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  Topic Review (Newest First)
2 Weeks Ago 11:11 AM
GTOM
Re: Long versus short overhangs

Valid points, more AND deeper ballast together improves the situation a lot. Provided the Swan is not really a "stix king" - the Nic43 could be even lesser of that.
2 Weeks Ago 10:47 AM
Jeff_H
Re: Long versus short overhangs

Quote:
Originally Posted by GTOM View Post
Allright, I found some relevant data: the Swan 43 (SWAN 43 (S&S) sailboat specifications and details on sailboatdata.com ) is 99% identical to the Nic43, and it has a stix of 39, AVS 121 degrees according to this list: https://www.hisse-et-oh.com/system/a...xls?1378928032 . Stix could be better for a boat of this length but neither values seem to be worrisome.
I can see why you might think that the Swan 43 might be 99% relevant as a basis to evaluate the Nic 43; after all they are of a similar length and displacement. But if you look at these boats with a thoughtful understanding of the various stability and motion controlling factors, you would see that they are really pretty different boats in a lot of key ways, particularly when it comes to evaluating the likelihood of capsize or their motion comfort.

Lets start with the numbers, in terms of stability, the Swan has 5% more ballast carried significantly deeper in the water. On boats like these, that means a lot more stability near the max. stability and a larger LPS (Limit of Positive Stability, a term that I prefer to AVS which can be a little misleading since it has been used in different ways over time). The Swan has a little more beam, and carries that beam for a greater proportion of its length, and so would be expected to have a bit more initial stability. From those two factors alone, you would expect the Swan to have a larger area under its positive stability curve (i.e. more force required to capsize the Swan).

But of greater importance might be the hull forms of these boats. During this period, S&S was tending to firm up the bilges on these designs increasing form stability. The Nic, like most British boats of the late RORC/early IOR era had very slack bilges. The firmer bilges on the Swan would tend to damp roll and increase initial stability.

There is a tendency to think that any form stability is a bad thing from an ultimate stability or roll comfort standpoint. That is really not true. While it is true that too much form stability typically results in a quick motion, and excess stability in the inverted position, moderate form stability is necessary to help dampen roll rate and angle, and keep the boat relatively synchronized with the wave face. Once a boat (too little initial stability) gets too far out of sync with the wave face the ends of the roll become more violent as the large volumes of the topsides collide more abruptly with the more rapidly rising face of the wave. This means a jerkier motion as well as larger roll angles as greater force is imparted into the roll.

S&S also tended to build some tumblehome into their boats. This was a mixed bag. Since most boats with flare or straight topsides tend to jerk to stop as the topsides hit the water, having some tumblehome meant a softer roll motion. It also meant that the toerail was less likely to dip, reducing the chance of 'tripping' in a large breaking wave. But the downside could be a small reduction in the LPS. But similarly, tumblehome also reduces the energy required to right a boat from an inverted position meaning a shorter inverted period.

So all that means that the ballast placement and weight, and the transverse hull form results in the Swan having a lot more stability across the full range of heel angles, and a much more comfortable roll motion.

But pitch is also very important to motion comfort, especially considering the hull forms of the era. Here again the Nicholson suffers having proportionately shorter waterline as compared to its length on deck which would tend to promote pitching through a greater angle, and more violent stops at each end of the pitch cycle. The waterlines in the bow of the boat on the Nicholson are much fuller and would tend to start to build buoyancy later and would then more rapidly (rather than progressively) build buoyancy as the bow entered the wave. This too would make the pitch more violent. In the end the greater form stability would provide better damping and the ballast position of the Swan would slow pitch rates further, and reduce the pitch angles as well.

So while these may seem like similar boats, sailed in the same conditions, I would expect the Swan to have a much nicer motion and considerably more stability. That added stability should make the Swan easier to handle requiring fewer reefs and sail changes. I would expect the combination of factors above would make the Swan more seaworthy, and I would expect the Swan not to grind down a crew as quickly as the Nicholson 43.

