Help with calculation for electric catamaran hull: speed, length, with, depth

Thank you, it's ok now.

It didn't work before.

 

What units used for LD ratio? I don't quite get how smaller number has higher resistance and LD ratio of 8 seems ultra light

 

In the 3rd Graph the "slenderness ratio" is used which is dimensionless, so the
units do not matter.

L/(DISPV)^(1/3) = 10 m / (10 m³)^(1/3) = 10 m / 2,15 m = 4,65

32,8 ft / (353 ft³)^(1/3) = 32,8 ft / 7,07 ft = 4,64

(difference = rounding error)

 

Ad Hoc, I accorded respect for your professionalism in my recent post. That I disagree with something is certainly not to be taken as a personal affront. Please take a deep breath and realize that your authority and generous council is not being repudiated.

 

None taken at all, I assure you

I just prefer to debate with facts where disagreements occur, rather than supposition emotions/feelings and conjecture. That's all.

 

Molland et al. 1996, shows a graph of residual resistance plotted against Fn. The monohull has significantly less residual resistance at all Fn. The difference is caused by hull interference of the cat demihulls. The plot accounts for centerline distances of 2p/L from 0.2 to 0.5. (p = centerline distance) The wider spacing being the lesser of the resistances but the mono continues to exhibit meaningfully less residual resistance even at Fn = 0.6 and beyond past 1.0. The subject graphic appears on page 43 of Faltinsen's Hydrodynamics of High Speed Marine Vehicles.

I am a willing subject who is anxious to learn where I may have somehow misinterpreted this kind of stuff. I am more than aware that "a little knowledge is dangerous".

 

Thanks for finding the publication and page. I would also like to say that I this is an important topic to me for human and naturally powered craft (wind, wave, solar...). I think a thorough battle of theories and experimental data would be valuable.

I think I found a copy here http://www.mojaladja.com/upload/Profa/Hidrodinamika superbrzih plovila.pdf

and I read it the same as you.


I have one question for the OP 'jon e' -the hull configuration you drew at the beginning is not a catamaran -I would call it a trimaran or more specifically a monohull with small stabilizing hulls. I have seen this configuration in many low power high speed displacement designs. It's a monohull -the small hulls just allow the main to be more slender than practical from a stability standpoint. In my study of the subject that IS the lowest drag hull configuration, and the only questions are the dimensions -start with displacement and thrust, estimate a drag, calculate a length with a minimum practical beam, try the theoretically optimal Cp, calculate length again...

BTW for hulls of this type and this speed I like start with "kayakfoundry". It has a drag estimator built in.

the other question is the placement of the stabilizing hulls -is it worth trying to place them to ride the wake of the main or just above the water.

I would like to pursue the graphs provided by Ad Hoc -what hull shape, size and speed produced them? I see the drag curves are very close but I wonder if it would be more meaningful to look at speed/power. The higher the speed the more the increased drag 'costs'.

 

We should all be willing to learn, no matter the source

On the one hand the graphs presented by Molland in Faltinsen's book are correct. So one might think, er...case closed?? Not quite.

Since like all research, it is just that, it is one piece of the puzzle, but everyone knows theory and practice are not always the best of bed fellows.

As such, if one has a fixed rigid SOR, and the choice is between a monohull satisfying that SOR and a catamaran, the resistance data 'suggests' a no brainier. But that is where it ends. To satisfy the SOR the design shall either be length driven or displacement driven. As I've noted many times before, a successful design is greater than the sum of its individual parts - in this case resistance curves.

Thus once the monohull is designed and built to those optimum resistance curves, it shall achieve speed X. Once the catamaran is designed and built it achieves speed Y. When comparing the two, you'll discover that the monohull has twice the power installed to that of the catamaran at the same target speed. Er...how so??

Well, there is more to design than a set of curves. Practical things like fitting in engines, stability a structure propulsion etc...all of which influence the final design solution.

It is not just mumbojumbo either. We have plenty of actual evidence from our own vessels that prove this too. Comparing our patrol boat monohulls with our cats, of same installed power or waterline lengths etc..the trend is very clear and obvious. Thus the theory helps considerably, it is 'our' yard stick and guide. But it doesn't tell the whole story, which is why a finished design should be the final yard stick.

So looking at that graph you note, the LD ratio is modest, it is 7.4. How many monohulls actually can achieve such....to satisfy an SOR, where the catamaran can. Which is ostensibly the reason for selecting a catamaran. Because the long slender monohull that is 'less' resistant with the modest LD ratio of 7.4, is unstable, can't fit the engines in and/or not enough deck space etc etc...whereas with a catamaran of 7.4....easy.

This is basically the findings of those that first performed the series of tests on those hulls, the Series 64, at the David Taylor Basin. It came about after the experiences of WWII. The R&D is excellent, it pointed the way fwd. But it was pure research...and when coupled to the practicality for a frigate of LD ratio circa 10+...unstable and dangerous. The benefits of such a long slender hull cannot be used in monohull format - owing to practical reasons. But very easy in cat mode

Does this make sense?

