semidisplacement catamarans??

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

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Again, his software does not include catamarans, so when working out for propellers the closest match is semi displacement. It is not displacement nor planing hull. Due to the lack of option in the software he suggests entering the boat as semi displacement as it gives the best results.

This is not a statement of the characteristics of a catamaran, it is merely a get around to work out prop size for a catamaran using his software.

A catamaran is not semi displacement, A catamaran is not semi displacement, A catamaran is not semi displacement, A catamaran is not se.............
 
I would be the first to admit I am not a hydrodynamist and there are limitations on my knowledge of this very complex subject. Selling these vessels means I have to convey to people the complexities of them in the simplest form I am able to. I do have practical experience on a wide variety of cats and have a "feel" for them. How much people value my opinion is up to them. I do understand the sqr rt rule is misleading people.

I am happy to debate any complex issue if it does not comform to my experience. For instance - I do not believe a cat with a larger distance between the hulls necessarily goes faster. I think this is also too much of a generalisation as many cats want to increase the pressure and create lift for optimum performance and comfort. Distance between hulls is so complex I get the feeling the designers can only tank test to be sure of getting it right.
 
Ultimately tank testing is the only way to confirm anything I agree, but in the design process most advanced designs arise from Computational Fluid Dynamics. Many small boats are of course "evolved" designs and not actually "designed" at all. Regardless of your acceptance or otherwise though, the principles of CFD are well known and up to a speed of approximately 15 knots on a sail driven vessel the distance between the hulls is very important. The limiting factor on beam is firstly a practical handling issue extreme beam being difficult to moor etc. but secondly one of dynamic stability and seakeeping which is often at conflict with the quest for speed. Assuming the beam is not too narrow for lateral static stability, longitudinal dynamic stability is compromised by extreme beam to length ratios putting the vessel in danger of burying the lee bow and pitchpoling. High pressure between the hulls on a displacement vessel will NOT cause the hulls to rise. In fact it will cause turbulence which will slow the vessel down. Therefore increasing the beam may well increase speed but compromise seakeeping. I think there is a danger that you are passing off your opinions as fact when you have little to substantiate them and confusing your clientelle rather than informing them. The situation regarding powered catamarans is entirely different from that of the sailboat. In fact the best sailing cats are often quite awful under power and vice versa. I believe you are confusing the known CFD data for powered vessels with sailboats. On a power driven cat the hulls are often assymetric and indeed some have foils and rails in the tunnel to generate lift. I have had a lot to do with such vessels and in fact the last major project I ran before I retired was to build 3 27metre fast ferries with waterjet drive. On such a vessel over about 25-30 knots the pressure between the hulls does indeed generate lift and enables speeds in excess of 50 knots to be achieved on the plane. Sailboats my friend don't go that fast and the trick is to reduce resistence and turbulence however possible as the power available is not infinite. Don't confuse one with the other please!
 
Hull speed ??? Does it restrict ??

<<Sailing monohulls are displacement craft who cannot exceed their hull speed except on ocasional surf but >>

Sorry but my heavy old tub exceeds hull speed under power easily .... there are witnesses to this fact literally each time I am out and about with the boat ..... she even exceeded it under sail last time in UK ........ that really blew me away !!

Admitted Hull speed is a "hump" to get over - but I tyhink too much emphasis is given to it ... as many a boat on these forums breaks the hull speed and regularly ...
 
I am glad you agree that ultimately tank testing is the only way because I hear leading builders get it wrong basing on CFD alone. I think cats are more complex because of the interaction between the hulls . I do agree CFD has its place but I am not sure it is understood well enough with cat design.

You say - "High pressure between the hulls on a displacement cat will NOT cause the hulls to rise. In fact it will cause turbulence which will slow the vessel down."

Can you explain why on my displacement cat that the whole boat lifts as I go faster, it sits higher out of the water and outboards are visibly higher in the water. It also goes faster. Higher pressure is marketed as a positive benefit by companies like www.worldclasscats.com


I think the following statement is a massive over generalisation - "increasing the beam may well increase speed but compromise seakeeping."

I do not understand why the situation regarding powered catamarans is entirely different from that of the sailboat. Surely the hull shape is key rather than the method of propulsion. Cant narrow beamed cats be powered by sail on a small rig to overcome stabilty issues?


