Quote:
Originally Posted by SailorChlud
Today I managed a good run in the bay- 12 nm or so. Using the aluminum
3 blade 15x17p prop we recorded WOT top speed of just 30 knots at 5200 rpm, so this seems to be the right prop for the boat so far. Seemed to cruise nicely with 6 people (our typical max load) and a full tank of fuel. That was with the bimini down. . .
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I respectfully disagree. Not sure of optimum rpm for your motor, how accurate your analog tach is, and if you've raised motor enough to get AV plate above water at cruise, but I think you should see at least 5500 rpm at WOT. (I believe optimum on the V-4 T-TECs, which use the same basic block, crank, rods, etc., is 5500-5750.) I ran a 13 3/4 x 15P SST prop on my old carb'd 115 which allowed it to spin up to about 5500 rpm with a very heavy Bahamas-type load and the top raised, and about 5700/30 kts with a light load and top down. I put well over 1000 hrs on that motor with no problems. When you lug down the motor by running a prop with too much pitch, it increases pressure and temperature in the cylinder, increasing loads on the bearings and rings and raising piston temperature that weakens the piston and makes it more prone to detonation or pre-ignition caused by a hot carbon deposit!
You can't hear detonation in a 2-stroke motor because there are no valves to rattle, which is what you hear in a 4-stroke motor that's "pinging", and is caused shock waves from fuel mixture which is exploding instead of burning! In the worst case scenario, the first sign of detonation in a 2-stroke may be when it blows a hole in the top of the piston! The lower speeds on an over-propped motor also reduce exhaust scavenging, leading to more carbon deposits on the piston, rings, heads and exhaust ports. Because the 2-strokes have nearly frictionless needle bearings, very short strokes and very rigid short cranks (good for about 9000 rpm on a V-4!), high rpm doesn't hurt 'em, and the harder you run 'em the more oil they get, so you're much better off to err on the high rpm side when selecting a prop!
Next time you're out, I would also test with bimini top up, which will further reduce your WOT rpm, so you may actually need an increase of about 400 rpm. That would typically require a pitch reduction of about 2", so when you get that dinged prop repaired, I would ask the prop shop if they could reduce pitch to about 15". You may not lose that much speed because the prop will be spinning faster with the motor higher up on it's HP curve, plus it will accelerate and climb on plane quicker. Once you get the aluminum prop dialed in, you'll then have a good baseline for selecting a more expensive stainless prop if you want a bit more speed and efficiency! I picked up about 3 mph with the heavily cupped SS prop because it allowed me to raise motor a couple holes.