The elements of boat strength

This has nothing to do with titling and registering a boat in Virginia. Boating Safety Education Requirement https://dwr.virginia.gov/boating/education/requirement/

 

I agree that IF the formula used draft. I've used LWL x BWL x Draft x Cb with a reasonable guess of Cb to obtain a preliminary estimate of displacement. But Gerr does not use draft.

Depth in Gerr's formula is the distance amidships from the sheer to bottom of the hull excluding keels and similar. For recreational boats height from the waterline to the sheer can range from less than the draft (some ballasted sailboats) to several times the draft (light boats with high topsides). So the correlation between Gerr's "depth" and draft is weak which means the correlation between his Scantling number and displacement is also weak.

 

Agreed.
So, one can only assume that he has taken this "into account" with his SN?

 

Based on the paper it looks like Gerr's rules rely heavily on regression of the scantlings against Sn using data from a number of boats. There is no evidence of any attempt to correlate displacement with Sn.

 

There is nothing ostensibly wrong with this approach, as that is how most NAs derive basic dimensions etc...a simple trolling through existing data to establish a rough best fit, where none exist and/or outside existing designs.
Trouble is, when I do this, I know what it is or is not based upon...and as such, the limitations of my assumptions and thus, require another but mroe detailed reivew in the deisgn sprial.

Do readers of D.Gerr's book, understand this? I doubt it, presumably they teat it as a carte blanche value.

 

Oh fellows! Take several thousand boats, and group them into sets based entirely on similar length and beam. Then see, by statistical analysis whether draft correlates with displacement. Since it is a physical impossibility that it wouldn't, that is how Gerr successfully derived his value with an extremely close correspondence with reality.

You're trying to argue that just knowing the length, width and depth of a box doesn't tell you the volume of a box. That would be true, if boxes had bizarre random shapes, but they don't. They have box shapes.

 

I agree that length x beam x draft will have good correlation with displacement. But that is not what Gerr uses.

Gerr uses depth of hull from sheer to keel, not draft. Take several thousand boats, and group them into sets based entirely on similar length and beam. Then see, by statistical analysis whether depth of hull from sheer to keel (not draft) correlates with displacement.

It is not the total volume of the box which is relevant, it is the weight of the box. To be consistent with Gerr the depth of the box is the same as the overall height of the box. Take a large number of boxes, made of different materials and with different contents. Knowing the length, width and depth/height of the box does not give the weight. But if instead of the depth/height you used the draft of the box when it floated then you would know the weight.

Gerr uses the depth of the hull from sheer to keel, not the draft.

 

No man, VA DWR actually comes out and inspects your boat; never said, implied or inferred that they check seaworthiness from photos … I’m blocking you , budroe !!! …rjw

 

… not hard , man … you guys just like to criticize … blocking you too !!! … rjw

 

Again, accurate but not correct. Hull above the waterline is determined by factors other than displacement. Sufficient freeboard for safety, so the boat isn't swamped by a light fog. Like that. Draft is determined by displacement. Again, you're arguing after the fact. He looked at a data set of literally thousands of boats and determined empirically that in reality the correlation was strong enough to give good results. Expressing an opinion before the data set is analyzed is reasonable. Trying to explain how reality must be at fault after the analysis has been done might be fairly described as less so.

 

rjwintl,

Block everyone, block out reality.

(BTW, it's poor form to tell someone you're blocking them.)

 

What correlation or analysis? There is absolutely nothing to suggest in either the paper https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH/BoatStrengthIBEX.pdf or the book that Gerr analyzed the correlation of his Scantling number (Sn) with displacement. The only claim I've seen from Gerr about his Scantling number (Sn) and displacement is from the book p 7:

"The Sn itself, however, applies consistently to all standard hull forms because it approximates total hull volume, which is closely linked to displacement."​

Nothing is provided to support the claim that total hull volume is closely linked to displacement. From my observations of boats this is not true.

While in the paper Gerr says he decided not to use displacement it is used in the form of the D/L ratio to modify some scantling rules. ( D/L ratio = displacement in long tons (2,240 pounds) / (waterline length (in feet)/200)^3)
Book p 7:

"Speed, type of service and displacement-length (D/L) ratio will be used in specific instances (in the following rules) to adjust the scantling results.
.........
no adjustment for displacement is necessary for vessels with D/L ratios between 275 and 100. This range covers the majority of ordinary craft. Adjustments for higher or lower D/L ratios are covered in each specific rule as they apply."​

The only set of rules I found with D/L modifications are those for bottom laminate thickness in solid fiberglass construction.

Gerr's scantling rules have been useful for many designers though they have a reputation for being heavier than needed for many boats.

 

Recognizing the great merit of these books and how useful they have been for many designers in the past, it is clear, I agree with you, that no one can claim to optimize a structure with such generic formulas.

 

Indeed not. But no one using this resource would be involved in optimizing a structure anyway. For my boat, optimization would be an oxymoron. I want it to be useful for everything. Therefore optimized for nothing. Gerr is ideal for me. But no one would suggest that an open ocean racer would be designed using Elements of Boat Strength. The boat would be far too strong, too heavy, too slow. Not competative. Designed by a professional.
At the other end of the scale would be a boom boat: a mini steel tugboat that muscles log booms around. Also designed by a pro. EofBS would make a boat far too fragile.

I'll just take a minute to point out the intellectual deficiency found in most designer's heads, because I've seen this played out countless times. Any calculations require inputs. Those inputs are educated guesses. Assumptions- all too frequently unexamined assumptions. Values taken from models that may or may not accurately depict reality. And yet I've had proper engineers ready to fight, over their numbers. They multiplied an estimate by an assumption, and firmly believe that because 6 times 7 really does equal 42, they have demonstrated something.

 

So, your statement HERE

.... how did they assess the seaworthiness of your boat then?

Must be wonderful being in your own echo chamber!