1988 2556 5.7 Cobra OMC stern drive- trim the boat while underway ok?

[Guest]

I have the original out drive - 1988 OMC Cobra. 5.7V8 Engine - 3 blade 13.5 X15 prop. Never had an outdrive before. Another boater was explaining how to trim the boat once on plane. I also have trim tabs. Can I use the OMC Cobra outdrive to trim the boat while underway or is this model only to be used with the drive all the way down? Need an education here. I'm getting 19 knots at 4,000 RPM's full tank of gas, with my tabs down and the drive completely down as well. Should I be bringing up the drive to get better trim and more speed? I see people with the same model getting much higher speeds than I am. Thank you!

[desuch]

I contacted bayliner on this very issue here is what they told me yes you can trim your drive unit it will move your bow up or down ,trim tabs are for sideways trim ,so i have tried trimming the outdrive never noticed much difference and did not feel comfortable doing it so i run with mine down all the way

[CptCrunchie]

Not sure what the protocols are, but this is what I do.

I lower the OD all the way, then give it three quick kicks up. Once I am on plane, the amount of trim tab I use is dependent on how fast I want to go. The slower, the more I use. I rarely have them all the way up when I go faster, but close. On a comfortable cruise, then I level the boat with the trim tabs. At that point, I tweak the OD up and/or down until my fuel consumption is the best for the speed I'm doing.

Not sure if you know this, so I'll tell you anyway. Pressing the right top trim tab button lowers the right side of the bow, but it does this by lowering the left trim tab. In other words, lowering the left trim tab by pressing the right button 'rolls the boat' to the right, .....and visa versa.

Some boaters start with their trim tabs all the way down, then raise them as they go. This allows them to quickly find the lowest speed it will plane at. Most usually go a little faster just to make sure they stay on plane. Then they tweak the OD to maximize their fuel consumption.

Keeping the OD all the way down will keep the bow down, particularly in rougher seas. But doing this also creates drag in calmer seas. An easy test is to pick a day when there is a little chop on the water. Put the trim tabs all the way up, and drop the OD all the way down. Get your boat on a nice fast plane, level the side-to-side by adjusting just one trim tab, then start raising the OD. Eventually, the bow will start bobbing. This is called, "Dolphinning', and that's what you want to avoid. Gradually lower the OD until the bobbing stops. Now you are on a very fast plane with the least amount of drag.

There are lots of YouTube videos on this subject.

[Guest]

Awesome explanations Thank you!!!

[Guest]

Tried this a few days ago in a mild chop. 3/4 fuel / water 1 passenger. Not fun at all. Could barely get the boat on plane without the tabs. Once I was on plane with each bump of the drive up the nose cam up and slowed the speed drastically. Went from 19 knots @ 3400 to 15 then 14 knots. Once I hit the tabs and drive down the bow came down and it sped up again. What am I missing here? Note: this is a 2556 5400lbs dry and the smaller 5.7 not the 7.4 engine.

[CptCrunchie]

"jrr4948" post=816929 wrote:
Tried this a few days ago in a mild chop. 3/4 fuel / water 1 passenger. Not fun at all. Could barely get the boat on plane without the tabs. Once I was on plane with each bump of the drive up the nose cam up and slowed the speed drastically. Went from 19 knots @ 3400 to 15 then 14 knots. Once I hit the tabs and drive down the bow came down and it sped up again. What am I missing here? Note: this is a 2556 5400lbs dry and the smaller 5.7 not the 7.4 engine.

Did you take your engine to WOT (wide open throttle)? If so, what was the RPM? This will tell you a lot about how your boat is propped. Trimmed out, you should be able to hit an RPM of between 4200-4600. If you can't get it that high, it could either be an engine issue (which I read that you have been all through) or prop that has too much pitch. If you can get it over that, your prop could have a little more pitch to it. Unfortunately, most prop matching is a best guess. However, with your trim tabs down, you should easily be able to get on plane.

