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Author Topic:   Trim tab confusion

elaelap posted 11-26-2011 11:38 AM ET (US)
It's always seemed counter-intuitive to me that one compensates for one's boat's tendency to veer to port by moving the rear end of the outboard's trim tab to port (and vice versa). This is exactly the opposite way a rudder works -- obviously to turn further to port one causes the rear of the rudder to move further to port (and vice versa). Who can explain this in words I can understand...in other words, very simply?
Tony
Binkster posted 11-26-2011 11:55 AM ET (US)
[This response misinterpreted the question and replied on a completely different topic. The response has been deleted.]
Tom W Clark posted 11-26-2011 11:57 AM ET (US)
Tony -- Good question.
Think of the trim tab not as a rudder, but as a rudder *for* the rudder (the outboard gearcase itself).
Most outboard powered boats will tend to steer to port or starboard due to the asymmetrical blade loading on the propeller caused by the motor being trimmed in or out. When an outboard is trimmed in it will tend to steer to starboard, when trimmed out it will tend to steer to port.
Let's examine what happens when a motor is trimmed in. The blades of a (standard rotation) propeller have a greater angle of attack on the downswing of its rotation when the blade is on the starboard side of the motor and less angle of attack on the upswing of its rotation on the port side of the motor.
This extra thrust on the starboard side wants to push the outboard's gearcase to starboard which, of course, will steer the boat to starboard.
When the boat is up on plane and trimmed out, the situation is reversed.
The trim tab on the motor's gearcase is behind the propeller and acts like a rudder for the gearcase. If the boat is pulling to starboard, it means the gearcase is wanting to swing to starboard. By moving the trailing edge of the trim tab to starboard it is directly water to starboard and thus pushing the aft end of the gearcase to port which compensates for the pull to starboard.
Clear as mud?
Binkster posted 11-26-2011 12:18 PM ET (US)
[Continued to respond with more on the wrong topic. Further changed the topic to discuss the decline in human intelligence. This nonsense was deleted.]
elaelap posted 11-26-2011 12:47 PM ET (US)
Binks,
Tom and I are talking about the little trim tab underneath the anti-cavitation plate aft of the propeller on an outboard motor designed to adjust for veering caused by prop torque.
And yes, Tom, clear as mud. It's gonna take me a while to puzzle this one out. Thanks for your help.
Tony
jimp posted 11-26-2011 02:11 PM ET (US)
Tony -
Set it once and forget it!
Hope you and Edy had a good Thanksgiving.
JimP
elaelap posted 11-26-2011 02:54 PM ET (US)
Jim,
Thanks and the same to you and yours. Edie and I were just talking about you folks after viewing a great PBS documentary, 'Smoking Fish.' Have you seen it?
And yeah, I've never had any actual problem using the little tab to off-set prop torque veer, it's just that I can't get my limited intelligence around how/why the damn thing works ;-)
Tony
contender posted 11-26-2011 02:58 PM ET (US)
Tom: I can agree with your last statement "The trim tab on the motor's gearcase is behind the propeller and acts like a rudder for the gearcase. If the boat is pulling to starboard, it means the gearcase is wanting to swing to starboard. By moving the trailing edge of the trim tab to starboard it is directly water to starboard and thus pushing the aft end of the gearcase to port which compensates for the pull to starboard." The gearcase is a rudder as well as the trim tab...
However, I do not understand how the prop blade looses an angle of attack from one side of the gear case to the other. The prop blade is fixed and does not move, or adjust and should grab the same amount of water on one side of the gearcase as the other. I could understand that the prop may force the water different from one side of the gearcase than the other due to the water being pushed against the gear case and then pulled away on the other side creating a void on one side, thus causing it to turn... Also would your theory/analysis be the opposite for a left hand rotation prop?

Bink: Darwin was right the problem is that these people can not fend for themselves and are protected. If they were in the wild they would be killed and eaten....sometimes even by their own.
