Fairline squadron 58
- Thread starter brannan77
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benjenbav
Well-Known Member
Bouba
Well-Known Member
Perhaps the best way to decide what makes a hull effective is to ask everyone here with a big shaft driven p or semi d hull; who can plane properly without the use of trim tabs. The winning hulls can then be forensically examined (and of course argued about more)
Nick_H
Well-Known Member
But as deadrise angle increases, so does surface area of hull to maintain the same beam. For sure, the vertical element of the force (what we call "lift") acting on any given area of hull will become lower, but there will be proportionately more hull for it to operate on. One cancels out the other.
Look at it another way. In the sketch below which hull generates the most lift at a modest speed?
The theory that horizontal rails increase lift suggests that the hull on the left creates more lift, in fact the total horizontal area of the hull is identical to that of a flat hull, so it follows that the lift must be the same as it would be for a completely flat hull. Now make the steps much smaller, so there are a hundred tiny steps, or a thousand or a million. The total horizontal area combined is still exactly the same as it would be for a flat hull, so no change in lift. Follow the logic to infinity, and it's clear that a V hull has the same lift as a flat hull, if all other things, including the beam, remain equal.
Apologies if I've misunderstood the argument
Portofino
Well-Known Member
But as deadrise angle increases, so does surface area of hull to maintain the same beam. For sure, the vertical element of the force (what we call "lift") acting on any given area of hull will become lower, but there will be proportionately more hull for it to operate on. One cancels out the other.
Look at it another way. In the sketch below which hull generates the most lift at a modest speed?
The theory that horizontal rails increase lift suggests that the hull on the left creates more lift, in fact the total horizontal area of the hull is identical to that of a flat hull, so it follows that the lift must be the same as it would be for a completely flat hull. Now make the steps much smaller, so there are a hundred tiny steps, or a thousand or a million. The total horizontal area combined is still exactly the same as it would be for a flat hull, so no change in lift. Follow the logic to infinity, and it's clear that a V hull has the same lift as a flat hull, if all other things, including the beam, remain equal.
Apologies if I've misunderstood the argument
It's to do with the direction of the force ,the full force
The reactionary force or ," Lift "from the molecules .Sure you can suggest a similar or equal surface area -(SA)- ,which you you have done .
In the RHS pure V a significant amount of force is lost I called it vectored off side ish ways therefore reducing the lift .
It can be added back by incorporating flat "lifting Strakes " or other flat surfaces .
Not wanting to sound too critical in fact if you unfold the LHS multi stepped hull ,because of triangulation it will actually have a much bigger SA -draw a few triangles to see .
As well as tons more lift the extra drag would kill that hull -the LHS
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stillwaters
Well-Known Member
But as deadrise angle increases, so does surface area of hull to maintain the same beam. For sure, the vertical element of the force (what we call "lift") acting on any given area of hull will become lower, but there will be proportionately more hull for it to operate on. One cancels out the other.
Look at it another way. In the sketch below which hull generates the most lift at a modest speed?
The theory that horizontal rails increase lift suggests that the hull on the left creates more lift, in fact the total horizontal area of the hull is identical to that of a flat hull, so it follows that the lift must be the same as it would be for a completely flat hull. Now make the steps much smaller, so there are a hundred tiny steps, or a thousand or a million. The total horizontal area combined is still exactly the same as it would be for a flat hull, so no change in lift. Follow the logic to infinity, and it's clear that a V hull has the same lift as a flat hull, if all other things, including the beam, remain equal.
Apologies if I've misunderstood the argument
I understood that the original hypothesis was that both the flat and the V surfaces would be equal. However, if you do as you suggest and both have the same beam, so that the V now has a greater surface area, it still doesn't work out. Inserting horizontal steps is misleading in this context, besides which, although obviously providing more lift than a flat angled surface, introduces other factors such as drag etc. Incidentally, millions of steps, as you suggested, could eventually become less effective if they were that small that they could no longer retain the water molecules that would be producing the reactionary force vertically. Much theory often only applies in certain circumstances, and is sometimes not black & white.
Back to your sketch with the non-stepped surfaces, though. To say that the equal beam scenario would produce equal lift is guesswork and it sounds plausible until you consider the following. As an easy example, assume that the V is at 45 degrees, much as your own sketch. Now assume that the beam of this V hull is 14.15 feet across. This would mean that the other two surfaces would each be 10 feet, or 20 feet total (sq.root geometry). However, because the angle of 45 degrees is the median between horizontal and vertical, only 50% of the lift is vertical, with the other 50% being lateral. So the resultant vertical lift is the equivalent of only 50% of the total 20 feet, ie 10 feet . However, the horizontal flat surface with a similar overall beam of 14.15 feet will have vertical lift applied to it right the way across its 14.15 feet. Another example to consider would be to think of each as steel plates (so that they sink in water) and hitch each behind a powerboat with the hitch above the surface of the water. At sufficient speed, each would rise and eventually begin to plane on the surface of the water. However, at the point that the flat plate was planing only on its rear edge, the V'd one would still be part submerged and would require still far more speed/power before its trailing edge was the only part of it touching the water. Can't prove it and never done it, but betcha it's correct.
D
Deleted User YDKXO
Guest
I always preface my posts in this theoretical threads with the proviso "I'm no expert" because I'm not and I'm probably going to get flamed for this but aren't you guys oversimplifying this question of lift by analysing it only in the static mode? Isnt hydrodynamic lift also about fluid dynamics as well as static mechanics? In NickH's example you could equally well put a round bilged hull form next to the smooth and jagged hull forms and argue that the round bilged hull should generate the exact same lift as the other 2 hull forms but that wouldn't be true. AFAIK round bilged hull forms are very poor at producing lift because there are no edges to induce flow separation which is why you dont see round bilged planing hulls. Hard chine planing hull forms have sharp edges at the side and the back precisely to induce flow separation to allow the hull to lift. Could it be that these lifting strakes you've been arguing about are not designed to produce static lift per se but to induce greater flow separation to allow static lift inherent in the hull (and its angle of attack) to be more effective? Is that why you tend to see these full length lifting strakes on narrow beamed planing hulls which dont have a large static lift component?
