Confused about overhangs...
Why is it that a traditional Alberg hull (like the Pearson/Alberg 35) shape is often described as having good sea going motion attributable to those long overhangs .. and other boats (like the Bristol 27) are sometimes criticized for having long overhangs?
I hope this isn''t a dumb question.
Confused about overhangs...
Long overhangs help smooth a boat''s motion through the waves by spreading out the center of gravity over a longer space. This same principle applies to a high-wire artist who uses a pole (often with weights in the ends) - or spreads out his arms - to steady himself. Spreading the center of gravity makes for a longer period of pitching in a boat, which is more comfortable and predictable for the crew. Racers nowadays avoid this approach because spreading the center of gravity tends to increase the amount of pitching (the inertia of the heavier ends makes them move more, even if the pitching is slow and more comfortable) and pitching slows the boat. Newer racing boats center as much weight as possible amidships and go for the maximum waterline length (plumb bows & transoms) for speed, not comfort.
Confused about overhangs...
(Warning! This is a long one)
I respectfully disagree with the implication of Paul''s conclusion that racers go for speed, not comfort and that the current crop of race boats offer uncomfortable motions because they have short ends. In fact, much study has gone into minimizing pitch and roll angles, AS WELL AS, harsh accelerations in the current crop of IMS type form boats. It may be that the prime interest is in improving the continuity of flow over sails, keels, rudders but the result is a very comfortable motion compared to race boats and race boat infuenced cruisers of the past. In the case of the better modern designs, great effort has been extended to decrease the amounts of inertia within the boat (your tight rope example is a high inertia model) and then use smaller dampening forces that progessively develop so that the accelerations can be much smaller. The fine ends on modern boats means a much smaller impact with waves and a more unformly progressive dampening of pitch than would occur with a long ended boat.
To address the questions contained in the original post, I do not know who told you that an Alberg hull (like the Pearson/Alberg 35) has good sea going motion attributable to their long overhangs but they are mistaken. I cannot emphasize too strongly that that long ends were an unhealthy, and excessive design tend that came out of efforts to beat the racing rules of the era (in the case of Alberg''s designs the CCA rule). In its day, this was a rule that was seen as producing slow, un-seakindly and un-seaworthy designs especially during the era when this rule had reached its most extreme. That said, these designs were seen as being seaworthy and offering a comfortable motion only as viewed in light of the excesses of the IOR rule that followed. On any objective these old short waterline CCA boats had a miserable motion.
Having sailed much of my life in long overhang boats in a wide range of conditions, as well as, on more traditional short ended craft and on the current crop of more modern offshore designs, both offer considerably more comfortable motions from the standpoint of rate of acceleration and from the amount of motion, present. Comparatively speaking these long ended boats have pretty poor motion comfort and their motion discomfort problems to a great extent result from their long overhangs and short waterline length.
Alberg 35''s on particular tend to hobbyhorse (pitch) miserably with a quick jerk at either end of each rotation that occurs when the fullness in their bow and the flat of their counter collides with each wave. They also tend to be real rollers, which comes from having a low center of buoyancy and comparatively high vertical center of gravity (exactly backward of what is desirable for motion comfort.) While the height of vertical center of gravity results solely from the construction of the boat, the low center of buoyancy is the result of trying to cram the necessary displacement into a very short waterline. This means a deep canoe body and that means a rolly motion.
I think it is a misnomer to call these long overhang boats ''Traditional''. The really are not when seen from a longer point of view. Long overhangs strictly result from loopholes in racing rules (and before that, shipping measurement rules) and only linger as aesthetic affectations. If you look at traditional working watercraft that worked in areas where they had to deal with rough sea conditions they almost never had long overhangs. Similarly if you look at modern offshore racing craft, where they also are no longer shaped by a racing rule, they never have long overhangs.
In a more general sense, long overhangs do nothing good for the motion comfort of a boat. To explain the physics of motion comfort of long overhangs, there are two aspects of motion that affect the perception of comfort of a boat''s motion. These basically are the change in speed of motion (acceleration and deacceleration) and the amount of motion. In US Navy studies of motion fatigue and motion sickness, there was a near equal split between those that can tolerate large amounts of movement but cannot tolerate rapid accelerations, those who cannot tolerate large amounts of movement but can tolerate rapid accelerations and those who can''t tolerate either.
