Trim Tab Operation

Aurelia · Posted: Tue Apr 28, 2015 4:57 pm Post subject:

We run slowly with tabs neutral and motor trim from 0-25% (from bottom) then throttle up and get on plane. Once planning, we use the tabs to bring the bow down to the level we like and then adjust the motor trim a bit up or down for efficiency. We don't generally need to trim laterally but when that is needed I do it last. Using the tabs to help achieve a plane never really occurred to me but it does seem like that need would indicate another issue with power or balance.

I love the tabs and use them whenever we are doing over 10 knots.

Greg

smckean (Tosca) · Posted: Tue Apr 28, 2015 8:13 pm Post subject:

Being new, I have a couple of questions:

1. I'm a little confused about "trimming up" and "trimming in" and all other combinations. Am I correct that with trim tabs, down means adjusting them so they are at an angle with the trailing edge lower than the leading edge; also that all the way up means the tabs are back parallel to the surface of the water (clam, not moving). I assume that trimming in relates only to the motor. When someone like thataway says he trims the motor in, does he mean that the bottom of the motor is moved closer to the transom (even past vertical). Motor trim out would mean that the gap btwn the bottom of the motor and the transom gets bigger.

2. In my CD25 with the Honda BF150, there is a motor trim gauge. I have never really known what the position of the needle means. Obviously, as the motor goes up, the needle goes up; but what do the markings on the gauge represent? My gauge has 3 marks: some sort of upper limit that is at approx 1 o'clock; a longer middle mark at 3 o'clock; and a down mark at approx 5 o'clock. On my boat to have the motor vane (or whatever that plate just above the prop is called) parallel to the water, I need to have the motor trim gauge very close to the 5 o'clock mark (say at 4:30).

ssobol · Posted: Tue Apr 28, 2015 8:40 pm Post subject:

Trimming up or down usually refers to the bow angle relative to the water. Bow down will require the tabs to be deflected trailing edge downward (might also be called extended). Trim tabs cannot usually tilt the bow up, they can only counteract the bow up tilt from the engine trim or water forces on the hull.

A lot of trim gauges actually have icons that indicate which directions on the indicator are bow up and bow down. You might have to look closely.

On the motor, trimming in will give you bow down. Trimming out will give bow up. These can also be referred to as trimming up and down and refers to what happens to the bow.

The trim indicator is not generally indicating any particular amount of trim (like degrees). It is just a pointer so you can set it to the same place when you want. Once you get the trim where you want it, you can note the indicator position so you can set it there again.

thataway · Posted: Tue Apr 28, 2015 9:40 pm Post subject:

smckean, To me trimming the motor in, or down, means toward the boat. Trimming out or up, means to the surface of the water. Each manufacturer may vary, but, generally the top hash mark at the topis when the outboard is trimmed all of the way up, the next one, is when the second ram, or trailering ram engages. Generally at this level, the engine intake will be out of the water, or out of the water shortly, and it is not advisable to run the engine with the intake out of the water (burns up impellers and engine blocks!)

At times I have to run the boat in skinny water, during winter low tides, I may only have 15" of water in the channel to my bayou. Then I will watch the engine water telltale, to be sure that it always has a good stream, as I trim the motor up, so the prop is partly out of the water.

Not all trim tabs are set up the same way--but many will have labels, such as port bow down, Stb Bow down, Port Bow up, Stb Bow up. If there is confusion, best to label. Almost all of the manufactures have indicators, most are LED lights which are available as an extra, to mark where the trim tab is located. Some of these also have a "auto retract feature" so that the tab is not left down when the boat is beached, or put on a trailer, potentially damaging the tab.

The tab going down, depresses the bow, and raises the stern. The starboard tab also tilts the boat to the port, and visa versa. So you have to remember which way it will react.

Often the tabs will be above level with the bottom of the boat when trimmed all of the way "up" .

I would make one slight correction to Leon's excellent post. That is I prefer that the prop be parallel to the surface of the water, and that may or may not be parallel to the bottom of the boat. (Remember the wedge we have on the Radar to get the beam down, because on a plane, there is often some bow rise.) Also different boats have various amounts of "rocker" this is curve to the bottom of the boat. This may also affect at what attitude the bow is, as well as if the bottom of the boat is parallel to the surface of the water. Generally there is less wasted energy, if the boat is driven straight, thru the water, not pushing up, or down….But the ride depends on the amount of trim also. There are times when the bow is trimmed up, or down, when the best ride may be with the prop in its most effective position, for that state of trim, and yet not be parallel to the bottom of the boat. I do agree that a great way to see your most efficient setting is to leave the throttle alone, and slightly vary the trim, to get the highest RPM, and and speed. As he says, this is by hearing and feeling the boat, as well as watching the tachometer or a fuel flow gauge.

