View Full Version : Is there a ratio?

Lowell Bernhardt
12-04-2002, 10:17 PM
Not to trivialize powerboat designing, I know I've stepped on some toes there before. But....

In designing a simple runabout for outboard power. Is there a ratio or a good guideline for the dementions? Say 2:1 lenght:width? Something similar to a John boat. Although, I would think that all the same principals would apply to others, ie: dories, skiffs.

I would assume that the further apart the numbers (just say 5:1 as an example)the better some of the boats chacteristics would become. While in the same token some would get worse.

Thanks, Lowell

12-05-2002, 05:49 AM
Check out www.svensons.com/boats (http://www.svensons.com/boats) they offer a load of free plans for all sorts of boats. You should be able to figure out a ratio for whatever dimension from the number of boats they have there. Hope you've got cable tho smile.gif

12-05-2002, 11:23 AM
There was a book now out of print, "Power Boat Design" (or something like this) by Lord. It was a real eye-opener.

He claimed the optimum beam was 42% of length for smooth water and 38% for rough water. He stated that one of the best hull designs was the Bertram, with its constant deadrise V-bottom. The dear old PT 109 hull wasn't good at all. It like many others had a hull whose bottom flattened towards the stern.

I don't know if this translates well down to small sizes like you are considering. Low deadrise towards the transom may be an important consideration for stability, if you sitting or standing in the stern while fishing.

I did a search on Alibris and Bibliofind and could not find the book.

Bruce Taylor
12-05-2002, 12:39 PM
ahp, would this be: _Elements of Powerboat Design_?

I found a copy of that one at www.bookfinder.com (http://www.bookfinder.com)

12-05-2002, 02:31 PM
Bruce, I don't know. I went to the website you gave, and yes they have "Elements of Powerboat Design" but no author is given. I am quite certain about the author's last name.

A search of Alibris and Bibliofind by this title found nothing.

12-05-2002, 04:05 PM
It'll be Lindsay Lord's;

Lord, Lindsay. Naval Architecture of Planing Hulls. (Cambridge, MD.: Cornell Maritime Press, Inc. 1963.)

Well spoken of, but I don't have a copy...

12-05-2002, 05:26 PM
Lindsay Lord on the Higgins PT design:

"A good feature of the design is the high forward profile of the chine. However, the excessive concavity of sections allows serious pounding just forward of amidships when the going is rough. Aft of amidships the plane warpage becomes so extreme that suction loads up to 8 tons are not uncommon, a full explanation of the poor trim and balance of these vessels."

He goes on to note (with dripping sarcasm) that Higgins patented the design, claiming that the squatting at speed caused by the suction load was an advantage of the design.

12-05-2002, 06:49 PM
I recall that he also stated that the suction near the stern was the cause of the rooster tail that is often attributed, wrongly, to propeller wash. It is wasted power.

He also critiqued the German WWII E-Boat. It was 106 ft long and about 16 beam and could achieve slightly over 40 knots with three 2500 hp diesels. If it were widened to 24 ft and other things kept the same, Lord claimed that the boat would have done the same speed with one less engine.

Lowell Bernhardt
12-05-2002, 08:16 PM
$$84.75!!!!! :eek: :eek:

"LIKE ZOIKES SCOOB" that's alot of money!! I was thinking quick and dirty, you know maybe not even having that much in the boat?? Maybe if I nudge Mrs. Clause hard enough she'll get the idea? ;) You're right, probably not.


12-05-2002, 10:01 PM

Sure there's a ratio of ideal form for minimum resistance at a given speed. But it is only peripherally to do with length/beam. Beam is only important because it controls hull form. That is; for a given displacement a wider hull will have less deadrise. Wider hulls with flatter bottoms plane with less power, but are hard riding and uncontrollable at high speed in a seaway.

This is where Lord's idea of making the German E (sic) Boats wider was unworkable. The German boats were running at very high speed (not 40 knots) in very rough seas. Lindsay's relitivly flat bottomed PT types planed at lower speed, but pounded themselves (and their crews) to bits in any kind of sea.

