I skimmed the photo from the article in the current Sailing World. Note how far back the masts are stepped. Apparently to reduce the chance of capsizing or pitchpoling, wouldn’t that wake you up out in mid ocean by yourself!!!
Sunfish Hull Speed...?
- Thread starter Simon Sexton
- Start date
A very good job was made in explaining the potential/theoretical hull speed. I may have missed it in all the posts, but it should be noted that the water line is not the static waterline length of a boat sitting at rest, rather the heeled and increased waterline. This line is a parabola that lengthens as a beamer hull heels.
Just pondering this as it applies to the AC75. Its sort of a classic beamer hull until the foil starts to lift the hull vertically which shortens the active waterline until this factor disappears entirely when the “hull” is no longer in the water at all, unless you start considering the foil as the hull??? Mind numbing stuff!
One of these?I still have a Bombardier " Invitation" that ate Lasers for breakfast.... yum, yum...~~(\_~~
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,556
@Hunter216 it does require your mind to stretch and that can be painful. Worse once stretched it is impossible to put the genie back in the bottle.
Here is a brief and general idea of what is happening.
The Length of the hull is used to calculate the hulls wave length. This calculation provides a theoretical "Hull Speed". Good so far?
The hull speed and boat speed are not the same. Boat speed is dependent on the amount of friction the boat exerts against the water surface. The greater the friction the less the speed. Power (be it wind, electric or fossil fuel sourced) in large enough amounts can over come friction. OR... if you are clever you can design something that reduces the friction of the boat in the water.
The foil idea is one that reduces the combined friction of the boat hull and the foil in the water. As you increase the power on a foil attached boat the foil lifts the boat reducing the friction of the hull, transferring the friction to the foil. The foil design has less friction than the hull. So the given amount of power is able to push the boat faster. Eventually to give the boat the appearance of flying over the water.
Oh the magic of science.
Here is a brief and general idea of what is happening.
The Length of the hull is used to calculate the hulls wave length. This calculation provides a theoretical "Hull Speed". Good so far?
The hull speed and boat speed are not the same. Boat speed is dependent on the amount of friction the boat exerts against the water surface. The greater the friction the less the speed. Power (be it wind, electric or fossil fuel sourced) in large enough amounts can over come friction. OR... if you are clever you can design something that reduces the friction of the boat in the water.
The foil idea is one that reduces the combined friction of the boat hull and the foil in the water. As you increase the power on a foil attached boat the foil lifts the boat reducing the friction of the hull, transferring the friction to the foil. The foil design has less friction than the hull. So the given amount of power is able to push the boat faster. Eventually to give the boat the appearance of flying over the water.
Oh the magic of science.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,556
Bobby... the surface I speak of is the surface of the boat against the water. The dolphin or a submarine, or tuna can have a calculated hull speed (so to speak). But now since they are totally submerged the calculation is less relevant. Of higher concern is the relationship of total surface friction to power capacity (energy burden)
Have you seen the documentary on Netflix about this? I got way into it a few months ago!
Eh, sunken beer, Sunfish; as long as they're not associated with each other, I think we'll be just fine!
I dare not question your expertise, sir, but am simply curious as to how a sailboat (say, something like a J-34) would be able to overpower the immense weight and pressure of the keel and climb up onto a plane. I assume that would only be possible with an immense amount of wind and sail area trimmed at just the right angle; thus, a keelboat sailing upwind close-hauled would never be able to plane out, simply because the wind isn't necessarily lifting the bow at any level. I may be wrong, but those are just my thoughts running the scenario through my head.
Nope head still hurts. Just kidding, I have invested (or perhaps wasted) time getting my head around this stuff so already stretched and it’s fun however I would much prefer investing my time actually sailing. Right now I can’t sail or really utilize Lake Ontario as it’s not yet frozen which would let me get out with a kite and skates or snowboard so I find myself just blathering on this site feeling jealous of my southern neighbours who are out actually doing it!
