Round Bilge vs hard chine

I had assumed this discussion was about multihulls, not monohulls

In a multihull the roll behavior is primarily determined by the hull centreline spacing (on a catamaran) and also by outrigger shape and height above WL (on a trimaran)

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Raps, here is a simple and proven way to compare a low wetted surface section with a skinny section-and you don't necessarily need a computer. And the Moth is like an 11' "model" with decades of results. Skinny does trump wetted surface!

Never having seen a skinny Moth I asked John Illett why they didn't use low wetted surface semicircular hulls like most fast cats did. He pointed out the substantial difference in L/B ratio-the skinny box being much better with a much higher L/B ratio. Since they've gotten on foils it doesn't matter as much as it used to. There are down sides that are obvious-like tacking-the box will be slower than the low wetted surface section BUT the pre-foiling results are in: the box is faster around a course time and time again.....

click on image:

 

Boat design applications like FreeShip are very useful but the drag calculations are only estimates, so comparisons between very similar hulls may not be valid. The formula may not recognize if the hull has hard or soft chines so the narrower hull was enough to give a smaller drag value.

 

Thanks Doug,
That is very interesting. All of the hulls I have proposed have been at least 10 to 1. It will certainly make building easier..

 

=======
Raps, note that the illustrations are extremes-they can be blended to almost any degree you want to-they just show the trend likely with a particular shape. Tacking problems can be addressed with rocker-at least to some degree. Most Moth hull sections have a slightly rounded bottom and slightly radiused chines -at least forward. The box shaped Moth hull has a lot of experience and racing results behind it even though it is ugly, looking sort of like an afterthought,imho. I was very surprised at the science behind it.
The longer the hull the more likely you are able to get the volume you want within a high L/B ratio and a low wetted surface shape. Experiment a lot during the design phase-do not rush to judgement! It is possible to get an easy to build shape that will perform with the best.
Have fun and good luck!

PS-remember: " skinny trumps wetted surface"

 

In very small boats, chines are appealing for displacement reasons. When drawing Slider, a 16 foot beachcruising cat, I realized that using multiple chines to approximate a semicircular section would mean that the boat would have insufficient displacement for 2 people and a fairly luxurious camping outfit-- unless I gave the hulls substantially more beam than the 10 to 1 hulls I ended up with.

Skin friction is only a significant source of drag at low speeds. Once speed exceeds theoretical displacement hull speed, it becomes a minor factor, and hull beam to waterline ratio becomes much more important.

Also, I think that chines on long skinny hulls are a much less significant source of turbulence than on short fat monohulls.

 

This is a very interesting thread - thanks to all the contributors.

To me it's always seemed obvious that if chines were aligned to water flow their resistance effect would be reduced to that of increased surface area. Perhaps wavemaking drag would be reduced. I am imagining some tool that could help one visualize water flow wrt chines to optimize chine placement.

Would it be reasonable to assume that increasing panel chord dimensions at each station proportionally to the square root of the increase in underwater section area would result in chines aligned with the water flow?

In other words, assume I have a box (i.e. square) section of 1 unit square at section 1, so that I have a underwater girth of 3. If I have a square section of 4 units square with a girth of 6 at station 2, would the chines then be aligned with the flow? In other other words, by keeping the shape of the underwater section constant, do I optimize the chine location?

Mike

 

The direction of water flow past the hull will change with speed and heeling angle. A designer should and probably can avoid gross flow angles across the chines in order to minimize turbulence, but it is surely impossible to eliminate completely in a monohull that will stay upright. Don't know about multis . . .

 

I agree 100%.

I agree, but have to make a little correction of the underlined thing:

• The Slenderness Ratio (or Length/Displacement, or short: L/D), defined as L/D^(1/3) ratio is a far more important factor for fast displacement speeds (also called semi-displacement ). High L/D ratios are beneficial for wave drag and allow an easier transition over the "resistance hump" speed region, up to nearly eliminating it when L/D>7 (approx.).
• The importance of the Length to Beam (L/B) ratio is more pronounced for low L/D hulls. For higher L/D ratios ( >6 approximately) the L/B has much less influence on resistance.

The conclusions and numbers above are given in the form of graphs in the paper "Resistance Characteristics of Semi-Displacement Mega Yacht Hull Forms" by Blount and McGrath, which can be found here: http://www.boatdesign.net/forums/hy...laning-hull-low-speed-35799-4.html#post427056 . A short summary of the principal results are given in the post #57: http://www.boatdesign.net/forums/hy...laning-hull-low-speed-35799-4.html#post427074 .

I also invite you to read Ad Hoc's illuminating (as usual) reply and comments to the above (post #67): http://www.boatdesign.net/forums/hy...laning-hull-low-speed-35799-5.html#post427307, which illustrates how the above numbers are to be taken as qualitative guidance only, since they will somewhat vary for different cases.

Cheers!

 

======================
Slavi, is there any comparative relationship between Length/Displacement and the Displacement/length ratio that you are aware of as a reference? I looked in Eric Sponbergs excellent paper on ratios and he has very little on the ratio you refer to-says he doesn't use it. I'd like to understand it better.

 

Wow, reading Daiquiri's posts is just like reading an intro to boat design book. Thank so much for consistently posting things to learn from. Much appreciated and a great thread.

 

Slavi

Sorry if I'm unclear.

I'm was referring to displacement hulls, and the difference between a chined or a round form for the same basic parameters and ratios.

 

They are inversely proportional to each other, plus the complication of the former using the SI units while the latter is using the Imperial ones, plus the further additional complication of the former using volume displacement while the letter is using the weight displacement.
It's more than clear how easily one can go nuts when reading technical reports written in different parts of the world in different time periods...

But it's not really so difficult as it might seem. Consider that:

Lwl (meters) = 0.3048 Lwl (feet)
D (long tons) = 1.016 D (cubic meters)

By substituting these into the two formulas (for LDR and DLR) and doing some math, you get the relationship:

DLR = (30.32 / LDR)^3

LDR = 30.32 / (DLR^0.3333)

Cheers!

 

Thank you, it is a great compliment indeed. However, fortunately you have used the word "intro", because Ad hoc's and Eric Sponberg's posts are real boat (and ship) design books.
Cheers!

 

----------------
Thanks ,Slavi!