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:

Yes, I've been pretty busy. I'll try to call tomorrow.

Brett

You are correct on your physics.
Also, cup affect the prop differently depending on where you add it. Very important....the amount of cup used and where. Additionally, different prop designs respond differently to cup too.

Brett

Brett

Brett, 2 props with the same amount of cup and pitch if one is an 3/4 to and inch smaller diameter would the smaller prop be worse at planing??

Most props I`ve tried just burn a hole in the water and will not get this heavy beast on plane until my current new Mercury labbed 17 3/4 X 27 . gets up no issues.
Ive run out of pitch and need to move up but planing is a huge concern. I`ve been finding some smaller props hence my question.

Yes the smaller diameter would be worse at planning if indeed you had the power to pull the larger diameter. More blade area will create more thrust.
It is harder to find propellers at 17.5 and above diameters.

Brett

It is harder to find 17.5" + 4 blades.
I would imagine if I did go down to a 17" prop but went to a 5 blade instead of 4 that should help things? ( more blade area)

Dan, what pitch are you looking for? I have two sets, you're welcome to try one.

Yes, 5 blades are still being made and do offer better planning in most cases.
I am supposed to be getting a number of 4 blade prop sets in for CR sales. Should be on the web site soon.

Brett

Frickstyle , sent you a PM

Is that how you add cup to a #6 style cleaver prop? I was always under the impression the blades on them were too thick to try and bend to shape, and thats why they grind out material on those style props? At least thats what mercury racing says regarding those cleaver style props.

Really, who at Mercury Racing told you that?
If Mercury Racing told you they grind out a hollow to add cup, Mercury Racing was mistaken in your question. I can promise you that unless something we developed over 2 decades ago has changed. And that is not the case.
You can bend and shape #3,4,5,6 drive propellers to some degree. It takes knowledge, background and talent. Some of that may have been lost or is being changed for easier production. But, it can be done.
Let's face it, the custom propeller ability is a fading art. Look at Mercury Racing, they don't offer custom propeller work anymore like when Dennis and I were managing the Laboratory. They offer one size fits all options now.

All the custom propellers we build at BBLADES come to you with 90 years of experience molding a handcrafted one off, if necessary, product.

Brett

I got that info from mercury racings website.

That's Andy and Mark in the photo . That's a pic from 12 years ago. Cool, I remember those days.

Oops I think it was 15 years ago..........

Thats probably around the time my buddys 17.75 28p four blade props with mercury lab finish were made. He had a set of 17.5 or 17.75 28p five blade props you reworked for his 38 cig a few years back. Those five 5 blades took a mile or two to even get the boat to lay over they blew out so bad. He tried adding spacers, switching prop rotation, nothing was helping. He bolted on those four blade merc props, boat planed off like a tournament ski boat, handled better, and gained a little top end with no cruise loss.

They must have done something right back in the good old days.

Yes we did some great stuff back in the past at that lab. Times have changed. I'm still a 4 blade cleaver fan. I would love to see those castings come back. And, would have voted to keep them in the line up in smaller inventory levels. There are still hulls that prefer that geometry.
Some old girls just don't like the stern lift the additional blades give those big V's no matter what you do.

Brett

How much pitch can be added to a prop? 17.5 x 24.... What can that be made into?

It depends on the prop.
On performance propeller for out boards, Bravo drives and #6's, you can not bend the root of the blade or 1/3 to 1/2 of the blade out from the root. It is stationary. Post that you are adding twist to the blade so the geometry is altered. It may not perform as a actual produced pitch prop will perform.
We do not recommend more than 1 inch.

Now if were talking 36 inch cruiser props, the answer is different.

Brett

Rolla 4 blade 19 spline????

Rolla props are super thick.
1" maximum and pounds of metal has to come off the casting to make it happen.

Brett

So are you avoiding against Rolla props?

No.
I advise you to get the best product you can to get you to your desired goal rather than taking whatever you can get and make it what it isn't.