Torgue Steer - vibration

Sarge · Posted: Wed Apr 25, 2007 7:59 pm Post subject: Torgue Steer - vibration

I tried searching on this topic and couldn't find an answer to my question, so I am posing it here...

Is it normal for a boat to have slight vibration when turning either with or against the propeller rotation. I'm getting a slight vibration when I turn to port. I have a Mercury Vorado 150. The factory rep says it's normal, but then again he's paid to say that. Is he speaking the truth or pulling my fin?

Thanks,

-Sarge

thataway · Posted: Wed Apr 25, 2007 11:39 pm Post subject:

I suspect that you may be having some prop cavitation or ventillation. It may be accentuated by the prop's rotation as the flow dynamics change with the turn. I can't say that I have ever noticed this in any of my boats, but have heard of it happening.

Sarge · Posted: Thu Apr 26, 2007 12:53 am Post subject:

Let me end my own thread...

For those of you with a Mercury Verado 150, here is the skinny:

http://www.mercurymarine.com/uploads/5014/1390/FourCylinderInsert.pdf

If you don't want to click on the link, it states:

Jazzmanic · Posted: Thu Apr 26, 2007 3:04 am Post subject:

Sorry Sarge,

I guess it would've been nice to know ahead of time... Mad

Peter

Wefings · Posted: Thu Apr 26, 2007 7:44 am Post subject:

The question is do you have hydraulic steering [Teleflex Seastar] ? If not , that a very heavy motor to be pushing and pulling by a cable . If you do ,and I believe its standard on the 25, the torque is affectd by motor trim angle and speed .A right hand rotation motor will be harder to turn to port . This will occur while on plane . You can correct some of that [if thats what it is] by moving the aft edge of the torque tab to starboard . Hope that helps .
Marc

Larry H · Posted: Thu Apr 26, 2007 2:53 pm Post subject:

Sarge,

Torque steer is when the motor tries to turn(rotate) opposite to the rotation of the crankshaft/flywheel. With cable steering without NFB (no feedback system) you have to hold the wheel or the boat will turn itsself. With hydraulic steering, the check valve in the helm holds the wheel for you. The trim tab can be adjusted to compensate for torque steer.

Vibration during a turn is NOT torque steer and is NOT normal(except Mercury claims its normal for them!) for any motor I have operated. It could be caused by propeller ventilation, propeller cavitation, or possible a bent prop. Bent props usually vibrate all the time more or less depending on RPMs.

Another cause of your problem could be an underwater protrusion on the hull like bait tank pickups or transducers. The hull must be smooth for at least 3 ft in front of the prop and within 18 inches or so either side of the centerline.

Sea Wolf · Posted: Thu Apr 26, 2007 10:17 pm Post subject:

Sarge-

I had a case of vibration off a really nice stainless prop once that was caused by inadequate clearance between the prop blades and the torque tab.

You have to have at least 3/4"-1" of clearance, or the intensely propelled water off the prop blade will "thump" as it hits the torque tab.

Usually, though, this type of problem works all the time, not just when turning, until you reach higher rpm and the thumping isn't noticeable because of the higher frequency.

They make special torque tabs that are flared back more to accomodate props that need the larger clearances.

Joe.

Sarge · Posted: Sun Apr 29, 2007 9:57 pm Post subject:

Thought I would add the following...

I took the boat on it's maiden voyage from Edmonds to Everett and then followed it the next day with a trip from Edmonds to Kingston, south to the northern tip of Bainbridge Island and back to Edmonds. The boat is not thrilled to turn to port but other than that I HAVE NO COMPLAINTS. The big Mercury just purrs. The motor is very responsive. I have the fly by wire throttle control and it is effortless. Actually took some time to get used to because there is absolutely no resistance.

I'm sad to say that I don't know what a torque tab is...

Also, to be honest it is more resistance and increased noise than real vibration.

-Sarge/Carl

Les Lampman · Posted: Sun Apr 29, 2007 10:06 pm Post subject:

Sarge,

The torque tab is the fin gizmo that hangs down from the anti-ventilation plate just aft of the prop. Generally there's a bolt that holds it in place (typically seen from the underside but sometimes under a rubber cap or such from above). With the bolt slightly loose you can twist the tab to help with torque steer. Move the back of the tab the direction that the boat is pulling just a bit, tighten it back up and give it another try.

Caveat; the steering is affected a lot by engine trim angle. For the most part try to run the boat with the engine level (usually the decals on the side of the hood parallel with the water) to minimize the torque steer induced with a lot of down trim. Use the trim tabs on the boat to control the bow. At slower speeds (10, 12, 15 knots, etc) you may also need some engine trim but at lower rpms you won't get quite so much affect from trim angle.

Sea Wolf · Posted: Sun Apr 29, 2007 10:19 pm Post subject:

Sarge;

The torque tab, or, more properly trim tab, is thje little triangular fin at the rear underside of the anti-ventilation plate.

The tab is important in helping correct for the tendency of the boat to want to not go in a straight line.

This occurs because of the effect of turning the prop iin one direction is to want to turn the whole engine in the opposite direction. (This is the "Equal and opposite reactions" of Newton's Third Law of Motion.)

As a result, the engine turrns or steers more easily to one side or direction than the other.

The trim tab can't eliminate this, just help steer in a straight direction.

Most steering systems have some sort of "anti-feedback" mechanism to deal with the steering wheels pull in the one direction at rest.

The tab is also made of a zinc or aluminum alloy to act as a sacrificial anode to control corrosion by controlling where it occurs.

Note trim tab at rear under anti-ventilation plate.

A trim tab removed from the lower unit.

Joe. Teeth

Dora~Jean · Posted: Mon Apr 30, 2007 12:53 am Post subject:

Joe wrote about the steering trim tab:

Sarge · Posted: Mon Apr 30, 2007 1:28 am Post subject:

Thanks Les and Joe for the fine explanation of what a torque/trim tab is and what it does. When my boat is out of the water to have it's bottom painted and trim tab anodes installed (that's another story Cry

) I'll see about adjusting it so that the rear of it faces starboard.

Much appreciated.

-Sarge/Carl

Sea Wolf · Posted: Tue May 01, 2007 6:10 am Post subject:

Dora~Jean · Posted: Wed May 02, 2007 12:39 am Post subject:

Joe (and Les), I too learned something, I didn't realize engine trim had so much effect on steering torque, left/right. Your Mercury manual reference says it's actually possible to make the steering torque reverse by tilting the engine up a fair amount. I'll have to try this next time I'm out! Thanks.

My reference to paddlewheel effect came from "Boat Handling Under Power" by John Mellor. He discusses the effect on both sail and power boats. I couldn't find the reference to the density of the water difference between the top of the prop and bottom, but here's an excerpt of what he says about paddlewheel effect: