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Old 09-14-2012, 10:28 AM  
Art's Avatar
City: SF Bay Area
Country: USA
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 9,283
Originally Posted by FF View Post
"Also - in average-good conditions, the same planing hull has less roll traveling at speed than a displacement hull traveling at 6-8 knots."

The problem is All boats roll , the shape of the hull will determine weather the boat is a bard bucket.

Roll which is soft (tho perhaps further ) when it checks and reverses direction is the easiest to live with.

A box bottom or hard chines is great at the slip , but underway the corners stop the roll much harder with a sharp quick motion.


A really skinney hard chined boat might do this less , but few are that skinny.


Guess itís because I like the feel of the sharp, responsive hard chine planning hullís immediate return actions to my piloting commands while I Capt a boat (especially a good design well powered twin screw craft)... while in ocean waves, confused inlet waters or inland water ways from huge wakes or stormy winds. And, cause I was blessed with real good equilibrium tween my ears so the word Barf never has come to mind or fruition during any boating circumstance. Iíve piloted D hull boats; at or below hull speed I found their roll a PITA for any ongoing duration as well as making it more difficult to stay on heading in some sea conditions. Now donít get me wrong... D, SD/SP, and P hull designs all have their high and low points for working with water conditions... and Dís surely offer the most economical capabilities... in the long run. However, I prefer the overall feel of hard chine bottom twin screw boats with rounded transom, slicing prow, and substantial upper hull flare that can be cruised quite economically at hull speed with one screw operating at a time... yet provide opportunity to fire up her twins and plane out for smoother ride and more mileage covered, with potential to run for minutes or hours at near WOT - - > to move like hell and get out of the way whenever necessary. Our Tolly gets approx 2.75 to 3 nmpg doing 6 to 6.5 knots on one screw (7.53 k is calced hull speed)... 2 nmpg on twins at hull speed... 1 nmpg at 16 k to 17 k twin screw plane... and, OMG Ė NO nmpg at 20 to 21 knots; WOT 22 to 23 k. All these figures are +/- depending on how loaded our Tolly may be.

Using 32í water line length; for a thumbnail of annual miles traveled/fuel cost comparisons:

Single screw D hull averages 3.5 to 4 nmpg operated at its hullís cruise speed of approx 7.5 knots. Twin screw P hull averages 2 to 2.25 nmpg when intelligently operated at its various cruse speeds (letís say an average of 10 knots). Boats are each operated 300 hours per year. Therefore: D hull travels approx 2,250 miles / P hull travels approx 3,000 miles - - > Figured on the best fuel mileage calcs Ė D hull uses 750 gals per year / P hull uses 1,333 gals per year - - > that means (at avg fuel cost of $4.50) that D hull costs $3,375 in fuel / P hull costs $5,999 in fuel - - > difference is $2,624 annual dollars Ė divided by 52 weeks = $50 per week extra fuel cost to own and enjoy the capabilities of a planing hull. Most boat owners can handle that. Also to look at this difference is in another light - - > if instead of the 3,000 mile travel figure used in these calcs for P hull, we reduced P hull to the miles traveled at 2,250 miles of the D hull (notably less hours for P hull to go same distance), and took the 750 mile reduction x 2.25 nmpg at $4.50 per gal cost then P hull annual fuel cost is reduced by $1,350. In that light it means P hull added annual fuel cost over D hull was only $1,274 which = $25 more cost per week to fuel a P hull in comparison to a D hull Ė Iíll gladly spend that $25 per week to get there quicker, experience better ride, and spend more time on the hook/dock-berth/mooring with a Planing Hull!

YRMV!!! Art
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