For a minimum resistance form you need to plug in the volume/Froude number (displacement/speed), projected chine length, and longitudinal center of gravity (which gives you running trim). These give you an idea about resistance, now you need to look at dynamic stability, both thwartships and longitudinally. Then there are practical considerations; at rest stability, load carrying ability, interior volume, ease of construction, etc.

You are correct that as a hull becomes narrower for a given length/weight it may be more easily driven, but only at some speeds, and other problems may arise.

Lindsay Lord's book is based on work he did during the Second World War. It was the bible of powerboat design 50 years ago. All the material presented is totally outdated. You would be much better to spend your money on the second edition of Larsson & Eliasson's Principals of Yacht Design. They have included a very good overview of high-speed hydrodynamics and their application to powerboat design.

All the best, Tad.

Tom Lathrop
12-05-2002, 11:04 PM

Thanks for the insight on some of the interrelationships of hull parameters. I do think you are being a bit hard on Lord though. He actually had praise for the sea keeping speed of the E boats relative to the British and American PT boats. I think the comments on beam of the E boats are out of context.

There has been lots of progress (and lots of recession) in planing hull design since Lord published his thoughts and data but to say that his work is all outdated is a great disservice. His recommendations for the basic hull sectional parameters are still perfectly adequate for most of us to use in designing a safe and efficient hull. Many of the boats being produced today would be much better if they just followed these relatively simple concepts.

If there has been an equally comprehensive book on planing hull design that is understandable by amateurs published since his last issue in 1963, I have not found it. MotorBoating's Ideal Series has much good information but it is just as dated as Lord's work and is not nearly as cohesive.

Granted that many boats being designed today have elements that go well beyond the technology of 50 years ago but most of us do not need these "advancements" in our boats. We build wood boats and mostly traditional (in spirit anyway). Heck, most of your boats stick to the traditional or classic lines too.

Lindsay Lord taught me far more than was available from any other source and I just had to speak up for his work. I am an admirer of your designs also and was a bit surprised at your post. Perhaps I just took it wrong? I wonder if you could point out any serious errors or misinformation in the book. I would be surprised if there were not some poor concepts but I have not reached the level that I could spot them.

I will certainly look for the book you mentioned.



12-06-2002, 12:55 PM
Hi Tom;

Thanks for calling me to task concerning the above, and “good on ye” for sticking up for Mr. Lord. The first thing I need to say is that everything I write is just my opinion.

You are right in that I was probably a bit hard on Mr. Lord's work; there is certainly some information in the book that is useful. I understand completely your frustration with the lack of a concise text on the design of powerboats. I have spent 30 years reading everything I can lay my hands on regarding powerboat design, and I’m still searching.

I have two main beefs with Lord’s book, and a number of minor ones. The major problem I see is that the book completely fails to keep up with development of powerboat design in the 17 years between the first publication in 46 to its third in 1963. I have the third issue and he still sticks to his WWII PT boat forms. He completely ignores the work of Bill Hand, Ray Hunt, Sonny Levi, and Peter Du Cane, among others. For instance in his Fig. 42 he calls the hull a Deep Vee Monohedron, obviously he was aware of Ray Hunt’s work, but does not acknowledge it or his derivation of it.

Second major beef is that in my opinion the hulls presented are just not very good. Of course this is in light of what we now know. They are all too full forward, especially between stations 2.5 and 5. This means they are hard riding, wet, trim more than necessary, and have higher resistance than necessary. Of course this is only my opinion and your mileage may vary. (Meaning there are many other variables that are pertinent) I have been thinking for a while that there are really two schools of planning hull design. One being the Lord/Blount school of low chine, high trim, “snowplows”. The other being the Hand/Hunt/Garden school of fine lined and more level running hulls. I very much subscribe to the latter.

I studied Mr. Lord’s book a great deal twenty years ago, but I came away unhappy. The boats just did not make sense to me. Part of this is certainly cultural bias, they are not of the type I admired while growing up. I learned a great deal more by studying Garden’s writing, that of Phil Bolger, and a paper in German on the relationship between speed and sectional areas/LCB’s. When I added this to more recent papers written by Daniel Savitsky, Eugene Clement, and Donald Blount things started to make sense.

