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  #1  
Old 07-27-2005, 11:32 AM
stevebrownell stevebrownell is offline
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Location: Pembroke, MA
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Default Numbers for a 20 I/O

I'm running a 4 blade aluminum 14.5 x 19 prop on my 20 I/O, 3.0 L. Mercruiser. Top speed is about 27 kts. Does anyone run the same setup, and would I gain anything on the top end if I switched to a stainless prop? Figure I'm looking at $350-$400 for a new prop, so if the gain would be minimal, it probably does not make sense. I understand that the 4 blade prop is considered a good all around compromise for this set up. Any insight the learned-ones on this board could provide is greatly appreciated - I'm stumped when it comes to the arcane science of prop selection. thanks.
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  #2  
Old 07-27-2005, 12:09 PM
nestorpr nestorpr is offline
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Default Re: Numbers for a 20 I/O

I run an aluminum 15 x 19 on my 20 Seafari with the original 140 hp Mercruiser and hit about 32mph at 4500 rpm. I guess I could get more from a SS prop but can't justify the expense for now.
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  #3  
Old 07-27-2005, 01:07 PM
stevebrownell stevebrownell is offline
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Default Re: Numbers for a 20 I/O

Thanks for the feedback.

that's interesting, my RPM's seem to be way low, considering you are pushing more weight with the same engine and getting 4500 RPMs. (I'm only seeing around 3250 RPM's)

Wondering if I should drop down in diameter a bit?
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  #4  
Old 07-28-2005, 12:46 AM
Briguy Briguy is offline
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Default Re: Numbers for a 20 I/O

Steve, If you are only topping out at 3250 you are lugging the engine. We need more information and you need to do more research. How is your holeshot? (no jokes here) What is your cruise rpm? Why the 4 blade prop? What is the HP of the motor and what is the manufactures wot rpm range? Is your bottom clean? (again, being serious) Is your prop in good shape? What is the gear ratio in your lower unit? All of these factors come into play when deciding what prop to use. 3250 rpm's is way too low unless you have a diesel. In my best guess you should be turning 3000 at cruise and 4200-4600 at wot with a full load. Talk to your dealer.
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  #5  
Old 07-28-2005, 12:13 PM
stevebrownell stevebrownell is offline
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Default Re: Numbers for a 20 I/O

I'm thinking that I need to first verify that my tach is reading accurately, but it reads around 750 at idle, so the low end seems right.

Holeshot seems ok, I'm not looking to snap heads back, so that's not really a concern...I guess.

Cruise RPM - not sure what you mean - my only experience is with a 90 HP 2 stroke outboard, cruised at 22 mph at 4200 rpm, this I/O is a different bird altogether. As I said, my max seems to be around 3200-3300, and I've played with the trim a bit, usually good for another kt., but that's it.

The 4 blade prop, (aluminum) came with the motor, and it is in great shape, no nicks, dings, etc.
I'm Perfectly willing to change out to another, Al. or SS.

Bottom is clean, painted w. anti fouling, but the boat is on a trailer, so there is no growth on there.

I'm also going to check the ratio of the lower unit, hoping it is not the lower ratio, that would suuuuuck, but explain a lot.

Thanks for the advice.

Damn, I really am dreading checking the lower unit....
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  #6  
Old 07-29-2005, 10:57 AM
stevebrownell stevebrownell is offline
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Default Re: Numbers for a 20 I/O

Outdrive ratio is 2.00, check this AM.
anyone have some suggestions on prop size? I know it's a rare bird, but is anyone using a different prop. on their 20 I/O, other than the 14x19?

Thanks again.
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  #7  
Old 07-29-2005, 02:28 PM
Briguy Briguy is offline
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Default Re: Numbers for a 20 I/O

Steve,

You may want to try a repost on thehulltruth.com. If no one here has your setup, start with a 15x17. Also there must be some way to hook up an external tach or check if yours is calibrated correctly. Have you spoken with the local Merc dealer.

Lastly, use your ear. I have been around boats all my life and can tell by ear if an engine is lugging or not. Most motors get "quiet" at their proper cruise rpm. They just sound "happy".

PS with your current prop, what RPM is the lowest you can maintain plane?
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  #8  
Old 07-29-2005, 10:47 PM
stevebrownell stevebrownell is offline
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Default Re: Numbers for a 20 I/O

Agreed, I'll bring it in. Have a great Merc. shop in my area.
The engine sounds like it is humming along, I'm beginning to think it may be the tach, but the prop. is probably not doing it any favors.

Thanks again for the advice, and nice refit/paint on your 23. Tremendous.
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