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...0Boat%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.