Re: Bob Perry's take on Wolfenzee's dream boat
I apologize up front that this is a very long one (even for me):
There are two questions that I would like to address, the first is Wolf's question about how much faster this boat would be, and the second is the question(s) about seakindliness.
To address Wolf's question, lets just say that if we give Wolf's boat the benefit of the doubt and suspend physics for a moment, and we might assume at best Wolf's boat is 30 seconds a mile faster than something like a 1960's era keel version of a Bristol 29/30, which in itself was a pretty fast boat for its day. That would give him a rating in the 180's.
I would expect the 'My version' boat should be faster than my current boat at 87, or a J-40 at 78, and probably be close to comparable to something like a Beneteau First 40.7 down in the 50's.
In other words, in 10-12 knots of wind, the 'my version' boat starts out a couple minutes a mile faster. That would be the wind range when the speed different is at its least and a 25% increase in actual speed is a huge difference in real speed when cruising. But the speed disparaity would grow even larger in light air, or in heavy air.
In light air, relative to Wolf's boat, the massive increase in sail area relative wetted surface means that the 'my version' boat would sail far better than its rating in comparison, and would hit its higher hull speed much sooner as well.
In heavy air, Wolf is right that his boat is a displacement boat, and therefore pretty much limited to at best sustaining its hull speed of 6.75 knots. A boat like the 'my version' is a semi-displacement boat.
My current boat is primitive compared to this design, but as a point of reference, it's not hard in wind speeds in the mid-teens for Synergy to beat at speeds in the mid-7 knot range and reach at higher speeds. Reaching in apparent winds in the 20 kt range, it is easy to sustain speeds well over 8 knots. My boat has sustained speeds over 9 knots in the right conditions and hit speeds in the high 11's without terribly big waves to surf.
That is the kind of relative speed difference that we are talking about. It means hours of time cut off of a day's passage and perhaps weeks off something like a trans-oceanic passage.
I also want to address Hartley's question about seakindliness and a similar point that Marty made.....Marty's first. Marty, to get a boat to plane, you do not need to make the forward sections flatter, and therefore you would not expect that a boat that can plane, would necessarily pound as much as your boat. The bow sections can be finer and have a fair degree of deadrise, which combined would reduce pounding.
That said, I do not expect this boat to plane except in very extreme conditions. At best I would hope that it would spend a lot of its life in semi-displacement mode. That is achieved by minimizing the size of the bow wave and therefore the drive required to achieve speeds above hull speed. It is the same principle that allows Catamarrans to achieve the speeds that they do.
But to the broad generalities of the seakindiness, I will apologize that I don't have time to write this from scratch and that much of this was written for another purpose and so may not directly apply.
But to touch briefly on this question specific to the two designs, there is a tendancy to 'read' boat drawings in a way that suggests that the boat is standing still. When you try to visualize the behavior of a boat, especially relative to motion comfort, you need to visualize that motion over time as the boat passes through the source of that motion.
Starting with pitch, if we visualize the bows of Wolf's Original boat ('WO' from here on) and 'my version' ('MV' from here on) passing through a wave, the fuller bow on Wolf's boat builds buoyancy very quickly. The 'MV' boat would slice into the wave and more progressively build buoyancy. Considering that these are boats of near equal weight, 'WO' boat's rapid increase means more force being imparted into the bow, a harder collision in effect, and therefore a more rapid rotation and deceleration surge. As the two boats rotate, the 'MV' stern progressively builds buoyancy at a higher rate than the 'WO' and so dampens that pitch more effectively better keeping the boat in phase and resulting in a smaller slower pitch angle.
Hartley, I would respectfully suggest that you are mistaken that the 'MV' boat has hard bilges. If you visualize the 'MV' boat rolling, a number of things happen, first of all, the cross sectional shape is intended to progressively shift buoyancy as the boat rolls, and thereby progressively dampen the roll, and avoid a lurch at the edge of the roll. In reality, this cross section is actually more rounded and shifts buoyancy more progressively than the deep deadrise on 'WO' boat.
But the 'MV' boat with its deeper keel and with its bulb centered 7 feet below the roll axis, and its much taller (albiet lighter) mast, also has a much larger roll moment of inertia which should result in a slower roll rate. A higher roll moment of inertia is usually associated with larger roll angles but in this case, the purposeful progressive dampening of the hull shape, and the tremendously greater dampening from the deep keel and rig should mean a smaller roll angle as well.
