Getting new prop have question

[logain]

I have a 16ft alumuacraft boat that I recently purchased. I have been unable to find the exact model as it was not on the title so I am unsure of weight of the boat itself. With 2 people, 2 batteries, 6 gallon tank and fishing gear and such add about 500lbs or so. Hopefully some of you smart guys will recognize it so I will post a picture of it. I switched my dad his long shaft Johnson 9.9 with my Mercury 9.8 since my Merc's shaft was a short shaft and not suitable for my boat and his long shaft motor fortunately was also not really ideal for his old 14ft sears semi v. Also easier for him to carry around the merc as it is much lighter. I mostly fish local 10hp limit lakes and until I can afford a different motor, I have to settle for the johnson 9.9 I have now. It is from, what I could tell, roughly a 1989 model. It has a 9.25 x 8, 3 blade prop that has shown a good deal of wear. My question before I buy, is if there is possibly a more suitable prop for my boat? I would not mind a little more top speed or fuel economy. Was thinking maybe a 4 blade prop would be ideal but not really sure.

 

[Borchik]

Re: Getting new prop have question

In a perfect world the 3 blade prop is the most efficient. I would see if they have a standard prop for that motor and start from there. Obviously you dont want to over-rev the motor and you dont want it running to slow either.

 

[steelespike]

Re: Getting new prop have question

Welcome,if you read the stickies at the top of the page and the helps on the prop store page it will help you better understand prop selection.
I couldn't follow your post;are you presently running the long shaft Merc on a 20" Long shaft transom?Is it 2 or 4 stroke? Regardless the best way to pick a prop is to get a tach(tinytach has a web site is about $47 and easy to install). and buy or borrow a gps for speed. Make some lightly load test runs and give us your results reminding us of the motor,and prop size.

 

[logain]

Re: Getting new prop have question

My Merc was a short shaft and is now with my dad. I am running a long shaft 2 stroke Johnson now. There seems to be a pretty wide variety of props for that motor but the most common, according to an info site I found, is a 9.5 x 10 which kinda had me confused as to why there was a 9.25x8 on it. My dad bought the motor off someone else that I think used it for a kicker. Kinda tight on money right now and that is why I did not want to buy a prop and it possibly not be a very good fit for my boat. I cannot seem to find the site now but there was one that said if I went to a 4 blade to go up 1 size in pitch over what you would use for a 3 blade.

 

[steelespike]

Re: Getting new prop have question

Is the boat fylly fitted out with a floor,live well and such?Or perhaps just bare bones?
Typically when going to a 4 blade you would go down an inch in pitch.However that would depend on the performance with the present prop.On a small motor a 1" pitch change is a lot.The 8" pitch would be for a kicker motor or a heavy application.You need to prop to the rated rpm range. Prior to 89 max is 6,000,89 on 6,500.I would run the 8" and get us the rpm and speed reminding us of the prop size.You can briefly exceed rpm long enough to establish an rpm.Rpm and speed is important to tell if the setup is working.

 

[logain]

Re: Getting new prop have question

Yes the boat is fully fitted with floor, front and rear deck and small live well. Sadly I cannot afford both the tiny tach and the prop right now. If I can find a proper GPS app for my droid I will try to at least get the speed. It is not all that fast considering I have seen bugs fly faster than me. :redface:

 

[logain]

Re: Getting new prop have question

OK thought I would bring an update on things. I finally have been able to take the time to take the boat out and used a GPS app on my DROID and it told me that my friend and I were going 11mph. Been focused mostly on school stuff as of late. I have not been able to get the money for the tiny tach just yet. Hoping to be able to get that in the next week or two. Also did some looking around and it appears my boat is a MV Angler model. I tried emailing Alumacraft and I got the first email about how it would take 3 days to get back to me and that was it.

