Next Fat Cat Project

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Deleted member 478

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This cruising life is all very well, except for the fact that the brain goes mushy after a while. So, I have decided it's time to start designing a new boat. This is how far I've got.
Length 17m (55ft 6in)
Beam 8 (26ft 3in)
Draft (dagger boards up) 0.50m (1ft 7in)
Draft (dagger boards down) (not known yet)
Hull Ratio 16:1
Estimated dry weight 8 metric tons
Standing Headroom 2m (6ft 6in)
Bridgedeck clearance 1.1m (3ft 7in)
Fresh water 200 litres
Diesel 600 litres
Watermaker 160 l/h
Electric motors 1 x 12Kva, 1 x 6 Kva (only used for manoeuvring)
Genset to run motors directly 20Kva
Bridgedeck Helm position
Carbon Mast

Design Brief:
Light, fast, cruising catamaran built in epoxy coated cedar strip.
Easy for an average handyman to home-build.
No doors on cupboards - all cave lockers (keeps cost, weight and build time down)
Sugar scoop access to port aft hull for storage of kayaks/dive gear etc

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200 litres of water doesn't sound very much for a friendly crew and cruise.


I like the idea of a genny driving the propulsion motors. Will you go for normal propellors or a water jet ? Not the Hamilton type but the 'ring main' one.

Standard sails, or a large power kite instead ?
 
I like the propulsion by electric motors driven by a generator, but dont you need two equal motors, one in each hull? My other thought is that is very big for anyone to home build, and also too big for road transport after the build. It might be a better project if it will scale down a bit.
 
One midships motor is fine with a smaller one for manoeuvring. Though originally Heavenly Twins diesel powered cats were supplied with an engine in just one hull and surprisingly produced almost no turning movement. What makes a catamaran turn on a 5p piece is two hull motors, one in astern, one in ahead, you have to see it to believe it.
 
ooh, this sounds fun!

When?
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Once I've mastered the CAD and the sums needed calculate boatie type of stuff. What you see took about a week, while someone that can use CAD would take about 20 minutes at the most to get that far. So this could take a while.
200 litres of water doesn't sound very much for a friendly crew and cruise.
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Am planning on a 160 litre per hour watermaker run off the generator.
I like the idea of a genny driving the propulsion motors. Will you go for normal propellors or a water jet ? Not the Hamilton type but the 'ring main' one.

Standard sails, or a large power kite instead ?
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high/low pitch (not sure of the correct term) for slow RPM propellers and standard sails - fancy sails cost too much
What CAD package are you using? I fancy having a go at this!
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Rhino3d
... but dont you need two equal motors, one in each hull?
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Underway, you only use one motor - even with diesel engines. This reduces the size requirement of the genset and hence the cost and weight. The second motor would only be used for manoeuvring in or out of a berth or in an emergency.
My other thought is that is very big for anyone to home build, and also too big for road transport after the build. It might be a better project if it will scale down a bit.
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sort of agree. My present catamaran is seriously overloaded with toys. The new one will have the same number of toys and being bigger, will have the huge carrying capacity. In terms of scaling down, I am working with a naval architect on this and there is a vague plan for him to do several models that would suit all pockets - much like Wharram. I will probably employ a team of about ten skilled labourers, so the project will only take six months to build. If I did it alone, it would take about six years. The rent saved over five and half years pays for the skilled labourers and they will definitely do a better job than me.
 
Muddy Paws said:
One midships motor is fine with a smaller one for manoeuvring. Though originally Heavenly Twins diesel powered cats were supplied with an engine in just one hull and surprisingly produced almost no turning movement. What makes a catamaran turn on a 5p piece is two hull motors, one in astern, one in ahead, you have to see it to believe it.
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A single motor won't work - the bridgedeck clearance is 1.1 metres min.
 
i think

200litre tank is still teeny. Bigger tank - that way you can fill up when water is free or just run it half-empty. 200litres means running the dang generator and watermaker far too often - some places you won't want to run the watermaker al the time, too. I have 500 which is ok.

I am scaredy of the hybrid thingy, which i think you are doing? Means when the generator is knackered - that's it! You can't even drive to somewhere that can fix things.

