Discussion: Pocket Cruising Power Catamaran Design for Recession Times...

They are relatively lightly built compared to some, but no lighter than their equivelent sized sailing multi's, that weather heavy weather just fine.

 

Note the high bridgedeck tunnel clearance?

This is why I said I have my doubts you would get a queen sized bed and the extras into a 25 ft hull and still maintain good seakeeping characteristics.

The height and windage would be getting out there too much for short waterline length

 

Australia has had arguably the best small waterline length powercat hull forms for many years, we have had Bruce Harris Shark Cats here for possibly 30 years used by the Coastguard and I am not suggesting they are the be all and end all of underwatershapes but the basic vessel shape is the same and similar to the pic above.

In 30 years they have never managed to lever a queensize bed (1500x2000mm) into one and still have a usable boat


IMHO, 30ft is about the minimum

Noosa cat is another Oz version

http://www.noosacat.com.au/2600.html

 

24 degrees at the transom is clearly wrong as the planning area towards the transom flattens considerably...

She does around 3 miles a gallon at 25 knots, according to an owner I contacted...

The beam of the cat under consideration is 10' and it will bank in turn...

I'm only interested in the hull form: it'll be a totally different boat that can have the same sort of accommodation as the Cape Cruiser shown in the previous post BUT not that of a 950 VT reduced to 750 in length...

Look at the 25' C- Dory Tomcat as evidence...

http://www.c-dory.com/boats/tomcat_255/cabin_photos.html

 

I'm pretty sure that Shark Cats have always had symetrical hulls. They're also no better on fuel that an equivalent monohull. Their great advantage is that they - like most cats - are a stable (if somewhat uncomfortable) work platform whilst at rest.

The particular cat that I'm referring to reputedly had 400gsm of glass over the duflex stripped hull. Whether he designed it that way I have no idea, but as far as I'm concerned that kind of skin thickness is more suited to a canoe than a 40 ft powercat.

The TomCat has insufficient bridgedeck clearance IMHO. Like Sabahcat, I can't see where you're going to fit all of your accomodations into a 25ft LOA and still provide bridgedeck clearance.

 

Yep, all had big HP to get speed

That is light, but, I am of the thicker core lighter skin crowd to some degree, I had 10mm wrc and 400gsm glass on my last sailing cat and it got flogged pretty hard and did a lot of miles with no sign of fatigue.(high speed and forces from rig) and bounced off a lot of things with no real damage.

Given and Tenant have many 40 ft racing machines (sail) that would be 20 year old plus built from 12 mm or less (I actually think 8mm) wrc and 200gsm uni (high speed and forces from rig)

I have a mate who has been cruising the last couple of years in a 37 ft powercat of foam and 600gsm triax and 37 hp diesels (low speed no rig forces)

I am doing my 50 footer from 16mm timber core and 600gsm DB, but again low speed and no rig forces.

Those that are doing high speed powercats with light layups could very well find issues if PUNCHING to weather without reducing speed for the seastate, but I reckon if they drop back accordingly and take the waves at an angle they would/should be ok.

Personally, I would not be happy doing 17 plus knots in a light powerboat of any size

These are the compromises that have to be made when wanting something relatively economical to build and run and have accomidation IMHO

Easy to build something heavier and bulletproof, but a what cost to load carrying capability, fuel, HP and ultimately $$

Also, what sort of speed does the tomcat achieve with a pair of 60's or 90's strapped on the back?

Nothing remotely like 25 knots I would think

 

Bloody hell! They are all very light....
The mono that I posted a while back features some pretty conservative scantlings as it is designed to be trailerable. It has a strip plank hull (Kirri) with a calculated 1470 gsm DB on the outside and about a 1000gsm on the inside, with no internal framing other than bulkheads and the main engine beds. These work out to be somewhat heavier than if I had used ABS rules, but not by that much. 1st principles will yield lighter scantlings again, but durability has to come into the equation as well IMHO. With a soft core as both WRC and Kirri are and 400gsm skins, I could put a dent in the boat with a good thump of my hand..... what happens when you drop a spanner on the deck?....
I have the scantlings calcs written by Gerr, originally for the manufacturers of Durakore, for both mono and multi's. If I get a chance I'll run a few basic numbers and see what I come up with. Once again, they'd be conservative, but not outrageously so.....

 

Agree that a trailerable boat needs a bit of extra guts for trailering load both in retrieval and bouncing down the road.

