24' pontoon needs prop advice

[Dusty18655]

I have a 24' pontoon boat that has an 84 Johnson 50hp outboard. The top speed is only 17mph according to my gps. The stampings on my prop are "13x11". Can someone explain this to me and possibly recommend a direction to take to improve my engine's performance until I can afford to repower with a larger displacement engine? Thank you

 

[ahicks]

I'm sure you've found pontoons are really not the hull of choice for speed and performance. You can double your horsepower for instance, and maybe hit 25 mph if you are lucky and have a light boat.. 23-24 top speed not unusual - and you would very likely be using twice as much fuel.

So back to your question. 17mph is actually pretty good for a 50. If the engine is turning max rated rpm at that speed (5500 or so) you're pretty much all the way home.

Here's a .pdf file with a chart (down towards the bottom) that I've found scary close when predicting pontoon speed for conventional 2 tube pontoons.

Note also, that if you were to double your horsepower, there would be a significant weight increase on the back of your boat. If the boat has large tubes (say 24"-25") it won't be a big deal. An older boat though, could have 20" (or so) tubes that would very likely be sitting lower in the water than many would like....

pontoonspeedchart.pdf

 

[Sea Rider]

You can better any OB performance by sitting it right along propping the OB to rev towards max full wot rpm range, will need to install a tach for that venture. Fom there can fine tune prop pitch once you know max wot rpm achieved on flat calm no wind water cond.

Happy Boating

 

[legalfee]

I have a 24' Sylvan with a 90HP Force. I also get around 18 MPH. I'm running almost 6000 RPMs at WOT and the motor is rated 5000-5500. I have a Quicksilver QA2032X which is 14X13. I'm considering going to a 17 pitch to lower my RPMs. My question is how many MPH would I pick up by going to a 17 pitch?

 

[Sea Rider]

It's not known how much wot rpm the OB will lose going in excess of 4 pitches up, only checkable on a wot rpm water test run on flat water. The rule of thumb that states -200 wot revs per one + pitch does not always applies, depends on many factors as hull drag, trim, OB-HP.

Personally would go per only 2 more pitches for a 14 x 15 prop. With a 14 x 17 prop will probably lose to much wot rpm range towards the lugging side of the OB. With a 15 prop wot rpm will drop accordingly while gaining slight more top end speed. Bear in mind that top hole shot will suffer a bit too.

Happy Boating

 

[ahicks]

I have a 24' Sylvan with a 90HP Force. I also get around 18 MPH. I'm running almost 6000 RPMs at WOT and the motor is rated 5000-5500. I have a Quicksilver QA2032X which is 14X13. I'm considering going to a 17 pitch to lower my RPMs. My question is how many MPH would I pick up by going to a 17 pitch?

Agreed, 2" of pitch, to the 15 would be my first try. Just realize it's not likely you're going to see much of an increase in speed. These 'toons play by their own set of rules.

I will say that I have a Honda 90 on my 24' toon w/24" tubes, and with the 13.5x13 it turns about 6K (top of suggested max) at 25mph.

 

[legalfee]

Thanks for the suggestions. I'll try a 15 pitch.

 

[Dusty18655]

Ok, Thanks for the chart, it does seem like my boat is running about right for speed. To get back to my other question, can someone explain what the 13x11 stamping means? I'll be the first to admit that I am dumb as far as understanding props.

 

[Dusty18655]

In addition, engine is turning at about 5000 rpm which is WOT, I've adjusted the trim and got the prop to bite the water better. Just wondering if it would be a wise decision to try a different pitch prop, and how I can tell from the numbers on my prop what the pitch is? It is an aluminum prop.

 

[ahicks]

Dusty-
We all start out pretty dumb. I hate it when I don't know enough about something to even feel like I'm qualified to ask the right question!

13x11 translates to 13" diameter and 11" of pitch.

If your engine is at 5000 rpm when wide open with this prop on it, you're pretty close to having the best prop. The next standard size prop down from the 11" prop is a 9". If you were to try that prop, you will likely find your engine is over revving and your boat is going slower.

Here's another chart for you. When you look up your engine, it shows the suggested rpm range is 4500-5500. At your current 5000, you are smack in the middle of the range.

If you wanted to mess with it, to get right at max suggested rpm to get max performance available from the engine, you would look for a prop that turns right at 5500 with a lightly loaded boat. The 9" might allow that, but it could put you over slightly just as easily - which would be a step backwards.

I'd stay put with what you have, especially with a pontoon boat. If this were a ski or performance boat, I'd be hunting around for a good prop repair shop that could customize a prop to do exactly what I wanted. Something that was effectively between the 9" which is likely a little too small, and the 11" which is just a hair big if looking for max performance.

 

[ahicks]

Forgot to add the chart. Here it is:

 

[Maclin]

I would consider your engine to be close to an over-achiever...:clap2:

 

[Sea Rider]

Was toon lightly, medium or heavily loaded when OB achieved 5 K wot revs ?

The best way to dial a perfect prop pitch for your application is boating with a fixed load which is my case and propping OB right to run middle or max wot rpm range, You decide if wanting slight better top end speed or slight better hole shot.

With a toon would preffer max wot rpm range, to be on its way faster than if OB revs towards middle rpm range.

Happy Boating

 

[Dusty18655]

sorry guys, was offline for a few days. The toon was about a medium load, 22 gals of fuel, 4 adults, and a cooler of dinner. I'd like to get it fast enough to pull my three kids on an inflatable, so I'm guessing from what y'all have said, my best course of action would be to step up to about a 75-90 hp engine. Thank for your advice and knowledge! I'm a little hesitant to run the engine right against the max rpm, I'm happy staying at about 50,000-51,000. Guess the engine upgrade will wait till next spring.

 

[Sea Rider]

Nothing wrong running any engine on any boat at max wot rpm range. Were designed to run that way. In your particular case, the only down issue is the variable load as previously loaded + considering adding 3 kids to ride on an inflatable boat which achieves excessive water drag when pulled. A single prop won't handle well both boating scenarios.

Prop right your engine to rum at max wot rpm range as previously loaded, remove extra boaters and pull 3 kids on the inflatable, a single maxed prop could take care of both scenaios but definitely not at same time, Untill not water tested it's pure theoretical speculation.

Happy Boating