question about varying twin engine trim and boat list

[Iced-D]

If one engine were brought up somewhat and the other left down, would the boat tend to list toward or away from the side of the raised engine?

I understand raising both engines when on plane but was wondering about whether raising one engine would be a good/bad idea rather than using trim tabs. (Have had this boat with twin engines for a month, and have not yet used the trim tabs.) I'm guessing the answer to the question posed is "toward," but I'm not sure, and I don't know how much the difference would have to be, and really don't know whether it's a stupid question to begin with.

 

[Legend]

I have had twins for years and honestly have never noticed any difference to the level of the boat if one engine was trimmed higher or lower - guess it it probably would impact speed and fuel consumption. The only way I have ever noticed any variance in trim levels is either a visual or gauge discrepancy.

 

[seasick]

Trimming an engine up changes the direction of the thrust and points it downward relative to the hull bottom. In theory that causes the stern of the boat to lift up. So in theory changing the trim angle of an outboard will cause the hull to tilt. In reality the force of the thrust has less effect on hull angle than trim tabs do, Trim tabs create more of a upward force than trimming the engine does. The engine spacing, hull weight ( actually righting force) are factors.
Also note that excessive engine trim can result in the prop losing bite or blowing out especially when turning.
The motors are separated closer to each other than the trim tabs are. That reduction in spacing reduces the lifting effect.
Your question raises another question; What effect on trim does running the motors at different revs with the same trim angle?

 

[DennisG01]

Use your tabs - they're a great tool. Kinda silly to leave them in the toolbox

 

[Iced-D]

Trimming an engine up changes the direction of the thrust and points it downward relative to the hull bottom. In theory that causes the stern of the boat to lift up.

So, in theory, if there is an effect (and the other variables eliminated), could trimming the starboard engine up somewhat, and leaving the port engine all the way down/in, cause the boat to list, with the bow lower on the starboard side? And therefore in no event would trimming just the starboard engine upward result in listing to port [i.e., with the port side of the bow appearing to be lower than starboard side of the bow]. (Or might I still have it backwards?)

Yes, I agree trim tabs are the right tool, but I've got a particular reason for wanting to understand the potential effect of trimming only one of the twin engines upward and leaving the other down, with both at the same revs.

 

[Sardinia306Canyon]

Trimming both engines out from perfectly vertical to diagonal up position send propeller wash upwards what pushes the stern down and bow up, thats why you should use a moderat trimmed up position at planning speed and engines tucked in to get bow down in choppy conditions.
Above planing speed if you use one engine more trimmed up than the other, then the transom on this side will be lowered and boat will list, if the props can produce enough downward thrust to make the boat list.

Yes it does, at least on smaller boats like my BW 23 Outrage, depend also of kind of propellers in use and how weight is distributed.
However using trim tabs is most likely the more fuel saving option than using outboard trim to do same.

Give it a tray and then you know as only trying it will tell you on your boat, it is not on all boats and propellers the same.

Chris

 

[DennisG01]

So, in theory, if there is an effect (and the other variables eliminated), could trimming the starboard engine up somewhat, and leaving the port engine all the way down/in, cause the boat to list, with the bow lower on the starboard side? And therefore in no event would trimming just the starboard engine upward result in listing to port [i.e., with the port side of the bow appearing to be lower than starboard side of the bow]. (Or might I still have it backwards?)

Yes, I agree trim tabs are the right tool, but I've got a particular reason for wanting to understand the potential effect of trimming only one of the twin engines upward and leaving the other down, with both at the same revs.

It's going to depend on the boat and the engines and distance they are apart from each other. So every boat is going to be a bit different.

Trimming up pushes the stern down... which causes the bow to raise. However, it will cause the opposite side of the bow to raise. Meaning, stbd stern down = port bow up. You can do the math on the other three scenarios from there

Curious... what's the reason? Trim tabs are so much more useful and efficient than using an engine to do this stuff.

