Engine size versus displacement

[Tony Dunkley]

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.

 

I would hope that it does, with that much power in a small boat.

The 'Dukers' motors ( MSC Bridgewater Dept. barges with 76 horsepower engines) would load between 80 and 100 tons and tow a dumb with 100 to 120 tons on board out of Salford Docks, or sometimes from Ellesmere Port, up the Irwell to Hulme Lock.

[Paul's Nulife4-2]

Yes, I don't doubt it, They are Massive things arnt they, and as With most things on the water, The only 'Real' power is required when Starting or Stopping.

But as a Livaboard, I've got Two 100a Alternators, that are pretty much constantly feeding a 2.5Kw inverter, so I would think that takes some of the HP from source so to say, and The Weather and Sea State tends to take the rest on occasion, Infact on a Winter Spring tide, when comming in from sea on to the River Arun 3hrs after H/W, it can and has taken me an hour to get to my mooring, and it's just over a mile up stream ! When it's running in excess of 6kts.

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

[Tony Dunkley]

Yes, I don't doubt it, They are Massive things arnt they, and as With most things on the water, The only 'Real' power is required when Starting or Stopping.

But as a Livaboard, I've got Two 100a Alternators, that are pretty much constantly feeding a 2.5Kw inverter, so I would think that takes some of the HP from source so to say, and The Weather and Sea State tends to take the rest on occasion, Infact on a Winter Spring tide, when comming in from sea on to the River Arun 3hrs after H/W, it can and has taken me an hour to get to my mooring, and it's just over a mile up stream ! When it's running in excess of 6kts.

 

With a displacement of around 13 tons and the length, beam and draught you've quoted, you must have a proper 'boat shaped' boat, and not one of these motorized skips that are passed off as boats on inland waterways nowadays.

[Paul's Nulife4-2]

 

With a displacement of around 13 tons and the length, beam and draught you've quoted, you must have a proper 'boat shaped' boat, and not one of these motorized skips that are passed off as boats on inland waterways nowadays.

It's only a little one, but it's home -

 

https://www.dropbox.com/s/zt651uxgs95wvyb/2014-06-02%2014.26.30.jpg?dl=0

 

https://www.dropbox.com/s/14mkh0tsqe37gz5/2014-06-02%2020.54.50.jpg?dl=0

 

https://www.dropbox.com/s/q8ha8cpqqgntaoq/IMAG0231.jpg?dl=0

[John V]

I have 66' oa x 16' 6" beam 4' draft approx 75 tons with a 6 cyl 8 litre normally aspirated 160 Hp driving a prop that is way over pitched. This gives me a tickover speed that is too fast for comfort. the prop diameter is determined by the deadwood and is unable to absorb the full power available from the engine. Speed of about 8 kts at about 1200 rpm Even if full power was available I doubt that with a very bluff bow much more would be achievable. One day if I feel rich enough I will get a new finer pitched prop.

The reason for such a big engine?.......I was offered a deal on it that I could not turn down, it worked out for a brand new guaranteed engine a similar price to having a restored Gardner or the like installed

[Neil2]

 

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.

 

I disagree, sort of. It's maybe not "determined" by the nature of the waterway, but generally you would have regard for the type of water being navigating when choosing the size of propellor.

 

As a matter of interest Honda sell different size props for their outboards, and it's to gain better control at lower speeds, which is why narrowboats generally have props that are, theoretically, too big.

[dmr]

Yes, but you are a special case, due to your beam you need that big engine to push the boat through the bridge 'oles.

 

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

[blackrose]

 

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.

 

What about stopping power? That's determined in part by prop diameter (according to Crowthers), especially on a wide boat like mine, so it certainly seems relevant to me!

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.

 

Are you sure it only displaces 13 tonnes? I'm sure i would be more than that.

 

My boat is 57' (54' WLL) x 12' x 2'2" draught and displaces about 32 tonnes.

Edited July 12, 2015 by blackrose

[blackrose]

 

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

 

Depending on what size prop he's got he might be able to reduce the pitch or diameter which might go some way to solving the problem.

[Detling]

Prop size and area is very important to achieve true power, that is why harbour tugs have huge slow revving propellers. Many years ago, when I were a lad we got a newfangled lifeboat sent to where I lived, It could do 15 knots (semi planing hull) via two 200+ horsepower engines you could not shout over at full throtle. The old lifeboat built in 1938 had two 45 horsepower engines running at just over 1000 rpm flat out and that drove the displacement hull at 8 knots through anything and you could whisper over. As an experiment the two boats were tied back to back and the tug of war took place, the old boat pulled the new one backwards at 4-5 knots. The fast revving small high pitch props on the new boat simply cavitated and made a lot of froth, but the big old fat blades slow revs just kept on going.

