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

[tdamico]

What an argument. On the one side there are builders like Kristen and KD Krogen who are positive that you should only venture out in an ocean passage in a soft bilge hull. Then you have builders like Cape Horn, and Marine Architect Lou Codega who are as positve that this is bunk and that as long as it is a full displacement hull, a hard chine is not only as good, but much better. Any experiences or thoughts? Thanks

[gonzo]

Some people have crossed oceans in an inflatable. I think that taking a single aspect of a design and making it the only one to consider is plain stupid. A seaworthy design can be heavy or light displacement. Planing hulls do fine too. I believe that the proof of seaworthiness is at sea. Theories are interesting to discuss, but boats designed according to necessity ( locally availability of materials or space to build) are used to work all over the world.

[tdamico]

While I would certainly bow to the greater knowledge that you poccess over me, Gonzo, even sombody as inexperienced as me knows that voluntarily traversing the ocean in an infaltable is also stupid. I don't doubt it can be and has been done. But that doesn't make it a good choice. If I was going to pick a boat, I would obviously base the decision on an plethora of facts, certainly not just one. Notwithstanding that a lot goes into the selection of a boat, there can still be dialog around the soundness of various hulls and the shapes. For example, the owner of Kristen states that while a hard chined boat, like the Mirage trawler for example, might be seaworthy, it would be like bouncing around in a box in rough seas. So, as you seem to have experience in quite a few different boats, with different hull shapes, designs, displacements, etc. Why don't you share your own personal experiences in those different hulls instead of telling me how stupid it is to base a boat choice on one thing. Of course its stupid! Tell me something I don't know.

[gonzo]

I don't advocate crossing the ocean in an inflatable. However, it has been done. Just as people did it in rafts and other odd craft. I didn't call you stupid. I reffered to those who base their opinion in a single characteristic. I think you are obviously trying to get the idea of what different combinations offer. Trawlers are hard chined and they work in very rough weather. Cargo ships have round bilges and work in the same kind of weather. Both, when well designed are adequate. I think that there are considerations like cost of construction, maximum speed, economy of operation and ride comfort that will define what kind of design better suits your needs. A chine hull is definitely cheaper, but more limited in shape than a round one. High speed requires a chine hull for planing. These and many other requirements help narrow the design. I suggest you make a list and then figure what each of them need to be satisfied. Usually you'll have to choose between several as they are incompatible. Boats are always a compromise.

[tdamico]

Actually I am in the process of doing just that. Once I narrow the list in the coming months, as previously mentioned, I hope charter several boats of the style that I ultimatly select. When I look at all of the various factors, it takes me to either a hard chine or a cat. The rollig motion, while at anchor, in a soft blige seems to be a concern. The solution to this, it is suggested, is a hard chine. But the round bilge folk don't believe its only a matter of motion, they use scare tactics about sea worthiness etc. Of course the hard chine folk say they are full of bunk. This is probably something a charter won't really show. So, what I am looking for is opinions from people who have been on both, and can comment on why a bilge hull would be inherently safer than a hard chine hull.

[gonzo]

Chartering a boat of the type you may be buying is a very smart thing to do. Cats are by far the most stable platform. The drawback, is that the rolling motion is less gentle. They roll and stop suddenly. I had a 34' Wharram, which have flexible mounts. Their motion is much reduced from a rigid cat. The compromise, is that the hulls are not connected internally, so you to go from one to the other it is neccesary to go outside. I sailed this boat in 25' plus seas and had no problem.
It seems to me that the main proble with unconfortable motion has to do with bad design. For example, a beamy round bilge hull can roll less than a narrow chine hull. Most trawlers, with their hard chine and full sections forward, pitch violently.
I think that your cruising speed expectations are one of the most important of the requirements. All the other specifications will be modified by it.

[tdamico]

Gonzo, I couldn't agree more. As far a cruising speed, (cat aside) 7-9 knots works fine.

[gonzo]

At that speed both designs would work OK. Are you familiar with the lobster boat type. There are many of them in NC, mainly set up for longlining. They usually use the beamier model with a lot of power for 25-30 mph speeds. However, at 9 knots, the power requirements are only about 25%.

[Portager]

It has been my experience that round bilge hulls are usually preferred for sailing vessels because they perform well over a wider range of roll angles. Hard chine hulls are preferred for power vessels for better roll attenuation and hard chine hulls are easier to build in metal that round bilge hulls. This explains why sail boats are predominately made of fiberglass.

In my humble opinion a hard chine hull is preferred for power boats due the roll attenuation benefits and the fact that powerboats are naturally more prone to roll due to the lack of a deep keel and the lower roll moment of inertia due to the lack of a large mast and rigging.

It is a common misconception that wider beam provides greater comfort. An excellent article on this subject is available at http://www.kastenmarine.com/beam_vs_ballast.htm . In fact Michael Kasten has provided a wealth of information at http://www.kastenmarine.com/articles.htm#On%20Boat%20Design .

