Join Date: Feb 2000
Location: Annapolis, Md
Thanked 234 Times in 185 Posts
Rep Power: 10
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 That is a bit of an outdated idea.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.