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  #11  
Old 10-20-2007
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sailhog has a spectacular aura about sailhog has a spectacular aura about
Sasha,
So downwind the bow needs to act like a sled to keep it from pig-rooting... I can get my mind around that. My C30 is mighty noisy below when sailing through chop, and I've always attributed it to the wide beam and the shape of the hull below the waterline toward the bow. Noticed your avatar has a plumb bow.
Thanks, Captain...
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  #12  
Old 10-21-2007
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Not buying it. It all sounds good til you consider looking at it from another angle. The boat with overhangs has a progressive, building dampening effect from reserve buoyancy. So, as it hits a wave, the force isn't all at once, but as the bow digs deeper into the wave, more of the reserve is tapped, thus dampening the impact gradually. The plumb bow has less impact with a wave, but also lacks buoyancy, so it will tend to plow. The broader overhanging bow looks like the clear loser in the drag comparison, but plowing drains speed too.
As the wave moves back along the hull, the bow will pitch up as the center of buoyancy is encountered, but a boat with overhangs will, while lifting the bow, also utilize the damping effects of an overhanging stern. In other words, the stern will settle or squat some as the lever arm moves back. So, as a consequence, as the hull lifts at the bow, it settles at the stern, smoothly with progressive resistance. This also limits the height the bow rises as the wave moves back. While the angle of motion is a given amount, the total deviation from center is divided between the bow and the stern. A plumb bow and stern, cannot do this nearly as well. As the lever arm moves back on a plumb bow, the wider (and typically flatter) higher buoyancy stern exerts all it's resistance at once. Ever leveraged something that was loose on one end (the bow) and rigid on the other (the plumb stern)? Since the stern is so much more buoyant from the outset, it doesn't settle into the water as the lever arm moves back. As a result, the stern acts like a hinge point on the water. As the wave moves back, the exerted leverage of the wave on the higher buoyancy stern, forces the bow to lift higher as a result, then fall further as the wave moves past the center of buoyancy. More of the angle of pitch is at the bow, with little at the stern. The sharp, low resistance plumb bow then slices deeper into the next wave perhaps submarining as the non-compliant stern rides over the wave and leverages the bow down hard. Having insufficient buoyancy to counter the stern, it plows even more. This begs the question. Since the wave moving past the hull is essentially a constant, why would you eliminate or reduce reserve buoyancy in the bow and have an excess in the stern? Great for racing. The fine, plumb bow slices cleanly through the water, and the high buoyancy stern keeps the boat up on the bow wave. I can see where Jeff's champion would be better for max speed in reasonably calm conditions, but toss and tumble? Forget it. I'll stick with some overhangs. Having been on one of these plumb bow/stern jobs recently in some gusty/slightly beyond choppy conditions, and feeling like I was going to be slingshot from the cockpit due to the pitching motion, I can say she was fast (all sorts of zippy in the initial calm, but not fast enough (to outrun the weather when things got more interesting....and it really wasn't that bad weather-wise), and not comfortable at all, because at the trough of each wave it was BANG, BANG, BANG, and it got a little damp too.
Thoroughbreds are beautiful and fast, but I'll stick with my Quarter Horse any day.
souljour2000, jameso and oldfurr like this.

Last edited by seabreeze_97; 10-21-2007 at 05:57 AM. Reason: typo
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  #13  
Old 10-21-2007
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I am not sure that the idea of a fixed hinge at the stern is accurate. I would think that the fulcrum would be further forward, at the centre of the horizontal plane.
As the bow comes up this would be opposed by the greater stern plane giving a smaller downward movement. While this depends on moments and the distance from the centre of buoyancy effects the angular movement, it seems quite possible that the rise of the bow is limited by the plumb stern.
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Old 11-15-2007
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Jeff,
I just read your very detailed post on the impact of the morc rules on performance cruising boat design.
I'm in the market for a performance/cruiser and I've been looking at 3 of the boats that you referred to:
Santana 30/30
S2 9.1
Kirby 30
Prior to reading your post I was unaware of the morc influence on these boat designs but I had by coincidence identified these 3 as candidates.
My mission is to sail the carib for several months or longer. I will make 3-5 day crossings and then layup until i want to change scenery and the weather cooperates. I expect to single hand much of the time.
In your post you referred to the Santana and the S2 as examples of late generation designs and the Kirby 30 as a middle generation boat. I would appreciate it if you would elaborate on that and describe the differences between the middle and late generations. Also, for my mission would you think any one of these boats would have any particular advantage? Or would you recommend something different altogether?
Thanks in advance,
Tom
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Old 12-26-2011
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I am inclined to agree with alot of this article by Jeff from a technical side as his explanations are detailed and demonstrate a firm knowledge of physics and boat design history but IMHO I am unsure of his conclusions in support of blunt bows, massive sail plans and aft buoyancy etc.. ... whether or not the new Open-class or other fat and plumb stemmed type boats are more comfortable for their crews still is the question only individual sailing conditions can answer...as it seems that the new boat are obviously faster and so their crews may experience more short term discomfort but may be sipping gin in the port tavern having arrived quickly while the guys in the old IOR boat are still "comfortably numb in a different fashion 50 miles offshore. Seems it will remain a "different horses for different courses" argument....but it is also lamentable that so few sailnetters have ever chimed in more on this older but quite relevant thread and it's subject matter... ...Morgan in Sw Florida

Last edited by souljour2000; 12-26-2011 at 10:51 AM.
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  #16  
Old 12-26-2011
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Step back a bit and read all the references to racing criteria and is should be clear that boats designed to win races are not optomised or even conceived with cruising in mind.

