Long versus short overhangs

[Alden68]

Chuckles, 60% of the time that is right all of the time...

[sailaway21]

Jeff...as to the statical stability curves where KG, keel to center of gravity, is varied while displacement remains the same in common hull forms, the vertical difference between the two curves of statical stability will be more significant than their horizontal seperation, particularly at the right edge of the curve. For those thoroughly confused, the curves appear as sine waves and the difference vertically between the two is GZ, or righting arm. The horizontal distance, measured left to right, is angle of inclination with the right edge at baseline, or "0" GZ commonly being somewhere past the angle of deck edge immersion. At those large angles of inclination the lateral shift of CB is going to be of more importance than the variance of the CG. This is most dramatically experienced at deck edge immersion or, on some boats, deck house immersion. The resultant loss of buoyancy has thus a far greater effect on determining where the righting arm, GZ, will begin to decrease and eventually become zero. I do understand your point though. The shape of the curves though is nearly identical and it's really, in the end, the area under the curve we're concerned with. I guess we're around to agreeing, as I re-read your original post (!), but I did think that you were giving short shrift to the buoyancy component.

The authors of Principles of Yacht Design in the section quoted by Keel-haulin make a point that must be read closely. In the first sentence they seem to refer to bow overhang unaccompanied by stern overhang. Thus the increase in pitching. The boat constructed with both will be induced to pitch more by the bow, which effect will be immediately dampened by the stern overhang. I had to read it a couple of time myself to cotton to where they were going. (And I've noticed that these guys write very carefully as the topic is no less contentious in the book world than sailnet! Except for Marchaj; he just let's fly!)

The bow flare with high freeboard and little overhang that Keelhaulin refers to as odd looking, and I tend to agree, can be seen on Giu's boat and many others. It's like you took a wedge and sharpened one of the vertical edges of it.

The advantage, on the other hand, of the flared bow, with overhang, or maybe better described as concavity and rake is that while it is being submerged in a seaway it is developing more buoyancy and longitudinal pitching moment as it sinks. This naturally results in pitching and, were the boat light, it might cause quite a hobby horse. The flared overhang though serves yet another perhaps more important purpose. Because the boat does not act in only the longitudinal axis, as the boat rolls it exposes the increased buoyancy of the flared bow to immersion thus increasing the righting arm, GZ. Those characteristics become more important as sea state increases. overhang at the stern is relatively less important, and may be even undesirable if carried too far, as the boat is generally moving forward.

In essence, as I see it, the raked concave flared bow, or overhang, shifts buoyancy upwards and forwards; desirable traits in heavy weather. the plumb bow with it's moderate straight flare will not achieve as significant an increase in buoyancy forward as the boat pitches and will be more prone to burying the bow which signals the end of any increase in buoyancy and the onset of more G right on the foredeck.

I'll agree with Jeff to the extent that most of these "rules' are racing derived and offer little towards an understanding of the ultimate seakeeping ability of one design over another. Even the more objective STIX standards are subject to wide debate. And, lest he thought it went unnoticed, I did catch his reference to where the boat carries her beam. I look forward to the debate on fat-arsed boats, if not the Portagee's defense of same. (g)

I'd quibble with seabreeze's Brewer reference in that increased beam only increases dynamic stability to the extent that it is accompanied by increased freeboard. Increase of beam alone only increases form, or initial, stability and that goes out the window in heavy weather as an important factor in maximum stability. I'm not sure anyone wants to debate boats dependant on form stability for offshore work.

[chucklesR]

Originally Posted by Alden68

Chuckles, 60% of the time that is right all of the time...

According to the Admiral that's only true if men are involved.

[Plumper]

Originally Posted by sailaway21

Increase of beam alone only increases form, or initial, stability and that goes out the window in heavy weather as an important factor in maximum stability. I'm not sure anyone wants to debate boats dependant on form stability for offshore work.

What about multihulls? Isn't that all form stability?

Regarding flared bows with overhangs and pitching, I understand your point of view but wonder if there is a "sweet" spot, so to speak, where that style of boat will pitch excessively. Once the distance between waves is longer than the boat length then as the wave approaches wouldn't the entire boat just lift and ride up the wave face? At some point in building seas the waves must become like hills that the boat must go up, not like short wavelength bumps that cause pitching. That makes it the pitching sound like a problem on certain boats only when the wavelength and hull length work together to create the pitching moment. On my boat, their is certainly a point at which the boat pitches most but that is on relatively small waves that are very close together (powerboat wakes).

[Jeff_H]

Sailaway, I think that I pretty much agree with all that you have written in that last post and it does clarify some of the apparent differences in our earlier posts. Nice work.

