[Fr. Frank]

Originally Posted by thealife

I am thinking about repowering my 18 with a new 115 yammi,

I repowered my '87 18' with an Etec 90. I have the factory 25" transom and the 25" 90 comes with the 4.75" lower unit gearcase. I'm running a 15x17. I cruise at 23 mph, and WOT is 34-35 mph. I can hold plane down to 15-16 mph and have no trim tabs.

The best part is an average 7.1 mpg. Over the course of a recent 122 mile fishing trip, of which half or more was trolling at about 7 mph, I burned 12.9 gallons, or just under 10 mpg.

I think my hull was re-placarded at some point. It says the right hull and manufacturer, but says max HP is 200. I lasered measured the hull which came out at 18' 6.4" LOA, with a beam of 7' 5.2"

I ran the numbers and the USCG algorithms show a legal max HP of 191.2 hp, (which is rounded to the closest 5 hp increment, i.e.; 190 hp) so the decal is wrong. BTW, it had the original Mariner 115 hp when I got it.

For those of you who want the numbers, it is LOA in feet, multiplied by the beam in feet, x2, - 90 = max HP.
USCG allows the LOA to be rounded up to the next higher foot increment, but the beam is always calculated to the nearest 1/10th of a foot including rub rails. If the boat is a sterndrive, the propulsion machinery (outdrive) outside of the hull is INCLUDED in the LOA calculation. Outboards motors are not. Swim platforms or outboard brackets are included ONLY if they include an amount of flotation that is greater that their displacement value.