 

[steelespike]

Re: Getting new prop have question

assuming the app is correct at 11 mph your hardly planing.if you and your buddy are average weight 150-180 I would think a healthy 9.9 could manage 15 to 18
in your boat with the 2 of you.Brochure indicate std prop was 91/2 X 10 your 91/4 X 8 should be pretty good for your setup.Again without rpm its difficult to rate your
speed though it seems low.If the 8" is worn and no money for both prop and tinytach get a new 8" prop.Be sure the motor is running on both cylinders and the throttle opens all the way.

 

[logain]

Re: Getting new prop have question

Throttle is opening up and it is running smooth on both cylinders. I think the rpms might be a little on the low side. The app AFAIK is pretty accurate withing about 1-2mph. I tried it out in my car one day and it was pretty close. I am about 220 currently and my friend around 140. I will try to get that tiny tach soon.

 

[logain]

Re: Getting new prop have question

Ok I have the tinytach now. My question is what setting do I use for the sparks per revolution?

 

[Sea Rider]

Re: Getting new prop have question

Ok I have the tinytach now. My question is what setting do I use for the sparks per revolution?

If it's a 2 strokes, 2 sparks per revolution. Install as recommended by the manufacturer.

Happy Boating

 

[logain]

Re: Getting new prop have question

I did some looking around and according to this it is 1 spark per revolution on a 2 stroke.

http://science.howstuffworks.com/transport/engines-equipment/two-stroke2.htm

 

[steelespike]

Re: Getting new prop have question

One spark per revolution for the one plug wire you will be working with.

 

[logain]

Re: Getting new prop have question

OK, took the boat out this evening. For whatever reason the motor was idling kinda high. WOT was 5800rpm and 12mph. From what I read, the Johnson 9.9's operating range is 4500-5500.

 

[logain]

Re: Getting new prop have question

So based on the info given, do you guys think I should go with a standard 9.5 x 10 then? From what I have read, 5800 is a bit high.

 

[steelespike]

Re: Getting new prop have question

At 12 mph your probably not planing.
Rpm seems good but your not planing?
Did you carefully run the wire so its not picking up any stray pulses?
My indication is prior to 89 5,000 to 6,000. 89- 5,500 6,500.
These motors can really spin the 15 is rated at 7,000.
If you could get it to plane a little more pitch may be needed.
Did you have your buddy move forward? Did you try timming in?
Maybe try it alone to see if it will plane and get the rpm.
Try raising the motor as much as the clamp will allow.Just hold it up with a stick till you tighten the clamps.If my math is right disregarding slip an 8" prop should produce about 16 mph. at 5,600. On a reasonably light boat should manage about 21 mph. with a 10" pitch.My figures may be all wet though the 9.9 is capable of about 21 mph.

 

[logain]

Re: Getting new prop have question

It does not fully plane with my buddy in the boat. He sits in the front all the time. I will recheck the wire and try again. Supposed to go out I think Saturday night. I have the motor trimmed in a far as it will. It is the only setting where it comes close to plane.

 

[Mi duckdown]

Re: Getting new prop have question

are you running a longshaft Motor (20 inches) on a 15 inch boat transom?
Had a hard time following the posts.
If so there is not a prop made to help you out.
For every inch below the proper cavitation set up, you will 2 to 3 MPH.

 

[logain]

Re: Getting new prop have question

are you running a longshaft Motor (20 inches) on a 15 inch boat transom?
Had a hard time following the posts.
If so there is not a prop made to help you out.
For every inch below the proper cavitation set up, you will 2 to 3 MPH.

No I had to have the long shaft motor because my short shaft mercury was too short. Prop would come out of the water when the boat tried to plane @ full throttle. Went out Saturday night with another friend out on the river. Up and down stream I got 5500rpm and 10-11mph. My idle problems earlier got solved. Had to readjust the slow speed knob. For whatever reason, it was backed out too far.

 

[logain]

Re: Getting new prop have question

I found this info on Johnson 9.9's. Based on my results what do you guys think would be the best fit? Would a 4 blade prop help planing issues at all?

The 1991 OMC parts manual lists the following aluminum 3 blade props -- 9" X 10", 9"X 11", 9 1/4" X 8", 9 1/2" X 10", 10" X 5", 10" X 7", & a stainless in 9 3/8" x 9"