How to tell drive gear ratio?

[Fordiesel69]

Is is an 89' wellcraft eclipse 233 23' boat. It has a brand new genuine merc alpha one gen one unit on it. I want to verify it has the correct ratio for the engine which is a 5.0L 305 V8. It seems the WOT RPM's are way too low for the 19P prop that is on it.

Would the serial number tell us this.

 

[achris]

Re: How to tell drive gear ratio?

Is is an 89' wellcraft eclipse 233 23' boat. It has a brand new genuine merc alpha one gen one unit on it. I want to verify it has the correct ratio for the engine which is a 5.0L 305 V8. It seems the WOT RPM's are way too low for the 19P prop that is on it.

I'm not surpirised. I would expect that combination to use something around 15", not 19".

Would the serial number tell us this.

The DRIVE serial number will, yes. Should be 1.5:1 for a 5.0litre. You can also take the top off and count teeth, or just turn the engine over with someone outside counting turns. The number of turns you did divided by the number of turns they counted on the prop is the ratio. (needs to be in forward gear for this to work )

Chris..........

 

[Fordiesel69]

Re: How to tell drive gear ratio?

Ok, I just searched and found where it is stamped. My bayliner has a 1.65R stamped in it, and others say 1.65 is what it should have if it is a V8.

 

[achris]

Re: How to tell drive gear ratio?

Ok, I just searched and found where it is stamped. My bayliner has a 1.65R stamped in it, and others say 1.65 is what it should have if it is a V8.

Some early 5 litres were fitted with 1.65, but the later ones are all 1.5:1.

I guess you can take your pick.

Chris........

 

[John_S]

Re: How to tell drive gear ratio?

I think the 1:65/1:62? ratio was for the pre-vortec alpha 305's. ie the "standard" one. Boat mfgs could also select from some optional ratios, to deal with heavier boats and elevation, etc. Ideally, the right gear ratio for a given boat would have a 19" pitch to be right in the middle of the commonly available prop pitches.

As you found on your Bayliner, the ratio is stamped on the drive, if you can still read it. Also a code is stamped on the input shaft that will indicate ratio. While this one is new, and probably accurate, on older ones, using the turn method or counting teeth is more reliable.

Are you looking at buying the Wellcraft?

 

[Fordiesel69]

Re: How to tell drive gear ratio?

No its a friend that already bought it. We are slowly trying to figure out why WOT rpms are so low. I just went over and looked at it:

Mercruiser service drive assy
1.62R 0A842579

Prop is a 14x19P

RPM is about 3700-3800 and the engine does not have a sound of being overworked.

 

[achris]

Re: How to tell drive gear ratio?

...RPM is about 3700-3800 and the engine does not have a sound of being overworked.

Then get another tacho (preferably a 'shop' tacho) and check that the dash tacho is indeed accurate....

Chris.....