Just for the Record.-

[Sea Rider]

Say you have (2) same diam props with same pitch, one is cupped, the other is plain. What's the documented pro and con between both type of props. Asking this as don't have any experience with non cupped props and their water performance. Current ones in use were factory delivered cupped.

Happy Boating

 

[Patfromny]

The published pros for cupped are holeshot and handeling. The downside of a cupped prop is loss of top end. Just what I have read in my epic journey through propsilvania.

 

[Sea Rider]

To better holeshot, handling, top end speed can be solved by going +,- one pitch size increment/decrement while powering light to medium light combos as experienced by changing a 9 to 8 pitch. The issue is that can't possibly have all 3 worlds in one prop, as top speed opposes hole shot and viceverse. For me top hole shot is preferred, achieving just 2 KPH less at full throttle is something you can live with.

Happy Boating

 

[H20Rat]

Cupped also adversely affects backing up ability to some extent.

 

[Faztbullet]

The downside of a cupped prop is loss of top end

Actually on a performance hull it will add speed as the prop "bites" better,lift transom and you can reduce wetted running surface which equals speed...

 

[Patfromny]

Ok, that makes a lot of sense. I guess my 77 Crestliner isn't exactly a performance hull then. Lol

 

[Texasmark]

I don't have lab test results but I have run a lot of high rake, cupped props operated near the surface on non-professional, stock boats under 18' and 125 hp. If you go to the www.go-fast.com site and dig around you will find a listing for a BAM prop slip calculator. That calculator involves the following items: gear ratio, engine rpm, prop pitch, how fast you would go if those numbers were applied to a "screw" in wood....no possible slip, how much slip is in your current setup, and how fast you can expect to go with the slip in the equation.

You can plug in numbers in all of these items except the one of which you are curious and hit the calculate button. You can work forward or backwards in the calculator depending on what you want to know.

You can also go to the top of this page and select Boat Parts and Accessories; scroll down to props; select the performance tutorial and read about all the prop parameters and how they affect the prop's performance.

I'll guarantee you, with first hand knowledge of running cupped props since the Ranger Bass Boat I bought new in 1989 that cupping improves prop "bite" which reduces slippage and reduced slippage results in MORE mph, not LESS, besides the fact that especially on boats that run out, say upwards of 35 mph, you get the effect of one extra inch of pitch........which if you think about it is just saying that it bites better than a non cupped prop! What this also does for you is that, if you are in the category where it can provide a notceable effect, it allows you to run a 1" lower pitched prop for the same top end but the effect of a lower pitch in the hole shot which will improve it and if pulling skiers, quicker pop-ups!

On a 14 ft john boat and 7 ? hp engine, I doubt it would do anything for you and I doubt they'd be available anyway.

 

[Patfromny]

Thanks Texasmark, I have been having fits with this boat and new engine set up. That site is worth a peruse with the info I have on the props I have tried. Does the program allow for drop of in HP? one of the props I have is 500 over the peak hp mark advertised by evinrude. The next size down which added 2 inches of pitch, was a 1/4 inch smaller in dia. AND was cupped, dropped the engine 800. 800 rpm from a 13 to a 15 pitch lead me to believe the cupping was responsible for the additional 400 rpm loss. I will know soon enough as I have had the cup taken out of the 15 and will test next time I get up there. The go-fast site might help me better understand my next move though if this newly uncupped prop doesn't do the trick. Thanks again, off to play with that prop slip calculater.

 

[dingbat]

The cupping gives a prop that low end boost and provides lift.

My current boat had an aluminum Michigan wheel prop on it when I got it. Bury the throttle and the slip went thru the roof. Lack of cup, blades flexing, don't know.

Changed that out for a 3 blade ss stiletto. Hole shot improve dramatically. Lots of transom lift but had a tendency to blow out in the rough, tight turns.

Swapped the Stilletto for a 4 blade, Renagade offshore and never looked back.

 

[Texasmark]

Thanks Texasmark, I have been having fits with this boat and new engine set up. That site is worth a peruse with the info I have on the props I have tried. Does the program allow for drop of in HP? one of the props I have is 500 over the peak hp mark advertised by evinrude. The next size down which added 2 inches of pitch, was a 1/4 inch smaller in dia. AND was cupped, dropped the engine 800. 800 rpm from a 13 to a 15 pitch lead me to believe the cupping was responsible for the additional 400 rpm loss. I will know soon enough as I have had the cup taken out of the 15 and will test next time I get up there. The go-fast site might help me better understand my next move though if this newly uncupped prop doesn't do the trick. Thanks again, off to play with that prop slip calculater.

Something else changed!

What else was different about the cupped "Next size down"? You had - 600 rpm on the books, with everything else the same. Change another parameter and you can change rpm. One thing that can change is how much boat is in the water with the change in prop....stern lift-no lift- bow lift changes the wetted area and drag coefficient. Engine torque curve????? What happened to it when you changed rpm? Having dropped in rpm one would have expected it to move up a notch or two as with most engines when rpms go up past the torque peak, torque drops....how fast depends upon the engine design.

