "tonnages" that appears odd to ordinary boaters because it relates to cargo volume.

 

I all honesty I doubt the weight of a narrow boat will make a significant difference to its prop size (it will make some) because skin friction and wave making are the two major factors for a displacement boat - now, if you fancy trying to get a flat bottomed narrow boat up on the plane weight may well matter and so will engine size - start looking at large straight 6 or V diesels fitted with both supercharger ands turbocharger as per Volvo. I expect 200 to 300 hp might just about do it :-)

 

 

Castle Marine in Wales has a prop size calculator you can download (when their site is up) and it will accept narrow boat type inputs, so you can play about with engine & prop sizes and gearbox reduction ratios. Without measuring a particular boat assume a modern cruising hull can swing something like a 16" prop.

 

Tony Brooks

[Timleech]

Castle Marine in Wales has a prop size calculator you can download (when their site is up) and it will accept narrow boat type inputs, so you can play about with engine & prop sizes and gearbox reduction ratios. Without measuring a particular boat assume a modern cruising hull can swing something like a 16" prop.

Tony Brooks

 

Can anyone explain to me a good reason why some volume builders now produce boats with 18" or less under the counter, when the 'finished' draught will be very likely getting on for two feet? I can't see why they do it, the counter drags in the water when under way creating inefficiency, and the choice of propellor size is severely limited.

Maybe it's just to cover up for a poor underwater shape, if the counter is deep enough in the water it'll mask that so that air won't get drawn in etc.??

 

I think we should be told!

 

Tim

[DHutch]

Can anyone explain to me a good reason why some volume builders now produce boats with 18" or less under the counter, when the 'finished' draught will be very likely getting on for two feet?

Stupidity? Carelessness?

- I guess if nothing else, there is an argument that says 3inches under is better than an inch above.

 

We draw about 2ft9 at the back, with the counter about an inch under when stationary, A little less if we're running the coal down.

- I'd say thats just about right really. It bassicaly never cavitates. Although when its very low on coal and you reverse hard it can be noticable.

 

But the other thing i dont get, is having got 18inchs to play with, why they then fit a 12" prop or somthing silly!

- I guess thats just down to cost? A smaller prop is a cheaper prop?

- Ours is a 26" crowther prop, which again works very well, but it wasnt cheep, worth it though!

 

 

 

Daniel

[John Orentas]

Can anyone explain to me a good reason why some volume builders now produce boats with 18" or less under the counter, when the 'finished' draught will be very likely getting on for two feet? I can't see why they do it, the counter drags in the water when under way creating inefficiency, and the choice of propellor size is severely limited.

Maybe it's just to cover up for a poor underwater shape, if the counter is deep enough in the water it'll mask that so that air won't get drawn in etc.??

 

I think we should be told!

 

Tim

 

 

Hi Tim.

 

I think you have not been told because no-one knows, hull design is very complex and very expensive, I very much doubt if any builder has ever done any development work at all though many will claim they have. There is certainly no rule which says that the counter should not run under water, in fact I and others have found to the contrary, that it needs to be well into the water to prevent cavitation.

 

Nor is there any good reason why a small propeller should not be every bit as efficient as a bigger one, certainly the adverse paddle-wheel effect will be lessened and it will suffer less drag.

[Timleech]

Hi Tim.

 

I think you have not been told because no-one knows, hull design is very complex and very expensive, I very much doubt if any builder has ever done any development work at all though many will claim they have. There is certainly no rule which says that the counter should not run under water, in fact I and others have found to the contrary, that it needs to be well into the water to prevent cavitation.

 

John

I'm sure you're right about a lack of hull development, though there was some work done by BW & Alvechurch on the subject.

Most pleasure narrowboats today are just scaled down copies of the working Narrow Boat form, some quite good copies & some pretty poor imitations. IMO the 'Small Woolwich' as built by Harland & Woolff was probably the best working NB form ever built, especially the stern shape. It's all been downhill since then

Needing the counter well under water suggests to me that the stern shape is less than ideal, that's what I was alluding to as a possible reason why builders do it, and running the counter substantially under water is bound to increase drag and/or turbulence.

 

Nor is there any good reason why a small propeller should not be every bit as efficient as a bigger one, certainly the adverse paddle-wheel effect will be lessened and it will suffer less drag.

 

It's well established that larger propellors are much more efficent when it comes to stopping & starting a boat, I believe there to be benefits when it comes to 'steady state' movement though I can't quote chapter & verse there. I'm not sure why you're concerned about increased drag, how can a prop create drag when it's propelling the boat forward? Yes it can be a problem with a sailing boat, but that's a different matter.