Second hand race boats have been used for cruising for a long time but the article makes it clear the compromises trying to win within design formulas seriously compromises the boat as a cruiser.

While I appreciate the value of technology evolved by yacht racing, the mistakes made in the name of a better rating formula are an unfortunate legacy for owners wishing to cruise.

Yachts like Freedoms were designed for practical use and racing aspects were largely ignored. I have trouble believing any yacht made to a racing formula is the best boat possible.
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  #17  
Old 12-26-2011
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"To some extent, the source of the inertia is important as well. For example, we can compare two otherwise exactly identical boats, each with equal roll moments of inertia, but one has a heavy mast but less ballast by the same amount added to the rig while the other has a lighter mast and heavier ballast. The high vertical center of gravity of the mast would tend to cause the first boat to roll further than the second boat for an equal roll loading, because the weight in the mast would be on the motion side of the roll axis while the weight in the keel would be on the opposing side of the roll axis."

Well, yeah, but why would anyone take a heavier rig and subtract the additional weight of said rig from their ballast? They wouldn't. You said it yourself, all else being equal. Removing ballast makes what you say true, but, again, why do that when it flies in the face of good sense. We aren't talking about dinghys on the pond here. So, as determined by the Fastnet committee, a heavier rig allows for greater resistance to capsize thanks to the very inertia you mention. A heavier hull also contributes to this, as does the lowest possible center of gravity, which comes back to the question of why would anyone remove valuable ballast because of a heavier rig? The Fastnet crews didn't abandon ship for motion comfort issues (fore and aft), they were rolling severely, some being rolled 360 degrees, with many having difficulty righting after capsize. While stability offers comfort, it wasn't a study on motion comfort, it was about stability and capsize.

As usual, Jeff, you just have to throw a jab at the CCA. It has been noted that most of the boats that got into serious trouble in the Fastnet race would not have met the minimum size limit of the CCA Bermuda race.... because size matters. It should also be noted that among the smaller boats, those built before 1975 survived with few problems, while many of the (then) new boats suffered badly. Something had changed and later statistical data confirmed that. "Boats everywhere were becoming lighter, beamier, and less stable."

Now, no doubt all this led to designers re-thinking things, but a plumb bow and fat behind cannot act in place of weight and the inertia that comes with it.

Last edited by seabreeze_97; 12-26-2011 at 02:50 PM.
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  #18  
Old 12-26-2011
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Nice job Seabreeze...your articulate gust of air you just gave us (which I mostly agree with) has offically rekindled this thread ....at least for the moment...
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  #19  
Old 12-26-2011
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I just discovered this old but very interesting post by Jeff. I have some personal experiences that differ from what would be predicted by some of Jeff's comments though, so I'll add my comments to the selected passages below.

Quote:
Originally Posted by Jeff_H View Post
Unlike earlier rules where a slower boat with a big rating advantage could win, in the early days of the IMS, there was no advantage to designing a slow boat because you could not cheat the rule.
To my mind this was the core failing of the IOR. I loved the look and windward ability of IOR boats but could never understand why, when it was revised so often, they never really moderated the most extreme effects of it. Why, for example, were keels mandated to be no wider at the bottom than the top? Bulb keels existed before in many shapes - my own Columbia 43 has one. They are obviously more stable than an IOR delta shape so why outlaw them? I can fully understand penalizing speed producing factors when handicapping - adding weight to a race horse is a well known example - but when the penalty actively encourages slow design aspects in new boats it is overdone.

Quote:
Originally Posted by Jeff_H View Post
Which brings us to fine bows, plumb stems and the afterward re-location of the center of buoyancy popularized by the IMS typeform. If you have ever beat or motored to windward in a short chop, try to remember the motion that you felt as the boat encountered each wave. Initially, there was a moment as the boat collided with the wave that to one degree or another you could feel your body thrown forward as the boat de-accelerated. And while you were feeling that force, you could feel the bow being be jerked upward, the whole boat rotating about the pitch axis of rotation. Then you could feel the boat heave (moving vertically) as the wave passed under the center of buoyancy. There is a brief moment during which the boat seems to hang in the air, before the bow begins to rotate downward. And lastly, you felt the bow jerk to a halt or even upward as it hits the back of the wave below.

What can be designed around is the amount of impact force imparted into the hull by the collision with each wave, and the amount of change in speed with which the boat pitches. To begin with, visualize two boats with equal displacement, equal longitudinal centers of gravity, the same deck shape when viewed from above and in profile, same depth of canoe body, same mid-ship cross section, and the same reserve buoyancy (meaning volume of the hull above the waterline forward of the center of buoyancy), but one has a plumb stem and the other has four feet of overhang at the bow. In this example, by its very nature, the boat with the overhang will be more blunt, less knife like, than its plumb stemmed sister. Instead of cleanly slicing through the wave, the boat with the overhang, actually collides with the wave with a greater impact force, and that impact force, both slows the boat down, and also is wearing on the crew.

