A question on hull shapes

[muddywaters]

Sorry if this might be a stupid question.

But it is in regards to flat and V shaped hulls.

Would I be right in assuming that a V shaped hull makes the boat sit deeper in the water.

[John Williamson 1955]

The depth at the keel will be greater, all other things being equal.

 

On the other hand, the depth at the side is correspondingly less, so you can get closer to a shallow edge.

 

V bottomed boats also roll more than flat bottoms as you move round inside.

Edited August 1, 2014 by John Williamson 1955

[Flyboy]

It all depends on the weight of the boat. If you have 2 boats that weigh the same, one with a flat bottom and the other with a V bottom. The V bottom will sit lower in the water at the keel and higher at the chine due to the displacement. The flat bottom will sit higher to displace the same amount of water as the V bottom hull.

 

So the answer to your question is yes but only if the boats are the same weight and therefore the same displacement.

 

The above assumes that both boats are the same length and beam.

Edited August 1, 2014 by Flyboy

[Hey Ho]

On this subject, why were Springers built with V bottoms? I'd have thought a V bottom would be more complicated to build than a flat one, although maybe they'd use a bit less steel due to cutting the corner at the base/side joint? Is a V bottom perhaps structurally stronger & therefore better with thinner steel? Trying to answer my own question but no idea what I'm talking about!

[John Williamson 1955]

It's a stronger shape in bending, slightly, and Sam did use thin steel. I doubt it's enough to make that much difference to the cost, though.

 

As I said ^^^ up there, with a V bottom and a soft chine, I can get closer to a shallow edge than a flat bottom boat with a hard chine, which may have been in the design brief. It's also possible he couldn't get wide enough sheets cheaply enough to make flat bottoms without the weak point of a weld in the centre, so made a virtue out of necessity by turning the V bottom into a selling point.

 

I'm sure there'll be a Springer expert along soon enough.

[John V]

"Chined" hulls (a V hull is a simple form) can be built without frames or in the case of Springer less framing. The "folds" of the the V part of the hull act as stiffeners.

[David Mack]

When Sam Springer started building his steel boats, virtually all the other pleasure craft on the cut were wooden or fibreglass cruisers, or converted lifeboats and the like, which would have had various forms of V-hull, and mostly more curvaceous hull sides. The only flat bottomed pleasure boats would have been ex-working boats and ex army pontoon conversions. Is it so surprising that he would have copied the conventional pleasure boat V-bottom, even if he did also copy the slab side of the working boat, which is easier to construct and gives more internal space?

[koukouvagia]

I'm pretty sure the V bottom in a Springer was to stiffen the base plate in lieu of transverse steel strengtheners. I seem to remember that the early Springers had a slight fold along the roof for the same reason.

I remember watching our first boat being built like this thirty years ago..

 

eta: Ah, I see others have the same thought.

Edited August 1, 2014 by koukouvagia

[benfordboy]

The depth at the keel will be greater, all other things being equal.

 

On the other hand, the depth at the side is correspondingly less, so you can get closer to a shallow edge.

 

V bottomed boats also roll more than flat bottoms as you move round inside.

 

 

Well, Dads old Springer doesn't roll when we move about inside - It does however roll splendidly on the Tidal Trent around High Marnham (Skiers!) & was even more spectacular on the Thames between Limehouse & Battersea (Oh dear now the cats out of the bag - Glad the Wicked stepmother doesnt read CWF!)

 

Certainly less movement on our N22 - but thats also far more maneuverable into the wash of fast craft.

[MtB]

"Chined" hulls (a V hull is a simple form) can be built without frames or in the case of Springer less framing. The "folds" of the the V part of the hull act as stiffeners.

 

Less framing?

 

No framing, more like, IIRC

 

FAR cheaper to build.

 

 

MtB

[John Williamson 1955]

Mine's got frames across the bottom of the hull and stringers? (Channel sections along the length of the hull) all welded to the hull and supporting the floor.

[Theo]

V bottomed boats also roll more than flat bottoms as you move round inside.

Not at all sure that this is correct.

 

The stability of a floating object depends, among other things, on the water plane area. (The water plane is the shape of the surface of the water that is displaced by the boat). More specifically the transferse stability of the boat depends upon the second moment of area of the water plane taken from the longitudinal centre line.

 

This means that what is below the water surface has no effect on the stiffness or tenderness of the boat. So the V bottom would seem to have no effect on it.

 

However the V bottom will have an effect on the vertical centre of gravity, which will be higher than the CoG for a flat bottomed boat of the same draught. This is because there is less mass at the lower depth so this might have the effect of making the boat more tender.

 

As they say: it all depends.

 

N

Edited August 1, 2014 by Theo

[Daiboy]

Not at all sure that this is correct.

 

The stability of a floating object depends, among other things, on the water plane area. (The water plane is the shape of the surface of the water that is displaced by the boat). More specifically the transferse stability of the boat depends upon the second moment of area of the water plane taken from the longitudinal centre line.

 

This means that what is below the water surface has no effect on the stiffness or tenderness of the boat. So the V bottom would seem to have no effect on it.

 

However the V bottom will have an effect on the vertical centre of gravity, which will be higher than the CoG for a flat bottomed boat of the same draught. This is because there is less mass at the lower depth so this might have the effect of making the boat more tender.

 

As they say: it all depends.

 

N

 

stiffness or tenderness

So the V bottom would seem to have no effect on it.

 

 

Oops sorry

 

[Theo]

 

stiffness or tenderness

So the V bottom would seem to have no effect on it.

 

 

Oops sorry

 

Hm...

 

Should have been more language aware!

[muddywaters]

Thanks only asked because I wondered if it was something to consider when looking at boats

[NigelMoore]

In support of all said above in terms of the strength and stiffening of the shape due to the folded sections. As to stability/rolling, my Springer is the most stable, rock-free narrowboat I have ever stepped on in over a quarter century in the inland marine industry.

 

Other practical advantages to the V-hull shape also, additionally to the ability to get closer to certain banks.

 

The only disadvantage is when drying out on hard beds, because you do tip over. if that happens on a steeply sloping riverbed, you could roll right over. I quite fancy a small pair of mid-position bilge keels to further increase stability while keeping the boat propped vertical on the hard.

[bassplayer]

Thanks only asked because I wondered if it was something to consider when looking at boats

Length is also a factor in how much a boat will roll. There are lots of short Springers.