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

[Jon E]

I have built a couple pretty crappy/slow designs on my own. It has been fun, but i do not know anything about hull design, obviously! I would humble ask you people for some of your time to please help me with some advice at this forum BEFORE I start building my next catamaran. 

Catamaran is the only solution, because I need a stable platform for my little dog… Length 5 to 8 meter. Easy to build "hard chime/box(?) shaped flat bottom hull", like the example below (Rick W.), but two hulls (cat). The boat will only be driven on the river, max wave height 50 cm, regular 20 cm.

I already have a Torqeedo outboard. Iwould like to get optimal hull speed at propulsive power 560 watts (input power 1000 watts).

Alternative heavy boat: Design weight 350 kg, max propulsive power 1120 watts (input power 2000 watts). I would like a design optimized for half throttle (propulsive power 560 watts (input power 1000 watts).

• 1 person = 80 kg.
• 2 x solarpanel = 40 kg.
• Torqeedo 2 kw = 15 kg.
• Batteries = 100 kg.
• Boat = 100 kg.

Alternative light boat: Design weight 245 kg, max propulsive power 1120 watts (input power 2000 watts). I would like a design optimized for half throttle (propulsive power 560 watts (input power 1000 watts).

• 1 person = 80 kg.
• Torqeedo 2 kw = 15 kg.
• Batteries = 50 kg.
• Boat = 100 kg.


My Torqeedo outboard engine: http://www.torqeedo.com/en/products/outboards/cruise/cruise-2.0-r/M-1230-00.html
Propeller: http://www.torqeedo.com/en/products...d-fins/spare-propeller-v30-p4000/1923-00.html

Can anyone help me, I would be very grateful.

 

[messabout]

Jon E, to reduce one of the significant drag factors of such a boat, give some attention to wetted surface of the hull or hulls. A flat bottomed boat ( which I am rather often attracted to) is the least efficient planform for a displacement boat.. The wetted surface is somewhere in the region of 11 % more than a three paneled bottom. In addition the chine eddy making of a flat bottomed boat is more severe than the eddy making of an eased chine. Not only that but a three panel bottom has a somewhat diminished average entry angle. Plus, plus, plus.

A three panel bottom can be described as a narrower flat bottom with angled sides of 30 to 40 degrees more or less, with the upper chine rising above the calculated waterline. The result of such a design is that you have a five panel build, includoimng the sides, instead of a three panel build. There is more! The three panel bottom reduces the width of the flat bottomed section which is of huge advantage in that it pounds less, it does not need as much lateral reinforcement, and so thinner panels can be used to minimize weight.

A three panel bottom has less initial righting moment than a wider flat bottom might have. That is certainly a design factor for a mono hull. Since you are doing a multihull that reality is not nearly so important.

Incidentally a three panel section can produce more area per inch of wet surface than a vee bottom. Tinker with the math for fun or for profit.

 

[Ad Hoc]

You have not noted a target speed. Thus what you are proposing is possible, since the end result shall be that it will do as 'designed'.

You need to be very detailed in your weights, no guesses, calculated weights and actual weights from eqpt suppliers and have a margin, a min is 5% and upper i'd say 10% on top of the weights you have noted above.

The heavier option has a length-displacement ratio (LD) of roughly 8.5 and the lighter option is roughly 9.5. The 8.5 is good, the 9.5 is very good. Thus shape will have little influence on the overall resistance at these higher LD ratios.

 

[Jon E]

You have not noted a target speed. Thus what you are proposing is possible, since the end result shall be that it will do as 'designed'.

You need to be very detailed in your weights, no guesses, calculated weights and actual weights from eqpt suppliers and have a margin, a min is 5% and upper i'd say 10% on top of the weights you have noted above.

The heavier option has a length-displacement ratio (LD) of roughly 8.5 and the lighter option is roughly 9.5. The 8.5 is good, the 9.5 is very good. Thus shape will have little influence on the overall resistance at these higher LD ratios.

Thank you for your reply. Target speed is pretty much given the design, i thought - since i already have an Torqeedo outboard 2 kw.

I would hope in the area of 8 knots half speed, and 10 knots max, but i have no idea if that is possible. Just hoping. The weight is possible to get exactly right, but if you think 10% more is realistic that is ok. I would hope the hull length of max 8 meter, but that is not an absolute criteria. Speed and easy build is most important.

 

[Jon E]

Any suggestions about dimensions and speed?

 

[Ad Hoc]

Jon E

As I previously noted, anything is possible. But..and it is a big BUT....you first need to establish an SOR, Statement of Requirements. In simple language, what is your objective or target?

You need to decide what is it you want from your boat...fishing...water skiing, leisure etc etc....and if you have any restrictions on this objective. One that comes to mind is your engine. Since the SOR is unique, no two SORs are alike...but once you start placing limitations,such as a certain size type and power of engine, your 'possibles', begin to narrow, to fewer and fewer options.

Thus you need to really thank about what you want from your boat...forget other plans for now...that's the easy part. What do you want?

 

[Jon E]

Jon E

As I previously noted, anything is possible. But..and it is a big BUT....you first need to establish an SOR, Statement of Requirements. In simple language, what is your objective or target?

You need to decide what is it you want from your boat...fishing...water skiing, leisure etc etc....and if you have any restrictions on this objective. One that comes to mind is your engine. Since the SOR is unique, no two SORs are alike...but once you start placing limitations,such as a certain size type and power of engine, your 'possibles', begin to narrow, to fewer and fewer options.

Thus you need to really thank about what you want from your boat...forget other plans for now...that's the easy part. What do you want?