My designs are mostly of a traditional style, but that does not preclude the work being informed by the vast body of work by others of a very different style. Your point on “appropriate technology” is just right. In some cases a more traditional form will do the job, new and different for its own sake is pointless. New and different that does the job better is what we are after. I’m thinking of Bolger’s cartopper as an example. But if all you have is Lord’s book and attempt a 50 knot offshore cruiser, you may not end up with a very great boat.

I think that you would be far better off to study the powerboat chapter in Principles, read Sonny Levi in Dhows to Deltas, read anything by Colin Mudie, Uffa Fox, and Cy Hamlin, then study Garden and Monk’s drawings. After that go back to Peter Du Cane and Lindsay Lord, just for perspective.

The Principles of Yacht design is available from WoodenBoat @ $44.95, a deal.

All the best, Tad.

Tom Lathrop
12-06-2002, 03:27 PM

You have spelled out many faults in Lord's book that are the result of the age and technology of the time. I agree with much of what you say and can not offer any explanation of why he did not update the book between 1946 and 1963. I have read both and attributed it to lack of time or resources to do the work necessary. One thing to remember is that the original work was done under contract to the US Navy to find out why PT boats performed so poorly. So, most of the drawings are of boats that were of inferior design, and this he does point out.

I understand the limitations of the low deadrise planing hullform but it has so many advantages for the ways that many of use our boats that we are willing to forego the higher speed in rough water that the deep V offers.

I found "Principles" on the net at less than $30 for the second edition and will order it.



John E Hardiman
12-07-2002, 12:30 PM
Tad & Tom;

A very nice discussion on a difficult subject.

One thing that struck me when reading Lord ( besides the very obvious axe he was grinding, go look at the history of the contracting for the development of the US PT boat) was that he totaly glosses over the effect of the two major design requirements, long range speed, and weapons capacity. It must be remembered that the displacement (and trim) of a WWII PT boat could change as much as 25% just in weapons alone and up to 50% if you considered fuel. I get a feeling reading the early edition of Naval Architecture of Planing Hulls (the one I have a copy of) Lord picks specific trials to emphise points. Work at DTMB in the mid 1960's shows that the trades of performance pre/post engagement for a small warship were too great. Off the design point on the ingress and you never deliver the payload, off the design point on the egress and you send the crew on a suicide mission. This lead to the push to hydrfoils, but they proved too fragile along with significant reduction in payload. The overall winner was OTH missile systems carried by larger ships.

What does this mean to the civilian designer? With a greatly diminished variable load (for moderately sized/ranged boats)the tradeoff now comes between absolute flatwater speed and sea speed for a given length/displacement/hp limit. Now there are many ways to skin that cat, depending on other requirements such as trailer width, hull depth, smooth riding, required volume, design sea state, etc.. One thing I think needs to be pointed out is that some of the more esoteric hull forms such as sea knives, SWATHs, low displacement cats, etc. are very sensitive to changes in displacement and trim.

So, in the overall assesment, the old hard chine v bottom may not be the best choice for any specific application, but it has the advantage of being able to compensate for large changes in trim and displacement by its squat angle. How an individual designer makes the trade between flatwater speed and sea ride, full load and burned out, should ensure many, MANY, hours of discussion of this forum. :D

Lowell Bernhardt
12-07-2002, 03:59 PM
Thanks guys, the dissussion is just a litlle over my head. I think I may try to find the Principals of Yacht Design book.

You mentioned that the flatter the bottom of the boat the harder it pounds, and the harder it is to control at high speed. Why is it harder to control at speed. My guess would be that there is less of the boat in the water to provide directional stability. However, wouldn't this be the same in a very fast semi or even deep V hull, runnning with no more than what seems to be the prop in the water? How would one correct this? Would a skeg or taller bottom strakes help to provide more stability?

My uncle always said that there's nothing faster than a garage door. And for a smaller boat (something small enough to be a life boat on a PT boat :D ) for running the lakes and stuff around here a flat bottom would be just what the doctor ordered.