Small Boat Stability


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Small Boat Stability

Initial Stability vrs Sea Keeping Ability.




No one wants to build a boat that is labeled as 'Unstable'. Small boats that are unstable are considered to be dangerous, and we want to avoid this flaw at all costs, when considering a design to build or purchase. The question is How do we determine whether a particular small boat design will be stable or unstable?

In considering the design elements at play here, we first need to come to terms with the 'having your cake and eating it too' problem that every small boat builder or designer must take into consideration. The fact of the matter, is that there are Two Stability Considerations that we must examine in All small boats, 'Initial Stability' and 'Sea Keeping Stability'.

Initial Stability in Small Boat Design:

Initial Stability can best be explained as follows: Initial stability is how stable the boat is when getting into or when exiting the boat, as well as how stable the boat is in regards to moving around inside of it.
If a small boat tips violently when entering the boat, it has poor Initial Stability. If the boat leans heavily to one side when the passenger moves to that side, the boat is said to have poor initial stability.

Bottom Width plays a very important role in initial stability in small boats. The tradeoff here of course, is to provide adequate bottom width for enough initial stability, without providing too much, so as to adversely affect seakeeping or to create excessive drag when rowing or sailing.

One can Further Define initial stability here, to include factors such as which small boats rail will go Under First, when heavily loaded to one side by a person literally stepping on the rail to try and enter the boat. One mention of the term Bank Dory ought drive this nail firmly in place, the semi-dory is another small boat design that may stand up to this test in lengths down to 15 feet.

Bottom Weight is also a help in increasing initial stability, to Demonstrate this in Your boat, try this simple and easy test. Step into the boat with your first foot placed right up against the inside of the garboard right at the beem. Now try it again, this time, first chucking 50 or more pounds of gear on the bottom of the boat and on the centerline. You should note a small increase in 'initial stability' with the gear first added.

Sea Keeping Stability in Small Boat Design:

The Statement, 'flat bottom boats are no good in a sea or swell', does have merit, but the gloster dory has a problem with the statement. A gloster, or a Grand Bank dory, in sizes ranging from 16 feet and up, if built along traditional lines, are Extremely Difficult to capsize actually, you really have to work at it. Why?

It is because the flat bottoms of these dories are long and slim, and they have sides that are either lightly rounded or multi-chined slightly at the top of the garboard, affording them Excellent sea keeping characteristics for a flat bottom boat. They also have the unique side flare that sets them apart as uniquely capable small workboats, excellent for hauling traps, and carrying heavy loads. They'll 'Begin' to tip, if you so much as Look at their side, but getting them to go Under, is frankly, Difficult.

The Key, to small boat stability therefore, is to find the Balance between Initial and sea keeping Stability, that best suits Your needs.

2007 smallboatinfo

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