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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
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  #11  
Old 03-13-2008
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I've got my popcorn...
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Old 03-13-2008
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Old 03-13-2008
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Old 03-13-2008
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Quote:
Originally Posted by chucklesR View Post
Jeff is correct of course, but they sure do look pretty when they splash around before they sink.

Just enough overhang so the anchor doesn't hit the bow going up or coming down. More than that and it's like a pot belly on a 50 year old. Saddly overdone and past it's prime.
However, one thing not really capitalized on about the overhangs... the anchor is one issue it solves - but it also extends out the Jib or Genoa and allowing for greater sail area... depending on course that it is rigged as such to utilize (most are)... in which case - beyond the drawback that overhang for marina billing uses is used against... can be an asset..................
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  #15  
Old 03-13-2008
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Plumper,

Its all a matter of degree. Typical CCA era boats have overhangs totaling 30% to 40% of their waterline lengths. Few modern boats have overhangs that total more than 20% of their waterline length. For what its worth, I am not advocating plumb bows, just short overhangs, but more later.

When you talk about ships, their overhangs rarely exceed 10% of their length but most have overhangs closer to 4% or 5% of their length. But ship's hull forms are shaped by all kinds of factors that are very different than those that shape sail boats, but comparatively speaking, ships have very short overhangs.

Submarines are shaped to to operate efficiently underwater and to leave a minimal turbulence track. On the surface they behave pretty much like preverbial 'submarines' with water flowing very far aft in rough seas, but in any event, again have little to do with sailboat behavior in rough seas.

I did want to comment on your comments: "The overhang provides a drier ride not a wetter one. It is plumb bowed boats that have a tendency to drive through the waves rather than over them." and "The point is that as the boat with the shorter waterline length (and consequently higher L/D) gets over run by a wave, the extra boat above the waterline comes into play and lifts the bow up."

People with boats that have long overhangs often say things like that, but its not borne out in tank testing or in observation of actual boats in a seaway. Let me try to use an example to explain why your statement is not particularly accurate.

Visualize two boats with equal deck plans, with equal longitudinal centers of buoyancy and equal displacement forward of max beam, only one has a nearly plumb bow and the other has a long overhang. You can visualize that the near plumb bow would have a straighter finer entry so that it has a gentler collision with the wave.

You can also visualize that as the boats encounter a wave, the entry on the plumb bow would enter the wave and start to increase buoyancy several feet ahead of the long overhang boat, and with the finer bow the buoyancy at the bow would increase in more progressive manner, so that rather than being jerked upward, the plumb bow would have a gentler rise and fall.

And when the bow of the long overhang comes down on the backside of the wave, its blunter shape does so with a greater impact throwing more spray.

I agree that you are right that there are some ill conceived plumb and short bowed modern boats out there where the hull was shaped to create interior volume rather than an efficient sailing shape (Island Packets and Catalina 380's come to mind). By the same token, there are longer overhanged boats that are carefully modeled to be drier and more comfortable underway (Herreshoff's hollow entry boats come to mind).

I also want to comment on "Proper modern bluewater cruisers still have overhanging bows if for no other reason than to prevent the anchor from flailing the hull and to sweep aside that log that you don't see". If you look at the current clutch of modern offshore cruisers such as the latest Hylas's, Moody's, Hallberg Rasseys or Tayana Annapolis 54, they have have fairly plumb bows for seaworthiness reasons, anchors are handled otherwise with shortsprits and the like. The extent of overhand found on these boats are not all that disimilar than those employed on such notable offshore working watercraft as the Bristol Channel Pilot Cutters (the originals and not the Hess minature) or the Colin Archer rescue boats.