Jeff
2 Weeks Ago 07:04 AM
GTOM
Re: Long versus short overhangs

Quote:
Originally Posted by GTOM View Post
Indeed a STIX value would be interesting. Unfortunately the available data on older, pre-standard boats is sketchy at best. Question for guesstimation: "Hull draft" is with or without keel? "Beam 2" is beam squared?
Allright, I found some relevant data: the Swan 43 (SWAN 43 (S&S) sailboat specifications and details on sailboatdata.com ) is 99% identical to the Nic43, and it has a stix of 39, AVS 121 degrees according to this list: https://www.hisse-et-oh.com/system/a...xls?1378928032 . Stix could be better for a boat of this length but neither values seem to be worrisome.
2 Weeks Ago 08:51 PM
hpeer
Re: Long versus short overhangs

Trying to read through this thread but realized I don't know what is "long" and what is "short" and what is "medium" with respect to overhand.

I'm presuming you all mean LWL/LOD.
SHORT is > 0.X
MEDIUM is 0.X to 0.Y
LONG is < 0.Y

just so I know what kind of tub I'm sailing?
2 Weeks Ago 03:53 PM
GTOM
Re: Long versus short overhangs

Indeed a STIX value would be interesting. Unfortunately the available data on older, pre-standard boats is sketchy at best. Question for guesstimation: "Hull draft" is with or without keel? "Beam 2" is beam squared?
2 Weeks Ago 02:22 PM
Jeff_H
Re: Long versus short overhangs

Quote:
Originally Posted by GTOM View Post
I recently looked at a Camper & Nicholson 43, which is quite an extreme with 40% overhang. Tom Dove's calculator gives quite impressive values (capsize ratio 1.7, Montion Comfort 35) better than some modern 60'(!!) boats....... Nor would I worry in the Nic43 much more in rough seas.
Personally I would be quite concerned with the motion comfort of the C&N 43 in rough going. I would like to point out that Capsize Screen Formula tell you absolute nothing about the likelihood that a particular boat will or will not capsize, nor does the Motion Comfort Index tell you anything useful about a boat's motion.

To explain, (quoting from an earlier discussion that I wrote on this topic) "It seems that as soon as someone poses a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. And no sooner thana poster questions the seaworthiness of some boat, that someone cites the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution.

But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous. Flipping a coin is way safer since you have half a chance of being right.

It is important to understand that both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. They were developed at time when a lot less was understood about motion comfort and capsize. They were developed at a time when it was difficult to perform detailed calculations of the real factors which impact motion and capsize.

These formulas therefore have very limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 200 pound weight at the top of the mast. (Yes, I know that no one would install a 200 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, the first one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design.

The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return. And this is where the Capsize Screen really fails, because under the Capsize Screen ultimate stability (AVS) is approximated from single dimensions, and ignore the impact of significantly higher stability at larger roll angles, roll moments of inertia's ability to either increase roll angle or decrease roll angle depending on the location of the mass and so on, the lessons which helped shaped boats in the wake of the Fastnet research."

Jeff
2 Weeks Ago 06:33 AM
GTOM
Re: Long versus short overhangs

Very interesting thread, after work I'll take my time to digest all 10 pages/8years of posts! I recently looked at a Camper & Nicholson 43, which is quite an extreme with 40% overhang. Tom Dove's calculator gives quite impressive values (capsize ratio 1.7, Montion Comfort 35) better than some modern 60'(!!) boats. To me, less space, worse Pounds per Inch Immersion, and high heel angles are the real negative factors. However, I doubt that the deck of a comparable size Bene 393 would collect dust on transatlantic passage nor would I worry in the Nic43 much more in rough seas.

Quote:
Originally Posted by Jeff_H View Post
Obviously you have not been reading my comments. If you go back to yacht design texts, working water craft histories and cruising books that predate the CCA era, there was a strong concensus that the CCA driven narrow beam, and long overhang designs have no place offshore. If you read CCA era design critiques, there was a real outcry against CCA type boats as being 'unwholesome' for offshore work. If you read some of the post Fastnet research and some of the pre- CE directive research on suitable offshore vessels, the short waterline lengths and long overhangs come into the crossfire for their negative impact on motion comfort, lack of stability, and poor carrying capacity.

At least amoung the current crop of offshore vessel designers there seems to be a near unanimous sense that long waterline/ short over hangs are the way to go from all perspectives; ease of handling, sea keeping, motion comfort, carrying capacity, not to mention overall performance.

Which also brings up a related issue. When you look at idealized values for such surrogate formulas as L/D, Motion Comfort Index, and Capsize Screen Formula, the numbers that we all are used to were based on CCA era short-waterline, long overhang designs.

