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Old 17-01-2010, 21:29   #1
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Displacement

I was just looking over some catamarans on yachtworld and noticed the displacement numbers for the following boats:

PDQ 36: 8,000 lbs.
Privilege 37: 14,500 lbs.
Manta 38: 13,000 lbs.

Can someone explain why the PDQ 36 is so much lighter than the other two, and if displacement numbers translate into purpose for the boat, i.e., lighter boats meant for coastal cruising; heavier boats for transocean voyages.
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Old 17-01-2010, 23:06   #2
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I think that the reason would have to be the beam. The PDQ is a small 17-18 compared to 21 and 22 of the other varieties. Also building materials could play a huge difference. I think that your idea about coastal cruising vs. blue water voyages could be valid.
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Old 18-01-2010, 09:21   #3
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Even though they are only a foot or two longer, I believe the Privilege and Manta are much roomier. The hull space necessary for larger accomodations results in more weight. I also have the impression that those two are more heavily constructed than the PDQ. That may or may not be necessary.
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Old 18-01-2010, 09:24   #4
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What's also curious is that the PDQ 32 is listed at 7200 lbs.; only 800 lbs lighter than the PDQ 36.
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Old 18-01-2010, 09:34   #5
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It is probably all in beam, fit-out and materials used.

Multis are supposed to be light. A light one is the fast one, unless you load it with your stuff. A heavy one is slow from the day it leaves the boatyard.

Good design, building and use of materials will result in a light, extremely strong and fast cat. Light equals weak only if the design, building or use of materials is/are off.

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Old 18-01-2010, 09:40   #6
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Heavy does not necessarily mean more seaworthy, especially with the high strength to weight materials we have now. ULDB's proved boats do not need to be heavy to be seaworthy.
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Old 18-01-2010, 11:22   #7
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Our Privilege 39 was 8000 Kg from the factory. That's over 16,000 lbs of boat. Privileges are heavy catamarans. Rigging is heavy. Masts are robust. Layup is heavy with a 15mm foam core.

Weight doesn't mean much to me. The quality of construction is paramount. I found only one construction error on our Privilege 39, and that was the small size of the backing plate on a genoa winch. I corrected it with a larger backing plate.

If you worship at the altar of speed, then pay close attention to the weight of the catamaran. I wanted a robust catamaran that would sail 150 miles a day, and it didn't matter to me that my Privilege came out of the factory at 8000kg.
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Old 18-01-2010, 11:25   #8
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Quote:
Originally Posted by snort View Post
I was just looking over some catamarans on yachtworld and noticed the displacement numbers for the following boats:

PDQ 36: 8,000 lbs.
Privilege 37: 14,500 lbs.
Manta 38: 13,000 lbs.

Can someone explain why the PDQ 36 is so much lighter than the other two, and if displacement numbers translate into purpose for the boat, i.e., lighter boats meant for coastal cruising; heavier boats for transocean voyages.
There is probably some truth in all the above responses. However, take most displacement numbers with a grain of salt. I believe the EU now requires a somewhat precisely defined displacement number be disclosed for new boats but that is only very recently.

Besides displacement numbers often being just wishful goals of the designer there are also several definitions, e.g. light, cruising, maximum etc.
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Old 18-01-2010, 12:41   #9
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Only trust the displacement of a boat hanging on a calibrated load cell empty , no water or diesel present and the differences between the brochure or planned displacements and the actual displacements can be a lot.
Often a designer plans a boat to be a certain displacement but the actual builder can make that boat a lot heavier than its design weight. It is a continues battle to keep the weight of a boat under control.
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Old 18-01-2010, 14:39   #10
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Also, the PDQ 36 is typically powered by twin light weight outboards such as Yamaha 9.9s. The diesel inboards found on Privilege and Manta cats are considerably heavier. But even taking that into account the PDQ is still significantly lighter.
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Old 18-01-2010, 21:12   #11
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Only trust the displacement of a boat hanging on a calibrated load cell empty , no water or diesel present and the differences between the brochure or planned displacements and the actual displacements can be a lot.
Often a designer plans a boat to be a certain displacement but the actual builder can make that boat a lot heavier than its design weight. It is a continues battle to keep the weight of a boat under control.
Absolutely! I heard about one cat that had a published weight of under 12,000 pounds, but the actual early production was closer to 18,000 empty dry weight. If performance is important to you and you're buying new, make your contract contingent on getting the boat weighed and meeting spec.
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Old 18-01-2010, 22:03   #12
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Quote:
and if displacement numbers translate into purpose for the boat
It is one indication of how much crap you can load and carry. Don't expect a light boat to carry a large load. Aside from that it's still only one dimension to a boat. It's easy to focus on one dimension as if it is the most important. If you choose to exceed the displacement then you have a show stopper of a problem. Displacement is measured empty with anything not nailed down removed.
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Old 18-01-2010, 22:15   #13
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Its too bad yachts do not come with stability and trim tables, like ships do. Then we would know the displacement by where the waterline is at the bow and the stern. Its a pretty easy calculation to make with the right info. Any idea why yacht manufacturers do not provide this data?
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Old 18-01-2010, 22:42   #14
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I would think David that the TPCI (Tonnes per centimeter Immersion) Table is too imprecise for small craft of a sailing shape
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Old 18-01-2010, 23:27   #15
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weight should be standard in any building contract , not only length , width , materials used tank sizes etc should be ion the contract but also a target weight that can vary no more than 5 %, if this target weight is not met a fine clause should be used for having more weight .
This target weight should be the bare boat plus any extra's ordered

Please keep in mind that the contract weight is excluding any options or accessory's and i have seen a weight being added in excess of 2200 kilo or 5000 Lbs in extra's

David , if going by the immersion rate and the exact measuring of the side of the boat, mistakes in excess of 500 kilo,s can be made on a 45 ft cat,I have found out it is not precise enough, measuring the immersion rate showed a weight of 8600 kilo for the last launched boat and the actual weight is 7940 on a calibrated load cell. that is a difference of almost 10 %
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