displacement cat v displacement mono

What you say supports the facts - there actually is a formulae - its the same as a mono hull displacment speed - the difference is that a displacement cat can go past the 'displacment speed' without as much penalty as a mono hull becaue the slimmer hull is generating a wave that is less powerful, and so require less energy to go faster - up to a point. After that point a planing mono hull would be better as regards efficiency.

"some 30ft semi displacement cats happily do 24 knots with small engines which seems to smash the displacement formula on waterline length."

I do not think that is right - they do not smash the formulae, the wave length genrated by the hull is the same and the formulae applies - its just that it requires less energy to go past that than is the case with a wider mono hull.

It really does seem that general conclusions from all that is interesting -

1. For most sized leisure boats 30 foot to 60 foot, Cats offer mid speed crusing, typically 10 to 25 knots that is very efficient compared to SD or planing boats. Displacment boats cannot practically do these speeds anyway.

2. The most useful cat hull is a full displacement one.

3. If you are going to add volume add it to the depth of the hull as only frictional resistance is added.

Your thread has been interesting as all this is new to me and I have learnt it all along the way - mind you I have had to consult an expert to get the info :-)
 
I am not expert enough to know whether the displacement formula breaks down with fine hulls or not as formula is usually regarded as a rule of thumb. I do not know whether block coefficients of hull, drag coefficent on hull, wetted surface area, fineness of entry, displacement, aerodynamics are factors applied in the formula. I cannot seem to use this rule when looking at cats. In fact given 2 displacement cats I know - the one with shorter LWL goes faster. I do agree that displacement cats look the more interesting for people happy with 25 knots max. More than this speed and mono looks more attractive. Given fuel issues in Uk and displacement being more sea kindly why is this superior hull form not being developed in UK? Why is there so little information? Why are so many commercial operators now using them and not leisure boat builders? With so little information it really is hard to understand. All I can say is that I am not a designer but have good practical experience on a wide variety of hull forms and I think comfort of a displacement cat in all weathers takes a hell of a lot of beating.
 
The site link you gave confirms what I have stated - the displacment formulae does not break down its just that the fineness of the hull means you can get past it more efficiently.

There will be other reasons why one boat goes faster than the other and that should be understood better by makers and dealers - not just left to be some form of black art. If you are involved with selling these things surely you must find out and be able to answer the whys and be able to explain?

Your general point of why these boats are not considered more is interesting - I can only guess that the public are ignorant (I was) and that the manufacturers are conservative and follow the public.

Cat makes seem not to help as they need the get some clear messages over to the public - they are not doing this. So may I twist the question a bit and throw it back at you. Why do you think it is that cat manufacturers/dealers have so far failed so miserably to get these points over to the boating public? Do they need a real kick? I am a technical sort of geek who finds technical stuff interesting and likes exploring issues to understand them yet i found your opening post totally confusing - I knew there was a message in there somewhere, nothing else was happening, and so I decided to 'Gludy' it a bit to try and get somewhere.

I have zero experience of cats so I would like to hear what the downside is – some folks on the forum must be able to provide the disadvantages for us to balance what has become a pretty impressive pro list?
 
I have recently ordered a new boat and I just asked myself why I did not consider a cat.

The answer was that apart from one power cat that did not impress me and was too small anyway, there seemed to be nowt on the market. The power cats you refer to are USA models - are they over here? Can people do sea trails on them? Have they had any UK mag reviews?

I think that if you want to change the market on this you really have to understand how little exposure the suppliers of cats are offering the market.

To sell properly in the UK market you have to have boats here and support here - you have to really be here before boaters will part with their hard earned cash - so it seems to me that the answer to your main questions is very close to home.
 
You will find some in MBM buyers guide. We do trials all the time. David harding of PBO did one reveiw in heavy weather and I understand bought one afterwards. Stewart irving did a test and recently voted one as best family boat in Boatmart.
Some reporters starting to understand. I have lots of figures but to be frank - I am not a nerd in this department
 
"You will find some in MBM buyers guide."
Not the way to convert a market!

Any chance of a peek at that heavy weather review? Month and mag would probably mean I could dig it up.

What is the biggest size you have in the UK?
Is it the USA boat from the kink you gave us?

"I have lots of figures but to be frank - I am not a nerd in this department."

I am not asking you to be a nerd - if you have figures on how fuel consumption varies compared to same displacment mono hull lets us see them.

You must be able to knbow enoughg to explain why the cat is better and for that you need performance figures.

I am pushing you here to provide figures which can only do you good.
 
Did you look at the Lagoon power cat then? How was it and what was wrong with the cat you looked at. I would have thought that it would have met your "make a cup of tea whilst doing 20knots+" criteria???
 
a couple at the boatshow I think.... this and previous years.

Personally thought the sailing cats from same firm looked much nicer - they'd tried to style the motor ones to be like a pastiche of 'orrible flybridge single hulls - We thought they looked odd.
 
I have to agree.
I really did not look much because the only one I knew about was too small and basic for what I was looking for.

It would be nice to have some 50 and 60 foor versions on the UK market - these look Ok on their web ite back in the States.
 
I think that the biggest barrier to Cat sales is marina fees. It is normal for marinas to charge either 50% extra or even double for multihulls. While they do this, I don't think many people have the incentive to look any further before buying a monohull!
 
I would agree with this if we were talking on the sailing forum. The best motor catamarans are not sailing boats with the mast missing. sailing cats need extra beam to support the rig. From what I have read it is generally regarded that the best motor catamarans are Lenght to beam ratio of no more than 3:1. Offshore high performence and efficient powercats are in fact very narrow. I have never paid extra mooring fees - in fact I have saved because I often beach and dont pay visitor fees
 
I have been to most of the marinas in the solent and in the outskirts and never paid any extra - why should I? I dont take any extra space.
 
My experience, from a background of working in a marina, was that if a vessel was wide enough that an entire "bay" was required for the vessel, then the vessel was charged at rate and a half, but that if a suitable berth, such as alongside a pier or on the end of a pontoon was available, then they would only be charged at the normal rate, which seemed fair. It may be that you've always been in that position with your berths and marinas, but I think in general the boat buying public are wary of this.

Personally, I'm all for multihulls, especially powerboats, and have been involved in a professional capacity with a large powercat abroad. I think they are probably the future, but I personally don't expect the market to move very fast.
 
I agree with this but it can be addressed with some design flair. i will send you some good design and then tell me they cannot be good looking.
 
erm, they're quite good for a cat, but still nowhere near as good loking as a birchwood, which itself is quite ghastly.

Quite lot of the point of a powerboat is for it to look good.
 
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