pontoon with low rpms

[fireman57]

Have a 1979 24ft pontoon that I can't get the rpms I should. The engine is in tip top shape and runs great. I have a 13 pitch prop on it now. Had a 9 pitch on it but it shook every bolt loose on the engine so went back to the 13. I actually lost speed when I went from a 17 to the 13 but have better performance. When there are more than 4 on the boat though it really bogs down. The setup on it is correct but still seems to be lower in the water than I would like. I put a new transom on last year but kept the same height. I would like to bring the front end up but am also at the end of my trim. I only turn around 2600 rpms and can get nowhere close to the 5500 I would like. Any suggestions?

 

[jbcurt00]

What engine is in tip top shape?

 

[fireman57]

oops, sorry. 1976 75hp Johnson.

I started with a 17 pitch, went to a 13 and slowed down, went to a 9 pitch and had better performance but shook the engine loose. Prop shop could find nothing wrong with it.

rpms increased slightly from the 17 to the 13. Thought they would go up at least 600rpms but only went up 350.

 

[jbcurt00]

13 pitch what diameter prop? Down from a 17 pitch what diameter prop?

And dropping the 4inches in pitch to a 13, then getting half the RPMs doesnt make sense. How are you reading RPM?

 

[fireman57]

12.5 x 13 12.1 x 9 12 x 17 Solas 3 blades

 

[steelespike]

I think a 13 is about as big as I would try. What was the rpm and gps speed with a 9" If it was vibrating something is wrong If not the prop something in the motor.
The motor must make about 5500 rpm at wot.
If the boat is low in the water check for water in the tubes.
It has been my experience that a pontoon doesn't respond much to trim except to combat ventilation.

 

[fireman57]

The rpms with the 17 were around 2400 but would eventually get to 18mph with GPS. the 13 would only turn around 2600 and top out around 15mph. The 9 when it was on turned only 2800 and 17mph. I liked the response of the 9 pitch. This is all with 4 people on the boat. I really need to add a 3rd toon to elevate the front but that would slow me down even more. The fuel consumption is huge. I wouldn't mind the fuel if I could get some rpms up.

 

[steelespike]

After some calculations I think your tach is wrong. If we assume a 2.23 gear ratio a 9" prop at 17 mph 20% slip we get about 5600 rpm.
You need to verify the tach. There should be a switch on the back note the position,operate it a few times to clean the contacts.
It may be set wrong.
Perhaps you could borrow a shop tach or buy an induction tach.
Your 75 will use fuel at over 9 gallons per hour at wot.
Throttling back 1 or 2 mph may surprise you how much you can save.
Do you know your gear ratio? What is the diameter of the tubes?

 

[Sea Rider]

All recreational boats should ride near or parallel to water level when on plane or at fast displacement speeds if impossible to plane, for that, boat must have weight evenly distributed on deck and engine trimmed to ride perpendicular (90?) to water level. If trim is set to other position prop will lose best angle of attack and forward thrust. Engine height is other tech issue that must be well dialed along pitch for OB to take all the horses out of that OB. Is transom height modifiable in case you need to do so ?

Happy Boating

 

[steelespike]

If it's a 79 the tubes are probably undersized compared to todays pontoons.Not likely to plane very well and adding weight puts the frames closer to the water;
more drag.At wot It will be like flushing a toilet full of gas but at 6 to 8 mph it could get better mpg than the same motor on a planing hull at the same speed.

 

[fireman57]

The tack is correct and I can hear the engine lugging. I think steelspike's last post is correct. I use about 8 gallons an hour at 75% throttle and 15mph with 4 people on there. My transom is not adjustable and I also believe there is too much engine in the water. I might redesign and build another transom as I have the capabilities in my shop. The problem is where we meet people is about 6 miles upriver and it takes a poopload of fuel and time to get there. I only carry 24 gallons with me and if we make any fishing runs I have to really watch the fuel so I can get back. It is an older toon and quite heavy but I should be able to get more rpms out of this engine. I had a 50hp on it when I got it and it actually ran faster but was so loud you couldn't hear anything. I know it makes no sense to me either and I have been around boats for about 50 years.

