Prop for Skiff with 25 'rude

makomike1

Recruit
Joined
Jun 21, 2009
Messages
3
I've got a J16 Carolina Skiff with a 1996 25 hp Evinrude (bought used together).
I spun the hub and took it to the local prop shop for repair. The aluminum prop was originally 10 x 15 but they recommended re-pitching it to 10 x 13 with a light cup.
Since they changed it, however, I've lost speed and it seems to maybe be cavitating at just under WOT. I don't have a tach or speedometer (hey - it's a basic just for fun boat used in the Intracoastal - no frills here!) but I estimate it used to run about 25mph and has probably dropped down to about 18-20 mph. Just for comparison, I was running along side a friend's same size Boston Whaler with a 15 hp with equal gear and passengers and could just keep up. The boat itself is supposed to weigh about 600-650lbs and 99% of the time its just 2 of us (slightly over 300lbs soaking wet combined), a cooler and basic gear.
How do I find out what the prop really should be on this rig. I'm looking for some better speed and performance

Thanks,
Mike
 

junior1113

Senior Chief Petty Officer
Joined
Jan 29, 2009
Messages
763
Re: Prop for Skiff with 25 'rude

can u post a pic of cavitation plate and bottom of boat sounds like motor needs to go down with a 13
 

makomike1

Recruit
Joined
Jun 21, 2009
Messages
3
Re: Prop for Skiff with 25 'rude

The motor doesn't have any adjustment up or down and I'm technically challenged when it comes to posting pictures. The cavitation plate is approx. 1.5" lower than the flat boat bottom and the center of the prop shaft is 8" lower than the boat bottom. What I'm looking for is there a way to find out for sure what the correct prop should be. It seemed to run a lot better when the prop was 10 x 15 but I'd like to know what really belongs on it.
 

junior1113

Senior Chief Petty Officer
Joined
Jan 29, 2009
Messages
763
Re: Prop for Skiff with 25 'rude

if your captivation plate is 1.5 inches under the bottom of the boat your beating a dead horse. it should prob be 1" higher than bottom of boat. you have to much drag. mine is up 4" from bottom go figure may need to modify transom to get it up if it has no adjust ments this will give optimum performance
 

makomike1

Recruit
Joined
Jun 21, 2009
Messages
3
Re: Prop for Skiff with 25 'rude

Everything else I've read says the cavitation plate should be even with the bottom of the boat or slightly lower, especially on a smaller flat bottom boat like this. If it's higher, won't the prop be sucking air?
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Prop for Skiff with 25 'rude

Mike, in theory what they tried to do with your prop was more or less correct, you should be running a 13" pitch prop with an all up weight of about 1200 pounds and a gear ratio of 2.15:1. But anytime they add prop cup to the trailing edge it adds theoretical pitch to the prop, and many shops do not know how to cup a prop the proper way so they mess up the prop and it doesn't do what it is supposed to do. My recommendation is to bring it to a first class prop shop and ask them what the REAL pitch is and to guess how much theoretical pitch was added with the cup they put on the prop. It is not unusual at all for people to have a prop redone and to lose speed and performance after they put it back on their boat. If you can borrow a 13" pitch prop try it and see what happens, I would say you will probably lose speed from the 15" pitch original prop but you will now be in the manufacturers recommended RPM range and you shouldn't lose but a couple miles per hour.
You should be having NO ventilation from a cupped prop, that makes me believe they did a bad job on the cupping of the prop. A correctly cupped prop has much less chance of ventilating than a normal prop.
And you need to purchase a Tiny Tach for about 40 dollars, you NEED a tach, it tells you if you are hurting your motor and if you have the correct prop on your boat, there is NO other way to know without a tach.

Best wishes
H
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop for Skiff with 25 'rude

The plate even wth the bottom of the boat is considered a starting point.
Ideal is as high as possible without venting(cavitation) problems.
But don't mess with the height until you get your prop solved.
I think they should take the cup out of the prop for nothing check that it has been repitched right.Then try it.I fully agree you need a tiny tach
and a borrowed gps to verify your setup is ideal. you'll improve hole shot
top end and load carrying ability.And a properly set up motor will live a longer healthier more economical life.
 
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