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Author Topic:   Effect of Engine Trim
gizzi posted 09-15-2003 04:11 PM ET (US)   Profile for gizzi   Send Email to gizzi  
I'm a rookie whaler/boat owner and I was wondering what the position of tilt too the prop will do depending on speed and weight in the boat to achieve the smoothest ride. I have a 1974 13'Sport with a 35 Hp motor - is there a general rule of thumb I can apply ? I've been mostly keeping it in the middle of the gauge since I'm not sure...


Moe posted 09-15-2003 08:12 PM ET (US)     Profile for Moe  Send Email to Moe     
I'm a newbie Whaler owner, too, but grew up with boats when I was younger, so let me take a stab at this.

When taking off with the trim in the middle, the thrust of the motor is raising the bow and lowering the stern, pushing the bottom of the boat into the wind and water. Trimming all the way in during take off lowers the bow and raises the stern, helping the boat to jump up on to plane much quicker. It also lowers the minimum rpm that the boat will stay on plane.

On the other hand, when the boat is on plane, being trimmed all the way in puts the boat bow down, putting more of the hull in the water, creating more drag, and making the engine work harder. It can also cause a dangerous condition called bow steering, where the bow wants to dart to one side or the other, especially going into waves, where too much in-trim also makes the ride a lot wetter and can cause the bow to punch into a wave face.

Trimming up from this point, you'll see the rpms increase and feel the boat speed increase as the engine is freed of pushing against so much drag. Once you've reached optimum trim, the rpms will stop rising.

If you trim out too far, the bow will rise up in your field of vision. If it rises too far up, the boat may start porpoising, with the bow bouncing up and down. The rpms may start falling again. With the bow too far up (trimmed out too much) in chop, the bow will ride up even higher on a wave, rather than cutting through it. At the extreme, too much out-trim could let the wind and wave blow the boat over backwards (it will usually fall off to one side).

The boat will also drop off plane easier when trimmed out too far at slow speeds. Trimming out too far can also lift the motor's cooling intake out of the water, especially if the motor is mounted high on the transom. Other clues that you're trimmed out too far is when the prop ventilates, i.e. comes out of the water in a turn, or comes out too easily in chop.

In chop, you want trim set, not so much for max rpm as you would on calm seas, but for a good compromise between in enough for the bow to cut through the waves and so not get lifted high by them, as well as in enough not to ventilate the prop frequently, and out enough to keep the bow from plowing into the waves and getting you wetter. Sometimes that does coincide with max rpm.

Besides the wind and water you're in, the appropriate amount of trim will also vary with boat load weight and weight distribution. Trim shouldn't be used to try to compensate for improperly distributed load, however.

It takes awhile, at least it does for me, to get a good feel for where the trim should be set. The tach is a good indicator, but I'm getting more attuned to the engine sound, and the feel of the ride.

Hope this helps,

Jarhead posted 09-15-2003 08:14 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
If I understand your question correctly you want to know the best tilt position for a smooth ride for your Whaler.

The best advice I can give is experiment until you find the best position for the condictions at the time. ie, smooth, light chop, rough ect. As well as speed. All these things make a difference in tilt position.

Jarhead posted 09-15-2003 08:22 PM ET (US)     Profile for Jarhead  Send Email to Jarhead     
Moe got his in while I was posting. Much better answer than mine. :)
gizzi posted 09-15-2003 09:41 PM ET (US)     Profile for gizzi  Send Email to gizzi     
Thanks guys - Moe when you say "trim in" does that mean tilting the engine/prop down more toward the stern of the boat and triming out means lifting the prop closer to the surface of the water ? (Like I said I'm new at this-thanks!)
Moe posted 09-15-2003 10:09 PM ET (US)     Profile for Moe  Send Email to Moe     
You got it, Gizzi!

If you're watching the top rear of the motor, it goes down when the prop goes in toward the boat, and up when the prop goes out away from the boat. Different ways of saying it.


jimh posted 09-16-2003 07:39 PM ET (US)     Profile for jimh  Send Email to jimh     
Most owner's manual provided with outboard engines cover the concept of engine tilt and trim. The explanations are also usually illustrated.
jimh posted 09-16-2003 08:29 PM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "Tilt of prop for planing."--jimh]
JohnJ80 posted 09-17-2003 11:50 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
One of the things that I notice is that with NFB (no feedback steering) as found on many whalers, that if the trim is in too far, it is significantly harder to steer. If you get it right, the wheel turns nicely and the clutch in the steering doesn't seem to need to enage as much (less torque). That is generally a clue to me I am into the proper operating range.


Cpt Quint posted 09-18-2003 04:50 PM ET (US)     Profile for Cpt Quint  Send Email to Cpt Quint     
My 2 cents. I like to trim until the wake breaker line at the hull is under your feet. If this line is in front of you than the boat is dragging your weight and if its behind your feet your weight can push the prop out(you dont want this). In short, if your sitting on the plane line than your weight is right at the midpoint, like a sea-saw and thus gives you a balanced ride front to back. ofcourse i could be wrong
Landlocked posted 09-19-2003 10:38 AM ET (US)     Profile for Landlocked  Send Email to Landlocked     
When I was first learning to operate a boat, I was told to look at the top of the motor cowling when on a plane at WOT. The motor should be trimmed until the top of the cowling is parallel with the water.

In the many years since operating my first boat, I have learned that this is not always the case but it is a good rule of thumb that will get you close to optimum trim.

With time, you'll trim by feel and sound alone.


kshoaps posted 09-23-2003 02:46 PM ET (US)     Profile for kshoaps  Send Email to kshoaps     
Another indicator of proper trim is steering effort.

When properly trimmed for a certain speed, load, water condition, etc., the steering effort will be the same for either turning to starboard or to port.


Moe posted 09-24-2003 03:43 PM ET (US)     Profile for Moe  Send Email to Moe     
My Merc manual says if it pulls to starboard, it's trimmed too far in and vice versa. So with no-feedback steering, that would mean if it's easier to turn to starboard it's too far in and vice versa.

However... that assumes the trim tab is centered. You can set that to give most any position of trim a neutral feel at a given speed.

But try as I might, I can't "feel" any consistent difference in ease of turning the wheel regardless of how it's trimmed or the speed. Maybe it's just me, but I find it a whole lot easier to use the tach... and increasingly the pitch of the motor.


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