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Sand crab · 01-10-2011, 16:51

Quote:

Originally Posted by hummingway

People have sailed oceans in 18 foot monos. Has anyone done it in a Hobey Cat yet? There is something to this even it makes some of the multi owners insecure.

These frenchmen that did a crossing in a 20" cat?
http://yachtpals.com/sailing-catamaran-7024
Hey people have rowed across the Atlantic but it doesn't mean I want to do it. This whole mono versus multi is pointless. They both have good and bad. I like cats because they don't heel, period. Or heel very little. But to each their own. BOB

**hummingway** · 01-10-2011, 16:57

Quote:

Originally Posted by Sand crab

These frenchmen that did a crossing in a 20" cat?
Crossing the Atlantic on a Beach Cat - News and Sailing Video | YachtPals.com
Hey people have rowed across the Atlantic but it doesn't mean I want to do it. This whole mono versus multi is pointless. They both have good and bad. I like cats because they don't heel, period. Or heel very little. But to each their own. BOB

I agree but mono v. multi is pointless but the original question wasn't although it was immediately buried.

Jimbo485 · 01-10-2011, 18:04

The OP already said he wants a cat (not a dog, nor half a boat). However it is very difficult to resist taking the piss once someone starts up the old debate.

Ok, so his original question is about length, not the number of hulls.

A really general answer would be:
20´ is doable but too uncomfortable, difficult and dangerous
30´ is doable and cheap but probably only sitting headroom in main cabin
40´ is a decent compromise for a tradewind RTW
50´ is better but more expensive
60´ is even better but much more expensive. Ring Alwoplast in Valdivia, Chile, and ask about the Atlantic 57. US$1.6M, but the US$ is going down the tubes, so maybe you can afford it this year, but not next year. If so, go for it! A true world cruiser.

It is difficult to give a better answer due to the sheer number of variables involved, which all seem to be interrelated - overall beam, individual hull waterline beam, waterline L:B ratio, b/deck clearance and length, inboards/outboards, single/twin engines, daggerboards/fixed keels, etc etc. The OP must do some more research on his own and then ask a more specific question.

Most of my cruising since I was a kid was on steel monos, so I spent years looking at this feline stuff and came down to the following, which ONLY APPLIES TO US:

Minimum waterline L:B ratio of 12
Minimum length of 40´
Twin outboards
Minimum 3´ b/deck clearance
Dagger boards and kick up rudders
Galley-up
Aluminium
A-frame rig with r/f main and jibs

But hey that doesn´t exist unless we build it ourselves!

So, since our time on this planet is too short for building, and considering that used boats are cheap, we looked around and decided we better make a compromise. So then we looked at Outremers, Kelsalls, Catanas, Atlantics, Wharrams, FPs. Ended up with:

correct L:B ratio
correct length
correct engine setup
incorrect b/deck clearance
dagger boards but no kick-up rudders
galley-up
incorrect hull material (FG, not alum)
rotating mast, not A-frame

So you take what you can get. Such is life!

The most important things are to:

Minimise your time in the rat race
Minimise your time in a boatyard
Maximise your time cruising

Victory49 · 01-10-2011, 18:04

The Tank study on capsize determined that a wave 55 percent of the length of the boat hitting broadside can cause capsize, as i recall From my understanding that rule applied regardless of beam.

I did not mean to start the whole mono vs multi debate- to each their own!

I dont believe either is safer than the other, anyone can cite examples for each case. But most examples of bad monohull disasters are nearly always lightweight and beamy racing boats, most similar to cats in design.

Design, skill, construction, and a little luck all factor in. There are some unsafe cats put there as well as many unsafe monohulls.

**hummingway** · 01-10-2011, 18:38

Quote:

Originally Posted by Jimbo485

Twin outboards

I'm curious about your preference for outboards Jimbo. Can you explain?

**estarzinger** · 01-10-2011, 18:53

Quote:

Originally Posted by owengg

What size catamaran is needed for a circumnavigation? I've read of the dangers of pitch polling in catamarans. From what I've read a circumnavigation shouldn't even be attempted in a boat under 40 ft.

