ryley
Super Anarchist
I always feel like you are saying something profound, if only anyone could fucking read it.
Hull speed - because it hasn't been debated enough
- Thread starter PurpleOnion
- Start date
DarthSailor
Super Anarchist
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fastyacht
Super Anarchist
- 12,928
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LOLz
I type pretty good on the computer. If I feel like it. Some other objet d'art are beyond my patience however.
fastyacht
Super Anarchist
- 12,928
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I should repair that haha
Not a bad article for introducing the sailor to our world.
I will note that half angle is loosely related to displacement / length^3 ratio and that is just as important to wavemaking. Regardless of half angle, the volume of the vessel over a length determines the transverse wave system. This is larger than the bow divergent waves when half angle is small and overall slenderness is more moderate.
Baldur
Super Anarchist
dogwatch
Super Anarchist
Once upon a time, your posts were interesting and readable. How long do you plan to continue this irritating affection?
floater
Super Duper Anarchist
lighten up - I'm in need of a new prescription myself. pretty clear its simply a case of small-screenitis.
I find that even after translation, I still cannot understand it. lol. but at least it sparks the imagination. After all, sailors do have a fascination for the wake left behind.
Lex Teredo
Super Anarchist
Yeah, I didn't understand that version either. LOL. But I have added a name to my list of people to buttonhole a little bit if I ever win the lottery and get to build a nice yacht to my own preferences.
fastyacht
Super Anarchist
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Small screens aka phones are new to me. Using them here to post on SA even newer. I didnt even get a touch tone bell telephone until 2002 much less a cellulaer radiophone
dogwatch
Super Anarchist
OK. It seemed to me you have been playing a game. I guess not.
Panoramix
Super Anarchist
According to this article, scows should be slow!
The Q
Super Anarchist
Not if you consider the shape at the actual waterline. It maybe rounded and broad above the water line, but that only takes effect if you stuff the nose in. The normal water line shape is a lot narrower.
JimC
Not actually an anarchist.
As with a wide stern, the angle can be in the other plane. The classic skimming dish in other words. But the challenge is then getting some displacement in.
12 metre
Super Anarchist
One of the problems with this article is the author brings up Mitchell's Equation - which IIRC was meant to apply to "thin ships" which most monohulls are not.
The author qualifies his comments by noting that the article applies to boats sailing in "displacement mode" (I am paraphrasing here).
I think the author puts the cart before the horse in discussing the entry half angle. The reality is that high L/B ratios is what results in lower wave drag (and a sailing scow typically has a high L/B ratio) and tends to result in a high Cp.
However a high L/B ratio pretty much necessitates a low entry angle (which I think is what mislead the author on the entry half angle thing). Conversely, a low L/B ratio necessitates a higher entry angle, unless you get into hollow bow shapes, which is less than ideal (which I won't get into here as it has been discussed in many other threads).
For offwind work, a bluffer bow shape tends to work better than a fine bow - in "displacement mode". Besides scows, in the early days of IOR Bob Miller's designs like Ceil II below ('74 Sydney-Hobart winner)) tended to do well in offshore racing but not so well in RTC racing and his designs had quite bluff bows
Panoramix
Super Anarchist
Yes but they should at least mention clearly that's this is the case as from simply reading the article, it is easy to come to the wrong conclusion!
12 metre
Super Anarchist
Sort of. Mitchell's integral, when combined with a skin friction model (ITTC 57 line, or similar) is surprisingly accurate for evaluating the drag of truly thin hulls: think kayaks, rowing shells, multihulls, etc. For beamier boats, the absolute numbers become less accurate, but the general trends should be reasonably accurate in most cases. Mitchell's integral is a pretty decent justification for arguing that angle of entry effects resistance, even for boats that violate the thin-ship assumption.
Generally, it's dangerous to reason too much about sailboat performance from a tool like Mitchell's integral. For sailboats, available power is related to hull shape, and this is something that is not considered by Mitchell's integral, which assumes that the power source acts through the CG of the vessel. No heeling or pitching moment from the rig are considered. In reality, you need to balance low resistance with high stability -- which is why your average monohull doesn't look like the skinny pencil you'd design if you considered resistance alone.
The scow bow comments illustrates this. A scow bow almost certainly has higher resistance than a traditional pointed bow. However, the major benefit of a scow bow is increased stability, particularly in certain downwind conditions. If it adds enough additional sail carrying power (and you can set enough sail to take advantage of that) then you can come out faster than the traditional bow.
JimC
Not actually an anarchist.
The problem, I think, is that people have the impression that hull speed is like the sound barrier - a definable velocity. Its not, its a zone in which wave drag rapidly increases to become the dominant source of drag - not a wall, but a hill, but a hill which may be too steep to climb. Waterline length defines where that hill will be, although waterline length is itself a zone, and fullness in the ends, for instance, will give a longer effective waterline length. 1.34 root L is just an approximation of hill location. Mitchell's integral, on the other hand, defines how steep that hill will be, and for a sufficiently slender hull the slope will be negligible. Then there's skin friction and other factors in the mix, which is why hull design is so interesting and so full of compromises.
