Hull speed

No.

I mean the Length of the Water Line.

That is about the best indication of potential Hull performance.

There are other factors too. They are important but less so than the Waterline Length.

Having a wide transom is definitely one of them. Having a low Displacement/Beam ratio is another.

Now, the Length of the Waterline while sailing is far more important than the Length of it while the boat is sitting level.

Early rule beaters had very short Water Lines, while sitting level. Once heeled, the Water Line got much longer. And it was that one that mattered.

 

This is a totally different realm to resistance per se.

That is about manipulation of a rule, the way 'length' is measured' in said rule, to gain an advantage, that is all.

 

There seems to be confusion between resistance of a hull and "hull speed". The curve for hull resistance will have an upwards inflexion at approximately the "hull speed" regardless of shape for what is usually referred as displacement hulls.

 

Yep, it happens again and again. In my 30+ years of practice, I found that every 5 to 7 years you have to totally re-explain basic concepts to your technical sphere of contacts because of turn-over and lack of use. So to repost from 15 years ago...
Expensive Hull Speed. https://www.boatdesign.net/threads/expensive-hull-speed.11342/#post-97099

 

You're probably right that it's informally used that way. But it has a formal definition. Hull speed is when the Froude number is equal to sqrt(1/2π).

 

doc can be found here
Small Craft Engineering: Resistance, Propulsion, and Sea Keeping https://deepblue.lib.umich.edu/handle/2027.42/91749

 

Interestingly, when one reads accounts by George Watson and WP Stephens, for example, they actually give different accounts about why their boats developed long overhangs. They were not ignorant of the measurement factor, but they tend to speak of the longer overhangs as the logical outcome of moving to a shallower, lighter hull form and reducing wetted surface area. As Watson said, once one goes to a shallow, wider hull then overhangs start to become the best way of creating fair hull lines, at least in a boat built to 1890s technology and concepts.

At a more practical level, a long overhanging counter had the huge advantage that you could reach the end of the boom, for stowing the sail, adjusting it, or putting in reefs. A long bow overhang allowed for a shorter, stronger and more easily worked bowsprit. Given that design and technology meant the use of vast rigs (by our standards) in many classes, and the difficulty of handling hemp or manila lines and cotton sails, bringing the deck closer to the end of the rig had a lot of practical advantages.

Designers like Clayton or Watson were not great fans of the rating rules that relied on waterline length - they were among the large group of top UK designers who formally asked the RYA to change the rule to create more "wholesome" boats, in a group submission about 1896. They were frank and outspoken against the rule, and about the way it could be beaten, so they were not trying to hide anything when they stated that the primary purpose of long overhangs was for reasons other than rule cheating.

In the more extreme classes, after about 1896, straight-out rule beating did become an object. But now that we can get access to lots of archived documents, the conventional wisdom that long overhangs were only there to cheat the rule doesn't really stand up.

 

Interesting anecdotes, thanks for that.

I think one would need to take a step back, and view through the prism of 19thC understanding of hydrodynamics. I suspect they didn't think it was manipulation of rules, for their aforementioned reasons, but unknowingly.
Given hindsight of today's greater understanding of hydrodynamics and its influences, I suspect they would concur re: manipulation for improved performance, rather than maintain their original ascertain.

 

Bluebell, you're some kind of mojo. 11 years of silence, you comment on it, and it takes off.

I think what Gonzo meant is that there's a point in velocity/ power curve, where the second derivative of that curve becomes rather abruptly negative.

 

11 years and 1200 views!

 

And 1200 people (give or take), who decided not to address the question "Has anyone studied how the longitudinal metacentric height is proportional to the resistance in speeds between FnL 0.3 and 0.5?".... Sooo, this is not an area of concern, or it is a doctorial thesis awaiting a brave soul. (FWIW, I see a quick and dirty one using the Series 62 Data back-calculating the required longitudinal metacentric height ... just tossing that out there.)

 

Two boats with the same total weight, the same length and width in the waterline and over all.
The big difference is the shape of the transom and thus the longitudinal and transverse metacenter radii.
This in turn creates less resistance in the speed range around "hull speed".
JS

 

Of course, both boats have different KMl, but a 20% difference seems too much difference, since the inertia of the float with respect to the transverse axis is not great. This is just an assumption since I have not performed any calculations.

 

My measurements are calculated, no guesses.
As I mentioned at the beginning, I have other examples with the rowing boats.
JS

 

I think you are giving me the reason when, to refute my "assumption" you are using a totally different figure from the one (post #27) I have commented on. But the subject is not the least important. As you say very correctly, the value of KMl is different, although it would not have been necessary to calculate anything.