Numbers
 

What kind of numbers should I expect to get from a 1975, 18 SF with a 98 Yamaha C115 two stroke & a Power Tip Ballistic 13.5" x 17P. prop. At the present time, I only have a tach., fuel gauge and a volt meter hooked up and have'nt installed any electronics yet since I'm still undecided on what unit to get. My w.o.t. is 5500 rpm as per Yamaha manual.
I primarily fly fish and use the boat on the great lakes & inland lakes as well as coastal near shore fishing for Albies, striped bass & bluefish and a yearly jaunt to South FL. for Tarpon/flats fishing etc.
Any input as well as advice on electronics would be appreciated.

Good set-up but unless your WOT all of the time, I'd swap the prop.

My guess is you'll hit around 40mph once the pointy end comes down.
Cruise about 28 at 4200 getting about 3 mpg.

Your rpm is right on, but me thinks the Ballistic is not the best prop for your rig. I believe efficiency and planing will be compromised under 20mph. That prop is designed to lift the bow, and run fast by reducing drag at the blade tips. Like most CCs you're going to want a prop that offers some stern lift to get on plane quickly, while having enough blade area to plane at lower speeds when the going gets rough.

Thanks for the input MG. I want to get another prop but unfortunately I don't have the luxury or access to try several props before I find the right one. I inquired about the right prop for my set up before and these are a few recommendations that I got. Solas Titan 13x17 semi cleaver, Quicksilver Trophy 13 1/4 x 15 or 13 x 16 (rare) , Black Max and Black Diamond in the same size of the trophy. I have yet to make a choice, what do you recommend ?
At 60 years I'm not exactly a speed demon so I don't mind loosing some top end for better efficiency and to get on plane quicker and at slower speed in the slop.

Roger that.
I think the black max/diamond is actually 13-3/4" diameter in the 15p but otherwise those are all solid. I think it's probably the best economy prop available for these hulls.

I think the Trophy is a stainless 4-blade and would do fine. Any boat shop would likely have a used black max/diamond available to try. Have you asked around or are you just too far away from shops?

Yamaha also has a semi-cleaver shape that would do fine but I think the the merc quicksilver has more blade area...

Great Motor! No oil injection on those commercial yamahas. The stock aluminum and stock stainless props on the yamahas are pretty fast (might even be faster than the ballastic) . . . You can play with the engine height, but your doing well with the setup if you are turning 5500 with the 17P prop.

a 17p prop, 2:1 ratio lower, and turning 5500 with 10% slip you'll be at 40mph.

http://www.go-fast.com/Prop_Slip_Calculator.htm

Here are a few archived 2 stroke yamaha 115 performance bulletins. The keywest 186 is very similar to your boat ;)

http://www.yamahaoutboards.com/sites...er-115TLRB.pdf

http://www.yamahaoutboards.com/sites...0111-CEN-Z.pdf

http://www.yamahaoutboards.com/sites...ef-115tlrc.pdf

Thanks for the input guys. Are the black max/diamond stern lifting like the 4 blade props ?
I have asked around to sea trial/try some props but most shops in my area don't offer such option and the only shop that does, doesn't have any suitable props for my application.

Yes, they are very good stern-lifters.
Here are a few reasonably priced options:

http://www.ebay.com/itm/Yamaha-13-1-...a9991d&vxp=mtr

http://www.ebay.com/itm/Mercury-Outb...ef8d3e&vxp=mtr

http://www.ebay.com/itm/Used-Outboar...0b9efb&vxp=mtr

Thanks for the links MG but I'm a little confused now. My existing prop is 13.5X17P and wot is 5500 rpm as per Yamaha specs, if I go to 4 blade shouldn't I drop in pitch or size ? (The trophy plus link is for a 17P !) and if I go with a black max/diamond would it be ok to drop to 15P instead of 17P ?

Sorry sidelock,
Didn't mean to confuse the issue. You could use the 15p but you'd probably wind it a few hundred rpm higher than you desire. Give you a heck of a hole-shot though. Just trying to show the props discussed are available for decent prices.

I'd try a 17p in any of those. Its very possible that you may gain or lose 100 rpm or more from prop to prop. Regardless, you're looking in the right direction.

You're right - I think the general convention is that you drop and inch of pitch going to a 4 blade. But a bow-lifting prop is usually harder to turn than a stern-lifting design, so that may be a wash...