Single or Double Chine?

I have had a copy of Plyboats for a long time. It is installed on and old IBM computer in my workshop. Plyboats works well on the ancient Win 95 computer. I use a Win 10 computer to hang around the forum and do almost all the other computer stuff. I dislike Win 10. The old 95 machine is a lot less capricious than my new fangled one.

I would be a bit hesitant to use PB as a Final design program. It is great for quicky drawings and it does furnish some elementary information like Righting moments, wetted surface area, section centroid locations, etc.

As for one chine or two chines, the single chine will give much better initial stability . Properly designed it will plane quicker than the two chine option.

 

That is because they are designed to carry huge sail plans.

To do that requires great form stability, and there are three ways to get that with a monohull.
1.) increase the maximum Beam.
2.) increase the average Beam by increasing the fullness of the water lines near the surface.
3.) decrease the Hull depth in relation to the Beam.

Modern materials science has allowed us to use the 3rd option more and more.

More chines allow greater performance over a wider range of wind conditions per given amount of sail area, as they allow less Hull surface area per displacement.

But this comes with diminishing returns. A "V" bottom with a single chine will likely be better than a flat bottom with a single chine. Add another chine and you will get a more slight but real improvement and so on.

Each chine added will improve the boat less and less.

I would have a double chine if I were designing a flat bottom boat.
The 2nd chine allows for a greater Water Line Beam while increasing the whetted area minimally. This may actually increase the initial stability of the Hull while cleaning up the hydrodynamics somewhat.

 

Yes, I think it would be better to call it double or triple chine.
Anyway, I would suggest the one with the lower number of chines.
In my opinion any chine under water is a bump that slows down the boat.

 

Interesting. This is what I was thinking myself, but with the extra chine the angles are less acute. There's much to ponder.

 

True! Thanks, good insight

 

Just an observation from testing and building a few kayaks. The multi chine version felt like it leapt ahead when you started paddling. The lazy man in me went with the single chine version for the good reason of wanting the authority with which the single chine Greenland pattern tracks straight when upright and turns when edged (legitimate). This same edged turning translates to weather helm in a sailboat. My paddling partner is my wife so I don't want-need the speed-efficiency of the multi chine boat (legitimate) Did I mention lazy? It was stupid to let that into the decision process (illegitimate)). Way more time is spent sanding than taping a couple extra chines.

 

Here's the plan and profile to help contextualise things.

 

Double chine sailboats were popular in the 70's, especially in alu. The most usual design was with the waterline at about midway beteween the 1st chine (in the water) and the 2nd chine (above the water), so to have a low waterline beam and wetted surface when upright or small heel angle (i.e.by light winds) and the 2nd chine in the water bringing more volume laterally , i.e. more righting moment, when heel 15° to 25° by breeze. A typical of such design was the Onvi 25' by Philippe Briand, linesplan attached. Of course, a hard chine brings more drag than a rounded bilge, but also more lateral shift of the hull volume when heeled, so more RM, and upwind by breeze that compensates and sometimes over.

 

Thanks for this. I have thought a lot about position of the chine in respect of the waterline. Given the small size of the boat (5m) I know that single handing will see her float above her marks. With a crew person or extra cruising supplies aboard she should gain stability of the immersed chine. My current boat (4.4m) has a narrow waterline for her beam and while is a delight to sail, doing anything aboard while on the anchor is annoyingly tippy. So I want more form stability in this boat. She will be a stable, friendly open boat for my old age...