Tom W Clark posted 11-26-2011 03:43 PM ET (US)

quote:
I do not understand how the prop blade looses an angle of attack from one side of the gear case to the other. The prop blade is fixed and does not move, or adjust and should grab the same amount of water on one side of the gearcase as the other.
Rick -- Yours is a good question too; angle of attack is not well understood by a lot of boaters.
You are correct that the angle of a propeller blade is fixed relative to its hub and thus the propeller shaft of the outboard too and, indeed, the entire outboard motor. However, the outboard motor rotates about its horizontal axis, literally the center it its tilt tube (Verados excepted).
The angle of attack of the angle between the propeller blade and the water. The water does not (except at one precise point) pass by/through a propeller along the same axis as the propeller shaft. Tuck the motor in as you might if you were trying for a quick hole shot and the propeller shaft will be angled down as viewed form the side.
Likewise, if the motor is trimmed way out as it might be during a WOT test run, the propeller shaft is angled up as viewed from the side.
Now visualize the propeller as it would be viewed if you were directly starboard of it looking at it from the side. Imagine the propeller shaft is perfectly level and level with the water it is passing through. If the propeller hub were invisible you would see the blades angled one way on one side and angled the exact opposite way on the other side, both at equal angles to the water, one blade going up and the other going down.
Not trim that motor it in your mind and view the propeller blades. The angle of the blades is fixed relative tot he outboard but the water is passing through the propeller at an angle. Because the outboard and the propeller have now rotated, the angle of the blade nearest you is traveling down (for a standard right hand rotation model) with more angle while the blade on the far side away form you is traveling up at a greater angle.
Because the blades are fixed relative to each other they are all necessarily moving at the same speed and the one(s) with the greater angle of attack will necessarily be moving more water and thus providing more thrust.
Yes, a counter rotation propeller will behave in exactly the same way but everything will happen in the opposite direction of what I have described.
jimp posted 11-26-2011 04:05 PM ET (US)
Tony -
I hate it when I spell names incorrectly. Apologize to Edie for me. Haven't seen "Smoking Fish". Have to ask Tom W how his was.
With the hydraulic steering, my 2003 225 Merc Optimax doesn't have a trim tab and tracks straight as can be with an occasional finger on the wheel. Who knows the answer to that? Why do some single engine outboard boats have trim tabs and others don't?
JimP
P.S. 45" of snow so far this month.
Binkster posted 11-26-2011 08:43 PM ET (US)
[Became cognizant he was on the wrong topic.]

Binkster posted 11-26-2011 09:15 PM ET (US)
[Gave an incorrect explanaton; deleled.] That is why most boats that are not center console, the driver sits on the starboard side. (extra weight on that side will help keep the boat level, extra weight on the port side will aggravate this condition if you don't have horizontal trim tabs.
Binkster posted 11-26-2011 09:28 PM ET (US)
[Changed topic to discuss the history of outboard engines having trim tabs on the anti-ventilationplate.] I think [use of trim tabs on outboard engines] began when through-the-hub exhaust was first used. I know my 1961 Evinrude V4 Starflite didn't have [a trim tab], the exhaust came out behind the propeller, and the water pickup both in a small fixed protrusion below the cavitation plate that was slightly turned to starboard and was fixed.
elaelap posted 11-26-2011 09:43 PM ET (US)
Actually, Binks, most vessels which have starboard-side off-set helms are designed that way for the following reason (or at least that is what I was taught in my licensing class):
_____________________________________
"Boat operator positions are normally on the starboard side of the vessel. The danger zone is an area that extends from dead ahead to 22.5 degrees aft of your starboard beam. This is the same arc illuminated by your green navigation sidelight. The concept of the danger zone is that if you see another vessel approaching yours from within this zone, they are likely to be privileged or have the right of way. Having the operator on the starboard same of the boat affords an unobstructed view of the danger zone."
