Noob question about twin engine

[Pete Meisinger]

In older boats, especially gasser V-8s most engines were counter rotating. More modern boats and in line diesels the counter rotation is often accomplished by the transmission.

pete

 

[Offline]

Go easy on a newbie here please.


On a twin-engine boat, assuming you're bopping along at 8 knots or whatever, are both engines turning the same direction, and therefore "forward" for one transmission is actually the opposite of "forward" for the other so that the props turn in opposite directions?

Assuming you are running in a straight course.
Yes they should be turning at the same RPM. Most boats today have synchronizers to make it easy to have both engines running at the same RPM.

If you turn the boat you will lose the synchronization. The outboard engine will have a higher RPM than the inboard engine. Once you return to a straight course they will return to a synchronized state.

 

[Xlantic]

counter rotating diesels

In older boats, especially gasser V-8s most engines were counter rotating. More modern boats and in line diesels the counter rotation is often accomplished by the transmission.

pete

In my older boat, while a not a gasser but a diesel (GM Toro-Flo), the engines are counter-rotating and the Velvet Drives transmissions (Warner Gear AS5-73C) reverse the rotation of the engines.

So, starboard engine turns left (when looking from the stern) and port engine turns right. The transmissions then invert this so the props turn the traditional way: starboard to the right and port to the left.

As mentioned, newer engines tend to only rotate in one direction and, in twins, it is up to the transmission to invert the rotation of one of the engines. This is usually done simply by the way the control cables are connected to the transmission: when control levers are in forward, one of the transmissions is actually running in reverse and vice-versa.

In my older Velvet Drives this is not possible: running in reverse can only be done for a limited period of time and power as the reverse clutches are not as sturdy as the forward ones. I believe this is true in all the Velvet Drives except, as mentioned, the CR2 where changing a gear in the transmission would make it rotate the prop in either the direction of the engine or the opposite.

I am now contemplating a re-powering and it looks like I will need to change the transmissions as well as the new engines will rotate in the same directions and my transmissions cannot be set-up to achieve counter-rotating props.

 

[Xlantic]

not only prop walk

In twins, in addition to prop-walk (sideways thrust at the stern as a result of the downward angle of the props axis) I would think there is also a force at play given the props are off-centre.

So, when the starboard engine is engaged, the off-centre thrust of the starboard prop will tend to push the stern to starboard. In conventionally-proped twins, this effect will add to prop-walk and magnify it.

If a twin-engined boat was configured the opposite way, i.e., in forward the starboard prop turns left, the two forces (prop-walk and off-centre thrust) would act against each other and the net sideways motion of the stern would be less.

So, it looks like in conventionally-proped twins, the aim is to magnify prop-walk, not diminish it. To me this makes sense as prop-walk can be our friend.

I will stop now as I am beginning to risk damaging my brain ...

 

[Trimjb]

Peter, Those WW2 units you refer to are actually called Twin Pac (6051). Never saw a quad Pac.
Twin Pacs were used in Tanks as well as heavy landing craft. The marine engines used were the pre Detroit Gray Marine HN9 and later 6-71. Beauty was you could disconnect each engine from the transmission and still have one engine coupled to the transmission.
Neat systems to work on in their time.
Landing Craft (LCU MK Boats) ran two HN9's and later 6-71 both right hand rotation all the way thru to the props. I run one as a Commercial Diving Work Boat.

 

[Scooby5959]

Go easy on a newbie here please.


On a twin-engine boat, assuming you're bopping along at 8 knots or whatever, are both engines turning the same direction, and therefore "forward" for one transmission is actually the opposite of "forward" for the other so that the props turn in opposite directions?

Counter rotation is used to offset all the forces you get when spinning a propeller . prop walk is caused by a downward angle of the propeller that causes prop walk .spinning the propellers the same way you would have to compensate for that sideways motion with the rudders which would cause a lot of drag which would not be good. Prop walk is just another one of those things you have to deal with like wind and current . in a perfect world there would be no problems . Prop walk ,wind or current the boat would just sit there motionless until you put it in gear . twins and a bow thruster would be wonderful. Twins to control the stern bow thruster to control the bow . get control of a horses head control the horse

 

[psneeld]

Wikipedia on prop walk...

"Propeller walk is caused by the water, moved by the propeller in an axial direction and in a rotation. The water, coming from the propeller, gets a cone shape, widening when it leaves the propeller.........."

 

[Scooby5959]

If you would of read further it stated that the angle of the propeller pushes water onto the bottom of the hull causing side motion or rudder effect. There is also a interesting article in passage maker that also claims angled propeller causes it for a different reason. Google it . Outboards,stern drives and pod drives do not suffer prop walk because there propellers are not angled

 

[koliver]

If you would of read further it stated that the angle of the propeller pushes water onto the bottom of the hull causing side motion or rudder effect. There is also a interesting article in passage maker that also claims angled propeller causes it for a different reason. Google it . Outboards,stern drives and pod drives do not suffer prop walk because there propellers are not angled

When trying to understand the difference in prop walk between fixed propellers and outboards, it is useful to note that the hull and keel surfaces that the rotating cone of reversing water can act upon are much closer to fixed propellers than to outboard propellers, so the effect is much greater.