When you talk about rotational motion (pitching, rolling and yawing) which are the primary causes of discomfort, short waterline boats tend to swing through wider angles than longer waterline boats of the same displacement. There are a lot of reasons for this but the basics deal with moments of inertia (momentum) and dampening (the forces quelling that momentum).
To explain moment of inertia in rotational terms, it is the combination of the each weight involved and that weight''s distance from the center of rotation. The bigger the weight and the further away from the center of rotation, the higher will be the rotational moment of inertia (momentum) of the object. So placing and anchor on the bow of a boat will generate a lot of inertia. Placing that same anchor on an extension of that bow will create even more inertia. Using a heavier anchor will also increase the inertia as well.
An object (like a boat) with a large moment of inertia will tend to accelerate more slowly but develop greater kinetic energy causing it to require a greater force to stop that rotation and therefore swing through a wider angle. A boat with a small moment of inertia will tend to accelerate more quickly but typically though a smaller angle.
In both pitch and roll directions, a centralized and low vertical center of gravity really helps in the kinetic energy department. When there is a lot of weight in the ends or high on the boat, this weight develops a lot of kinetic energy in the direction of the motion. Objects that are above the axis of rotation, the force of gravity works to dramatically increases the tendency to rotate further. Keeping weight low and out of the ends means that is a lot less energy trying to make the boat rotate in the direction it is moving, and the force of gravity works against the force of rotation on those items that are low in the boat.
The reason that the angles of rotation vary has to do with the forces that arrest the rotational motion. A boat would spin in a circle if there were no other forces opposing this rotation. Gravity, in much the same manner as a pendulum, comes into play to arrest the swing in one direction, but in doing so the kinetic energy is stored at the end of the swing, and without dampening of some kind the pitching and rolling would continue indefinitely. To one degree or another, dampening comes in the form of frictional resistance of the water against the hull, the aerodynamic dampening of the mast and rigging arcing through the air, the hydrodynamic dampening of the keel and rudder as they rotate through the water, and the shifting of the buoyancy in the hull (both athwartship, and fore and aft).
When you talk about motion comfort, the ideal damping increases progressively, but at a uniform rate of increase so as to not create a jerky motion yet arrest the amount of movement quickly. This is where long overhangs come into play. In terms of pitching, the weight of the bow and stern overhangs hang past the waterline and create a long lever arm for the weight of the hull, hardware and anything else contained in these regions. Comparatively speaking, there is a higher pitching moment of inertia for a boat with longer overhangs as compared to a boat of equal weight and waterline, but shorter ends. The primary means of dampening pitch comes in the form of a shifting of the center of buoyancy in relation to the center of gravity. (This is also the usual definition of form stability, which is more normally associated with athwartship stability.) In a long ended boat, the shift in buoyancy tends to be abrupt as overhanging portions of the hull are submerged in the water. Alberg''s designs are particularly notorious in this regard, as they tend to have a lot of fullness in the ends just above the waterline.
In terms of rolling, obviously one way to arrest a rolling motion is with form stability. An equal weight, ballasted plank laid flat would roll through a narrower angle than an equal weight, ballasted cylinder. But arresting motion with high form stability comes with a price; that price being a much quicker, jerkier motion. From a motion comfort viewpoint, you do not want to round a bilge because you would roll through a wide angle or too flat a bottom because of too jerky a motion. (BTW Multihulls are extreme form stability vessels which is why they hardly roll but their motion tends to snap back and forth through these small angles.) Traditional working watercraft (with some notable exceptions), CCA designs, and the current crop of IMS type form offshore racers and the designs influenced by the IMS type form tend to have a comparatively low form stability.
Fortunately there are other ways to dampen roll. One of the best is rotating a tall rig through the air and the comparatively flat sides of a deep keel through the water sideward. Here again long overhang boats do not do that well. If you take two boats of equal length and displacement but one has a shorter waterline, the boat with the shorter waterline would have to have a deeper canoe body and/or fuller submerged ends to carry the weight. That means a hull that is rounder in section (more prone to a wider roll angle) and with less dampening surface than a boat with a shallower canoe body, and equal draft.
The reality of long ends is that, while they are beautiful to look at they are not a great idea for an offshore cruiser or even a coastal cruiser for a venue where you are likely to encounter a lot of chop.
Confused about overhangs...
