I hear occasionally that the wetted surface on the M is greater than the X because of the curved hull on the M and the "flat" bottom on the X. My egghead side says this logic doesn't make sense. The surface area of a curved container per volume is less than that of a rectangular container. For example, a cylinder has less surface area for the same volume as a box of the same length. A little calculus shows that the same trend applies given that both containers are displacing the same amount of water (ignoring the ends). i.e. a round hull displacing the same volume has less wetted surface area than the flat hull. Simplistically, a 55 gal drum will have less wetted surface area than a box of the same length and width and weight (again ignoring the ends). I realize my assumptions are simplistic. The M hull is more rounded at the front and I doubt that the X hull is completely flat. I'm also ignoring planing. So what's going on? This assumes the X is sailed flat so maybe that's what makes the difference.
Another example: I like to race scows (C's and E's). I heard an interesting history on the design of the scow hull (many decades ago). I can't prove the story but it makes sense. The scow was designed to fool the handicap formula used at the time. Since the scow has a flat bottom and rather wide beam when flat, the calculation would result in a rather high number. "The fooling the formula part" came when it was sailed at about a 23 degree angle. The rounded sides of the hull (hyperbola shaped) then would provide a very efficient shape. There are even 2 lee boards (port and starboard) that are oriented vertically when the boat is sailed at the proper angle. It works, anyone who races scows knows that they're dogs when sailed flat. This means even with little wind the crew heels the boat so it's not flat; sometimes even leaning it into the wind. Of course the modern handicap numbers adjust for this. Nevertheless; if you sail these boats flat you'll loose.
Can someone explain this M vs X wetted surface to me? Thanks.
BB
M vs X wetted surface: I'm confused
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baldbaby2000
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ALX357
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Given the shortest distance between 2 points (on either side of the hull at the waterline) is a straight line, see that the V shape (of the M) traces a longer line (than the flat hull = straight line X does) between. Multiplying by the length below the waterline, and get a larger square footage of wetted surface. Maybe you were considersing that the X sat lower at the chines (edges) than the M in the water, but it draws less, rated at 9" nominal draught for the X, and 12" for the M.
....to borrow a stance from one who goes before, Me carpenter, no engineer, but that's how i figure it.
....to borrow a stance from one who goes before, Me carpenter, no engineer, but that's how i figure it.
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baldbaby2000
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Norbert,
ALX357,
Good point. I was looking at a rounded hull not the V hull. Remember that you need to consider the surface area of the sides of the square boat and add that to the surface area of the bottom. If I do a similar calculation(albeit with simplistic assumptions) with a completely V hull vs a completely rectangular hull, the V hull still has less wetted surface area until the angle of the V gets bigger than about 74 degrees. This seems to be a pretty deep V. I don't know what the 26M has. I'd go look at mine but it's getting bottom painted now.
My real question I suppose is where the information came from that says the M has more wetted surface area than an X.
BB
You're right! For those that have a blue hull, probably subtract another 10% from the wetted surface area.you didn't consider the hull color in your calculation
ALX357,
Given the shortest distance between 2 points (on either side of the hull at the waterline) is a straight line, see that the V shape (of the M) traces a longer line (than the flat hull = straight line X does) between. Multiplying by the length below the waterline, and get a larger square footage of wetted surface. Maybe you were considersing that the X sat lower at the chines (edges) than the M in the water, but it draws less, rated at 9" nominal draught for the X, and 12" for the M.
Good point. I was looking at a rounded hull not the V hull. Remember that you need to consider the surface area of the sides of the square boat and add that to the surface area of the bottom. If I do a similar calculation(albeit with simplistic assumptions) with a completely V hull vs a completely rectangular hull, the V hull still has less wetted surface area until the angle of the V gets bigger than about 74 degrees. This seems to be a pretty deep V. I don't know what the 26M has. I'd go look at mine but it's getting bottom painted now.
My real question I suppose is where the information came from that says the M has more wetted surface area than an X.
BB
The X and M are neither round nor flat, but if anything, both are much closer to the latter. They're more shallow V hulls, and while MacGregor has specified the deadrise at 8 and 15 degrees respectively, there's some changing radius rounding, especially at the soft chines, that makes the hull form surface difficult to measure. When you add to the complex hull forms, the different displacements and drafts of the boats, I don't think you can easily calculate the wetted surface area. At least I can't.
What I've done when I've said that is to fall back on the end result, and on the generally accepted principle that for the same boat, the deeper the V, the greater the drag and horsepower to overcome it. That seems to be the case with the X at 24 mph and the M at 22 mph, with the same 50 horsepower. It's also why I recommend for the M, the 60HP version of a same size and weight motor as the 50 HP that would be used on the X.
You do point out something interesting, however, with the heeling factor, where the one side of a deeper-V bottom moves toward being a flat-bottom, and one side of the bottom and chine of the flatter boat moves toward being a deeper-V. Perhaps this does give the M less drag/speed disadvantage, or maybe even an advantage, when heeled.
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Moe
What I've done when I've said that is to fall back on the end result, and on the generally accepted principle that for the same boat, the deeper the V, the greater the drag and horsepower to overcome it. That seems to be the case with the X at 24 mph and the M at 22 mph, with the same 50 horsepower. It's also why I recommend for the M, the 60HP version of a same size and weight motor as the 50 HP that would be used on the X.
You do point out something interesting, however, with the heeling factor, where the one side of a deeper-V bottom moves toward being a flat-bottom, and one side of the bottom and chine of the flatter boat moves toward being a deeper-V. Perhaps this does give the M less drag/speed disadvantage, or maybe even an advantage, when heeled.
--
Moe
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wiehan
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Re: M vs X wetted surface: I'm confused
You agree that a Round bottom for the same length has less volume than a flat bottom. So, as a result, a round bottom must go in deeper in the water than a flat bottom to displace the same volume to keep the boat afloat. As a result, it has more surface that touched the water than the flat bottom in order to displace the same amount of volume !! If I understand your question correctlybaldbaby2000 wrote:I hear occasionally that the wetted surface on the M is greater than the X because of the curved hull on the M and the "flat" bottom on the X. My egghead side says this logic doesn't make sense. The surface area of a curved container per volume is less than that of a rectangular container. For example, a cylinder has less surface area for the same volume as a box of the same length. A little calculus shows that the same trend applies given that both containers are displacing the same amount of water (ignoring the ends). i.e. a round hull displacing the same volume has less wetted surface area than the flat hull. Simplistically, a 55 gal drum will have less wetted surface area than a box of the same length and width and weight (again ignoring the ends). I realize my assumptions are simplistic. The M hull is more rounded at the front and I doubt that the X hull is completely flat. I'm also ignoring planing. So what's going on? This assumes the X is sailed flat so maybe that's what makes the difference.
Another example: I like to race scows (C's and E's). I heard an interesting history on the design of the scow hull (many decades ago). I can't prove the story but it makes sense. The scow was designed to fool the handicap formula used at the time. Since the scow has a flat bottom and rather wide beam when flat, the calculation would result in a rather high number. "The fooling the formula part" came when it was sailed at about a 23 degree angle. The rounded sides of the hull (hyperbola shaped) then would provide a very efficient shape. There are even 2 lee boards (port and starboard) that are oriented vertically when the boat is sailed at the proper angle. It works, anyone who races scows knows that they're dogs when sailed flat. This means even with little wind the crew heels the boat so it's not flat; sometimes even leaning it into the wind. Of course the modern handicap numbers adjust for this. Nevertheless; if you sail these boats flat you'll loose.
Can someone explain this M vs X wetted surface to me? Thanks.
BB