In reality, that result will vary with wave height, steepness, and period, but as a broad generality the MV boat should roll and pitch a slower rate and through a smaller angle.
The rest of this was exurpted from something that I wrote for another purposes but I added some comments in italics to try to explain how the generalities apply:
Much of the 'common knowledge' concepts about motion and light boats came out Marchaj's book, "Seaworthiness, the Forgotten factor" but that book was written at a time when our understanding of motion and weight was at a very primitive state of study and does not reflect the 30 years of research that has occurred since. Marchaj's book clearly explained most of the dynamics of motion but many if not most of his conclusions were based on studies of light boats of the IOR type form, which tended to have beamy hull forms with pinched ends. small ballast to displacement ratios and high vertical centers of gravity. His conclusions about the causes of problems with this type form were right on target, but the IOR type form was not a very basis for designing light weight boats. Since then a much better understanding of how to design a light weight boat has emerged and so have light weight boats that offer exceptional seakeeping and seakindliness while advancing the speed of these boats as well.
To explain further, in the late 1980's and into the early 1990's, designers of IMS and Volvo type performance boats came to understand that motion was a major un-rated factor in the performance equation because large roll angles and sharp accellerations disrupt the flow over the sails, keel and rudder, creating drag and limiting the production of lift. There was a huge amount of study that went into developing an understanding of motion control and being able to computer model motion. Full sized boats were instrumented and that data was used to calibrate, validate or discredit the various theories floating around.
In the end, the predominant factors that control the faster motions of a boat are:1. minimizing the forces which might generate motion, (roll, surge, and pitch and to a lesser extent surge), 2; to be weight and bouyancy distribution (roll, surge, and pitch), 3. dampening (both static and dynamic) (roll, surge, and pitch), 4. remaining in phase (roll, heave and pitch), The overall weight of the vessel has minimal impact on the roll or pitch speed or angles.
In response, the better designed lighter weight boats (and the 'My version' boat) are purposely designed have dramatically lower and more concentrated vertical centers of gravity, progressive dampening, and hulls modeled to minimize sudden increases in buoyancy. As a result these designs tend to have gentler rotational motions (roll, sway, and pitch) through smaller angles than more traditional heavier weight boats.
Heave, which generally tends to be one slower of the six degrees of motion, is the only direction that modern light boats generally do poorer in but even there it is only in some conditions. And even in this case the greater rate of motion is partially the result of modern bouyancy distribution rather than being simply weight driven.
The current theories on heave is that the relationship between the heave acceleration to the wave configuration is proportional to the weight of the vessel per waterplane area. In other words, the force of a rising wave acts on the waterplane of the boat. The more weight per square foot of waterplane, the slower the boat will accellerate vertically. Since modern designs tend to have a lot of waterplane for their displacement, they tend to be affected more quickly by heave; pretty much following the contour and speed with which the wave surface is rising.
Heavier boats per square foot tend to expeience a kind of delay. It takes longer for them to feel the upward force and change direction, but once they do they store more energy and so momentum takes them higher than the wave surface at the top of the wave. This delay can be very helpful in a short seaway but can be a real liability in steep seas where being out of phase can mean a pretty harder landing. (been there, done that, have the broken toes.) This is a good example of how getting out of phase can be detrimental to motion comfort.
I also want to bring up this issue how light vs. heavy boats seem to be defined, and particularly as it seems to be done on this forum. This goes back to a point that I have raised here before. If we size boats by length then we seem get into an endlessly circular discussion of the merits of light vs. heavy boats.
I suggest that displacement is a much more accurate indicator of the ‘size’ of a boat. This is especially true when you talk about a boat intended for long distance voyaging. In other words, while it is tempting think of a boats size boat solely on a length basis and the need for specific accommodations, the displacement of a particular boat says a lot more about its 'real' size.
And it is here that I have the problem with the apparent implication that light boats are inferior for offshore voyaging or that they have poorer motion. If we size the boat by its displacement and compare to equal displacement boats, one being longer for its weight than the other, all other things being approximately equal, the longer boat will offer better motion comfort, be more seaworthy, be easier to handle, have an ability to carry more supplies, and be quite a bit faster. In most cases and for most costs, if the boats are of equal weight they will have a similar cost to buy, and maintain.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 05-17-2013 at 09:53 AM.