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Old 09-08-2007, 15:15   #1
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Catamaran Hull Speed

I have the formula for figuring hull speed for monohulls, but how does that figure compare to a catamaran. With less wetted surface it seems like it would be higher but how much?
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Old 09-08-2007, 15:27   #2
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cat hull speed

The hull speed equation considers that the hull moving through water creates one large wave with it's high points at the bow and stern and the trough at mid hull. A cat with narrow hulls does not create this wave and does therefore not have the same limitations. As the hulls get wider they probably start to react much like two monos. I don't know if a formula exists but I have sailed my 32 foot cat at twice the hull speed of my former 33 foot mono so clearly the mono formula does not apply.
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Old 09-08-2007, 15:47   #3
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The squareroot of the waterline X1.34 applies to most hulls including multihulls. However if the hull width is narrow then a higher speed can be achieved, before the resistance produces a bow wave.

If the hull/hulls have a length to width ratio of 16 -1 "the displacement rule" does not apply and these boats do not form a bow wave even at high speed (30 knots) (Commercial ferries) The difficulty is that it is difficult to design a practical boat under 200 ft using this principle.

I have also found that by designing a hull in the shape of a wedge with a very narrow angle of entry and a wide stern I can also "Break" the displacement rule and achieve high speed without forming a bow wave.

I am building a 40 ft Power trimaran based on this principle which is powered by two 30 hp Honda outboards and will be able to cruise at 20 knots. I will be in the water in about 4 weeks.
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Old 12-08-2007, 20:47   #4
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Narrow hull Cats aren't all that attractive to cruisers. Yes they're fast but as we all know they can't carry a significant load. Racing cats obviously benefit.

If one was to consider performance formulas on a cruising boat it would require an LOA > 50 feet, ie... a Gunboat.

Consider that at 16:1 a 32 foot cat would have hulls 2 ft wide. Even at 8:1 they'd only be 4 ft wide which is almost enough room for a single berth and a walkway to the head.

I'm a catamaran guy and love my boat. She's wide, comfortable, stable and predictable but shes not real fast and points about as well as a barge. I just read a post about a 16 knot PDQ???? I'm familar, as we sail with a couple of them around here. His boat is obviously much faster than the ones we sail with.
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Old 14-08-2007, 07:34   #5
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narrow hulls

re PDQ comments - The PDQ32 hulls are quite narrow and as result load capacity is limited but more than adequate for our current needs. The boat is actually a center cockpit layout with sleeping accomodations on the bridgedeck behind the cockpit. It does provide a lot of very usefull living area.
As far as performance goes I have not seen 16 knots but she was doing a respectable 13.5 on a broad reach in 20 knots of wind the other day. Not quite twice the speed of my old mono but close enough ......
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Old 15-08-2007, 19:56   #6
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I sailed a PDQ 32 down in St. Vincent and yes it can be very fast. I obtained over 14 knots twice also on a broad reach, once with a spinaker from a Kirby 30 which I borrowed. However, going to windward is the pits. Most of the time I ended up motorsailing which helped to improve pointing and cut down on the drift to leeward. It was a fun boat and very open .
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Old 15-08-2007, 20:17   #7
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PDQ32 to windward

When I first got my PDQ32 I was disappointed with windward ability however once I had the mainsail recut and stiffer battens added + I got to know the boat a lot better I find it points quite well - it sails nicely at 30 to 40 degrees apparent - it does its best in lighter air with relatively flat water - I do not find the need to use engine help to windward as a rule.
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