Leon · Posted: Wed Apr 29, 2015 1:07 am Post subject:

Sorry Bob ....the prop runs parallel to the bottom of the boat, it is a constant. On a perfect plane, on smooth water the bow will be slightly higher, and the motor with be a bit forward to the water. if you make the motor parallel to the water, you will be moving it back and a up, it will bring the bow up even higher.

smckean (Tosca) · Posted: Wed Apr 29, 2015 11:06 am Post subject:

Thanks thataway and ssobol. Your explanations confirm my intuition. My boat operates almost exactly as thataway described. When fully up, the trim tabs are a bit above the water level. I have the LED indicators and I know how to use them. I also have the auto retract feature (which can be annoying when I turn off the key to hear that whine thru its entire cycle even though the tabs are already fully up).

At first, I would get confused because to move the port tab one must use the starboard switch. I intellectually understood why that should be, but it didn't "feel" right. Now it feels natural (push the upper part of the left switch to push down the port bow). My only problem now is that the LED unit is mounted too low for me to see it properly; so I have to crane my neck awkwardly to see it. At some point, I will move that instrument up higher on the helm bulkhead for better visibility.

My motor gauge seems to work exactly as you described thataway. I sometimes have to retrieve at the ramp I use with the prop just out of the water as you mention....I keep my eye on the pee stream. If I remember correctly, the motor switches to the trailering ram just when the needle in the gauge is at 3 o'clock as you mentioned.

ssobol · Posted: Wed Apr 29, 2015 12:36 pm Post subject:

I disconnected the auto-stow feature on my tabs. Resetting them every time the engine shut off got old. The way my boat sits on the trailer, it is not a issue if the tabs are still down when I retrieve the boat.

smckean (Tosca) · Posted: Wed Apr 29, 2015 2:20 pm Post subject:

ssobol · Posted: Wed Apr 29, 2015 6:43 pm Post subject:

On the Bennett tabs there is a 22 ga orange wire with an inline fuse holder coming from the controller to power. Just take the fuse out. Since the fuse clips can float in the holder and possibly touch I put an insulated butt splice in place of the fuse. You could put a blown fuse in the holder if you want.

There is also another orange wire without an inline fuse holder. Leave that one alone.

Before doing this, check how your boat rides on your trailer. You do not want to rest the boat on the extended tabs. Removing the auto stow feature can leave you open to this happening if you forget to manually stow the tabs before retrieving your boat. In my case the bunks and the tabs do not align when the boat is on the trailer so it is not an issue for me.

thataway · Posted: Wed Apr 29, 2015 7:30 pm Post subject:

Will-C · Posted: Thu Apr 30, 2015 7:59 am Post subject: Trim Tab Operation

We didn't often use trim tabs. Our previous boat had a Permatrim and with that we trimmed only needing to move the motor with the trim switch on the throttle. We did use trim tabs when taken wind and waves on the front quarter to level the boat when shifting weight around could not get it done. I think in normal situations people might want to take note of where the water is coming off the bottom of the boat by looking out the window at you optimum trim and throttle settings. For us that usually meant seeing water coming out from under the boat in about the helm area in terms of front to back location. When everything is right most can feel the happy zone. The right rpms, trim right for current conditions and load. We seemed to get better mileage at higher speeds as more of the boat is out of the water I guess. Obviously the best mileage comes at displacement speeds.
I think inboard boats rely more on trim tabs as they can't change the attitude of the prop.
D.D.

Sunbeam · Posted: Thu Apr 30, 2015 8:50 pm Post subject:

Rain · Posted: Thu Jan 21, 2016 9:59 am Post subject:

This thread is getting old, but not stale! I am relieved to read that some take years to learn how to best utilize their trim tabs. It certainly is not yet intuitive to me. The setup of the toggle switches in relation to the direction the tabs move takes some getting used to. I can see why they are that way; push the top of the starboard switch and the bow and starboard side come up and so on. The problem for me is that the trim tab position indicator lights show where each tab is. My brain automatically compensates for what each tab will do to the boat according to its position and/or change of position. Forcing my brain to ignore the position of the tabs and focus on what a push of a toggle switch does to the boat instead of what it does to a tab and consequently to the boat is taking some time!

Add a Permatrim and motor trim to the equation, and it gets complicated.

Sunbeam · Posted: Thu Jan 21, 2016 11:36 am Post subject:

thataway · Posted: Thu Jan 21, 2016 11:47 am Post subject:

The best way is to observe the way the boat's attitude is, as it runs. Think of bow down and bow up--on each side. Although the lights might seem like a good idea--for many they are confusing.

The only time I found the lights helpful, was on a larger boat than our C Dorys, where there was not the almost instant response there is with any C Dory.

It becomes a reflex, just like balancing a bike, or the helm of a sailboat/tiller of outboard motor--or the wheel of a car or boat...