[Paul's Nulife4-2]

 

What about stopping power? That's determined in part by prop diameter (according to Crowthers), especially on a wide boat like mine, so it certainly seems relevant to me!

 

Are you sure it only displaces 13 tonnes? I'm sure i would be more than that.

 

My boat is 57' (54' WLL) x 12' x 2'2" draught and displaces about 32 tonnes.

Hi ya,

Yes, pretty sure, Don't forget Nulife4-2 is built completely out of Aluminum Alloy, and has NO fitted internal Ballast, it takes on Water Ballest in the Motion Dampening V Hull.

So Basically ALL of this -

 

https://www.dropbox.com/s/6wmrse8hvdicye1/IMAG0209.jpg?dl=0

 

Is filled with water anyway, Which makes her a very safe boat, as you can Hole the outer Hull, and it makes NO difference !.

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

[magnetman]

I think Black Rose is slab sided with a full or very nearly full width baseplate so its tons per inch immersion figure will be quite a lot higher than a vee bottom depending on the depth of the vee.

 

The math is too much for me at this time of the day (or the life)

 

 

Edit to add aspace

Edited July 12, 2015 by magnetman

[Paul's Nulife4-2]

I should really try and do the math on mine I suppose,,,just so I know, At the moment I'm really relying on original Boat Building figures.

[Jim Riley]

 

I disagree, sort of. It's maybe not "determined" by the nature of the waterway, but generally you would have regard for the type of water being navigating when choosing the size of propellor.

 

As a matter of interest Honda sell different size props for their outboards, and it's to gain better control at lower speeds, which is why narrowboats generally have props that are, theoretically, too big.

Yes, just bought a Honda 20 for my 32 ft narrowboat, it had an old Honda 10 originally, adequate but not enough for adventures to ripon, Nottingham etc. I came across and bought the high thrust prop, it's now a treat to drive, will pootle along nicely but has that bit extra for manoeuvring and stopping.

[GUMPY]

I should really try and do the math on mine I suppose,,,just so I know, At the moment I'm really relying on original Boat Building figures.

Or get it craned out most cranes have weight gauges on them.

[Tony Dunkley]

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.

 

Alan

 

I don't know how you've worked that out, but it's wrong.

Narrowboats ( genuine ex-working boats with a typical hull form, that is) can attain a speed/length ratio of 1.3 to 1.4 when running single and empty with quite modest horsepowers.

One particular ex FMC motor which I know well, having worked it for some years, was timed on the Thames at 12 mph (10.4 knots) with a 27 bhp Lister ( FR3 turning 26'' to 27'' 3 x bladed propeller at 600 rpm).

There was nothing unusual or special about the boat, engine or sterngear, but all the requirements for good performance were in place . . . . . good hull form driven by a large diameter, slow revving propeller.

If you were referring specifically to most modern so-called narrowboats being unlikely to ever exceed 7 knots, then you are absolutely correct, because in that case all the requirements for bad performance are in place . . . . . poor hull form, resembling a tapered ended skip, and a small diameter, high revving propellor.

An interesting, and relevant, footnote to this is that the Woolwich and Northwich built boats supplied to the GUCCCo in the 1930's were designed to achieve 6 knots (towing a butty) with 18 bhp.

Edited July 12, 2015 by Tony Dunkley

[Theo]

Prop size and area is very important to achieve true power, that is why harbour tugs have huge slow revving propellers. Many years ago, when I were a lad we got a newfangled lifeboat sent to where I lived, It could do 15 knots (semi planing hull) via two 200+ horsepower engines you could not shout over at full throtle. The old lifeboat built in 1938 had two 45 horsepower engines running at just over 1000 rpm flat out and that drove the displacement hull at 8 knots through anything and you could whisper over. As an experiment the two boats were tied back to back and the tug of war took place, the old boat pulled the new one backwards at 4-5 knots. The fast revving small high pitch props on the new boat simply cavitated and made a lot of froth, but the big old fat blades slow revs just kept on going.

 

I remember seeing a prop sitting beside St John's Lock (just down from Lechlade) it was, IIRC, about 18" in diameter and had 5 (I think) blades which had such a big area that they overlapped one another by a good margin. I supposed that it was from a tug.

 

N

[magnetman]

 

I don't know how you've worked that out, but it's wrong.

