Re: Bob Perry's take on Wolfenzee's dream boat
I had hoped that this thread would trigger these kinds of “what if’s?” and “howz abouts?” that help explain how design decisions are made. In that regard I like where this thread is going but I also think that some of these discussions, if they continue into a lot more detail, may deserve their own threads. But the two topics currently running interspersed appears to be questions about 1) the choice of rig, and 2) questions about the choice of keel and rudder configuration.
Speaking for myself, since I have not discussed this with Bob and he may have his own point of view that it in a different place than my own, here is my take on these points. (I apologize in advance that I feel a very long pair of answers coming on.)
I will spit this into two separate discussions and posts. The first is on the choice of rig....
Choice of Rig:
As I have often said around here, I see well designed boats being developed as a system of decisions with each decision informing the other. If we start out looking at Atkin’s original design for Wolf’s boat, relative to what we now know about hyrdrodynamics, we see a boat with a lot of wetted surface, an inefficient hull and keel shape, and comparatively little stability for its drag. Wolf’s descriptions of how he sails his boat, basically support these descriptions, which can also be ‘read’ from the drawings if you have a trained eye.
Atkin’s rigs somewhat respond to the realities of that hull form. They are comparatively low aspect ratio, spread out horizontally, for example aft of the transom by the long boom, and so carries its sail area in a way that can generate a lot of power reaching, but which is not optimized for its about to use that sail area efficiently.
And that leads to perfect example of a decision loop. The boat cannot carry a tall rig since it lacks the stability to stand up to that rig so it gets a lower aspect rig. It would not benefit from a more efficient rig, since even with a more efficient rig, its ability to point would still be limited by the high wetted surface and inefficiency of its keel. Therefore the decisions made sense one to the other.
But as we are improving the hull, and keel, reducing drag, increasing stability, and improving the lift to drag characteristics of the keel, if we kept the original rig, it would be the rig which would be holding the boat back from being able to point, or reach efficiently. So the next step is to move to a more efficient rig.
To gain that efficiency the rig got taller, and the large overlapping sails, were designed out of the sail plan. The changes being made to the hull and keel (and which are still in progress when I get a few minutes to draft Bob’s latest comments), are greatly increasing stability, especially a small heel angles, so the boat can carry this bigger sail plan with less of a heel angle.
Considering, the suggestions to consider retaining either a Cutter or Ketch rig, The while both rigs have virtues in some applications, both would be inconsistent with the hull and keel as it is evolving. It is important to go back to the decision loop thought process. Both cutters and ketches are at their best in boats with lots of drag for their stability. These rigs allow the sail plan to be stretched horizontally, and to deal with changing conditions by reconfiguring the sail plan, (i.e. increasing wind- Ketch: going jib and jigger or Cutter going staysail and reefed mainsail, decreasing wind: both adding big genoas and in the case of the ketch, maybe adding mizzen staysails). But these rigs start out with performance limitations in terms of the relative inefficiency requiring a lot more sail area for the drive than a more efficient sail plan, and limitations on pointing ability.
So seen in a decision loop, the high hull drag means that the boat will have limitations on pointing ability, and so the inefficiency of a cutter or ketch rig does not hurt all that much, and besides, the comparatively small stability compared to drag, limits the ability of the designer to design a more efficient rig and expect the boat to stand up to it. And if the boat cannot stand up to its rig, it will be reefed more frequently, and once reefed the rig will once again be reduced to a slightly less efficient sail plan.
But as the boat design evolves so that it has less drag, more stability and more lift out of the keel, then it cries out for a more efficient rig. Because Wolf sails in both coastal and offshore conditions he needs a rig which can sail well in a board range of conditions and which can quickly respond to changing conditions. It needs to sail well in light air, and it needs to weather a storm.
Perhaps as an associated goal, the rig not only needs to be efficient, but be able to adapt to changes in wind and sea state without actually make sail changes (i.e. putting up down wind sails or big genoas at the lighter end of things, and changing to smaller jibs and dousing deep wind angle sails as the breeze builds.
And those are the factors which resulted in the design being shown with the fractional rig. (I don’t want to speak for Bob, and I have to admit that I drew that rig without consulting with Bob, but I knew that both Bob and I sail fractional rigged boats and so rightly or wrongly believed that we would both probably be on the same page regarding the virtues of a fractional rig. )
But there is the theory on this decision seen through the filter of the decision loop. To begin with, the rig became taller and with a little more sail area. As the rig gets taller, and sail are increased, it generates more drive with less aerodynamic drag. This comes at the price of greater side forces which disburse as leeway and heeling forces. The lower resistance hull can actually take advantage of that greater drive and move faster through the water, the greater stability, allows the boat to stand up to the heeling forces, and the more efficient keel, reduces leeway so more of the force can be used for forward motion.
In light air, the combination of a bigger taller sail plan with its minimally overlapping headsail, and reduced hull resistance should allow the boat to move better than the old boat even with its big genoas. There was some mention of fractional rigs needing downwind sails more frequently than masthead rigs. I am not sure that is really true. In very light air, even with a spinnaker up, it makes no sense on almost any boat with a reasonably efficient hull form to head dead down wind. The apparent wind speed when dead downwind becomes so low, that it becomes much faster to sail on broad reach, which the apparent wind speed is much greater, and on that point of sail, a fractional rig gives nothing away to a masthead rig, and frankly typically does better since most of its power is in its mainsail rather that in its jib which on deep wind angles act in the dirty air of the mainsail.
As the wind picks up, better sail shaping gear and the geometry of the fractional rig, allows the sail plan to be easily depowered. This ability to depower easily results in less heeling, a balanced helm and increases the wind range before a reef needs to occur.
But at some point there is too much wind for any rig. It is at that point that the merits of a fractional rig come into their own. Almost all boats develop increased weather helm with increased wind speed. That weather helm is generated by the lever arm between the forward force on the sail (drive) versus the aft force of the hull (drag).
In a typical cutter rig, dropping the headstay sail, moves the center of effort aft increasing weather helm. To offset that and balance the helm, the typical heavy air strategy results in mainsail needing to be reefed when ever the Headstay sail is doused. Because of that, the typical sequence of dealing with heavy air on a cutter is to reef the mainsail, then if more sail area reduction is needed douse the headstay sail. That results in a nice snug rig.
What happens on a fractional rig, is that the location and size of the typical fractional rig AP jib is such that it effectively is sized between the size of the forestaysail and a combined headstay sail and forestay sail. Because of that, the first reef on a fractional rig is typically slightly bigger than on a cutter, but the fractional rig remains more effective because the frac is still able to carry more sail area with less weather helm and heeling. (bigger vertical and horizontal change in CE in the right directions.)
The idea that the Cutter’s forestay sail is its storm jib, can work, but generally doesn’t. The Cutter’s forestay sail is usually too large and of too light a cloth to properly perform in those extreme conditions where a true storm jib is required and so for ocean crossings, a true storm jib would be required on both rigs.
The Ketch rig comes into its own on relatively high drag, low stability boats in venues which are predominantly reaching. They do not do well upwind and downwind, or in lighter air. Therefore a ketch rig would not have the broad range of virtues that would be consistent with our design brief to develop a well rounded design, one that would sail well on a broad range of conditions.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 10-29-2013 at 09:54 PM.