Cupping question

[Frank Acampora]

Simple question, but here's the set-up: It is at speed in the avatar.

I have a 10 foot Cougar Cub tunnel hull powered by a Chrysler 50. It is swinging a bronze 10 1/2 X 14 two blade prop, trailing edge fully cupped from hub to outer edge of the blade, right into where the edge transitions into the leading edge.

The boat goes 40 MPH at 5,000rpm WOT, which is right in the middle of recommended rpm range, so I don't have a performance issue.

However: Above 35 MPH, it starts to porpoise and sometimes chine-walk a bit. The major problem is the porpoising though.

Does cupping increase bow lift? And, if I reduce the amount of cup (the length of trailing edge cupped, not the degree of cup), do you think this will reduce the porpoising?

BTW: As an aside, it burns about 5 to 5 1/2 gals. an hour running full throttle.

 

[Dhadley]

Re: Cupping question

Cupping can add bow lift, transom lift or overall lift depending on where it is. Is your prop a clever style or a round ear style?

Those boats did tend to porpoise a bit. At 35-40 the boat may not be going quite fast enough to pack enough air to create the natural lift a tunnel should provide. Set up will be critical.

 

[Frank Acampora]

Re: Cupping question

Thanks, Dad---Round eared prop--old Michigan AJC style. The cup extends all the way into the round part. About 4 inches on each blade.

Your air comment makes sense as the sponsons ride deep in the water, that is , deeper than I had anticipated. BUT: the engine weighs 160 and I weigh 160 too, so that's a lot of weight in the stern for a boat that is only 4 feet wide at the chines, and almost half of that is tunnel.

 

[sho3boater]

Re: Cupping question

Also try moving weight in the boat and see what it does, it may want it in a different place to balance right. Check for hook on the bottom, but those little hulls have this issue often. Last, if you can find a good prop guy they might be able to fix you up. Very common to have to customize the prop on a custom hull, or if used the prop might be modded for some other craft. Sounds like a fun boat though!

 

[hwsiii]

Re: Cupping question

Frank, here is a drawing showing the different parts of a prop and what cupping will do if it is in that area.


Blade Tip, Leading Edge and Trailing Edge

A - Blade Tip- The Tip is between the Leading edge and the Trailing edge of the prop.
1 - Cupping the blade here increases the Rake of the propeller which also increases the tendency of the prop to have more bow lift.

B - Leading Edge- This is the edge closest to the boat and it is the thinnest edge and cuts the water more like a knife with a clean edge.
2 - If the pitch is lower here than on the trailing edge this is called a Progressive Pitch.

C - Trailing Edge- The edge farthest away from the boat where the water leaves the blade.
3 - Cupping the blade here adds Stern Lift, more Pitch and helps control Cavitation and Ventilation to lower these factors considerably.

To design a prop for overall lift you have cup starting at the root of the trailing edge and continuing all the way around the blade tip and stopping right before the leading edge.

And if it was me I would be real careful of that hull at speeds and running into wakes or making turns, because I think I remember those boats tend to to turn over veryyyyyy easy.


H

 

[Frank Acampora]

Re: Cupping question

YEP! Tell me something I don't already know! I have already flipped it three times. Each time in a wake. First time was a big wake from a wakeboarding boat--I figured I would hit it fast, jump it, and get a little air under the boat---NOT! Second and third times were in progressively smaller wakes. Yeah, I am dumber than a rat in a maze--keep making the same misteake (sic). It is extremely unforgiving and flips before I can react. It is a fun ride, but expensive. Now, if big boats are on the water, I take it back to the dock.

Thanks for the drawing. I have the prop cupped from C to A, right where you drew the arrow and it seems to perform exactly as you say: A bit of chine walk due to stern lift, and porpoising due to bow lift.

So, I may decide to reduce the cup a bit and see what happens. Or, I may just install my 13 pitch three blade Columbian prop.

 

[Dhadley]

Re: Cupping question

Do you have to use a lot of up trim to lift the hull?

 

[Frank Acampora]

Re: Cupping question

No power trim. Trim bar is third hole out (of four holes) from transom. Cavitation plate is just slightly out or up from parallel with the bottom. Anything further in and she is too tight on the water, speed drops. Never tried the last hole out since on the hole shot I already need to lean way over the steering wheel to get her to drop onto plane. I figure that with that much trim, it will make it even more difficult to get the bow down.