Regards;
Mike Schooley

[Willallison]

Lets assume that at the end of the day, you decide that what you need is a full displacement passagemaker. Regardless of construction type - be it steel, aluminium, or GRP - there are companies that build them in both hard chined and round bilge form.
At the end of the day, a well designed and built hard chined vessel will outperform (in every aspect - efficiency, comfort, stability etc) a poorly designed / built round bilged boat....and visa versa.
Unless you are planning to custom build, then the argument is almost irrelevant....

[Tohbi]

td's question does address the ancient argument of hull shape. ancient mariners seemed to have accepted rounded hulls as being the most efficient. i guess that would depend on your definition of efficiency.

i think "seakindliness" may have been foremost in many minds in olden days. today, we think of ultimate speed. the two designs are widely dissimilar.

so, for a precise handling, comfortable vessel in a seaway, we may be looking at slack bilges, but for a faster one or one designed for initial stability in calm waters, a chined hull shape may suffice.

remember. our forefathers didn't have the benefit of internal combustion; they designed in a fashion that was thought to be in harmony with the elements. personally, i like that idea.

[Palmer]

The ancients probably built round bottomed boats because they are easier to plank.

I infer from the initial question that you think any seagoing vessel must be fulll displacement whether or not it is hard chined. I don't think that's the case. Take a look at your local commercial fishery. There will be a pretty wide variety of hull forms. All of them capabale of going to sea and coming back alive. The Gulf of Alaska has an awful lot of hard chined semi-disp. boats working it.

Quality of design and construction is more important than underwater form.

I would only go with a full disp. boat if I planned on crossing oceans. But just because of the added volume required for fuel.

I think semi- disp. is more versitile. It is likely to have a more comfortable ride, get decent economy at cruisiong speed, and be able to run for cover if you decide the need has arisen. The reduced draft is something to keep in mind costal cruising.

[Tohbi]

palmer wrote: "The ancients probably built round bottomed boats because they are easier to plank."

this hasn't been my experience, but i'm willing to learn your technique. how do you make planking curved frames easier than straight ones? thanks

[tdamico]

I actually am coming the the conclusion that, assuming a high quality of design and build, or in other words, in a apples to apples comparison, a full displacement boat regardless of soft bilge or hard chine, is the superior boat, on average, for most conditions. Most espcially for boats that will be used for sea crossings.

If it is my desire to run at around 7-9 knots most of the time, then having a semi-displacement hull that can go faster means having engines that can propel it faster. So consequently, if I run those more powerful engines at slower speeds I compromise those engines. At least this is what I have distilled from most of my research to date.

If my conclusions are in error, please point it out to me. If not, then I have to come to a decision on soft bilge or hard chine. I believe that while hard chine might be much more harsh in its movements, generally speaking, that will only be while in conditions that are choppy. On the other hand, while soft bilge will have a more easy going motion in choppy seas, at anchor even the slightest motion sets it rolling with the edge going to hard chine.

So the question is how will I use my boat? Answer is on the hook 80% of the time. Under power in coastal waters or traveling to the islands, 15% of the time, crossing oceans, 5% of the time. Given this set lof criteria, I believe that a full displacement boat with a hard chine is the best compromise. (unless I go with a cat)

I would very much welcome rebuttals to this analysis.

[gonzo]

I think that there is a misconception that boats that travel at hull speed or less have to be of heavy displacement. It is, in my opinion, safer and more econmic to build a light displacement boat that travels at no more than hull speed.

[Doug Carlson]

Gonzo,

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

Doug

[tdamico]

Why safer?

[gonzo]

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.

[Doug Carlson]

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

[tdamico]

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

[Guest]

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.

[Tohbi]

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.

[Portager]

Deleted duplicate post.

[Portager]

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

[Tohbi]

"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.

[Portager]

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

[Guest]

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.

[tdamico]

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.

[Tohbi]

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."

[tdamico]

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.

[Tohbi]

i don't know where to find definitive, empirical evidence about this subject. i know that i prefer my lightweight, shoal water boats for inland sailing because they tend to slide over obstacles, are beachable, and have less impact momentum. but the boat i'm building will be deeper and heavier and will ride better in a seaway.

if you're looking for maximum safety, the coast guard has parameters for their craft, and i know ice breakers are tres heavy. boats in alaskan waters tend to be built of metal for impact resistance. so, that's kind of empirical.

[Willallison]

The reason that there are such varied opinions on these topics is because there is such variation in the shapes of each type of hullform.
Take the (not very well drawn) example below. On the left is a round chine vessel, on the right a hard chined one. Clearly here the round chine vessel will have greater initial stability - it will rock less at anchor but will probably have a more snappy motion underway. Hence my earlier comment that well designed and built - to suit your particular needs - is a more important consideration than hard or soft chine, even heavy or light. Or put another way - once you have truly listed all the requirements you have for the vessel, the decision may well have been made for you

[Portager]

Tom;

Centuries of empirical evidence and basic physics support the conclusion that heavier weight vessels are more comfortable in heavy seas. Experience has shown that people find vertical and transverse acceleration discomforting. Physics tells us that force equals mass times acceleration, therefore acceleration is equal to force divided by mass. The vertical force acting on a boat is due to the increase in buoyancy caused by waves suddenly changing the water level or displacement. For small perturbations the vertical force is proportional to the waterplane area times the height of the wave, so vertical acceleration (AKA heave) is a function of waterplane area divided by mass. Therefore, to reduce heave you want to reduce waterplane area and/or increase mass. Since in equilibrium, mass is equal to displacement, which intern is equal to waterplane area times the average draft, in general better comfort at sea equates to deeper draft, so shoal draft boats are rarely very comfortable in heavy seas.