Respectfully,
Jeff
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Old 03-13-2008
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And then there's the direct approach. I don't wanna go off into a debate over what he meant. I related my impression, but that could be wrong, so why not just ask? Fired off an email to Ted Brewer asking for clarification. Will pass along what he says.
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Old 03-14-2008
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Quote:
Originally Posted by Jeff_H View Post
Plumper,

Its all a matter of degree. Typical CCA era boats have overhangs totaling 30% to 40% of their waterline lengths. Few modern boats have overhangs that total more than 20% of their waterline length. For what its worth, I am not advocating plumb bows, just short overhangs, but more later.
Jeff,
I agree with everything you said. It is all a matter of degree. I guess I misunderstood you earlier and thought you were advocating plumb bows on cruising boats like some of the latest racing machines have. Regardless of the advantages, I think that a plumb bow on a cruising boat is more of a liability. on the other hand, I skippered a 102' boat with a 63' waterline for a long time and became fully aware of the limitations and squirreliness of those long overhangs. Finally, I own a Garden design with a short overhanging (and hollow) bow and love the nice dry ride.
Your response was clear and accurate.
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Old 03-14-2008
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Regarding ships, which do not ride the same as sailboats, Jeff's logic would be completely backwards. The plumb bow is not seen on ocean going vessels for just the original reason Plumper pointed out, the tendency to bury the bow, loading tons of water on the foredeck. Rather than pounding, the flare of the bow, as the bow descends, provides increased buoyancy resulting in not only the bow rising but a tendency to part a sea versus knife through. The deeper the bow plunges, the more she'll tend to rise and as she does slamming becomes a factor. No vessel can take prolonged slamming of the bow for long. The tremendous clipper flare to a ship's bow resists this quite well. You'll notice on a destroyer, an extreme example, that the fineness of entry is carried nearly 25% of the vessel's length aft at the level of the keel. Given their extreme motions in a seaway this is essential to prevent slamming which might be considered as the slapping of bottom plating on the sea as the bow descends.

Very few sailboats have a true plumb bow as seen on ships. Most do not have the clipper type bow seen on most modern ships, either. Ships tend to have a much finer and longer entry than on sailboats. The spoon bows seen on many sailboats are effective in wavelengths of greater length than the vessel and that does not often occur on ships.

If you examine ships of the early twentieth century you'll see that their plumb bow had very little flare and this was their death knell design wise. When you start burying the bow of a ship you're going to encounter structural damage sooner than later.

I too am somewhat sceptical of the current plumb bow on sailboats if only for the wet ride. That they do not bury their bows
more than they do I'd ascribe to the fact that they do not carry their fineness of entry very far aft and that they're relatively small in proportion to wavelength. Like many, the aesthetics leave me ambivalent. (g)
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  #19  
Old 03-14-2008
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Quote:
Originally Posted by sailaway21 View Post
Regarding ships, which do not ride the same as sailboats, Jeff's logic would be completely backwards. The plumb bow is not seen on ocean going vessels for just the original reason Plumper pointed out, the tendency to bury the bow, loading tons of water on the foredeck.

If you examine ships of the early twentieth century you'll see that their plumb bow had very little flare and this was their death knell design wise. When you start burying the bow of a ship you're going to encounter structural damage sooner than later.

I too am somewhat sceptical of the current plumb bow on sailboats if only for the wet ride. That they do not bury their bows
more than they do I'd ascribe to the fact that they do not carry their fineness of entry very far aft and that they're relatively small in proportion to wavelength. Like many, the aesthetics leave me ambivalent. (g)
The reason we see plumb bows on new sailboats is the same reason that CCA boats had short LWL's. Racing design rules. Since LOA is now the main design factor it makes little sense to reduce the LWL except maybe for very light air to keep the hull's wetted surface minimized. But with the shift (for performance reasons) to light displacement and bulb keels; the longer LWL helps these boats get on a plane when sailing downwind; therefore they make it as long as possible.

In terms of sailing characteristics; the long overhangs (which usually also equates to heavier displacement) will always have better motion comfort. New boats carry so little draft that the first 5-6 feet of the hull at the bow may only be 4-6" deep. When they go over waves they tend to slap that first few feet onto the next wave rather than knife the bow through it. It might be fine for short distance racing but if you are going for an extended offshore cruise or race I think it would become very tiresome. Lighter displacement of the hull also equates to more tendency to pound (the wave will transmit more acceleration force to the hull due to the boat having less inertia).

