30% prop slip?!?
 

Sup fellas, I have issue going on here with my powerplay xlt-185.... I dropped a 406 sbc with 390-400hp in her a while back and just finally got it all broken in. I started out with a 21p mirage and was over revving up too 5300k... builder told me the motor should only be going to 4400-4600... ok... so i put a 23p on it and still revving at 5k at only 51mph!!! how can this be? The boat pops right on to plane with the 23 like its nothing and at 3k I will hammer it down and seriously it will throw me back into my seat, but when i start getting to the 4700k mark it just looses its power. I will put a 25p on it this weekend but the prop slip is really bugging me... how can this be? Am my being a newbie and should just prop up?

You might get 100 different opinions here, so here is 101!
 

Find a good prop (generally repair) shoppe and tell them what the boat is doing and what you want it to do. Give them all the information you can as to hull shape, weight, drive specs... everything!

The perfect combination of all aspects of a prop is seldom available, but if it is a good shoppe they can let you know you might be sacrificing one thing to gain another.

If you are close with a "boxed" prop the repair shoppe can add or subtract to the prop to "tune it" in. I have had props stepped on so hard (de-tuned) you might think I was nuts, but the end product worked perfectly!

Generally these guys are truly professionals in their field and make it a lot easier to get to the end objective faster than experimenting with multiple props.

fixx
 

sounds like this was a v6 powered boat or a small v8 like maybe 305.. if your still running the drive for thease 2 engines then you may have the wrong gear ratio causing too much torque for the prop to hold the water..v6 ratio can be from a 1.65 which should be ok if your running a 3 blade with alot of cut or a 4 blade prop..but if its the 1.92 ratio then that is way too much and will need to be regeared to a 1.50...

4400-4600 rpms for a 400hp SBC is way low. I highly doubt the engine will make anything close to 400hp at that rpm. Any well built SBC should be able to run 5500rpms.

we are assuming it is an alpha drive? the diff between the 1.5 it needs and the 1.92 or 1.98 it could be is about 30% the first thing you need to do is check gear ratio. especially as a used alpha costs less than a new prop...

Yes, i have an alpha drive. Can i check the gearing to be sure by popping the cap off the top and counting the teeth?

I did at 406 in an 18 ft formula, used a Hydromotive 27" 4-blade, above 70 chine walk was incredible no pad bottom just rocking back and forth, there was nothing flat to get up onto to keep it stable. Pito tube mount was on side of transom half way up, when at speed it would come out of the water. Would drag the tabs just to try to get some stability. I Ran out of sack, bottom just plainly not designed for such speeds and dangerous. Be careful.


I ruined one engine due to poor fuel delivery, Pickup in the tank and fittings, lines, water separator, and fuel pump had to be modified. Melted #8 piston top when it leaned out at speed, hard lesson. Destroyed the alpha the next week just from loading ABSOLUTELY NEVER a HOLE shot, just slowly get on plane, slowly push stick forward. About the third or forth run it spit teeth out the side of the lower, as in a hole in the housing leaking lots of drive fluid into the lake. Exxon Valdez display, glad I was near home, limped in with all the knocking beating going on in the drive.

It should be stamped on the side of the drive upper.

I took a look this morning and its 1.50... Im going to try a 25p mirage this weekend and see what happens.

So... i thought i had counted correctly and then my uncle took a look at the gearing and he came up with 24/20.... 1.98!!! So this explains everything, and my uncle what like " how do you have a masters degree?" Anyways... ill be putting on my 1.5 backup drive on this weekend and try a 23 mirage + i have.