Re-gearing OMC 800

[Crisco87]

I am a new boat owner (about a month now) and I am slowly creating my "To-Do" list for my 1979 Bellboy 28'.

Currently I have a 305 Chev with an OMC 800 foot. There is a label saying 1.5:1 gear ratio. I posted earlier asking how to make it faster and I was told I would need a lower ratio. How would I go about doing that? What would you all recommend? Right now I don't even get on a plane...I just muscle through the water. I am not trying to turn it into a speedboat...but by using my GPS speedometer it shows I am barely breaking 8 mph.

Any recommendations?

Crisco

 

[mkast]

Re: Re-gearing OMC 800

A 28 foot boat with a 305 engine, no amount of re gearing will help.
You need a six pack of torque and a six pack of horsepower,
read bigger engine, lots bigger!

 

[HT32BSX115]

Re: Re-gearing OMC 800

Howdy,


Your problem may NOT be drive ratio.

You could have other problems ranging from wrong prop pitch to a boat that is seriously over weight (due to water under the deck etc) or an engine that is not developing full power.

Also, as old as it is, your drive ratio may not be what is indicated since someone in the last 30 years could have rebuilt that drive many times and used whatever gear set that was available. (not 1.5)


Things to check first.

Engine compression, plugs, points condenser, carburetor etc.

DETERMINE the drive ratio. 1.5:1 means 3 engine revolutions to 2 prop revolutions. (in gear, coil wire removed) move the engine with a wrench 3 turns and have someone count blades on the prop)

Then determine the prop pitch. It's usually stamped on the prop but that prop may be re-pitched.....


There's info above in the "adults only sticky" that are generic in nature. and there's service manuals for Mercury engines and drives that have trouble-shooting sections that talk about engines that cannot achieve full power that would apply to your engine.


Good luck...


Rick

 

[HT32BSX115]

Re: Re-gearing OMC 800

A 28 foot boat with a 305 engine, no amount of re gearing will help.
You need a six pack of torque and a six pack of horsepower,
read bigger engine, lots bigger!

Although I agree that 28ft boat is seriously under-powered, .....My former next door neighbor has a 26ft Fiberform with a 305 and a stringer that (although not a hot-rod) actually planes.

So this boat should do (a little) better than 8 knots...

 

[Howard Sterndrive]

Re: Re-gearing OMC 800

I posted earlier asking how to make it faster and I was told I would need a lower ratio.

No one said it would be faster
http://forums.iboats.com/showthread.php?p=2071191#post2071191

1. Do you even know what gearing is actually in the drive other than the decals?
2. Have you remedied all of the safety deficiencies? If you still have that automotive distributor in there, it's part of the power problem but it's a bigger safety concern.
3. The 350 in my 1974 Chev with the smog era cam was rated at 145HP.
I have no idea what an early 80's 305 would have... 135 maybe?
With the big ol' EGR sticking up there, and the vacuum advance on the distributor, it could very well be that you have an automotive engine transplanted in there before the sale to you (to make it runnable) and therefore you would have about half or maybe 2/3 the power you would require to plane that boat.

And, we're going on someone's word that it even is a 305... It could be a 283, or a 267 cubic inch or something even smaller
Might want to get the casting number off the block

 

[Crisco87]

Re: Re-gearing OMC 800

I have not looked into the actual guts of the foot or anything like that. Like I said in my first post, I am still very new and am doing my best to learn. What would you recommend, then? Refitting the engine with marine components or just buy a new engine/foot?

 

[Howard Sterndrive]

Re: Re-gearing OMC 800

Refitting the engine with marine components or just buy a new engine/foot?

I would climb in there with a flashlight and try to figure out what the engine is by it's casting number.

If it's a 350, I would probably compression test it.
Then try to remove the timing cover and see what I could figure the camshaft was.
If a marine cam, I'd leave engine in place and replace all the electrical and fuel system parts with marine approved components.

If an auto smog cam, I'd get a marine cam into it. Those car cams from the pollution controls era can cause an engine to suck it's own exhaust water into the cylinders and cause hydrolocking at idle and destroy the motor.

I wouldn't give the drive any thought until I knew what I had for an engine.

 

[Crisco87]

Re: Re-gearing OMC 800

Howard,
I really appreciate your help thus far. How would I know if it's a "smog cam" or "marine cam?"

