On 07/20/2016 10:11 AM, casioqv@usermail.com wrote:
I'll second that, theoretical hull speed is no hard upper limit, it's only the point where it starts to take a lot more sail energy to go faster due to the bow wave having a wavelength longer than the boat, and the stern falling down off the wave crest.
Maybe getting a bit OT here for a Monty list, and...for the 'big picture' enthusiasts... If you get hull length:beam ratio around 12:1 and up, the bow/hull wave drag limitation decreases dramatically. Here an article by James Wharram (Wharram catamarans) that includes that info, as well as freeboard and windage effects: http://wharram.com/site/how-we-design/freeboard-and-windage Search for 'wave drag' on the page to cut to the chase on that part (about half way down). There's a bit more on wave drag/hull speed in this article: http://wharram.com/site/how-we-design/catamaran-stability ...read starting just above Figure 1 to cut to the chase on how monohulls evolved in terms of stability. The rest is about cat stability but as he points out the formulas are not different, just the results in terms of boat design. And also as he points out, humans figured all this out hundreds to thousands of years ago... cheers, John S.
I've often sustained 8.5-9.5 knots per GPS surfing downwind in my Catalina 22 (6.0 knot hull speed) coming back downwind from Catalina Island on a windy afternoon. The C22 rudder isn't quite powerful enough to control the boat at those speeds, and it will broach easily.
This article by our list member Dr. Judy Blumhorst does a good job of explaining why this is possible: http://potter-yachters.org/manyways/hullspeed/
Tyler
----- Original Message ----- From: swwheatley@comcast.net To: "For and about Montgomery Sailboats" <montgomery_boats@mailman.xmission.com> Sent: Wednesday, July 20, 2016 9:38:21 AM Subject: Re: M_Boats: 1) BBQ 2) m23 speed
You guys are talking about theoretical hullspeed, which is more like a rule of thumb than a law of physics. Boats with displacement hulls can and do exceed their theoretical hullspeed all the time, including by heeling (which lengthens your LWL), or by surfing along the front of a wave (which turns your displacement hull into a planing hull, albeit briefly). For an M23, 6.5 kts heeled and 8 kts surfing are both very plausible.
-- John Schinnerer - M.A., Whole Systems Design -------------------------------------------- - Eco-Living - Whole Systems Design Services People - Place - Learning - Integration john@eco-living.net - 510.982.1334 http://eco-living.net http://sociocracyconsulting.com