Prop pitch vs. RPM vs. MPH

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J-Dub

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Apr 7, 2006
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Ok, so I've gotten my boat recently repaired and I'm pretty fired up about getting it back but now I'm wondering if there's yet another problem with the dang thing. I took it out on Melton Hill for a bit last night to fish and let my kids do some swimming. Fishing was slow as expected and the kids had a ball swimming. Anyway, I finally got the motor broken in and can finally open her up to see what she can do. Not nearly as well as I'd thought and I'm thinking I need a different prop? Here's the breakdown- I'm not sure what a motor should turn rpm wise but I have a '95 Johnson 150 on a '95 Javelin 389. The tach says it was pulling 6300 rpms which seemed high to me and to add to the confusion the gps marked me only going 45 mph which was way slower than I had figured I would top out at. My old boat that was a 1977 Champion with a 120 johnson went every bit that speed with about the same amount of gear, myself and two kids so now I'm scratching my head. emoScratch I don't know the pitch of the prop as I bought it used (long story) but it's a raker prop. Any suggestions? I really appreciate it.
 
J-dub,I`m presuming the 150 is a V-6....6300 would be max rpm and you probably don`t want to run it at that very much. What is the size of the boat ..I`m not familiar with model numbers. Most V-6 should run between 5300-5800 ideally but with the proper water pressure and prop maybe a couple hundred more. You might want to check the tach for accuracy too... You should be able to find the prop numbers under the washer on the hub of the prop somewhere. I would think and 18-19 footer would run 23-24-25 pitch with that engine. # or 4 Blade...my memory says rakers are usually 3.
 
The boat's 18'-9" and it's a 4 bladed prop. It comes out of the hole great but it just feels like the top end is really lacking. Now the guy I got the boat from claimed that it ran close to 60 mph for him so if I'm only getting 45 mph something's not quite right in that equation. I had it around 55 before I had motor problems so I'm really confused on what it could be.
 
I have 1996 389 TE, it has 175 johnson on it, it also has 4 blade 23 pitch ss prop, it will run 60 mph loaded down. rpm at 5900. What kind of work did you have done on the motor? your prop may need a new hub kit, maybe slipping. Your rpms are to high if they are correct. I don't think your boat will do 60 mph with the 150 hp, maybe 52 to 56. Just my .02.
 
I thought the claim of 60 was too high but figured mid 50's would be more like it. I had to get two cylinders bored out and fixed as they went bad after I'd had it out just a couple of times. This boat just doesn't seem to like me and I'm seriously entertaining ideas of selling it, sitting on the $ and trying to save some more and get another one sometime next year. I just hate to dump the boat after putting so much $$ into the dang thing but each time I take it out it just doesn't seem like the boat for me.
 
Ive got a 24 in raker you could try thats for sell. Its an OMC prop with 3 blades. I think it should put you in the right rpm range.
 
motor may be set up to high above the pad, which will raise rpms and slow you down. 5600 rpms is about all you want out of it, and it should do at least 50. What is the boat rated for?
 
Don`t get too excited yet J-Dub....If it`s truly turning 6300....it`s probably just a set- up problem. Did they pull the engine off the boat or just the Block. or otherwise tinker with the engine mounting. Look at the prop fit up to the lower unit. Some motors require a particular , usually plastic ,adapter to fit between the lower unit and th prop hub. If it will turn that prop when properly set up it should get around 55 ..just a guessin` !
 
With the correct prop and and one person on board the boat should run in the mid to high 60's.  Probably need to check the setup.  FA
 
Evidently I was so bored at work that I made stuff up- the prop is only 3 bladed and when I looked it's 24. A guy at work told me to put the motor in gear and see if I could turn the prop by hand, which I found out I can. It's not real easy to turn but when it does I hear what sounds like some sort of rubber squeaking. Is that a loose fitting or something like that?
 
Probably going to have to take the boat back to the shop that fixed it for me as I've been doin' some pondering about it all. It ran 55 and was in the right rpm range before the motor went out. I haven't changed the prop or done anything else to make it change so it has to be something they did. In a club tourney I fished out of my boat on Saturday and my partner and I both agreed there's no way the motor's turning that high of rpm that the tach is saying so something's still not set up right so something changed in the shop. The motor sounds like it's just on cruise not screaming from turning rpms. I don't know if there's something that can be adjusted internally but it's like the throttle's not opening all the way up. Boat issues sure are frustrating especially when I only get to get on the water about once a month.
 
<p align="justify">I have a 2006 200 Pro Xl Stratos. 225 Yamaha. 25 Yamaha prop that has been cupped. (twice) I can only get 57-58 hundred RPMs and 71-72 MPH. I raised it this weekend a 1/4 inch and saw nothing change on my set up except for a much bigger rooster tail. </p><p align="justify">Thjere are boats identical to mine (with Yamahas) running on average 74-76 MPH. Do I need to try a differant brand prop or move into a 26? Still, why dont I see and feel a change in adjusting the jack plate. I would think a 1/4 would do allot. </p>
 
Billy,

your boat turned a lot more than 5800 when I drove it with you. a 26 Tempest would be perfect for your boat. You are welcome to try mine. I just need to get it off of my boat. The T1 you are running is actually a good prop.

BS
 
<p align="left">I should have mentioned that too Lance. </p><p align="left">The day we went, the boat did turn high but recently I found that if I tap the gauge, it settles back down. Still only seeing 71-72 and one heck of a rooster tail now. Something just isnt right. Didnt we try a 26? </p>
 
J-dub, the tach could be bad or failed. They can do that. I would have them check the tach to make sure it is right. Then I have the whole motor checked out. If the tach is right then you have set up problems. If you want to check it out there is a formula that you can use that will get you in the ball park for figuring out what you should be running.</p>

 RPM / RATIO X PITCH X 60 / 12 / 5280</p>

Take the known rpm and divide it by the ratio of the foot. Multiply this number by the pitch of the prop. Multiply that by 60. Then divide that by 12 and then divide that by 5280. That will give you most that the set up could possibly do. Then you can subtract around 10 percent from that and it will give you close to what you will be doing. The 10 percent is an estimate of slip from the prop.</p>

 I usually start with the prop set up a couple of inches below the bottom of the pad. Then run the boat. Write down the results. Then raise the plate 1/4" at a time while watching the water psi and the speed. Until the water psi starts to drop (try to keep it around 15 psi. at wide open ).  You can then tweek the set up as needed to get the right setting. When you get to the magic place when you raise the motor more it will do as BBass said, it will give a bigger rooster tail with out better results. For best performance the rooster tail should be not much higher than the top of the cowl.</p>
 

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