Read more at Suite101: Avoiding Boat Collisions: COLREGS and Navigation Rules of the Road | Suite101.com http://alansorum.suite101.com/avoiding-boat-collisions-a20399#ixzz1erwttKdH
______________________________________
Obviously in small skiffs, such as my classic Montauk, one soon learns to shift one's weight to port or starboard to compensate for veer. In fact, when running downhill it's fun to see how far one can travel without touching the helm, slightly changing directions by moving slightly from one side of the console to the other.
Tony
Binkster posted 11-26-2011 11:55 PM ET (US)
Tony, I think the starboard side steering thing for reasons of the danger zone, are for larger inboard vessels. To prove my point, when you are driving your
Montauk alone try standing first on the starboard side of your center console,(the boat will run level-side to side) and then on the port side,(the boat will list slightly to port,) considering that your fuel tank is centered.
fno posted 11-27-2011 09:25 AM ET (US)
Tony, something else that may not have been mentioned yet. A prop is not only forcing water directly behind itself axialy. It also forces a great deal of it radially to the sides. In the case of normal rotation (clockwise from the rear) The water behind forced radiall from 6 o'clock to about 11 gets thrown up to the surface of the water. 10 o'clock to 1 hits the cav plate. From 1 to 6 the water is forced to the right and that water pushes the boat to the left. The trim tab is just trying to offset that force and all of what Tom said. Not sure if it makes sense, but that explanation removed some of the mud from my glasses.
wezie posted 11-27-2011 09:46 AM ET (US)
Tony, it is backwards, and likewise each time I think about it, I have to start from the beginning. Stand behind the engine looking forward and plan the adjustment, steering the back of the engine. Looking at it from the bottom just confuses me more, Many times I have adjusted it in the wrong direction.
History: In the 70s, a couple of guys marketed a steering system called "True Trim". They replaced the drive trim tab with a steerable triple tab that actually steered the rear of the outdrive/outboard. Worked very well, even though they placed speed limitations on it at about 55mph. Finger tip steering.
The first few degrees of steering wheel turn steered the tabs; after that the steering turned the outdrive/outboard as usual.
Mercury bought the company and tried it for a short time and then killed the concept. Other than Hydraulic steering, it was the best I ever used.
Hope your Thanksgiving was Full Filling!
boatdryver posted 11-27-2011 10:48 AM ET (US)
When I bought my new whaler in 2008 I was surprised to see that the 175 HP Verado did not have a trim tab. I first thought the factory had forgotten to install the thing, but on closer inspection it was clear that there was no provision for one.
The boat comes with hydraulic (not powered) steering so there is no feedback. with this type of steering one needs to touch the steering wheel only occasionally to maintain a very straight course.
I'm still surprised about this; what if one of these motors gets installed on a boat without "no feedback" steering?
JimL
gnr posted 11-27-2011 11:32 AM ET (US)

quote:
Obviously in small skiffs, such as my classic Montauk, one soon learns to shift one's weight to port or starboard to compensate for veer. In fact, when running downhill it's fun to see how far one can travel without touching the helm, slightly changing directions by moving slightly from one side of the console to the other.

Ran 10 miles yesterday with my hands in my pockets. Flat conditions with just a very slight breeze pushing me. Zero traffic to be concerned about.
Made two intended course corrections simply by shifting my weight, one coming out of the bay and one as I passed the last island to get to where I wanted to set up my first trolling run.
I love this boat.
Binkster posted 11-27-2011 01:12 PM ET (US)
[Offered another incorrect interpretation.]
Tohsgib posted 11-28-2011 10:50 AM ET (US)
My 14' Carolina Shitt had a center console and I could literally lean left and right and steer the boat quite well.
gnr posted 11-28-2011 11:31 AM ET (US)
Sorry Binks.
I wasn't alone and had the kill switch lanyard clipped to my jacket.
There was no alcohol invlved.
LOL
contender posted 11-28-2011 11:45 AM ET (US)
I moved the kill switch from the Yamaha 25 outboard to the console and made a plate to hold it in place, I attached a cord with a slip knot to the key, When my son rides in the boat he attaches the switch cord to his ankle. If he falls out of the boat or falls off his seat the engine will be cut off...