Any effect of a lesser angle of that cone is also going to be much less when the rotating cone of water is further away from those surfaces.

 

[Scooby5959]

When trying to understand the difference in prop walk between fixed propellers and outboards, it is useful to note that the hull and keel surfaces that the rotating cone of reversing water can act upon are much closer to fixed propellers than to outboard propellers, so the effect is much greater.

Any effect of a lesser angle of that cone is also going to be much less when the rotating cone of water is further away from those surfaces.

I don’t particularly want to be rude however when I read this all I could do was laugh

 

[psneeld]

As one once said...."if you read further"....there are many other factors that affect prop walk which include distance from hull, pitch, diameter, depth of water, etc....... some of which can affect it greatly, as much as angle in my experience.

 

[ProMaritime]

Most twin engine boats do not use left and right hand transmissions or left and right turning engines, they use left and right hand props....much simpler.
US engines all turn clockwise when view from the rear whereas British engines were always counter-clockwise, excuse me, anti-clockwise. Don't know about today's Brits.
Same with propeller driven airplanes...left rudder on take in the P-51 (and lots of it, even with the US Packard built Rolls-Merlin engines) and right rudder on take off in the Spitfire.
I saw a three-engine outboard setup the other day. I notice that two props turned one direction and one turned the opposite direction so I ask the guy on board how he deals with the middle engine. When docking leaves it in neutral. Makes sense. But I would think that at speed with all engines at the same RPM she would pull in the direction of the single left-turning engine. Note this was accomplished by propeller pitch only not by having different engines or transmissions. That would be really stupid and expensive on maintenance side....but I sure someone has tried it.

 

[KnotYet]

Most twin engine boats do not use left and right hand transmissions or left and right turning engines, they use left and right hand props....much simpler.
US engines all turn clockwise when view from the rear whereas British engines were always counter-clockwise, excuse me, anti-clockwise. Don't know about today's Brits.
Same with propeller driven airplanes...left rudder on take in the P-51 (and lots of it, even with the US Packard built Rolls-Merlin engines) and right rudder on take off in the Spitfire.
I saw a three-engine outboard setup the other day. I notice that two props turned one direction and one turned the opposite direction so I ask the guy on board how he deals with the middle engine. When docking leaves it in neutral. Makes sense. But I would think that at speed with all engines at the same RPM she would pull in the direction of the single left-turning engine. Note this was accomplished by propeller pitch only not by having different engines or transmissions. That would be really stupid and expensive on maintenance side....but I sure someone has tried it.

I'm not trying to nit-pick here; if the engines both turn the same direction then one
transmission will have to reverse that rotation to its shaft.
The propellers are then pitched to match that.
Just putting a left pitched prop on a right turning shaft would result in reverse thrust.

 

[Scooby5959]

Like Forrest Gump said I’m not a smart man but I know how to dock a boat without prop walk

 

[rslifkin]

Like Forrest Gump said I’m not a smart man but I know how to dock a boat without prop walk

You can absolutely dock without it, but on a boat that has significant prop walk, you learn to work with it. In some situations it'll fight you (more so with a single than twins), in other situations it can be used to your advantage.

 

[Caballero II]

My two Detroits run "inboard" to cancel torque steer. (Port counter clockwise and SB clockwise facing the front of the motors ie looking towards the stern).

Then the transmissions reverse both rotations so as to have outboard turning props. Outboard/forward inboard/reverse turning props amplify the asymmetric thrust with prop walk and help in maneuvering. Inboard forward turning props can give a little more speed.

So I have a left and right turning engine and a left and right transmission.

Wow. I like it, but am admittedly a little challenged in the understanding. That sounds like a whole undergraduate course. I'm going to have to think about this. Tell me more.

 

[Caballero II]

Like I said: Inboard turning engines to counteract torque steer, outboard turning props for maneuverability.

Define "torque steer".

 

[oscar]

Define "torque steer".

Ever put your foot on the gas in a muscle car? One front corner trying to jump off the ground? Same thing, and worse, would happen if you had two engines in a boat spinning the same direction. Shaft/flywheel etc accelerate, the block around it wants to go the opposite direction. And since the boat is attached to them it goes with them. So you shove the throttles forward and the boat wants to turn. Counter rotation negates that.

 

[toocoys]

I love twins....


... I think these are counter rotating.

I supposed if you can do that, then I can do this.


These are more my speed.

 

[Wayfarer]

I supposed if you can do that, then I can do this.





These are more my speed.

But are they counter rotating?

 

[toocoys]

But are they counter rotating?

