semidisplacement catamarans??

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pW2

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Thanks to whoever for the link Castlemarine propcalc (I suspect I´ll be sending them some cash before long)
But what are "semidisplacement" vesels? Certainly not the vaste majority of catamarans be they sail, passenger ferry or cruise ship. Cats for the most part are straightforeward displacement craft and follow the hydrodynamic rules governing such craft, so why do I have to call my boat "semidisplacement" when she aint no such thing?
The only craft I can think of which would be "semidisplacement" are craft using hydrofoils too small to give full hydrofoil lift and I can´t for the life of me see what the advantage would be in doing that
W
 
I can understand the label, I think.

Sailing monohulls are displacement craft who cannot exceed their hull speed except on ocasional surf but catamarans can and do exceed hull speed which implies that they can sail partly on the water, rather than in it.
 
You mean like this

039_101.jpg
 
I think you will find that all sailing cats sit on the water rather than in it, this accounts for their diffferent motion, and that the formulae for speed is not so relevant.
 
I definitely wouldn't go that far - a cruising catamaran doing 3 or 4 knots is definitely in the water and acting as a displacement craft - it can't be planing at that speed and probably isn't hovering.
 
Virtually all multihulls are displacement vessels. They stay 'in' the water as opposed to planing.

I imagine the use of 'semidisplacement' in this context refers to vessels exceeding hull speed but not managing full planing speed.

'Hull speed' does not apply to vessels with length-beam ratio over about 8:1 which covers most (but not all) multihulls.
 
The semi-displacement hull, generally seen on larger cruisers, has some lift capability. This is shown by the fact that as speed increases, the forward part of the hull lifts up and allows the boat to exceed its maximum displacement speed. Some disadvantages to semi-displacement hulled boats are that they need much bigger engines and consume much more fuel. -this is true for motor cats

and for a number of sailing cats - even the old Catalac!

Are you saying that your boat is incapable of exceeding the hull speed (which is the defined limit for displacement boats)

for example B&Q doing 36 knots in south atlantic - surely this equates to a semi-displacement definition!
 
[ QUOTE ]

'Hull speed' does not apply to vessels with length-beam ratio over about 8:1 which covers most (but not all) multihulls.

[/ QUOTE ]

That's a new one to me and might help explain my confusion because when cruising cats are going above hull speed they don't feel as if they're planing until they actually start surfing.

Have you a handy link to somewhere which explains how the 8:1 thing works? I feel like learning something new today, preferably with a bit of maths to wake my brain up.
 
No, I'm saying there is no such thing as hull speed for narrow hulls. Hull speed is an effect of the wave-making and a fine hull does not make those waves, they just go on getting faster as more power is applied. Multihulls at speed don't rise out of the water in the way that planing or semi-displacement mobos do.

I said that this applies to most cats. The Catalac, along with most of Bill O'Brien's designs are exceptions, their broader hull forms do genernate large bow waves which limit their speed. When driven hard they will generate lift and show signs of planing.
 
I am sure you are right in the strictest interpretation of the terminology, but it is a tad confusing to have one definition for mobos and another for some multis and not others. Especially when the effective performance of all the multis especially in comparison to normal mobos, is to place them within the semi-displacement category.
 
You don't really need much maths - just watch a rowing eight at full speed. It's doing something like 12 knots yet it leaves barely a ripple. On the other hand watch a trawler of the same length at that speed: big bow wave, big stern wave and the boat sitting in the hole between the two, unable to climb out.

If you want a bit of maths, here's a couple of formulae to compare speeds of multihulls:

Sail: 5 x sqrt(LWL ft x Sail area sq ft / Disp lbs)

Power: sqrt (lwl m x HP / disp tonnes)

The constant 5 in the first formula tweaks the answer to give something like reaching speed in a force 4.

these are empirical and operate within reasonable limits. They are known to be optimistic in larger size ranges. Also they apply only to fine-form hulls with length:beam of 10 or more
 
Ok, now I've seen some of the theory and maths. Hull speed does apply to catamarans but much less. How much less depends, for a given displacement, on the length multiplied by the square of the beam. The beam matters but a higher ratio of length to beam actually slows you down as a doubling of length doubles the resistance but only increases maximum hull speed by about 1.4.

In other words a 2 foot wide hull produces a sixteenth of the resistance of an 8 foot wide hull, so it needs far less power to climb over its maximum hull speed bow-wave.

I think.
 
No, I think you misunderstand me. Forget the old 1.4 root LWL formula. A slender hull simply doesn't make a bow wave that needs to be climbed over. Hull speed just doesn't happen. The speed resistance curves for fine hulls and fat hulls are fundamentally different shapes.
 