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Author	Topic: Trim tab confusion
elaelap	posted 11-26-2011 11:38 AM ET (US) It's always seemed counter-intuitive to me that one compensates for one's boat's tendency to veer to port by moving the rear end of the outboard's trim tab to port (and vice versa). This is exactly the opposite way a rudder works -- obviously to turn further to port one causes the rear of the rudder to move further to port (and vice versa). Who can explain this in words I can understand...in other words, very simply? Tony
Binkster	posted 11-26-2011 11:55 AM ET (US) [This response misinterpreted the question and replied on a completely different topic. The response has been deleted.]
Tom W Clark	posted 11-26-2011 11:57 AM ET (US) Tony -- Good question. Think of the trim tab not as a rudder, but as a rudder for the rudder (the outboard gearcase itself). Most outboard powered boats will tend to steer to port or starboard due to the asymmetrical blade loading on the propeller caused by the motor being trimmed in or out. When an outboard is trimmed in it will tend to steer to starboard, when trimmed out it will tend to steer to port. Let's examine what happens when a motor is trimmed in. The blades of a (standard rotation) propeller have a greater angle of attack on the downswing of its rotation when the blade is on the starboard side of the motor and less angle of attack on the upswing of its rotation on the port side of the motor. This extra thrust on the starboard side wants to push the outboard's gearcase to starboard which, of course, will steer the boat to starboard. When the boat is up on plane and trimmed out, the situation is reversed. The trim tab on the motor's gearcase is behind the propeller and acts like a rudder for the gearcase. If the boat is pulling to starboard, it means the gearcase is wanting to swing to starboard. By moving the trailing edge of the trim tab to starboard it is directly water to starboard and thus pushing the aft end of the gearcase to port which compensates for the pull to starboard. Clear as mud?
Binkster	posted 11-26-2011 12:18 PM ET (US) [Continued to respond with more on the wrong topic. Further changed the topic to discuss the decline in human intelligence. This nonsense was deleted.]
elaelap	posted 11-26-2011 12:47 PM ET (US) Binks, Tom and I are talking about the little trim tab underneath the anti-cavitation plate aft of the propeller on an outboard motor designed to adjust for veering caused by prop torque. And yes, Tom, clear as mud. It's gonna take me a while to puzzle this one out. Thanks for your help. Tony
jimp	posted 11-26-2011 02:11 PM ET (US) Tony - Set it once and forget it! Hope you and Edy had a good Thanksgiving. JimP
elaelap	posted 11-26-2011 02:54 PM ET (US) Jim, Thanks and the same to you and yours. Edie and I were just talking about you folks after viewing a great PBS documentary, 'Smoking Fish.' Have you seen it? And yeah, I've never had any actual problem using the little tab to off-set prop torque veer, it's just that I can't get my limited intelligence around how/why the damn thing works ;-) Tony
contender	posted 11-26-2011 02:58 PM ET (US) Tom: I can agree with your last statement "The trim tab on the motor's gearcase is behind the propeller and acts like a rudder for the gearcase. If the boat is pulling to starboard, it means the gearcase is wanting to swing to starboard. By moving the trailing edge of the trim tab to starboard it is directly water to starboard and thus pushing the aft end of the gearcase to port which compensates for the pull to starboard." The gearcase is a rudder as well as the trim tab... However, I do not understand how the prop blade looses an angle of attack from one side of the gear case to the other. The prop blade is fixed and does not move, or adjust and should grab the same amount of water on one side of the gearcase as the other. I could understand that the prop may force the water different from one side of the gearcase than the other due to the water being pushed against the gear case and then pulled away on the other side creating a void on one side, thus causing it to turn... Also would your theory/analysis be the opposite for a left hand rotation prop? Bink: Darwin was right the problem is that these people can not fend for themselves and are protected. If they were in the wild they would be killed and eaten....sometimes even by their own.