That would explain why most planing hulls are thin and stout towards the bow and the stern is often wide to offer stability while planing. I have planed hulls with a different design, and the thing was yawing like it had consumed said sunken beers. Rigs for planing boats are also very different from basic keelboats: The mast is often shorter, but the sails are wider and able to catch more wind (thus generating the necessary push). Most of them are also rigged with a jib and genoa on separate furlers for different points of sail and wind conditions. You know this, I'm sure. Twin rudders, asyms, thinner keels and even newfound designs for swing keels are all being incorporated into the planing sailboat design. Very interesting, indeed.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,556
Simon... While the idea is one of not likely, the concept is possible. It is all about the application of power. Most likely not a very good planing hull and in all likely hood beyond unstable. With the application of enough power you could get the hull. not likely the keel (it might even rip off the boat in the process) mostly out of the water and moving faster than the displacement 'hull speed'. That said not sure why you would want to, other than to test the theory. Note that hydroplanes lift out of the water and ride on their prop, based on their prop speed.
With a displacement sail boat relying on sail power the only scenario that comes to mind would be riding down a Southern Ocean 60 foot swell (ok gravity is also at play) with a 50 knot wind pushing the boat and the sails are spread (right before they tear to shreds) and the boat crashes into the trough. The bow buries and the boat pitch poles.
Vivid imagination eh?
With a displacement sail boat relying on sail power the only scenario that comes to mind would be riding down a Southern Ocean 60 foot swell (ok gravity is also at play) with a 50 knot wind pushing the boat and the sails are spread (right before they tear to shreds) and the boat crashes into the trough. The bow buries and the boat pitch poles.
Vivid imagination eh?
In a different sense, I believe they would. The concept in my mind is similar to that of a falling object from any certain altitude reaching its terminal velocity. Granted, that incorporates gravity and wind resistance, but the overall point is that an object can only go so fast depending on its design. A dolphin, I believe, can swim at a maximum speed of 25 miles per hour. A tuna, so I've heard, can swim up to about 50 miles per hour. This might be confusing to the overage person, seeing as the dolphin has more muscle and seems to be more streamlined and powerful than a tuna. A tuna, however, has certain design features that work to its advantage: Instead of skin, it has scales which are more slick and often covered in a sort of lubricating slime. The tail, instead of being horizontal, is vertically situated and thinner, almost like a crescent, allowing more energy to be preserved through shorter, sudden bursts of speed. Therefore, a tuna's stroke is much shorter and more sleek than a dolphin's, conserving more energy and generating more speed at one time. A dolphin has to move its tail up and down in a greater range of motion, causing more resistance and spending more energy. A dolphin is also not quite as streamlined as a tuna, considering the differences in fins, body width, etc. Now, I'm sure that if there were some ways dolphin and tuna could be propelled through the water at higher speeds, it would be able to go much faster. However, I think that, when reaching a certain speed, the body would be uncontrollable and start wobbling and eventually spin out of control, sort of like a motorcycle. This is all just my opinion, though.
Very vivid, sir, and quite extreme! I do suppose, however, those are the conditions that it would take to test that theory. The keel ripping off is very possible; one time, I was sailing a Sea Pearl 21 (gaff-rigged schooner with a swing keel). While on a beam reach sailing across Galveston Bay, a storm rolled in and we began going to fast that the keel started humming and eventually began to partially fold back up into the trunk. Noticing our sudden instability, as turned to point downwind toward the docks and sailed wing-on-wing the rest of the way. Interesting experience.
there is that. also in cases such as my old alden/cca vintage vessel with it's long overhangs, the waterline will lengthen standing straight up with no heel. then there are these new designs that have long waterlines from stem to stern that have no overhang for which to grow.
when the hull falls off the stern wave, your at hull speed
Our Sea Scout ship used to have one. Skipper and I found it in some guy's backyard heading to Kemah one day and asked if he still wanted it. He said no, and that we were more than welcome to have it. It was in terrible condition, but we restored it, and it did outrun Lasers and Sunfish like a monster. A ton of fun. Unfortunately, we had to retire it once the mast broke in half in a gale (may or may not have been my fault...).
https://sailboatdata.com/sailboat/a-scow
wide bow/ huge rig = lots of speed with this design and no increase waterline length
wide bow/ huge rig = lots of speed with this design and no increase waterline length