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Hull speed - because it hasn't been debated enough


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Not a bad article for introducing tje sailor to oir world.

I will note that half anglecis loosely related to displacement lengthcratio and that is justvss important to wavemaking. Regardlrss of half anfle tje volime of the vessel over a lwbfth determines the transverse wves system. This is larger than the bow divergent waves wjenchalf abgle is small and overall slenderness is more moderate.

 

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1 minute ago, fastyacht said:

Not a bad article for introducing tje sailor to oir world.

I will note that half anglecis loosely related to displacement lengthcratio and that is justvss important to wavemaking. Regardlrss of half anfle tje volime of the vessel over a lwbfth determines the transverse wves system. This is larger than the bow divergent waves wjenchalf abgle is small and overall slenderness is more moderate.

 

The steepness of the drag curve is related to displ length ratio. And half angle is meaningless on scow forms of light displacement. Indeed a barge scow bow even. Yoi have butto mck flow and the wavemaking iscinflurn ed by pressure differential neR tje surface

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14 minutes ago, fastyacht said:

The steepness of the drag curve is related to displ length ratio. And half angle is meaningless on scow forms of light displacement. Indeed a barge scow bow even. Yoi have butto mck flow and the wavemaking iscinflurn ed by pressure differential neR tje surface

Taking spelling lessons from Snaggs?

NTTAWTT

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47 minutes ago, fastyacht said:

the drag curve is related to displ length ratio

I thought it was a very good article but there's a limit to what a short essay can cover. They almost get to displacement when they mention the energy in a wave is proportional to H^2. Maybe if they'd gone with the energy in a wave per unit area is proportional to the height of the wave squared the link to displacement would have been more obvious? The boat pushes water up which takes an effort related to the amount of the water and the height. That's related to displacement and distribution of displacement. Then the water falls down and some of the energy may be recovered by the boat. The trivia of optimizing these relationships is left as an exercise for the student. ;)

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A very interesting Article, I wish I had read it 25 years ago, When designing and  building, the only little boat I'll ever actually make to  my plans..

She is very narrow at the water line  LWL / BWL = 5.3, but to get the crew space, the mast was placed fairly far forward, so the beam is carried further forward than this theory would like, so the half angle will be quite blunt. I'll do some measurement on that when the plans are got out again.. 

The aft end should be OK as a fairly long shallowing / narrowing shape was achieved..

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12 hours ago, weightless said:

I thought it was a very good article but there's a limit to what a short essay can cover. They almost get to displacement when they mention the energy in a wave is proportional to H^2. Maybe if they'd gone with the energy in a wave per unit area is proportional to the height of the wave squared the link to displacement would have been more obvious? The boat pushes water up which takes an effort related to the amount of the water and the height. That's related to displacement and distribution of displacement. Then the water falls down and some of the energy may be recovered by the boat. The trivia of optimizing these relationships is left as an exercise for the student. ;)

And Gutelle likes to point out Delft (IIRR) towing studies, shallower hulls showing less resistance.  Depressing the interface rather than splitting it?

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5 hours ago, The Q said:

A very interesting Article, I wish I had read it 25 years ago, When designing and  building, the only little boat I'll ever actually make to  my plans..

She is very narrow at the water line  LWL / BWL = 5.3, but to get the crew space, the mast was placed fairly far forward, so the beam is carried further forward than this theory would like, so the half angle will be quite blunt. I'll do some measurement on that when the plans are got out again.. 

The aft end should be OK as a fairly long shallowing / narrowing shape was achieved..

Would a narrow deep v in front have worked?  Or Wetted surface would s l o w  t h I n g s  d.     o       w.       n.     ?

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Perhaps the geatest problem with this article is a lack of perspective along witj oversimploficatuonscwhich carry assumptions. The thames barge being one of them

There os no proper discussion of upwind vs reaaching vs downwind speed.

The while narrow waterline half entrance is severely overplayed. Yes impertant. Yes. In zome classes it is greatlyvdesirable to minomize it. Bit not all.

He also does not discuss thecvasrlt different results of optimosayion relative to sail carrying pwer. An A 18 is very differemy from a cruising boat

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2 hours ago, Amati said:

Would a narrow deep v in front have worked?  Or Wetted surface would s l o w  t h I n g s  d.     o       w.       n.     ?

With needing the mast forward in the original design, a narrow v wouldn't have carried the weight unless I made the hull deeper.  That was not wanted, as the boat is designed for river sailing, where tacking every minute or so is usual up wind. A deeper hull is more resistant to tacking.

So the hull shape was a compromise between:

A shallow hull for easy tacking, allowing a slightly longer keel.

A narrow, very round, low skin area hull for easy driving  through the water, in light winds, common in our area.

Even true planing performance dinghies rarely do so at our club as we don't get the winds, and when we do get them they tend not to be on a beam reach but straight down the river..

 

20:20 hindsight, a change in class I'll be sailing in, plus now sailing single handed, means I could have placed the mast about 15% further aft. Reducing the need for carrying buoyancy forward, but a rebuild of the forward half of the hull is a step to far..

If I'd been designing for open waters, I'd have loved to have long elegant narrow overhangs, she would have appeared much slimmer and prettier, even though she would actually be the same max beam.

 

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18 hours ago, fastyacht said:

Not a bad article for introducing tje sailor to oir world.

I will note that half anglecis loosely related to displacement lengthcratio and that is justvss important to wavemaking. Regardlrss of half anfle tje volime of the vessel over a lwbfth determines the transverse wves system. This is larger than the bow divergent waves wjenchalf abgle is small and overall slenderness is more moderate.

 

I always feel like you are saying something profound, if only anyone could fucking read it.