Tom W Clark	posted 11-26-2011 03:43 PM ET (US) quote: I do not understand how the prop blade looses an angle of attack from one side of the gear case to the other. The prop blade is fixed and does not move, or adjust and should grab the same amount of water on one side of the gearcase as the other. Rick -- Yours is a good question too; angle of attack is not well understood by a lot of boaters. You are correct that the angle of a propeller blade is fixed relative to its hub and thus the propeller shaft of the outboard too and, indeed, the entire outboard motor. However, the outboard motor rotates about its horizontal axis, literally the center it its tilt tube (Verados excepted). The angle of attack of the angle between the propeller blade and the water. The water does not (except at one precise point) pass by/through a propeller along the same axis as the propeller shaft. Tuck the motor in as you might if you were trying for a quick hole shot and the propeller shaft will be angled down as viewed form the side. Likewise, if the motor is trimmed way out as it might be during a WOT test run, the propeller shaft is angled up as viewed from the side. Now visualize the propeller as it would be viewed if you were directly starboard of it looking at it from the side. Imagine the propeller shaft is perfectly level and level with the water it is passing through. If the propeller hub were invisible you would see the blades angled one way on one side and angled the exact opposite way on the other side, both at equal angles to the water, one blade going up and the other going down. Not trim that motor it in your mind and view the propeller blades. The angle of the blades is fixed relative tot he outboard but the water is passing through the propeller at an angle. Because the outboard and the propeller have now rotated, the angle of the blade nearest you is traveling down (for a standard right hand rotation model) with more angle while the blade on the far side away form you is traveling up at a greater angle. Because the blades are fixed relative to each other they are all necessarily moving at the same speed and the one(s) with the greater angle of attack will necessarily be moving more water and thus providing more thrust. Yes, a counter rotation propeller will behave in exactly the same way but everything will happen in the opposite direction of what I have described.
jimp	posted 11-26-2011 04:05 PM ET (US) Tony - I hate it when I spell names incorrectly. Apologize to Edie for me. Haven't seen "Smoking Fish". Have to ask Tom W how his was. With the hydraulic steering, my 2003 225 Merc Optimax doesn't have a trim tab and tracks straight as can be with an occasional finger on the wheel. Who knows the answer to that? Why do some single engine outboard boats have trim tabs and others don't? JimP P.S. 45" of snow so far this month.
Binkster	posted 11-26-2011 08:43 PM ET (US) [Became cognizant he was on the wrong topic.]
Binkster	posted 11-26-2011 09:15 PM ET (US) [Gave an incorrect explanaton; deleled.] That is why most boats that are not center console, the driver sits on the starboard side. (extra weight on that side will help keep the boat level, extra weight on the port side will aggravate this condition if you don't have horizontal trim tabs.
Binkster	posted 11-26-2011 09:28 PM ET (US) [Changed topic to discuss the history of outboard engines having trim tabs on the anti-ventilationplate.] I think [use of trim tabs on outboard engines] began when through-the-hub exhaust was first used. I know my 1961 Evinrude V4 Starflite didn't have [a trim tab], the exhaust came out behind the propeller, and the water pickup both in a small fixed protrusion below the cavitation plate that was slightly turned to starboard and was fixed.
elaelap	posted 11-26-2011 09:43 PM ET (US) Actually, Binks, most vessels which have starboard-side off-set helms are designed that way for the following reason (or at least that is what I was taught in my licensing class): _____________________________________ "Boat operator positions are normally on the starboard side of the vessel. The danger zone is an area that extends from dead ahead to 22.5 degrees aft of your starboard beam. This is the same arc illuminated by your green navigation sidelight. The concept of the danger zone is that if you see another vessel approaching yours from within this zone, they are likely to be privileged or have the right of way. Having the operator on the starboard same of the boat affords an unobstructed view of the danger zone." Read more at Suite101: Avoiding Boat Collisions: COLREGS and Navigation Rules of the Road \| Suite101.com http://alansorum.suite101.com/avoiding-boat-collisions-a20399#ixzz1erwttKdH ______________________________________ Obviously in small skiffs, such as my classic Montauk, one soon learns to shift one's weight to port or starboard to compensate for veer. In fact, when running downhill it's fun to see how far one can travel without touching the helm, slightly changing directions by moving slightly from one side of the console to the other. Tony
Binkster	posted 11-26-2011 11:55 PM ET (US) Tony, I think the starboard side steering thing for reasons of the danger zone, are for larger inboard vessels. To prove my point, when you are driving your Montauk alone try standing first on the starboard side of your center console,(the boat will run level-side to side) and then on the port side,(the boat will list slightly to port,) considering that your fuel tank is centered.
