# Hull speed

Discussion in 'Boat Design' started by HJS, May 9, 2010.

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### HJSMember

In this forum often is written about hull-speed relative to waterline length.
It often ends up in a confusing mess of arguments.

Should we relate to something more relevant and yet easily accessible?

When I did some investigations on two rowing boats that would go in the critical so-called "hull-speed", I found that the resistance curve changed drastically, if I had a transom. The larger width of the stern revised the longitudinal metacentric height very considerably.
One advantage of studying the longitudinal metacentric height is that we also include the boat's shape above the waterline.

Has anyone studied how the longitudinal metacentric height is proportional to the resistance in speeds between FnL 0.3 and 0.5?

js

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### BlueBell"Whatever..."

Oddly interesting that this has garnered no response in 11 years and 1200 views.

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### jehardimanSenior Member

It is probably because correlation is not causality. There are many factors that can be calculated for any hull form that have very specific uses, but are meaningless when compared between hull forms. Thus it is naval "ART-ca-tecture".

DogCavalry and fallguy like this.
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Interesting observation!

Then in your research one assumes you discovered that there is no such thing as "hull speed" It is a misnomer used by non-NAs.

As JEH notes:

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### HJSMember

In the introductory post, I have used the wrong term by mistake.
The longitudinal metacenter height shall rightly be the longship metacenter radius.
That is a significant difference.
Sorry
JS

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### gonzoSenior Member

I think that "hull speed" is used informally as the speed at which the power curve has an inflection point upwards. It has a practical application for design.

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### HJSMember

What I am pointing out is that "hull speed" is normally related to the waterline length, which is misleading. I believe that longship's metacenter radius is a much better dimension.
JS

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### gonzoSenior Member

I think that hull shape has the most significant influence. For example, straight buttocks and a full stern can generate sufficient dynamic lift to get the vessel on planing mode. On the opposite extreme, a circular currauch will have the bow wave break over the bow and sink with increased speed.

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### TANSLSenior Member

The power-speed curve is a parabola corrected by various factors and with various addends in the total equation. Therefore, for positive velocities there cannot be an inflection.
Hull speed, to put it quickly, is nothing.
It would be very frustrating if the hull shape did not have "significant influence".

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Why?

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### sharpii2Senior Member

Actually, IMHO, Displacement Length (D/L) ratio is just as effective and easier to calculate.

Low D/L boats make less waves than high D/L boats do.

And, because of this, they can typically out sail the traditional "Hull Speed", which is usually defined as: 1.34 * ((LWL ft)^0.5).

It is easy to have two boats, which have the same Hull Length and Displacement, but have much different D/Ls. This is because it's the Waterline that is counted, not the Hull Length.

Not only that, but the D/L is a cubic relationship, just like a longitudinal meta center is.

A method I use to determine the D/L of a boat is: (28.6 * DISPL. cu) / ((WL lu*0.10)^3).

cu = cubic units
lu = linear units

A D/L of 200 or less is considered low.

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### clmangesSenior Member

Do you mean that you're measuring the perimeter of the waterline instead of it's length?

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### jehardimanSenior Member

No, he's using the waterline length, but cubing it to get the same volume unit as displacement. This makes the factor dimensionless.
Anyway, standard practice for all small craft hull forms and propulsor types was mostly settled back in 1970's with the 1971 publication (and 1977 revision) of Small Craft Engineering, Resistance, Propulsion and Sea Keeping, Pub No. 120 from UofM. While some more modern paper dive into the niches and specialty hull forms, this publication covers all the basics and even has a section on waterjet performance by Jacuzzi.
Small craft engineering: resistance, propulsion, and sea keeping https://agris.fao.org/agris-search/search.do?recordID=XF2016073850

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If they are conventional hulls and not planing hulls, that is not correct.
The datum is always the Lwl.

The datum for comparison remains the same (one does not mix and match), which ever hull 'type' one is using. Ergo, same displacement and same "length" = same LD ratio.

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