Transverse Chine Angle

[penningtonjeff]

Hi,

I'm working with a NA on a lower-speed planing runabout, and the current question is how would reverse chine benefit the her at relatively slow (< 20kt) planing speeds?

She is 19'10" at her waterline, 6'8" beam, weight somewhere < 2000lbs with motor. Her intended speed is 15-18 knot cruise with a 50-60hp four stroke. Top end speed is not important. She'll be carrying precious cargo (knee-high daughters), so a comfortable (non-pounding) ride at this speed is a high priority.

Her hull shape is 'developable'. She has a pretty sharp forefoot, and a pad on the keel toward the stern. The current design includes a ~3" chine flat. How would angling this flat down affect her? What are the pluses and minuses of reverse angle?

-Jeff

[Tim B]

It would probably reduce the wave pattern from the chine a little, but I suspect it would give an increase in pressure further aft. pressure on a hull means one of a two things (simplistically) lift or drag. The angle of the chine-flat will lead to a cosine-type reduction in lift, so I suspect that you'd end up with equal or less lift and more drag. I can't say more than that without doing some rather hard sums on it.

If you want to get rid of the slamming problem you need very fine sections in the bow, and since you want to keep it planing, pretty flat sections in the stern. You're almost asking for a 10 degree or so deadrise stern and a yacht-type bow.

Good luck,

Tim B.

[tom28571]

Tim, If I follow your argument correctly, I have a question. Say there is 10 degrees of aft deadrise and the chine flats are given a transverse down angle of 2 to 3 degrees. Won't there be an increase in both lift and drag? If the lift gets the boat up on plane earlier, the net effect may be all to the good.

One negative of too much immersed chine on a fast and maneuverable boat is tripping on the chine in a fast turn. Not what you need with your precious ones aboard. A down turned chine will also cause the boat to bank less in turns, making the tripping tendency worse. All this is probably not a problem at the speeds you intend but 60hp may be more than you need.

All speculation at this point.

[Chris Ostlind]

Hi Jeff,

I think the hull you want to emulate would be a mirror of these, the Tolman Skiff hull. I wouldn't mess with the negative chine angle as it's going to be about increased drag if anything.

The Tolman planes early, has a beautiful ride in chop and can get it on with a smaller than typical engine. http://www.xyz.net/~mgrt/

Chris

[Raggi_Thor]

You could also go the other way and have a look at John Welsford's Rifleman,
http://www.jwboatdesigns.co.nz/plans/rifleman/
It planes at low speed because it has a (narrow) flat bottom, while it has a sharp entry and narrow waterline beam to slice through the chop.

[bodenbp]

First the craft must be designed for the function it has to preform. This must be specified by the owner and he cannot keep changing what he wants. Having said this I assume(one should never assume) that you will be using a 60hp 4 stroke outboard.On the right size boat,with a monothederal constant deadrise hull of about 15 degrees at transom this will give you 18 knots.Now regarding the downangle chine. Remember water is 800 times more dense than air. Tank testing has proven that on most planing hulls there is an increase in efficiency with a 5 degree down angle chine strake at about 80mm wide on a 6 metre boat. its a bit like the jumbo jet liners,thay have winglets on the end of the wing to improve the efficiency of the wing. it may not be a lot but as most aircraft can spend up to 16 hours a day in the air,the fuel saving adds up over time. Its a simple as that,just put 5 degree downangle at the chine and forget the rest.