fno	posted 11-27-2011 09:25 AM ET (US) Tony, something else that may not have been mentioned yet. A prop is not only forcing water directly behind itself axialy. It also forces a great deal of it radially to the sides. In the case of normal rotation (clockwise from the rear) The water behind forced radiall from 6 o'clock to about 11 gets thrown up to the surface of the water. 10 o'clock to 1 hits the cav plate. From 1 to 6 the water is forced to the right and that water pushes the boat to the left. The trim tab is just trying to offset that force and all of what Tom said. Not sure if it makes sense, but that explanation removed some of the mud from my glasses.
wezie	posted 11-27-2011 09:46 AM ET (US) Tony, it is backwards, and likewise each time I think about it, I have to start from the beginning. Stand behind the engine looking forward and plan the adjustment, steering the back of the engine. Looking at it from the bottom just confuses me more, Many times I have adjusted it in the wrong direction. History: In the 70s, a couple of guys marketed a steering system called "True Trim". They replaced the drive trim tab with a steerable triple tab that actually steered the rear of the outdrive/outboard. Worked very well, even though they placed speed limitations on it at about 55mph. Finger tip steering. The first few degrees of steering wheel turn steered the tabs; after that the steering turned the outdrive/outboard as usual. Mercury bought the company and tried it for a short time and then killed the concept. Other than Hydraulic steering, it was the best I ever used. Hope your Thanksgiving was Full Filling!
boatdryver	posted 11-27-2011 10:48 AM ET (US) When I bought my new whaler in 2008 I was surprised to see that the 175 HP Verado did not have a trim tab. I first thought the factory had forgotten to install the thing, but on closer inspection it was clear that there was no provision for one. The boat comes with hydraulic (not powered) steering so there is no feedback. with this type of steering one needs to touch the steering wheel only occasionally to maintain a very straight course. I'm still surprised about this; what if one of these motors gets installed on a boat without "no feedback" steering? JimL
gnr	posted 11-27-2011 11:32 AM ET (US) quote: Obviously in small skiffs, such as my classic Montauk, one soon learns to shift one's weight to port or starboard to compensate for veer. In fact, when running downhill it's fun to see how far one can travel without touching the helm, slightly changing directions by moving slightly from one side of the console to the other. Ran 10 miles yesterday with my hands in my pockets. Flat conditions with just a very slight breeze pushing me. Zero traffic to be concerned about. Made two intended course corrections simply by shifting my weight, one coming out of the bay and one as I passed the last island to get to where I wanted to set up my first trolling run. I love this boat.
Binkster	posted 11-27-2011 01:12 PM ET (US) [Offered another incorrect interpretation.]
Tohsgib	posted 11-28-2011 10:50 AM ET (US) My 14' Carolina Shitt had a center console and I could literally lean left and right and steer the boat quite well.
gnr	posted 11-28-2011 11:31 AM ET (US) Sorry Binks. I wasn't alone and had the kill switch lanyard clipped to my jacket. There was no alcohol invlved. LOL
contender	posted 11-28-2011 11:45 AM ET (US) I moved the kill switch from the Yamaha 25 outboard to the console and made a plate to hold it in place, I attached a cord with a slip knot to the key, When my son rides in the boat he attaches the switch cord to his ankle. If he falls out of the boat or falls off his seat the engine will be cut off...