 

[Iced-D]

Curious... what's the reason? Trim tabs are so much more useful and efficient than using an engine to do this stuff.

I may have engines that are not aligned, or when raised together are not rising in the same degree (with starboard higher). The Yamaha CL7 readout shows different levels when they should be the same. I can't see a misalignment looking at the engines, so I suspect it's a calibration problem, but there also seems to be a small port-side list sometimes.

When both engines are fully down/in, port reads level 1 and starboard reads level 2. At one point starting gradual raising both engines together, the port side still reads 1 when starboard goes to 3. Still can't "see" the difference when looking at the engines. Will be checking calibration at the shop, but wondered whether there "could" be an effect on list.

 

[DennisG01]

Gotcha. Thanks for the clarification.

Something to think about... if you're referring to the list to port primarily when the engines or down (or mostly down), that's quite normal for nearly all planing-style hulls. There's too much hull in the water causing too much drag and that's exactly what happens. Very normal.

It sounds like you're on the right track with checking calibration. You can also do some easy measurements after, say, 2 seconds of holding each trim "up" button and maybe again after 4 seconds. That will tell you for sure.

Another thing to do... just go play! Purposefully trim one engine up higher than the other and see how that impacts your particular boat.

 

[Iced-D]

Something to think about... if you're referring to the list to port primarily when the engines or down (or mostly down), that's quite normal for nearly all planing-style hulls. There's too much hull in the water causing too much drag and that's exactly what happens. Very normal.

Just to be clear, you're saying with a deep V hull there could be normal a list to port specifically, or rather a list either direction from time to time, based on wind or current?

 

[DennisG01]

Just to be clear, you're saying with a deep V hull there could be normal a list to port specifically, or rather a list either direction from time to time, based on wind or current?

From my experience, it always seems to be to port. I'm guessing it has something to do with prop spin/weight distribution/wind/current as I don't think there's something inherent about the hull, itself, that determines which way it goes. But it has always seemed to be to port for me, anyways.

 

[Iced-D]

From my experience, it always seems to be to port. I'm guessing it has something to do with prop spin/weight distribution/wind/current as I don't think there's something inherent about the hull, itself, that determines which way it goes. But it has always seemed to be to port for me, anyways.

Prop walk doesn't seem to happen with counterrotating twins, so I can't understand how the props could lead to a list to port. I tried looking at the bow from in front of the boat when tied to the dock (on the starboard side), and it actually looked like it was listing to port a little.

The helm and head are on the starboard side, so I don't know what has been counterbalanced in the construction. At this point I may just be imagining things. First thing is to check the CL7 calibration, which is the prime suspect.

 

[Sardinia306Canyon]

Prop walk doesn't seem to happen with counterrotating twins, so I can't understand how the props could lead to a list to port. I tried looking at the bow from in front of the boat when tied to the dock (on the starboard side), and it actually looked like it was listing to port a little.

Yes, you are right on twin outboards with one counter-rotating prop it should not happen, but inertia from same direction turning engines may do.

However, i started to use my livewell and when seacock is open it fills with water what is normal.
But what i am surprised about is: how much list the full livewell provokes, yesterday i needed 3 bars of stbd tab down to correct that.
Yes i have a big livewell, but livewell is rather near center keel, about same as when i boat alone sitting in captain chair and no list.
You may do a test to shift some weight on the opposite side to see if list at planing speed is reduced or goes away.
Chris

 

[DennisG01]

Prop walk doesn't seem to happen with counterrotating twins, so I can't understand how the props could lead to a list to port. I tried looking at the bow from in front of the boat when tied to the dock (on the starboard side), and it actually looked like it was listing to port a little.

The helm and head are on the starboard side, so I don't know what has been counterbalanced in the construction. At this point I may just be imagining things. First thing is to check the CL7 calibration, which is the prime suspect.

I was talking generalities regarding the hull steer phenomenon I was referring to that you quoted. I wasn't referring to any specific boat. Just talking about the hull steer list that happens to pretty much all of them in case that's what you were experiencing