This is an old one I wrote 5years ago, but it is probably useful to someone needing a rock guard for their prop.
If you are like me you’re hunting areas with a prop that are good some times and other times trying to break your skeg or grind you propeller to shreds. I hail from the mountains in Western Maryland and one thing we have plenty of is Rock ridges. We have them in the farm fields and in the rivers. The river depth can go from 6 feet to 12 inches in 1 foot of space. You can race up the river and have your lower unit hit one of those and you just cringe every time you hear it. So wouldn’t you like to build a rock guard that can save your lower unit from all that trouble?
The following is a little instructional tutorial and some history of what not to do. You will learn how to build yourself an easy rock guard that will not take too much of your power away. But if you’re like me, it better to only do 16 mph and save your skeg and prop than to do 20 to 21 mph and split you skeg up into the bullet area, again, or trash yet another prop. Now, even when this rock guard is finished you still should not be slamming your lower unit in to rock ridges or the bottom. That sudden shock vibrates all the way up to my shoulder when holding the tiller handle, so I know it’s still hard on the motor.
I started out with a yard sale rock guard I found. I thought “This will get me into all those lower water spots and I’ll really get into the birds.” Well, it was better than nothing, but it had it downfalls, shoot, almost pitfalls. It was made from 1” angle iron that would be bolted to the cavitation plate. That one had an old pitch fork that was welded to an angle iron drop in front of the lower unit bullet. The pitch fork was welded on a 45 degree angle back toward the motor to roll up the motor on contact with rocks.
Now this worked, but had some serious flaws. You don’t have to be a rocket scientist to know a pitch fork in front of your lower unit is going to slow you down. So many pieces causing turbulence and taking power away from your prop, but the angle iron drop was the worst. The drop had the closed end of the angle pointing ahead of the motor and made a eddy behind it and made lots of turbulence. If I got up near half throttle the prop started to cavitate so bad it almost sounded like the motor was out of gear.
This was an easy fix. So I cut out the angle iron drop and replaced it with a ½” cold rolled rod. Welded the pitch fork back on and went for a drive. Well, I cut the turbulence problem, but I still had all that drag from the pitch fork and now the pitch fork needed a stabilizer strap to the skeg to keep it from bending back on hard contacts. So I said that’s it and I going to build another one, but I’m and only going to build it with what I have in my shop or one of my buddies will give to me. You can’t beat a free rock guard.
So off I went digging. I found some angle iron that I could use on the top of the cavitation plate. I had a coffee can of old and new bolts from other projects I really did not want to use cold rolled steel after the last job and it started bending on harder contacts. So I asked my buddy Jerry and he said they had some old rebar lying around that he would use for trapping stakes. I got several feet of that and headed home to start the build. So now the following is a list of what I used to build the rock guard.
4’ – of ½” rebar
4 – of 1” angle iron
8 – of 1” wide, 1/8” thick flat steel
1 – 5/16 x 1” bolt and nylon lock nut
4 – ¼” bolt and nylon lock nut<br><br>
The Angle iron was cut on one side in three places and bent to fit around the motor above the cavitation plate. The two end cut where bent about 30 degrees and the middle cut was bent about 120 degrees or so. A little scrape metal was laid over the cut areas and welded to stiffen them. Some fill rod was welded into the front area to thicken it up for where I wanted the rebar to go. The rebar was already bent for laying into concrete so all I had to do was open the bent you see in front of the bullet. I tried to make the bend you see at the bottom of the skeg but rebar is tool steel hard and with out a torch I could not get it to bend. So I cut the tail and the two side arms (cut and welding on a 45 degree angle) and welded them all together.
You can see in the above picture how the guard can grab rocks and push the motor up as long as you have it on tilt. My wife likes to refer to the rock guard as the “chicken foot”.
The only thing left to secure the rock guard and make it work properly was to weld some 1” 1/8” flat stock to connect the guard to the skeg. Then I drilled with a 7/32” bit through the flat stock and the skeg and bolted it all together. I then cut off the remaining bolt hanging past the nut to remove more drag. Then I let my daughter help and prime and paint the entire thing. The guard got two coats of Ford blue from tractor supply.
Now this does slightly impact performance but it makes sure the skeg stays on the lower unit while using it on a rocky river. Another draw back is even though it is round it does affect the water intake for you water pump some so you have to monitor your water stream on the motor and the water to make sure it stays cool. Have fun and try and make your own Free Rock Guard.