Cupped props, good or bad at slow speed?

[bobk]

A previous owner of our 48MY had the props cupped but the diameter and pitch are factory. During survey they turned up to WOT spec so I wasn't concerned. But now, I'm wondering if the cups help or hurt efficiency at our normal 8 knot cruise (1000 RPM). Any one know? It appears to be just a small cup, but I have no other experience with cups so my estimation may be way off.

By the way, we had the bottom cleaned ten days before starting our trip, so I started and ran the engines in gear at 600-700 rpm in the slip each day to try to keep the gear clean until we could leave. No luck, the running gear grew many small barnicles in that time, and it increased fuel burn from 5.6-8.3 gph and speed dropped from 8 knots to 7.5. Big difference percentage wise.

Bob

 

[Traveler 45C]

Is your gear painted?

 

[bobk]

It was painted with Trinidad in October, but that is long gone after all the miles. Thinking about 'Prop Speed' next time, but the diver tells me it doesn't prevent barnicles, just makes it easier to remove them

Bob

 

[Brian Degulis]

I'm thinking a cup would neither help nor hurt slow speed performance or efiecency. Larger diameter with less pitch would improve economy at lower speeds.

Brian

 

[Paul45c]

It was painted with Trinidad in October, but that is long gone after all the miles. Thinking about 'Prop Speed' next time, but the diver tells me it doesn't prevent barnicles, just makes it easier to remove them

Bob

I think your diver is wrong, Bob...I had personal experience with it on 2 applications. If applied correctly (which, sadly, was not the case for my 2nd app) it definitely keeps the barnacles from attaching. I was a full 8 mos. w/o anything other than a light coating of moss you could wipe away with a dive glove, even with this bathwater warm S. Florida water.

 

[bobk]

I'm thinking a cup would neither help nor hurt slow speed performance or efiecency. Larger diameter with less pitch would improve economy at lower speeds.

Brian

We run a lot in shallow water in the bays and ICW. This boat squats and loses speed in proportion to water depth. Would a larger prop help or hurt?

Bob

 

[Trojan]

Cupping helps at any speed. It helps hold the water on the blades giving them more push with less pitch. Which is good. Most or all Hats pick up speed in shallow water. The water compressing off the bottom helps lift the boat. I run with the trim tabs fully down 99.9%. of the time. My 43DC runs great this way. If I don't run with full tabs down the boat will not get up on plane.

BILL

 

[Bill Allen]

Ditto to what trojan posted. Run my 43 mostly at 7 kts. Light cup, I think it also important to not over wheel and to much cup can have that effect, early death to the motor. It's a fine balance and can be a pita to get the wheels just right. Bill

 

[Brian Degulis]

We run a lot in shallow water in the bays and ICW. This boat squats and loses speed in proportion to water depth. Would a larger prop help or hurt?

Bob

Definetly Hurt you will squat more in shallow water with a larger prop

Brian

 

[Trojan]

Bigger wheels won't make any difference unless your way off on WOT. Its just the nature of the beast. Try it with your trim tabs full down. It should give you some stern lift.

BILL

 

[chris]

Bob

Ditto on the tabs down. My 48MY runs 7.7kts @ 1000rpm with the tabs up. Tabs down it runs 8.5 kts. It seems to run better with the bow down.
Chris

 

[bobk]

Bob

Ditto on the tabs down. My 48MY runs 7.7kts @ 1000rpm with the tabs up. Tabs down it runs 8.5 kts. It seems to run better with the bow down.
Chris

Chris, are you stabilized? That's a big difference. I'll have to check for it.

Bob

 

[chris]

Bob
I am stabilized. I am sure it cost me some speed, but I gain speed with the bow down.
Chris

 

[Trojan]

I'm going to order new 3 blade props next week to replace the 4 blade on the boat now.


BILL