Twin Outboard Toe-in Part II: Performance Losses

[ClassicGuy]

Does anyone have an idea of how much performance or fuel economy would be lost from having a pair of engines that was an inch or more incorrectly toed-in? Is it likely to be neglible (say 1% or less) or is it the kind of thing you really notice when you correct (like going 2 kts faster WOT or burning 5% less fuel)?

[auguste]

What kind of boat, engines, top speed etc.? Need much more info.

[trouty]

Classic guy - you can actually answer your own question...

Try to think of it like this.

What if you tied to the dock witgh a hawser and slowly took up the strain and kept increasing revs?

You'd sit still and move a lot of water - but basically your props would slip...and you'd get zero forward momentum.

When you toe in too much your increasing drag (like being tied to the dock only not so bad).

When your stationary, at the end of the hawser - you have 100% prop slip!!!

If you have too much drag from toe in - you can "calculate" your prop slip % from a few simple calcs...

1. Engine RPMs
2. Gear reduction ratio of lover unit
3. Prop pitch

What you do is take engine rpm's times the reduction ratio, multiply the answer, times the pitch of your prop (in inches) - and work out how far (in inches) you should travel in one hour at WOT (Max RPM's) if the prop were in a semi solid medium with no slip based on it's forward movement thru the semi solid with each revolution of the prop.

Convert this to knots or MPH or Kilometers per hour - depending on what units you run your GPS in.

This your "Theoretical" Tops speed...based upon RPM's and prop pitch allowing for the gear reduction ratio - if you had no prop slip.

Now - what is your actual GPS WOT top speed?

Compare the two - and however much you are below the theroretical Tops speed in actual tops speed is the % prop slip...

If it's over 9% you have probably a little too much slip - and it can be attributed to either drag from incorrect toe in,

or,

too little surface area of the props due to worn down dia -

or

too small a blade surface area,

or

not enough blades etc..

Basically - you should be able to compare your theoretical WOT Top end speed, against WOT actual top end speed - before and after the toe in adjustment ..to find out how much the toe in correction reduced slip - this was the figure your after!

Darn it now I'm all confused - what was the question again?

Cheers!

[Mumblerone]

Interesting question. The specification for my boat is 3/4" - 1" toe-in. We had a tornado just miss the docks where I slip my boat last year. The motors were in the up pusition and hit the rear of the slip and bent my port steering arm. This gave me a toe-in of 2 1/4"! The motors looked like they were heading in different direction. Well, almost. Just for the heck of it, after checking for other damage, I took the boat out and noticed no difference in performance...I was amazed. But it still looked like hell. I took all the SeaStar steeing off and rebent the steering arm to spec. This is antidotal, I understand, but it might give you some insight!

Boston Whaler, "MUMBLER", 24' Outrage, twin 175 HP Evinrude Ocean Pros. Snowball, the cat...

[FishEye]

This is not at all a scientific way to adjust toe-in, but here is how I did it. My previous boat had visibly noticable toe-in. Also, the outboards caused spray outward from each motor. I adjusted the toe-in closer to neutral until the spray was gone. Actually I looked from the top of the motors an compared the spray from each side of each outboard. The engines visually looked to have zero toe-in at that point.

[ClassicGuy]

This post grows out of a friendly argument between me and an old friend. We know that his toe-in is not correct by at least an inch. To fix the propblem will be a bit of a pain since the bar will have to be cut down. He doesn't think it is worth it. I think that the benefit will be significant (for the reasons Trouty identified, the water flow over the props will be clearer and rsult in less slip), so I thought I would try to find others who might have corrected their toe-in and seen performance changes. The boat is a 26 Regulator with twin Yamaha F225s.

[Mumblerone]

ClassicGuy...who set the toe-in to begin with; or did you hit something. Not the kind of thing that changes by itself.

Boston Whaler, "MUMBLER", 24' Outrage, twin 175 HP Evinrude Ocean Pros. Snowball, the cat...

[ClassicGuy]

Not sure how it got that way -- the boat was repowered and my friend thinks the dealer screwed up in reusing the old tie bar when it needed a shorter one. He had never checked it until a couple of weeks ago.

[DiamondDave]

Has anyone contacted Regulator and asked about the toe-in. They might be that way for a reason.

[ClassicGuy]

That's how we know it is wrong -- he asked Regulator what they recommend.

[Mumblerone]

Is it Teleflex/SeaStar...do you have the Tie Bar Instructions? they tell you exactly how to cut bar to the proper length. I'd be glad to send you a copy, only one page, if I get your e-mail.

Boston Whaler, "MUMBLER", 24' Outrage, twin 175 HP Evinrude Ocean Pros. Snowball, the cat...

[ClassicGuy]

We have them, thanks.