Drive Angle Vs Trim Tabs, What's The Relationship?

[Squidward]

Brought my boat in for drive service today, and right after the mechanic finished up it turned out the solenoid that lowers the drive is messed up. He had to lower it manually by shorting it with a screwdriver.

I can fix the solenoid, but it got me wondering about the actual usage of the drive angle mechanism. When do you adjust the drive angle instead of the trim tabs, and vice versa?

[Pcpete]

It’s an interesting relationship. On my previous boat I had twin 4.3’s with counter rotating Alphas. I would fully extend the tabs bow down, advance the throttles to about 3/4, and as the boat came up on a plane I’d retract the trim tabs and reduce throttle as the boat dictated. Most of the time the cruising configuration was drives all the way down and tabs just so the boat was running very level and throttled at between 3200 & 3550. If I ran in to light chop I’d bump the bow down to take the liveliness out of it. The only times I’d trim using the drives was above 3800 to lift the bow a bit. At 4800 it was very touchy keeping the props from cavitation. If they did the rev limiter on the engines would kick in with sounds I really didn’t like. I was doing over 45mph plus on the gps in an Encounter 2950 Sunbridge, canvas up and dinghy on the back.
You can find the best combination for your boat through experimenting. By doing that you’ll find that different conditions will dictate different combinations. The big challenge for me was getting the boat all trimmed and one of the kids would walk across the boat.

[Jim_Gandee]

When you have both trim tabs and adjustable drive I believe most use the drive adjustment for bow up and down while the tabs are used to level the boat right and left.

[Centerline2]

If the tabs are big enough, they can do all the work by themselves, but if they are the minimum size for the boat, the outdrive trim helps a lot..

once you get up on plane, set the tabs for the best performance for the conditions.. then trim the drive to get the smoothest operation... when the outdrive is counteracting the lift of the tabs, you can feel/hear it.. and also when the drive is trimmed too far down/in...
get the tabs and the outdrive adjusted together and the boat will run its smoothest and perform most economically... and if a bit of adjustment is needed after getting them set together, use the tabs to do it...

If you have a trim indicator, mark the sweet spot for future reference, because that will be close to perfect every time you get on plane, unless you have a severe loading change...

Only experimentation and experiance can tell you how your boat reacts to them when used together, and how much of either one works the best in the given conditions... ALWAYS keep the tabs fully retracted in a following sea, as it can cause the boat to bury the bow into a wave, which is a quick way to cause a broach/rollover..

[builderdude]

I do as Jim stated in post 3. I use the drive trim often to adjust the attitude of the boat and the trim tabs to correct whatever list is present. Every boat model probably acts a little different but I’ll say trimming my Bravo drive has a massive impact on the how the boat runs through the water.

[BL2859]

This is interesting. I’ve always had outboards up until last year. I have been trimming my 2859 with tabs, and leaving drive all the way down. Someone told me it was a lot of wear and tear on the drive if you raise and lower it often.

[talman]

When you are driving into the wind and waves is when you really can appreciate tucking the outdrive in. It pushes thought the waves more aggressively, especially when tabs full down. Honestly, I never mastered the art of proper outdrive trim, except when I was getting the crap pounded out of me in thick seas. It was easy to see if the drive was too far out; bow way up. Trimming down from there was never something i felt like I mastered. Flying a 2556 at 25 mph seems like it allowed a lot of slop on outdrive trim. Tab trim was always the minimum required to level out the boat, except on those hard, into-the-wind legs.

[builderdude]

This is interesting. I’ve always had outboards up until last year. I have been trimming my 2859 with tabs, and leaving drive all the way down. Someone told me it was a lot of wear and tear on the drive if you raise and lower it often.

The drive was designed to be trimmed during use. There is a limit switch incorporated into the system to keep the drive from being trimmed up to far and you'd likely hear some cavitation way before you reached the set limit.
My 2556 pushes a ton of water with the drive full down. I always start there but once she begins to plane I bump the drive trim up (about 4 seconds) and she lifts up to a nice attitude where the bow spray is reduced, speed and rpm increase and if it's flat water it gets raised a bit more.

[BL2859]

Thanks! I can’t wait to try!

[JRR4948]

I have a 1988 2556 with a newly rebuilt 5.7 engine and an SEI conversion Alpha 1 Gen 2 drive. I still do not understand how to trim the drive properly and would appreciate your guidance please. What RPM's are you running? I usually run at 3200 RPMS. With the trim tabs down all the way and the drive all the way down I get the most speed at 16 knots on average. When I trim up the drive the boat slows to 13 knots. Was the same when I have the old Cobra OMC drive. What am I missing? Should I be running at a faster rpm? It is usually just my wife and I with a days worth of gear so we are not overloaded with guests. My tabs are original with the wings. Thank you for your advice.