Narrowboats ( genuine ex-working boats with a typical hull form, that is) can attain a speed/length ratio of 1.3 to 1.4 when running single and empty with quite modest horsepowers.

One particular ex FMC motor which I know well, having worked it for some years, was timed on the Thames at 12 mph (10.4 knots) with a 27 bhp Lister ( FR3 turning 26'' to 27'' 3 x bladed propeller at 600 rpm).

There was nothing unusual or special about the boat, engine or sterngear, but all the requirements for good performance were in place . . . . . good hull form driven by a large diameter, slow revving propeller.

If you were referring specifically to most modern so-called narrowboats being unlikely to ever exceed 7 knots, then you are absolutely correct, because in that case all the requirements for bad performance are in place . . . . . poor hull form, resembling a tapered ended skip, and a small diameter, high revving propellor.

An interesting, and relevant, footnote to this is that the Woolwich and Northwich built boats supplied to the GUCCCo in the 1930's were designed to achieve 6 knots (towing a butty) with 18 bhp.

One of the fastest narrow boats I have seen was the Cowburn & Cowpar Skylark which had a Gardner Semi Diesel in it I think and I assume a decent underwater shape as it was going like a rocket up the Thames when it was on red boards. Quite impressive and made all the more so by the sound of the engine

[MoominPapa]

 

One particular ex FMC motor which I know well, having worked it for some years, was timed on the Thames at 12 mph (10.4 knots) with a 27 bhp Lister ( FR3 turning 26'' to 27'' 3 x bladed propeller at 600 rpm).

 

Did that have a 3:1 reduction box, or 2:1? I guess 3:1, as that would allow the engine to run at the maximum revs of an FR.

 

MP.

[George94]

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.

 

7 or 8 knots at Limehouse?

 

Hmmm.

 

I know it's on the outside of a bend, but the river is pretty wide at that point. It must have been a highly unusual set of circumstances to have generated such a current. The PLA recreational users guide says: "Be aware that the river can flow at speeds of up to four knots".

[Tony Dunkley]

 

Did that have a 3:1 reduction box, or 2:1? I guess 3:1, as that would allow the engine to run at the maximum revs of an FR.

 

MP.

 

It was a 3:1 Reduction Box (FRM's rated at 1800 rpm) . . . . . a 2:1 Reduction Box would have been swinging only a 21'' diameter at most (diameter derived from the standard formula with restricted channel / shallow water correction applied) and the boat speed would have been a little slower, although nothing like as slow as modern boats due to the underwater hull form and shape.

Working boats were invariably equipped with the largest diameter propeller that could be swung, in terms of the clearance under the counter and / or what the engine could cope with . . . . . completely wrong if you listen to the theories expounded by some members of this forum, but nevertheless, found to be correct, and upheld in practice by every barge, tug, ship and commercial narrowboat designer, builder and operator since Brunel fitted a screw propeller to the SS. 'Great Britain'.

Edited July 13, 2015 by Tony Dunkley

[Tony Dunkley]

 

7 or 8 knots at Limehouse?

 

Hmmm.

 

I know it's on the outside of a bend, but the river is pretty wide at that point. It must have been a highly unusual set of circumstances to have generated such a current. The PLA recreational users guide says: "Be aware that the river can flow at speeds of up to four knots".

 

I think there is a tendency for people to estimate current and tide speed from whatever speed they believe their boat to be capable of, however unrealistic that may be.

I would say that the PLA estimate of approximately half of the figure estimated by the boater in this instance is going to be fairly reliable.

Edited July 13, 2015 by Tony Dunkley

[GUMPY]

 

7 or 8 knots at Limehouse?

 

Hmmm.

 

I know it's on the outside of a bend, but the river is pretty wide at that point. It must have been a highly unusual set of circumstances to have generated such a current. The PLA recreational users guide says: "Be aware that the river can flow at speeds of up to four knots".

Indeed Parglena (my Duker barge copy) would do a max of about 5Knts and could punch the tide coming in to Limehouse even on springs. Edited July 13, 2015 by Loddon

[magnetman]

Having come into Limehouse in a variety of boats I have to say that I don't think you get 7 knots outside Limehouse Lock.

On the 4 occasions I have entered Limehouse in my barge on an emptying River out from Teddington I have only had to give it about 1800 or perhaps 2000rpm (flat out is 2600) which on calm water would be about 5 or 6 knots. There is a fair bit of water coming down but nothing particularly shocking.

I am Not saying 7 knots there is impossible but I think it is implausible.

[Pen n Ink]

So does this all mean I am wrong in calling for a 1 knot "safety" margin? Doesn't seem excessive to me.