Now that I have said all that I would like to quote Henry David Thoreau who wrote, “All generalizations are false, including this one.” What I mean by this is a lot can be done with shape and form to improve the comfort of a vessel and a poorly designed deep draft boat can be less comfortable than a well designed shoal draft, however a well designed shoal draft will not be as comfortable and a well designed deep draft, heavy displacement vessel of comparable design (i.e. monohull to monohull or catamaran to catamaran).

On the subject of light displacement craft being safer, I will agree that a light displacement craft can be made safe however it can not be made as comfortable as a heavy displacement craft of comparable design and if construction technique and quality are equal, I doubt the light displacement vessel could be as safe as the heavy displacement (unless you are talking about positive buoyancy).

Although they are really talking about sailboats, I think the following quote from http://www.macnaughtongroup.com/designin.htm states the differences between light and heavy displacement boats very well, “While offshore boats are typically of heavy displacement, it is perfectly possible to design a light-displacement boat that is safe offshore. The chief objection to them is that they have a quicker, jerkier motion, which most people find uncomfortable and tiring. This effect can be held to a minimum if the boat is narrow and relatively V-shaped, with a deep keel, which are all good characteristics for the safety of the vessel as well. Most light-displacement boats are given shapes that encourage surfing, but for offshore use it is important to prevent surfing, which can cause a loss of control. Even so, the light displacement boat is apt to be too fast when running off. Therefore, rudders should be large and mounted on the keel or a large skeg, probably mounted under the hull rather than on the transom, so as to prevent ventilation from the surface. The only advantage to the light displacement type for offshore use is in her ability to carry full foam flotation with minimal loss of interior space. This is so easily accomplished that it is a mystery to us why more light displacement boats don't have it. Since a fast, self-righting, unsinkable boat is possible for those willing to accept the motion, this removes any argument in favor of the offshore multihull, as far as we are concerned.
Heavy displacement is the "natural" choice for offshore use. The comfort level is much greater, and conventional proportions produce the right combination of stability, self-righting, motion damping and non-surfing behavior for maximum average speed and safety without gimmicks.”
If you want the ultimate in safety and comfort in an offshore voyager, then I recommend a long and narrow vessel with high displacement (relative to its waterplane area) and a low center of gravity or metacenter. Some excellent example of vessels of this type include; Tad Roberts Passagemaker Lite http://www.tadroberts.com/ , many of the designs by Michael Kasten http://www.kastenmarine.com/power.htm and Dave Gerr’s Ironheart http://www.gerrmarine.com/power_70.html#ironheart , although Will and I agree that the interior arrangement of Ironheart could be improved. The biggest problem is boats like these are not available as production boats that I know of. You might want to consider a custom design. In the price range your talking about you could have a custom design for less that the brokerage fee on a production boat and, if it were metal to eliminate the cost of building molds, the total cost would be very competitive. My preference is to have the boat designed around my needs and desires rather than the production boat one size fits all approach.

Returning to the original subject of round bilge versus hard chine hull forms. I have see unsupported claims and insinuations that round bilge hulls are more comfortable in heavy seas, but I have never seen any supporting data for this position. The only advantage I know of for a round bilge is efficient at any attitude, where as the hard chine hull is only efficient at one attitude and as roll angle increases powering efficiency degrades. This is very significant for sailboats which operate at heel angles most of the time, however for a power boat the degradation over a passage is generally less than 1%. So why do some manufacturers emphasize their use of a round bilge for powerboats. My theory is that it is because most Trawler owners are ex-sailboat owners who are already convinced that round bilges are better (although they have probably forgotten why). I believe that for a powerboat, the hard chine hull is superior because it provides better roll attenuation.

Regards;
Mike Schooley

[Tohbi]

mike wrote "a long and narrow vessel with high displacement (relative to its waterplane area) and a low center of gravity or metacenter" is the basis for offshore designs. if you'd like to participate in the discussion in the sailboat section under 'ballast vs sail area" you'll see pics of a 12-meter design that pushes this envelope.

i'm listening to all the advice available about developing the model so don't be shy about chiming in. thanks

[gonzo]

I think that one of the strongest arguments for a light displacement boats is that fast boats are fun. Surfing and planing make it all worth it. Also, in the subject of safety, I'll take a light displacement fast boat to get away from a lee shore any time. Heavy boats are slow and clumsy. A lot of their weight is unnecessary and doesn't make them stronger, just heavier. The shape of the boat will make more of an impact on the heaving and pitching than displacement. Also, a heavy mast can control speed of movement.