need to find recommeded wot RPM for 81 merc 70hp ( 3cyl)(

[pghwrench]

currently the motor is running at about 4200 rpms at wot throttle on 28 ft pontoon. I want to get a new prop as it is showing some wear and tear.and wasnt originally on this boat. thanks in advance for the help as i'm new to the subject of boating.

 

[steelespike]

Re: need to find recommeded wot RPM for 81 merc 70hp ( 3cyl)(

If you read the stickys at the top of the page and get us as much info as possible
we can get you set up.
The motor is rated at 5,500 rpm.We need to know your prop size,near the base of the
blades or under the prop nut, your rpm and speed.

 

[pghwrench]

Re: need to find recommeded wot RPM for 81 merc 70hp ( 3cyl)(

1) Performance issue you are trying to correct. = none really would like a little more speed
2) Current prop manufacturer, model, aluminum or stainless as a minimum. = it is the original prop aluminum will try and get size pitch tommorow.
3) Current prop diameter and pitch (required). again original equip will try to find numbers tommorow.
4) Wide open throttle RPM and speed with an average load (very helpful) = Right now its wot rpm is 4400 with average load
5) Engine/drive make, model, year, and HP = 1981 mercury 70hp, 1987 landau 28 ft pontoon striped down to 4 swivel seats, helm, bow mount trolling motor, fish finder, two batterys, and about 200 lbs tops of gear, removed all stock seating etc. and cut back the hard top to cover a little less than half. I have no idea of actual total weight. but know it sits much higher in water than it orignally did . 16 gallon alluminum fuel tank full speed is a best guestimate at 20mph cant get speedo to work and i hear there not all that acurate anyway. i use it primarily as a fishing vessel just would like to get there a little sooner and sleep a little longer. thanks again
6) Boat make model, year, length and weight

 

[steelespike]

Re: need to find recommeded wot RPM for 81 merc 70hp ( 3cyl)(

70 hp 28 footer isn't likely to win any speed contests but being extra light may help.
Your rpm needs to be near the 5,500 rpm I mentioned both for performance and the health of your motor. Get us the prop size,usually near the base of the blades or under the prop nut.
Perhaps you can borrow a gps or get an app for your phone for the speed.

 

[pghwrench]

Re: need to find recommeded wot RPM for 81 merc 70hp ( 3cyl)(

prop size is 10 and ?/8ths r 14 ! should i go down to on the pitch to get the rpms up to close to 5500? (11) You do go down on pitch, to raise rpms right? Also i noticed here on i boats parts i could go up to a 11 3/5 or 11 1/2 diameter prop would that help. or a 4 blade? Update tried 11 1/2 x 9 prop got 5000 on tach but after downloading app (thank you) i am traveling at 17mph so lets work from there. I have a friend at the marine store (parts) whom is letting me try out any prop he has in stock, as long as i dont damage it . really would like to get most speed from it while hitting that 5500 mark not over.thanks again