200 RPM Rule

[shoundog]

How true is this rule in prop selection. I have a 1,700 rpm difference in what my 50 Johnson 4 stroke is turning at wot then what the book recommends.
18 ft Triton 2 toons
50 hp Johnson 4 stroke, 11 3/5 x 14p alum prop wot 4,500, GPS speed is at 11.5 mph.
Manual says rpm range should be around 5,900 to 6,500 rpm wot and 6,200 at power.
The old prop was beat up pretty bad so I took it off to replace. Well I look all over it for the numbers and there were none to be found so I got out the manual to see what they recommended which was 11 3/8 x 14. So using the present 4,500 and 6,200 at power figure I get a difference of 1,700 rpm. So what should I be running at 14p now and dropping a pitch for ever 200 rpm that puts me at around 6p. Am I over thinking this. I don't have a dealer around here that has a large range of props in stock and I can order one on line for a lot less than what they get. I know I am not happy with the 14p now.
Thanks, Doug

[malone1956]

You probably should include more information with your post. I'm new to pontooning myself, but have 30 years with bass boats. Bass Boaters will spend their last dollar trying to tweak out their prop and worry about the rent later. LOL Anyway, it really sounds like something else is going on. Information should probably include boat model, length and maybe weight. Also, is this motor new to you? Did you re-power this boat, etc? Is it new to you and you're starting from this point? Those answers will tell us all a lot. There are so many reasons a boat motor may be laboring hard.

[rancherlee]

The 200rpm rule is garbage for lower pitch props used on pontoons an 11pitch of the same brand/type will put you at ~5700rpm. 14" / 11" = 1.27 difference or 27% gain in RPM. 4500 x 1.27 = 5700 and probibly more since your going to get into your rated HP range!

[Bamaman]

I looked online at props at the Big O in Fayetteville, NC.

They carry aluminum props as low as an 8 pitch. If the 200 rule would hold true, you'd need a 9 or 10 pitch prop for your boat.

Have you checked around your local dealerships to see what props they're putting on comparable pontoon hull/engine combo's?

[shoundog]

Malone, I bought the pontoon used and it is a Triton Summit 180 SCR-F Gold. It did come with the Johnson 50 on it but like I said the prop was beat up real bad hence the new one. My problem came when I could not get any number off the old one so back to the manual to get a starting point so to speak. Its just my son and I that use it for fishing sometimes the wife will tag along. Not sure of the weight of the boat along with motor.

Rancherlee, Thanks for the information I was just looking at a 11 4/5 x 11p of the same brand prop that I now have. Also a 12 x 10 and a 11 2/5 x 10 four blade. These are all Solas Amita props and in same price range. Which do you think would be the best bang for buck?

Bamaman, I have not ask local dealers around here. They seem to be more interested in selling me a new boat instead of giving information. I guess they make more money at selling . There are a few prop repair shops about 25 miles from me I was thinking of talking to them and see if I can get some information. Thanks Doug

[Bamaman]

I'm fortunate to have Jim's Prop Shop in town to lean on if there are any problems. His expertise is free if you buy from him.

Fortunately I have not had a single bent prop since I switched to stainless steel props in 1980. I used to go through one per year. Thankfully our lake is very deep and wide.

[rancherlee]

I'd go with the 11 pitch 3 blade or 10 pitch 4 blade, both should be pretty close in RPM. If your not aware your engine is Suzuki DF50 with white paint and Johnson stickers, so you can look around for DF50 pontoon combos also to see what they are running.

[MH Hawker]

9, 11 and 13 are very common toon props, the 200 rule dose not really apply to pontoons, its more like 200 to 400.

[ROLAND]

9, 11 and 13 are very common toon props, the 200 rule dose not really apply to pontoons, its more like 200 to 400.

MH Hawker.... NOW YOU TELL ME!!!!! You are soooo right about this... Two years ago I dinged up my prop on my toon so I went back to the dealership... I don't remember the specs on the prop, but the guy in the service dept. sold me a prop that was not the same pitch as my original and told me it would only make a 200 rpm difference... put it on and took it out the next weekend and man was he wrong!!!! @ wot she was turning 5200 rpm's... with the original it was right at 6000 rpm's.... 800 rpm difference....so me, being a boating novice thought "great, turning 800 less rpm's, less rpm's = less stress / strain on the motor". When I posted those results, almost everyone on this forum told me 'WRONG".. said I was over propped and was actually making the motor work harder... So I called Yamaha and talked to a tech there who told me the same thing..... went back to the dealership and returned their prop for a new one with the same specs as my original... back to 6000 rpm's..... another reason this forum is so good... they called this one right from the get go. There's no replacing experience.

[shoundog]

Thanks to all. I am going to try the 11p ordered last night so will get it on Friday. Now to sell the 14p on Craigslist.
Doug

[jimrs]

I think you will find the 200 rule to be for regular plaining hulls not toons. This rule has been around sense before I started boating and that was in 1960. Every hull type has a new set of rules. The 200 was the one remembered by most. Primacy what is learned first is retained longest.

[curtiscapk]

So true JIM!

[HandymanHerb]

I stick to the 300 RPM rule for pontoons, I can't believe I was getting 22 MPH with a 13 x 21 prop that came on the engine, I knew it was to big even though the tach was showing 6000, we finely took the prop off and checked the pitch, it should have been bogging the engine out.

But anyway I went to the prop shop and traded it in on a 13.5 x 15, I need to call and see if he got it fixed and ready to try on the toon, then I'll order spares from iboats for a 110. a piece.

I was looking at their four blade props, they said you need to drop on inch of pitch to use it, but that puts you in the even numbers with only high dollar SS to chose from, you think if they know you need to drop an inch the four blades should come in even numbers

[shoundog]

Ok so I went ahead and ordered the 11p from Iboats. Well It showed up on a Thursday and when I opened the box much to my dismay it was a 10p. It was a toss up as to order the 11 or 10 so I checked my order and sure enough they made the mistake not me. I decided to go ahead and try the 10p instead of sending this one back and waiting for a new one. Guess what, I am very pleased with the 10p boat ran at 5900 rpm and 20 gps. The 10 turned out to be a little bigger than the 11 also which I think helped. I guess a 9 would put me in the 6200 rpm range but not sure I would get anymore speed. I'll stick with the 10 for awhile and if I run across a 9 sometime I might give it a try. Thanks for all the help.
Doug