Engine size versus displacement

[NB Lola]

Well throw me a kipper because mine does

[GUMPY]

Well throw me a kipper because mine does

Over 8mph? You must have a very special hull/engine combination. Most modern boats are lucky to get 6mph flat out due to having the hydrodynamics of a brick.

even Loddon which has long double curved swims front and back would struggle to make headway and would need to be flat out.

Edited July 11, 2015 by Loddon

[Tony Dunkley]

As a part of a discussion I'm in the middle of having, the horny old question of engine size has come up and I don't know the answer but I'm sure that someone will.

 

Assuming a modern engine (Beta, Shire etc) revving at oomphty thousand rpm as opposed to one of the old timers, can anyone advise on the appropriate engine size for the following, assuming that the boat should have enough power to fight (almost) the worst tidal stream it is likely to encounter on the inland waterways.

 

70' narrowboat, with a draft of 30". Originally set up for a 23" propeller. I've worked out in my thumb-in-bum calculation that the displacement will be around 28 tonnes.

 

So - what does the panel think - recommendations for engine size, gearbox, and propeller size?

 

To make the best and most efficient use of the engine power available in any slow displacement vessel requires the largest diameter and slowest revving propeller that the hull design will accommodate.

Propeller diameter is determined from shaft horsepower and shaft rpm.

Propeller pitch is determined from boat speed and shaft rpm.

Having first decided on the maximum speed you wish to achieve the next step is to estimate the horsepower required to achieve it for that particular vessel, you can then check that the required horsepower is within the ideal range for your propeller diameter.

There are formulae for determining diameter, but you will need to re-hash one of them and work it backwards to give SHP as the result instead of diameter.

[magnetman]

 

There is a rule thrown around for leisure barges of 1.5hp per tonne of displacement.

 

By that logic we should have about 100hp. We have 30hp, which is what she was built with, and she moved 100 tonnes of cargo with that setup.

 

I guess nobody ever gets in trouble for erring on the powerful side, so the recommendations creep upwards and upwards over time.

1.5hp per ton but at what rpm is that HP rating?

 

I don't know what engine you have (JP3 maybe?) but if it is 30hp at 1000rpm for example that's the same torque as a 90hp running at 3000rpm

[blackrose]

Over 8mph? You must have a very special hull/engine combination. Most modern boats are lucky to get 6mph flat out due to having the hydrodynamics of a brick.

even Loddon which has long double curved swims front and back would struggle to make headway and would need to be flat out.

I agree with Julian.

 

I'm not sure why anyone would want to make headway against a 7 knot current for any length of time on a narrowboat. It's asking for trouble in my opinion. It may be possible in theory given an adequately powerful engine, but most people who actually venture onto tidal rivers regularly tend to use the tide to their advantage rather than battling against it.

 

I've turned into a 7 or 8 knot current at Limehouse on my widebeam but only for a very short time just to get into the lock. Even flat out I was unable to make any headway until I got into the slower moving current on the other side of the river.

 

Punching the tide is sometimes necessary until it turns in one's favour, but you don't try to punch a 7 knot tide unless you're on a powerful cruiser or speedboat.

Edited July 11, 2015 by blackrose

[Tony Dunkley]

1.5hp per ton but at what rpm is that HP rating?

 

I don't know what engine you have (JP3 maybe?) but if it is 30hp at 1000rpm for example that's the same torque as a 90hp running at 3000rpm

 

Neither torque, nor the engine, or shaft, rpm is a consideration. The horsepower per ton figure is a simple rated engine horsepower to displacement ratio.

[magnetman]

Fair enough I was just pointing out why I believe a 30hp would be ok.

 

If I am specifying an engine for a boat I am definitely interested in the rpm and torque curve

 

Can't just say it needs xxxhp that's not enough info in my opinion.

[Tony Dunkley]

Fair enough I was just pointing out why I believe a 30hp would be ok.

 

If I am specifying an engine for a boat I am definitely interested in the rpm and torque curve

 

Can't just say it needs xxxhp that's not enough info in my opinion.

 

But if you consider torque and rpm together, then you are considering horsepower . . . . it's a function of torque versus rpm, and is a measurement of work done in a specified period of time.

Engine rpm is a factor to be considered only in relation to choosing the reduction gear ratio.

[magnetman]

 

But if you consider torque and rpm together, then you are considering horsepower . . . . it's a function of torque versus rpm, and is a measurement of work done in a specified period of time.

Engine rpm is a factor to be considered only in relation to choosing the reduction gear ratio.

OK

[Pen n Ink]

I agree with Julian.

 

I'm not sure why anyone would want to make headway against a 7 knot current for any length of time on a narrowboat. It's asking for trouble in my opinion. It may be possible in theory given an adequately powerful engine, but most people who actually venture onto tidal rivers regularly tend to use the tide to their advantage rather than battling against it.

 

I've turned into a 7 or 8 knot current at Limehouse on my widebeam but only for a very short time just to get into the lock. Even flat out I was unable to make any headway until I got into the slower moving current on the other side of the river.

 

Punching the tide is sometimes necessary until it turns in one's favour, but you don't try to punch a 7 knot tide unless you're on a powerful cruiser or speedboat.

Totally agree. The idea comes from the desire to have a safety margin built in - in other words "if it ever became necessary to do this daft thing for whatever reason, then I know I could". Doesn't mean I choose to do so!

Edited July 11, 2015 by Pen n Ink

[J R ALSOP]

Everyone should give their engine a name, mine is called "Hector", you will have to read Lillian Beckworth"s books to find out why. Their might be a prize in it.