 

 

Going back to the original question, just as an example for the OP, the only boat I've built from scratch (I'm a repairer/engineer) is 35' long, it's 20 years old now, fitted with a Vetus/Mitsubishi 3-cylinder engine (with 3:1 reduction!), the present owners have had it from new & there's never been the slightest hint that they've thought it underpowered. Mind you, they haven't tried to go to the moon with it . The counter sits flat on the water when static, BTW, & there's never a problem with cavitation or drawing air into the blades. ISTR those engines were well under 20bhp when they first came out, later versions were 20 & then 22 bhp. It's also smooth running. The current Vetus 3-cyl is quite a bit more powerful, & a 2-cyl version may well have enough hp for a 30' boat but I can't comment on how smooth or quiet they might or might not be.

 

Tim

Edited May 20, 2007 by Timleech

[Adam]

What size of engine do dutchbarges have say 57 x 12 ft?

[John Orentas]

Hi Tim.

 

I somehow think we are in general agreement, that Alvechurch boat was about the only attempt at development in all my 30 years involvement with canal boats and even that was a watered down copy of a seagoing craft and I am not sure that sort of thing can be directly scaled anyway, I rather think they missed the point of what should have been a protruding bullet shape bow. My gut feeling would be that the underwater shape of the stern is more significant. I am sure you are right about the small Woolwich being the optimum shape but then that was designed probably 90 years ago, hardly the white heat of technical innovation.

 

The woolwich design incorporated many three dimensional curves which would probably add at least 100% to the build coat of a boat so even if great improvements could be shown I am not sure the design would catch on and a slippery straight life performance would no doubt have other costs.

 

Thirty years ago 30 horsepower was considered ample for even the largest boats, in the working days that kind of power would have been regarded enough for a tug pulling a couple of barges, yes I know all the arguments about 'torque' but most of it is a matter of perception, having a nice relaxed, slow revving engine, and there is a lot of rubbish said about torque anyway. A moped engine with the right gearing would push a fully loaded working boat along just as effectively as a Bollinder. Horsepower is horsepower.

[Timleech]

What size of engine do dutchbarges have say 57 x 12 ft?

 

I can't comment directly on Dutch barges. I had a share for some years in Humber barge (Keel or Sheffield Barge), 61' x 16', & ballasted to about 4' draught

That when first motorised in the 1930's had a 21hp Lister, she was 'modernised' in the 1960's and fitted with a 33 bhp Lister HA3.

In that form we took her across the North Sea (Boston to Ostend, non-stop, 40 hours run) and travelled extensively in France, Belgium & Holland.

We decided to change the engine before venturing down the Rhone, partly because the Lister gearbox was getting tired & partly because we wanted to be sure we could get back up the Rhone. We fitted a Gardner 4LW (about 60 bhp) with a 2:1 gearbox, which was well matched to the Lister propellor as the Lister had a 3:1 box.

 

Tim

Edited May 20, 2007 by Timleech

[Timleech]

Hi Tim.

 

I somehow think we are in general agreement, that Alvechurch boat was about the only attempt at development in all my 30 years involvement with canal boats and even that was a watered down copy of a seagoing craft and I am not sure that sort of thing can be directly scaled anyway, I rather think they missed the point of what should have been a protruding bullet shape bow. My gut feeling would be that the underwater shape of the stern is more significant.

Agreed absolutely

 

I am sure you are right about the small Woolwich being the optimum shape but then that was designed probably 90 years ago, hardly the white heat of technical innovation.

70 years actually

 

 

The woolwich design incorporated many three dimensional curves which would probably add at least 100% to the build coat of a boat so even if great improvements could be shown I am not sure the design would catch on and a slippery straight life performance would no doubt have other costs.

 

Yes the swim plates have 3-D curves, which are expensive to make, but just having a good length of swim with smooth curves gets you a long way there. The counter shape is particularly good, IMO better than the Josher, but I haven't seen many serious attempts to copy it even though it wouldn't be horribly expensive to do.

 

Thirty years ago 30 horsepower was considered ample for even the largest boats, in the working days that kind of power would have been regarded enough for a tug pulling a couple of barges, yes I know all the arguments about 'torque' but most of it is a matter of perception, having a nice relaxed, slow revving engine, and there is a lot of rubbish said about torque anyway. A moped engine with the right gearing would push a fully loaded working boat along just as effectively as a Bollinder. Horsepower is horsepower.

 

Again, I agree entirely.

That 'gearing' bit is vital, though, so as to drive the right size of propellor

 

Tim

[Edders]

Hi Tim.

 

I somehow think we are in general agreement, that Alvechurch boat was about the only attempt at development in all my 30 years involvement with canal boats and even that was a watered down copy of a seagoing craft and I am not sure that sort of thing can be directly scaled anyway, I rather think they missed the point of what should have been a protruding bullet shape bow. My gut feeling would be that the underwater shape of the stern is more significant. I am sure you are right about the small Woolwich being the optimum shape but then that was designed probably 90 years ago, hardly the white heat of technical innovation.

 

The woolwich design incorporated many three dimensional curves which would probably add at least 100% to the build coat of a boat so even if great improvements could be shown I am not sure the design would catch on and a slippery straight life performance would no doubt have other costs.

