Prop Advice - 21' Wellcraft

[Turbo-Duck]

Hi all,

I need some advice / opinions on prop selection. My boat is a '97 Wellcraft 21' Cuddy (21SL) Dry weight listed at 2900 Lbs. - 5.7 Mercruiser / Alpha One Drive. (I've looked all over that drive and can't find a gear ratio anywhere.) We were running with a pretty full tank of fuel - 2 average size people and not a lot of extra gear.

When I got the boat, it had a 14 x 19 on it that was pretty well twisted up and looked small, so I picked up a 14.25 x 21 Aluminum prop. Boat planes easily at 1/2 throttle, and tops out at around 52 at just over 5k RPM's WOT. Running WOT, can't get much over 1/2 way trimmed up (by the gauge) - it seems to cavitate or spin real bad when adding much trim. Because of that, I"m thinking I can go bigger on the prop, or maybe go to a 4 blade?

Not really looking for top end speed, but actually trying to improve my cruising range.

We went out on the Ohio River last Saturday - The boat cruises at about 30 MPH (by GPS) @ around 3200 RPM - What I'm really trying to accomplish is to be able to cruise at that speed at a lower RPM, and hopefully extend my cruising distance. (Fuel mileage) It's definitely not working the motor hard, I'm thinking it's slipping a lot....

I talked to a guy at a Marina that suggested a 4 blade prop of the same size. And I've read that Stainless makes a big difference. I"d be willing to spend the extra $$ for SS, but it gets kinda spendy trying different props. Unfortunately I don't have a Marina near me that will let me try out props before I buy...

Our plan is to spend a week on the Ohio River next year, and fuel consumption will be important.

 

[steelespike]

My info indicates 4800 is the max wot rpm rating.1.47 seems to be a popular gear ratio. Your slip is at 23% at 52 mph I always suspect the tach when slip is so high.Is your speed by gps?The only way to be sure speed is accurate.What do we know about your present prop?
Perhaps before a prop change You could verify the tach.Borrow a shop tach or get an induction tach.
If we assume the tach is accurate A 21" 4 blade should put the rpm closer to where it belongs. Solas Amita or Rubex props have a good reputation.
It should grip better and speed per rpm should improve.
You don't want to force the motors rpm lower by prop selection.The motor needs to be able to rev freely within its rpm range.
The 4 blade should have better hole shot(saves fuel),Should stay on plane easier at reduced throttle( saves fuel).
An added bonus it should respond better around the dock.

 

[Turbo-Duck]

Thanks for the info. I haven't verified the Tach yet, but it sounds pretty close to accurate. (I've been wrenching on cars and boats for over 30 years, and have a pretty good ear / feel for engines) I think your numbers are pretty right on, as it feels and sounds like it's slipping a lot more than it should.

Nobody talks much about diameter on props, how does that factor in? What happens if I stay at 21" pitch, but increase the diameter? Like 15" x 21 in a 4 blade, or go to a 14 x 22 4 blade.

I also ran the Mercruiser prop selector, and it recommends a 23" pitch - I'm thinking that might be too much, but not sure.

-Gary

 

[steelespike]

Diameter usually just follows the design but increasing diameter about an inch could lower the rpm some.
Adding cup can lower rpm,I would guess your present prop doesn't have much cup.
Typically going to a 4 blade it is suggested to decrease pitch an inch So going to a 4 blade in the same 21" pitch
could lower rpm 100 to 200 rpm.