1979 Mercury o/b 90 hp prop question

Hello all. Motor does not have the original prop install. As you can tell the, I think its called a anode, has been modified. I'm not familiar with motors at all, but I did install a brand new harness in this thing. Motor sounds great. But anyway, I want to replace the prop with the one that belongs on it because I'm affraid this one will ruin the drives. Any info on the size and the pitch would be greatly appreciated.

What is the perpose of the anode? Just curious. I will replace it also.

I will get rid of the prop I have. Looking to trade for one like the size I need or sell on CL. Thought I would give the guys on IBoats first dibs since you guys give great advise for a great price.
Not sure of its worth but I know they are not cheap, the prop that is.


Thanks guys

Re: 1979 Mercury o/b 90 hp prop question

View attachment 113811

The PO never ran the motor in the water with this prop. I'm not sure but that may have been a good thing

Re: 1979 Mercury o/b 90 hp prop question

The anode is a trim tab that is adjustable to cancel out steering torque, you can buy one that is offset for prop clearance. The prop should have a part Nr. and the pitch, diameter stamped in it on the side and/or on the front of the hub behind the nut. If it's a 21 or 22 pitch I'd be interested in buying.

Re: 1979 Mercury o/b 90 hp prop question

merc850 said:

The anode is a trim tab that is adjustable to cancel out steering torque, you can buy one that is offset for prop clearance. The prop should have a part Nr. and the pitch, diameter stamped in it on the side and/or on the front of the hub behind the nut. If it's a 21 or 22 pitch I'd be interested in buying.

Click to expand...

I will try to remove it tomorrow. Might as well ask, any special tool needed or would wrenches and channel locs be fine?

Re: 1979 Mercury o/b 90 hp prop question

Remove the plastic plug at the top of the driveshaft housing above the trim tab and you will see a bolt underneath, it should be a 12 pt. socket, or an Allen head, bolt - just undo and the tab will come off.

Re: 1979 Mercury o/b 90 hp prop question

One thing to remember the late 70's 90hp inline 6's are rated @ 4500-5000 RPM at WOT.So depending on what type of boat you slap that puppy on you would need to find the optimal prop size for your needs.For example I run a 15 ft trihull 800 LBS. plus 300 LBS 90hp inline 6 2 adults 350 lbs. 100 lbs. gas,cooler etc,etc. So at around 1500LBS. I use a 13.5 x 17 pitch SS prop spinning WOT 5000 RPM and GPS 45 MPH.Also my SS prop sticks out just like yours does and I choose to use a race zink anode that is a flush mount on the cavitation platte.Good Luck and have fun.These 90hp inline 6's are quick out of the hole and have the torque to pull .

Re: 1979 Mercury o/b 90 hp prop question

Gomer50, thanks alot. Great info. My set up is pretty much the same. 15 1/5 trihull bassboat. So would this oversize prop damage the drives? The motor is in great shape and I would like to keep it that way. I don't know if a prop puts strain or just over works the motor. I know what kind of anode you are talking about. If the prop will work just fine on the motor, I will purchase one of those.

You are right about this thing being quick. For just 90horses, it sure surprized me. Guages don't work, not yet anyway, last time I was on it with PO, I bet we hit 60 or better.

Again, would this prop be perfectly fine? Thanks alot

Re: 1979 Mercury o/b 90 hp prop question

Jitter, That prop has more rake than a standard prop. This usually means it will perform better. There are actually three (maybe more) anodes that will fit that motor. One is standard, with the fin i the center of the anode and is what has likely been hacked down by the PO. The second has the fin set back. Raked props will usually ork fine with these,a nd I recommend you try one. If that fails, there is an anode with no fin at all, which will definately work.

Since the fin can offset torque to the steering wheel, having a fin of some sort is recommended. Try out the prop before you sell it and go to a standard one. You may just like it!

Re: 1979 Mercury o/b 90 hp prop question

Ok, I take it that the prop is just fine on the motor. I will use the prop and look for the proper anode (with a fin if possible). Thank you very much guys.