You make another interesting statement
"In fact the best sailing cats are often quite awful under power and vice versa"

as the majority of power cats are simply sailing cats without a mast and bigger engines. I agree that it is not ideal but cost savings come to mind and it is the reason why so many motorcats are wider than necessary.


This is also an interesting statement
"On a power driven cat the hulls are often assymetric and indeed some have foils and rails in the tunnel to generate lift."

- can you provide me with some examples in leisure industry?

This is an interesting statement "I ... build 3 27metre fast ferries with waterjet drive. On such a vessel over about 25-30 knots the pressure between the hulls does indeed generate lift and enables speeds in excess of 50 knots to be achieved on the plane"

- Does this vessel really plane!!!!!! Seeing 27m out of the water must have huge power requirements.


I think next statement is a huge generalisation - ask ellen mcarther !!!

"Sailboats my friend don't go that fast and the trick is to reduce resistence and turbulence "
 
Re: semi displacement catamarans??

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I do not understand why the situation regarding powered catamarans is entirely different from that of the sailboat. Surely the hull shape is key rather than the method of propulsion. Cant narrow beamed cats be powered by sail on a small rig to overcome stability issues?

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Now I am not able to throw lots of facts and figures in here, but narrow beam cats in the 60's and early 70's, the ones with big floats on the masts were case in point for not compromising the beam, the minimum industry standard for sailing is 2:1. The early beta test cats, like the bobcat and Hirondelle were a little tippy. In fact this era is probably the main reason cats have such a bad reputation today. Supposition only by the way.

The hull lifts because of the bridge deck, open slatted catamarans do not lift as the speed increases.
 
Re: semi displacement catamarans??

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The hull lifts because of the bridge deck, open slatted catamarans do not lift as the speed increases.

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Absolute garbage... Try sailing a modern dinghy type cat... the hull lifts then alright! The reason th hull lifts is due to the rig forces overcoming the righting moment. The reason it may continue beyond the vertical can be because wind catches the bridge deck, but the lift is entirely due to rig forces
 
Re: semi displacement catamarans??

yes in 60s cats got a bad reputation because a lot of them were home built in the garden by enthusiasts and broke up. Good cats need to be lightweight and built professionally - the development of lightweight composites has made the boats perform well and be able to withstand the stresses involved.
 
Re: semi displacement catamarans??

I took that from an article by James Wharram, I either misunderstood what he was saying or he is wrong.
 
You don't seem to understand that sailing hulls and power hulls are designed differently. Look for your examples to the racing powerboats. They are assymetric hulls. Look also at The Prout Panther range.(now out of business) 61 ft and upwards assymetric hulls and waterjetdrive. These boats have NOTHING in common with sailboats apart from the fact that they have 2 hulls.
Yes of course you need a lot of power to plane a 27m metre fast ferry at over 40 knots. A total of approx 1400 shaft HP actually. What did you expect? A bleedin' outboard motor?
I am glad you find my other statements interesting. I am losing interest in yours I am afraid.
 
Re: semi displacement catamarans??

Actually what Jimmy Wharram meant I think is that low aspect ratio fixed keel boats of that type should be designed to lift their bows in a gust and sideslip the keel effect that would otherwise cause them to trip over their leeward keel and lift the windward hull. Thats one reason why Prouts have their mast so far back which privides lift from the genoa. Funny old feeling when you have too much sail up and the wind gusts on the crest of a wave and 6 ton of snowgoose is picked up and turned around downwind and slammed back down again! Only happened too me 3 times in winds over 30 knots but it aint arf nice to know it happens if you overdo it!
 
Re: semi displacement catamarans??

You are confusing lifting the bow (both bows) with flying a hull (very near to disaster point on a cruising cat). We like to keep our keels wet at all times!
 
I am interested in your statements and try to remain objective. I obviously hope my views are respected. I dont think it is correct to say that power and sail cat hulls have nothing in common. I think for speeds up to approx 25 knots majority look very similiar to me. Many of the sailing cats hulls are also marketed as power cats without the mast. eg Lagoon an many others in US/Southern hemspere. Look at glacier bay, motorcat, worldclass cats, excitecat as powerboats and then look at fountaine pajot, gemini, broadblue ......... are they not similiar.?? These are all big brands overseas.

I may be wrong and persuaded otherwise - but to my eye power and sailing cat hulls often look similiar.

Actually your fast ferry is interesting as I think it might be possible as a displacement cat to get 40 knots with an outboard motor with very fine hulls and light composite construction.
 
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