Round v. Hard chine runabout hulls

[Sands]

A general question on round v. hard chine planing hulls.

It would seam that many of the planing hulls out there have hard chines. Though there are some round chined hulls (the Ocean Pointer and Lyman boats come to mind) there are not as many.

What are the general advantages and disadvantages? HP/speed to plane? Harshness of the ride? Turning ability?

I'm specifically looking at plans for 30's runabouts. Many of the Hacker plans are hard chined (thinking about 'White Cap' or 'Irene'), while a GF Crouch designed 'Grayling' has a round bildge.

Thanks!

[Eric Sponberg]

In general, hard chine boats run faster for the same horsepower than round bottomed boats. That is because the chine sheds the water off the hull, allowing for less drag.

The boats of the 1930s were very flat-bottomed, and had concave bow sections forward. As a result, they rode hard and were kind of wet. Turns were a little unstable. The advent of the deep V hull in the late 50s and early 60s gave us a deeper deadrise and convex bow sections that offered a still-fast ride, but with less pounding in waves, a drier ride, and better stability in turns. These are generalizations, of course, there are some bad examples of deep-V boats, and these usually have to do with too much curvature in the bow sections and an improper placement of the center of gravity (not in the right fore-aft location, and too high). But in general, deep-V is better, and that is why you don't see too many round-bottomed planing boats these days.

My own Cherubini Classic 20 design (formerly the Independence Classic 20) has a very traditional look above the chine, but a modern deep-V underbody below the chine--deep-V with 20 deg deadrise at the transom, and nicely shaped convex bow sections. She runs like a champ, very fast and very stable. You can see more about it at http://www.sponbergyachtdesign.com/IC20.htm.

Eric

[RANCHI OTTO]

What nice runabout....!!

Congratulations....

[Sands]

I did a little reading about dead rises rates and what not. It would seam that about a 15-20* deadrise (beam dependent) is a decent starting point. I was thinking about playing with some CAD/Hull software to see if I can't carry the deadrise from about the middle of the boat (station 5 or 6 on the Hacker plans I have) to the stern. Need to tweak the chine to get the right boyancy though.

What about adding a small return rail at the chine to direct spray down a tad? Something simple like that might reduce the spray. Seams that they are used on many hulls today....

[Eric Sponberg]

A down angle on the chine up forward is usually employed, say 7-10 degrees or so. This angle then reduces to zero (horizontal) by about amidships, and stays flat the rest of the way to the stern. A little down angle aft would not hurt. Overall, however, you don't want too much down angle because the spray then hits the water hard further away from the boat and it deflects back up with more spray into the boat. This is particularly true if there is too much down angle on the chine forward.

It is the turning of the water as it crosses over the chine that provides dynamic lift to the boat--lifts it further out of the water sllightly. This reduces drag and increases speed. So what you suggest to do is common practice.

Eric

[Sands]

That makes some sense. I'm at the 1st stages of requirements development for what I would like to do. The company that I'm at right now is getting purchased, so any plans that I have about major cash outlays are on hold for at least 6 months. I guess I have some study time.