Hull Design

[John Hughes]

My school science profect is to answer the question of how the design of a boat's hull determines the effectiveness of its application. Does anyone know of any books or articles that discuss the basic hull types and their uses and what is required in their design to make them effective for their chosen use?

[Jeff]

Sail or power? Smaller boats or ships, or both?

There are many many factors which go into a boat's design, so I'm guessing you will end up dividing it up into categories:

For example, selecting a basic hull shape might involve the following criteria:

1. Speed (how fast does the boat need to travel, cruise and maximum)

2. Water conditions. (inland, open seas, wave height and type of waves)

3. Cargo (what type of cargo needs to be hauled, people or freight, large cargo requiring large beam or height. How large should the boat be for the desired use - 30', 50', 100', 200')

4. Cost / Durability or materials and construction or desired materials/construction

5. Economy of Operation

6. Special considerations (low wake, special docking requirements)

One way to approach the question is to apply general criteria like this to the broad forms (for power, maybe: high speed planing, air entrapment multihull, semidisplacement monohull, semidisplacement multihull, full displacement) and then after that look at the special types/hybrid forms in each category.

[John Hughes]

Jeff, thank you for your reply. The problem is finding materials that I can cite. I assumed when picking this topic that there would be books on design issues of the type that you reference in your reply such as a textbook or something similar. So far, I have found nothing except for general design books on power and sail by Gerr and Edmunds and an old book on the mechanics of marine vehicles by Clayton and Bishop. If you know of any titles that might provide me the information that I need, please provide me with them and if you know, where I might be able to obtain a copy. Again, thank you for your reply. John Hughes.

How about books like
Introduction to Naval Architecture by E.C. Tupper ISBN 0750625295
Preliminary Design of Boats and Ships by Cyrus Hamlin ISBN 0870333917
or Designing Power & Sail by Arthur Edmunds ISBN 1892216051

Here is a list of books picked by this website. Amazon is a great site to search for books too where you can see the cover and get an overview. The prices aren't bad and the format of the Amazon link includes the ISBN number after ASIN (look for ASIN/ISBN#) so you can often get the books through interlibrary loan too.

[Stephen Ditmore]

When Olin Stephens spoke to my design class at The Landing School in '91 he surprised many of us by saying, "The most important thing is that a boat be strong enough, the second that it be stable enough, and the third that it be controllable. A low resistance hull shape is well down the list of considerations." With that in mind, hull shape optimization and resistance prediction is certainly a part of naval architecture.

Where powerboats and ships are concerned the standard technical reference is Principles of Naval Architecture, Vol. 2, available from http://www.sname.org/publications/catsearch.pl. You can also get a book by Jay Benford with a much less technical discussion, Naval Architecture for Non-Naval Architects, from the same web page or from Jay Benford's own site at www.tillerbooks.com.

The leading American researcher/theoretician on planing hull performance is Daniel Savitsky at http://www.dl.stevens-tech.edu/. Resistance prediction software is available from Don MacPherson <http://www.hydrocompinc.com/>, Dick Akers <http://www.shipmotion.com/>, Robert Schofield and others, and CFD codes that model the free surface such as Splash <http://www.panix.com/~brosen/> and Shipflow <http://www.flowtech.se/> are now being used to optimize hull shapes (see also http://www.hsva.de/services/cfd/index.html) Finally, there are going to be some seminars related to hull shape at IBEX - you can check those out at http://www.pbbtradeshows.com/ibex/

When I hava a chance I'll also post a reply to Fernando's Displacement vs Semidisplacement vs Planing question which may help you as well. If I can be of further assistance please feel free to contact me at sditmore@juno.com

Good Luck!
Stephen Ditmore
New York

[Stephen Ditmore]

Your question’s caused me to think about the issues involved in designing a hull to a target speed. One factor is the bottom shape in profile: whether it is curved (“rockered”), straight, or has an inverted curve, as sometimes occurs in combining V’d sections forward with flat sections aft. Another issue is prismatic coefficient (how much volume is in the ends of the boat) and a third is the location of the center of gravity. I got to wondering whether some single simple value would tend to capture all of these variables, and I think there is one – the amount of transom that is immersed when the boat is at rest.

This is complicated a bit by the differences between monohulls and multihulls, with multihulls generally enjoying an advantage at semi-displacement speeds (speed/length = 1.34 to 3.0, Froude# = 0.4 to 0.9). I’ve seen two interesting sources of comparative data recently. One was a graph in an article on the Morrelli & Melvin design firm <http://www.morrellimelvin.com/> in a recent issue of Professional BoatBuilder Magazine <www.proboat.com>. The other was a paper published in the mid 1990s by Art Anderson Associates <http://www.artanderson.com/> comparing various candidate fast ferry types.