Is it a pretty easy to do a compression test?

 

[Howard Sterndrive]

Re: Re-gearing OMC 800

How would I know if it's a "smog cam" or "marine cam?"

hmmm....say a little prayer that there would be a part number on the front end of it under the timing gear about the only way without pulling it out.

Again...you are planning too many steps down the road.
If the casting number says engine's a 267 p.o.s. out of a Monza, then it's gotta come out anyways. doesn't matter what the cam is.

Is it a pretty easy to do a compression test?

yes
I'll bet there's a video tutorial on youtube

 

[Crisco87]

Re: Re-gearing OMC 800

Following your advice Howard,
I climbed in there and looked around for some number. I found GM 3570010 towards the back of the engine. Am I to assume, then, that it's a 357?

 

[boatguya1]

Re: Re-gearing OMC 800

Following your advice Howard,
I climbed in there and looked around for some number. I found GM 3570010 towards the back of the engine. Am I to assume, then, that it's a 357?

Yes the 357 and I think the 0010 after it indicates it's the magnum marine model.

James

 

[boatguya1]

Re: Re-gearing OMC 800

OK, so that wasn't nice, I did get a chuckle out of it.

I looked on a couple sites with casting numbers and could not find any that started with 357. I don't usually have to ID motors on casting numbers so I could be missing it. Google GM casting numbers for some lists.

James

 

[Howard Sterndrive]

Re: Re-gearing OMC 800

Following your advice Howard,
I climbed in there and looked around for some number. I found GM 3570010 towards the back of the engine. Am I to assume, then, that it's a 357?

could the 5 be a 9?
3970010 is a common 350 block from the 70's
I have that one in my Chevelle actually.

3970010 casting was also used for 327 and 302, but in limited numbers.
It was available as 2 or 4 bolt main, but almost all were 350 with 2 bolt main.
I think marine 350's from the 70's probably also used that casting mostly.
This is positive!

It's worth replacing the distributor with a Marine one and seeing how the power is. I PM'd you a good example of what you need.

 

[Howard Sterndrive]

Re: Re-gearing OMC 800

Yes the 357 and I think the 0010 after it indicates it's the magnum marine model.

James

Rattle Can of black paint and a fancy cover for the flame arrester and he pretty much has it 'cept for the vortec heads....

 

[boatguya1]

Re: Re-gearing OMC 800

Now that looks very nice. (wipes droolfrom chin)
James

 

[Crisco87]

Re: Re-gearing OMC 800

Howard,

So if I were to purchase that item you messaged me, that would replace all of the safety inefficiencies you found on my pics?

Assuming this is a 357, it would be worth rebuilding then? Possibly investing some money to get a new outdrive...other than OMC?

 

[boatguya1]

Re: Re-gearing OMC 800

I don't think you have a 357. I would guess that Howard Sterndrive is correct and your 5 is really a 9. I did not find a 3570010 casting number on any of the charts. As he mentioned the 3970010 casting would be common for a smallblock.

A marine distributor is a good start. You also need to make sure you have a marine carb, starter, alt, fuel pump, and fuel lines. Due to liability I don't suggest it but a guy could replace the distributor and see how it runs. At this point it might be cheaper for you to find a good used complete marine engine with accessories. Or possibly a complete "donor boat" that you can transplant vital organs from.

James

 

[Crisco87]

Re: Re-gearing OMC 800

Either way, if it's a 350 or a 357...a 350 is still good enough to power my 28 footer, right?

 

[wvwarlock]

Re: Re-gearing OMC 800

do you have the ID plate on the port side valve cover that gives you the engine model number? if so post that number and it is easily cross referenced to let you know the engine size along with the horsepower rating that it should be unless it has had major overhauling. i will gladly look them up in my factory manual and give you the stats if you can provide that model number.

good luck

 

[boatguya1]

Re: Re-gearing OMC 800

do you have the ID plate on the port side valve cover that gives you the engine model number? if so post that number and it is easily cross referenced to let you know the engine size along with the horsepower rating that it should be unless it has had major overhauling. i will gladly look them up in my factory manual and give you the stats if you can provide that model number.

good luck

From pictures in another thread it looked like a complete transplant from a car or truck.

James