1983 35hp Johnson prop lost rpms's and speed

[Al-53]

I have a 1983 Sea Nymph bass boat its same stlye as a tracker semi v hull flat bottom...and a 1983 Johnson 35hp on that runs like a champ

Ok here is the problem The motor had a 11x9 prop on it and it gave me 22 mph at 5200 rpm's WOT...I got a SST prop at a good price its a 10x13 I installed it and took for a run..well it seemed so doggy WOT was only 4200 rpm's and speed was 16.8 I lost rpm and speed...

I have read in here some are running a 10x15 on same engine with gained speed but a slight drop in RPM I lost both speed and rpm...does sound right...I was looking to gain just a little more speed is all for fishing the bigger lakes wanted to get there a little faster but wont happen with this prop...

Putting the old one back till I see what I did wrong...

The boat loaded i would say weights close to 1000 pounds 2 batteries gear 2 people and fuel...

Any help would be helpful and if U think another prop will do do it let me know


Al

 

[steelespike]

Re: 1983 35hp Johnson prop lost rpms's and speed

Man I just spent 40 minutes writing a post and lost it.arrgh!
You should read the stickies at the top of the page and the helps on the prop store page.
Basically you did everything wrong.lol Went the wrong way in pitch. You just tried to make your motor do it all in overdrive.Even if you did the ss prop perfectly,
about the same pitch, your results could have been a few tenths to a few mph.
Forget that prop.
Try raising the motor to a point just before venting(cavitation) becomes a problem
If you have to raise the whole motor just add sticks on top of the transom to support the motor till you tighten the clamps.under ideal conditions you could gain a couple of mph.Unfortunately it may not change much either.Sometimes a jack plate lets you get more height and setback.Setback gets the motor in cleaner water (less turbulance)Your asking a lot of your 35 and I think your present prop is close to right.Your motor needs to run near the top of its rpm range 5,500?
If batteries and gas are at the stern try moving them forward to about midway.
The boat will handle differently with weight midway than if it is at the extreme ends.
Testing will determine what is best for your setup.
Try to eliminate weight;use a light weight danforth style anchor,plastic oars/paddles.try to get the boat ballanced fore and aft so the motor doesn't have to force it to plane right.eliminate non essential gear,carefully adjust motor trim.
Be sure the motor is in excellent tune,throttle plates opening all the way.Prop in excellent condition.Boat bottom clean.If you are presently poorly setup you might gain 4 or 5 mph.Let us know your results we may be able to suggest a prop change.
For instance a 4 blade will allow maximum height with better hole shot and less power required to stay on plane.Resists venting.

 

[Al-53]

Re: 1983 35hp Johnson prop lost rpms's and speed

Thank you for getting back so fast....

Ok the motor is on a jack plate with trim CMC 130 plate...the cavation plate on the motor is level with the keel with the 11x9 prop it planes out perfect with just using trim...I put the 11x9 back on went to the lake and back to running great again planes out fine and 5300 rpm's and 22mph....so I guess thats the prop for my conditions...just don't understand how others are using a 10x15 or 10x13 and getting more speed hmmm....and planing out ..I just don't understand the size and pitch ratio's I guess...I read and read and still confused lol....maybe my boat is to heavy for any of the other props dont know...

now would a 10x10 or 10x9 make any difference just asking since having a hard time understand the inches in ratio to pitch is all

Al

 

[steelespike]

Re: 1983 35hp Johnson prop lost rpms's and speed

If you have a jack plate usually the motor can run higher than on a transom.
If your antivent plate measures about even with the bottom you probably can raise it 2 or 3 inches to a point just before venting becomes a problem.If your running with it even it could be actually worse than being even with it on the transom.
Boat motors are working like they are going uphill all the time.If you add weight
obviously its harder for the motor.Selecting a prop is like picking a single gear in a car to do everything. Lets call your 9" pitch 2nd gear its easier for your motor to push up that "hill" so it can get to its max rpm easier.Just as with a car it is slower with the 9 as a car would be in 2nd gear.if you go to say a 15" your pretty much in high gear asking your motor to get up the"hill" so the motor ends up
lugging outside of its rpm range. On a boat that weighs say 300 lbs, less you and your gear, you could probably easily run a 15" prop.and go 30 to 35.because the motor can get up to its rated rpm and the prop goes farther with each revolution. Take that same 15" prop same boat and try to ski with it and it may be sluggish drop to say a 13" like 3rd gear skiing is easier but max speed will be slightly reduced.But the key is the motor will be able to run within its rpm range. The faster and more responsive the boat is the less fuel you will use. Could you measure the difference? Probably not without a flow meter.
If you make your destination 3 minutes faster obviously you will use less fuel
if you can make your old speed at less throttle it saves fuel.blah blah,blah.
Happy boatering