If you look at an equal length on deck boat from the CCA era vs one from today, you'd be surprised that the overall weights of these boats are not all that different, but the waterline lengths of the newer boat is typically as much as a third longer than those of the CCA era boat. The newer boats also often have greater depth and higher ballast ratios as well, meaning lower VCG's relative to the Vert center of Buoyancy)

What this 1/3 longer waterline does is make the equal weight modern boat seem overly light (in other words, what we would consider a moderate displacement boat of today with an L/D of 170 would be the same weight and length on deck as a CCA era boat with an L/D of roughly 350 which would have been considered quite heavy)

Historically a LD of 170 would be considerd too light for offshore work, unable top carry adequate supplies, and the other formulas would suggesting less stable/ seaworthy, and prone to a less comfortable motion, when in fact the longer waterline/equal weight boat should actually be less prone to capsize, have a more comfortable motion and have greater carrying capacity.

But beyond that these CCA era almost by necessity are sailed at very high heel angles, and compared to more modern designs tend to scoop up a whole lot of water over the bow and be pooped over the stern making them miserable to sail in heavy going.

It is for that very reason that I cringe whenever I see someone suggest that boats like the Alberg's, Ariels, Bristol 32 and to a lesser extent 40, Triton, Vanguards and the like make any sense of offshore work.

Respectfully,
Jeff
03-01-2016 02:48 AM
S/VPeriwinkle
Re: Long versus short overhangs

I think you want some overhang. Look at W.B. Crealocks designs, which are famous for fabulous sailing qualities, including in terrible weather. Of course those designs don't have extreme overhang, but enough to give the bow added buoyancy when it comes down into the water, thus keeping it afloat. No overhang and you just don't have that.

Periwinkle
--
Boat for sale ad removed per forum rules- Jeff_H SailNet moderator
11-01-2015 11:21 AM
Lokiyawl
Re: Long versus short overhangs

This thread has been inactive now for 5+ years but I just found it and have a few things to add. Having owned and sailed relatively long overhang/narrow/deep boats over the past 40 years it seems in this discussion that that a few of the advantages of the design has been either forgotten or misunderstood. First, while the CR uses waterline, beam and displacement, I think that it might be more useful to compare what I call water loading, which is analogous to wing loading on aircraft. With aircraft, the comparison between a light wing loading plane and a heavy wing loading (commercial jet) flying through the same air clearly shows the advantage of higher wing loading. These older designs of course have the higher water loading for the reasons that have already been discussed and I personally find the motion to be the best of any of the designs I have sailed to date. I do think that boats with long overhangs tend to pitch more than the boats with shorter overhangs, but the pitching motion is not all that uncomfortable but definitely does reduce speed, mainly to windward in a chop which is a big negative. With regards to yaw control, I have personally found that when well designed that these types of boats can have excellent control but not for the reasons that were being discussed. Keel hung rudders are certainly less efficient in generating a yaw moment than a detached rudder of the same area, however the narrow hulls generate much less helm than the modern wider designs. In fact if the relationship between the width and the depth of the hull reaches a certain value where the depth curve is more significant than the beam curve, the hull will generate a lee helm…in other words try to yaw against the direction heeled. While this might sound alarming (and can be if taken too far) remember that the rig generates weather helm that increases when heeled and it is quite possible for these two forces to partially or fully cancel which creates a neutral helm. Regardless of the exact relationship, this type of hull design can lead to having a boat that has a very light helm in all conditions. With the helm pressures very light, a keel hung rudder with the heel raked forward such that the rudder actually gains effectiveness when the boat is heeled, plus the fact that keel hung rudders act as a cambered foil and retain effectiveness even when stalled, control is very good on these boats even at extreme angles of heel. I have a couple times been knocked down to 60 degrees or more and still felt that I had good directional control. Sailboat design is fascinating stuff and I enjoy learning all that I can about it. Just for the record, I am now looking at more modern designs mainly for the increased stability but as is always the case I can see that I will have to compromise in some other areas so not sure yet if I want to make a change. Thanks for all of the dated but interesting discussion! James
10-09-2010 10:33 PM
souljour2000
Overhangs and cliff-hangers...

D'sestini....glad you posted on that old thread...I had missed it somehow. This was one of the best threads I've seen in here...I for one would welcome anybody who has new angles or anecdotes concerning the albeit wide compass points of this thread to weigh in...
This thread has more than 10 replies. Click here to review the whole thread.

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