Question on deeper deadrise on a light hull

[Wayne Grabow]

Question: How important is it to have the chine well-imersed in a power runabout?
Background: I just built a 18' variable deadrise hull with a fine entry transitioning to 6 degrees of deadrise at the transom. It performs as you might expect; planes easily with low power, fast (for 50 hp) in flat water, but the bow rises enough when planing that we get wave slap about 1/3 back from the bow when going upwind in a chop. The hull is rather light; bonded West system with no fasteners. With a monocoque hull, the "slap" resonates like a violin.

So now I am thinking of a new design with 14 degrees of deadrise at the transom maintaining constant deadrise for the aft 50-60% of the length then transitioning into a fine entry. This should keep the bow down and present more of a "V" aft to cleave the waves with less hull slap Problem is that with deeper deadrise on a light hull, the projected weight is less than the displacement when the chine is immersed. With two people aboard the chine will be slightly exposed; with four people it will be slightly immersed. Is this a problem? I don't want to reduce the beam due to accomodations and static stability concerns. I already plan chine flats so that the"V" deadrise is less than the chine beam. Maybe I am overly concerned, but it is better to ask questions than to make mistakes.

[gonzo]

You will have very little initial stability. Also, it will probably chine walk or even worse, trip on the chines making the boat get out of control.

[Wayne Grabow]

Thank you, Gonzo; just what I needed to know. I had considered initial stability, but, with little powerboat experience, hadn't considered the likelihood of chine tripping. So the deep V monohull by definition must be a fairly heavy boat. I need to either add some weight or reduce deadrise slightly.

[gonzo]

Deep vee fast boats are normally narrow.

[PAR]

Match your immersed volume with the anticipated weights you'll encounter and draw up a hull shape to conform to the SOR.

Slapping and pounding will happen on any hull form and the faster you go, the more likely you'll experience it. If you over drive a warped bottom, you run into this issue pretty quickly. You'll need more speed to see the same affect on a deeper V hull form, but you'll eventually run into to it on them as well.

[anthony goodson]

Wasn't there a detailed discussion about this idea a few months ago ? I can't find it ,does anyone else remember?

[Wayne Grabow]

Thanks all. I reduced the deadrise to 11.75 degrees which better matches the anticipated weight of the hull with minimal crew and equipment. Should be fine now. Although I don't plan on high speeds, I can understand (after Gonzo brought it to my attention) why it would be better to have the chine consistently immersed and engaged, rather than catching unpredictable waves, for predictable performance.