Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

[bell_man]

I just bought a 1993 SeaRay 170 3.0 Mercruiser/Alpha One Gen II. Right now at WOT im running very close to 3900RM. Right now the Boat has a 15.25x21p prop on it. I was told by a boat mechanic to get a 15x16p 4 blade dose this seem right? We will be using the boat to pull tubes and ski.

 

[bnicov]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

Dropping to a 17pitch prop should raise your rpms by around 3-500 so you should probably be in the middle of your recommended rpm range for your motor, check on where you should be running. Don't just take the word of one mechanic, unfortunately there are mechanics out there and then there are MECHANICS.........

 

[steelespike]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

Apparently there is a 115 and 135 version of the 3.0.Which do you have?
Normally a 5" pitch change would net 800 to 1000 rpm.
Also what was your speed at 3900.
A 16" prop is pretty close to what you would put on a hard to push boat like a pontoon.

 

[smokeonthewater]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

definitely work through the details with steelspike BUT I agree with the mechanic..... your boat would tow tubes like a tractor with that prop.... a 14.5x17x3 blade would also be a potentially great choice and would likely get you a couple more mph on top while still pulling very well

 

[steelespike]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

Assuming it is a 115 version 3.0 a 16" 4 blade probably is what you need.
It will give you the power for skiing and such but may sacrifice some speed.
motor is rated for 4600 rpm prop calculator indicate about 33 at 4600.
You may need to watch the rpm with a light load.
At 3900 a 21 indicates about 36 with about 10% slip.Probably lacks
enough power to ski easily.

 

[bell_man]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

Assuming it is a 115 version 3.0 a 16" 4 blade probably is what you need.
It will give you the power for skiing and such but may sacrifice some speed.
motor is rated for 4600 rpm prop calculator indicate about 33 at 4600.
You may need to watch the rpm with a light load.
At 3900 a 21 indicates about 36 with about 10% slip.Probably lacks
enough power to ski easily.

Yes it is a 115 version 3.0L. I dont know what the speed was didnt bring my GPS and my speedometer is not working due to a pinch tube on the drive. Is the 33 and 36 you mention MPH?. Also when testing my WOT the trim was not all the way down. When testing WOT do you keep trim down? What is slip?

 

[steelespike]

Re: Help with Prop choice 1993 SeaRay 170 3.0 Alpha One Gen II

33,36 was mph. actual could be more or less. When running on plane you allways trim up to the fastest speed/rpm for the selected throttle setting.
This could vary depending on the load and conditions.As you slow you would nornally trim down to help stay on plane.
For the best hole shot you would start trimmed down and gradually trim up for best speed/ride at your throttle selection.
Slip: Prop pitch is in inches.A 12" pitch theoretically moves 12" in one revolution but through a liqiud it is less, that is measured in
percentage of slip.On a reasonably fast recreational boat slip would range typically from about 3% to 12% Usually something under 10% wouold be expected. The slower the prop turns the higher the slip. Heavy or hard to push boats boats usually have higher slip.