Why are monohulls under 40 feet considered safer? The physics don't make much sense to me.

This is a complex question. The Wolfson unit University of Southampton is the leading authority, and their most readable report is here. There are several other useful/interesting stability reports on the same site.

But in short . . . there is no hard cut-off size for safety. There have been many long passages safely completed on 30' multi's and we know of at least three done on 18'er. But the smaller the vessel the more the skipper needs to look after the vessel. These small multi-hulls were ground zero for the early para-anchor experiments and development in order to stabilize the vessel in storm situations.

The logic for smaller mono's to be viewed as 'safer' is: the probability of a capsize increases in small vessels (both monos and multis) and with a small mono it is a recoverable event while with a multi it is (generally) not. In very small vessels the risk of capsize statistically overwhelms the 'hole in the hull' sinking risks (Where the multi is generally arguably 'safer' than the mono). A secondary factor is that it's also the case that it is (generally) harder to carry all the appropriate 'safety' equipment (ground tackle, fresh water stores in case of dismasting, tools and repair parts, etc) on a small multi vs a small mono.

Jimbo485 · 01-10-2011, 19:11

Hummingway, the following applies only to us. Conventional wisdom says have an inboard, but outboards have come a long way...

PROS

No thru hulls for shaft, nor cooling water, nor exhaust. (Simple. Safe. Cheap.)
Easy removal for maintenance either in the cockpit or ashore. We have done both. (Also simple, cheap and sooo convenient.)
When sailing, we kick up both outboards and remove our "sailing handbrake". (Performance.)
They do not take away from below decks volume. Nor pollute the below decks atmosphere, like engines beneath the aft bunks. (Comfort.)
The 3rd world fishing fleet operates with outboards. (Spare parts. Mechanics.)

CONS

Take away volume from cockpit lockers. (Not significant, but should be considered.)
More exposed to the elements than inboards. (Use that WD40, mate!)
Props can suck down some air and cavitate in extreme conditions. (Minimal but it depends on outboard length and longitudinal position - forward and deep is obviously better.)
Easy theft.

Ian McD · 01-10-2011, 19:31

Length would not be my main worry. Ultimate stability is the trick. I have talked to many sailors who have sailed offshore for years and never come across the conditions I had off the Oregon coast a couple of months ago. Luck and planning has a lot to do with it but in conditions where ultimate stability matters, mono hulls do have an advantage. This is simple physics. However this doesn't answer your question. In short, bigger is always better but its how you use it that matters most. I have a good 40 foot mono and have on two very memorable occasions been well over 90 degrees, mast in water which would be a real problem in a cat. Whether a cat would take a lot more to get into that situation I really don't know. I would love to have the room a cat provides but can't seem to accept the stability issues.

Ian

smj · 01-10-2011, 20:29

Quote:

Originally Posted by Ian McD

Length would not be my main worry. Ultimate stability is the trick. I have talked to many sailors who have sailed offshore for years and never come across the conditions I had off the Oregon coast a couple of months ago. Luck and planning has a lot to do with it but in conditions where ultimate stability matters, mono hulls do have an advantage. This is simple physics. However this doesn't answer your question. In short, bigger is always better but its how you use it that matters most. I have a good 40 foot mono and have on two very memorable occasions been well over 90 degrees, mast in water which would be a real problem in a cat. Whether a cat would take a lot more to get into that situation I really don't know. I would love to have the room a cat provides but can't seem to accept the stability issues.

Ian

You've had your mono beyond 90 degrees a couple of times and your worried about a catamarans stability? Yes it would take a lot more force to take a cat to that point, but once at that point on a cat there probably isn't a lot of coming back.

Jimbo485 · 01-10-2011, 20:35

Check out Chris White´s book. It is an excellent read for the OP.

He states that capsize resistance (roll moment of inertia) is proportional to the cube of size. So doubling your length from 25´ to 50´ results in 8 times the capsize resistance. There are a few assumptions in there, so read it yourself and then come back here.

dennisail · 01-10-2011, 21:01

Quote:

The logic for smaller mono's to be viewed as 'safer' is: the probability of a capsize increases in small vessels (both monos and multis) and with a small mono it is a recoverable event while with a multi it is (generally) not.

