Question: D/L ratio for large vessels.

Hello everyone.

This is my first post in the forum. I am a student on Naval Engineering in Madrid, Spain. I am working in my final thesis project. I have to desing a 30m sailboat.
Excuse me if I don't speak english very well.
I have been reading this forum long time and learning a lot.

I have a problem with the D / L ratio. In technical books of yacht design you can see a ranking of this ratio:

Displ. / (LWL/100)^3

(D in tonnes (or lbs * 2240), LWL in feet)


D/L Category
>450 Very heavy
400 Heavy
350 Medium heavy
300 Medium
250 Medium light
200 Light
<150 Very light

I saw it in “Seaworthiness, the forgotten factor”, Marchaj. And maybe in other books.

In gosail.com I saw this classification:

>380 Very heavy
320-379 Heavy
250-319 Medium
120-249 Light
50-119 Very light
<49 Ultra Light

You can see a great diference. Could be this because the first one was studied long time ago, when vessel was designed with great displacement (V hull)?

Another question I have is, Is this only valid for medium length vessels? For example from 7-20 meters (22-65 feet)?

I have made a database for vessels of 30 meters. If you make the study of this ratio you can see that more or less all this lengths have an “ultra light” ratio in the first classification, or “light” in the second one. The ratio of the database of 30m is between 50 to 200.

What would you do? Would you make a conversion from the limits [150,450] to [50,200]?
Or really megayachts have an “ultra light” ratio?

Best regards.

 

If you have "Heavy Weather Sailing" (Peter Bruce, Adlard Coles) you can see in page 3 one graphic that explain that the D/L ratio is [150,500] for 20 feet vessels and this range decreases to [50,200] for a 70 feet vessel.

That is the reason because I said before "to make a conversion" from one range to the other.

 

It should be a dimensionless number to let you compare different size boats. That is where the cube in the formula comes in.

 

Yes, I realize that. The length has more importance because it is raised to 3. Then the great lengths they have a lower ratio than smaller ones. My question is whether that classification is or is not worth for large vessels because megayachts are always light according to this classification.

 

probably this post from eric sponberg is of any help to your question:
http://www.boatdesign.net/forums/bo...ulation-implications-30857-10.html#post340222

to my understanding it is just a ratio to compare vessels with similar ratios despite having different length...
the classification does not matter in that case (explained in the mentioned post) and this classification changed over the decades because new materials allowed for lighter builds and the classifying had to be redone...
it probably will have to be redone in the future again...

 

Thank you Capt Vimes.

Eric must be a person with much knowledge.

As Eric says, DLR would be constant between different size vessels if they were geometrically proportional.
I understand that this ratio decreases when the length increases, Because for example beam is not linear with length. I also understand that designs have changed and the materials have evolved in recent years.

I understand "Why", but I still do not understand what to do.
My idea was to transform the limits of that classification from small vessels to the results for large vessels.

In this picture you see what I mean. It is from an article by Olin J. Stephens II, in "Heavy Weather Sailing".



You can see [150,>450] in small boats and [50,200] in large boats.
Then the transformation would be:

>200 Very Heavy
175 Heavy
150 Medium Heavy
125 Medium
100 Medio Light
75 Light
<50 Very light


Do you think this reasonable or it is an aberration?

 

Navalex,

It does not really matter what you call the different D/L categories. What matters is how does your 30M design compare with other 30M designs? You probably have data on other designs of a similar length, and there usually is enough information in published particulars (displacement and waterline length) to calculate the D/L ratio of those designs. You can plot them on a graph, D/L ratio vs. Length or vs. Displacement. You'll get a scatter of data points on the graph. Typically, you want your new design to be somewhere close to that scatter. If you want to be lighter or heavier for boats of similar length, then go ahead and do so, and be able to justify your decision with what it is you are trying to do. It does not matter what category you are in, really. You can call your design a "lightweight design" or a "heavy design" if your design is lighter or heavier than your comparison data. If your choice is in the middle of the data, then you can call your design a "typical"design.

Eric

 

Eric is correct. Plotting various boats on a graph is a good way to visualize comparisons.

I often use a SA/D vs D/L chart to compare boats.

In the attached example I was mainly looking at offshore race boats, with a couple of other controls thrown in.


I can't imagine anyone designing a sailboat in this modern age that would have a D/L greater than 300. Even 250 seems excessive now. Those older references (300 = Medium!) are very archaic.

 

In the "class" that I am leading on the link that Capt. Vimes mentioned:

http://www.boatdesign.net/forums/boa...tml#post340222

I am going to be discussing SA/D ratio (to be posted this coming Sunday or Monday). And in my design work, I also do what you suggest, plot SA/D vs. DLR to get a picture of overall performance. That, actually, is going to lead to another discussion the following week to introduce a new concept called The S Number (S#). This is a way to rate all sailboats on a performance scale of 1 to 10. We can overlay categories of S# over the plot of SA/D vs. DLR. It's really pretty neat.

Stay tuned!

Eric

 

Thank your Eric.

Now I know what to do. I have to forget the numbers.

I studied DLR with my database. With points on the graph above and below you can see the limits of DLR for this length. If I want my boat to has a mid-heavy ratio, compared to the database, I will have to choose the information that is in the top half of the points.

I will stay tuned waiting for that number “S#” that you mentioned before

Best Regards

 

Eric, I'm looking forward to see your book at Amazon.com.
If you haven't thought of writing one yet, I think the time is ripe. You have a lots of things to say and to teach.

 

Thanks for that thought, Daiquiri. I think enough people have written books on basic yacht design already. The book that I want to write, someday when I am retired, is a review of some of the screwiest boat ideas you ever heard of. And the title of the book will be: "I want to build a boat, but I don't have any money."

Eric

 

I agree.