Does it matter what direction the 2 props turn on a dual-motor/prop catamaran?

[jlanders001]

I searched for this answer and could not find anything. Please forgive me. This is a newbie question, but I'm no newbie to RC.

Does it matter what direction the 2 props turn on a dual-motor/prop catamaran? Should they turn opposite ways? Which directions are best?

Thanks!

JTL

[Yamadori]

If you imagine a prop like a car-wheel...than you will notice that except propelling function, prop has lateral rotational momentum that is trying to twist/rotate/lean the boat sideways. That is called "prop walk".
On dual motor setups...advantage is twin prop momentum which neutralizes each other as long as props turn in oposite directions. That's why there are left and right props. I think it doesn't matter which side they turn as long as they turn oposite sides...clockwise and counterclo kwise.
Please, senior members...correct me if I'm wrong

[Neveryachtz]

Dual Propellers should ALWAYS turn in opposite directions.

However outboard (top of props turning out) or inboard (top of props turning in) is something I am not completely sure about. I came across an article the other day on another boating forum that discussed this, however they are more referring to displacement hulls.

outboard-vs-inboard-turning-propellers

I am also in the process of setting up a twin, so hopefully someone with more experience with twin prop FE boats will chime in.

[Neveryachtz]

Actually the more I think about it, the more it seems like outboard props are the way to go. Referring to the article I linked above, the outward transverse flow is definitely present in boats with a large transom deadrise. So outboard props (surface piercing like most FE) turning inwards at the bottom to meet the outward transverse flow should be more efficient.

Sound right?

[nobbiworld]

Makes little difference. Have run all 3 ways and all work. All twin nitro riggers run both props in the same direction. You will get a bit of prop walk with this set up. Steve

[Johnnysplits]

You have to test both ways. Generally, props turning outward will give better handling while props turning inward will give more top speed.

[nobbiworld]

Sorry, just read question again. Have found outward slashing works better on counter rotating. Try both. Steve

[iridebikes247]

makes a tremendous difference. on SAW boats 99% of the time props will turn inward. if you run at 50 mph you may never notice a difference but once you reach 70 the differences become exaggerated in a big way.

for cats setup for oval props spinning out help with cornering, really suck the rear end down. for SAW boats spinning in is the way to go low speed handling suffers though.

these are general guidelines both should be tested if you're boat isn't established in the world of SAW or oval racing.

[Rumdog]

It all depends on what hull you're running, and how you like to run. It's easy enough to try both ways

[flraptor07]

I was wondering the same thing, I'm setting up a R2Hobbies Sober twin Hydro it's the one with the motors in the sponsons. Nobody has really had a lot of experience with this boat so I'm sure I'll just have to test to see what works best with this one. I'm going to try props spinning outward to start with, I'll let ya'll know what works best for it.

[jlanders001]

Thanks guys. I'll be setting up this 36" cat just for some good 'ol hobby level fun. No racing, however I want it to be as fast as possible. I'll try both directions and see what happens. Outward first to get used to the new boat. Inward later when I want to max out the top speed.

Thanks for the help!

[jlanders001]

Oh one more thing...

About what size(s) props are best for a 36" catamaran? Is there a rule to figure that out? It will come from the store with 34mm x 25mm x 13mm props, (also has an omega sign and a "4" beside it...shaft size?) but I want metal props so I'll need to know what to buy. I'll be using 2 Tekin 7.5 turn 3000k motors on 6S if that helps. The cans a longer than normal so I think they will have a little more torque than some motors.

What is roughly the best pitch for fast electric cats? Is there an ideal range?

[TheShaughnessy]

Those tekin motors are great for cars, not so much for boats, at least in that kv range on 6s in a cat this size. You'll be searching for the fire extinguisher before the radar gun

[martin]

Yep even on 4s your in SAW territory at over 44000rpm unloaded.

[jlanders001]

OK. I'll rethink the motor kv choice.

About what kv would be good for a 36" dual motor cat? I could do 3S, 5S, or 6S.

Could ya run smaller props to compensate for more lipo cells and higher kv motors?