Quote:

capsize resistance (roll moment of inertia) is proportional to the cube of size. So doubling your length from 25´ to 50´ results in 8 times the capsize resistance. There are a few assumptions in there, so read it yourself and then come back here.

Once you get small enough, both boats will have a decent risk of capsizing in a storm. Is anyone really going to refute this? If not, logic follows that there will be a point when downsizing that a mono will be safer as stated. (assuming an upturned cat is less safe than a mono that has capsized and righted without filling with water since the skipper was not sailing in a storm with the companion way doors unsecured)

Also scaling laws are full of cubed and squared parameters. The smaller you make a boat, the stronger everything can be (in comparison to the loads). http://www.av8n.com/physics/scaling.htm#sec-strength That's why when you see small boats/dingys capsize they hardly ever loose there masts, but large yachts almost always loose theirs. A small strong mono may capsize, but it has a good chance of coming up with the mast if all the gear is in good condition.

Once you get up to a certain size cat the chance of capsizing becomes so small the risk becomes minor compared to other dangers. I have no idea at what size these matters come into play, but I am certain they do. But some cat guys will continue to argue cats are safer no matter what, and some mono guys will continue to say cats are unsafe no matter what, despite any logic presented.

I would feel totally safe in a 40ish foot cat and I would not be scared to sail a smaller one around the world via the tropics. If I had to sail around Antarctica on a 28 foot boat, I would choose a mono since the chance both might capsize would be high enough to be a concern.

Victory49 · 02-10-2011, 06:09

Quote:

Originally Posted by dennisail

Once you get small enough, both boats will have a decent risk of capsizing in a storm. Is anyone really going to refute this? If not, logic follows that there will be a point when downsizing that a mono will be safer as stated. (assuming an upturned cat is less safe than a mono that has capsized and righted without filling with water since the skipper was not sailing in a storm with the companion way doors unsecured)

Also scaling laws are full of cubed and squared parameters. The smaller you make a boat, the stronger everything can be (in comparison to the loads). http://www.av8n.com/physics/scaling.htm#sec-strength That's why when you see small boats/dingys capsize they hardly ever loose there masts, but large yachts almost always loose theirs. A small strong mono may capsize, but it has a good chance of coming up with the mast if all the gear is in good condition.

Once you get up to a certain size cat the chance of capsizing becomes so small the risk becomes minor compared to other dangers. I have no idea at what size these matters come into play, but I am certain they do. But some cat guys will continue to argue cats are safer no matter what, and some mono guys will continue to say cats are unsafe no matter what, despite any logic presented.

I would feel totally safe in a 40ish foot cat and I would not be scared to sail a smaller one around the world via the tropics. If I had to sail around Antarctica on a 28 foot boat, I would choose a mono since the chance both might capsize would be high enough to be a concern.

Well put!

Factor · 02-10-2011, 07:43

Excellent - I have no plans to sail round Antarctica so I need not trade my boat ona 28 ft mono. I am most relieved

barnakiel · 02-10-2011, 07:46

Quote:

Originally Posted by dennisail

(...) Once you get up to a certain size cat the chance of capsizing becomes so small the risk becomes minor compared to other dangers. I have no idea at what size these matters come into play, but I am certain they do.(...)

You are probably correct, in the cruising boat aspect. Please note that big and fast cats (e.g. racing ones) are very easy to capsize with no help from the waves - it is all in the relationship of the boat's size/weight and her SA.

So, big cat = NOT safe, big cruising cat = very safe. Or something like this

Quote:

Originally Posted by dennisail

(...) If I had to sail around Antarctica on a 28 foot boat, I would choose a mono since the chance both might capsize would be high enough to be a concern. (...)

One should not sail to such places in a 28 footer AT ALL. In fact, a 28 footer is generally only marginally safe also on the coconut milk run either.

If people sail to places in TOO small boats they do it for economical or sport reasons.

b.