Hi guys, I’m trying to get my head around a few concepts and have a few questions about aspect ratio? First off, how do you calculate aspect ratio? Do you measure aspect ratio by the wetted surface of the board?
Here is a quote from Rusty’s hull blog on surfline
“let’s start with the assumption that the wetted area will more or less reflect what the overall plan form aspect ratio is.”
“Let’s take a 6’2” x 19" board. Steve Coletta’s outline (which is what I
have to work with) has 1060 square inches. The span is measured as the
width of the shape or perpendicular to the flow, parallel to the
stringer. In this case the span is 19", the square of 19" is 361. The
span squared 361, divided by the area of 1060 is .3405. .3405 is the
aspect ratio of the outline shape."
Is Rusty saying that the area of 1060 is wetted surface area or overall area?
I am really confused here. Isn’t aspect ratio simply width divided by length. Couldn’t he have just said it simpler since:
width squared / area = aspect ratio, and area = width x length, so (width x width) / (length x width) = aspect ratio So to simplify; width / length= aspect ratio Right?
So another question or maybe observation is in lord’s naval architecture of planing hulls there is a planing angle relationship with the aspect ratio.[img_assist|nid=1049591|title=planing angles|desc=|link=none|align=left|width=640|height=183]
Should we design rocker to have an angle through the wetted surface that corresponds to these angles? Will these angles provide the most efficient planing? Should a wider board have more rocker and less for a narrow board?
Here is a rocker diagram with the a board with .34 aspect ratio and the planing angle of 2.2 degrees
[img_assist|nid=1049592|title=rocker angle|desc=|link=none|align=left|width=640|height=120]Should I add concave behind the center to get closer to the planing angle, or try to keep it a natural centerline rocker?
This should have more replies. I don’t agree with abproud that aspect ratio can’t be applied to surfboards… because all the boards I’ve seen seem to use the aspect ratio in different ways.
From eye balling lots of boards I can generally guess a ratio that looks ‘right’. Perhaps this works better than trying to get technical.
However…
I want to go wide but making hundreds of boards to get the ratio just right isn’t possible. A bodyboard ratio looks about right for the chest area - it seems to match up with the ratio I have in my head.
I generally want a chest area good for low speeds for drop in. I wonder what that ratio is around the chest area - the actual wetted area. The rocker means that a faster, messier wave will wet more of the board. Keeping it simple with flat bottom, uniform waves.
The interesting bit of me is the tail. Generally we see the rail/tail cut in at the sides for turning. What if the rail/tail is seen more as an area of different aspect ratio? Boards that are good at high speeds have gently curving sides whereas a mini simmons doesn’t. Understanding this a bit better might help find the optimum compromise between low speed boards and high speed boards (keep it simple - talk about speed characteristics)
edit: useful quote from Bill T:
"As a ballpark value, Simmons used a’‘release angle’', at the tail, of 15 degrees. Starting at a point 15 percent of board length, up from the tail. Actual tail width ended up whatever that exercise dictated. A classic Simmons balsa, 24 inches wide and 10 feet long, ended up with a tailblock approximately 16 inches wide, plus or minus 2 inches. Various of his boards have variations in tailblock width, as he played with that relationship over time. "
Exact measurements do not make sense in surfing.
No-one can tell you the real world values of AR for a surfboards, they’re not even static values.
Trying to calculate it make no sense.
What does makes sense is that understanding the theory allows you to think about trends.
You’ll be able to understand that a wider board will give you a higher aspect ratio and you can then use the theory to estimate what the effect will be on performance.
It’s all about trends, if you understand what effect which design change has, you’ll be able to iterate faster with your designs.
A good example of this is Airbus (the airplane builder). They were able to reduce the drag of one of their airplanes by 10%-20% using a very inaccurate modelling technique some 30-40 years ago. This technique overestimated the lift forces significantly, but it allowed them to spot trends in the drag forces so they were able to reduce it without having to build many prototypes. Today much more accurate modelling techniques exist but performance improvements made with them are orders of magnitude lower.
I agree with hans. Raw numbers based on length and width are rather meaningless when applied to a surfboard being ridden on a wave. For one thing, only portions of the board are actually in contact with the water. For another thing, stuff like water flow direction changes across the bottom of a board. In my opinion a person might be better off examining one of the Maurice Cole single concave bottoms and how the concave interfaces with the tail rocker. This has been addressed before but in a nutshell, a straight edge held across the bottom at an angle resembling actual water flow direction might show a dead flat section through the tail… Bill thrailkill has pointed out in the past that the fastest bottom is flat. What interferes with that basic concept is that a surfboard shape is a multitude of compound curves. By mixing that blend of rocker and concave you get… a flat in the midst of all those curves. This video might be worth a second (or third) look. https://www.youtube.com/watch?v=887oTdGshzM
He says his boards are going 80 kph, that’s enough for takeoff in a light plane !
And he also says at 45 degreees there’s a flat line across the hull, why 45 degrees ?
Hang on. Hydrofoils still have a Hull before they’re up and planing. The Hull there is narrow. Without confusing matters Does The hydrofoil have the same design for all speeds?
Oh yes, a hydrofoil has a fixed shape but it’s effective lift area is variable and based on the shape of the wave.
People think waves are very complex and focus on the measureables but all Waves break in the same shape based on their environment. Not the same speed, depth or height but the same ‘shape’… and that’s what the hydrofoil is interacting with.
To foil/fly, the foil must remain underwater. If the foil breaches to the surface, it loses lift and sinks.
If the foil was moving over the surface, it would be planing rather than foiling.