That was me. I bought my props from Castlemarine and sat in the office while we worked out the required size.

There is no option for catamaran and as there are so many different cat hull forms it would make the software pretty unusable. So for sake of argument he always states sailing catamarans as semi displacement as near or damn it the figures will come out correctly. Though his figures said I would only get 5kts and I am getting 6.5kts with the throttles wide open, cruising at about 5.5.

This was not a statement suggesting catamarans were semi-displacement, it was just the easiest method to calculate prop size using the standard methods.

Hope this clears it up.
 
What you're saying makes sense but the sites I've looked up today don't seem to quite say that.

Some of the sites talk about "fineness" but the ones that do explain the logic behind it do seem to say that a hull is a hull, whether attached to another or not - and narrow hulls have so much less resistance (thanks to the resistance being related to the square of the beam) that the hull speed effect still happens but is so minor that it can be easily driven past.

However, I don't really know enough about the subject yet to comment much further until I understand it a bit better, so you are probably right, and thanks for starting off the train of thought.
 
By way of example, my own boat has LWL 36 ft, waterline beam around 3 ft, i.e. 12:1

'hull speed' would be 8.4 knots but under power she accelerates smoothly to 11 knots and has done 14 under sail (17 running in big seas). The only evidence of wave-making is a row of standing waves astern.
 
I sell catamarans and can help with a little with the theory. I will make a new thread and offer more explanation for the forum on this subject. The displacement formula just calculates the speed at which the distance between the crests of the bow wave and stern wave are equal in distance to the waterline length. In that sense "hull speed" is identical for a monohull or a cat of equal waterline. This speed in knots is 1.34 X square root(waterline in feet).

This is also maximum speed for most displacement mono boats. However, a very narrow hull makes a small bow wave and thus the power needed to exceed "hull speed" is reduced. The fineness ratio of a displacement cat hull is important in speed calculations.

Why bigger displacement cats do not increase in speed easily due to weight (volume). This is why buying displacement cats is opposite thinking to monos - the better cats use more expensive light composite materials. Monos are sold on basis that heavier build is better. Cat thinking is the exact opposite.

Cat displacement cruising is therefore possible at 25 knots in many sized boats.
 
You are fundamentally correct in what you say SL but it's wrong to associate it specifically with a "bow wave"
The best way to interpret it is to stop at the point where people talk about "square root rules" and recognise that these are now't but convenient approximations of an average monohull's hydrodynamic resistence and have no real basis in science.
When the length to beam ratio gets greater, the formula becomes less accurate and with a cat form does not really apply at all.
Basically its the difference between pushing a football through the water and a pencil and the resistance comes as much from the drag behind the football as the resistance in front of it.
You are also entirely correct that all the cats that we sail (assuming we are not flying a hull!) are displacement vessels. The use, by some, of tables referring to semi-displacement vessels is only a convenient way of saying "a cat is less resistance so we will use the semi-displacement table cos we don't have one for cats and it's nearer what we want" Many cats will lift their bows when going fast, especially hull forms like Talbots. This ain't planing though. Anyone who has a RIB knows what planing is. It's when the boat goes "over the hump" and lifts clear of the water. When even my little Avon does this I can throttle back and still maintain 15 knots but it takes a fistfull of throttle to get it up there. "Semi-displacement" means nothing to a hydrodynamicist. A hull is either displacing water or planing.
until a hull lifts clear of the water however much lift it has it as still in displacement mode.
 
With due respect, being an agent for selling boats does not qualify one as a hydrodynamicist. The square root rule is entirely missleading. Actually the hull form of each multihull must be studied in three seperate ways for any proper analysis. Firstly the horizontal lines (mainly at the waterline) of each hull section.
Secondly the vertical lines (anyone with a lift keel will tell you the effect of lifting keels downwind.) And thirdly the interference pattern of each of the inner faces of each hull with the other. This latter consideration is extremely important and very complex but is why a cat with a larger distance between the hulls goes faster than a narrow one. The generated pressure is less. I do not mean to criticise your well meaning contribution but any attempt to over simplify multi-hull hydrodynamics is frought with danger. I have been involved with multihull designers and builders and as a chartered engineer with a smattering of hydropdynamics have listened very carefully to eminent hydrodynamicists on many occasions. I only understand this complex subject partly though and am not an expert. I would advise anyone with a general understanding to stick to generalities as there is already too much pseudo- science being talked. And, by the way, the half-wave theory relating standing waves behind the boat to waterline length is also a generalisation and affected very significantly by the distance between the two hulls......
 
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