Prop Question

[fastimz]

Alright guys, I have beat this around with every boat guy in town. I want to get some outside input and see what your opion is. I have a 19p Mirage on my 27' Sun Runner. The boat has a carburated 454 with a Bravo 1 outdrive. Went out last Sunday for a sea trial and here are the results. My cruise speed is about 27-30 mph at 3,300-3,500 rpm. No problem there, would like it a little closer to 3,000 but will work. To acheive that I would go to a 21p prop, right? Well wot I am only turning 4,000 rpm. This would lead me to believe I needed to drop to a 17P prop, right? Right! So, do I settle with the 19 that gives me a decent cruise, go up to a 21 to improve my cruise(and kill my wot), or go to a 17 to get the wot? I am at a loss, kinda

 

[90stingray]

Re: Prop Question

Move this to the prop section. Folks there will be able to give you better advise.

 

[Bondo]

Re: Prop Question

Right! So, do I settle with the 19 that gives me a decent cruise, go up to a 21 to improve my cruise(and kill my wot), or go to a 17 to get the wot? I am at a loss, kinda

Ayuh,... Ya probably should go to the 17"...
Yer already lugging the motor with the 19" now
Going to the 21" will Severely lug the motor...

Lugging causes detonation...
Detonation turns runnin' motors into Anchors in No time flat...

 

[steelespike]

Re: Prop Question

Boats are not like cars they like to be able to rev easily within their rpm band.
A low cruising rpm doesn't necessarily mean best economy.Best cruising speed could be just on plane up to 500 to 1,000 rpm higher.Only way to be sure of best cruising speed is with a flow meter.The lower pitch will make it easier to stay on plane as well.

 

[fastimz]

Re: Prop Question

But if I go to a lower pitch my cruise speed will decrease or my rpm's will increase. It doesn't seem to be under a strain while at 3300. Just doesn't top out. It comes on plane fairly quickly. I did run the test fully loaded with 150 gallons of fuel and 35 gallons of water, so it will be a lot lighter once all that gets used. I have a 17 that I tried a few weeks ago, it wasn't a Mirage and it seemed to turn way too many rpm's throughout the range. It was turning around 3300 but I was only cruising at 20 mph, just on plane. If this was a 350 I wouldn't mind turning the rpm's but don't want to kill this BB. While I cruise at 3300 with the 19 the 4bbl are closed and begin to open around 3600. I get what you are saying, Im just playing devil's advocate!

 

[walleyehed]

Re: Prop Question

This would more than likely be a great candidate for a 4-bld in a 17P. The 4 would provide more over-all lift on that type of rig. It doesn't matter what engine it is but if you run (prop it) at the top of the manufacturers recommended range, your cruise economy will follow....the reason I would suggest a good 4 bld SS.

 

[Bondo]

Re: Prop Question

I get what you are saying, Im just playing devil's advocate!

Ayuh,... Then run the 21"....

When you've burnt a Hole in a piston,...

We'll be down in the Merc- I/O forum to help ya pull the motor for replacement...

 

[steelespike]

Re: Prop Question

Boat motors are allways straining like they are going up hill.
Because your motor is running at a higher rpm doesn't necessarily mean a higher throttle setting.
Props are not all created equal.For various reasons one may run faster top end or have a better hole shot and some props just aren't right for certain applications.
If the motor freely revs it will have better hole shot,stay on plane with less diddling and will have a better top end.Right now your missing about 500 rpm and except at no wake speed your motor is struggling up that hill with a trailer in overdrive.

 

[fastimz]

Re: Prop Question

I like the idea of trying the 4 blade 17p. I will have to see if any of the shops around here have one I can give a try. Sorry to get you all bent outta shape there Bond-o. Not disagreeing with you just trying to get all the variables and options out there.