Prop Question for the pros

[Scott Savoy]

I finally recieved my new rig last week!! :lol: and broke the prop on day 3 :angry: We were turning about 2500 rpm on a semi choppy lake, hit something and chipped the prop...up on the trailor it went. Talking to the guy at the prop shop, he says I was rigged with the wrong prop pitch. I was still in the break in period, so I dont have the WOT info. I'm probably ready to go wide open now, but I need a prop first. Several people have told me that I should have atleast a 17 pitch... 19 pitch possibly.



Please set me straight.



115 Optimax Mdl 1115D83HY S/N 1B913074



Prop from factory 77342A45 15P



Boat Tracker Targa 18 WT



Thanks

 

[Bob Bourget2]

The boat should have come with a 13 7/8" diameter x 14" pitch alum from the factory. Don't let the "other" guys tell you what's right and wrong.

 

[Scott Savoy]

Thats why I came to the pros. Why are they telling me that I will be over straining the motor by staying with the smaller pitch? Would I see more performance if I went to a bigger pitch?....and would that hurt anything?

 

[Bob Bourget2]

Actually, it has the opposite effect. More pitch is like adding another gear to your car...reduced rpm, less torque {more strain} is a recipe for pre-ignition / detonation. There was talk some time ago about doing away with the Makers "range" that they give you, and the target should be the top rpm number. You want to prop your boat to allow it to turn as close as possible to the high-end of the scale. With the junk fuels we have now, the motors need all the help they can get. Let it spin. Outboards in general will not be harmed from excessive rpm....but....lug them too much and their lifespan could be drastically reduced.

 

[tightline 700lx]

That makes a lot of sence. I use to work in a warehouse and I would not get as tired trasfering a pallet of 40# bags as I would trasfering the same weight with 80# bags. Double the "RPM" but it was easier...



As always...



Thanks Bob B

 

[Scott Savoy]

BOB, The guy at the BPS that "delivered" my boat to me said that I should never get it over 5600 RPM. Will 5600rpm red line the engine? He also told me for "break in", to do the throttling between 5000-5100 rpm.....according to the manual, it is supposed to be done in the 4000 rpm range.

 

[Bob Bourget2]

The "redline" for that motor is 5750. You can safely prop right up to that number with NO issues. A few of my Customers with this series engines {propped correctly}on the best day of the year, cool, water temps low, might get an errant "beep" from the over-rev protection. Break-in according to the operators manual please. In the event of a failure, we can tell exactly how it was broken in {or broken}. You don't want someone to tell you that "you did it wrong".

 

[Scott Savoy]

Got my new prop put on today. We tried a 17p but it ran 39mph at only 5200rpm wide open. Droped down to the 15p, and topped out at 36mph at 5750rpm<---real close to wide open. If i pushed it anymore, the over rev protection alam sounded. Of course I backed it down immeditaly!!!! It put the boat back in the same grooves as before with the factory prop. My hole shot did improve due to the ports on the new stainless prop.I plan on cruising mostly between 4000 rpm and 5000rpm (to conserve gas). Does all this sound right to you Bob? If not let me know! Thanks again for all your help!!!

 

[Bob Bourget2]

With the 15"...are you getting the WOT rpm from a "mid-trim" position or a "high" position. Rememeber to ONLY trim until speed does not increase any longer. Excessive rpm from more trim is just increasing slip, due to ventilation. You need a long stretch to test. The 15" is not the same prop as you listed before? {77342A45?}

 

[Scott Savoy]

I was getting the WOT rpm at a "mid trim" position.--> I could get the boat on plane and trimmed properly at 5500rpm then increasing the throttle.



No not the same p/n

The new prop is a PowerTech.

 

[Bob Bourget2]

Ok that makes more sense. All props are not created equal. Whenever pitch is discussed, it is with the same prop maker. All of Mercs props are "pitch" compatible. In other words, you could change from a tempest to a trophy with similar wot rpm readings. Your power tech is prob not quite a 15" pitch....