Some initial prop/set-up questions UPDATE and QUESTION

[ezmobee]

I've been thinking about replacing my prop for a while and it now looks like I'm having some slippage so I going to pull the trigger. I'll have the old one repaired to keep as a spare. I have no idea what prop is on there now. Hopefully it will have some numbers on it when I pull it tomorrow. Where should I be looking for those?

Right now my '73 Johnson 85 on my 16' Starcraft closed bow aluminum boat runs in the high 30's at 5200 RPM at WOT. I have a fantastic hole shot. I don't really ski so I don't have a lot of use for that great hole shot and would like a little extra top end. According to my factory manual the full throttle operating range of this motor should be 4500-5500 RPM. I'm thinking I'm good at 5200, do I need to try to be getting to 5500?

As I said I don't know what prop is on there now. I'll remove it tomorrow and try to figure out what it is. According to the manual the stock prop was a 3 blade 13 1/4"x17" aluminum. If that is indeed what is still on there, what should I consider as an upgrade for top end performance?

 

[steelespike]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

Before getting a new prop optimize your setup.If you have had little or no problem with ventilation then you can probably raise the motor a hole or so.
The antivent plate even with the bottom is considered a good starting point.
You might get the prop fixed then work on the setup.Make a prop decision based on the setup results.I'm not an expert but I would consider a stainless prop in the same size.A stainless may allow even more height.Numbers should be on the barrel -- X -- (diameter X pitch).The pros should be along with more specific info.

 

[ezmobee]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

Update: My current prop is a 13 3/8 x 17. Any suggestions as to what I should go to for better top end performance?

 

[Silvertip]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

You were just given the information on what to do. If you increase pitch to gain more speed you end up lowering rpm, so the situation gets worse. Stainless is more efficient than aluminum so slippage would be a little less. Top speed is what you get when you have the engine propped so it revs at or near the upper end of the recommended RPM band. You are at 5200 now. Less pitch raises the rpm but keeps the speed the same unless the engine is so overpropped it can't get over the hump so to speak. Propper setup will very likely get you the rpm you need and a couple of MPH. I don't know what you are thinking a new prop will gain you in speed since the engine is already close to the upper end of the band.

 

[ezmobee]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

I'm a dumbass. I guess now that I think about it, it's not like a prop is a gear that I can change to suddenly get more speed out of the same RPM. Reading the "how to select a prop" on the iboats store seems to indicate that I'm right where I should be RPM wise. Unfortunately I cannot afford a SS prop at the moment. However, my prop is a little work so perhaps just replacing it with a brand new aluminum prop will help. All the 17 pitch props for my engine are 13 1/4". Is that 1/4" going to make any difference? I'm thinking no.

 

[Irv964]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

I'm a dumbass. I guess now that I think about it, it's not like a prop is a gear that I can change to suddenly get more speed out of the same RPM. Reading the "how to select a prop" on the iboats store seems to indicate that I'm right where I should be RPM wise. Unfortunately I cannot afford a SS prop at the moment. However, my prop is a little work so perhaps just replacing it with a brand new aluminum prop will help. All the 17 pitch props for my engine are 13 1/4". Is that 1/4" going to make any difference? I'm thinking no.

Nothing that will be noticable, 1/4 inch less may mean you will be getting a 1/4 longer blades which will help some.......Irv964

 

[chris0061]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

I would go with stainless. You get about 200 more rpm than alum. at the same pitch, and that would put you at your top range. You can usually find a good stainless on eBay for the price of a new alum. prop. Performance wise it is well worth it.

 

[MikDee]

Re: Some initial prop/set-up questions

Re: Some initial prop/set-up questions

I don't know what brand or model to recommend, but, It has been my experience, that a smaller diameter prop with less blade area, and better design, will give you more speed, unless your boat is heavy like an I/O, but that's not the case here. You may lose some in your hole shot, but you say yours has a great hole shot, meaning you've got the power there, you just have to redirect it towards speed. The trick is, to find the right 17" pitch prop. There is a certain brand aluminum prop I've heard of here that duplicates a stainless one, delivering great results, but I forgot the name?

 

[ezmobee]

Re: Some initial prop/set-up questions UPDATE and QUESTION

So I was going to be taking 4 friends out last weekend and I was pretty concerned that my partially spun prop would be up to the task so had to buy a new one immediately which sadly meant I had to overpay for a BRP prop at a local dealer. I went with one that should have been damn near the same as my current one. My current one was 13 3/8 x 17. The one I bought is 13 1/4 x 17. My performance has really suffered!! I lost a couple MPH and a bunch of RPM. Had the boat out on Friday with only me in it and could only hit 4800 RMP at WOT? What the heck?

 

[haskindm]

Re: Some initial prop/set-up questions UPDATE and QUESTION

You have discovered that there is much more to props than just diameter and pitch. To get the most out of your boat you will probably need to experiment with different manufacturers and configurations, or work with a shop that will customize a propeller for you. It is tough either way. You will either need to find a dealer that will let you experiment with different props, or spend the $ to work with a custom shop. Look into iboats rite prop program.