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Spun the hub out on the water today. Been thinking about a new prop anyways. I have a 1995 17' cobia bay boat with a 2000 df70. Just me on the boat today with approximately 15 gallons of gas and 75 lbs of gear, it runs WOT at 54-5500 rpm's and 31-32 MPH. Anti-ventilation plate is even with bottom of hull. I have a whale tale on it to help it get on plane better, which it does. The current prop is a suzuki stainless 13 1/4 × 16. Any ideas for a better prop size? Maybe 4-blade? Or Do these numbers sound Ok?
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You have a couple of options here: 1) A good prop shop can re-hub your existing prop as well as take an inch of pitch off at the same time. That should get your RPM’s up where they should be ~5750 RPM. Check your Owners Manual for rated horsepower RPM and shoot for the high side of that. 2) You could also raise the outboard one hole to get the anti-ventilation plate above the bottom of the boat and re-hub the existing prop without changing pitch. Raising the outboard a hole will reduce drag and increase RPM. Your numbers look good but could be tweaked a bit to raise RPM @ WOT. Remember you only get 70 horsepower @ the rated RPM. (Again check the manual) If you can’t reach that RPM you aren’t getting the horsepower you paid for. -
If you want to try a 4 bladed prop be aware that pitch AND DIAMETER affects RPM. If you find the same diameter prop (13 1/4) in a 4 blade version you will want to lower pitch by 2 inches (14P). 4 blades offer more stern lift and better hole shot but you lose top end speed due to the added drag of the extra blade. It’s a compromise. I’ve also found 4 blade props to run smoother (less vibration) and give better directional authority when in reverse. If you change to a different DIAMETER propellor you must account for the change by adjusting pitch accordingly: Even a 3/4 inch DIAMETER change is significant. A smaller DIAMETER prop behaves like a prop with less pitch and a larger DIAMETER prop behaves like a prop with more pitch. Here’s more info on the subject: http://www.actionoutboards.com/3bladevs4blade.html
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I actually dropped it off at a prop shop yesterday morning to have the Hub replaced. it's done I just have to go pick it up. the guy at the shop recommended dropping an inch pitch also so I will see if maybe he can do that to my prop before I look at buying a new one with a slightly less pitchComment
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