As far as taking a knock, that first one I had with the 10mm WRC during her lifetime she

Hit a 40ft steely during a race , cosmetic paint damage to bow
Hit a reef at 14 knots enough to park the boat and lift that hull out of the water, minor damage to board and 12mm crease in outer hull skin on trailing edge of case
Hit a rock doing 6 knots motoring through a river in North Qld, 50mm crease in outer skin, did break the skin, no core damage
Countless biffs and belts into jetties and reefie bits over the years at walking pace, no damage apart from scratched paint and antifoul.


Over the years I have been involved with a series of destructive drop tests in the past for survey requirements.

4x2 hardwood was used for 1'st drop which failed on 1'st drop

One of the panels I made was from my first boat, 10mm wrc 400gsm DB and epoxy (not for survey)

Foam panels for survey had multiple layers of biax, woven rovings, chopies and polyester getting up to around the 1800gsm from memory

Others were balsa with similar layups in both Poly and Vinyl.

These panels did take more hits to failure than the 10mm epoxy one of mine, but it was something like 4 hits to break back skin 7 hits to full failure compared to 9 hits to full failure on the foam and balsa.

The foam did absorb the hit better than the balsa

Both of these balsa and foam panels were considerably heavier and more costly then the WRC panel

Tests done recently for my own benefit , not as accurately as the test above, included One 15mm Balsa 600gsm biax panel from a previous build (furniture) that had a drop test done on it had a rather neat hole the same size as the weight punched through it after 4 drops

My 16mm Kiri 600gsm DB took 6 hits to break the back skin but would have only let a trickle of water through.
Complete failure was on drop 8

I put it down to there being more longitudinal strength/stiffness in strip planking compared to the lack of longitudinal strength/stiffness of balsa and foam. For example, I , at a guess, have say 300gsm of unidirectional strength running for and aft in planking and the DB gives another 600 in +45 and -45

Where as balsa and foam lack the for and aft strength, or any other strength, so the 600gsm Biax is trying to do all jobs, so, IMHO, they probably need something like 1000gsm triax to compete, at a guess

I have the results for those first tests somewhere as they were published in a magazine, I will attempt to dig them out and scan them when I get a new printer/scanner.
They arent really anything scientific, just observations on panel performance of one type of hit

 

Hmm - interesting.. it'd be interesting to know what others think on the subject, but it's certainly difficult to argue with your experience....

I don't want to hijack the thread, but what methods were used for your scantling calcs? And I note that you're a fellow Kirri convert... it seems to be a far better core material than wrc to me....

There's a thread about both kirri and skin thicknesses around here somewhere.... perhaps they'd be a more appropriate place to continue the discussion....

 

Here's the skin thickness one...

http://www.boatdesign.net/forums/boat-design/composite-panels-bulkheads-decks-etc-20491.html

 

And here's the Kirri one...

http://www.boatdesign.net/forums/materials/kiri-properties-19012.html

 

For comparison my project, a 30' mono motorsailer, has 1500grm outside 20mm strip planked core and 1200 gsm inside. Calcs made with Gerr's scantlings..

 

Thanks for those links Will,

Where did I get the scantlings for mine? Cheated a bit and just made observations over the years of what similar non-survey, non-production vessels were using and went the same with arguably a bit more localised reinforcing in strategic areas. As you would be aware, furniture, benches shelves floors add greatly to the stiffening/strength of a panel, so that has to be taken into account as well.

 

Big lump of lead trying to escape from the bottom of the boat would add a bit of extra stress/load that a Multi wouldnt have.

As an example the 40 ft Crowther supershockwave in the pic is 20mm foam and 200gsm kevlar in and out with 400gsm biax for abrasion resistance on the outside.

This is a cruising boat (a very fast cruising boat) and has been getting around for close on 12 years now, others with similar layups have been getting around for a lot longer

 

The cat that i posted pics of was built by one of our "local" boatyards Coplan Boats. The boat is hand laid GRP polyester with balsa core stiffening. Essentially it is a one off design, semi-dispplacement with relatively large flat planning areas at the sterns which gives extra flotation. The corrected top speed for the boat is more like 20 knots WOT. My friend owns the moulds and is busy modifying them. The next build will be longer and wider.

However at under 10 knots the boat is a absolute gem and the 50 E-tec's are superb.

I am going to the boat for the weekend of 30 Jan 09 and will take lots of pics and post them with decriptions, and i will get more technical info and figures then