Catalina 30 Universal M18 underpowered

Dec 6, 2010
50
Catalina 30 City Island
I thought the big work I was going to do was replace the prop shaft and cutlass bearing but I got sidetracked by electrical issues.

Further research into my power situation, I'm over propped. Max RPM should be 3200, cruising range 2100-2600. Last season full throttle all I could get was 2100 in gear, in neutral I could spin the engine up to 3200. The reputed prop for this boat is a 13x10. The prop on the boat is 13x12. Ive seen 2 inches down of pitch will add approx 500 RPM. I inherited with the boat a 15x8 prop. The 15 dia wont fit on the boat, but the pitch of 8 would put me at the right rpm if the 500/2in is correct.

Does it make any sense to modify the 15 in prop? Reduce the dia, trim an inch off each blade? Does this muck up the pitch? Or is the curvature and rotational progression not affected by the area away from the center of rotation, I'd thing not. I know the balance would be something. I'd have a pro do it.

Any thoughts?

Thanks,
John Rolka
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
John, it's a very good question. Perhaps some here could answer it. If it was my boat and/or prop, I'd ask the professional prop shops.
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
John:

I would highly recommend that you look into a Campbell Sailor prop. By the time you pay to have your existing prop reworked you will almost have a new one.
 

CalebD

.
Jun 27, 2006
1,479
Tartan 27' 1967 Nyack, NY
John,
There are a bunch of Catalina 30' owners over on the Moyer Marine forum and almost all of them use a 12" x 6" - 2 blade prop, or perhaps the Indigo or Campbell Sailor.
Yes, the Atomic 4 is not an M18 and in theory can deliver up to 30 HP but most of the time Atomic 4 owners are getting more like 18 HP, give or take.
I bought one of these 12" x 6" props from a C30 owner. I had a prop shop modify the prop to 13" x 8" for my Tartan 27' (has the prop in an aperture - not free wheeling like the C30). A good prop shop can add or subtract material and change the pitch as you request. I believe I paid $150 to have the prop modified for my boat. The used 2 blade prop I bought only cost me ~$75.
I'd be on the lookout for a used 2 blade 12" x 6" prop which should allow you to reach higher cruising rpm's.
You are not under powered, you are over propped!
 
Dec 6, 2010
50
Catalina 30 City Island
Thanks for the comments, I have talked to a prop manufacture about this and a prop shop since my hair brain idea to modify this 15x8. There's very little information on line about this Campbell sailer prop. It apperently three blades thin, I've found no price for the thing. My baseline is the 13x12 2 blade coventional prop on the boat, I don't know what effect the thin or 3 blades will have. My previous boat had an atom bomb, the problem with those are they are direct drive so the prop is spinning very fast, that's the reason for having such a small diameter with the thing, it's far over powered in terms of thrust, with the old boat with the prop I had before I got the indigo, I could do nearly hull speed with a clean bottom in nearly neutral making maneuvering and docking a real nightmare. And being a gas engine the thing too want to run at slightly higher RPM.

The shop I talked to about modifying the 15x8, the guy just said it was a lot of material to take off. I kind of wondering if I do this as a DIY project, fire up the grinder and cut some bronze. What I couldn't do is balance the thing and don't know if there would be an issue with modifying a possible temper in the metal...

My plan is stick with the type prop I have, because that's the source of the data I have and find the right prop / pitch for the least amount of money. Already got a nice light air/down wind sail and furler I'll put on a removable bowsprit.

Thanks again
John
 

CalebD

.
Jun 27, 2006
1,479
Tartan 27' 1967 Nyack, NY
John,
Making your prop bigger is only going to slow down your max rpms and make your engine struggle.
I stand by my assertion that a Catalina 30' with a 12" x 6" is about all you need. Your prop is in a strut in nice clean water so you do not need extra size to propel your boat.
If you really want the larger radius prop I'd try a 13" x 6" first. If that is still too much prop then change it to 12" x 6".
 

CalebD

.
Jun 27, 2006
1,479
Tartan 27' 1967 Nyack, NY
Actually you did not specify what type of transmission you have.
If it is direct drive then I stand by my assertion that a smaller prop will be better.
If it is a reduction drive tranny then it may be able to take a slightly larger prop.
If there is a Hurth transmission then it is probably a 2.x : 1 reduction drive.
 
Dec 6, 2010
50
Catalina 30 City Island
Actually you did not specify what type of transmission you have.
If it is direct drive then I stand by my assertion that a smaller prop will be better.
If it is a reduction drive tranny then it may be able to take a slightly larger prop.
If there is a Hurth transmission then it is probably a 2.x : 1 reduction drive.
Hi Caleb,

Thanks for the thoughts, I'm not proposing a larger prop on the boat, I'm over propped already, the modification I was looking at of the 15x8 was trimming an inch off of each blade, 13in dia is the largest thing that will fit under the hull, making it thus a 13x8. If the 500RPM/2 inches of pitch would put me in the right place.

The question is how much would it cost for a pro to do the modification and if I elect to do it myself,will the thing be properly balanced so as not to kill my cutlass bearing in the course of the season.

Regards,
John ROlka
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
1. how much would it cost for a pro to do the modification and

2. if I elect to do it myself,will the thing be properly balanced so as not to kill my cutlass bearing in the course of the season.
John,

1. Why not ask the pros? They'd know better than anyone here (I don't recall ever seeing a prop shop pro on this forum). Like I suggested earlier.

2. Really, really, really a poor idea. One of those "if you have to ask..." kinda questions.

Good luck.