displacement cat v displacement mono

hobiecat

hobiecat

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The main negative is that costs are higher for a catamaran for a given length. With the two hulls there is simply more surface area and fibreglass needed. Systems must be duplicated in both hulls. Pumps, engines, electrical and plumbing systems, etc. However you end up with is more usable space for a given length. Fuel costs are considerably less. Tank testing was recently carried out and simulated huge breaking beam seas and the power catamaran could not be rolled over but the similar sized monohull power vessel could. In addition, in following seas, the relatively narrow hulls of a catamaran track straight and true with none of the usual monohull broaching tendencies. Although the displacement catamaran will have a wave resistance-determined hull speed just like the monohull, this hull speed will be very much higher. The speed of the displacement catamaran is largely a function of the fineness ratio of the hulls. Realistically, the upper speed range for a displacement catamaran hull is about 20-24 knots. It is advantageous in terms of fuel economy/horsepower requirements to keep your top speed, and particularly your cruising speed, as far down the resistance curve as is practically possible. The cushion of air created between the hulls acts like a shock absorber and creates a far more comfortable ride. Juggling the variables, inevitably, seems to mean a fuel efficient, semi-displacement catamaran hull which achieves cruising speeds in the 15-20 knot range as a better boat with a proviso on price. Better performance and comfort is achieved on catamarans with modern, lightweight strong construction methods.
 
I am not a designer but I have more experience than most of cats. My understanding is that the max displacement speed is around 24 knots for a cruising motor catamaran. I understand the normal displacement formula breaks down and the fineness/width of the hulls is the important factor. Wafer thin hulls will go faster but not much good for cruising.
 
Sorry that cannot be- the displacment speed must vary with the size of the boat.
Otherwise a 200 foot cat will have the same speed as a 20 foot cat and that is not the case.

I know nothing about this subject but will find out more and come back to the thread.

A word of caution ... if you are going to discusss something on this forum and you are right ... you will get ripped to bits. If you are going to discusss something and you are wrong ..... they tear you to shreds .... if you are wrong and its your subject .... well you have no idea how bad it can get! :-)

I once posted a thread that simply stated 1 + 1 = 2 and they disputed it!

So make sure you get the info from the boat designers asap.
 
You wouldn't be thinking of the thread where there was posted a website that included measurements which contradicted the stated theory and which went totally ignored, would you?

Rick
 
I am posting the corrections to that site using their on figures.
You cannot get arounf the basic physics and its frankly amusing that you do not accept them but do not openly deny them either.
 
Glad I'm amusing you. FWIW, I don't know whether a spinning prop causes less drag or not. I have 2 bits of evidence, one from a website which seems reasonably plausible considering the debate on the T&T list - and you on here that keeps stating "its a fact". Frankly, at the moment, you're less believable. The fact you steadfastly ignored that post twice (and then said you were investigating, but never came back), and now you're "sending them corrections" doesn't strengthen your argument, I'm afraid.

Besides, if you're so convinced you are right, why did you even bother posting in the first place? I think thats what most people remember about every gloody thread.

Rick
 
I have spoken to a boat designer and he told me this:-

Before I tell you what he said ... please keep in mind that it will be at this point that the thread takes off in all sorts of twist and turns and those amongst us who do not live fully on this earth never feel the need to follow the laws of physics.

1. A cat of the same length will have the same displacement speed as the mono hull - the formulae only depends on waterline length - nowt else.

2. The wave that you create and push against (that normally limits the max speed on a displacement hull form to what we know as the max displacement speed, is less with the cat and so the cat can go faster without the same penalty as the mono hull.
In other words a cat with a displacement hull can go past its displacement speed and go faster because the wave it creates from the slimmer hull form is less than it is on the same displacement mono hull.

3. The cat though has more surface area and greater frictional drag and so there is a limit past which this can become significant - hence the extra speed does tend to have a ceiling. it is not friction that stops D hulls going faster - its the wave they create in front and need to push through.

4. there are some complex wave cancellation factors involved depending on the distance of the hulls apart - not too significant for other than large ferry type cats who actually design that into the equation.

5. A displacement cat hull should try to get it volume by depth not width,

6. There is a super efficient 60 foot displacement mono hulled thin with only 5 foot width that managed to go around the world on little fuel - that was because it was thin but deep and long.

