behaving and handling of sharp bows

[Hornblower]

I like the sharp bows of river inspection launches (for optical reasons). But how is their bevaviour and handling compared to the "normal" more bulky bows of most of the narrowboats?

One might think the sharp bows are more efficient, cut with less resistance through the water and make less waves. But I think it is the other way round, isn´t it? And is the handling different?

[rusty69]

I always thought the arrows were the sharp bits, not the bows

[Athy]

13 minutes ago, rusty69 said:

I always thought the arrows were the sharp bits, not the bows

I'm quivering with mirth.

[Sir Nibble]

Interesting fact. An equilateral triangle pushed through the water flat face forward will have less drag than one moving point first.

[OldGoat]

A fine bow is less robust than a bluff one and is longer, also doesn't have much effect until the vessel is moving fast(er) through the water - none  of which are very suitable for canal use.

My guess.

[cuthound]

A sharp bow has the potential to cause more damage to any boat or structure that it hits (unless properly fendered) than a bluff bow.

[Bee]

At canal speeds probably doesn't make much difference but big ships have a big bulb bow under the water so sharp doesn't always mean best. For practical purposes the usual narrowboat bow is less likely to hook under lock gates and lift them off / sink the boat.

[cuthound]

12 minutes ago, Bee said:

At canal speeds probably doesn't make much difference but big ships have a big bulb bow under the water so sharp doesn't always mean best. For practical purposes the usual narrowboat bow is less likely to hook under lock gates and lift them off / sink the boat.

ABC introduced something similar to their hire fleet over 10 years ago.

it was supposed to save fuel and minimise damage to the canal banks and bed. All that happened was the hirers went faster, nullifying the fuel savings and had more accidents, increasing insurance costs.

[Murflynn]

the blunt bow or bulbous bow of a cargo ship is to fool physics into thinking the hull is longer than it really is, when applying Froude's number to calculate the maximum economic speed of the vessel.

as a canal boat more than 10metres long will never approach 'hull speed' this relationship is not really relevant.  A sharp bow and stern, and a smaller displacement (weight) that is characteristic of an inspection launch that was not designed to carry cargo, will offer less resistance and need less power for a given speed.

in simplistic terms, 'hull speed' is the speed at which there is a single bow wave that crests again at the stern, supporting the hull horizontally.  Go any faster and the stern crest moves behind the boat, which is now trying to climb a hill all the time.  That speed will be approached by a small yogurt pot or a mini-narrowboat on a canal, and will make you very unpopular with continuous moorers.

Edited April 25, 2017 by Murflynn

[WotEver]

Interesting article about bulbous bows here:

http://www.marineinsight.com/naval-architecture/why-do-ships-have-bulbous-bow/

[Iain_S]

Not a lot of difference in handling at canal speeds, but I found sharp bows a nightmare when breaking ice. The boat just followed the line of least resistance, along the crack to the nearest bank, despite any steering input.

[Hornblower]

Thanks for your thoughts.

That the bowshape doesn't make so much difference at canalpace will be right. But I would use it on canals and on rivers. I remember the turbulent water of the Thamse near limehouse basin.

Another similar question:

We just returned from another great journey, our 2nd on the Thamse. Beside our hired narrowboat we met a lot of grp-cruisers. It seemed to me that they were creating more waves than we did even when going at about the same speed. The only reason I can imagine is that a glide-hull seems to create more waves than a displacement-hull (at the same displacement speed). Am I right? (Not relevant for my future boat, I'm just curious.)

[Murflynn]

it is because all those yogurt pots have a planing hull, with a gurt big immersed flat slab transom; just look at the turbulence created immediately behind any of those boats. It is as if the boat is trying to pull against a vacuum in the water.  The bow wave is actually not too bad, but there is a stern wave like a big swell forming behind the boat which creates a significant disturbance.  They are good at planing, rubbish at cruising at less than 20mph.

I have recently modified my little yogurt pot by adding a 'sugar scoop' to the transom.  Google it if you are interested.  It fills in the hole behind the transom, providing a smooth exit path for the water displaced by the boat.

Edited April 26, 2017 by Murflynn

[FadeToScarlet]

31 minutes ago, Hornblower said:

Thanks for your thoughts.

That the bowshape doesn't make so much difference at canalpace will be right. But I would use it on canals and on rivers. I remember the turbulent water of the Thamse near limehouse basin.

Another similar question:

We just returned from another great journey, our 2nd on the Thamse. Beside our hired narrowboat we met a lot of grp-cruisers. It seemed to me that they were creating more waves than we did even when going at about the same speed. The only reason I can imagine is that a glide-hull seems to create more waves than a displacement-hull (at the same displacement speed). Am I right? (Not relevant for my future boat, I'm just curious.)

If you are thinking about cruising on the rougher stuff near Limehouse, a more bluff bow, I've heard, works better than a more fine one.

As a case in point, Joshers with lovely fine double curvature bows tend to cut through the waves, whereas big Grand Union boats (some of which were designed to work on the Tideway) bob upwards more easily.