Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[ntg06]

Not sure where to post this, so please move to the appropriate section.........

A brief history...I purchased a "flying fish" from ebay (http://www.qi-remote-control-toys.co.uk/catalog/product_info.php?products_id=66), as a tester to see if I liked the rc hobby. I had a few hours of fun with this boat, but wanted more speed. I went out and purchased a couple of offshore warriors from Astec. These have been great fun to scream around the lake with my son for the last year or so.
The flying fish has been sitting around gathering dust, so I decided to strip it out and add a brushless motor and rudder. Took it to the lake on Sunday for the "launch" and all is not well! Plenty of revs, but practically zero forward movement. Obviously there is something wrong with the setup of the boat so would like some help with what to change?
Why did I chose to modify the flying fish?? - I like the look of the boat, and it was a cheap, quick way to learn some building techniques.

Basic setup:
3600Kv brushless (http://www.giantcod.co.uk/bl2845-3600kv-watercooled-inrunner-motor-p-404992.html)
60A ESC
3S Lipo

Boat is just under 1m long, and around 300mm wide.
Weight with lipo is 2.2Kg
CG is around 40% from rear, so this is a little high. As the boat is so light, I was going to just add more weight to back end?
The depth and angle of the original flying fish props have been copied, but, because the motor is now a single centre mounted one, I think I should have mounted it deeper (I think the "V", blocks the prop)?
I bought a 31, 34 and 38mm prop from Giantcod (cheap to try different setups), but none really made any difference.
I have attached some pictures so you can see the basic layout of things.
Any help and comments will be greatly appreciated.

[gwa84]

drop the prop shaft to the lowest part of the v and get it as level as possible with the bottom of the boat where it is its just going to churn water 

[triumphjon]

need to get the shaft angle as shallow as possible and as low into the vee of the keel as you can , you can also move the battey pack aft which willl alter the c of g

[ayrshire andy]

Hi,

Cracking looking boat methinks a quick solution to yor lack of thrust problem would be to fit one of your props to a lengthened shaft running inside the current tube in order to, as fellow mayhemers have suggested, move the prop rearward out of the shadow of the transom...would save you messing about with the current tube/coupling/motor setup unless you have already...you would have to keep the unsupported shaft length within reasonable limits say 75 to 125mm in order to reduce or prevent excessive vibrations or even the shaft whipping about or breaking dangerously at higher revs...also if you are using a plastic prop whose blades deflect under the thrust produced by high revs (and they usually do!) a simple swap  to a carbon fibre, glass filled nylon, aluminium, brass or stainless steel prop may be all that you need before setting about your pride and joy with hacksaws etc...whatever the answer is it's all very fixable do not despair the flying fish is a cracking boat well worth your meaty powerplant!

Best wishes,

Andy

[ntg06]

Thanks for the advise guys.
I have decided to reposition the prop as suggested (not too much work), so just waiting for new shaft to arrive.
Interesting point about the plastic props, I will keep that in mind. I just bought these cheapies to find out the best size, and then I was going to swap it out.
If I am lucky I may be able to get mark II in the water this weekend.
I will let you know the outcome.

Thanks again.

[ntg06]

Just a quick up-date.....got everything put back together friday, and then disaster, the brand new motor has water pumping out of the windings. Waiting for a replacement!

[ayrshire andy]

Maybe they sent you a hydraulic motor by mistake...you can't get the staff these days...seriously more likely a leakage at the motor cooling jacket probably where the inlet or outlet tubing pushes onto the jacket nipple(s)...a bit of epoxy or a tight mini cable tie at either or both joints may be all you need? The fast electric folks will have umpteen solutions to this one if need be: keep us informed!

[TomTheKraut]

G'day,
... one further thing: check the prop. If the prop blades are soft enough that you can bend/flatten them by hand with no or almost no effort, chuck the prop in the bin and get yourself a non-flexible of the same diameter and pitch.
For the current test stage I would recommend Graupner K-series props (black plastic reinforced with carbon fibers) which sound more expensive than they are.

Reason for this move: if the prop blades are too soft they will distort under load and the prop efficiency will change to "nowhere fast". Additionally, soft props more often than not do not feature a "proper" i.e. hydrodynamical correct blade geometry.

Once you have established the best prop for this boat and the setup you are going to use, you can decide whether to continue with K-series props (as quite some people do, even when racing seriously) or to go for brass props (Octura, Prather, PropShop or some such).

Meanwhile I'm keeping my fingers crossed for your project...