Wrong prop, but got a new outdrive with different ratio

[pmill]

My boat currently has a Mercruiser 470 with 1.81 ratio and a Mercruiser 48-78122A4 21p 13 3/4" prop. It's a 21' cuddy and loaded weights about 4000 lbs. Getting on to plane in good conditions can take over 15 seconds, slow. At full throttle my rpm's are at 3800 39mph. WOT range for this engine is 3800 to 4200. Any suggestions on a prop to help with getting on plane? I don't care about top speed, I need power to get up and go quicker.

On a side note, I just bought another outdrive that's geared at 1.47 and came with a Mercruiser 48-66452A4 15p approx 15" prop. What would happen if I bolted this up and took it out as is? Is there a prop I can get that will make this gear ratio work with my engine?

Thanks

 

[buckinrut25]

I think the swap will only hurt what you are trying to do. If your current outdrive is healthy keep it. I always recommend checking the cheap stuff first. Compression, timing, basic tune up items, fuel filter and make sure carbs are getting all the way open at WOT. If the enging checks out I would drop to a 19p 4 blade Solsas Amita. That should keep your RPM in range and help get you on plane better. You should not lose much or any speed with the swap. Just a side note with your current set up is only showing a 6% slip which is really low for this style boat you may want to verify your speed with GPS and check your tach. You can spend a lot of money chasing a good prop set using untrustworthy gages.

 

[jestor68]

I would suggest Mercury's aluminum 15-1/2 X 18 pitch. That prop produces very good out of the hole thrust to push your boat up onto plane quickly.

The 1.47 gear ratio would not be compatible with the 470 motor.

 

[pmill]

I really appreciate the feedback. The fuel/water separator and points distributor changed over to electric pertronix and timing reset this spring. I need to check compression, carb butterflies at WOT and verify speed with GPS, have not done that. Tach was partially rewired with the ignition and appears to be accurate (best I can say at the moment).

A couple things I forgot to mention is this is an offshore/bay fishing boat and I have a 30 gallon bait tank mounted to the swim step and it's always full of water/bait. If I try to trim up at cruise speed the boat starts to porpoise so I've given up trimming at all. I'm in the process of moving the bait tank forward towards the cockpit to better balance the boat.

A search on Solas website with generic data for my engine recommends 19x14.25. I do like the 4 blade prop recommendation. Does this sound good for the situations I've described?

Thanks