Torque Tabs - best method in design?

[Sonik]

I've been running this through my head the last few weeks in my typical overanalytical ways. I have seen many different styles of torque tabs ranging from factory to weld on, rivet on, and home made. The allison cut tabs seem to be the most popular for all intents and purposes but how does one know what is the best/ most effective without overcompensating for any given situation or application? What height, length and degree angle works best for a given setup?

How much does the overall gearcase length, propshaft height, # of blades, rake, cup, and pitch play into the amount of prop torque needing to be mitigated by a tab? At what point does a tab become counterproductive by introducing air across the blades at speed and would this create a blowout condition much sooner than it would without the tab?.....removing the gearcase factor and trim angle of course.

In a nutshell, I just want to know if it is simply a matter of trial and error, or manipulation and testing, or is there a proven mathematical method that produces the best possible end result....minimal drag, maintaining minimal steering angle, and maximum torque / paddlewheel reduction.


Jeff

[specboatops]

I had Titus rework my gearcase last fall, I've only run it 1 time this summer BUT what a difference it made, his torque tab worked very well for my application.

Chris

[Sonik]

I had Titus rework my gearcase last fall, I've only run it 1 time this summer BUT what a difference it made, his torque tab worked very well for my application.

Chris

What specific questions did they ask you when you sent it off??? Propshaft height, power, hull weight, prop style, etc etc etc??

[Forkin' Crazy]

Choppers = more torque than cleavers or semi cleavers. More diameter increases torque. The more blades, less torque. Just from my limited observations.

What are you calling an "Allison cut tab"? I have heard of an Allison cut skeg.

There is a wedge that rivits or welds on, and then there is the weld on tab that actually makes the gearcase skeg longer and is suppose to be a bit faster.

I have one that is home made. It has a slight bend to it and is kind of built like an airfoil. Haven't run it yet though.

[Jacob]

What are you calling an "Allison cut tab"? I have heard of an Allison cut skeg.

Check this out guys
http://www.byuboyz.com/forums/attachment.php?attachmentid=21203&d=1231982075

[specboatops]

What specific questions did they ask you when you sent it off??? Propshaft height, power, hull weight, prop style, etc etc etc??

....All along that line, He asked what hull and horsepower I was running, he wanted to know the application ( race/Ski/Pleasure) he asked what type of prop or props i was running I believe he even asked fat or small shaft. He asked for approx weight of hull, and how well it planned. Top end is day and night from what it was, I guess before I was experiencing blow out, though it wasn't violent, the nose would lift....lift.....lift it would really start to stretch it's legs, then the nose would just drop, softly but would drop and scrub top end numbers.


Chris

[transomstand]

I'm not crazy about holes in skegs, not wild about any type of welding either, so I made mine from thickened epoxy, reinforced with carbon fiber.

I intentionally made it too large and thick, and I'm reducing it's size until I get what I want. Originally, it was just a wedge, and not very effective, so I reshaped it to a concave shape, and WOW, what a difference. Need to cut it down, when you chop the throttle now, it feels like you threw the anchor out:eek:

[Sonik]

What are you calling an "Allison cut tab"? I have heard of an Allison cut skeg.

Poor choice of words on my part but yes, I meant an ally cut skeg with the top notched and a built in tab.....

So does a longer radius and less angle of attack yield more mitigation and less blowout than what a short, high-angle tab would? Conventional logic would say yes but what did titus do differently over and above what you originally had on the case??? I'm curious.....

[Forkin' Crazy]

Are you doing an Johnrude case? They are already swept back like that. I know a few people that have modified Johnrude skegs to be larger like the Merc sporty for less torque.

[Sonik]

Are you doing an Johnrude case? They are already swept back like that. I know a few people that have modified Johnrude skegs to be larger like the Merc sporty for less torque.

I'm referring to no case in particular. If you were to draw an imaginary line from front to rear horizontally along the skeg, the radius created by the tab bending towards starboard is what I'm referring to.

Viewing the skeg from the rear, water will travel over the skeg and be pushed to the right or starboard to counter torque, conversely, water traveling along the port side of the skeg will have to follow a contour opposite that of the starboard side, and can/will create a void inducing some type of blowout at the prop given enough speed and trim angle.

And with that, how does one know how tall, how wide, and how much angle to use when determining the need for a tab based on any specific setup without becoming counterproductive....that is my question.


P.S. If I'm overanalyzing this, just tell me to go pound sand :)

[Forkin' Crazy]

Heck no dude! I wouldn't tell you to do that. Might tell you to go jump in the lake though! ;) :)

IMO the wedge type leave a void like you say, and the weld on type like Bob's would not.

[Sonik]

Heck no dude! I wouldn't tell you to do that. Might tell you to go jump in the lake though! ;) :)

IMO the wedge type leave a void like you say, and the weld on type like Bob's would not.

In reality, both types will leave a void given the proper circumstances, the wedge / flat trailing edge being the worse of the two obviously. I understand there are alot of variables that play into blowout.....bullet diameter, gearcase profile, skeg profile, etc etc but I was hoping maybe someone from Bob's or Alabama Custom Marine and Welding would pipe up and either share a mathematical approach or simply tell me to stop wasting my limited brain power trying to qualify something as simple as a small chunk of aluminum. :D

[transomstand]

or simply tell me to stop wasting my limited brain power trying to qualify something as simple as a small chunk of aluminum. :D

OK:p:D:D

Drive the boat first, establish a baseline, then determine it's needs.

[Sonik]

Ok.... [/end rant] :thumbsup:

[RONNIE]

put a sportmaster on it and stop jerking around!! hows that sarg