New to me pontoon - prop not right???

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
18' Bass Buggy, 70 hp Johnson 1975. Today I took out the pontoon with the new prop 13.5X15 after changing the original prop 13X19. The original prop was pushing 4000 rpms WOT. According to mph gauge around 14 mph. After reading all the info on props, I thought I needed to bring down the pitch to get the rpms up. I tried the 13.5X15 with high expectations. I could only pull a little over 3000 rpm WOT with the new prop. I thought I would increase the rpm by 800 approximately to get closer to the rated 5000 operating rpm as stamped on the engine ID. I am thinking of trying to lower the motor down one notch. The motor bracket sits one notch above the top of the transom. Lowering it would put it resting on the transom top. Any ideas??? This is the first pontoon I have owned and today was the third time I have taken it out. Previous owner put the 70 hp Johnson on it to sell with the boat, he never used it with the Johnson on it. By the way, great info on the forum.
 

Attachments

  • 18-ft-bass-buggy-pontoon-3500-3 (1).jpg
    18-ft-bass-buggy-pontoon-3500-3 (1).jpg
    49.3 KB · Views: 0
Last edited:

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
more than likely the 19 wasn't getting any sort of a grip and the 15 is and a 11 or a 9 is more like what you need
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
You may need to raise the motor vs lowering
 

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
7,985
First thing you need to do is have the motor looked at has it run good at all for you. If we do the math and your 15 is at 3000 rpm to gain the 2000 RPM you need. If we take 1" pitch=200 rpm as a rule of thumb you need do drop 10" of pitch to get you there never seen a 5"pitch prop for that motor. Since nobody ran the boat with the motor that you bought it with, it's condition is suspect as it sounds like you may be on 2 cylinders instead of 3. I think you need to do a cylinder spark test and a compression test before you buy more props
 

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
I just got the motor tuned by the mechanic. It fires right up and runs smooth. I am going to raise the motor so the cavitation plate is just below the transom to see if it makes a difference. I have read that two tube pontoons should sit at the lowest possible setting and that tri toons motor plate should be set just below transom. I will try both to see if it makes any difference. I still think that going from a 19 to 15 should have been a little closer to being right for an older 70 hp Johnson, so I am not sure if tach is accurate. Will do some more trial and error.
 
Last edited:

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
The anti cav plate should be just below the water in both cases
 

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
Ok. I know when cruising the upper plate is just below water and the cavitation plate just over the prop needs to be there. I will go up a notch and see if any difference.
 
Last edited:

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
If rpm went from 4000 with a 19P to 3000 with a 15P (even factoring the 1/4 inch diameter difference and the difference in manufacturer) something is wrong and it is likely more than just the prop. Both the 15 and the 19 are wrong for a pontoon, but going from 19 to 15 should have brought the rpm up, not down. Before you continue to blow money on props, make sure the engine is healthy (compression and leakdown tests). Tune ups do not make a mechanically unhealthy engine healthy. Verify the tach accuracy with a shop tach that is known accurate. Verify speeds with GPS, not the terribly inaccurate dash speedometer.
 

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
I think I found the problem. As suggested by flying scott, I started with the ignition. I checked the coils with an inline spark tester and found that the middle coil was not firing. The motor still cranked right up and idles fine on two cylinders. The motor is on a hose so not able to rev it up. I took off the coil and tested it with a meter and confirmed its bad. I ordered a new one and will do an update when its on.
 

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
I was a little concerned when I put on the new coil because I was getting spark on all three but also a little hesitation when idling. It was running off the hose. I took it to the lake this morning and it fired right up. I turned on the GPS speedometer and was going 18 mph at 3800 wot. I know its a little low but the water was choppy and it was windy. It did not miss while under load. I went fishing and had it on and off several times and it started with no problem. I did notice a very faint backfire, I think, when I started it before I took it in. I checked the volts while cruising and was reading between 12 and 13 volts. So far so good. I still want to get the low cruise speed a little smoother. I added some liquid carb cleaner to the gas mix. After watching the motor while cruising, I can see the water about 3 inches above the anticavitation plate, so I will raise the motor up a notch. So far so good. Thanks
 
Last edited:

pearlvette

Cadet
Joined
Jun 18, 2010
Messages
7
No Title

I got the new prop 14x11 and was able to try it out today. The first thing is that there is only a few millimeters clearance from the prop tip and the cavitation plate. I was a little concerned but I did order the correct prop. Its just that it is too close for comfort. I noticed a little bit more vibration during low speed when loading and unloading. It did not seem that smooth. As I got out on the lake the rpm's at wot were about 5000 before I pulled back a little. The mph were around 17-18 wot. It seemed to be smoothest around 4500 at about 12 mph. This prop seems to have a lot more turbulence especially at lower speeds. I kinda like the 15 pitch better because it seems smoother on acceleration and at low rpms, but just cant get the rpms up enough at wot. I am very new to pontoons and have never been on one to compare, it just might take some getting used to but it seems that I am close to getting it close to being right. I have one slot I can lower the motor down on the transom. I think I will do this to see if the turbulence gets better. Anyone think the prop clearance is the cause of the turbulence?? Anyone else have a similar setup? Here is a pic of the wake at 4500. It seems ok?? The second picture shows the cavitation plate just below the water so lowering one slot should be ok.
 

Attachments

  • photo248967.JPG
    photo248967.JPG
    226.4 KB · Views: 0
  • photo248970.JPG
    photo248970.JPG
    176.7 KB · Views: 0
Last edited:

BatDaddy1887

Chief Petty Officer
Joined
Jan 18, 2009
Messages
463
I had similar issues with mine as I have a 115 Johnson that came with the typical prop, a 3 blade 13.25x17p. The performance and speed were not up to par on the 24' pontoon boat, I was getting 4900rpm when the engine calls for 5500. I bought a 4 blade 14 x 14p and life is quite good now. RPMs are up to 5700 (i throttle the engine speed to the recommended setting of 5500) and it lifts the boat up out of the water a bit more than before as well as getting more speed than I was getting. I too noticed one centimeter of clearance, and was a bit concerned, but it has not hit the motor or had any adverse reactions. Too bad you've already purchased a new prop, as I think the 4 blade 14x14 would work out for you too. Have fun out there and welcome to the forum.
 

Silver Eagle

Master Chief Petty Officer
Joined
Mar 16, 2010
Messages
852
Raise your trim. Your pushing water.Your forcing the front of your boat down.
 
Top