Twin drive cats.....a discussion.

[Simon.O.]

By now a few of you wll be thinking I am well double dipping as far as threads go but it is not so. I am trying to keep my information in separate folders if you like.
So here is the new discussion.

Cats on twins.

Props.
Is is absolutely necessary to have props counter rotating ?
If they are both L hand what will happen ?
This is for pure pleasure on my local pond, No racing, No rules.

I am after good acceleration, o.k I am keen to go from standstill to airborne/planning instantly, read drag boat type takeoff. I know that I will trade off some top end for this.

I would be keen to go for a 32" hull similar to the H&H Agitator or the Chinese knockoff of it.
I will draw and build the hull myself.

There Will be more questions on this topic.

[Rex R]

Re: prop direction
I note that the full size 'PT Boats' ran 3 props, all spinning the same direction, and I understand that they needed steering input to counter prop walk...
rex

[Eyekandyboats]

One thing i can help you with
If your props counter rotate then it will be Very hard to turn.
If they swing the same way you will have a little bit of prop walk( big deal...) but you can turn alot better.

[kevinlew211]

To me, one direction on twin cat is NEVER can be done, thats was a 1st thing i did with my 1st hpr and did try it again with my 2nd one, of couse i alway hope its running ok, choosing right handed prop is pain in the a**, my cat rudder almost 50% negative to made it run straight doesnt mater what i did, lower strut, set them not level.... anything you can name it, and i have twin rudder also, its alot of drag but its not all, when you slow down or give less throttle, without a warning the boat will fishtail and might flip, about turning, if you want to turn wider, do outner rotation, and inner for tighter turn, i always thought about twin nitro rigger running so fine with same direction, god know why, maybe cus they are rigger, with turn fin and those props are set too close to each other??

[Flying Scotsman]

To the best of my knowledge, as Kevin has stated. The theory is to use counter rotating props that turn INWARDS.
Edit
That is the rotation on my scale Dauntless 49" boat and it turns well and runs straight

Douggie

[Eyekandyboats]

flying.
how can you say INWARD is theory.
Inward and out ward rotation effect different hulls each way, and the distance the prop is from the tub wall effects it as well.
any also the "theory" about how inward and out ward rotation work, is not a theroy it is a fact and has been proven many times.

[kevinlew211]

More you raise the prop is more prop walk you will get
Inner: both prop walking inside means easier to turn ......tighter.
outner: both props walking outside mean harder and wider in turn but easier to control the boat without fishtail, just require bigger pond i guess
The reason i choice outner for my cat because i running twin rudder, i try to avoid "fishtail" problem by turning to tight since i have alot of ....rudder.
Note: i did not notice speed nor lift diff at all on both setup.
back to the topic, if you choice to do twin for your cat then L/R props is the best choice, dont worry too much about 100% right props because you will never can get them unless Octura, Abc, ...etc willing to make right handed props on ALL sizes and types later, but your speed and handling still MUCH BETER than your rudder is 50% .....off with L/L props

[kevinlew211]

Oh.. and one more thing you should know about, it was my lil mistake and i have to live with it until now.
choice your motor KV a lil bit higher than single setup, so you can use Octura right handed props range, remember, they skip from 47mm up to 57mm on their right handed props. Also you can use bigger prop with twin setup without heat attack.
I ran 1527 825kv, 10s1p, props 3.3 total pitch, motor temp was 140+ degrees, esc was +/-120, lipo is HOT on single setup, same prop with twin same setup PLUS 20 inches longer cat, twice of weight, almost double running time, its was 10mph faster, motor, esc NEVER get to 100 degrees, lipo 120 degrees.

[Simon.O.]

I am not sure how to explain this.
Do twins help each other by unloading the props.
Given the data from FeCalc that I use as a ball park figure, it usually is high on amps and that helps for speedy selection.
Can I assume a little less Amps and a bit more rpm given a similar prop.
I said this might not be easy to explain.

I'll do a few FeCalc scenarios soon and we will discuss it.

[Flying Scotsman]

flying.
how can you say INWARD is theory.
Inward and out ward rotation effect different hulls each way, and the distance the prop is from the tub wall effects it as well.
any also the "theory" about how inward and out ward rotation work, is not a theroy it is a fact and has been proven many times.

Taylor, you previously stated ."One thing i can help you with
If your props counter rotate then it will be Very hard to turn.
If they swing the same way you will have a little bit of prop walk( big deal...) but you can turn alot better".
So are you saying that using two conventional turning props are the way to go on a cat

Douggie

[Eyekandyboats]

no i am saying build the boat how you like but be warned on how you have it setup because prop choices effect the way your boat will handle.

[Flying Scotsman]

That is Simon's question. What is the best prop suggestion for his proposed cat hull design. I suggest counter rotating and inwards. The prop design is another matter...pitch etc. and I have no idea what to suggest.

Douggie

[kevinlew211]

I am not sure how to explain this.
Do twins help each other by unloading the props.
Given the data from FeCalc that I use as a ball park figure, it usually is high on amps and that helps for speedy selection.
Can I assume a little less Amps and a bit more rpm given a similar prop.
I said this might not be easy to explain.

I'll do a few FeCalc scenarios soon and we will discuss it.

you can use same KV motor with bigger props on twin setup OR more KV motor with same pitch prop, its up to you, just.... remember that Octura skipped right handed props from 47mm jumped up to 57mm and im stuck right in the middle with my setup, thanks god Abc props is there, i agree with you about Amps = power but since you touch the motor amp limit and go OVER that, its kinda amps we dont need and its "burning" amps, and how we notice that we go over motor amp limit without data log? to me by heat, im not happy to see me 1527 heat up to 160 degree.
Btw, my goat is high speed and low temp on everythings.

[Gerwin Brommer]

Eyekandy :

http://www.swissmodelpowerboatteam.c...06_Victory.wmv, props are turning outwards.

[lomdel]

From the same site a few pics of the 4x Gas engine 2,33m HPR cat with outward turning props and some of the engines running... http://www.swissmodelpowerboatteam.ch/ct04vic.html