Re: Force 125HP Lower Unit
The prop will either have numbers on the front of the hub--under the nut and washer that holds it on, OR-- it will have the numbers stamped on the outside of the hub between the blades. If the prop has been painted, they can sometimes be difficult to see. There may be other numbers on the inside of the hub, but the numbers you are looking for are: With a stock Force or Chrysler prop for example 476 21. The 476 is the factory part number and defines the prop and the 21 is the pitch in inches. In this case, 476 is a 12 1/2 inch cupped cleaver prop with a 21 inch pitch. I only used it as an example because it was the one sitting next to me as I wrote this.
More than likely, if your engine is on a boat larger than 18 feet long, you will have a prop like a 523 17.
If you do NOT change the prop to a larger pitch, two things will happen:
1. acceleration will be excellent and perfect for pulling up a water-skier. However, top speed will be slightly less than with the old lower unit. You just won't have enough pitch to attain the top speed with the prop turning slightly slower through the water.
2. The engine MAY -- depending upon where the wide open throttle RPM was with the old lower unit-- rev up higher than the factory recommended WOT RPM. This will only be on the order of about 400 RPM more and will not immediately ruin the engine. You will however get slightly less fuel economy and possibly slightly less engine life--But remember, we are not talking about a lot of RPM here.
You engine was rated to run at the RPM range of 5000-5500 RPM at WOT. If it was running at say, 5000 before, then it will run somewhere around 5400 with the new lower unit -- Still within the recommended RPM range. SO; What you really need to do is purchase a tachometer if you don't already have one and see just where the engine is running at wide open throttle with the new lower unit. Now: if the engine is turning too fast, It is not absolutely necessary to purchase another higher pitched prop. You could "cup" the prop you have and this will reduce rpm by about 200-400 RPM. It will not give more speed, but will increase the prop's "holding ability" It will tend to ventilate less and give better performance in sharper turns. If you already had a larger pitch prop and the engine was turning less than 5000 RPM then no change of prop is necessary and you will get better performance out of the engine. Again, a tachometer is a very useful tool and really a necessity on outboard engines.
Cupping is the slight turning outward ( toward the prop nut direction or the direction the prop is trying to push the water.) of the last bit of the trailing edge of the prop blade--say, between 5-10 mm. It is not necessary to send it to a prop shop for this procedure. I use a good hard 2 X 4 on end. I put the trailing edge on the end grain and tap with a 16 oz hammer to bend the trailing edge. I work slowly and do not try to do a complete cup at one shot--rather I do it in stages. I start near the root (where the blade joins the hub) and work outward to the tip of each blade. Just be sure to closely eyeball it so it is essentially equal along all three blades. If I find after running that I have not put in enough cup I simply add a little more Or, conversly I take away a little. Even though the blade is cast aluminum (or stainless steel) It will not break or chip if you do the bending in small stages and work slowly. Get impatient--Work too quickly and you will chip the blade though.
