CD 30/MD7A proper prop

[John MacArthur]

I have my 3-bladed prop in the shop for fixing the nicks on the tips, and the guy says it's too far gone to fix. It's a 13" (dia) by 12" (pitch). The guy says that if I replace it, I should go with a 13 x 9 or 10. He thinks the 12" pitch is too steep for the motor. My boat ran fine with that prop, moving along quite nicely at 1800 RPM, and I was able to rev it up to 2200 or so before I felt it was straining. Now here's the question. How many of you with 30's and/or MD 7A's know what your prop pitch is? Or, alternatively, what was the manufacturer's recommended pitch. I'm interested mostly in what you in fact have on your boats, but will listen to/consider all theorizing and opinion.



jmac@laplaza.org

[Leo A, MacDonald CD30K]

John,
I have a 1977 CD30 Ketch with a MD7A. The prop is a 2 blade 13 LH 13.
We can get approx. 6.5 knots at 2600 rpm on flat water. I would think a 3 blade 13 __ 12 is a bit aggressive. Can you get to the 2600 rpm range at all?
Good luck, Leo MacDonald CD30K #57 Heather Ann
(on the hard in Groton, CT)

[John MacArthur]

John,
I have a 1977 CD30 Ketch with a MD7A. The prop is a 2 blade 13 LH 13.
We can get approx. 6.5 knots at 2600 rpm on flat water. I would think a 3 blade 13 __ 12 is a bit aggressive. Can you get to the 2600 rpm range at all?
Good luck, Leo MacDonald CD30K #57 Heather Ann
(on the hard in Groton, CT)

As I understand it, 13 x 12 is a little *less* aggressive, although the prop is 3 bladed. I can get to 2600, but it overheats if I keep it there for very long. However, I have recently completely cleaned and repainted the bottom, and that may help. Also, the prop was pretty eroded on the blade tips, so I am sure I was getting cavitation at the higher revs. Do you know if yours is the original prop? Thanks for the help.

Johnny MacArthur (CD-30C "Stork", on a trailer, anxiously awaiting a prop so we can leave for Mexico)



jmac@laplaza.org

[Leo MacDonald CD30K]

As I understand it, 13 x 12 is a little *less* aggressive, although the prop is 3 bladed. I can get to 2600, but it overheats if I keep it there for very long.

The three blades are "moving" approx. 150% of the water than a 2 blade with same Dia. And Pitch. That 150% takes energy, i.e. HP. I would think that may be a factor in the overheating problem.
From working on the drive line 2 years ago, I get the feeling every thing was original, Bronze shaft and prop, no pitting, etc. Also the shaft to transmission flange had just a shear pin w/o a key in the keyway???? with a rubberized glue?? holding the shaft to flange.
If I were going to a 3 blade I would reduce the pitch a little and get someone REALY knowledgeable involved.
I remember some postings on this about a year or so ago, maybe a search would be come up with more info.
Enjoy your trip,
Leo MacDonald CD30K #57 Heather Ann




macdola@gwsmtp.nu.com

[John MacArthur]

snip I would snip get someone REALY knowledgeable involved.
I remember some postings on this about a year or so ago, maybe a search would be come up with more info.
Enjoy your trip,
Leo MacDonald CD30K #57 Heather Ann

Again, thanks for the help. I've been trying to access Michigan Wheel's Propeller analysis page, but I keep getting a "Netscape error" message. I alerted them to this, and they said "nothing's wrong with our site". I searched all the available postings on this list, and didn't come up with anything about MD7A prop specs. I'm sort of flying blind, now. Maybe later I'll try some different search parameters. For this trip, I'm getting the shop to do what they can with my prop (grind off the de-alloyed metal, and refair it). After all, it is a sailboat...



jmac@laplaza.org