Sd/fd

[BruceK]

This SD/FD thing is a bit like the Jack Russell Terrier dog breed. Lots of variations and no hard and fast rules.
Eric, do you really think Bob`s hull is planing? Looks more FD than anything else, though a bit flat bottomed, and no chines I can see. I think a lot of hulls fall somewhere( to use an autism term) "along the spectrum", trying to box them in with labels is fraught with difficulty.

 

[FF]

"What speed would you say had equal drag from wave making and surface area drag for a 36 foot trawler."

It is usually figured as SL, the sq rt of the LWL

Depending on the boat the easy to push multiplier is somewhere between .9 and 1.15.

Light skinny boats do well ,but it rakes a length to beam ration ( L/B) of above 6 as in a cat or tri to get the bow wave small enough at speed to push through it..

The ease of a multihull to go fast is balanced by the higher skin friction (more surface area) and the requirement for more expensive light construction and the load limits.

A real planing boat will be far lighter than a displacement boat ,

Displacement–length ratio - Wikipedia

https://en.wikipedia.org › wiki › Displacement–length_ratio

The displacement–length ratio (DLR or D/L ratio) is a calculation used to express how heavy a boat is relative to its waterline length. (Rousmaniere, 1999)

 

[psneeld]

Mostly. ..these discussions about hull form are in a vacuum of theory over practice. A good example is using 1.34 as the absolute in calculating a hull speed. My readings have shown that that can vary a little bit based on slight variations in hull shape.

Take sectional barges that are completely reptacular and squared off below the waterline.
They have flat bottoms and have hard chinees but are no way ever designed to be planing hulls. In fact they are the slowest towing objects I've ever had to tow.

There are many principles to hold design and what they ultimately will be used for. Many other factors and just QBBL or prismatic coefficient or whatever you want to pick..... you can't just pick one and say what a hull is or should be.

It reminds me of navigation rule arguments where somebody's reads them, picks one, applies one to a particular incident and says that's what you must do. Any experienced operator knows differently.

 

[Nomad Willy]

It’s not hull speed it’s wave speed and that’s not theory. That’s fact.

 

[Nomad Willy]

This article explains it in an understandable way.
(BTW, I noticed he is talking about sailboats)

https://www.boats.com/reviews/crunching-numbers-hull-speed-boat-length/

Soo-Valley,
OK I read it just now.
A very good intro to the ways of hull speed thinking.
Most everyone on TF should read this. As SooValley has stated it’s easy to understand and reasonably comprehensive.

Great Lakes ore boats operate w multiple waves in their length. Some operate w a returning wave having returned several times and finally comes up under the stern. Their bulbous shape/hullform prevents them from going fast enough to take advantage of the first returning wave. Usually we only concern ourselves w that first wave but if we slow down the first wave may hit the hullside about amidships .. bounce off and return at the stern .. but w reduced force and amplitude. Cruise ships probably operate on the second wave.

Listening to other opinions and seeing facts arranged in a different way will most often lead us to greater understanding.

Thank you very much Soo-Valley for the link.

Here is an example of the link text.
“To understand why this is and where this mysterious multiplier of 1.34 comes from, you first need to understand that the term “displacement hull” refers to a hull that travels through the water rather than on top of it. Because such a hull displaces significant amounts of water as it moves along, it inevitably creates two series of waves in so doing--one at the bow and another at the stern. These waves are governed by a law of natural physics, which states that the speed of a series of waves in knots equals 1.34 times the square root of their wavelength, which is the distance in feet between the wave crests (WS = 1.34 x √WL).

 

[psneeld]

yep . ...everything in the universe falls exactly into place into every scientific equation. ... there's no overlap, there's no outside the lines, there's no margins of error, do standard deviations, there's nothing but absolutes.

Any possibility that although waves fall into that equation that variations in hull design may create waves slightly inside and outside that absolute formula?

 

[SteveK]

yep . ...everything in the universe falls exactly into place into every scientific equation. ... there's no overlap, there's no outside the lines, there's no margins of error, do standard deviations, there's nothing but absolutes.

Any possibility that although waves fall into that equation that variations in hull design may create waves slightly inside and outside that absolute formula?

math does explain a lot in the universe.
Up until the article I linked the math did not make sense either. It was too finite. How can a boat exceed the math calculated speed?
Well I got to thinking about those speed boats I used to run around in after reading the boat length calculation gives you the minimum-maximum speed, referred to as the hull speed when the distance between the bow wave and the stern wave equal the boat length it made sense.

Back to the speed boat, as you apply throttle the bow wave stays put but the stern wave gets further and further away behind the transom. IF you then could measure the distance between the bow wave and the stern wave (now behind at some distance) then it would give you the current speed.

Now it makes sense that a SD/FD hull speed can be exceeded if enough power is applied as the stern wave moves aft.

FD/SD is in the eye of the beholder as is a planning hull.

also found this link useful when used with wave length instead of LWL
https://www.vicprop.com/displacement_size.php

 

[Wayne]

'Lo All,
My old 46' sailboat was a Starratt & Jenks 45 (mold length was 45'6" (?), but w/ anchor 's bow roller was a little over 46'). It was designed by Charles Morgan as an IOR racing boat. Sitting still in the water it had one waterline length (WL) (I forget exactly what it was), but when sailing it had a longer WL. The reason was that when it heeled under sail, the long bow and stern overhang were now in the water, effectively increasing the WL length and thus its theoretical maximum speed. When IOR measured the boat, it was sitting still (upright), but when racing, it was, of course, heeled over most of the time. I was told that IOR changed its method of measuring to stop "Morgan's cheating" due to the boat's added waterline length when heeled. Charlie Morgan and his Morgan 45 (not to be confused with Morgan Yacht's Morgan 45 cruising sailboat) were separated from Morgan Yachts and the molds were later sold to Starratt & Jenks. My old sailboat was easily driven to hull speed - required only about 15 KT winds when on a broad reach. Wind speeds over that and I had to shorten sail. I have a picture of her with sails full, gliding through glassy smooth water. Thanks for the memories of a great boat. Last I heard, she is still sailing in the Tampa Bay area.