Identical engines are loads cheaper/easier for spares? Just don't have turbos. Is the hybrid idea cos you had a sh!t time with those alternators?
 
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Gerry said:
How about designing it on a collective basis?
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No way. There is only one thing that is almost as inefficient as a Labour Government and that is a committee. The likelihood of it being successful is about as likely as the good folk from this fine parish ALL agreeing on WMD, ColRegs, Anchors and MMGW at the same time.

Saying that, all suggestions are appreciated.
 
tcm said:
200litre tank is still teeny.
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I sort of agree, but being like I am, the water tanks will always be full so I will always be carrying an extra 400kg if they are 600 litre tanks.
I am scaredy of the hybrid thingy, which i think you are doing? Means when the generator is knackered - that's it! You can't even drive to somewhere that can fix things.
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Just like a monomaran then?
Identical engines are loads cheaper/easier for spares? Just don't have turbos. Is the hybrid idea cos you had a sh!t time with those alternators?
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No major spares for the electric motors I am looking at (only bearings).

This gets complicated because there are so many factors:
- a Genset should run at about 80% of it's capacity for most of the time.
- Running two motors does not give a huge increase in speed in most conditions (yes I know, this is a very dangerous generalisation).
- Running both motors at a decent rpm means I need a bigger Genset.
- The smaller 2nd motor is only used for manoeuvring which is seldom done at high revs, so you can get away with a smaller GenSet, which saves weight and initial cost and cost of spares down the track.
 
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p.s.

tcm said:
... when the generator is knackered - that's it! You can't even drive to somewhere that can fix things.
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In fact, it's not quite as bad as being in a diesel powered monomaran. At least I could switch off all the demanding stuff like the freezer, autohelm etc and run one electric motor from the house batteries. Probably not for long, but certainly longer than not having an engine at all.

I am sure someone that knows electrickery can do the sums for me:
- 9kW electric motor @ 144 volts
- 600Ahr of 12 volt batteries which I'd like to assume are at 90% of their capacity.
How long can I motor for at say 60% power? Thanks.
 
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stingo said:
No way. There is only one thing that is almost as inefficient as a Labour Government and that is a committee. The likelihood of it being successful is about as likely as the good folk from this fine parish ALL agreeing on WMD, ColRegs, Anchors and MMGW at the same time.

Saying that, all suggestions are appreciated.
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Maybe I didn't make the concept clear! The idea behind localmotors is that they are built collaboratively http://www.local-motors.com/howItWorks.php

So rather than simply making a one off design you would have a product that appeals to a wider market and you could leverage the work and thoughts that you harness, call it advice you receive if you like.

It's designing by 'open-source'.

I understand that it may not be for you but maybe it's an idea that the boat industry could pursue??

www.gerryantics.blogspot.com
 
You can buy a set of plans off the peg for a boat like that. Just contact Derek Kelsall in New Zealand. It would have to be built in foam sandwich though but as that's easier and faster than cedar strip I don't think that would be too much of a departure.

8 tonnes is very light for a 55 footer. It could be done in foam sandwich but would need everything stripped to the bone.

It's possible to use a single diesel with hydraulic or electric drive to two small propulsion units in the hulls but I prefer twin engines as you have a spare when one packs up.

Just 200 litres of water could be a problem for a lengthy passage (over a week) if the watermaker packs up. They are pretty unreliable items.
 
8 tons is the dry weight. Add 600 litres of fuel (500kg), several sets of dive gear, a kayak, windsurfer, bicycles, several cases of beer, provisions, a create or two of pilot books bla bla bla and suddenly you're at 12 ton.

On long passages, I'd stock up on fresh water jerry cans just in case the watermaker has a tizzy-fit.
 
I would caution against Cedar Strip construction. I owned a Roberts 395 built using Cedar Strip and epoxy and despite the boat being well constructed and only 10 years old when I bought it, it suffered from rot, keeping on top of the maintenance was like painting the Forth Bridge. I much prefer the idea of C-Flex or foam sandwich construction - easier construction process too I imagine and probably a similiar weight.

Cheers, Brian.
 
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