Wrong prop ?

[eightpointup]

I picked up a Stratos 256V. it is a 1989 16' bass boat with a 50hp Johnson on it. The prop on it is a 11 3/4 x 17. its a little slow coming out of the hole but I can live with that. But after getting on plane I can only trim it until I hit 4600 rpm before it starts porpoising and I have to cut it back ton 4500. This seems low? Should my top end being turning more rpm? What is the rpm range I should be operating at WOT ?

If this is the case where do I start to correct this? wrong prop(11 3/4 x 17)? motor height not right? (currently in second hole from top)

If more info is needed just tell me

Thanks

 

[usskydvr]

Re: Wrong prop ?

Porpoising means trim/ motor height is not set correctly. Before changing props make sure you height is set so the anti-cavitation plate is the same height and parallel to the bottom of the hull. Use a yardstick to check. After checking /correcting that, check your tilt pin settings. You may need to try several to find the correct one. pay attention to how the load in your boat is distributed, ie. too much weight in the rear.

 

[Silvertip]

Re: Wrong prop ?

that engine really likes to run at 5500 and 5800 won't hurt it a bit. You cannot begin to think about props until you get the porpoising taken care of. Distribute some weight up front if possible. Until you can run wide open and not engine rpm, we can't help. What I can tell you is that a 11-3/4 x 17 is a little bit much for that boat. Most 40 and 50 JohnnyRudes run best with the 12-1/4 x 15P. But that all depends on the load, hull type, etc. A couple of boats ago I owned a 16 foot MirroCraft that with a 40 Evinrude that was propped for 5800. That motor never missed a beat and jumped out of the hole with two aboard and only slightly less snappy with three aboard.

 

[eightpointup]

Re: Wrong prop ?

I am not sure I worded things right. I do get to WOT and on plane. I continue to try to trim it up and at 4600 rpm the front of the boat starts bouncing. If I back off the trim slightly it stops. Is it possible that this is as far as I should be trimming it up as I have no trim gauge?

usskydvr "check your tilt pin settings" I have no idea what pins your talking about unless your talking about the pin that the motor rest on when trimmed down. It appears it has recently been moved to the inside most hole but you can see it was used most the time in hole 2. This is also the spot where the anti-cavitation plate and pad on boat seem to sit parallel. I'm guessing PO was trying to get more hole shot.

Weight distribution is not something I can change.

I just got the book from previous owner and it appears 4500 rpm is the bottom of WOT range. It shows the motor is positioned in the original position and original prop spec's.

All that said We know I am not tuning enough rpm's. So how do I go about trying another prop without buying one? I know our local shop has a lot of used ones.

Silvertip I will try your suggestion if possible and stick a 12-1/4 x 15P on it.

 

[usskydvr]

Re: Wrong prop ?

Ok, it sounds like you have found your max trim point. ( when you start porpoising) You are correct in your assumption about the tilt pin setting. I wanted you to make sure it wasn't in the end position, but should normally be in the 2 or third hole. I agree that if was in the first hole the previous owner may have been looking for more hold shot, but that would have caused the bow to ride down. The correct setting is for it to be in the hole that positions the anti-cav plate parallel with the bottom. As far a trying before buying, I hope you fine a good prop shop ( or possibly know someone who you can borrow their prop from)

 

[eightpointup]

Re: Wrong prop ?

usskydvr thanks you. I have called around and no one has a used one. If I bought a new one we know rpm will get closer to our proper range but what negative effects are possible such as loss in hole shot or loss of top end speed?

 

[RicMic]

Re: Wrong prop ?

There won't be any negative effects. The lower pitch will give you a stronger hole shot and since the motor will gain a couple hundred rpm, it will be coming more into its optimum hp range and you might gain a bit of top end.

 

[usskydvr]

Re: Wrong prop ?

RicMic is correct, although you probably won't gain any top end speed.

 

[eightpointup]

Re: Wrong prop ?

I ordered a 12 1/4 x 15 and hope it helps top end. PO said it ran 37 mph and today GPS showed 28.2 mph at 4600 rpm

 

[steelespike]

Re: Wrong prop ?

Did it run 37 with the present prop?If so then you don't need a prop change.
The anti vent plate just above the prop should be visable at speed.Should be at least even with the bottom of the boat.
Most end up about an inch above.In extreme cases this could add 2 to 3 mph.
Won't solve your problem but will improve performance.

 

[eightpointup]

Re: Wrong prop ?

PO said with this prop it ran 37 mph. I am betting he never used a gps

 

[eightpointup]

Re: Wrong prop ?

Ok I bought a new 12 1/4 x 15p ans ran it today I gained 2mph and still good out of the hole. 30.3 mph 5000 rpm @ WOT. It will have to do for now.

I am at the middle of what johnson says my WOT rpm range is 4500-5500. Silvertip you said 5500-5800 is what your setup liked. Is there a better prop for mine still? and if so what would I gain from turning another 500 rpm?

Thanks for the help,
Ron