prop selection

I have a 16.5" bass boat with a 115hp late 60's early 70's vintage "tower of power" I-6 and was wondering if you all could help. Current prop is a 14x21 ss mercury 3 blade. It gets out of the hole fine and runs out to 45mph at 5100rpm. I would like more top end, is there anything I can do?

Re: prop selection

Moved this to the Propeller section. You'll get more help for this here . . . Good luck!

Re: prop selection

More pitch = more speed but if your not reaching max RPM's then the equation begins to degrade itself. I would drop to a 19 and and grow a 1/4" or 1/2" in diameter if it will fit. That should pick you up about 400 rpm and maybe a few more mph. Once you get to the point your at, you are in the fine tuning stages of what is commonly refered to as the "Black Art" of prop selection.

Note: Just a reminder if you test with a aluminum prop for cost purpose give the prop room to grow because they will flex out on you.

Re: prop selection

I don't know what the gear ratio is but that's probably too much prop.
Is the motor in excellent tune, carefully timed?Is the throttle opening all the way.Have you raised the motor as high as possible on the transom?
Raise to just before venting(cavitation)becomes a problem.Be sure it pumps water.You could drop to a 19" pitch Would put you at ABOUT 5,500.
If you have a good prop shop they may be able to remove some of the cup of the 21" prop. to gain some rpm and speed.
Some years ago I read that the 115 needed to be timed carefully And at some point the timing was retarded slightly.Johns Old Mercury Web Site may be able to shed some light on the subject. May be able to help with the prop as well.

Re: prop selection

More pitch = more speed ONLY if the engine has enough power without over reving to twist more pitch. Gaining 400 RPM by decreasing prop pitch also does not guarantee an increase in speed because you move less distance (2 inches/rev theoretically in this example) so the increased RPM will result in the same speed but hole shot should be greatly improved. I don't know what Merc recommends for WOT rpm on that engine but it does appear you are over propped with a 21P. Before you start spending money on props, experiment with engine height. You might pick up speed that way as there is less lower unit drag. Just don't go so hight you lose water pressure.

Re: prop selection

The engine seems to run very well, the throttle is going "wide open" but I don't know the timing. How would I check that? The motor is as high as it can go, I already have cavatation issues in choppy water. Would a spacer plate help to gat the motor into "cleaner water"?

Re: prop selection

thephoenix said:

The engine seems to run very well, the throttle is going "wide open" but I don't know the timing. How would I check that? The motor is as high as it can go, I already have cavatation issues in choppy water. Would a spacer plate help to gat the motor into "cleaner water"?

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Use of spacers or Jack Plates needs to be carefully evaluated. Try to find out if the manufacturer of your boat has any recommendations. Most likely they will tell you that the best configuration is as it was tested and sold.

Offsetting the engine 5-6 inches aft in many cases can get you into better water but on some boats with deeper V's it is actually worse. It can also cause you to porpose because of weight distribution differences so you might find yourself needing to add trim tabs also.

The least expensive would be a simple offset plate and if it works for you then you might try a Jack Plate that you can fine tune.