Off center single engine

"Prop walk" makes a boat turn slightly based on which direction the propeller rotates. On some older single inboard designs, the shaft was slightly angled to the centerline to cancel out this effect.

On single outboards the same happens, but the helmsman just turns the wheel till the boat goes straight. Since there is no indication of the angle of the engine to the angle of the keel this goes unnoticed.

If in this situation the outboard is theoretically pushing the boat at a slight angle to which it is traveling wouldn't it be more efficient if the outboard was mounted slightly off-center and therefore could point parallel to the keel while the boat was going straight?

Most difficult question is, how much? Has anyone done this calculation? Does anyone have a guess based on experience? Is it 1/8" , 1", or 8"?

Why? Who cares? Me. I'm naturally curious.

 

If you were to specify a particular hull/outboard, and the speed that you wish to target, a more straightforward answer might be available.
Most outboards have a trim tab behind the prop that is designed to correct the problem, and it may take some fiddling around to get it set correctly, but it will only work as intended at high speeds.
I don’t believe there’s any real advantage to offsetting an outboard, or an inboard for that matter, unless a specific hull design or quirk heavily amplifies the problem.
The propeller operates in a liquid medium, subject to many variations, and constantly changing, so accurate engineering of such forces is impossible, and the best “compromise” position is straight ahead on the centerline.
A slight nudge on the helm easily corrects prop walk at speed, and a decent autopilot will do it better than a human can.

 

I've had a boat with an offset inboard I liked because of the exaggerated prop walk effect when put in reverse. It could really turn on a dime because of that. Mind you, only in one direction, but it could.

You also could also approach a dock almost perpendicularly, put it in reverse and it would gracefully "skid park" in really tight spots.

 

If you had a power cat with assymmetric hulls like the one in my avatar, you would not notice any difference re an offset engine - in her early days, when she had 2 x 115 hp O/B motors, she was running for a while on just one engine, as something major happened to the other one. They were initially fearing the worst, thinking that she would just want to turn in circles, as her beam is 16', but they were surprised to find that she still tracked perfectly straight on one engine, with no correction needed on the helm.

 

Maybe I should have mounted Serenity's single 250 off center, away from the bubble stream.

 

The tab on the cavitation plate is intended to correct feedback at the helm. Not the exact same thing as prop walk.

If a boat has a right-hand outboard and the motor is parallel to the keel the boat will steer to port. The bottom half of the propeller throws water to the port pushing the aft end to the starboard which steers the boat port. The top half of the propeller does the opposite, but it is impeded by the cavitation plate on an outboard (the bottom of the hull on an inboard). The bottom of the propeller has a larger effect.

This sideways force gets transferred to the boat through the steering so it will cause feedback at the helm. But other things also affect feedback at the helm. The torque of the motor can create feedback by itself.

I find it interesting that we have "No feedback helms" but no one runs an outboard without the tab to correct feedback.

If a true "no feedback helm" existed, moving the engine off centerline (to port for right-handed outboards.) Would correct for prop walk and be more efficient.

Outboards are different than inboards. Angled-to-centerline prop shafts were pretty common back in the day. Efficiency was more important back when horsepower was heavier and more expensive. Atkins Rescue minor is an example below. The drawing shows the propshaft angled 4 degrees to centerline.

I thought it would be a helpful design feature in boats that had an asymmetrical layout (i.e. side consoles or asymmetrical walkarounds)

 

Well, I have 4 outboards, only one of which has a trim tab. And I know it's a nitpick, but the flat plate above a prop that is commonly misnamed a cavitation plate is correctly called a ventilation plate, since it has nothing to do with cavitation, but does prevent ventilation in typical use.

 

Actually many of the helm companies have no feed back helms. Seastar and others. The newer cable systems and most certainly the hydraulic systems are no feed back.

 

"No feedback" but if you take your hand off the wheel it doesn't stay straight long. If it worked, why do all the outboards have the tabs installed? They make flat versions for twin applications.

 

Agreed. Haven't trained myself to change the term yet

 

The hydraulic are 100 percent no feedback you quit moving the wheel, the valves close and the steering is locked.
The new rotaries are advertised as no feedback and I have had them on outboards up to 120hp with no issues.

The old rack and pinion crept and the old cable and pulley set , there are some of us lurking around here who know what I am talking about, required a hand on the wheel.

 

Why are the tabs installed?

 

I have never adjusted outboard tabs. The last two boats were hydraulic seastar with either twin 300's or twin 250's
In all of our jets, with the exception of the first one (rack and pinion) we used rotary. You could put the jet into the turn and not feel that the jet thrust wanted to straighten out. Ie little or no feed back.

Additionally I just checked Images for Merc outboards and did not find a tab on the larger outboards and for some reason the Yammy site had picts of some with the adjustment and some without

So my question to you is this
Why do some engines have them and some do not?
Seems if it is a significant problem then all of them should have the adjustable fin.

 

Well hopefully someone has some idea. Curiosity is killing me.

 

http://www.seastarsolutions.com/products/mechanical-steering/nfb_tilt-rotary/