Prop help

[71prostreet]

Looking for some help I have a 90 Rinker Captiva 236 with a merc 454 mag and bravo 1 with 1.50 ratio with side hull exhaust so no exhaust or water are expelled from my drive it only picks up water from drive, curently I have a Mirage 21p Rh prop. When I stab the gas from a stop or a slow speed the bow stands up and the motor revs up it does not lag. It takes a long time to get up on plane and behind the boat water is just getting thrown around. when it planes the prop feels like it bites and the boat takes off and starts pulling hard. right now it hits 47 to 50 mph in the bay on gps with some choppy water with 10 to 15 mph winds. some people tell me that I should put a smaller prop on it like a 19p but my rpms are already at 5000 with a big block and that seems a bit high to me I figure it should be from 4000 to 4600 max. I think the prop is slipping and not getting good bite would larger prop bite better. I want to keep the boat quick but want to improve lower end to get up on plane fast right now it takes a long time too long to plane. any help would be great thanks.

 

[junior1113]

Re: Prop help

19 p 4 blade would surprise you

 

[hwsiii]

Re: Prop help

Street, if you will give me the Horsepower of your motor and the recommended WOT RPM range as well as the diameter of the Mirage plus you are running I will see if I can't figure out the correct prop for you. I have come up with an allup weight of your boat of about 4,600 pounds, this includes about 75 gallons of gas 4 people and other items, the base weight of the boat I show is 3,200 pounds. And right now I show you with a prop slip of 28% at 48 MPH and 5,000 RPM and that is kinda high, and I show you should be turning a smaller pitch prop than what you have now.

H

 

[71prostreet]

Re: Prop help

From what I find the rated max rpm is 4200-4600 and I'm at 5000, there is so many sites that list prop shaft hp at any where from 310-368hp The prop I have is a part number 48-13702-21p it looks like it is 14-3/4 I know alot of people put to big of a prop on boats, and many say I should use a smaller prop but if I do go smaller my rpms will go to 5400 if I go to a 19p, I have had the boat out twice and both times if I try to stab the throttle with the drive all the way (in or down) it just stands up the bow really high and wants to plow through the water, if I back off on the throttle the boat drops down on plane and starts pulling like mad then if I move the drive (out or up) I can get it to pull some more. I know it might sound crazy but is the motor over powering the prop and it does not want to let the prop bite. Thank again

 

[hwsiii]

Re: Prop help

Street, the first thing we need to do is be absolutely sure your tach is showing the correct RPM. If it is, it will not be hard to find the correct prop, so you will not have such a massive bow lift and very slow acceleration to plane and high speed.
If the tach is reading the correct RPM you will need a 4 blade stern lifting prop with zero rake to lift the stern more and the 4 blades for more surface area with less prop slip. It would also be better to have a prop with a larger diameter as well as that will increase blade surface area as well as give you more bite in the water, and trailing edge cup will also help control the water better off the end of the prop and thus create less prop slip.
If you can verify that your tach is correct I will be very happy to recommend a prop that will control the slip better and thus increase speed.
Yes, the prop is being overpowered by the HP and torque of your motor if the RPM you give are correct.

H