Am I splitting hairs?

G'day people, my first post, though I have been lurking around for a couple of years...

I have been working on a medium-heavy 30' cruiser, of which I plan to start construction early next year. After a few years of self study in design, with books ranging from Skene's Elements of yacht design to Larson and Eliasson's (sp?) Principles of yacht design (all up a full shelf on the bookcase) and figuring out both paper drawings and FreeShip, my plans are nearly complete. A boat not to most people's liking, but it's what I want! A 30' LOD topsail gaff cutter yawl centre cockpit/aft cabin. Almost plumb stem, moderately cut-away forefoot, long keel and short counter stern. Hull loosely based on Essex smacks and pilot cutters like Jolie Brise. Wood strip construction, sheathed in 'glass. I am not after a race winner (obviously) but a very strong and comfortable long distance cruiser. I have all but completed the hull design on the current version, which is the last...! Comparing the two best variations on an excel spreadsheet I put together prompted the following question - which boat would be a better ghoster, i.e. still able to make headway in the lightest winds, yet still maintain the best high speed potential? Here are some stat's.

Boat Name_____________mm30a____________ mm30b
LOA_____________________29.899 ft_____________ 29.901 ft
LWL_____________________27.313 ft_____________ 27.415 ft
BOA_____________________9.413 ft______________ 9.413 ft
BWL_____________________8.670 ft______________ 8.670 ft
Draft____________________4.700 ft______________ 4.700 ft
Draft BOA/8______________ 2.206 ft______________ 2.206 ft
Displacement_____________ 15,232 lbs____________ 15,459 lbs
Ballast Ratio______________0.40_________________ 0.40
Ballast___________________6,093 lbs_____________ 6,184 lbs
Sail Area Total(SA)________ 800 sq. ft____________ 800 sq. ft
Midship Area (Am)_________15.706 sq. ft__________ 15.698 sq. ft
Wetted Surface (SW)______286.050 sq. ft_________ 288.650 sq. ft
Area Lateral Plane_________99.948 sq. ft__________ 100.810 sq. ft

DLR___________________333.73______________ 334.94
SA/D__________________20.91_______________ 20.70
Hull Speed_______________ 7.00 knots______________ 7.02 knots
Velocity Ratio____________ 1.17____________________ 1.17
Capsize Ratio____________ 1.52____________________ 1.52
Motion Comfort___________42.29___________________ 42.81
Roll Period_______________ 3.87 seconds____________ 3.91 seconds
Roll Acceleration__________ 0.02 g's________________ 0.02 g's
Stability Index____________ 1.35____________________ 1.36
Prismatic Coeff________ 0.555________________ 0.561
SA/SW_______________2.80_________________ 2.77
GM (approx)_____________ 0.97 ft__________________0.953 ft
Lat. Plane/SA____________ 12.5%___________________12.6%
AVS____________________ 174.5 degrees____________ 175.3 degrees

Boat mm30a has slightly faster DLR and SA/D, and the slightly lower Prismatic Coeff would benefit the lower speed performance, but detract from the upper end. mm30b's higher Cp would help upper end, but with the slighly slower DLR and SA/D, would it make any difference? The SA/SW ratios indicate that mm30b may be slightly more slippery. I know that these two hulls are very, very similar. Am I wasting my time agonising over the options? I think I am leaning towards mm30a. Any thoughts or suggestions?

Cheers, Pat

 

You are splitting hairs. Good luck building

 

I immediately thought of this boat, one I used to charter. The reason is the centercockpit, one of the best layouts I've experienced. I highly recommend the layout seen in the drawings. While the Magellan is a bit heavier at 17,000 lbs, it's close enough to do some comparisons.


http://www.serenitymaritime.com/Sailing.html

 

Indeed splitting hairs. A barnicle on the bottom or a 1/4" difference in easing or tightening a jib sheet will make as much or more difference. Building a boat is going to call for more than a few decisions to be made in the field, beware of analysis paralisis. Every once in a while you're going to have to fake it and hope for the best.

 

I find most non-production boats are off by a couple inches here and there anyway. Keep an eye on your weight, much more important.

 

G'day fella's, thanks for the replies. I'm going to go with mm30a - everything else is pretty much equal, but mm30a saves 227lbs of ballast, which reduces the amount of wheel weights I need to collect from the local mechanics!

Alan - You're spot on. The Magellan 31's hull profile is very similar. I too have drawn it with no drag on the keel to make careening easier and more comfortable, though mine has a much more plumb stem and the counter extends about 2.5' aft of the aft perp.

TollyWally - "Analysis Paralysis"!!! I love it! Yes, I know the symptoms all too well! That's the reason I posted, I could justify both models as being the best of the two.

mydauphin - Weight is an issue, yes. The construction is going to be heavy as it will be built in good Aussie hardwoods, which tend to be around 1000kg/cu. m (Western Red Cedar is around 360kg/cu. m). The shaping of the timbers will be much harder than if I used softwood, but I have 1200acres of assorted Eucalypts and a chainsaw!

From the data and descriptions given, would anyone like to share their opinion of this boat? A functional, old school design that will get me around in style or a very expensive and time consuming lemon that I will want to scuttle?