Outboard vs inboard turning propellers on a catamaran... What are the benefits?

[boilo56]

Hi everyone! I decided to experiment on way props are spinning on a cat. Don't think i'm an expert! I'm way from being that. But still i wanted to see what would be the results.

Here are my observations from what i have noticed on my brand new C1 Flowmaster twin motor cat.

-speed increase,0.4 km/h.(not big, but it is there)
-Better turning abilities.(the lifting effect of the props going outward the boat gives a lifting effect that keep the boat to flip over( at some point, remember, it is a cat.))
-Again, beter turning abilities. ( The water being pushed upward, in the middle of the transom, gives more bite to the rudder.)
These are my first observations about this setup.

If anyone have more info,experience with twin motor cats, observations, or theories, please comment!

[lt130th]

In full scale cat's, I believe inboard rotation gives you more top end speed. Since we're considering RC boats that are driven at wide open throttle the majority of the time, and are traveling at an outrageous scale speed, it makes sense for you to actually see a slight top speed increase with inboard rotation. Outboard rotation may give you better control at really low speeds, in a full scale boat, but I don't think too many guys are concerned with docking their RC boat.

[Brandon Galles]

A Theory.... In a twin engine boat spinning outboard or , RH on right side, LH on left, the torque of the propellers tends to return the boat to level when one side or the other leans outward. When props are spun inboard RH on left, LH on right, or counter rotated the torque tends to make the lean worse. This occurs because as a boat(Cat or Vee) leans to one side, the prop on that side buries deeper and exerts more force, while the prop on the other side rises (in some cases out of the water) and exerts less force. So...... If a RH prop is spinning on the right side of a boat leaning to the right, the prop tends to push the hull back over to the left. (spinning Outboard). If, however, the left hand prop is on the right side in a right hand lean , the prop tends to continue to roll the boat over. I submit that this is why there has been such a dramatic increase in barrel rolls (even in straight-aways) in races in recent years. When I flew over races for 16 years, I could always spot the counter rotated boats, because they behaved very differently (and definitely not better)than their properly spun (outboard rotation) competitors.
The reason that many people spin them "backwards" is the fact that on some hulls the boat may gain a mile or two per hour at top speed....or..... the boat has a bottom tune that needs more bow lift. The speed can generally be achieved in other ways without using an unnatural rotation as a "bandaid" (This is particularly problematical on stepped bottom boats that skid first, then catch suddenly, then roll outward......get the picture?)
One can re-tune most bottoms to add rocker to provide bow lift without resorting to ill advised prop rotation changes.

[boilo56]

A Theory.... In a twin engine boat spinning outboard or , RH on right side, LH on left, the torque of the propellers tends to return the boat to level when one side or the other leans outward. When props are spun inboard RH on left, LH on right, or counter rotated the torque tends to make the lean worse. This occurs because as a boat(Cat or Vee) leans to one side, the prop on that side buries deeper and exerts more force, while the prop on the other side rises (in some cases out of the water) and exerts less force. So...... If a RH prop is spinning on the right side of a boat leaning to the right, the prop tends to push the hull back over to the left. (spinning Outboard). If, however, the left hand prop is on the right side in a right hand lean , the prop tends to continue to roll the boat over. I submit that this is why there has been such a dramatic increase in barrel rolls (even in straight-aways) in races in recent years. When I flew over races for 16 years, I could always spot the counter rotated boats, because they behaved very differently (and definitely not better)than their properly spun (outboard rotation) competitors.
The reason that many people spin them "backwards" is the fact that on some hulls the boat may gain a mile or two per hour at top speed....or..... the boat has a bottom tune that needs more bow lift. The speed can generally be achieved in other ways without using an unnatural rotation as a "bandaid" (This is particularly problematical on stepped bottom boats that skid first, then catch suddenly, then roll outward......get the picture?)
One can re-tune most bottoms to add rocker to provide bow lift without resorting to ill advised prop rotation changes.

Ok! So i was right with my observations! The boat was more stable in high speed turns! And the extra lift generated, helps to get the transom out of the water more efficiently.

[centralrcmystic]

Ok soo which way should I be spinning my props inward or outward

[boilo56]

For my boat, outboard is the way to go, but you have to experiment, as different boat hulls won't behave the same. to me, it seems that, for my boat, outboard makes it more stable in high speed turns and also in straight line.

[MarkF]

Inward is for saw boats. Outward is for oval and makes for a better turning boat.

Mark

[Fluid]

This thread is eight years old, the OP hasn’t been back to the forum in years, he won’t see your post.



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[paulejr]

its kinda like dihedral in an ac wing .. forcing the fuselage to center up versus a symmetrical wing which has less lift and does not try to center the ac... The F4 Phantom, for example, has a very radical airfoil that is extremely acrobatic ( as a fighter should have ) dihedral in the wing and down facing ( anti dihedral ) horizontal stab. now on a boat or hydro .. yea not so much, as the props are the way to make it behave compared to a single, for cats and ??? . Now , if they figure out how to fix a submerged foil like the Sail GP boats use ... all bets are off.

[Paintendave]

So on a mono step hull inwards and on a Cat hull outwards ? Thanks

[fweasel]

So on a mono step hull inwards and on a Cat hull outwards ? Thanks

no. different hulls like different setups. the answer is try it both ways and determine what runs better.

[Paintendave]

Okay thank you!