Because both boats have equal reserve buoyancy, the deck will stay equally dry, but because the waves act more suddenly upward on the blunter ends of the boat with the longer overhangs, there is more concentrated rotational force imparted and applied more rapidly (i.e. more of a collision than a gradual application of force) into the forward end of the boat with the longer overhangs. All other factors being equal, greater concentrated force applied further outboard forward means a greater rotation angle; more rapid application of force means a more rapid change in direction. Greater rotation angle and greater speed of motion means a larger flow interruption over the sails and foils and less comfort for the crew.

On the boat with the modern hull form, as the bow moves upward, the fuller stern sections build reserve quicker than the overhanging sterns of more older hull forms and that earlier progressive building of buoyancy serves to further dampen the speed and amount of rotation.

At this point the boat with the longer ends is moving upward at a greater speed and will have greater kinetic energy causing it to rise higher than the shorter overhang boat. Because it has farther to fall, and the acceleration of gravity is a constant, it will hit the water later on the wave, and with a greater impact force.

Another factor that further improves the motion of the IMS type form is that the center of buoyancy is located further aft. If you visualize two boats having their bows lifted an equal distance by a wave, but one boat rotates about a point that is further forward than the other boat’s axis of rotation, the angle of rotation on the boat with the longer distance to the axis of rotation will rotate through a smaller angle than the boat with the axis of rotation located further forward. And since the boat with the center of buoyancy located further aft ends up with not only small rotation angle, but with a shorter distance from the axis of rotation to the cockpit, there is less vertical distance experienced by the crew in the cockpit.
This is the commentary my experience differs with. I owned a fairly extreme, mid 70's, custom IOR quarter tonner for many years. This was a 26' boat with a 20.6' WL, 9.5' beam and 4300 Lbs Disp. It had the typical IOR diamond plan shape with an extreme "stinger" stern. Internally that boat had nothing but a head in the bow back to the mast and nothing in the stern aft of the cabin top - ALL the weight was located over the keel.

That boat had the least tendency to pitch of ANY boat I have ever sailed. As the fine bow (with typical overhang for the type) met a wave, it would slice into the wave without rising. As the crest of the wave approached the beamy midship section the boat would start to rise, levelly since there was very little polar moment due to the centralized weight. As the crest rolled aft the radically tapered stern wouldn't rise, or rather, wouldn't depress the bow, for the same reason the bow hadn't risen originally - the relatively low percentage of total buoyancy in the ends. The boat never slammed when descending either although waves sometimes "slapped" under the highly flared topsides

The net effect was the boat basically only heaved over waves, it didn't pitch much at all - it would rise up, over and down, staying very level the whole time.

Now, on the other hand, I have a fair bit of time on a modern cruising boat with a near plumb stem and a very wide stern with a bit of a sugar scoop - not a race boat though.

That boat pitches relatively more although not badly but it slams mercilessly. Generally, when motoring into head seas, the slamming is so bad you have to tack - under power - to make the ride bearable. It has been so bad I worried about the rig, it was banging and shaking so badly - to be fair, that was motoring into 40 knots and fairly short seas.

The boat has a shallow wing keel so generally its motions are quite soft and comfortable but the contrast in pitch & slam between the two boats would seem to conflict greatly with Jeff's commentary.

Any comments?
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Last edited by SloopJonB; 12-27-2011 at 01:48 AM.
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  #20  
Old 12-28-2011
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I'm no expert either in terms of technical or actual water time in a wide spectrum of conditions but it seems more than reasonable that there is no perfect hull shape when it comes to sailboats...only a multitude of various kinds of sailors and types of sailing requirements tasks, etc. and expected performance parameters...the new boats we are seeing for offshore/nearshore racing and cruising seem to spring from the drawing boards of designers who are paying close attention to speed...pure and simple...along with a similar focus on spacious "Scan-design" interiors which often produce what only what might be termed 'initial comfort".
We won't see the likes of the CCA and IOR boats again in production and those boats IMHO are better all-around sea boats in all conditions...they had IMHO ... in their more traditonal designs ....more solid built-in safety factors for the average Joe or newbie sailor who is in Long island sound with his family on a given weekend in November experiencing the "learning curve"...so to speak.

On the other hand... Many of us would enjoy surfing at 8 knots in one of the newer offshore designs...in good conditions...but that doesn't ease my trepidation of what happens to "crew lucidity" in these types of boats when speed is no longer of any help in dealing with a raging sea. So I guess I'm skeptical whether the speed of these boats offsets their shortcomings...but I will gladly concede that speed is a huge factor...and it's bonuses vs.older, slower and more sea-kindly boats cannot be quickly dismissed....and I tend to think in the big picture they are probably at least as safe as the older seakindlier CCA IOR boats overall...

Last edited by souljour2000; 12-28-2011 at 02:15 PM.
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