Too much cup???

[bigtwin]

I have a 14 1/2 foot alum. semi vee fishing boat, it weighs about 750 lbs (6gal. gas, 2 batteries, motor, trolling motor, and gear)
It is powered by a 1998 25hp twin cyl. Evinrude (last winter I installed 30 hp parts) I am running a 10 1/8 X 15 Ballistic prop., the motor is mounted so the a/v plate is even with the bottom of the hull. And I have smart tabs installed.
Last week I took it out for a performance test (me and both my daughters 360 lbs.), there was a slight breeze (2-3 mph) and 75 deg. temp.
I made several runs, and found my top speed (GPS verified) ranged between 33 and 34 mph., at 5750 - 5800 rpm., however I found the prop. ventilated badly in turns, so I had to reduce the throttle to less then half, if I applied the throttle too quickly coming out of a turn the motor would bounce off the rev. limiter and then hook-up.
The Ballistic prop. in this size has no cupping, so I brought the prop. to the local prop. shop to have the prop. cupped to reduce the amount of ventilation, and I know that cupping should cause a drop of about 100 rpm.
The next day when I picked up the prop. I noticed the prop. now had heavy cupping on both the trailing edge and the blade tip. I ask the guy at the shop if he thinks that amount of cupping would drop my rpm excessivly, he told me not to worry.
So I took it out again for further test, and now I can report the ventilation problem is now gone, however my top speed is down to 30 mph (with just me in the boat), and my motor is now struggling to hit 5400 rpms, also the handling is squirrelly, there is alot of tourque steer and the tiller handle jumped right out of my hand in a corner, this caused the boat to take a sudden, hard right turn, and that caused me to take a very long swim, on a very cold day!!! (37 deg.)
I am going to bring the prop. back, should I have him reduce the amount of cupping?
Remove the cupping from the trailing edge and leave the cupping on the blade tip??
Or just remove all the cupping???

thank you

 

[rndn]

Re: Too much cup???

Another very good reason to always wear a PFD. If you had hit your head your daughters would have watched their father in the water and unable to help him.

 

[bigtwin]

Re: Too much cup???

I know, this time I was lucky, and I have learned a hard lesson.
I was by my self when it happened at 7:30 am last satuday, the outside temp. was 37?. and the water temp. was 52?.
I went in about 75 yards from the shore, and I had to swim through the thickest milfoil I have ever seen.
I wasn't wearing my life jacket and I didn't have my kill switch tether attached either.
My boat took off across the lake in circles untill it finally got caught up in milfoil on the other side. It's a good thing my unguided “missile” didn't hit anyone or anything.
A bruised ego, a bruised butt, a cell phone that can only be used as a paper weight, and a $400 pair of prescription eye glasses were the only casualties.
I find it strange how such a small change in the prop. could make such a huge change in the behavior of the boat?
I would like to keep some cupping but not at the expense of 400 rpm or predictable handling.

 

[John_S]

Re: Too much cup???

IMHO, you are wasting too much time and money (and almost your life!) on such a small outboard used for fishing. Stay with low cost aluminum. Those rocks in MN hurt just as much as in NY.

 

[Dhadley]

Re: Too much cup???

Small change? That's a huge change.

One of the benifits of cupping - depending on where it is on the blade - is that it will allow you to raise the motor which I don't think you did. I do think the prop shop went "overboard" (sorry, couldn't help it) with it. Now, if you went to them and said to put in as much cup as possible it sounds like they followed instructions. If you asked them for just enough cupping to prevent the venting - don't need more lift etc - then they went way beyond what was expected.

Welcome to the world of propeller modification.

 

[bigtwin]

Re: Too much cup???

IMHO, you are wasting too much time and money (and almost your life!) on such a small outboard used for fishing. Stay with low cost aluminum. Those rocks in MN hurt just as much as in NY.

imported John S you are probably correct, I have a 10X15 aluminum Michigan Wheel prop. that cost me more then the SS Ballistic, and it can't touch the performance, besides I also have a lot of free time on my hands. the way I look at there are rocks everywere, up here in MN we just have more water covering them.


Dhadly thanx for your input, the "overboard" comment cracked me up, I hear you will be playing up in the Poconos all next week!?
I remeasured the motor height today and the A/V plate is a half inch above the bottom of the transom. I found if I raise the motor one mounting hole higher I start loosing water pressure.
I think I will have the shop take out most of the cupping from the trailing edge and leave some on the blade tips, if I understand it correctly taking off the cupping from the trailing edge will reduce the effective pitch, and leaving some cupping on the blade tip will increase the effective rake thus providing more bow lift.
If I am wrong in my facts please chime in and tell me otherwise. The reason I posted in the first place is know were I should go from here, and tell the guy at the prop shop what I need to get done.

 

[Dhadley]

Re: Too much cup???

I wish I was going to the Poconos next week!