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1 hour ago, ryley said:

I always feel like you are saying something profound, if only anyone could fucking read it.

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.

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22 hours ago, Danceswithoctopus said:

Taking spelling lessons from Snaggs?

NTTAWTT

Snags is way easier to read than that shit. Nice try. 

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On 12/15/2021 at 1:52 AM, fastyacht said:

Not a bad article for introducing tje sailor to oir world.

Once upon a time, your posts were interesting and readable. How long do you plan to continue this irritating affection?

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36 minutes ago, dogwatch said:

Once upon a time, your posts were interesting and readable. How long do you plan to continue this irritating affection?

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.

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16 hours ago, fastyacht said:

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.

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. 

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3 hours ago, dogwatch said:

Once upon a time, your posts were interesting and readable. How long do you plan to continue this irritating affection?

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

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43 minutes ago, Panoramix said:

According to this article, scows should be slow!

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.

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59 minutes ago, Panoramix said:

According to this article, scows should be slow!

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.

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1 hour ago, Panoramix said:

According to this article, scows should be slow!

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

 

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2 hours ago, JimC said:

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.

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!

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1 hour ago, 12 metre said:

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

 

Forgot to attach the photo of Miller's Ceil III

b16b111419f83dd379d9d23c3d3796a7.jpg

Ceill 73.jpg

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1 hour ago, 12 metre said:

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.

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.

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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.

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You can talk about fine bow angles, but in all designs, there are also shoulder angles and the transition to an exit angle and it is not relevant only in plan view (ie waterlines). A moving boat pushes some water aside, which is all drag, and some water underneath, which has dynamic lift benefit. And their relative amounts change with the heel of the yacht, so angles and transitions should be optimised in all planes.

Jim Young’s Rocket 31 is a good example where all lines, angles and transitions have been refined and smoothed out, in waterlines, buttocks, diagonal, even the curve of areas, as much as possible.

The principal thing which works in all planes, especially waterlines, is L/B ratio. The larger the L/B, the finer the possible entry and exit angles, the smoother the transition in between. Most monohulls struggle to get better than 4:1. The Rocket 31 is ~ 4.2:1. Edmund Bruce did a lot of research into hull resistance back in the seventies, and his general conclusion was that if you can get an L/B ratio above ~ 8 wave making and resistance falls off rapidly until you get up to ~ 18:1 or thereabouts, when there is negligible overall gain due to the greater increase in frictional resistance. Even narrow 100 ft maxis like WOXI don’t go much beyond ~  7:1. “Fat” Comanche is ~ 5:1. Scows and Metre boats can dramatically increase their L/ B ratios  and reduce their “angles” when heeled optimally. Thomas Tison scow example below, static L/B: ~ 4.7, Optimally heeled: L/B: ~ 12.0. 

As others have indicated upthread, Displ/L and SA/Displ are also factors. And certainly, the lower the Dipsl/L ratio, the easier it is to get good angles and transitions at least under the hull. There are fag packet formulas (OMR, MOCRA and TEXEL rating rules to start with) which predict ball park speeds solely on Displ LWL and SA, with no hull resistance factors. To use my boat (windward hull flying) as an indicative, albeit extreme example:

L/B: ~ 14. Displ/L: ~ 34, SA/Displ: ~ 40 (white sails), Hull Speed: ~2.10 upwind. Theoretically capable of > 4.21 off the wind.

To “restrict” it to a Hull Speed factor of 1.34 upwind for the same sail area, I would have to increase Displ by > 5 times, if you could, giving respective figures of: L/B: ~  12:1, Displ/L:  ~ 190, SA/Displ: ~ 13.

But you could still increase “Hull Speed”, especially off the wind, by adding a disproportionate amount of sail, if you could, and you could get back to the original Hull Speed numbers from the > 5 times heavier displacement, if you  increased SA by > 360%.

Pick any Hull Speed number you want?

FWIW, the first part of 3 articles: https://thomastison.com/scow-hulls-part-1-the-pros/

A26F3C2A-717B-4593-A72C-2008F0531063.jpeg

262F1ACE-B4FE-4DCF-A8FD-0C299AC288A5.gif

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On 12/16/2021 at 8:18 AM, floater said:

After all, sailors do have a fascination for the wake left behind.

So that’s my problem!   Always fascinated by the hole that Anna dug for herself to hide in while sailing in heavy winds. (And the pile of water building up at her stern).   

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6 hours ago, Santanasailor said:

So that’s my problem!   Always fascinated by the hole that Anna dug for herself to hide in while sailing in heavy winds. (And the pile of water building up at her stern).   

Bob Perry explained the similar problem my 1/4 tonner had - once you balance the ends on a wake where the wavelength is equal to the waterline length - the beam now lies in the trough. so, there is no lateral support there to hold the boat up. Hence, steering an IOR boat downwind is like walking a tightrope - except a tiller seems a poor substitute for a balancing pole.

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Herreshoff also knew how to dig a good hole. Go sail a bullseye and you will see.

On the other hand he also knew how to actually go fast. That catamaran. And Reliance wasn't so bad--but she too dug a tremendous hole. I suspect that had the Boston syndicate been a little more together, INDEPENDENCE would have been quite something.

In my dinghy sailing the 505 was the moment of extraordinary clarity as a kid. Same weight as a GP14 (which I sailed a TON) and yet the 505 would not dig a hole. It would gently go faster and faster and you'd watch that wake flatten out. On the other hand in the GP-14 you definitely knew when you were planing!

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On 12/14/2021 at 6:09 PM, Amati said:

3A723F17-4F8F-4E2F-8BE6-939412D55BB2.jpeg

Would be annihilated by more recent IMOCA 60as with wide bow angles.

That article is one of the worst I have ever read on design and speed factors.

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