Thank you for your reply, Ad Hoc. I am sorry, my English are not so good. I hope i understand your question correct.

I have owned several boats from 8 to 27 feet, sail and speedboats, from 5 to above 70 knots. Day and weekend cruisers. I have 40 years on the sea.

My catamaran project's is just an odd hobby. I like to build things, but unfortunately i do not have the knowledge of calculating effective hulls - that is why i am asking this excellent forum for some guidelines/suggestions.

I live by a quiet river with some current (1 to 4 knots). It is 20 kilometers long and around 100 meters wide. I do not want this project to anything specific, other than to drive me and my dog around and observe the life ashore and enjoy life a nice sunny day - at some speed(!)... in quiet. Like a water scooter. If i am getting wet on my feets, that is okay. Some speed is all i want.

My homebuilt boats until now has been is too slow and heavy. Around 5 knots, and that is boring! I would like to double it, if possible - with the given Torqeedo engine. I think that is possible.

I do not need space aboard for anything, except my butt and my small dog. This is not a family craft. It is only from A to B - in "high" speed. I need speed! And some maneuverability, off course...

 

[Ad Hoc]

Jon E

Thanks for the reply. In that case then, just build the vessel as light as you can and whatever size suits you. Place minimal epqt on board, and then, she'll do whatever she does. No point breaking your bank or your blood sweat tears trying to eek out 1 knot here or there and designing it like it is a space shuttle! Enjoy your hobby in building..

Just build it, and see what you get...since if you are convinced it is as light as YOU can make it...that's all you can do. So long as the LD ratio remains high, shape plays little effect on the resistance. That's it, enjoy

 

[Jon E]

she'll do whatever she does

Well that is not very helpful, and could have been mine quotation. That is why I asked this forum for some help to calculate my hulls.

I have supplied accurate weight, my Torqeedo engine power in watts and hull length - but ok...

 

[Stumble]

Jon,

Given your goals I would suggest taking a look at a beach catamaran or two. The hulls won't be ideal since they will be designed for sailing not motoring, but it's a good starting point.

Off hand my thought would be to scavanage an old Tornado platform and convert that. Running the calculations are beyond me, but I would guess 10kn would be realistic. If you go thru the work of eliminating the rocker and other sailing specific design issues you could probably better that but at the cost of a lot of work. Maybe a 10% speed increase?

 

[messabout]

Jon E; You have already established that you intend to use the Torqueedo. At an out put of 560 watts that amounts to three quarters of a horse power. You want to make 8 knots more or less with that power output. That is optimistic but perhaps possible if every detail is attended to.

The first detail is weight. The overall weight of the boat, motor, batteries, you and the dog must be the absolute practical minimum. Sorting through the weight factors we see that the only variable is the boat. In that case the boat design must concentrate on boat weight. A catamaran is not the most easily limited in that regard. Are you open to the idea of a monohull, perhaps 6 meters long and not much more than one meter wide? It would be possible to build a skin on frame boat of that sort to weigh less than 30 kg. which is a long way from the weights that you have suggested for the cat.

If not skin on frame then perhaps a combination of 6 and 4 mm Ocumee. where the weight would be in the region of 40 to 45 Kg.

The mono hull will have less wetted surface than the cat. I am normally most respectful of Ad Hoc who after all is a professional. In this case I do not
agree with him that the shape of the sections are not important. One designs the low energy boat to have the least wetted surface and to
minimize eddy making surfaces or intersections. I refer you to the stupendous amount of expensive research and development of low powered (human powered) but fast boats of the Olympic classes such as K1, K2, Pairs rowing shells, etc. Shape matters.

I certainly agree that length displacement ratio is an important determinant. That is what we are dealing with by making the boat as light as possible within the bounds of reasonable comfort and safety. Incidentally a mono will let you sit in the boat rather than on the boat. That can make a big difference in the pleasure derived if you are no longer a teen ager. The dog may like it better too.

 

[Ad Hoc]

That is why I asked this forum for some help to calculate my hulls....

Then perhaps you need to reread my comment above several times to fully understand the implications.

The mono hull will have less wetted surface than the cat. ..... In this case I do not agree with him that the shape of the sections are not important. One designs the low energy boat to have the least wetted surface and to minimize eddy making surfaces or intersections. .

It bothers me not if you disagree with me. But your position is not supported by any evidence, quite the contrary, hence my reply.

Endless tank testing by ourselves and others clearly show that length displacement ratio is THE most important factor. Playing about with shape has very low influence:



And looking just at LD ratio...it is clear increasing this reduces the resistance:



And once you get over an LD ratio of 8.0 (which the two options noted above are..), shape plays next to no influence. As noted by Molland again too here:



Ergo, shape plays very little influence, despite what others may "think" or "feel"..it is not supported by any evidence. Only low LD ratios does shape begin to play a part.

When looking at monohulls, the effects of LD ratio are even greater:



And, again contrary to "popular" opinion...a catamaran, of same length, and LD ratio and when in higher Fn > 0.6, has approximately 50% less resistance than an equivalent monohull. See Molland et al for same results.

Engineering is about facts, not supposion. Take your choice..

 

[Heimfried]

Hi, Ad Hoc

thank you for posting the graphs. But they are much to small for reading.

 

[Ad Hoc]

thank you for posting the graphs. But they are much to small for reading.

May be it's your browser??...as when I click on them...i get a large image.

 

[Boat Design Net Moderator]

Click on the thumbnails for a larger image.

Edit: sorry, posted at the same time.