Cal's Evals » PAHI 63

Release Date: 1/5/2006

PAHI 63

It takes a lot of courage to evaluate one of James Wharram's catamarans, because they are SO different. A reader needs to study James' design concepts in his "Design Book" or some other writing to decide whether he agrees with them or not. To say the least, they are radically different from what is the norm in modern catamaran design. And James makes no apologies.

Nonetheless, the Pahi 63 can be evaluated in the same fashion as other boats in the Multihull Dynamics, Inc. data base, but with an exception…the extreme bow and stern overhang of the boat begs the question of whether to compare it with 63 foot overall length boats or 51 foot waterline length boats. This boat is 24% longer LOA than its Lwl, compared to an average of 4-6% for other boats near its size. That is extreme.

Not being willing to miss an important comparison, I will compare using both Loa and Lwl. The table below shows how the Pahi 63 compares in each parameter with 62-64 foot overall length boats (in brackets before the slash) and 50-53 foot waterline length boats (in brackets after the slash).

Displacement: 12.5 tons [12.03/9.41 tons] Displacement is similar to larger boats

Length Overall: 63 feet [62.95/53.15 feet] LOA is similar to larger boats

Length at Waterline: 51 feet [59.64/51.03 feet] Lwl is similar to smaller boats

Beam Overall: 28 feet [32.30/28.93 feet] Boa is similar to smaller boats

Sail Area: 1400 sq. ft. [1889/1491 sq. ft.]SA is similar to smaller boats

SA/Disp Ratio: 24.29 [38.03/32.24]SA/D is significantly lower than both

Hull L/B ratio at waterline: 10.46 [14.74/12.37]

Beam between hull centerlines: 24.06 ft. [27.48/24.06 ft.]

Similar to smaller boats

Base Speed: 11.66 knots [14.57/12.90 knots] Slower than both

Texel Rating: 119 [98/106] Handicap allows for slower expected

Performance

Bruce Number: 1.24 [1.52/1.42]Conservatively rigged

Stability Index: 5.22 [3.90/3.60] High stability in comparison with both

Stability Speed: 24.39 knots [20.57/19.98 knots]High stability

The high stability of the Pahi 63 is indicated by both the Stability Index and Stability Speed is probably understated even by these numbers because the boat has a schooner rig rather than the Marconi rig assumed in the formulas that approximate these indices. Her Base Speed shows that she is not a racing boat. This, of course, is predicted form the SA/D ratio that shows she is under-rigged for a boat her size, whether by comparison of overall length or waterline length.

Wharram defends the extreme bow and stern overlap as adding to seaworthiness. Similarly, he defends the canoe stern, a feature that is decried by the universe of modern multihull designers as being a major contributor to hobby-horsing, or pitching. If, indeed, the Wharram boats do not suffer from this, it is because other design features (low rig, concentration of mass in the center of the boat) compensate for it.

Using Charles Kanter's rating system which is based on Lwl/Bh, this boat might fall in either the Cruising/Racing or Cruising category. Considering all of its conservative design features, the cruising category is probably generous. Given that, some of the living features of the boat are intriguing, particularly the "flexi-space interior" design.

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net