flat bottom hulls

Hi everyone, I'm a newcomer to the forum from the UK where we have a huge variety of boat designs to deal with the huge variety of waterways in our small island.
First of all, I admit to knowing next to nothing about boat design, and you might well think that the question I am about to ask confirms this...

Here in the UK there is a strong tradition of building boats with flat bottom hulls and straight sides basically to maximise cargo space. Received wisdom has it that such boats are fine on inland waterways, but at sea are unsafe. I am trying to understand if the concept of a flat bottom hull is inherently wrong when exposed to sea conditions, and why.

Many years ago there was a class of boat which used to cruise up and down the west coast of scotland known as a Clyde Puffer. These ships were built with flat bottom hulls as very often the only way they could unload goods was by beaching. More recently there have been intrepid individuals taking to sea in traditional english "narrowboats" which are flat bottom craft of less than 7' beam.

I can see why a flat bottom hull would be less efficient than a v section but I am struggling to understand why they are thought to be dangerous at sea - especially when there are examples of such craft having coped with sea conditions.

I suppose the question strikes at the heart of basic hull design but as I said, I know nothing about the subject, I'm here to learn.

 

If you provided some reference to the "Received wisdom" it might help in trying to understand the point.

Welding a dead flat plate will almost certainly cause some buckling. It helps to have at least a little curvature to give the plate some inherent resistance to buckling. Likewise with ply and composite flat panels. A bit of curvature improves stiffness.

A beamy hull with a flat entry will have a tendency to slam a bit more than something with a V entry but becomes more of an issue at higher speed.

The central hull on this trimaran is essentially flat with nicely curved chines:
http://www.strategypage.com/military_photos/military_photos_2008120422651.aspx

Rick W

 

Hello,

If you look at the post #12 of this discussion:
http://www.boatdesign.net/forums/bo...-small-boat-slow-speeds-30931.html#post333661
I had included few pics of a traditional small fishing boat called "batana" - in use in the northern adriatic sea, Istrian peninsula.
The post #15 at the same page contains more historical info on the boat. The fact is, they have been used for coastal fishing for centuries, and still are. It means that they have performed the required task very well.

The required task. That is the keyword. It means simple and cheap construction (fishermen are poor), big payload capacity for a given length and beam, and the possibility to navigate under oars, sails or (recently, of course) with a small outboard mounted to a narrow transom. A good part of the deck (some 50% or more) is closed, a feature which makes it suitable for use in seas, near the coast.

The most important feature, which determinates whether the boat can be used at sea, is not in the hull type but in the sailors' heads, of course... They are able to evaluate if the weather and sea conditions are appropriate for the use of this kind of boat.

Hope that helps a little. You can find more about this type of boats here: http://www.batana.org/ (the site is entirely made with Flash so the pages will load a bit slowly, be patient)

 

Neal,

You will find much discussion about flat versus other hull bottoms on this forum. The flat bottom is supposedly easier to construct than the V or arc bottom. Rick is being generous when he said that the flat bottom tends to slam more in waves or chop. I'd make it a much stronger case than that although there are some things that can be done to reduce this serous shortcoming.

A flat bottom is not inherently less seaworthy than other types but the answer depends on many other parameters. I am surprised that you find flat bottom hulls to be so plentiful in England and suspect that you are only talking about inland waters and, particularly the many canals. Its true that at least one narrowboat has ventured across the channel to the continent. However, that couple chose to have the boat shipped back home instead of making the return on its own bottom. I met the boat and its owners here in my town on the US east coast a couple years ago and discussed this with them. This 22 ton, 60' LOA by 6' 10" boat had issues with the water in our local sounds and is no match for the ocean.

A wide flat bottom is much harder to make rigid than even a slight V or arc. A panel or beam stiffness (a prime desired characteristic in hull panels) varies inverse exponentially with the span and wide flat bottoms need a lot of interior structure to stiffen them.

Flat bottoms make hulls with the highest initial stability and load carrying ability and are great for barges, oyster tongers and crab pot working boats. In Grand Banks dories, they were the best overall solution to a particular set of conditions. For most boats, there are other better solutions to be found among the many alternative bottom shapes. A full understanding of these shapes and the mediums they operate in will take some effort on your part and will never end. None here have learned all about this subject.

Edited to add. Daquiri got a post in while I was typing but there is nothing contradictory between the two.

 

flat bottoms are usually safe if there deep enough, & there is adequate freeboard, canal boats often lack these details

 

Ok so I said I know next to nothing about boat design but I do understand that to make a flat surface stiff you need additional strengthening and why bother when you can just make it curved and give it some inherent strength without a weight penalty.
It's true that many canal boats/barges are built with very little freeboard because they don't need it and that in itself would make the boat unsuitable for the sea.
The guy with the 60' narrowboat is something of a folk legend in the UK and polarises opinion because of his somewhat eccentric demeanour. Much less well know is a group of individuals who in the same type of vessel have crossed the irish sea and navigated the north west and north east coasts of the Uk and in the same boat have cruised officially disused waterways some not much wider than the boat itself.
I take the point about "received wisdom" In general boats built for the UK inland waterways are not safe for sea use for a variety of reasons - what I am trying to understand is to what extent this is because of their hull shape.