That said, I relied on my experience and intuition to come up with a candidate formula for the relationship between immersed transom area and target speed. It is:

target V/L^0.5 = 1 + T/M + L/20M^0.5
and can be rewritten as:
optimum T = MV/L^0.5 - ML/20M^0.5 – M

where:
V = speed in knots
L = waterline length, static ½ load condition
T = transom area immersed in sq ft, static ½ load condition
M = midsection area in sq ft, static ½ load condition


I emphasize that this formula has not been validated. It is pure conjecture at this point, but I think this formula is of the right general character, and could be tweaked to fit empirical data.

I hope this is useful. Good luck!

Stephen

[Stephen Ditmore]

You might find the discussion at http://www.sciam.com/1097issue/1097giles.html helpful. The author has essencially designed a semi-displacement cargo ship. The lines of his ship are published as part of his patent - you can see them at www.delphion.com

I take back what I said about posting a response to Fernando's question. Enough already!

[John Hughes]

Thank you for the information. I will try to locate the referenced materials. Regarding the issue of resistance, that seems to me to be an important consideration but one more of economy rather than actual use. Regarding the issue of target speed, how would that work on a vessel that does not have an immersed transom such as seen in some monohull sailboats or multihulls?

[Stephen Ditmore]

Lack of an immersed transom would tend to indicate a realtively low target speed, except in the case of long, slender or light displacement hulls, in which case it would be consistant with a medium target speed. The target speed of a sailboat might not be its maximum speed - it will usually be designed to sail well in light winds as well as heavy. The ammount of immersed transom might also be influenced by a rating rule. Whitbred/Volvo 60s and Open 60s sail similar courses, but the Whitbred/Volvo 60s carry their transoms higher because they can make the boat longer under that rating rule if they make adjustments in the shape of the hull. Most open 60s would have a little bit of immersed transom if you filled 1/2 of their ballast tanks.

Most sailboats have higher prismatic coefficiants than, say, rowing shells or canoes, and most high performance catamarans have higher prismatic coefficiants than monohulls.

Stephen

[Stephen Ditmore]

There's an excellent article on catamarans and other fine hulls in the current (Dec-Jan) issue of Professional BoatBuilder <www.proboat.com> by Dick Akers <http://www.shipmotion.com/>

[Jimboat]

This site might be of interest to you...for high performance powerboat design information, check out the Secrets of Tunnel Boat Design book and the "Tunnel Boat Design" software at AeroMarine Research.

[Stephen Ditmore]

Now that this thread is up at the front of the que, let me mention that I've added a term to my candidate formula for the relationship between immersed transom area (at rest) and target speed. It now reads:

V/L^0.5 = 1 + T/M + (20T^2)/(ML^2) + L/(20M^0.5)

and can be rewritten as:

optimum T = (MV)/L^0.5 - (ML)/(20M^0.5) - (20T^2)/L^2 - M

where:
V = target speed in knots
L = waterline length in feet, static 1/2 load condition
T = transom area immersed in sq ft, static 1/2 load condition
M = midsection area in sq ft, static 1/2 load condition

I'll reiterate that this is still merely conjecture, and has yet to be checked against empirical data.

Stephen: Have a look at my graph of curves of areas and S/L ratios at www.tadroberts.com. You will find it in the articles area and originally appeared in WoodenBoat #137. It covers the idea you are working toward with the above equation. This graph has worked well for me in a number of hull designs, and it agrees with LCB locations from Garden and Fexas hulls. I believe this to be the most important idea in power boat design. Get the LCB in the right place for the hull's speed, and place your weights so the boat floats there, and it will be a pretty good boat.

All the best,

Tad

[Stephen Ditmore]

I'm favorably impressed by what I see on your web site, Tad. Some of the photos are of vessels I thought were Bruce King designs... are you associated with King? I don't have time right at the moment to look for your LCB/LCG stuff, but I will as soon as I have a chance. Thanks.

[Tad]

Stephen;

Yes, I spent 14 years with Bruce King Yacht Design, 1987-2002. I moved back home to B.C. last spring and opened TR Design. Currently I continue working on the 154' ketch building at Hodgdon Yachts in Maine and several local projects.

I include the graph mentioned above. Stations are numbered along the bottom, right to left, zero to ten. On the left side are percentages of the midsection area, 10-100%. Note how the percentage of midship area increases at station ten at higher S/L ratios. The LCB locations are the dots along the bottom line. LCB at station 4.75 relates to a S/L of .8, at this speed transom volume is zero. LCB at station 5.1 is for a S/L of 1.1, etc., up to a LCB at 5.95 for a S/L of 3.0, at this speed transom volume is 65% of the hull's midship section area.

I am sorry this scan seems unclear, you may have to go back to the magazine to see it clearly.




Tad