Square chine/round chine

I'm an experienced boat guy who knows enough to make a bad decision, not nearly enough to make perfect decisions.

I am presently hooked on small, flat bottomed puddle duck types. Say 15 feet LOA more or less. The one on my drawing board is skinny at 37" on the chine. It'll carry its' forefoot slightly above the WL. I reckon it will row poorly, sail fairly well, and accept a small 2 to 4 HP motor if all else fails.

Here is my question surrounded by too much explanation. Consider some section of the body plan about a third of the way aft. Focus on the immersed part. Draw some vectors. The horizontal ones will be longer than the vertical ones if we consider divergence angles only. That suggests a circulation from the sides toward the bottom. But wait ! Bernoulli is at work, and the pressure at the sides will be less than the pressure at the bottom. Hmmmm... The circulation may go in the opposite direction. Velocity will no doubt play into this. Suppose we try to soften the vertical/horizontal transition . We can round the chines a bit. (we'd do this by using strip construction at the rounded part and plywood in the flat parts ) Is it worth the trouble ? Does anyone have information or opinions of a qualitative or quantitative nature. How much radius must we use in order for the affect to be worthwhile ?

Gene

 

chines or not

Gene, in the highly competitive Moth class Andy Paterson(bloodaxe boats) has written that he considers the benefits of the higher(narrower) beam to length ratio with a rectangular section outweigh a semicircular section with its lower wetted surface for equal displacement. Most of the seahugger Moths have hard chines.Seems like that shape can't be too bad drag wise...

 

Doug;
Thanks for your input. Many moons ago I built and sailed a "Cates" moth. It was the hot number at the time. Severe deadrise, boat would go fast if you had gut enough to hold it flat. A dog when heeled. Todays moth is apparently in a whole different world than the oldies. Foils were illegal when the Cates type was in vogue.
I've read many of your erudite postings on this site as well as the AMYA site. I fiddle with experimental models but not on the level that you pursue. I have towed a bunch of models trying to learn by observation and home brewed drag measurement gadgetry. Fun but sometimes confusing.

Presently I'm on a kick with sharpie like flat bottomed boats. Their heeled immersed body is fascinating. Scrumptious splinter like waterplane with perfectly aligned section centroids when at optimum heel angle. The chine interest is to explore the possibility of reducing rowing or sculling effort. Radius must be kept small in order to keep the heeled advantage. If 2 or 3 inch radius has only minimal effect, then its not worth the construction complication. I have noticed that several designers have flirted with this. Bolger has experimented with this idea and John Welsford (new Zealand) specifies eased chines often.

Doug, since you are a foil guy, Have you read Bernard Smiths "The 40 Knot Sailboat" ? Smith arrived early on the Aerohydrofoil scene. (Library of Congress catalogue number 63-18980) Published in the early sixties. Other boat nuts like us may enjoy it too. One last bit...Are you related to Lindsay Lord (Naval architecture of Planing Boats) ?

 

relative reading

Gene, I haven't read Mr. Smiths book yet but I want to and no,I'm not related -that I know of, to Mr. Lord.
Good luck with your design exploration!

 

A lot of high performance skiff type boats have round bilge forward, but in the last 1/3 or 1/4 of the boat have a chine or some sort of sharp edge to help a clean spray release when planing.

The idea is that the rounded fron end will minimise vortices which a chine would tend to promote (as you have noted).

But at the back end a round bilge tends to project any spray upwards as it tends to stay in contact with the hull. If spray is projected UP - there is a net DOWN force on the boat. Thank Mr Newton.

So many fast boats have a sharpish chine towards the tail of the boat so the spray will break off cleanly heading in a more horizontal direction.

At lower speeds the chine is either out of the water or closely aligned with water flow (chines are sorta parallellish to the direction of travel. Best to not have them completely parallel as the stern gets much too wide unless the boat is very narrow like a skiff moth.

Boatmik

 

Boatmik;

Thanks for your input. My notions are to build exactly as you describe....Eased chine forward, sharper aft. My boat may never plane, though I will do the run of the bottom in such a way that it has the capability.

You mention the word plane, which is, in the american impression, obligitory for Aussies. We know about the crazies who sail those 18 footers and even the 10s with enough sail to drive a tea clipper. I wish I had the courage (and youth) to try something like that.

Best regards

Gene

 

Hi Gene,

You are slightly mistaken.

It is not about Courage and Youth.

It's about lots of Sail and
as little Weight as you can get away with!

Do this and you can't keep a good boat down!

MIK