Need help with selecting corrrect prop

[wildhorse]

Hi all,

I’m hoping for some guidance with prop selection. Our story is that we inherited a 21’ RC V Drive 3 years ago. It used to run some horsepower, but just before my Father in Law passed he replaced the engine so that he could just run pump gas and have more of a cruiser of sorts. The boat is used as the family station wagon at a Lakehouse and gets driven more for transportation than for a typical recreation type of mission. A 10 mile ride to and from a restaurant, etc..

As stated just before he passed he did the engine. When I got the boat the Hobbs showed that he had run the new engine less than 30 minutes. I know he ordered a prop trying to run something better suited for a lower horsepower application, but I never got to speak with him about it. I installed the new prop and have run it for 3 years now.

Last month we spent time on a friends pontoon boat on Lake Powell and I realized right off that this gynormous boat was WAY faster than my Schiada. And I mean it would spank it badly.

Today I carried a portable GPS and collected the following numbers on a mirror smooth lake. The boat always feels like it needs a taller gear so to speak with the forward motion not being right for the excess of power that seems to be being wasted for each speed. It feels like it will easily pull a taller gear for lack of a better wording on my part.

Here’s the numbers, and I stopped at 3,600 rpm because it seemed pointless to keep going with this. For each RPM I played with trim to get the highest readout on the GPS FWIW... Also I’m assuming that the old prop is all wrong or he would have not ordered a replacement and stopped driving it after only 20 minutes or so. It’s a big effort to get the trailer off the hill behind the garage and take the boat out of the water repeatedly with me just guessing with the prop. Also, I’m embarrassed to say that I fon’t Know the numbers on either prop. I’m hoping for a good starting place... Thanks in advance for any help..

2,000 rpm = 23 mph
2,200 rpm = 26 mph
2,400 rpm = 30 mph
2,600 rpm = 33 mph
2,800 rpm = 35 mph
3,000 rpm = 36 mph
3,200 rpm = 38 mph
3,400 rpm = 41 mph
3,600 rpm = 43 mph

 

[BajaMike]

What engine does it have?? It’s a V drive?

Lake Powell is a different story, because of the altitude, most boaters have to prop down (I did).

If you have a normal load of people, fuel, beer, water and toys, if can you get to the rated RPM (probably around 5000 RPM) quickly, your prop is probably right. If it will easily over rev under those conditions, you might need a higher prop.

 

[BamBam]

Those numbers sound pretty good for stock power in V-drive 21'
Hard to make any recommendation for a change until we know what you're starting with. It would be nice to know HP, Gear ratio in v-drive, current prop dimensions #of blades and material to help. I've always said that V-drive props are like black magic and voodoo because 2 props with the same dimensions will work differently. I don't have a complete grasp on it, but I understand the basics. Try to post up some more info and you may get the help you are looking for.
If you are happy with the performance of the prop (how it carries the boat) then a gear change might be a better choice t lower the RPM's (which don't sound too bad anyway)

 

[wildhorse]

Those numbers sound pretty good for stock power in V-drive 21'
Hard to make any recommendation for a change until we know what you're starting with. It would be nice to know HP, Gear ratio in v-drive, current prop dimensions #of blades and material to help. I've always said that V-drive props are like black magic and voodoo because 2 props with the same dimensions will work differently. I don't have a complete grasp on it, but I understand the basics. Try to post up some more info and you may get the help you are looking for.
If you are happy with the performance of the prop (how it carries the boat) then a gear change might be a better choice t lower the RPM's (which don't sound too bad anyway)

Oh crap, should’ve listed the hp. My bad... 454 Chev. -down to 550 hp now

 

[wildhorse]

What engine does it have?? It’s a V drive?

Lake Powell is a different story, because of the altitude, most boaters have to prop down (I did).

If you have a normal load of people, fuel, beer, water and toys, if can you get to the rated RPM (probably around 5000 RPM) quickly, your prop is probably right. If it will easily over rev under those conditions, you might need a higher prop.

We keep this in CA, elevation 1,200’.

 

[DRIVE BYE]

If you are just using her as a cruiser a 12x15 3 blade should be the go to right all?... and like Bam Bam said v drive props ARE black magic...

 

[BamBam]

If you are just using her as a cruiser a 12x15 3 blade should be the go to right all?... and like Bam Bam said v drive props ARE black magic...

I agree a 12x15 three blade would be a good place to start. He may be running one already, but we don't know cause he didn't get #'s off of the prop.

 

[wildhorse]

I agree a 12x15 three blade would be a good place to start. He may be running one already, but we don't know cause he didn't get #'s off of the prop.

I just looked at the prop and it’s a 12x14.

 

[2FORCEFULL]

first thing I always do is figure out what gear it has...mark the shaft on top of the upper and lower shaft, spin the top one full turn and see iif the bottom goes the same one full turn,.. if it does you are one to one...if the top goes one full turn , and the bottomgoes less than 1 it's under driven... and if it goes more it's over driven... look and see how far past one turn it goes on the bottom and that's the percent over..

 

[2FORCEFULL]

one other thing that happened on one of my schiada's….the trans wasn't going to high gear.... that was a head scratcher..

 

[2FORCEFULL]

your numbers look like the gears are straight up....did you check????

 

[2FORCEFULL]

and for what it's worth, tach don't mean dick if you don't know the gears....just say'n...