On cupping in the hole shot, with my last boat I could shoot the hole with the engine trimmed out beyond vertical....0 degrees trim. With it out that far and at low speed the boat porpoised but that worked in my favor as every time it went nose down, the prop ventilated and rpms shot up [Hp = (Rpm x trust)/5252]. When the bow popped back up, with the significant increase in rpm, the available thrust (cupping assisting in prop bite) was much higher and the boat shot forward. About 3 of those rapid cycles and I was long gone. Was a lot of fun.

 

[Patfromny]

Same boat and passengers. Tests run on the same day. Just a prop change. Weird. The only other difference in props was that the 15 (cupped) pitch prop was 13.5 and the 13 pitch was 13.75. Diameter. This difference should have also worked in my favor dropping down in size.

 

[Sea Rider]

Don't have experience whatsoever playing with 2 same prop pitches, one cupped while the other uncupped, as to know how much wot rpm will dial between both.

But when playing maximizing cupped props by 1 -,+ pitch, do find that OB will achieve somewhere -,+ 400-600 wot revs, the issue is that boaters are giving too much blind credit to the Rule of Thumb of + 200 wot revs which most of the times doesn't dials in. Playing with pitches and their wot rpm results is something that will surprise and even break your head.

Happy Boating

 

[Texasmark]

If you don't think this subject is complicated, just Google Marine Propeller design and get ready for an eye full. To give you an idea as to what all is involved, "water density" is one of the inputs along with temperature and on and on.

 

[Sea Rider]

Dialing a prop to match your boating requirements is darn complicated, too many tech issues are involved. That's why to each boat his own prop. What works top for your "personal requirements" will not work same for other boater, no matter if having same exact combo. Load and rpm works close to each other.

Happy Boating

 

[Patfromny]

I am truely finding that out the hard way. Lol. Delving into customizing props just to get into the right rpm range is a step I didn't want to take. This means I will need to buy the exact prop and have it reworked each time I need a new one. Something I see happening often due to the aging in of my nephew and neices. They will all be able to take the boat out by themselves next year once they take their boating courses. I wake up at night in cold sweats. Lol

 

[Texasmark]

Delving into customizing props just to get into the right rpm range is a step I didn't want to take. This means I will need to buy the exact prop and have it reworked each time I need a new one. They will all be able to take the boat out by themselves next year once they take their boating courses. I wake up at night in cold sweats. Lol

If your "sweet prop is off the shelf and a reliable source (iboats for one) you ought to be able to get another when you need it.

On the last 2 sentences I prevented that by being the "escort". Didn't take long that the "family" and INVITED GUEST were enjoying the boat together. Guarantee you, its not like borrowing your car to to run down the street to Burger King or such. Way move complicated animal and with the cost of repair parts, I just decided that they'd have to do it chaperoned with the rest of the family. If they wanted to go it alone they can.......when they can shell out the bucks for their own rig.

 

[Patfromny]

I agree there Texasmark but the whole reason for this rebuild was for them. You see, this was my dad's boat and my brothers and I had full access throughout our childhoods. We did plenty of damage, most of which I have repaired. I wanted our kids to have the same taste of freedom of jumping in the boat and ripping to the other end of the lake (5 miles long). I only get up there 2 or maybe 3 times a year so it was for me but mainly for the kids and my brothers as well. I never expected to fall into an etec but with the good fortune comes additional expense of repair. My Brothers have offered to help with my costs and so far I have been able to shoulder it. If there is any damage the responsible parent will pay for repairs but the headache of buying a new prop and then getting it uncupped or possibly re-pitched and cupped (my next option with the 13 pitch if this doesn't work out) is a major pain and time waster. I am thinking of just getting two customized so I have one as a back up once I figure out which one is right. That way I won't have the lead time factor to get back on the water. Still looking for a prop that will work off the shelf but it isn't looking good. Thanks for the interest and explainations, I am learning more trying to find a prop then I have throughout my whole build. Lol

 

[Patfromny]

I have been riding with them for the last couple of years and teaching them the lake and it's hazardous areas. There are quite a few. But the mind of a teenager is somewhat sieve like at times. I have made the boat easier to operate with the new one arm controller and the old motor had no trim so that should come in handy for the neices when running through the shallow bays. To be honest, I'm more worried about one of my brothers, he is quite careless. He drove the boat last year while the kids were tubing and I had to mutiny and throw him out of the captains chair. We use my other brothers pontoon to launch and retrieve and the menace brother was coming in way to close and way to fast. He also never, not for a second, looked forward while pulling the tubers. Drove me nuts. Kicked him out. Lol

 

[Texasmark]

Wish you the best with your good intentions.

 

[Patfromny]

Thanks, kind of a sticky situation but I can't explain the excitement and joy I see on the kids faces each year when they see my improvements. I have told them since the first year that this is their boat. They are overwhelmed each year by its progress and that has given me more joy and satisfaction than if I was just doing this for me. The speed/ prop thing is purely for me...and the pontoon bro. I think the 13 pitch, even though over revving, will satisfy the majority. It's the two of us boat nuts that want to see how fast she'll go. And my anal streak needs to see this through to its end. I can't wrap my head around "good enough". Just one of my many faults I guess.