42ft Ply / Epoxy Cat

[ausnp84]

Hi guys,


Looking at a cat in Amsteram and wanted the group's thoughts on it...
https://www.boat24.com/en/sailboats/...detail/436014/


12.8m long, 6.4m wide, draws 1.2m, reasonable kit list, needs a smidge of work but that's not a big deal.


It also has thin hulls and quite a wide, flat bridge deck (think FP Louisiane) but it does have 60cm of clearance and has been thoroughly tested in the North Sea (which isn't exactly known for being smooth water sailing).


My big concern is it only weighs 2t (2,000kg). To me that seems waaay too light for a large cat - am I overthinking this or is there some merit in a cat needing a certain displacement?


n

[tkeithlu]

Looks like a lot of boat for 30K Euros. Have fun with it.

Cats use beam where monos use weighted keels. That means cats have more immediate righting moment at zero degrees (upright) but zero at 90 degrees and back to their immediate moment at 180 degrees. In other words, on their side they can go either way and anti-fouling up they are darned difficult to turn back over.

OK, enough pontificating. As long as you keep it to less than 90 degrees, variation in displacement really should not matter in a cat. If this one has been bouncing around the North Sea and you are not taking it where cats should not go, have at it. Just don't knock it down.

Cleat the sheets close-hauled and turn broadside, and you'll learn a bunch about ultimate righting moment very quickly.

[ausnp84]

Originally Posted by tkeithlu

Looks like a lot of boat for 30K Euros. Have fun with it.

Cats use beam where monos use weighted keels. That means cats have more immediate righting moment at zero degrees (upright) but zero at 90 degrees and back to their immediate moment at 180 degrees. In other words, on their side they can go either way and anti-fouling up they are darned difficult to turn back over.

OK, enough pontificating. As long as you keep it to less than 90 degrees, variation in displacement really should not matter in a cat. If this one has been bouncing around the North Sea and you are not taking it where cats should not go, have at it. Just don't knock it down.

Cleat the sheets close-hauled and turn broadside, and you'll learn a bunch about ultimate righting moment very quickly.

Haha thanks. This isn't my first time at the catamaran rodeo - sadly the last one was written off by an errant fishing trawler.


I guess my concern is that lightweight means lighter built and does that mean more prone to failure? Sure, race boats are super lightweight and use carbon this and that, but this is ply & epoxy... would that be lighter than a similar size GRP cat?


n

[tkeithlu]

Yep. Fiberglass resin has a density of about 1.5 relative to water.

I apologize for going flaky on you.

I sailed on a 26 ft plywood and fiberglass cat in the 1960s, and knew another one pretty well. I'm sure that they have rotted away by now, and I guess that's something for you to watch, but strength was not weight. Strength was adequate framing and stringers for the flexibility of the building material. Therefore, you could have a plywood boat much stronger than needed, or one that flexed on every wave. How far apart are the frames? how far apart the stringers? How rigid are the panels in between when you step on them? How well is the stress on one hull shared to the other? What does your surveyor say about the structural strength? Is the standing rigging such that it transfers the stress well?

You'd ask that of any boat, and I'm sure that you will.

Again, my best to your endeavour.

[Franziska]

Looks like a Peter Spronk design. Or at least inspired by his designs.
Interesting designs, especially for the time.

[valhalla360]

I'm betting it weighs a lot more than 2 ton. From the pictures, it doesn't look like they used a lot of high tech weight saving measures, so unless they went ultra thin on everything, I'm not sure how they could get it down that low.

Our 34ft cat had a brochure weight of 4 ton. An a couple of travel lifts we used had scales built in and the operators told us 5 tons loaded.

Weight does have an impact. For the sails to flip the boat, they need to create more overturning moment than the hulls can resist.

If we assume the numbers are correct, maximum righting moment will be around:
(6.4/2)m * 2t = 6.4 m-t of righting moment.

For comparison, our much smaller cat with only a 3.6m beam
(3.6/2)m * 5t = 9 m-t of righting moment (almost 50% higher with a narrower beam).

If you get into light weight racing cats with tall rigs, the assumption is the crew is constantly on guard ready to spill the sails before she goes over. On a cruising cat, you want a more conservative rig, so if you look away for a minute, she doesn't go over.

But again, I'm not buying the 2 ton figure.

[boatman61]

Its still a bit on the low side, for open water sailing around 30in or 76cm is best for that size..
Minimise the slamming as much as possible.

[ausnp84]

Originally Posted by valhalla360

I'm betting it weighs a lot more than 2 ton. From the pictures, it doesn't look like they used a lot of high tech weight saving measures, so unless they went ultra thin on everything, I'm not sure how they could get it down that low.

Our 34ft cat had a brochure weight of 4 ton. An a couple of travel lifts we used had scales built in and the operators told us 5 tons loaded.