I can't say I agree with the blanket statement that a CCA era hulls are not offshore capable. The "International Offshore Rule" hulls were designed to be "offshore" boats but we all know that there are major issues with many of those designs also (IOR is AKA the "broach coach" of hull designs). I think it depends on the particular IOR or CCA hull.

There are lots of Pearsons, Allbergs, Newports, etc. sailing on SF Bay and beyond (in addition to even older fleets that are still racing like Birds, Knarrs, Folkboats, Int. One Design). Some days the winds are 30-40kts in the slot; and these old boats are racing with the crew hiked out and the lee deck buried up to the doghouse. (Yeah; they are probably not sailing them optimally, but it sure is fun to watch a lapstrake hull Folkboat sail at 60deg heel).

Last edited by KeelHaulin; 03-14-2008 at 04:04 AM.
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  #20  
Old 03-14-2008
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I think that this has turned into a good discussion of this complex issue. I did want to touch on a couple of Keelhaulin's discussion points.

He is correct in saying that many classes such as the Volvo Series boats, and open class boats are LOA driven, boats racing under the old IMS and IRC rule, where we first saw race boats with nearly plumb bows, are measured in a way that that essentially digitizes their lines above and below the waterline and attempts to predict the performance of that hull at varying heel angles.

In the case of these IMS/IRC designs, their near plumb bows had little or nothing to do with downwind performance. At the time that the IMS became popular, there was a feeling that there was little that a designer could do to beat the rule in terms of rig or hull design. Consequently, designers began to look a motion as an under penalized area of design.

It had been known that large roll and pitch angles and sudden motion changes interrupted the flow over sails and underwater foils hurting performance upwind and close reaching. A lot of effort went into studying motion dampening and the results of those studies really changed the hull forms of race boats. The biggest impact of the research was on the cross sectional shapes employed, movement of the longitudinal center of buoyancy further aft, the use of finer entries and near plumb bows, lowering of the VCG, and careful design of foils to increase dampening roll dampening. The real gain on the race course with these fine bows occurs upwind and when power reaching rather than downwind and when sailing in a shorter length wave, or in vertical seas.

As these near plumb bows are rendered in the current generation of race boats, they have U shaped sections that are moderately flat on the bottom and so the more extreme versions of the current crop of IRC race boats will pound a bit on a hard beat in short seas.

Cruising boats eventually began to pick up on the potential for improved motion comfort that came out of these newer hull forms. But unlike the shapes employed on purpose designed race boats, as these motion controlling principles have been carried into cruising boats with some moderation, particularly at the bow, where cruising versions tend to have more flare in the forward sections, tend to have Vee'd sections below the waterline, and tend to have a deeper forefoot. All of which means cruise versions have a drier ride and a more comfortable motion than would be found in the full race versions, where wetted surface mitigates against vee shaped sections.

I do think that Keelhaulin may have missed the point that I was raising about displacement of new vs. older boats. The point that I was bringing up was that there is a perception based on L/D numbers that modern performance cruisers are substantially lighter than the boats that preceded them.

As I was trying to point out a modern 35 footer is not all that much lighter than a CCA era 35 footer (perhaps 5%-10% at the most, and often the newer design is actually heavier) but they tend to have substantially longer waterline lengths and substantially lower VCG's, both of which would tend to make the boat more seaworthy, have motions that are easier on crews and increase carrying capacity as compared to an earlier era equal displacement/shorter waterline boat.

The point that I was making is that there is a problem that arises in comparing equal weight boats modern boats to older designs using these older formulas. The problem is with the expected values which were based on older style short waterline boats. When you apply these standard range of values to modern designs with their long waterlines, modern boats appear to be much lighter than they are and so also appear to be less seaworthy.

I gave a similar example above, but just for the sake of clarity if we compare say a modern 35 footer to a CCA era 35 footer, both would weight roughly 12,000 to 13,000 lbs empty and would have similar weights fully loaded, but the modern boat could easily have 5 or more feet of waterline, both static and heeled. But since the LD, CSF, and MCI use waterline length, the equal weight modern boat would appear to be extremely lighter and less seaworthy, when in fact it is the same weight, and if properly designed, more seaworthy.

Respectfully,
Jeff

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