As for the cup, adding or taking it away in specific places has more to do with lift than pitch and it won't change the rake angle. But yes, tip cup will provide bow lift.

 

[bigtwin]

Re: Too much cup???

Dhadly thanx again for your input, I will take it back to the shop tomorrow to reduce the cupping.

 

[Texasmark]

Re: Too much cup???

I wish I was going to the Poconos next week!

As for the cup, adding or taking it away in specific places has more to do with lift than pitch and it won't change the rake angle. But yes, tip cup will provide bow lift.

D, got me confused man. If tip cupping doesn't make for the 1" pitch change that you attribute to cupping when comparing/selecting props, then where is the cupping located that does that?
-------------------------
Big Twin, FYI:

I have 3 Ballistics for my 90 hp Merc (17/21/24XP) and all have the classic Ballistic chopped tails with "slight" cupping on the tail and between the tail and the hub. The blade comes off the hub in normal fashion, but at the end, rather than follow the normal geometry that other HP blades use, it looks like the blade (tip) was made to extend a couple of inches behind the rear of the hub and then it was chopped off square with the prop......clears the trim tab that way for one thing.

I don't know what the prop for your engine looks like.

I can get blowout on tight turns trimmed out for WOT operation, but I don't loose the engine (have no rev limiter) like with catastrophic ventilation. The engine gains about 200 rpm's (which it holds) in the turn and a couple of seconds after I straighten out it loads back up normally (no ports in the prop).....I only do this in calm water with no more than a few inches of chop....never tried, nor wanted to, in rough water.

I have it on an alum boat with 1 notch of jacking off the transom (no plate). Sets the vent plate about 1" above the pad. The hull is 3 step padded and being alum there is a center seam which is covered with a piece of angle material running the full length of the boat. Both of these things makes for a lot of turbulence in front of the prop when executing the tight turn, but dig in nicely for the turn.....no sliding across the water.

Reducing the throttle (1000 rpm's) or trimming in stops the "controlled" blowout but I usually do it intentionally for effect so why stop it. Grin

Mark

 

[rndn]

Re: Too much cup???

You can never have too much cup. I prefer a "D" cup myself.

 

[Dhadley]

Re: Too much cup???

Mark, I don't buy into that theory that cupping adds 1" for just the reason you mention. It all depends on where it is and how much it is and what style of cup they use. Adding cup will add bite in most cases, and decrease slip in most cases if it's added or placed properly.

My buddy who just won the Nationals in Mod-U had some cup removed from his prop and the boat got faster because it changed the lift.

 

[bigtwin]

Re: Too much cup???

Texasmark the prop. you discribe for your 90 hp sounds like the same thing as what I have (sans cupping).

I understand why you hold the throttle open in turns.
a) it sounds cool having the motor at warp speed through the turn.
b) you know when the prop hooks up again your motor will already be in it's max hp range.

I have decided to hold off bringing the prop back for now, I want to raise the motor another half inch to see if I could gain back some rpms and to take advantage of the extra cupping. I also blocked off the top half inch of my water intake screens to prevent the lose of water pressure, and I will bring along a wrench to adjust my steering trim tab to counter the increased tourque steer I noticed the last time out.
I will also remember to wear my PFD and attach the kill switch tether.

BTW rndn a firm "c" is alright by me

 

[Ben Paquin]

Re: Too much cup???

I found your story extremely funny! because you survived. You could have easily died in there.

Too much cupping and you take a dipping !

 

[Texasmark]

Re: Too much cup???

Thanks D. One of the areas in question is doing slippage calculations. Like for mine, the number is unbelieveably low if I don't throw in some cup pitch. Another would be someone trying to change from a non-cupped prop to a cupped one. I realize changing props is SWAGging, but you have to start somewhere.

On less cup, that's interesting. My 24P XP prop actually has (appears to have) less than the 21 and the 17, all built by the same mfgr. I originally thought they goofed in the design, but maybe not.
-----------------------
RNDN, caught me off guard and got a good chuckle out of that.

Mark

 

[bigtwin]

Re: Too much cup???

I found your story extremely funny! because you survived. You could have easily died in there.

Too much cupping and you take a dipping !

I had hoped my little tale would have informed more then entertained.
I didn't know that adding cup could change the handling, and cause excessive torque steer, however I did know, to always wear a PFD, and to attach the kill switch, shame on me!

I was going to post on the stupid human tricks while boating topic, however I think I have taken enough greif on this post.

This morning I entered my speed, rpm and pitch (adding 1" for cup), and fond my slip had increased after cupping, it was 12-13% prior to cupping now it is 18-19%, either Dhadley is right that cupping dosn't increase pitch, or adding cup dosn't always decrease slip.

 

[Texasmark]

Re: Too much cup???

If you noticed, he said that his racing buddy got more speed by removing some of his cup.

Mark