11 February 2012 02:00 PM
Judging by my title and the forum within which it exists, no doubt this sketch is going to come across as tubby to some. I have great respect for the “less is more” mindset but decided to depart from it for this.
To explain where this concept is coming from, it is my mashup of the accomodations of a cruising trimaran with the cockpit concept of Phil Bolger’s birdwatcher.
I am very curious what sort of all-around performance this would yield; comparable to a cruising trimaran or much worse leaving a proa this plump an oxymoron?
Comments welcome.[ Edited: 11 February 2012 02:07 PM by Adam ]
11 February 2012 02:04 PM #1
12 February 2012 07:05 AM #2
The issue here I think is going to be load capacity. Proas are happiest when lightly loaded, my former 23’ P52 was designed as a two man EC challenge boat and could carry two guys and a minimum of gear just fine (handling and a few other misc items notwithstanding) was even better later solo for a couple of Texas 200’s.
Take a serious and honest look at weights for a 4 person weekender, the boat might want to be longer (proas always seem to want to be longer).
Nice drawing and approach.
12 February 2012 10:58 AM #3
Thanks. Yes I am keeping an eye on weight for sure. The main hull is drawn with 2300 lb displacement to DWL, owing to a generous 26” draft with fins up. I’m not sure if that’s adequate or not yet; it’s probably cutting it too close as I’m trying to avoid composite construction.
Another thing I’m wondering about is how much the windage will affect performance. The cabin top is 4’7” above the waterline.
12 February 2012 08:38 PM #4
this is a lot like what i was trying to do with the whole box keel idea. on a fixed water line and draft you get the most displacement with a box section. I had read in a design book by Kelsall written years ago that a fast multihull had a maximum draft of 1/16 of the waterline length. I used this ratio for the WL beam ration as well, then started putting “cheeks” on for inside living volume and some lee pod effect. I was thinking the length of the lee side pod combined with the more gradual entry might get me the reserve stability on a narrower overall beam as well compared to Russ’s pods. Never really know until I get off my ass and build it though.
I should note that he wrote this book quite a while ago, and I’ve seen some argument for staying somewhere around 12 or 14 to 1 with the waterline more recently.
I’m with skip. as long as you don’t get your weight to far out of hand it looks like a nice design.
Work to live, don’t live to work.
13 February 2012 02:21 PM #5
That’s a lot of space! I’d be worried about putting weight in the ends of the boat. Its probably not a good idea to encourage that by having accommodation out there.
I LOVE the birdwatcher derived concept, though.
Maybe you could lower the roofline overall to reduce some of the windage—since you have unlimited headroom in the middle? How do you closer up that big hole when the boat’s going to weather into chop? What about watertight compartments?
And of course all of that skin weighs something.
Can you calculate your overall square footage? That might give you some idea of how much weight all that skin adds. That would be way WAY to much windage for a trimaran. Should the windage on a proa be even lower?
What do you guys think?
Looking forward to seeing more,
14 February 2012 05:11 PM #6
Tom, thanks for the WL beam ratio reference. I’m at 12 so I guess it’s pushing the limit of performance tradeoff.
Chris, the hole is to be closed by a rigid flap with overlapping edges. As for piloting, I was thinking about moving the hole off centerline on both ends, and leaving a 3 foot portion uncovered at all times and sectioned off as a designated rought weather cockpit with interior dividers and self draining floor for that section. Having control on both ends is easily solveable with rudders of course but not with sheets yet.
I’m in the process of moving more toward the birdwatcher concept as far as sitting headroom only being in the main hull rather than on the wings throughout. May end up with sitting headroom in the wings in the very center, or maybe just with a tent.
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