Let me put it another way. If you were to build a flat bottom boat with sufficient strengthening, deep enough draught and freeboard, would it be safe to use at sea?

 

Captain Cook did a very big trip in a flat bottom boat through some of the worst seas on the planet.

 

Sure, they are called tankers. Larger ships get away with this form because the size and displacement are so great relative to the waves. This includes most of the big cruise ships.

My son was, for a while, a hydrographer on the NOAA ship Rude which was 90', 220 tons and flat bottom. When doing survey work, they always tried to come into port at night because the ship was so active in a seaway. You can interpret "active" as uncomfortable.

On an unrelated note, he and his co-hydrographer located most of the wreckage of Pan Am flight 800 off Long Island with Rude's side scan SONAR. He also participated in the de-comissioning ceremonies of the Rude in Norfolk last year. No one who had served aboard was sad to see it go.

Sure Capt Cook and many others sailed the oceans in ships that were relatively flat bottomed. These ships were ballasted and had sails which helped to stabilize them. Any sailor can tell you that sails are a great stabilizing force at sea.

 

If you get the design right, flatbottoms work exceptiionally well in seas. Speaking strictly for myself, there is no way i could run these seas in a V bottom.


What i did was to make the widest part of the bottom 2/3 hull length forward of the transom (width at transom 8" less). By doing that i created an impact zone where i want the hull to crush the water and throw it outward. To accomplish that i put all the fuel (54 gals) in the fwd 1/3 of the hull and I incorporated a SST lifting strake on the transom. That SST tab goes across the transom with a cutout for the lower unit and is 11" long. The hull gets lift (and weight for crushing) from the bow area being wider than the transom, and it also gets exagerated transom lift from the strake which is perfectly tuned to augment the lift of the lower unit trim

the secret~~~~Lift at both ends of the hull

 

Weren't the Viking ships very beamy and much like a canoe, mostly flat on the bottom? I've seen barges here in SE Alaska in rough weather taking a beating and wondered what it would be like being aboard. If you want to hear good things about flat bottoms in the nasty go to Atkin boat plans.

Easy Rider

 

Perhaps a better source of information would be anyone who served on an LST.

 

Safe is a broad term. A flat bottom may be safe from sinking and at the same time be so lively as to throw you around and beat you to death on the furniture. A flat bottom will be more affected by waves, in that it cannot rotate so easily as a rounded shape, it cannot act as independently from the shape of the waves compared to a round bottom. Say waves are coming from the side, a flat bottom boat will roll farther side to side and it will do it faster, and the change from rolling one way to the other will be "snappier".

 

The idea of moving the widest beam forward has been explored earlier in a boat called the mid planing hull. Its claim was smoother riding at a lesser angle of hull trim. The downside was that it is critical to weight balance and must have the CG fixed well forward. I suppose Brad (loveofsea) countered this by installing a large trim tab aft called the SST. Its easy to see that the spread of these two lifting forces can maintain a constant level of trim and avoids having the hull bottom slam into waves.

Why these factors favor a flat bottom is not clear to me. It would certainly make a flat bottom more acceptable in chop in my view, but better than the same ideas applied to a normal warped V, double wedge, etc? I am skeptical about that. I actually use an aft hull bottom shape to similarly hold the bow down on some of my boats but would not give up the sharp angle bow to smooth entry into waves or chop.

 

Hello Tom

One of the problems with drawing the bow to a V in an otherwise flatbottom is that the V entry will slide down the swell rather than carve into it. Picture a surfboard with a V bow, there's no way it will stay attached to the slop of the swell. A V bow may very well do better running straight up swell, but it has serious disadvantages when running down swell. With exagerated transom lift a virtual necessity in a flatbottom, running in following seas with a V entry may produce 'bow-steering' where the pressure against the bow overcomes the effect of the rudder or lower unit and that can lead to loss of control and even broaching.

The good skiff uses a tiller and when i am running in substantial seas i use the tiller (and articulate throttle) to work the forward edge against the angle of the swell. I look for the highest peaks of the swells as indicated by the cresting portions and i run to those and surf down the face, peak after peak . This flatbottom prefers to quarter up the swell rather than down it.

I agree that weight balance is critical.

Sam, i have to respectfully disagree with you about 'a flat bottom boat will roll farther side to side and it will do it faster.' When a flatbottom tilts, the buoyancy increases exponentially because more of the hull is submerged forcing it back to center. In a v bottom there is much less resistance to rocking side to side.

I can't say that i blame anyone for thinking flatbottoms are undesirable in seas, i have never actually seen another one that has it right.

 

This is because your too arrogant to admit your simplistic and narrow minded approach. Sounds like an "inherent design flaw" with your thought processes.

Of course you'll never qualify that comment, but you will manage to insult and insinuate as you make meager attempts at changing the subject.