Trim Tabs Operating the Wrong Way?

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rgsmg53

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This may end up as a 'stupid question' thread so please forgive me in advance.

I acquired a twin-screw motor boat earlier this year in need of a bit of TLC so I have not used her a great deal and only got round to experimenting with trim tab settings the other day. In short, they seemed to make little difference in the trim up to 20 knots (the boat's WOT speed).

This led me to check things more carefully at 0 knots whilst alongside my marina berth. Whilst I hung over the edge of the bathing platform, my wife operated the controls as requested. To my surprise, when 'bow down' was selected both the tabs went up. I might be missing something here but from fluid dynamics theory I think that in order to trim a floating body 'bow up' an aft transom-hung flap should go up at its trailing edge (ie the aft-most edge) trying to push the stern down. Similarly, to trim 'bow down' the tab should go down at its trailing edge (or trying lift the stern).

To add to the mystery, there is also a six-way joy stick control on the flying bridge which appears to operate the tab on the opposite side of that labelled on the switch but in the correct direction for that situation. For example if Port bow down is selected the Starboard tab goes up!

Am I having a serious senior moment or is my theory basically correct and the boat's trim tab control mal-functioning?

Any thoughts gratefully received.
 
Re-posted - originally posted in PBO Reader to Reader

This may end up as a 'stupid question' thread so please forgive me in advance.

I acquired a twin-screw motor boat earlier this year in need of a bit of TLC so I have not used her a great deal and only got round to experimenting with trim tab settings the other day. In short, they seemed to make little difference in the trim up to 20 knots (the boat's WOT speed).

This led me to check things more carefully at 0 knots whilst alongside my marina berth. Whilst I hung over the edge of the bathing platform, my wife operated the controls as requested. To my surprise, when 'bow down' was selected both the tabs went up. I might be missing something here but from fluid dynamics theory I think that in order to trim a floating body 'bow up' an aft transom-hung flap should go up at its trailing edge (ie the aft-most edge) trying to push the stern down. Similarly, to trim 'bow down' the tab should go down at its trailing edge (or trying lift the stern).

To add to the mystery, there is also a six-way joy stick control on the flying bridge which appears to operate the tab on the opposite side of that labelled on the switch but in the correct direction for that situation. For example if Port bow down is selected the Starboard tab goes up!

Am I having a serious senior moment or is my theory basically correct and the boat's trim tab control mal-functioning?

Any thoughts gratefully received.
Read more at http://www.ybw.com/forums/showthrea...s-Operating-the-Wrong-Way#AVFrWXlyI8MAMxS9.99
 
Tabs down = bow down.

Some boats will have the port switch operate the stb tab. This is so when you press the port switch to "bow down" the stb tab lowers, to bring the port side bow down.

Clear ?
 
In my experience, pressing "starboard bow down" should depress the port trim tab down and vice-versa. Similarly "port bow up" should lift the starboard trim tab, and so forth.

Sorry - I've just realised Paul Rainbow beat me to it!
 
The typical error one finds is that the wiring behind the switches has been deliberately changed by a previous owner to suite personal preferences or was never installed properly to start with.

If you press 'BOW DOWN ' both trim tabs should move down to lift the transom and lower your bow.
Likewise if you press the starboard 'bow down' it's the port tab that moves down and vice versa.

See here: http://www.boatus.com/boattech/articles/trim-tabs.asp

Start with telling us what make or trim tab you have and what type control switch , then maybe we can help you.

If you have a Bennet model I have a document that can show the wiring diagram for the different switches if I can get it to upload somehow.
IMG_0326.jpg
 
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Thanks Benchmark, I think you may have provided the answer.

In this case, it seems the two Bennett rocker switches in the wheelhouse raise the tabs when asking for bow down (which is wrong) and whilst the 6-way joystick on the fly-bridge operates the opposite-side tab (correctly) it appears also to operate the 'wrong' way. ie If I select Port bow down the Starboard tab goes up.

If I've understood the replies so far correctly then it's Bow down = Tab down on same side (which makes good sense to me) and also Bow down = Tab down on opposite side (which doesn't make sense to me). Is this correct or should it be Bow down = Tab up on opposite side?

I also realise I need to do a few more tests on the joystick to check all 6 positions.
 
Bow down = trim down on opposite side

Quote from Bennett website
You can easily bring the boat level using your control. If the port bow is high, push the port side “Bow Down” direction (this lowers the trim tab on starboard side). If the starboard bow is high, push the starboard side “Bow Down” direction until the boat is level (this lowers the trim tab on the port side).

This will help you understand how they work better
https://bennetttrimtabs.com/how-do-i-use-my-trim-tabs/
 
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I had very similar issues.

You may find the upper helm is wired into the lower helm as there are only 4 wires coming of the pump so look for a problem there first.

At a previous time, the wiring between the lower helm and the pump had been "repaired" but the colours mixed up when rejoining which is handy!!

It should be as follows.

Both buttons bow down = both trim tabs down.
Port bow down = starboard tab down
Starboard bow down = port tab down

Port bow up = starboard tab up
Starboard bow up = port tab up
Both buttons bow up = both trim tabs up

It sounds and looks complicated at first but is very straight forward. The pump runs either way to suck or push the fluid and there are port and starboard solenoids to control the fluid to one tab or the other or both.
 
Another thing to remember is windage in the sence of a strong beam wind tilting the boat say on the port beam here you need the starboard down
.
I had " oppersite effect " type on the sun seeker .bow up / down labelled switches
Most confusing !!PITA !!

The Itama has two switch just up / down - and guages -2 to 0 to + 5 labelled L + R
Easy to Phathom.

No mention of bow and correctly sided .
 
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Thanks to all for their input but especially Benchmark and FARSCO.

I'm relieved that my intuition was largely correct but I had not bargained for the correct use of tabs on the opposite side. I shall now be able to continue my tests with confidence - there is definitely something wrong and my money's now on crossed wiring between the joystick on the fly bridge and push switches in the wheelhouse.

Thanks again,

Richard
 
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