Oh yeah... they rotate. One goes to the left, the other to the right.

 

[oscar]

I supposed if you can do that, then I can do this.


These are more my speed.

Which reminds me..... I can't find my red Speedos. Wife denies all knowledge but mumbled something about "slightly overweight balding old farts in inappropriate swim wear......"

 

[FF]

On a boat with a single engine , and nice big prop, and good prop walk , the boat will have a preferred side to dock. No problem at all.

 

[Fish53]

Inboard turning props when in ahead obviously turn outboard when in reverse and thus provide "prop walk" to their respective side. To anyone that's operated large single screw boats "prop walk" is an invaluable tool well worth being familiar with. Most oil supply boats used to be set up like above although now with bow and stern thrusters and dynamic positioning I'm not sure how they're setup.

 

[Fish53]

On a boat with a single engine , and nice big prop, and good prop walk , the boat will have a preferred side to dock. No problem at all.

Right on!

 

[Scooby5959]

Inboard turning props when in ahead obviously turn outboard when in reverse and thus provide "prop walk" to their respective side. To anyone that's operated large single screw boats "prop walk" is an invaluable tool well worth being familiar with. Most oil supply boats used to be set up like above although now with bow and stern thrusters and dynamic positioning I'm not sure how they're setup.

I don’t know exactly what you mean by Oil supply Boat . Here on the Columbia river they use tugs or pushers whatever you wanna call them to push barges with fuel while up-and-down the Columbia river . and I Can assure you they don’t use prop walk they have a lot of horsepower variable pitch propeller‘s and a tremendous amount of thrust . I think all of them are singles . when I am in the locks they always ask me to leave first so that they don’t aerate the water and sink me that’s how much thrust they have I can assure you they do not rely on prop walk . Next thing you’ll be telling me freighters wouldn’t be able to dock without prop walk they’re all shingles big propellers lots of horsepower you guys need to get over your prop walk it’s more of a hindrance than ever is a help if the wind or the current or whatever all works together were you can actually use prop walk to push yourself into a side tie . I use the thrust from my propeller and my Rudder.

 

[psneeld]

If you only really knew.

Most here and most pros I know including published authors disagree.

The pusher below (labeled as Columbia River pusher) obviously has twins like most pushers (towboats) I know and have driven. Many more images I researched do too. (Photos from Google).

 

[Fish53]

I don’t know exactly what you mean by Oil supply Boat . Here on the Columbia river they use tugs or pushers whatever you wanna call them to push barges with fuel while up-and-down the Columbia river . and I Can assure you they don’t use prop walk they have a lot of horsepower variable pitch propeller‘s and a tremendous amount of thrust . I think all of them are singles . when I am in the locks they always ask me to leave first so that they don’t aerate the water and sink me that’s how much thrust they have I can assure you they do not rely on prop walk . Next thing you’ll be telling me freighters wouldn’t be able to dock without prop walk they’re all shingles big propellers lots of horsepower you guys need to get over your prop walk it’s more of a hindrance than ever is a help if the wind or the current or whatever all works together were you can actually use prop walk to push yourself into a side tie . I use the thrust from my propeller and my Rudder.

I'm afraid your knowledge of vessel maneuverability is somewhat lacking. I admit I have no experience with pushboats beyond the Mississippi where most use flanking rudders and push 1000ft tows. I won't bore you with a description of those, I'll leave it to you to educate yourself on that. Oil field supply boats are 175 to 300 hundred foot vessels used to, obviously, supply offshore platforms in the oil and gas industry in various places around the world. As the utility of having good maneuverability in confined space seems to have eluded you it would appear the why would be a waste of time. The how it seems is also foreign to you but in a simplified way I'll describe it for you. When a propeller rotates it creates asymmetrical thrust, especially in reverse where the bottom of the rotation effectively moves more water thus creates sideways thrust. This thrust and the resultant pressure against the hull as the blades reach closer to the top of rotation has the effect of moving the boat sideways. Now I know an accomplished seaman such as yourself has no need of such information but many of us mortals do. With a little way ahead on a twin screw supply boat and the judicious use of reverse on the inboard engine and the bow thruster it's pretty simple to walk a boat to the dock. with no fuss. the same goes for pivoting the vessel using bow thruster and opposing engines. I'm confident you can probably do this by using the strength of your will but the rest of us need help. I would suggest some reading material but that doesn't appear to worth the effort. As for your oddly ill-informed comment about "freighters" I'm afraid I don't have time or interest, get back to me if would like to discuss. I'm glad to answer questions or enter into a respectful discussion but when the demeaner of a comment seems more to create contention than conversation I'm afraid my sarcasm demons are lost from my control, please forgive them.

 

[oscar]

you guys need to get over your prop walk it’s more of a hindrance than ever is a help.

Hmmmm........

 

[psneeld]

I was just appropriately spanked for my reply, I applaud our forbearance.

I liked your post #58.