Prop selection problem

[exdog]

Boat essentials:<br />Thundercraft Magnum 160 16'<br />Tri Hull<br />Yamaha 90<br />Standard load is 2 adults and 2 small children and a cooler.<br /><br />I had a Johnson 60 with a 15p prop until the #3 pistin ring let loose. I decided on a bigger engine hence the Yam 90. I also had a Michigan Wheel 2 piece prop, got a hub for the Yamaha and put the 15 pitch on.<br /><br />I checked with the dealer that WOT was 5400 and Im between 53 - 55, and that I dont hit 90HP until 5000. But when I try to take off, I get massive ventilation. Ive tried those whale tails and trimming all the way down but it still does it. The ventilation plate sits just below the bottom of the boat. <br /><br />I considered jumping to a 19 to eliminate the ventilation, but according to what I read it would drop the RPM to right at 5000. A 17 might be better, but I dont want to suck air. <br /><br />Any other suggestions or am I on the right track?

 

[Mettaree]

Re: Prop selection problem

Where is the AV ( cavitation plate ) in relation to the bottom of your boat ? Above ? How high ? Do you have any transom add ons, i.e. speedo pick up tube, depth transducer, etc. mounted close to motor ( inches away ) that could cause turbulance getting to prop. Check height first. Even to maybe 1 1/2 inches above keel at transom.<br /><br />

 

[Dhadley]

Re: Prop selection problem

Going to more pitch will drop your top rpm's as you suspect. Exactly the oppsite of what you need. Venting on acceleration has to do with the overall condition of the prop and the original design. <br /><br />If its worn it will do as you describe. Which is what we suspect as you said your AV plate is slightly below the bottom now.<br /><br />You havent mentioned diameter. We suspect its small also.<br /><br />Mounted that low could certianly explain the broken rings. The rpms need to come up for longer life.<br /><br />It appears a new prop is in your future. A good design that will allow you to raise the motor. That alone will increase your overall efficiency and prolong the life of your new motor. We would target 5800 with an average load.<br /><br />Good luck!

 

[exdog]

Re: Prop selection problem

JK, I stand corrected, the AV plate is dead even with the bottom of the boat with the motor sitting as low as it will go. The speed and depth pickup are both more then a foot away from the motor.<br /><br />DH, The RPM range for that motor according to Yamaha is 4500 - 5500. 5800 is way over red line, and I dont want to chance overreving.<br /><br />The prop is in like new condition, and it a two piece prop from Michigan wheel. According to their prop selection chart, they call for a 21p prop which is even bigger that I figured. <br /><br />I mentioned it earlier, but it only happens during takeoff and goes away once it gets on plane.<br /><br />Info on RPM range found here: http://www.boat-props.com/info/prop_info.cgi?page=rpm_yamaha <br /><br />Info on prop size found here: http://www.miwheel.com/MIWheel/html/products/outboard/PSearch.asp <br />Based on these selections:<br />Propeller Type: Outboard<br />Manufacturer : YAMAHA<br />HP / Model : 90 HP<br />Year : 1984-PRESENT<br />Boat Size : 15'-18' BOATS - ALL LOADS, SKIBOATS<br />Material : ALUMINUM<br /><br />Blades: 3<br />Diameter & Pitch Rotation: 12-5/8 X 21 RH<br />Part number: 101035<br />Style: MICHIGAN MATCH<br />Comments