Thank you for the thoughtful answers. My wife and I are shopping for a larger retirement boat. Our Catalinas''motion doesn''t please her on rough days (hobbyhorses). I want comfort and performance, she wants stability and motion comfort. By the way, Jeff... I print and save them for reference. Please let us know if there is ever a book. I''ll want to buy a copy.
Confused about overhangs...
To highlight the excellent description of the forces affecting a boats motion that Jeff has taken the time to post... (Hmmm I wonder if he has these saved up and just cuts and pastes them where necessary...)
When things get bumpy on the sound, there is a boat at my marina that never seems to mind. A Nonsuch 36! While many would not find them "Pretty" in a traditional sense of the word. Traditional is just what they are! The design harkens back to the days when small craft (under 60'') were working vessels. Those days, form followed funtion, and a boats handling and ability to be worked in a wide variety of conditions were paramount. Overhangs? It has none. Simplicity of rig? Just a main, nothing else to worry about.
While other vessels are cowering at the dock, I see this boat always going out with the owners smiling. I have never sailed on one, but have heard that they are stable and swift, if not overly fast in light air. When I have chatted with the owners of these boats after a cruise, I have never found a group of more enthusiastic individuals regarding their boats.
I would be interested in Jeff''s comments on this design, as it seems to meet many of his requirements when discussing the merits of a properly designed cruiser, albeit one that could go on a diet!
Confused about overhangs...
Nonsuch boats were featured in the latest issue of "Good Old Boat".
Confused about overhangs...
Thank you for your kind words. To answer Silmaril''s questions, my post above was written specifically to answer the question being asked. I often write answers to questions that are likely to come up again using MS Word. (I am a little dyslexic and so left to my own devices my spelling ability is a bit nightmare.) I have kept these as a file and will sometimes pull out an answer that had been used before. I also wrote articles for a local sailing club newsletter and occasionally will grab one of these articles or a part of one of these articles and will use them to answer a relevant question.
As to the Nonsuch, I basically like these boats for coastal cruising. They are quite a clever design in a lot of ways and I am always pleasantly surprised at how well they sail. There is a lot to like about these boats.
On the other hand while loosely based on the Cape Cod Catboats, there is very little that is really ''traditional'' about the design of these boats. Cape Cod Catboats were really not intended to be all weather offshore boats. They were the sailing equivelient of a center console fishing boat. Literally dozens of these hearty little boats would be lost when a squall line would go through the fleet.
Cape Cod Cats were an extreeme example of form stability. Form stability is a bad idea when it comes to motion comfort. Nonsuch''s are also major league form stability dependant but are a bit finer than a Cape Cod Cat. As such they really do not have a very comfortable motion. Because of the weight of the mast at the stem of the boat, they carry a lot of fulness forward, this means a jerk pitching motion that was also typical of the Cape Cod Cats for the same reason. They also have a prett quick side to side motion which results from their extreme beam.
The best motion comfort comes from boats with comparatively small inertias and well balanced dampening.
Confused about overhangs...
I think Jeff H is in serious contention to capture the "resident amateur physicist" title here on the board (which I had aspired to)! ; > }
If you think about automobile ride comfort, inertia is a big component in perceived comfort, hence Jeff''s discussion of inertia is quite appropriate. In layman''s terms, a big boulevard cruiser weighing a couple of tons (think Cadillac) will ride over washboard and small potholes more comfortably than a little one ton econocar, because it''s inertia will resist the deflecting input fed to the car''s suspension (springs) by a bump. The other important component for that cushy "boulevard ride" that car enthusiast magazines sneered at back when I had time to read them is damping, as Jeff points out: Sooner or later, the Caddy will hit a bump or series of bumps that get its stiff springs bouncing; if not damped, you''ll be "hobby horsing". Hence shock absorbers connected between the wheels and the car frame.
So how is a monohull''s motion damped? Aside from the damping action of the sails motion during rolling or pitching, and the keel resistance to rolling that Jeff mentioned, it seems the primary job of damping pitch and roll falls to the hulls, a task for which they are not particularly well suited. There are probably two effective mechanisms that could potentially damp the energy of a pitching hull: vortex generation (turbulence generatation) and wave generation (pushing a hull section up and down in the water will generate waves, which requires energy). Sharp edges can generate turbulence, but tend to do it also when you don''t want it (moving through the water under sail). Tapered hull shapes probably generate waves better than flat sided hulls. In terms of damping, Jeff''s experience with "overhang" boats is reminiscent of attempts to damp pitching motions using bulbous formations on the bow. Overall, though, there''s probably not a lot of potential for motion damping in a monohull form, unless you''re designing a houseboat that''s not meant to go anywhere.