[NB Lola]

I have decided on Stallone lol

[GUMPY]

Mr A Is your engine a romeo or does it have a lisp?

Edited July 11, 2015 by Loddon

[cuthound]

OMG. I haven't worked on Detroit Diesels for years.

Or what about EMDs then?

Another piece of brilliant American engineering. NOT!!!!

Back in the mid 80's I put in a 2000kVA genset with a 2 stroke V16 Detroit Diesel. Love?y sound

[magnetman]

Back in the mid 80's I put in a 2000kVA genset with a 2 stroke V16 Detroit Diesel. Love?y sound

Did it run the washing machine properly?

[Theo]

 

There is a rule thrown around for leisure barges of 1.5hp per tonne of displacement.

 

By that logic we should have about 100hp. We have 30hp, which is what she was built with, and she moved 100 tonnes of cargo with that setup.

 

I guess nobody ever gets in trouble for erring on the powerful side, so the recommendations creep upwards and upwards over time.

 

I was chatting with a boater a while ago who was moaning that his boat was overengined. Even on tickover he could not go slow enough and was causing alarm and despondency to those moored boats that he passed. He had to keep knocking his engine out of gear. Sounds like a real pain to me.

 

N

[Neil2]

I would have thought something like the 40 or so hp Kubota would be enough for that boat but if you're thinking of river use a 23" prop would be way too big. Narrowboats are almost always overpropped to gain the benefits of a big propellor on canals but in most cases it means the engine can't develop its maximum power on open water. You can't, as a general rule, have it both ways.

 

Most folk don't mind as the number of occasions when you are going to push a strong current out of necessity are very few. But if you really do want a speedboat you have to get a powerful engine up to maximum revs which means a smallish prop. I reckon with a 2:1 reduction ratio you could have a prop around 18" which is about as small as you would want to go without being embarrased on canals.

 

Bear in mind that most engines used in boats are not continuously rated ie they are only meant to use maximum revs for short periods (with long intervals between). So the Kubota would get you out of a spot but if you really wanted to push a 7 knot current for several hours you need something quite a bit bigger which would of course be overkill on the cut.

 

A Bukh DV36 would do the job, that is a motor that is designed to run flat out all day, though as a Bukh owner I'd be the first to admit they aren't a natural canal boat engine.

Edited July 11, 2015 by Neil2

[Henhouse]

As already stated, adequate cooling for whatever size engine you choose is the most important issue. However, don't forget that a few of your horses will be spending their time driving alternators, and won't be available for propulsion.

[Tony Dunkley]

I would have thought something like the 40 or so hp Kubota would be enough for that boat but if you're thinking of river use a 23" prop would be way too big. Narrowboats are almost always overpropped to gain the benefits of a big propellor on canals but in most cases it means the engine can't develop its maximum power on open water. You can't, as a general rule, have it both ways.

 

Most folk don't mind as the number of occasions when you are going to push a strong current out of necessity are very few. But if you really do want a speedboat you have to get a powerful engine up to maximum revs which means a smallish prop. I reckon with a 2:1 reduction ratio you could have a prop around 18" which is about as small as you would want to go without being embarrased on canals.

 

Bear in mind that most engines used in boats are not continuously rated ie they are only meant to use maximum revs for short periods (with long intervals between). So the Kubota would get you out of a spot but if you really wanted to push a 7 knot current for several hours you need something quite a bit bigger which would of course be overkill on the cut.

 

A Bukh DV36 would do the job, that is a motor that is designed to run flat out all day, though as a Bukh owner I'd be the first to admit they aren't a natural canal boat engine.

 

Propeller diameter is not determined by where, or what type of waterway a boat is being used on. The ideal diameter is derived from two factors, horsepower and shaft rpm, therefore it's the reduction gear ratio, the engine's rated continuous output and the rpm for that maximum continuous output that are to be considered, nothing else has any relevance.

[nb Innisfree]

Blade area is what transmits power to water, the larger the diameter the larger the area, if diameter is restricted by hull shape then wider blades must be used if blade area is insufficient. Blade area (BAR) is seldom mentioned or considered yet it is the basis of propellers.

[Tony Dunkley]

Blade area is what transmits power to water, the larger the diameter the larger the area, if diameter is restricted by hull shape then wider blades must be used if blade area is insufficient. Blade area (BAR) is seldom mentioned or considered yet it is the basis of propellers.

 

That's not strictly correct, and rather an over simplification of a complex subject. The primary reason for the need to increase blade area is to maintain blade loading below the value when cavitation commences.

[Alan Saunders]

See Wikipedia - Hull Speed. It is unlikely that any 70' nb can exceed 7 knots however powerful the engine. With increasing power there are rapidly diminishing returns of speed through the water.

 

As you already have a 20" prop. a long stroke engine is more likely to meet your requirements than a high revving engine even with a finer pitched propellor and a 3:1 gearbox.

 

Alan

[mark99]

No ones mentions propeller slip yet - and whether high rpm induces more slip and hence an engine with characteristic of max torque/power at lower revs is better.

Edited July 11, 2015 by mark99

[nb Innisfree]

One thing's for certain, trying to find the correct prop by asking on a forum will only muddy the waters (groan) I decided to let Crowthers sort it, they were spot on, lovely wide bladed 17" job that achieved just under max revs and no sign of cavitation.

[Paul's Nulife4-2]

My 42x11 ft Wide beam goes at about 13 ton, Drawing about 1150mm (Fully loaded) Has got an 85hp 5 cylinder turbo engine, and plods along very happily at anything around the 7 kts mark all day long, with maybe a kt or 2 in reserve if needed.

Edited July 12, 2015 by Paul's Nulife4-2