You might also see if you can shift any weight forward. My boat is only a foot shorter than yours, and the 5.7 works amazingly well at all speeds. I started with a 15.25x15 prop, then moved to a 15x17 with excellent results. Mine originally came with a 5.0, which most of them did back then, and it could get on plane too.

Not sure what to tell you at this point, just that you shouldn't be having the issues you are having.

[vthartford]

CptCrunchie, I am looking at a similar boat to what you have.  What are your GPH at cruise and WOT?  What is cruise speed and WOT?  Thanks!

[aluxury1]

When you run with the drive all the way down, it is actually a negative angle of propulsion. Like minus 3 - 4 degrees.You are forcing the bow down rather than straight ahead, good for getting on plane but not for ride and economy. And the trim tabs are not just for side ways trim, they will do more than outdrive trim. MY .02

[watchthis]

If you hit 19 knots at 3400 rpms on your boat with your 5.7, I think you ought to be stoked and are doing fine (my .02). In my opinion, the trim adjustments you made after that took you out of optimum trim and bogged you. Do what you did to get on plane and cruise 19 knots at 3400 rpms and have fun.

[Guest]

I have a 13.5 X 15 prop on it. I think this was the original size it came with. WOT is about 4,100 - 4,200 keep in mind this is an original Bayliner 1988 tachometer so I'm probably close to 4,200

[builderdude]

"jrr4948" post=816929 wrote:
Tried this a few days ago in a mild chop. 3/4 fuel / water 1 passenger. Not fun at all. Could barely get the boat on plane without the tabs. Once I was on plane with each bump of the drive up the nose cam up and slowed the speed drastically. Went from 19 knots @ 3400 to 15 then 14 knots. Once I hit the tabs and drive down the bow came down and it sped up again. What am I missing here? Note: this is a 2556 5400lbs dry and the smaller 5.7 not the 7.4 engine.

I have a different power package than yours but in choppy water I start with both trim tabs and drive trim all the way down. Once I'm up on plane I'll bump the drive trim up till I hear the rpm come up a bit. Then I'll start retracting the trim tabs a bit until she's riding where it feels good and level.

[desuch]

Factory prop for the 2556 is 15.5 x15 with the 5.7 I use 15.5 x13 4100 rpm at 26 mph

[CptCrunchie]

CptCrunchie, I am looking at a similar boat to what you have.* What are your GPH at cruise and WOT?* What is cruise speed and WOT?* Thanks!

WOT of 4600, max speed is around 36kts, but it is burning 26GPH. I typically cruise at 3300rpm with moderate trim tabs, using a 15x17 prop on my 5.7OMC Cobra, and I get around 20kts at just over 8GPH.

[Ruffryder]

13.5" diameter prop is too small for that boat...
going to need 15.5 or 15" diameter at least.

[Chief_Alen]

Here's the list, check it off one at a time ... The reason I am posting it is maybe your engine is not heathy, or a cylinder is not firing. Prop is the wrong size for one but recommend wot for your year engine is 42-4600 rpm. Read the list hidden water maybe. Post 8. https://www.baylinerownersclub.org/f...es-gctid392600

[JRR4948]

Hi all - Well shame on me I screwed up!!! I decided to take out my prop from storage now that April is on its way and it is time to uncover the beast. Low and behold the prop size I have is 15 X 13.5 not 13.5 X 15...Therefore it looks like the size of the prop is correct. What about the pitch? So here we go again Command Bridge 2556 5400 Lbs dry + 4 people and 200 pounds of gear and 50-75 gallons of fuel. Anything more than 4 people and she will not get on plane unless someone is on the bow and I wont have that. WOT is 4100- Rochester 4 bbl and new Jasper Engine. 3300 RPS = about 17-18 knots. Should I switch to a 15X15 for more speed. Would this impede my WOT and getting on plane? Possible the carb needs adjusting to give it more fuel? Fuel Pump and fuel lines are new, one way fuel safety valve new. There has been some disagreements on this site for timing. Manual says 3 degrees others say 5 degrees or even 8 degrees BTDC. My profuse apology for the mistake in my earlier posts. Thank you.