I have a 1983 Omega 20' open bow I/O with a SBC in it. Bought it a couple months back and its my first boat. I'm just curious as to how fast I should expect this boat to go? Im not at all out for speed, but I feel it should go faster than what it goes at the moment. I GPS'd it at 36mph going up river out at Fisher's Landing.

When I first got the boat it ran like crap from bad plug wires and such. So I tore into the motor and put a Summit hydraulic cam and lifters in it (282 duration with .465 lift), new true roller timing chain, a Weiand Single plane intake and 650 Edelbrock carb was put back on. Also put brand new HEI dist. with new wires and plugs. The motor runs great in my opinion. The boat has a brand new 14X21 pitch SS prop on it that came on the boat when I bought it. It has a Mercruiser Alpha One series one outdrive.

Is thinking that my boat should go closer to 50 mph wrong? Is a dual plane intake better for my boat? One issue that I haven't gotten to yet it that the tach isn't working so I'm unsure as to what the motor revs to at wide open. I have used some of the online prop calculators but am unsure about a few things. I don't know the drive gear ratio....I just used 1.84? I don't know the RPM the motor is turning.....so I just put 4500? and I don't know the prop slip %.....so I used 12%?

Am I pissing in the wind? lol

Am I pissing in the wind? lol

The first problem is most of the guys here don't know much about i/o's and really don't want to.

GT is going to be your best source of information and he should chime in soon.

The cam specs seem a little off to me, most 280 series cams have around 480 lift.  I think you would have a better combination with a dual plane intake manifold (like an Edelbrock Performer RPM) than the single plane for the cam you have.

Good luck and welcome to SCJB

Thanks I'll have to get his opinion. Hopefully he sees this thread

I actually have a Performer intake which is why I asked. I think it probably would be better too since my boat doesn't have so much done to the motor nor is it a top end boat.

As for the cam, the specs state 282/292 duration with .465/.488 lift. Maybe my motor combo is wrong....not sure

I plan on hooking up a tach and running it this sunday to get a baseline.

Bonneville,

That's exactly what I was thinking. I feel like its just not going the speed it should. Im planning on taking the boat out this weekend with a tach so I'll let you guys know.
Thanks again for the response, check back in a few days and Ill post the results.

Jordan

Have you checked for water saturation under the floor?  It could easily be a couple hundered pounds more than it is supposed to be.

In the picture it sure looks to be setting deep in the water.

 

Just got home from work and can't sleep...Saw the thread and think you have more than one problem...

Is this a Mercruiser or an Obsolete Marine Corporation deal?

I feel you have a bad torque coupling or a bad propeller hub and this causes a major slippage not unlike a slipping clutch in a car. The problem I have with blaming the prop is you have tried two different ones....

Second, providing you have a 350 Chevy/260 HP Merc, the ratio should be 1.50:1, you should also be running in the 4200-4500 rpm range with a 21" pitch prop...If your getting 5K, that is one good running SBC....

If you feel the tachometer is even close, leave the carburetor alone...

Three questions..
1. Do you know how to properly trim the boat out? You need to trim the drive "up" while under way and get the hull out of the water, if left "down" it will plow like an icebreaker and not perform worth poo.    When trimming the drive, while on plane trim up until the hull starts to porpoise, then bring it in (or "down") a tick, this will be the optimum point for top end performance...

2. How much crap/people did you have in the boat? The load needs to be proper to ensure a sound center of gravity...

3. Have you weighed the boat and trailer?...A water logged hull will make you chase your own butt...


My last Alpha I (Merc) boat had a 260 HP 350 with some mild mods including a square bore carb, a 1" carb spacer (4 hole), a dual plane intake, through transom exhaust and a giant hillbilly flame arrestor and it could do 65 MPH at 5000 rpm all day with a Quicksilver High Five 21" pitch wheel.

You should be able to get into the low 50's without much fuss...

GT

The torque coupler iss attatched to the crankshaft of the engine at the flywheel...Annual alignments are necessary to keep it from failing...

Trimming the boat out should be worth 10-15 MPH...

GT

10-15 really? holy moly....blame it on the driver then...haha

So everything is so cramped towards the back of the boat that Im not sure if I can see the coupler. Is it rubber? I suspect I would need to pull the motor to even check or replace the coupler?
I can see the shaft coming out from the flywheel to the outdrive and looks like a round (maybe rubber?) coupler thing. Not sure if that is what you are talking about.

You cannot see the torque coupler...It is inside the flywheel cover...

You need to retest with your new found trimming skills 

GT

TRIM IS GOOD .  .  .  words to live by

So I took the boat out again this afternoon....

The trim did help some, with the 19 pitch prop the boat went 40 mph at 5200rpm. I put the 21 pitch prop on, bimini down, took out anything that didn't need to be in the boat (chairs, cooler, etc) and the boat went 43 mph at 4600 rpm down river. If I could get the boat to rev to 5200 or so with the 21 pitch prop I'm sure Id be in the 50's.

In the midst of all this I ended up towing a 26ft Pontoon boat the had the water pump go out in it. lol

So whats your thoughts on speed etc? Should I swap the intake to a Edelbrock Performer? Try a holley 750 vacuum? I have those sitting at my brothers. Or you think the coupler is bad?

Propeller slippage is out of control...(excess of 25%)   

The easiest way to check a torque coupler is to put a breaker bar on the front crank bolt of the engine and put the outdrive in reverse gear then stand on the prop in a forward motion (don't cut your leg off) engine off obviously, there should be no rotation of the prop (I do not think this is your problem....)..........

You either have a serious hull deflection in the bottom, a ridiculous hook, or your GPS or tachometer are full of shit....

You should be in the high 50's (58.2 AT 7% slip) with a 21" prop and a 1.50:1 gear ratio at 4600 RPM....Something is amiss.

GT

checked the torque coupler like you said. I cant stand and push on the prop and sucker doesn't budge. Haven't checked the deflection or hook yet though....