Soft bilge hull vs. Hard chined. Are both seaworthy?

Gonzo,

Are you saying displacement or semi-displacement running at hull speed? In a light displacement boat do you advocate ballast?

Doug

 

Why safer?

 

I think light displacement boats are safer because the slamming forces and overall stress are less. The may have ballast but is not as necessary as in a heavy boat, because the machinery, tanks, etc. are a large percentage of the boat's weight. All these things are installed in the lower part of the hull and usually work well to keep the CG low. Another characteristic of a light displacement boat that makes them safer, is that all the gear is lighter too. That means that anchors, lines, chains and any other part of the working gear is less dangerous to work with. A lighter boat needs less power so it uses less fuel too.

 

Gonzo,

A boat that comes to my mind is Bolger's Mikim which is an East coast lobster type, powered to achieve no more than hull speed.

Do you have any examples of boats that you feel are in this category?

Doug

 

My ingorance is frustrating me. I wish I was more knowledble. By light displacement do you mean semi-displacement?

 

Apples and oranges. Light displacement means light in weight.
Semi-displacment means faster than a displacement hull (light or heavy) and slower than a full on planing boat.

 

td, i think the argument for round bilge concerns the motion; it is easier than the snap-roll effect of hard chine hulls. the boat seems to sit more "in" the water than "on" it. also, they're faster, having less wetted area.

to some extent, the same applies to gonzo's theory. heavier boats have an easier motion and will bust through rough seas easier than a light craft. however, as he points out, the light boat has less momentum and, therefore, won't damage itself as much in a collisiion, and will be easier on the rigging.

boats like america cup racers carry so much ballast [70% or so] in order to stand up to their massive sail plan that enormous stresses are placed on the hull and rigging. but they sail like ferraris. each has it's advantages for sailing but at anchor gimme a heavy boat.

 

Deleted duplicate post.

 

As Dave Gerr points out in “The Nature of Boats” pages 106 & 107, comfort at sea is related to the rate of heave, or the vertical acceleration of the boat as it hits waves or as waves hit the hull. Heave rate is a function of the waterplane area to displacement ratio. Another way to look at it is if the boat is too light for its waterplane area it will bounce around like a cork. Dave Gerr provides a graph on page 107 that shows a curve labeled “Best Comfort” versus Displacement and Waterplane area. He says for good comfort you want to be on or below the curve. That means for a given waterplane area you want to increase displacement (weight) to improve comfort, which for a fixed length and beam required increasing draft. Likewise for a given displacement, you want to reduce waterplane area (i.e. reduce length and or beam) while maintaining displacement.

Regards;
Mike Schooley

 

"Likewise for a given displacement, you want to reduce waterplane area (i.e. reduce length and or beam) while maintaining displacement." in other words, a heavy boat is more comfortable?

but maybe the theory shouldn't be taken too literally because, while reducing length will reduce waterplane area, it will also make the boat more prone to pitching, which is uncomfortable. so, i guess it depends on the tradeoff.

beam, otoh, slows a displacement boat so narrow is good, except when powering or anchoring when it contributes to rocking. of course, a ballasted sailboat doesn't have that problem when sailing, but you have the problem of more wetted area due to the keel. of course, that is necessary to prevent leeway and yaw, and yaw can be a problem for powerboats too. then, there's the question of freeboard.........it's purty complicated and i'm getting dizzy. seems there is no one, right answer.

 

Your right, you can not take any rule of thumb too literally, but in general higher weight and lower waterplane area improve comfort due to heave rate.

You are correct that longer length increases pitch moment of inertia, which is the integral of mass times distance to CG squared. Longer length also reduces sensitivity to shorter wave length waves such as chop.

Higher length to beam ratios increase powering efficiency, but there is much confusion regarding bead and roll comfort. Increased beam will increase initial stability, but it does not improve comfort. A wider beam boat experiences harsher roll characteristics and larger inputs since the roll disturbance is increased by the larger leaver arm.

The factor that most people overlook is the detrimental effect of height.

In general the longer, narrower, heavier and lower a boat is the better its sea keeping characteristics. Most production boats available today are far too wide for good comfort at sea and fuel efficient operation. They are optimized around minimizing slip fees, not for offshore operations.

Regards;
Mike Schooley

 

Look at what Yards are Building

If you really want an answer to this question look at what the big Yacht yards are building. I don't mean these locale shops either. Look at Feadship and Lurssen look at some of these yards that have been doing it for over a half of century. They have definatley put the time and money into the research and development of hull and they have very picky customers.

I understand that alot of comercial boats use hard chine for obvious reaseans there is more storge in a hard chined boat then in a soft bilge, but in the same respect they are not built for comfort they are built to work. The other side of this coin is that they have devleped new technologies that help all boats at anchor. Which will probably make this a moot point very soon. I know that FEADShip has developed new technology for most of there vessels.

You could also talk notes from some of the bigger boats out there as well. Like alot of the cruise lines what are they running on. There primary goal is comfort at sea. I take pointers from anyone who is able to weather a level 4 hurricane underway.


Just my thoughts hope they help.

 

A question. Given good quality and design. What type and displacement hull would "generally" be more comfortable at anchor? Underway? And would that answer change as the seas picked up.

 

an architect friend designs lovely buildings but he doesn't know squat about boat design. a building is static but boats are dynamic. the problem is no, one dynamic can fit all conditions.

in calm water a wide, chined hull will be less tender. but in any kind of seaway, i'll agree with mike [portager]: "In general the longer, narrower, heavier and lower a boat is the better its sea keeping characteristics."

 

I understand, fully, that no one dynamic can fit all conditions. I actually get it! That is why I made sure I put the work "generally" into my question. But there appears to be a disagreement on weight. Some say heavier is better some say lighter is safer. Which is right? Is there any facts on this that exist? Not opinion, but a scientific study of the properties of hull design and weight in varying sea conditions? I can't seem to find anything factual in any kind of search or book.