Counter Rotating Outboard Engines

CORRECTION!!! I stated the boat has 3/8" toe in*, this is wrong. It now has 3/16" toe in, measured from the tip of the nosecone and the back of the props.
This is on a hull with five degrees deadrise at the transom.

*Note to self, measure twice, post once...

 

Good question!

I'm inclined to say at the front of the motor like with stern drives, so you can still use individual trim and tilt. But I've seen external tie bars as well, with a common switch for trim/tilt, mainly on very powerful engines.
If one tilt motor fails, something is bound to snap or bend.

 

Yes with a shallow dead rise the toe in will be less . If the dead rise was greater so to would the toe in be more .

The tie bar on modern out boards is the tube that goes through the pivot so would be at the front .
Racing twins and triples have tie bars at the back as it changes the geometrics of the turning of the outboards . Like streering in a car the rake is at the back of the axles never at the front !!

 

OF course the other difference is when in front it will be connected to the tiller arms which are rubber mounted to the engine.
Behind it will be sold mounted
If the steering is only connected to one engine then you can have either tie bar as you might then be able to full tilt an engine depending on how far apart they are.

 

Tunnels
outboard rotation on a planing hull greatly help in stability
inward rotation is sometimes used as you get more bow lift from this but many would say its dangerous on a fast boat

Toe in even or out depends o the boat speed generally
toe out is better on fast boats as it makes the sterring more stable
Mercury would say 70mph+ toe out

 

Can you (or anybody) explain the theory connecting deadrise angle and toe in?

I'm trying to understand the "why" of toe in.

 

the water flow ( at low speed) has to converge and meet behind the boat to fill in the hole the hull made.
If the hull was flat bottom they would be parallel yes?

 

That makes perfect sense and my mind can grasp that

Thanks powerabout.

Then speed = less boat down in the water = less angle to water converging at props = less toe in, am I right so far?

But what is the "why" to help me visualize changing to toe out? at a certain speed?

 

helps with to prevent chine walking

 

Toe in has the tendancy to raise the nose of the boat so to make it go down fit trim tabs to do a final trim .
If you have a boat where there is nose up you need toe out this will lower the nose .
Its easyer to use trim tabs to lower the nose than to have a natural nose down and try to raise it . Have to shift weight or realign the motors if they are inboards !!
The same thing is done on Aircraft with multi engines . Racing outboards they play with the set up to make the boat perform to its max all the time

 

Thanks again powerabout.

I've seen a number of reasons cited for chine walking.
Category 1) Motor mount play, steering play...
Category 2) The prop's torque on smaller like bass boats
Category 3) Once the boat is riding on a small v section or a pad it's now flying and not aerodynamically stable. It tips a tiny bit until a chine hits. This then violently forces it over to the other side and the effect gets worse with each hit.

Is the prop itself in play for causing chine walk on a twin?

Or if it's down to category 3, how exactly does the toe out help prevent it? I'm having trouble visualizing the "why it works" again.

 

Also when you change to toe-out is there any small but measurable speed penalty for going against the logic of post 22?

 

if you have toe in when the boat rolls the low engine is now trying to turn the boat back.
With toe out the engine now fully submersed is trying to hold the boat to that turn hence its only a small force but can make the difference as you are right in that there are many reasons and cures for chine walk.
Post 22 rules are for slow boats, 70mph and above being fast but that depends on the size an 18' boat at 70 has not much hull in the water a 50' boat at 70 has quiet a bit.

 

Thanks again powerabout.

Another thing I did not think about before. I have heard people talk many times about changing prop rotation to change bow or stern lift. I never heard before that toe in of the drives would also lift the bow. Thanks tunnels.

 

me either