 

Thirty years ago 30 horsepower was considered ample for even the largest boats, in the working days that kind of power would have been regarded enough for a tug pulling a couple of barges, yes I know all the arguments about 'torque' but most of it is a matter of perception, having a nice relaxed, slow revving engine, and there is a lot of rubbish said about torque anyway. A moped engine with the right gearing would push a fully loaded working boat along just as effectively as a Bollinder. Horsepower is horsepower.

 

Hi John,

 

Yes, I expect a moped engine, suitably geared with a matching prop could push a boat along but torque is very important.

The old London Transport RT bus weighing in at 7.5 tons (empty) had a 9.6 litre engine developing just 100bhp. No doubt a smaller engine with similar power may have sufficed, but would it have been as driveable or reliable?

 

Edders

[Maverick]

Thanks lads. Went off topic and started talking Technical B***ocks that went straight over my head. Guess you should have just said you didnt know.

 

Thanks to the early replies that at least attempted to answer my question.

[John Orentas]

Hi John,

 

Yes, I expect a moped engine, suitably geared with a matching prop could push a boat along but torque is very important.

The old London Transport RT bus weighing in at 7.5 tons (empty) had a 9.6 litre engine developing just 100bhp. No doubt a smaller engine with similar power may have sufficed, but would it have been as driveable or reliable?

 

Edders

 

 

"Driveable or reliable" that's another matter altogether. Torque is no more than the 'twist' exerted on the crankshaft, that force can be very easily increased with the addition of gearing.

[Timleech]

Thanks lads. Went off topic and started talking Technical B***ocks that went straight over my head. Guess you should have just said you didnt know.

 

Thanks to the early replies that at least attempted to answer my question.

 

Actually I did try, amongst the pseudo-tech stuff, to give some sort of an answer.

 

Tim

 

Hi John,

 

Yes, I expect a moped engine, suitably geared with a matching prop could push a boat along but torque is very important.

The old London Transport RT bus weighing in at 7.5 tons (empty) had a 9.6 litre engine developing just 100bhp. No doubt a smaller engine with similar power may have sufficed, but would it have been as driveable or reliable?

 

Edders

 

The torque requirements for propelling a bus and a boat are totally different. The boat needs the max torque at max rpm, the bus doesn't.

The moped engine probably wouldn't last more than a day or two, but that's not really the point.

 

Tim

[blackrose]

What size of engine do dutchbarges have say 57 x 12 ft?

 

My 57' x 12' widebeam (not Dutch barge) is about 30 tonnes and only has a 55hp Izusu which is well below that recommended on the Beta guide above, but I've never felt short of power even on the tidal Thames.

 

Someone told me a bigger engine wouldn't make much difference in terms of speed because the boat is limited by its hull speed, which I assume means the combination of water resistance & drag. Does anyone know more about this?

Edited May 20, 2007 by blackrose

[magnetman]

Canal boats have a very high 'block coefficient' which is a percentage comparison to a rectangular block of the same length, breadth and depth as the boat. This means increasing the engine size after a specific point will not increase performance. I would argue a canal boat would never plane because hull form is wrong. Early les/bob allen boats had a similar stern end to a woolwich and swim sweet

[grunders]

Someone told me a bigger engine wouldn't make much difference in terms of speed because the boat is limited by its hull speed, which I assume means the combination of water resistance & drag. Does anyone know more about this?

 

 

There is a wonderful equation for the maximum hull speed of a displacement boat: 1.34 x SQUAREROOT OF(length of waterline)

 

This site has more information on it if you are interested: http://home.golden.net/~capone1/wsc/calc.htm

[blackrose]

There is a wonderful equation for the maximum hull speed of a displacement boat: 1.34 x SQUAREROOT OF(length of waterline)

 

This site has more information on it if you are interested: http://home.golden.net/~capone1/wsc/calc.htm

 

Interesting, thanks.

[carlt]

IMO the 'Small Woolwich' as built by Harland & Woolff was probably the best working NB form ever built, especially the stern shape. It's all been downhill since then

So the bluff front end of a woolwich is better than a Nurser or Josher? I don't think so.

The back end I'd agree (though it'd be a toss up between a little Woolwich and a Ricky, IMO)

 

Back to the question though. A 6 or 8hp (proper hp that is) single thumper (Lister SR1, Armstrong Siddeley AS1 or Petter PH1 for example) would look and sound lovely in a 30 footer and push the boat along nicely. Modern jap sewing machine engines, not my dept., I'm afraid.

[Maverick]

So the bluff front end of a woolwich is better than a Nurser or Josher? I don't think so.

The back end I'd agree (though it'd be a toss up between a little Woolwich and a Ricky, IMO)

 

Back to the question though. A 6 or 8hp (proper hp that is) single thumper (Lister SR1, Armstrong Siddeley AS1 or Petter PH1 for example) would look and sound lovely in a 30 footer and push the boat along nicely. Modern jap sewing machine engines, not my dept., I'm afraid.

 

Thanks Carl, thats the nice simple language us beginners understand.