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[Mad_Mike]

This is probably a question for either Jan or F1madness or nick75au but any helped offered is more than welcome

I want to build a catamaran. Ive got the plans ive found off the net and im going to shrink them a bit but what to depends on the motor. I currently have an 60cm FE mono with a 3600kv motor 37.5k series prop and 3s batteries, the combination works very well for this boat and it would work for a 60cm catamaran hull but the propsizes i can use are limited and i would like my cat to be 66cm and faster. I would prefer to stick with 3s lipos as i allready have a stash but if not then im open to suggestion. Can I have some advice on what motor to run please. Ive seen some possibles i would like your oppinions though please:

I currently run one of these, works well does the job but if theres a better alternative:

http://www.ebay.co.uk/itm/RSC-Model-3600-KV-RC-Ship-Boat-Hobby-Water-Cooled-Brushless-Motor-IM294-/390417939702?pt=UK_ToysGames_RadioControlled_JN&hash=item5ae6b8fcf6

like this alternative:

http://www.ebay.co.uk/itm/130704508887?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648

or maybe this one. I think the kv is too low for surface peircing prop on 3s though.

http://www.ebay.co.uk/itm/170843627984?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648

a bit steep but maybe:

http://www.ebay.co.uk/itm/110864264303?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2648

or:
http://www.hobbyking.com/hobbyking/store/__17700__XK3665_B_2300KV_Brushless_Inrunner.html

thanks to anyone who offers me help

mike

[Mad_Mike]

looks like i was on my own with this one

anyway ive decided to go with this:

http://www.ebay.co.uk/itm/130678261783?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2648

and a nice pair of 3ah 2s lipos to go down the hulls.

[pompebled]

Hi Mike,

Sorry for the late reply, but one can only frequent so much forae a day...

For a cat, you aim for 30.000 rpm or more at the prop loaded, so it's a matter of doing the math with the motors you've selected.

I am currently modifying a Academy Triton with the stuff I have available and I'm using a 2300Kv 3656 motor on 4S, turning a Ø35 mm threeblade plastic prop.This gives me about 30.000 rpm under load and even though the boat is rather heavy, because there's a second 5000mAh 4S lipo on board to balance the boat, it is scaringly fast for an ABS hull that's rather narrow (offshore design).


Where you say: "I would like my cat to be 66cm and faster", should I read 66 mph?

If so, that is very fast for a 60 cm cat, I'm not saying it can't be done, but I'd rather build a bit larger hull for that kind of speed.
Which design are you going to build, if I may ask? (link?)

The 2000Kv outrunner you've selected would't be my first choice, as the rpm on 3S would be low, requiring a large prop and on 6S SAW-like high (40.000 rpm), requiring a sharpened and balanced metal prop.

The Leopard would be indeed a bi t overkill in such a small hull, but with a small prop it should work fine, as the rpm are right in the sweet spot on 3S.

Regards, Jan.

[Mad_Mike]

You dont need to apologise for my impatience, i just thought everyone had left me in the dark

Anyway enough of that. I designed and built a monohull last year with your advice and the results were sastisfactory roughly between 25 and 30mph. Due to the small nature of the 3600kv motor i imaptiently purchased it limited the size of my boat. The upside is that this small boat can be easily transported on the bus, the downside is that the boat handles like a pig. This time i want to build a cat that is 66cm. Slightly larger, but still small enough for transport, but im hoping the slightly larger boat will help a bit with the handling and the characteristics of the cat will give me more speed.  Im aiming for 40mph range anythng over that is a bonus. Ive bought a 37mm diameter 54mm length 2000kv brushless outrunner. I have also bought a pair of 2s lipos to run a total of 4s.

4s or 14.8 x 2000 = 29600rpm

fullycharged 4s 16.8 x 2000 = 33600rpm

I remember from last time you teaching me the magic 30k rule, Ive noticed it come up a lot of other forums. Im going to run the batteries in the hulls.

Im using the same plans as this fella has used, mine have been shrunk by a freind.

http://www.rcgroups.com/forums/member.php?u=334941

ive noticed he has used the rudder behind the prop method. I was going to use the offset rudder method like on my mono. what do you recommend?

[pompebled]

Hi Mike,

A 66 cm cat can run 40 mph, no problem, my 80 cm Arowana runs that on 4S too (scratching on the 50 mph mark), but the cat in the link is the Maxcat IMSMW, which is originally designed for a Zenoah and scaled down for electric propulsion.
Scaling down a cat can't be done without consequences for the handling.
The smaller, the narrower, the less it can handle chop, a tennisball on a fishing pole is going to be your most frequently used piece of equipment...

If you haven't started the build yet, only shrink down the length and height and keep the beam, so your cat will be substantially wider compared to the length.

The 2000Kv on 4S will only reach 26500 rpm under load, thats what a monohull would run, to get 'cat like' revs you'll need to run on 5S and have 33000 rpm under load.

A rudder behind the prop need to be longer to work in undisturbed flow and therefore have more drag, an offset rudder can be shorter and has noticeably less drag.

Regards, Jan.

[Mad_Mike]

 i havent got a 5s battery, unless i combine 3s and a 2s, which unless i get the 3s more central than the 2s will be unbalanced. I havent started yet but i really dont want to keep the beem the same cos then it will just look well......  I would rather go larger on the overall boat. 70cm at most which will give it a beam of roughly 20cm. i have a dedicated rescue boat for reftreivals.  If i were to go 70cm can i still use a 4mm flex drive?

Ill stick with the 4s till i can go 6s and get the batteries and a metal prop to match, until then ill just use a bigger prop and steeper pitches.

thanks

this boat is 66cm long if i can get it to run like this ill be happy

http://www.youtube.com/watch?v=2zYHyPnYW0Q

Ill stick to plan you can say told ya so later