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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Tender or Stiff
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Thread: Tender or Stiff Reply to Thread
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Topic Review (Newest First)
09-06-2012 11:09 PM
PaulinVictoria
Re: Tender or Stiff

There's also the Dellenbaugh measurement which gives you some idea of stiffness of a particular boat. It's a calculation as if 1 pound of force was pushed against a square foot of sail (with some other specifics around it being sheeted central etc). Not an exact measurement but does give some indications around how easily a boat heels. Here's a graph for C&Cs for example, http://cncphotoalbum.com/technical/stability.htm shows you that something like a Mk1 30ft is very stiff, whilst the Mk II turned out to be quite tender. Both sail excellently, so it's not really an indication of how well they sail.
09-06-2012 09:41 AM
LinekinBayCD
Re: Tender or Stiff

A lot of boats particularly older designs are intended to heel at a certain angle increasing their waterline and thereby their speed. Hence the reason for being initially "tender".
09-06-2012 09:08 AM
Jeff_H
Re: Tender or Stiff

Quote:
Originally Posted by davidlaing View Post
Light displacement is not, in itself, a cause of tenderness. A low ballast/displacement ratio is, but the distribution of the ballast matters, too. A boat with a given B/D ratio and a deeply-positioned ballast will be stiffer than a boat with the same B/D ratio but with a more shallowly-positioned ballast.
You are right that "Light displacement is not, in itself, a cause of tenderness." But a boat's ballast ratio really has almost nothing to 'stiffness' as the term as it is used in naval architecture. As classically defined Stiffness is primarily about initial stability (form stability).

Jeff
09-06-2012 07:38 AM
davidlaing
Re: Tender or Stiff

Light displacement is not, in itself, a cause of tenderness. A low ballast/displacement ratio is, but the distribution of the ballast matters, too. A boat with a given B/D ratio and a deeply-positioned ballast will be stiffer than a boat with the same B/D ratio but with a more shallowly-positioned ballast.
04-19-2006 01:52 AM
nolatom In ships, tender and stiff tend to refer to rolling periods, with the former having a long rolling period and the latter a short (quick) one.

If you're carrying cargo, tender may be better, the ship rolls more slowly and gently, the movement is easier on the cargo. Tender here means having a low metacentric height (vertical distance between buoyancy and gravity);

Stiff ships tend to snap back upright quickly, with a short rolling period. Lots of military ships are stiff, since the quick rolling period brings the decks (meaning gun decks) back level in a hurry. That 'snappish' motion is hard on cargo and passengers, though, it beath you up.
04-18-2006 01:11 AM
Faster Thanks, Jeff, for the excellent clarification.
04-14-2006 09:33 AM
Irwin32 Good explanation, Jeff. Tender boats are not bad or unseaworthy. For years I owned a Paceship Eastwind that was pretty tender, but the nicest sailing boat I have ever been on. The helmsmen had to be dead not to be able to feel when this boat was in the groove.
04-13-2006 06:20 PM
Jeff_H Although 'Faster' was closest, so far you have not gotten an answer that reflects the traditional naval architectural definitions of tender and stiff. These terms are strictly used in reference to initial stability (form stability) and generally are not thought to include other forms of stability. A boat that is said to be stiff has a lot of form stability. (Stability that comes strictly from the shape of the boat.) Tender refers to a boat without much form stability.

Faster explained quite well the shape of a boat that produces lots of form stability, but in a general sense, as they heel, boats with lots of form stability shift their center of buoyancy to leeward more quickly than tender boats. Since the center of gravity does not shift relative to the boat itself, as a boat with a lot of form stability (a stiff boat) heels, initially, the lever arm between the center of gravity and the center of buoyancy increases rapidly, creating a lot of initial stability. The problem occurs at steeper heel angles, Unless the boat has a very low center of gravity, as a stiff boat heels the center of buoyancy rapidly moves back toward the center of gravity. As this occurs the stability of the stiff boat can rapidly decrease.

Stiff boats tend to have very quick motions but rock through smaller roll angles. They also tend to have a smaller angle of ultimate stability and so are more prone to be able to capsize and stay over longer.

Under the traditional definition of tender or stiff, neither term has anything to do with the overall displacement of the boat, the depth of its keel, or the amount of sail area that it carries. Traditional heavy displacement boats tend to be tender (lacking form stability). Modern race boats tend to be a bit on the stiff side and 70's era race boats tended to be excessively stiff.

What confuses people is the frequent coloquial missuse of the terms. People assume that tender means that a boat heels easily. That really opens a whole can of worms because at that point, a boat that appears stabile may be so for a lot of reasons varying from a low center of gravity (which is good for performance, seaworthiness and motion comfort) to a lot of form stability, (which is not bad for performance but comes at the price of motion comfort and seaworthiness) or simply short of sail area (which does nothing good for the boat in the long run).

Respectfully,
Jeff
04-13-2006 01:41 PM
SailorMitch It's also a subjective thing. What one person considers to be a tender boat may not be to someone else. For example, to my wife anything that heels more than 5 degrees is tender. To some folks, you don't start really sailing until the boat is heeled over 15-20 degrees.

Most designers have a target range for their boats to heel, so some boats will heel more than others in the normal course of things. It's good to know the designer's intent because you don't want to push the boat beyond that amount of heel or the boat will want to round up. To prevent that, you have to turn the wheel to keep the boat going straight and then the rudder starts to act as a brake slowing you down.
04-13-2006 01:16 PM
Faster Hull form is a factor also. Many round-bilged boats are initially tender, heeling easily in light breezes, but firm up as the righting arm of the ballast increases.

Flat bottomed boats with hard turns (or possible chines) in the bilges provide what is known as "form stability" and initially resist heeling due to the bouyancy of the hull sections. These types of boats, if insufficiently ballasted, are particularily tender once the form stability has been overcome, and rely heavily on crew weight & placement for righting moment.
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