That is my opening salvo of matters i am assured are facts. Now its up to the fla t earthers amongst you to pull them apart.
 
Rickup
1. The evidence from the university that directly measured the drag gave the static prop 30% less 0 you ignore it.

2. The evidence from the 42 foot boat that monitored consumption and was detailed on the thread gave the static prop less drag - you ignore it.

3. The theory calculations done by the designer showing what to expect from a given prop also showed that extra wasted energy in the free prop 0- I also offer to email the long clacs for this to anyone who wanted them - you ignore it.

4. The evidence from props on planes on many sites and what pilots are taught - you ignore.

5. The only bit of evidence that purports to show other actually contain errors - I have the details now and will post them on the thread as soon as I can lay them out better that was given to me.

All the evidence except for one little bit that actually contains errors - I will show that it actually shows the exact opposite at some speeds and is very dodgy - that piece you cling to ....... I declare you a ’flat earther’, fully qualified.
 
Rickup
"Besides, if you're so convinced you are right, why did you even bother posting in the first place? "

Please read my openeing post it was not asking about the fixed prop bit it was asking if anyone had experience of how much the actual saving was. Again and again I pointed out that I had not started the htread to ask what i already knew, I started it to find out what i did not know.

I think flat earthers have a tendency to do selective memory reading.
 
Paul, I know what your original question was. I posted figures. You ignored them. Now you're resorting to insults - so I know exactly your level and don't think I need to respond to you anymore. Grow up, please, or leave the site again.

Rick
 
The 'flat earth' bit is a fun way of telling you that you are ignoring the facts. No offence was meant ... and I am sorry if it offended. I was simply trying to point out that by you not accepting basic physics you could be classed in such a manner.

Really the argument on this is not with me - its with Issac Newton.

What I am stating is basic physics accepted by the world - the evidence on the site supports it - in a few minutes I will even show that the figures you pointed to also support it.

"Besides, if you're so convinced you are right, why did you even bother posting in the first place? I think thats what most people remember about every gloody thread."

That is a quote from you. A personal attack, a sort of smear about the way I deal with threads rather than deal with the points I made. I have chosen to deal with that by clearly answering you and asking you to re-read my opening post - you now refuse to answer and want to take your ball home. If you are going to make such untruthful staments about me then be prepared to back them up.

You accused me of posting a question when I knew the answer. You stated thatI have stated that is not true - so if you know my original question why do make such false claims?

I will choose to stay or leave the site when I wish, not when you tell me.

I have left before because I was fed up with the childish behaviour of a clique - I am back and I am not giving into it again.
 
The statements you make are true. I can only speak from experience of happily driving cats and some 30ft semi displacement cats happily do 24 knots with small engines which seems to smash the displacement formula on waterline length. The formula is only a rule of thumb after all. I asked a few foreign cat designers which is a specialised market and 1 wrote the comments below. It seems to confirm my first thoughts that a planing cat has little advantage over a mono planing boat. However, an efficient displacement cat can go at very respectable speeds and has huge potential. A simple method of telling whether it is a good cat is to compare speed with the size of engines. Some are truly awful just like good and bad monos I guess.

"Please remember that boats are much more difficult machines to move than cars or planes because they move trough two medium ; air and
water.Running trough the air is simple but try to run in the 4-5 feet swimming pool and you will find out that the forces are brutal.Boats from that point of view are
very difficult to design .Mono hull overcame this force by planning and power (fast boats=inefficient ) , brute force of power ( large semi displacement boats) or
slow displacement boats (hull speed =efficient).
Please also remember once you air borne (planning) boat is subject to pounding .
Most cats are overcoming this force by brute power as well (Glacier Bay ,Noosa etc) by basically doing the same what monos do.Therefore they are not different
than monos (see attached study I did in 2005).Smart Cats like MC 30 ,PDQ or Maryland are not planning but efficiently displacing water avoiding pounding since they are rarely lifted out from the water.(see my article to MM in Boston).
The problem is very few people understand this and writers have no clue what they writing about since they do not consult with right people .This leaves
people totally unaware of the good designed cats.Selling the idea is difficult since you have to educate buyers (slow and time consuming"
 
[ QUOTE ]
This leaves
people totally unaware of the good designed cats.Selling the idea is difficult since you have to educate buyers (slow and time consuming"

[/ QUOTE ]

Which is why you have started these threads /forums/images/graemlins/tongue.gif
 
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