# L/b Ratio Formula

Discussion in 'Boat Design' started by FAST FRED, Dec 8, 2014.

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### FAST FREDSenior Member

Years ago (1960 era) the AYRS was the leader in reporting on all things multi-hull.

At that time one contributor felt that the Hull Speed formula SLx 1.34 was nonsense as it was worked out by measuring commercial vessels with usual L/B of 3-1 or 4-1.

AYRS came up with a simple method of figuring "hull" speed
for a variety of boats.

S = L/3b X SQ.RT. (L)

When run for some theoretical hulls it may be a bit fast , but seems to work.

Today with 50 years of building and great computers , what say?

Is this a rational formula ??

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### TANSLSenior Member

I do not think any of these formulas can be generalized for all boats. They may work in a very specific case but nothing more.
On the other hand, think that computers have not invented anything. No new formulas because computers. The formulas are the same.

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### AlikSenior Member

Well, due to computers it is easy to make regression formulas now... But done by men, yes.

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### AlikSenior Member

2FastFred

This formula makes no sense. For high displacement speed, the primary factor of resistance would be slenderness ratio, i.e. length to displacement ratio. All other factors are secondary, including L/b.

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### Richard WoodsWoods Designs

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### MikeemcJunior Member

Aieee, when ships were made of wood and fast men were made of iron.

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### FAST FREDSenior Member

I was hoping a simple formula would be of help to the endless folks that want to build a fast efficient skinny boat.

If a L/B ratio theoretical hull speed is figured , frequently 75% of that would be a good long range cruise speed.

Might help some folks that have really unique desirements.

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### daiquiriEngineering and Design

The term "hull speed" was invented to indicate a point where the resistance curve of some types of hull visibly bends steeply upwards. Fully-displacement hulls with low slenderness ratio are characterized by this kind of behavior, for example. Once arrived in proximity of this speed, any further small increase of speed requires a disproportionately large increase of shaft power.

But boats with high slenderness ratio have nothing that could be defined as a "hull speed", because their resistance curve increases smoothly over a vast range of speeds. More power simply translates into more speed in this case.

So I don't see a usefulness of defining such a parameter for what you call "skinny" boats.

Cheers

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### groperSenior Member

Agree with daiquiri... Low displacement to length ratio boats, regardless of the number of hulls, don't exhibit a significant hump in the resistance vs speed curves. Thus there is no point in using the term " hull speed" with these kinds of vessels...

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### TANSLSenior Member

Maybe I do not understand what is being said. Leaving aside the term "hull speed", which I think everyone can use as he likes, Ddo you think that there is a valid formula for the speed, directly proportional to L / B?.
I have always believed in the value of the drag was a very important part dependent on the third power of the speed.
As said earlier, I may be misinterpreting something. Sorry if this is so.

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### Leo LazauskasSenior Member

I doubt it would be useful for the total resistance.
The wave resistance component of thin hulls should vary like beam squared, so
you might be able to use L/B in some type of composite formula for the total
resistance.

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### groperSenior Member

No, there is NOT.

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### MikeemcJunior Member

Thanks Fred , always liked you posts. I guess it all comes down to how fast and when do you want to get there. Years ago when we long lined , I had lots of time on my hands. Sooo I did some figuring on how to get somewhere without starting the engines or putting up a sail no matter what kind of boat you have. I put my theory through the test on a dead reckoning corase for 5days a whaala I was there in a shorter time than I plotted. I modified a very large sea anchor with steering baffles and used the current to pull me along at 10kts , yep 10kts. Using the gulfstream and dropping the weight 200feet down all I used was fuel for the generator. Yes it was a calm 5days with great weather. The stream is known to flow at 25kts . Long liners save fuel at all costs, happy fishing !

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Mike; I have messed about in the gulf stream but I had no idea that the stratified velocities were so variant. I did have to do a bit of calculation to get from Ft. Lauderdale to Bimini and back, but not nearly that much correction. We live and learn.

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### groperSenior Member

What a load of rubbish.... All the recognized scientific bodies state the gulf stream current as highest near the surface and peaking at no more than 5 - 6mph. Absolute fallacy if I ever did hear it!

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