All IMHO and waiting ready to be flamed
Portofino
Well-Known Member
I always preface my posts in this theoretical threads with the proviso "I'm no expert" because I'm not and I'm probably going to get flamed for this but aren't you guys oversimplifying this question of lift by analysing it only in the static mode? Isnt hydrodynamic lift also about fluid dynamics as well as static mechanics? In NickH's example you could equally well put a round bilged hull form next to the smooth and jagged hull forms and argue that the round bilged hull should generate the exact same lift as the other 2 hull forms but that wouldn't be true. AFAIK round bilged hull forms are very poor at producing lift because there are no edges to induce flow separation which is why you dont see round bilged planing hulls. Hard chine planing hull forms have sharp edges at the side and the back precisely to induce flow separation to allow the hull to lift. Could it be that these lifting strakes you've been arguing about are not designed to produce static lift per se but to induce greater flow separation to allow static lift inherent in the hull (and its angle of attack) to be more effective? Is that why you tend to see these full length lifting strakes on narrow beamed planing hulls which dont have a large static lift component?
All IMHO and waiting ready to be flamed
It's all like going down the aisle in a supermarket with a trolley you can choose and hull designer pick ,n mix what features you want to put in that hull .
The hull behaves differently at different x sections due to different deadrise and other features -strakes ,rails etc
Drag is a killer of speed and you may want to introduce air to seperator the water from the hull .
Water is 100 more dence and draggy than air - the flat bit at the hull when it's planning that just is exposed to the air may wash out and at that 1/2M actually loose lift .
But the stern submerged flat bit of the chine , at more or less 90 degrees and Allways submerged ,may be wider will generate lift .
The builder will take the drag saving as good thing for those bits that airate .
The chine are usualy flat an on planning boats generate lift at the back say 1/3 rd if they are submerged and not airated .
But do create drag,hence the increase in Hp needed .
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Nick_H
Well-Known Member
I thought the hypothesis was always both hulls having the same beam, so that just the addition of spray rails/lifting strakes was in question?
I'm certainly working on a distant memory of A level physics here (so more than happy to be shown to be wrong!
), but you've calculated based on the vertical element of the force being 50%, but isn't it the sin of the subtended angle x the force? Sin45deg is 0.707, not 0.5, and when you apply that to your 20 feet across, you get an equivalent 14.14 feet?
Nick_H
Well-Known Member
I always preface my posts in this theoretical threads with the proviso "I'm no expert" because I'm not and I'm probably going to get flamed for this but aren't you guys oversimplifying this question of lift by analysing it only in the static mode? Isnt hydrodynamic lift also about fluid dynamics as well as static mechanics? In NickH's example you could equally well put a round bilged hull form next to the smooth and jagged hull forms and argue that the round bilged hull should generate the exact same lift as the other 2 hull forms but that wouldn't be true. AFAIK round bilged hull forms are very poor at producing lift because there are no edges to induce flow separation which is why you dont see round bilged planing hulls. Hard chine planing hull forms have sharp edges at the side and the back precisely to induce flow separation to allow the hull to lift. Could it be that these lifting strakes you've been arguing about are not designed to produce static lift per se but to induce greater flow separation to allow static lift inherent in the hull (and its angle of attack) to be more effective? Is that why you tend to see these full length lifting strakes on narrow beamed planing hulls which dont have a large static lift component?
All IMHO and waiting ready to be flamed
I agree Mike (and btw I should preface my posts with the same disclaimer!), but I think we're ignoring flow separation and drag, because the claim was made that the spray rails also add Newtonian lift
baylabayla3288
Well-Known Member
One of my earlier boats was a Princess 34 flybridge. I loved the boat's interior space for it's size and the design however it had a major problem in my opinion. It was underpowered with the already tired KAD 42s. On top of all the running angle was very bow high without the use of maximum tabs and full drives in trim even at planning speed. I wanted to fix the issue and I even vent so far that I called the designer Bernard Olesinski! He was very helpful and explained that the original line drawings did not have prop tunnels however in the production boats, tunnels had been added. The reason for incorporation of tunnels was to allow the KAD 42 engines to be installed higher up to meet the Volvo specs. This decision was done by the builder. Well, the consequense was reduced lift in the stern and the yard tried to compensate this by installing oversized (read wide) tabs. I did not like the handling of the boat and I wanted to address the issue somehow so I contacted mr Olesinski and, long story short, when I asked wether extending the strikes all the way to the transom would address the lack of lift, his ansver was clear, "extending the strakes will not help as they are only there to control spray! What is needed is the addition of a concave section on the back of the hull's bottom panels in such away that the concave angle increases with the distance from the keel to achieve also improved transversal stability. The boat also had a tendency to over lean into turns. I finally ended up installing vertical trim interceptors on the transom and this improved the ride clearly. The hight of the interceptors was only 15mm but they eliminated the need to use the tabs for correcting the running angle. Conclution, strakes does not add any lift at these speeds for this type of boat. Extreme high speeds are a different story. The above is off course for dynamic situations. Now without being an expert I cannot imagine that the small volume of the strikes add any static stability. Hard and wide chines and reduced V angle adds stability vers deep V and rounded shapes because the center of the buoyancy is moved further away from the keel the more the boat heels.
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