Which prop

[TQ]

I bought a 21ft Rinker last year and the guy I bought it from gave me two props, an aluminum with 19" pitch and a SS with 21" pitch. He said he liked to run the 19" pitch because it gave him better performance for water sports but I found at WOT with the 19" pitch I was over reving with a light load. I tried the 21" pitch and at a light load condition I was just below the engine max rpm, so ok good. Two question:

-If I run the 21" pitch with full gas and say 6 people, I don't reach the max rpm at WOT, so this is overloading the engine right? As long as i stick to slower speeds (well below WOT, say 3000 rpm, the normal cruising speed), when I have this "heavy load" this will not over load the engine for the trip right, only if it tried for WOT?

-Also if i put on the 19" prop from water sports and then I want to go cruise somewhere, as long as I dont go WOT which over revs the engine, there is nothing wrong with this, right?

This is just a sanity check getting ready for the ever slowly approaching boating season......

 

[dan t.]

Re: Which prop

sounds reasonable to me

 

[steelespike]

Re: Which prop

We need details max rpm and speed with a light load with both props.
rpm and speed with a load.Don't be afraid to over rev long enough to get a reading.
If the heavy load is an occasional thing no worries but this does depend on your actual readings.No problem running the 19 for everything just stay out of the throttle.Fuel use may be slightly more with the 19 due to less speed for same rpm also the stainless would usually be more efficient.

 

[Silvertip]

Re: Which prop

A prop that is wrong at wide open throttle RPM is also wrong at any throttle setting less than that. Why? Because the engine is still loaded more than it would be at the same speed you would have with a correct prop. Fuel economy is also worse. That practice may not harm the engine, but it certainly would not be operating at best efficiency.

 

[tmh]

Re: Which prop

there is no "perfect" prop for sports boats, all are a compromise of some sort. So, I think the OP's thinking on this is fine. 19" is better for some things, 21" for others - nothing unusual there. Just don't normally over-do with either and you should be fine. I guess the 21" may be worse for the engine when heavily loaded tha the 19" when lightly loaded (assuming you stay away from WOT with it).

Now, if i were you (and spending YOUR money!) I'd get me a 20" 4 blade SS Stiletto bay Pro 2 for that boat. I run that on my boat and it has hole shot like my 19" had and top end like the 21" had. Best of both worlds, except less $$$ left in my pocket ..... So I'm going to sell the 21" SS and keep the 19" as a backup prop. The BP II has great overall lift so a bit less drag.

 

[TQ]

Re: Which prop

Thanks for the responses. If fuel stays cheaper than last year maybe I can afford to compromise and get that 20" SS. If not, I guess this year I may switch props more often then last year depending on what we are doing.

I didn't realize how adding a few hunderd pounds to a 21 footer changes performance so drasitically.

 

[Tail_Gunner]

Re: Which prop

Id slow down on a 20p baypro it will act like a agressive 21p 3 blade. Its best you post your wot and speed... with you and a some weight in the boat.

 

[TilliamWe]

Re: Which prop

A prop that is wrong at wide open throttle RPM is also wrong at any throttle setting less than that. Why? Because the engine is still loaded more than it would be at the same speed you would have with a correct prop. Fuel economy is also worse. That practice may not harm the engine, but it certainly would not be operating at best efficiency.

Exactly right.

On a small block Chevy with Vortec heads, you MUST be able to acheive your recommended WOT RPM range, or you WILL damage the valves. Don't ask me how I know, I just know!

 

[TQ]

Re: Which prop

I will have to wait till spring to get the exact numbers again. I know when I checked speed and rpm with just two people in the boat i was right at the engine recommended rpm.

So going back to my original post, when i add a few people and gear, that makes it so I can not get to my original top speed and rpm, from when it was lightly loaded. So when you said "On a small block Chevy with Vortec heads, you MUST be able to acheive your recommended WOT RPM range" do you just mean at my light load test condition, or ANY time I take the boat out?

I doubt I am going to buy any new props, I just want to determine which one I have is best overall, and when I need to switch them out.

 

[TilliamWe]

Re: Which prop

do you just mean at my light load test condition, or ANY time I take the boat out?

I mean any time you take it out. If you can't reach the max wot rpm range, then you are overloading the motor.
You have to prop the boat for the way that you are going to use it the most. Or get used to, and real good at changing the prop!