Another aspect of seakeeping/seakindliness which needs to be mentioned is resonance. Any harmonic system (like Jeff''s pendulum) can be driven to resonance with input at the right frequency. Like a parent pushing their child on a swing, if you have a boat whose natural pitch frequency is close to the typical frequency of waves you encounter in the waters you frequent, you are going to have a hard time of it. Various add-on systems (anti-roll tanks, bilge keels) have been designed to try and supplement the minimal resistance to rolling that a hydrodynamically efficient hull shape offers.
There''s a good basic discussion of seakeeping and various attempts to improve seakindliness by John Waterhouse at:
See also the preceding chapter on "stability" which contains the Figures Waterhouse refers to in his article:
So what sort of design would be most comfortable for you? Ultimately, you are wading into a compromise situation when considering sailboat hull design no matter what direction you approach it from (sailing speed and efficiency, motion comfort, economy, cargo capacity, ergonomics, etc.) Long, slender hulls may be resistant to pitching, for example, but shorter, wider planform hulls will resist rolling better.
Regarding the Nonesuch, I haven''t sailed on one, but as Waterhouse says, "vessel mass is a key factor in the equation. For a given wave height a heavy vessel will have
lower accelerations, or move less, than a light weight vessel." Comparing the Nonesuch 26 to my Helms 25, for example, the waterline length is comparable, but the Nonesuch has more than twice the displacement, and a very substantial ballast keel weight of almost 3000 lbs., which will keep the vertical center of gravity low. I have no doubt that this design would be more seakindly than my lightweight, centerboard coastal cruiser in many conditions.
As Jeff notes, some people have more intolerance of acceleration (quick motion), others of amplitude (big, slow rolling or pitching). Perhaps the best thing to do would be to crew on some deliveries to get real world experience of how different hull shapes make you feel out on the open water.
Confused about overhangs...
One additional consideration regarding boat motion and overhangs is how they effect the boat''s motion at anchor. A cruising boat spends a lot of time anchored. Even a cruiser/racer is likely to spend a lot of time anchored. In any kind of chop, the flat sterned "sugar scoop transom" is an abomination to those who intend to sleep onboard. I''ve had this experience with friends'' Catalina 320 and Beneteau 36. It''s like being in Shamu''s belly during an all night belly-flop contest. Sometimes, you can here the stern slap from another of these boats from a couple hundred yards away. An example of a boat that behaves nicely at anchor as another friend''s 80''s Ericson 32. The boat has moderate overhangs, is usually considered to be initially tender, but when anchored with the nose into the chop, it acts like an elevator car. There was minimal, soft rolling, very litle pithching, and no bone-jarring slaps.
Another observation not directly related to boat motion is reserve buoyancy, especially at the bow. Many of the go fast boats today have fine entries and plumb bows which are fast and good for the boat''s motion, as has been mentioned. But they are also largely wave piercing. It''s a staple of sailing magazine covers to show a fast boat head-on, the bowman getting ready for the windward mark, just as the boat punches through a wave, which is now blasted up and over the foredeck crew. Not just spray, but practically the entire wave!
There are trade-offs, and deciding which is appropriate for your needs is why you should, as has already been mentioned, sail lots of boats. And anchor them, too.
Confused about overhangs...
I think this is an excellent discussion. There are a lot of good ideas being floated that I would like to add to, starting with the other "resident amateur physicist", Allen''s comments. I somewhat disagree with the idea that weight in and of itself is good for motion comfort. That is a bit of an outdated idea. While overall weight does come into play when considering motion comfort due to linear accellerations (heave, surge and sideward accelerations) linear acceleration tend to be of lower magnitude on sailboat that rotary accelerations. Here the sheer amount of weight has far less of an important role than its distribution. (The Cadillac''s boulevard ride came not so much from the weight of these cars but from the design of their dampening systems and the weight distribution that came from placing a cast iron straight or V- 8 that far forward in the chaisis. SUV''s generally weigh far more than the boulevard cruisers of yore, yet have considerably harsher rides than a modern lighter weight sedan.)