 

Jon E, I had a 6hp 2 stroke on a tornado (6.2m long, I'd guess 200-250kg with the motor and me). It did 11 knots. It was a bit faster when I forced the bow down, with windsurfer fins on the motor or with ballast on the bows.
Do not get a beach cat or tornado, it is much more and heavier than you need. My 5m cold mold (?) wood kayak is 9kg.
Look at Gary Baigent's build threads, I believe he uses 3mm plywood - the elliptical hulls are stronger and lighter than chined hulls. Some moths are even built in 2mm plywood but with many light foam frames. A light glass sheath on the outside is good.

I think we can sum up the previous discussion and say that if you can get away with a slender mono (with one or two outriggers), then other things equal it will be lighter and less draggy than a cat.

I wish I could tell you what length and beam to shoot for, but I can't. The 'hullcalc' spreadsheet by Malcolm Smith suggests, for a 250 kg fore aft symmetrical mono, that optimum beam is about 0.35m and length 8m. This is for a speed of 10 knots, which would take 0.9 brake kW with a propulsive efficiency of 0.7. 8 knots is 500w.
The cat is a meter shorter, 0.3m beam, and it uses 1.2 kW at 10 knots. But additionally there is the interference drag of which I have no idea. I'm going to try Michlet one of these days.

Are you east, west, north, south in Norway?

 

Thank you for your reply, Sigurd. It is a relief to hear that your catamaran did 11 knots with only 6 hp. That means my goal 8 knots cruise might be possible.

I agree that the catamaran should be as light as possible, so i hope someone with the calc knowledge would give me some approximations/numbers about the hulls. Length (6 to 8 meter), width and depth. Rocker or not, i have no idea...

I have calculated the boat weight to 100 kilo, so i should have a good margin on the hulls - to reach the total design weight at 245 kg.

I live in Skien, south in Norway. Near a long protected river called Skienselva.

 

I hope some more number guys and hpv guys come around,
but what you may do is look at rowing shells and find one that is close to your target weight (boat and crew). http://www.resoluteracing.com/
(then copy)
The 2 man light for a cat or 2 man heavy for a mono seems to be in range. It was interesting to see the effect of wattage on length - the difference between the heavy 1 man and the light 2 man.
Here is someone who's bothered to gather lots of data about rowing shell dimensions.
The rowers do 10 knots or so with similar power and weights - so 8 knots cruise doesn't sound impossible in a cat. I think some dogs like outrigger canoes too

 

Thank you again, Sigurd! It looks that the other members of this forum are not interested to help my with my needed calculations, but discuss technicalities that i do not understand.

I tried the spreadsheet by Malcolm Smith. I found that a katamaran (250kg) at 7 meters and 30 cm width is the best choice. By 8 knots, it draws 0,8 kw of flywheel (efficiency of 0,56) - and 10 knots at 1,6 kw. Is this spreadsheet quite accurate?

In that case, that is all i need of information.

I do not know why this forum's members does not bother to help me with the numbers, but so it is... Why i asked for help in the first post, is that i do not want to build an expensive boat that do not perform.

 

I think they went off a bit on a tangent, maybe forgot the original request, it's what usually happens here. Good will I presume
Question: 'How many boat designers does it take to change a light bulb?'
Answer: 'How many do you want it to take?'

I do not know the accuracy of the spreadsheet. At the very least it depends on the propeller efficiency input being correct - on a Torqeedo, with 32cm propeller, what could that be, I wonder? 0.7?
Where does the electrical - mechanical efficiency number, 0.56, come from?

yoo hoo human power guys, is it way off to assume a world class rower puts out about 500 watt over six minutes for a 2000m? And assume they get a 0.6 propulsive efficiency?

Rocker.
I think in this length, with double ended hulls, it'd be hard to get the rocker very wrong. It affects several things, such as maneuverability (won't be a problem with an outboard IMO), hobbyhorsing (hobbyhorse=gyngehest), wave making drag, trim resistance and wetted surface. At least, be sure to not lift the ends out with too much rocker.

Build methods.
Elliptical sections 3mm ply is pretty tough and light - my kayak has neither frames nor stringers.
Rick W have used aluminium sheet. I think he said he used 0.6mm for the attached boat. He says the angle that comes in the waterline from the 'pinched tube' construction technique is not a problem at all.

I'm trying Michlet today, I'll let you know if I find something out. How much beam do you want for the whole cat?

 

0,56 is from the technical data (Maximum overall efficiency in %): http://www.torqeedo.com/en/products/outboards/cruise/cruise-2.0-r/M-1230-00.html

How much beam do you want for the whole cat? I really dont know, Sigurd. Is the total width important?

 

I'm not sure yet. I'm only halfway through the Michlet manual. It can model the interference between two or more hulls. The divergent wave interference is affected by the lateral spacing of the hulls, so maybe there is a sweet spot somewhere within your desired range of hull spacing.

Hullcalc prefers the Sears Haack shape, I have only experimented with the PEP hull in Michlet so far, it is a hull that has parabolic waterline and rocker, and elliptical cross sections. M likes a longer hull than HC, but it has a way to optimize things so we shall see. Rick W seemed to think M gave him accurate results. It generated a hull which then went and broke a human powered distance record, IIRC.