Just for the record, I never have advocated dead plumb stems for offshore cruising. Instead I prefer raked or short spoon bows, a hollow or straight entry, and a reasonable amount of flare, a bow form which can be seen in both modern and historic cruising boats. In the end, much of this comes down to the specifics of the particular design.

But if we get back to the original topic, about whether CCA era boats make good offshore cruisers, I still think that they are a poor choice for offshore cruising, even considering their price. My basis is that I think that their long bow and stern overhangs are less than ideal offshore because long overhangs negatively impact carrying capacity, tracking and motion comfort. I think that CCA rigs are poorly proportioned for offshore work in that they were designed around huge 170% genoas and wide headed spinnakers, both of which are ill-suited for offshore sailing, but without them, the boats are underpowered even in moderate conditions, so you have a choice of some combination of slow passages, lots of fuel consumption, or lots of sail changes. I think that their keels with their cut away forefoot and steeply raked stern posts and with thier attached rudders (what would have been called fin keels with attached rudders in their day) result in the worst of all worlds, poor tracking, heavy helms, and placing thethe rudder in the most vulnerable of all positions, near the point of maximum draft.

But then again, that's just my opinion.
Jeff

[Jeff_H]

Plumper, that was the point that I was raising earlier that in sinoidal waves of a certain longer wave lengths, heave becomes far more critical. But in the most extreme conditions where the waves are steeper and breaking, long overhangs and a propensity to pitch once again become a liability.

Jeff

[sailaway21]

Plumper...from the point of view that I am arguing this issue, the wave length may always be considered longer than the boat. Shorter wavelengths do not present the same issues.

And multi-hulls exhibit all that is good and horrendous about form stability.

[Valiente]

Interesting stuff, gents, and more than theoretical in my case, as I am dropping my 100 gallon x 2 SS water tanks from their position hanging beneath the deck to HDPE tanks, 4 x 50 gallon, sitting in welded, boltable frames atop the stringers either side of the engine.

This decision, which is largely intuitive, comes from noticing a certain tenderness at dock and a lateral roll I attribute to the elevation of the current tankage, plus a desire to break up the tanks into more easily maintained and accessed (from inspection ports on top) units. Also, the boat sailed obviously differently with near-empty water tanks than with full, and if one was draining faster than the other, you could detect a slight list.

I will also lower the hot water tank while I'm at it.

I lose "gravity feed" to the taps, which doesn't matter as a Flojet pressurized the water anyway, and I am going to dual feeds with foot pumps for "at sea" use. I gain useful space above the tanks I can use to hang line, chain, secured anchors, and so on...but the main reason for this is that I feel lowering approximately one ton of water this close to the CE of the boat will stiffen her up a fair bit, and long, low, trapezoidal tanks (still with a couple of feet gap either side of the engine) should get the weight where I want it.


The addition of 4 8Ds just after of the mast above the middle keel fuel tank completes the picture of apportioning "ballast" in aid of "comfort motion".

So these fairly academic and theory-based discussions are of great interest because they alert me to the wider scope of keeping a boat safe and seakindly, given the absolute design elements that can't be changed.

Sailing frigates used to regularly shift cargoes, sometimes to a significant degree, in order to attain an instinctual "ideal trim", which would manifest as speed and quickness in wearing and tacking. While modern boats are largely calculated to a higher degree of awareness of such details, they still affect performance in sometimes unpredictable ways.

[sailingdog]

Valiente-

What the H*** were they thinking putting the tanks just under the deck? Yes, I'd imagine that moving the water tanks and adding the four 8D batteries is going to seriously improve the boat's stability. I hope you're setting up a way to transfer water from one tank to another, to help with trimming the boat out. Each tank will be about 500 lbs. full, and being able to transfer water between them will help you keep the boat in proper trim.

[artbyjody]

Val - do the current tanks have baffles built into them?? The baffles are designed to prevent significant sloshing... which generally will cause that tendency of tenderness in your vessel if they are not in the tanks (for instance on mine none of the tanks are baffled)...depending on H x W x L

Baffles are inserts - or if you look at it from a layman's perspective much like those cardboard carriers for wine...there are cardboard slots that criss cross the container so you can put the wine in the car without the effects of motoring wildly and having them explode on you...baffles in a tank have slots at the bottom for liquids to to take quickest route to the bottom but works the under the same principle as a wine carrier...the chambers prevent the fluid from reacting to rolls...

You should be careful in concentrating tanks as probably as part of the design of the boat the weight was calculated to be there...and therefore may effect one of those numerical factors...it may be cheaper and more efficient to install baffles into the tanks your currently have installed otherwise...if they currently do not have them - and may save your bottom line costs... maybe

AFOC comment probably but thought it may be somewhat important to your situation...