Prop Cup
 

my boat has problems getting on plane with offshore cleavers. The previous owner added 2" spacers to no avail. After some research it appears likely that i could benefit from adding cup. It is my understanding that this would yield results similar to increasing pitch will reduce rpm and slow the boat down. If true i will then need additional mods to regain the rpm and speed. Now with that said is it likely that the additional mods be able to recover all of the lost speed?

2nd question. Are there pro's/con's to bending the prop cup vs removing some material (by grinding) to create the cup?

Straight bottom deep V with notched transom, #4"s with shafts mounted even with the bottom without spacers, prop shaft's currently 2 inches below via spacers.

im running Bblade massaged 28 x 16.5 4 blades merc labs, #4's with 2" spacers, boat planes like a runabout at 3K rpm, props appear to have major cup added by bblades through grinding/thinning as best I can tell, but talk to Brett, he helped me tremendously and couldn't be happier

Moxy

Grinding in the cup is a complete no-no! I almost laugh and then cringe when I see that. The water can't flow correctly when you grind out a hollow for the water to run into it. We add cup via heat, dies hammers and many moons of experience.
Call me and we can talk about your issues.

Brett

I worked in a prop shop as a kid. Cupping is definitely a "black art" sort of thing.

Thanks Brett. I figured you're quite busy these days so I sent you a PM with my number, thought it might be easier for you to call me when you have a minute. Would like to know if this is going to cost me top end and how much.

FWIW, I have talked to a few people on this subject, who have been there before. It's a combination of things, but from what I gathered, "cupping" the prop will help lift the boat onto plane. I was going to try spacers also, but I am going to leave things as they are and have the props cupped by BBlades. I have a pair that run great, IF I can get the boat on plane, and I can't.

I have a similar setup, SSM5s old heavy 38ft set even with the bottom, running 4 blade Merc props. Good luck and please post results so that we can all learn something from it.

Here in lies the question, do you mess with props that have been dialed in for top speed or do you get a 2nd set of general purpose props that provide best all around performance? If the top speed is within a couple mph I don't care but if it's 5+ I will get a 2nd set.

I started with a baseline - 30p props that were available. I knew they were too short for my setup, but just wanted to get it running and on the water.

Came across a set of bigger diameter (same diameter as the 30s) 32p props that run probably 5mph faster, better cruise, but cannot get the boat up on plane without a 1/2mile burnout, and having people move up in the cabin to the bow for ballast. Going to get the 32p props sent over to BBlades and see if they can add the cup to help it plane. I don't really care about top end seeing as it's just bragging rights at the dock, and you rarely ever hit top end. I am looking for a faster cruise. My boat will never be "fast" anyways, 100+mph.

Cracks me up when people ask "how fast is it?" I just say I don't know, never hit the top end, which is true, I have no idea, never been over 5100rpm.

Yeah, I'm with you on most of that Frick, If I didn't have props that were already dialed in I would probably just have mine bent up and that would be that, still might do it anyway... I just wish I could get an answer on what the negative impacts of cup might be.

Someone who knows alot more about it than me will chime in.

Just considering the physics of it, seems that it will reduce overall prop slippage, and bring down your RPM due to the fact you are "grabbing" more water. Not sure if you would actually lose much top end, and the prop should be more efficient, proving more thrust, in turn requiring more hp to turn it to max or target RPM. IE: you may lose upper RPM, but gain back a little because you reduced the slippage. maybe? :confused: