Innie vs Outtie

[Nightsky]

My twin screw boat has outward turning props when in forward. I read somewhere that you need inward turning props in order to "walk" the boat sideways. I tried testing this and was unable to walk the boat whether I had the rudder turned to one side or the other. My questions are: 1) what is the advantage of outward turning props? Vs inward turning? And 2) has anyone here switched from outward to inward or vice versa? If yes what was your reason and did you get the results you were hoping for?

 

[Gabe n Em]

It's just a difference of whatever direction the prop will walk when in reverse. you need to reverse on the port engine to walk to stbd and vice versa.

switching would require swapping engines and props or at least gearboxes and props depending on whats in there. no reason to though.

mine are outward turning as well - no problems walking

 

[Benthic2]

I believe a twin engine boat has counter rotating props to eliminate prop walk. In order to "walk" you'd need to only use 1 engine.

 

[Comodave]

Twins will usually walk unless you have a fairly large keel. My President does not want to walk probably due to the deep keel. Planing hulls without keels will usually walk. By walking, people mean getting the boat to move sideways without a lot of forward movement. You put the helm hard over away from the desired walking direction. Then put the engine on the side you want to walk to in reverse and the other engine in forward. The forward engine prop discharge will hit the rudder trying to push the stern away while the engine in reverse will hold the stern from moving forward and the discharge current does not hit the rudder since the engine is in reverse. The result will be that the force left over will be a sideways force and the boat will "walk" in that direction. Hopefully I explained this ok. You just have to try it and see if your boat will walk or not. And if it will walk, how much throttle is needed and how much the boat does or does not move forward.

 

[Nightsky]

Thanks Comodave. I have no keel to speak of. The way I thought it would work was like this; to walk to port turn rudders hard over to stbd put stbd engine in forward port engine in reverse. Boat just wanted to spin in ccw circle. Maybe rudders are too small to have much effect or I didn't apply enough rpm on stbd engine. I was in a narrow fairway and maybe didn't give it enough effort. It got me to thinking that maybe the prop rotation being inwards (while in forward) would have greater force acting on the rudder from the stbd engine and the port prop would walk sideways to port with more traction.

 

[Comodave]

Rudder size does play into this. You may be able to play with different power settings to overcome it. From your discription it soulds like your reverse engine is overpowering the forward engine. Maybe try a bit more power on the forward engine. Get away from everything so you have room to move (and errors) and just try different settings and see what happens. Good luck.

 

[Peter B]

Thanks Comodave. I have no keel to speak of. The way I thought it would work was like this; to walk to port turn rudders hard over to stbd put stbd engine in forward port engine in reverse. Boat just wanted to spin in ccw circle......

That didn't work because to 'walk' either way, ie when basically stationary, the rudders are almost irrelevant. Sure a burst in forward gear with rudders hard to starb'd or port, will push the stern in the opposite direction, something useful in say swinging the stern in the desired direction when you have enough room, especially if reversing out of a slip. I do that all the time to turn my single with no thrusters. But at slow speed or stationary, to walk the stern, if you put the engines in opposite gears, they tend to cancel this movement out, or turn the whole boat on it's axis, as you found.

If you want to gently walk the stern to port with your set-up, and outward rotating props in forw'd, then you would just use the starb'd engine in slow reverse to walk to port, and vice versa to walk to starb'd. Visualise the prop in this situation sort of acting like a wheel gripping the water and moving it sideways, rather than as a propellor pushing it forwards or backwards. This then only works until the 'push' effect overcomes the 'walking wheel' effect as the boat's inertia is overcome. Does that help..?

 

[psneeld]

Hmmmmm.....a lot of stuff I cant agree with.

Outboard tuning props are common because the prop walk on single egine use (on a twin engine boat) is amplified by the offset thrust, making the boat turn faster without rudder use.

Though I have only seen inboard turning props on racing boats, something to do with the wash for speed.....I do believe they do work well for a more direct walk sideways.

If the starboard engine on a inboard turning prop is put into reverse the stern walks to starboard but the bow might swings a bit to port. If the engine is goosed a bit, the prop walk increases and so does bow swing. As the prop starts to gain and move the boat backwards, a bit of forward on the port engine keeps the bow straight. To accentuate the starboard push from the port engine...full left rudder will negate some forward motion and give even more push to starboard. To go to port, switch engine directions and rudder.

On an outboard turning propped boat, to go to starboard, the port engine is reversed to pull the stern to starboard. The bow will swing to port pretty rapidly, so the starboard enging has to be in reverse to counter the twist. Now both engines are in reverse...not as effective for moving straight sideways. So the rudders can be hard right and bumping the starboard engine slightly may keep the twist out with the prop walk and asymetrical reverse thrust on the port engine may be enough to pull the boat sorta sideways. But with the bumping in and out of gear, its more of a ratchet sideways with some fore and aft movement.

Now, boats can be funny.....with keels, no keels, size of rudders, props, etc....some outboard turning propped boats may go sideways pretty well. But in my experience most dont, so you use the greatly accentuated assymetrical thust bu nosing in to the forward end of your spot and by using the outboard engine in reverse to slow, it pulls the stern right in.