When dealing with rotational motion, weight that occurs high in the boat can actually result in a less comfortable motion and contribute to excitation rolling (and pitching) which is the harmonic rolling anbd pitching that Allen mentioned above.
I somewhat disagree with Allen''s statement, "it seems the primary job of damping pitch and roll falls to the hulls, a task for which they are not particularly well suited." This is one of those areas where magnatude of the dependence on the hull for dampening very much depends on the design of the boat. In most traditional watercraft and also modern IMS typeform offshore designs, the hull plays a very small role in dampening rotational motion. While both types would have small amounts of form stability, roll dampening generally came from rotating the large area of a full keel sideward through the water on a traditional design, or rotating the deep fin of a modern design sidewards through the water. In the case of pitch, it is the bouyancy distribution on both modern and traditional designs that really dampens pitch. Turbulence and wave making really do not come into play here as this is pure Aristotle (buoyancy) and Newton (inertia and impact) physics.
"Long, slender hulls may be resistant to pitching, for example, but shorter, wider planform hulls will resist rolling better." Again this deals with form stability issues and as such with buoyancy distribution. In a strict sense, a long slender hull may not be expecially resistant if the ends are very fine and the bouyancy is clustered amidships where it provides little help with pitching (late IOR type form).
In evaluating motion comfort there is always a need to balance quick accelerations with large rotational angles. If we look at the long narrow model, too much buoyancy in the ends will result in too quick a motion, and too little buoyancy in the ends will result in too much pitching. This is also the problem with using beam as a way of dampening roll. "shorter, wider planform hulls will resist rolling better" and so will roll through narrower angles, but will have much faster accelerations. If you look at traditional watercraft that are intended to work offshore or at modern IMS typeform designs, you will note that neither are especially beamy.
I also want to touch on the points raised by Chad. I thought that Chad raised a very good point about how boats sit at anchor. In a bouncy anchorage a boat that tends to pitch will be harder on its anchor rode when it comes up short as it rears upward against its rode. Again this makes sleeping aboard a longer overhang boat somewhat less comfortable at anchor. The other point that Chad raises about slapping and slamming is an important one. Flatter cross sectional shapes tend to slap or pound more than deeper Vee''d shaped sections. When you look at the counters on many long ended boats these will often pound in much the same way the flat stern sections of many production cruisers. Unfortunately fairly flat aft sections are important for reaching performance so that there is a trade off between a quiet stern and performance. It does help to have shorter ends so that there is less area above the waterline for waves to hit.
The other issue that Chad raised has to do with reserve buoyancy in the bow. As mentioned above, the amount of buoyancy in the bow needs to be balanced against the amount of momentum it needs to resist. Too much resistance and the boat has a corky ride and collides violently with each wave, and too little buoyancy and you are sailing a submarine with a mast. Some flair to the topsides forward is a helpful thing. Of course the wettest boats in terms of blue water over the decks that I have ever sailed were old short waterline boats whose bows extended far forward of their waterline plane.
That said, I have noticed that on more modern designs you sometimes really need to slow down a little or bear off in a steep chop. I think that I told this story here before, but last fall my wife and I were beating out of the Chester River in Maryland on a day where there were gusts reported into the mid to high 30 knot range. I was under a reefed mainsail and kevlar blade. We were going upwind at somewhere just below 9 knots. At first I was pointing quite high. This was right after I had gotten my boat and so I was really trying to sort out what she was about. I was really surprised at her motion. Instead of coliding with waves she tended to knife through in much the same manner as cahd mentioned. There was little slowing and none of the harsh collisions with each wave that I would have expected out of a 10,500 lb. 38 footer. I was having a ball and the boat was amazingly dry until we punched into a wave that was a foot or so above the stem. I was stunned to watch a foot or so of water roll aft across the deck, and still there was no feeling of a collision with the wave and minimal slowing in speed. The water rolled aft but went off the deck to leeward before it reached the cabin. After several more of those, I dropped the traveler a little and eased jib sheet and moved the lead forward just a tick and bore off a bit. The speed went up nearly half a knot but we were no longer knifing the tops off of waves. I think that a more traditional design would not have been moving at that speed or that close winded but I also think that a more traditional design would have been more tolerant of the angle to the waves.
|All times are GMT -4. The time now is 03:19 PM.|
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
(c) Marine.com LLC 2000-2012