 

[Sea Rider]

In order to correctly modify transom height to dial best propulsion, must know exactly where's water flow passing by at lower leg at fast displacement speed, the fastest way is performing a visual check pulling head out transom while combo is under way. Will indicate if in need to raise or lower OB a bit to perform better along maximized pitch.

Happy Boating

 

[jbcurt00]

I dont think transom height or depth of leg below ideal effects rpms.

Might post the prop you tested and its mph and rpms at WOT. If the conditions and boat load werent the same every time, results will be skewed.

As I understood your original question, prop swapping lugged the motor quite a bit and rpms were very low.

 

[steelespike]

The prop calculator says you can't go 17 mph with a 9" prop at 2800 rpm.
Do you know your gear ratio? I believe it is 2.23 You can pull the plugs and turn the motor by hand
to figure the ratio. Motor turns a little over 2 times to the props one.
If the motor can't reach close to max rpm with a 9" prop you have a serious problem either in design or motor health.
Most motors have holes in the mount so you can raise the motor. It doesn't need to rest on the top of the transom.

 

[Sea Rider]

If a pontoon boat is already one that has more water drag compared to traditional recreational hulls, imagine one with badly distributed deck weight, OB badly trimmed & height seated, you're increasing pontoon drag more than ideal consequently wot revs will lessen in proportion, worst if OB is underpowered, over pitched or with hub issues.

Is that OB the max HP that toon will take ? If not, there's a max number of passengers that toon will "transport" well. If with 3 performs well, not with 4, will need to get did of the fourth one, unless combo is state of the art tunned to perform better, probably not top though.

Happy Boating

 

[fireman57]

There are two batteries and 24 gallons of fuel that sit on the back of the toons behind me. It is an older model, 1979, with old logs. I do have to drain about 5 gallons of water after 20 hours or so on the water. I'm sure there are baffles in it and they might be waterlogged but am not ripping them apart to find out. I will be using air compressor this spring to find and seal leaks. I'm sure that I am underpowered when there are more than 4 on the boat but I would still like the rpms up when underway. I have visually looked at it underway with 4 or more people on it and the water comes up to around 4 inches below the engine so I know it needs to come up some. I will experiment and see what I can figure out.

 

[Sea Rider]

Place 2 boaters up front to have a well ballanced hull, on flat calm no wind water cond, while on plane or fast displacement speed, pull head out transom, if its a center console have somebody else, check water flow pasing under lower leg, ideal is for flow to skim right under small upper water deflector plate, check to seee how close passes by.

Wll need to rise or lower engine a bit to achieve that condition. OB must be set to a trim position to ride perpendicular to water level along AV plate ridding parallel. Will achieve best prop grip, blade angle of attack and top thrust. Get rid of water inside tubes for the test if any.

Once dialed you can go straight for a prop maximization to get all the horses out of that OB and probably fourth passenger not be left stranded on peer. LOL. Perform that simple test and report back your findings.

Happy Boating

 

[steelespike]

"Polson Enterprises Prop calculator", 17" prop,2.23 ratio,25% slip,18 mph = about 3300 rpm, 13",2.23 ratio,25%,15mph = 3400
9", 2.23, 20%,17= 5500 rpm. Note that the 17 and 13 badly oversized props produce about 3400 rpm And the motor certainly is lugging.
The 9" prop is 8" smaller than the 17 and 4" smaller than the 13 yet is faster than the 13 and only 1 mph slower than the 17.
And produces about 5500 rpm. While 17 mph is a little on the slow side todays 24 footer with a 75 would probably run about 20 -24 mph.
Note that the 150 - 200 rpm per inch rule of thumb closely fits the rpm change with the 17" to 9" change.

 

[jakedaawg]

When proping this boat tests should be done with an empty boat, not four people in it. I apologize if I read the posts wrong but wanted to mention that. I have an old 24' toon with a 70 on it and it spins an 11x13 with 19"Dia toons.

 

[steelespike]

I like the tests to be light load, but he had already worked with 4 aboard. My inputs Prop size is accurate,gps speed should be accurate.
For my calculations. Gear ratio is a guess 2.23 , slip is a guess at 20%.Prop calculation is only as accurate as the inputs. So his actual; rpm could vary quite a bit. Thanks for your numbers. I figure his tubes are about the same as yours. Do you have a gps speed and rpm to go with your numbers? What year and brand motor?