Reduce Pitch or Raise Outboard?

[Mike Robinson]

Hello All<br /><br />I have an 5.6 meter (18 1/2 ft.) welded aluminum cabin boat with a 1994 150 hp Johnson outboard (commercial model). When I bought the boat, WOT was 4800 rpm with a 15 by 17 three bladed aluminum prop. I have since switched to a 15 by 15 three bladed aluminum prop and removed the hydrofoil from the leg. I now get 5300 rpm with a light load with the motor and trim tabs trimmed up and a speed of 33 knots. <br /><br />The cavitation plate is just shy of about 2.5 cm. (1 inch) below the keel and the motor is mounted in the bottom holes of the outboard bracket.<br /><br />We use the boat for ocean fishing and troll with a kicker (9.9 Yamaha). We run about 30 miles to the fishing grounds through sea conditions from smooth to 2 meter swells.<br /><br />My question is; Should I consider raising the motor or use a smaller pitch? Fuel prices being what they are I want to be efficient as possible.<br /><br />PS. I am also considering buying a Stainless Prop and keeping the Aluminum as a spare. Any comments on this would also be welcomed.<br /><br />You feed back is much appeciated.

 

[walleyehed]

Re: Reduce Pitch or Raise Outboard?

You say the AV plate is 1" below??? should be about 1" above for starters, but you also say the bolts are in the "bottom" holes...???<br /> Verify and post back on where the top bolts are..Of course it would be best to raise the engine to optimize the set-up...always more efficient to do it that way, and it usually always responds better to changes in heigth with the larger diameter prop, or atleast allows more heigth.<br />If it would turn out that the engine truely is in the Bottom holes (top bolts) and the AV plate is still 1" below boat bottom, I'd say we have a 25" engine on a 20" transom.

 

[Mike Robinson]

Re: Reduce Pitch or Raise Outboard?

Sorry Kenny<br /><br />The outboard mounting bolts go through the top holes, I can easily go up. I can go up three holes (3 inches). Any suggestions on how far up I should try, two holes?<br /><br />Maybe I stated the prop size/pitch in the wrong order. I went from 17 to 15 in pitch.<br /><br />Thanks!

 

[walleyehed]

Re: Reduce Pitch or Raise Outboard?

It's best to move one hole at a time and test.<br />No, You said 15 by 17 and 15 by 15..I missed it, but you said it right...There "may" be a tach issue with this...that 150 otta crank the wrinkles out of a 15P on that boat, and we need to be up at 5800-up for RPM.

 

[Mike Robinson]

Re: Reduce Pitch or Raise Outboard?

Is there any way to test the tach? Do I need to take it to a shop? Maybe if I look up and calculate prop slip I would have an idea if the tach is accurate?

 

[Mike Robinson]

Re: Reduce Pitch or Raise Outboard?

Ok, I did the prop slip calculation and I'm not sure I did it right. <br /><br />Here are my results;<br />15" pitch times 5300 rpm = 79,500 inches per minute<br />divide by 12 = 6,625 feet per minute<br />divide by 6076 = 1.09 nautical miles per minute<br />multiply by 60 = 65.4 knots<br />multiply by gear ratio .5384615 = 35.215 knots which is the theoretical speed<br />actual speed is 33 knots (GPS)<br /><br />That leaves a slip of 6.218%<br /><br />Comments?

 

[walleyehed]

Re: Reduce Pitch or Raise Outboard?

I figure a bit over 16% slip... not bad, but could be better.<br />Could likely lower that by going SS...it may drop 2-4% with a quality prop and we have to watch the RPM because an SS prop will "usually" drop the RPM around a hundred in the same pitch, sometimes more.

 

[Mike Robinson]

Re: Reduce Pitch or Raise Outboard?

Thanks Kenny<br /><br />I am going out this afternoon and will see how the boat performs raised one hole. Once I have the height worked out then I'll look at the idea of getting a stainless prop.<br /><br />(Can you believe it, my wife actually wants to fish on Mother's Day!)

 

[Mike Robinson]

Re: Reduce Pitch or Raise Outboard?

No luck fishing and there wasn't much change raising the outboard one hole. WOT was 5300 with a heavier load. Tomorrow I will try raising it one more hole.<br /><br />Thanks for your help Kenny!