[builderdude]

I have a 1988 2556 with a newly rebuilt 5.7 engine and an SEI conversion Alpha 1 Gen 2 drive. I still do not understand how to trim the drive properly and would appreciate your guidance please. What RPM are you running? I usually run at 3200 RPM. With the trim tabs down all the way and the drive all the way down I get the most speed at 16 knots on average. When I trim up the drive the boat slows to 13 knots. Was the same when I have the old Cobra OMC drive. What am I missing? Should I be running at a faster rpm? It is usually just my wife and I with a days worth of gear so we are not overloaded with guests. My tabs are original with the wings. Thank you for your advice.

JR, I changed your comment to a post so it doesn’t get overlooked by others.

You may be running a bit low on rpm at cruise. Most of us run in the 3600-3800 rpm range while at “cruising” speed. You want enough rpm to get up on plane and then some.
Once I’m up to speed I’ll retract the trim tabs then re adjust to compensate for list, any down tab or drive trim will slow me down substantially.

[Centerline2]

see post #4.... when trimming, the rpm of the engine matters less than the speed of the boat... you will either be on plane, or not, and although there is some benefit to the tabs and outdrive angle at very slow speeds, whet you seem to be referring to is when running on plane....

post #4 explains it about as simple as it can be, but every boat is different, and every boater has their boats loaded differently, so other than a basic explanation of how they should work, experience is the only thing that will teach you what you need to know....

with tabs full down, outdrive trimmed in fully.... then get up on plane. and then pull the throttle back just a little so the engine isnt over revving.... then start raising the tabs by pressing the buttons for about a second, giving it about 15 seconds in between each button press for the change to take place before pressing the button again...you will feel the bow rise and the boat will seem to speed up as the water slips past the hull with less resistance... you will know when you've gone too far with the tabs, and then press the buttons the other way to lower them a little bit.

ultimately, what you are looking for when trimming is finding where the engine rpm raises to its highest rpm, and the boat is going the fastest by making changes to the trim, when running at any given rpm that is LESS THAN the WOT position....

once you find the ultimate tab setting at that given speed, then trim up the outdrive the same way.... you will notice the engine will smooth out a little, which is what you want, but if you raise the outdrive too far, it will ventilate the prop and so you will have to quickly trim it down again....

at any throttle position, or change of throttle position, the boat may need to be re-trimmed to get the best efficiency from it...

[JRR4948]

I guess maybe I am not running the throttle up high enough. Once we get back in the water next spring I will try what you said and run it at 3600 -3800 and try to trim it. Thanks for the advice. I truly love the design of this boat. The only boat which comes close is the Cutwater command bridge. I have about $20K into my 2556 however it has a new engine, new outdrive, completely rebuilt like new Tarpon twin axle trailer, white leather interior, new holding tank and hot water tank and a new fiberglass stern deck. The Cutwater is north of $300,000! I am happy with all of the amenities of the 2556 as well as the way it handles. Plus I save $280K HAHA. Thanks again and Happy New Year.

[Metrodriver]

Tabs down, is that in line with the hull or sticking down more?

[Centerline2]

down is extended, lowered as far down as they will go.... in-line with the hull is retracted and fully up....

[Metrodriver]

Thanks!

[builderdude]

Tabs down, is that in line with the hull or sticking down more?

Down below the plane of the hull.

[Metrodriver]

This is all new to me...

[captharv]

This may help you:
Guide to Trim Tabs - Bennett Marine (bennetttrimtabs.com)
What I do, which is based on 40 years of different boats w trim tabs.

The tabs provide lift to the stern. When the boat is squatting, and the bow is high, the ride suffers. The pointy end of the boat is designed to cut thru the waves, not the middle part plowing thru.
I set my drive so its somewhat parallel to the water, giving the best efficiency, and set the tabs to 1. Get the right running attitude for running comfortably, and 2 athwartships level. When slowing for a no wake zone or some Bozo fishing in the middle of the channel, I run the drive all the way in for getting on plane. Then when at cruising speed, i trim out for efficiency.
A friend bought a 2859 with the diesel (200 HP) the instruction manual said not to run over 3000 RPM for more than 10 minutes at a time. At 3000 it was almost on plane, bow high enough that seeing over teh bow required a step, as he (and I) are only 5-7. We removed the 18" tab s and put on 40" tabs. Had to cut down a set of 48" and add the second ram. Now the boat planes at 2600 RPMs and at 3000 its doing 23 MPH. Granted, this required extreme work, but hey, a 28' boat with a diesel engine, generator, water, holding and fuel tanks, water pump, water heater, and a lot of batteries, all in the very aft section, needed this.He claims the fuel usage went down a lot, as level is what he boat wants to be underway.
My personal take is I won't own any boat w/o the trim tabs.
To the OP: replace BOTH solenoids o the power trim unit, and the solenoid on top of the engine called "auxiliary starter solenoid/relay". They are said to be "sealed against moisture." Don't believe it.