Weight does have an impact. For the sails to flip the boat, they need to create more overturning moment than the hulls can resist.

If we assume the numbers are correct, maximum righting moment will be around:
(6.4/2)m * 2t = 6.4 m-t of righting moment.

For comparison, our much smaller cat with only a 3.6m beam
(3.6/2)m * 5t = 9 m-t of righting moment (almost 50% higher with a narrower beam).

If you get into light weight racing cats with tall rigs, the assumption is the crew is constantly on guard ready to spill the sails before she goes over. On a cruising cat, you want a more conservative rig, so if you look away for a minute, she doesn't go over.

But again, I'm not buying the 2 ton figure.

I questioned the seller and he’s sticking with the 2t lightship figure... to me that just seems too light and from looking at more pictures, the hull doesn’t seem to have much material on it.

Thanks for confirming what I was thinking tho... my last cat was heavily overbuilt and this one seems quite underbuilt. Just need to find a good middle ground now!

N

[catsketcher]

You can do a quick calculation of weight using a tape measure and some graph paper.

Get inside the boat and measure waterline beam and hull depth at lengths along the boat. Work out an area at each measurement point. (Often this is a section of a circle). Then plot each area as a point along a baseline on the graph using the position along the hull. The area under the graph is the volume of the hull.

For me I am dubious about the weight, if it is built of 8 or 9mm ply it is going to be 3-4 tonnes. If it is 5mm ply it could be less. I think the rig looks a bit underdone on this boat. Also one thing I always see people forget is the sinkage. Check whether the hull has lots of flotation left so that you can add at least 1000kg of stuff when you go cruising. You can calculate the immersion rate by working out the waterline area - just measure the beam at the waterline along the length and work out the area of the waterplane - that graph paper is good stuff. Or you can just use a prismatic coefficient around 0.7 -0.6. LWL x waterline beam of hull x 0.7 should give a rough figure. For this boat say 12 x 1 x 0.6 = 7.2m^2 for each hull. So it would take 72kg to push each hull down 1cm. Work out how much load you need to put on the boat and then use the immersion rate to work out how far down she will go.

Also - how stiff is the deck - there are no stringers. Is she honeycomb on the deck?

cheers

Phil

[ausnp84]

Originally Posted by catsketcher

Also - how stiff is the deck - there are no stringers. Is she honeycomb on the deck?

cheers

Phil

There are fore and aft stringers beneath the bridgedeck - seller emailed me more pics last night.

N

[ausnp84]

And one more...

[Sos]

Looking at the advert pictures I must admit I thought 2 tonnes was a bit light but looking at the latest pictures she has very narrow hulls and with the outboard and stuff all off maybe 2 tonnes is correct? The owner should know since the mobile crane could tell the weight when lifting.
Not sure if it is the perspective of the pictures but that does look like a modest sail plan which will help if cruising. Mind those narrow hulls will get overloaded with too much stuff on board

[ausnp84]

Originally Posted by Sos

Looking at the advert pictures I must admit I thought 2 tonnes was a bit light but looking at the latest pictures she has very narrow hulls and with the outboard and stuff all off maybe 2 tonnes is correct? The owner should know since the mobile crane could tell the weight when lifting.
Not sure if it is the perspective of the pictures but that does look like a modest sail plan which will help if cruising. Mind those narrow hulls will get overloaded with too much stuff on board

Yep. And then add in 300kg of water and various bits and bobs... probably looking at 3-3.5t fully loaded. I questioned the owner on the modest sail plan and he said it was indeed, and that they’d comfortably had 15kts out of the boat.

So this all brings me back to the original question - does the 2t lightship figure indicate the boat has been lightly built? Just thinking about materials - an 8x4 sheet of 12mm ply weighs about 20kg. The surface area of the deck alone is 775sqf, which badly equates to 25 sheets... so that’s 500kg alone. Then there’s the hull sides, bottom of the bridgedeck, etc etc - gotta be up to 1500-1800kg at that point. Now add in wiring, plumbing, steering, perspex windows, the outboard, stringers, plus the epoxy to glue it all together, and I cannot for the life of me work out how it only weighs 2t.

Unless of course it’s been underbuilt in lightweight ply....

N

[PineyWoodsPete]

From what I can see the hulls, at least, look clearly moulded - not a multi-chine plywood design. And the specs show "plastic" construction - no mention of wood. Maybe decks and house, though?

My FP Antigua 37 w/ moulded monocoque constuction, lightweight interior mainly FG, large cockpit came in at 5 tons. 42x21' w/ large deckhouse/small cockpit - no way two short or long tons...

[ausnp84]

Thanks for your thoughts guys. An FP Louisiane 37 has come up for a similar price so I’m going to go have a look at that instead and leave this as a plan B... it’s a good boat, but the FP is proven blue water whereas this has a lot of questions surrounding it.

N