Moderated Discussion Areas
ContinuousWave: Whaler Performance
|Author||Topic: Double cupped?|
posted 08-27-2003 02:14 PM ET (US)
I've been reading lots about props on this and other forums. What is the difference between a cupped prop and one that is double cupped? I have two stainless props. One is a 13.75 X 15 Solas that looks to have a little cupping. The other is a Honda peformance 13.25 X 16 that has lots of cupping and vent holes. With only one inch difference in pitch, I see over 300 rpm difference at WOT. I thought the rule of thumb is 200 rpm for one inch of pitch. What accounts for the difference in rpm's? Does the extra cupping actually increase pitch and how can you tell if a prop is double cupped?
posted 08-27-2003 02:50 PM ET (US)
Double cupping adds 1" of pitch....for the most part. Looking at a prop is hard to tell how much cup is there.
posted 08-27-2003 05:35 PM ET (US)
When I bought my new motor, the dealer only had a double-cupped prop instock (14.25x18). We put this on so I could fish a tournament that weekend. I was only revving to 5400rpm and moving about 34 mph (medium load). After trying many different pitches, I decided on an uncupped of the same pitch. Now I turn 6000 rpm and run about 42 mph (similar loading). This was a HUGE difference. Much more than anyone expected...
posted 08-28-2003 09:55 AM ET (US)
Here are some prop term definitions as I understand them: The angle of the blade at the hub determines the base pitch; After that, twisting the blade gives it variable pitch moving toward tip and this is referred to as "cupping"; now, if the trailing edge is bent up to direct water more straight back it is referred to as "double cupping" (this is a very visible feature); now, if the blade tips are pulled rearward (reducing the swept diameter) it is called "rake". I play around with props almost every day and one thing for sure, I have a lot to learn and experimentation is the only way to select the perfect prop. It's not just the prop but the combination of all...hull design/shape/condition, engine foot shape/size, mounting height/set-back, etc, and of course the engine condition/power output...The prop is arguably the most important item in the entire set-up (remove prop and see how far/fast you can go!). All the advice and prop tables/formulas only get you in the ball park... happy Whalin'... clark.. Spruce Creek Navy
posted 08-28-2003 10:26 AM ET (US)
Clark....I menat looking at you can't really tell if it is cupped or double cupped.
posted 09-04-2003 10:17 PM ET (US)
I find the cupping most useful in turns (especially hard turns) where the uncupped would normally tend to lose bite, the cupped shrugs off the extra aeration, and continues to hold. This is especially noticeable if you start raising the engine on the transom. Cupping will also tend to allow you to trim further out than an uncupped prop.
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