prop selection advice needed

[brdmh44]

1. I'm currently trying to fix an over rev (too much rpm) and gain grip. The boat comes off plane when turning and revs way too high. Low top end speed. 2. Mercruiser aluminum 3. 14.25x19 4. 5300 rpm (verified) 41-43 mph (gps verified) 5. Mercruiser Chevy 305 2bbl 210hp 1995 alpha 1 6. 1995 conquest 202 23' 3300 lbs l for meaningful response to most questions. 1) Performance issue you are trying to correct. 2) Current prop manufacturer, model, aluminum or stainless as a minimum. 3) Current prop diameter and pitch (required). 4) Wide open throttle RPM and speed with an average load (very helpful) 5) Engine/drive make, model, year, and HP 6) Boat make model, year, length and weight

 

[Chris1956]

So she looses grip when turning? Usually that is caused by mounting an outboard too high on a deep vee hull. In your case (Sterndrive), the only way I know how to fix it is with a prop that will hold the water better. I do not have a recommendation for you, but someone will come along with one.

What is the max recommended RPM for your motor?

 

[Starcraft5834]

You need to increase pitch in 4 blade application. Your rec rpm range is? 44-4800? Increasing pitch should bring your rpms down

 

[steelespike]

Using a 1.61 ratio 5300 rpm at 13% slip would make 51.5 mph at 43 your slip is 27% Is the prop in nasty condition?
Are you saying the motor revs and the boat slows in general or in turns?
Does the boat get up on the water when you trim up.
Sometimes it is necessary to trim down for turns.

 

[brdmh44]

We just purchased this boat from a broker and this weekend was its initial test. I feel like this boat is severely under propped as it is 4400-4800 rpm max. I only opened it up all the way for testing purposes. Would a 21 be a good weight prop then perhaps a 23 for 2 person speed runs? We played with the boat more and she seems to.be holding turns now (my cousin doesn't know how to trim properly) I currently run the trim all the way down as she'll cavitate if you try to trim out. Conquest boats went belly up in 2000 and I haven't found anyone else's prop recommendation. 14.25 21 4 blade or 3 blade? Can I pull a 23 with 1 or 2 guys? Using the 200 rpm for every number of pitch basically says to go with 21. Any recommended widths? A local marina has a ton of new aluminum merc props at great prices, but they won't give me a recommendation.

 

[brdmh44]

Also, the current 14.25 19 prop is in good condition. It seems like this hull can pull speed as the motor really doesn't have to work. She comes right out of the hole almost too easily, and hits top speed almost instantly with 7 adults.

 

[Starcraft5834]

23ft. 3200lb boat... 4blade21p that should remedy your issue assuming your engine is healthy

 

[brdmh44]

Any certain width?

 

[brdmh44]

I'd like to buy two props, one for family days, and another for speed runs. Motor is in excellent condition according to leakdown and compression tests. I called a few local shops and once I said aluminum, they did not want to work with me.

 

[steelespike]

Need to get things clear.Do you not trim up at all? Does it ventilate when you trim up in a straight line?
If you don' t trim up that would account for lack of top speed.
How did you confirm your tach is accurate?It would not be surprising for your 19 year old tach to not be accurate.
If your not trimming up it would be odd to experience over revving unless the boat is acting strangely from the lack of up trim.
​ Assuming all is accurate it appears a 21" prop would put you at about 4900 rpm. You will lose some hole shot.
A 23" will slow you further.
What is the prop number under the prop nut?

 

[Bamaman1]

A Conquest 23' boat with the Mercruiser 305 cubic inch motor is certainly not overpowered. If you're overreving @ 5300 rpm's, you should go out and buy a 21 pitch 3 blade aluminum propeller--a modest investment.

With 2 people and 1/2 tank of gasoline, see what performance you get out of the new propeller.

At a dead stop, trim your motor down. After the hull planes off, hold the trim button until the nose of the boat rises and it starts rocking slightly. Then, tap the outdrive trim down slightly--until the rocking stops. That's your best outdrive trim. Your goal should be to try to get it to 4800 rpm's with the boat properly trimmed lightly loaded.

You will find that you'll be at your boat's optimum speed. You're never going to have a 55 mph boat with a 305 engine and 3300 lbs.

 

[brdmh44]

I feel like my messages have added some confusion to the original question. 1. Yes, I am trimming up after coming on plane. I barely get any lift before experiencing ventilation. However the prop looks like it has actually experienced cavitation. There is no way this prop is close to being right. 2. I understand this is not a speed boat. 3300 pounds was what the broker guessed. I believe it may be lighter, but there is no way to tell as information regarding the boat is long gone. Any information regarding these boats disappeared when they closed up. 3. Going from a 19-21 pitch should drop me from 5300 to 4700 in theory. Would the pitch change not add more speed? 4. The RPM was verified with a knowingly good tachometer, it was within 50 rpm of boat tach. I just want to gain a little top speed, 50 mph seems like a doable number while maintaining modest hole shot. I'm not opposed to a 14.25x21 4 blade, but is that too wide?