about the chine?

[adla]

i have a question about the chine. i took vacation this week and didn't work on the boat until tonight...and the chine or what would be the chine has me a bit puzzled. should it be rounded? so far, i have not done that...sanded and filled but left the edge pretty *square*. I am mostly curious about the way the boat sails and how the edge works and what it should be like.

a.

including some pics of the current condition of the chine.

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[Frank Hagan]

With a hard chined boat, the sharper the edges the better. But you do have to round them slightly to make it easier to lay the fiberglass cloth around the corner.

I rounded mine enough so that a scrap piece of cloth could make the bend easily without puckering. If you are using the 6 ounce cloth the plans call for, that's just a slight rounding ... I used my belt sander and a light touch to do it. If you are using a stiffer cloth or tape, you may need to round it a bit more.

Hard to see how sharp the corners are in this picture, but I think you can see there is just slight rounding, and the cloth lays over the edge nicely:

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[Dave R1]

Adla, my chines are about like Frank's, rounded enough to allow the cloth to make the bend but no more.

[capt jake]

Round to appoximately a 3/8" radius. This will assure your cloth lays out nice. )

[MikeStevenson]

We do our chines and keel radii closer to 3/4", maybe 5/8" or so.

Mike

[Florin {VACATIONER}]

The radius on our keel was made with a 3/4" router bit. We tried to duplicate it on the chine, but we've had to use the beltsander this time, for the router wouldn't work there due to the various angles involved. The 3/4 " radius worked quite well on the chine and the glassing went well.

Florin

[Florin {VACATIONER}]

Correction!

The router bit used for radiusing the keel is 1 1/4".

Sorry

Florin

[MikeStevenson]

So you really rounded your keel. We usually did ours with a little (3/8" or so) flat spot in the center. We used a Surform shaper to do ours, so the radius was freehand. This also for the chines.

Mike

[adla]

Thanks everyone for the input...i didn't round my keel to a 1 1/4 though, mostly because i don't have a round over bit that size. i am pretty sure it was 3/4 .What i needed to know is whether to round that chine some or not and now i know. thanks!

I ordered the glass for the bottom on thursday! it should be here next week!

i am very excited!

a.

[Frank Hagan]

I'll bet we could get into a very detailed discussion of what effect the rounded keel has over the less rounded keel. Not sure I know if it makes a difference at all on the keel ... but looking at the big old boats around here, the keels are not rounded very much!

Mine has a radius on the edge, but I think its 3/8 or 1/2", and it was with a Surform shaper.

On the chines, I had always thought you want them sharp enough to really bite into the water. But I'll bet its one of those details we stress over, but doesn't really matter as much as sail trim.

[adla]

oh sure...so much for all my easter dinner calories...the sure form shaper and i have rounded the chine...it looks great...laid a bit of cloth that i have to see how it sets and things look good.

thanks for all the help..my arms feel like jelly.

a.

[Florin {VACATIONER}]

Yep, Mike I reaaallly did round it. I was always under the impression that bit was 3/4".... not sure why, I could have read the print on the box like I did a couple of days ago. Turns out I don't own a 3/4" bit, or if I do I can't find it. The chines aren't quite as round though. I think the keel looks allright with those radiuses there .

We mainly wanted to do a larger radius than per plans because we wanted to put that kevlar on the keel. We all know how that turned out now (don't even wanna hear about that for another 10 years ) We've also had a couple of layers of 6oz glass cloth there but it all got sanded off in the process of removing the kevlar.

Florin

[adla]

how did it turn out...did you have a bad time with the kevlar? i am interested since i have been considering using kevlar on my keel...do tell....

a.

[adla]

never mind florin...i found your long post..what a horror story.

a.

[John Teetsel]

If you want to have a sharp chine, glass the bottom as usual then slather some thickened epoxy on the edge and sand it back down to a sharp edge.

[Frank Hagan]

If you need a "sacrificial shoe" on the keel I would go ahead and use a hardwood like white oak (a traditional keel shoe material!) Some people will use a brass strip also.

[capt jake]

We do our chines and keel radii closer to 3/4", maybe 5/8" or so.

This more approximates what i did. 3/8" is a minimume.

[Barry Pyeatt]

According to the tank studies my Marine Architect alluded to, it isn't so important that it be sharp at all for good performance. It is the "sharp" angle between the side and the bottom that makes for the efficient flow.

Any sharp entry is not as good as a rounded entry with a sharp trailing edge. Sharp entries tend to make things track in a particular fashion that makes them hard to turn when you want to. Flow studies essentially come back to the "airfoil" shapes as being most efficient. Blunt, rounded entries and sharp exits on rudders and centerboards. On long traverse runs they all should be rounded rather than sharp for the most efficient flow over them.

Or at least that is what the tank studies show.

[MikeStevenson]

I agree; It feels like the panels are the active force in a "hard-chine" boat, not the actual edge. We always think of water as a sticky, viscous liquid (which it is...) and try to make it easier for the water to get past the boat. Water likes big round shape better than sharp ones, so try to keep the radii big. Especially on inside curves like the keel fillets...water doesn't like low-pressure spots.

Mike

[Barry Pyeatt]

In underwater videos taken during tank studies and in actual full scale use, sharp edges and corners have a lot of bubbles building off of them. That denotes drag. Smooth flowing contours have no bubbles trailing off of them denoting an efficient flow of water along the hull. Pressure sensors attached to hull models will also note that there is lower pressure where ever there are sharp corners such as keel/bottom joints. That causes drag to occur and slow the boat down. The radius is important but on a boat such as these, anything larger than about 3/4" will work out fine. Water gets sticky in a flow along a hull. The more even (fair) the hull curve the smoother the flow and consequently the better the hull will be at achieving its potential.

The bulbous nose on a sub is a good example of how water flows better over a blunt shape than over a sharp one. The trailing edge of a rudder or centerboard would ideally be an infinitely sharp edge to allow for ideal flow. Any blunt edge on a trailing edge creates turbulance and that creates drag.

Amazing studies about hydrodynamics and hull shape/efficiency. This chine/keel relationship is a wonderful relationship for efficient flow and control.

Think of an extended "W" shape and just round over the sharp bottoms. It tracks thru the water like it is on a rails. But with the right front to back contour, it also allows for very controlled turning and efficient flow over the hull. The tunnel in effect replaces the centerboard by channeling the water and keeping it on track along the hull. A rounded hull form has no such control, just efficient smooth flow front to back if it has a "fair" profile. The smoother the curve and more even the surface, the more efficient it is. Flat spots or pockets along the curve cause drag and turbulance. So time spent making the hull "fair" and smooth before painting will pay off in more efficient flow and better hull speed. Little bits can make large differences in ultimate performance.

But these are fun boats, and we arn't building performance boats.

So maybe this is too much to go into in any depth.