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  #1   IP: 192.186.122.174
Old 10-29-2018, 08:53 AM
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Prop Sizing and Placement

Morning Folks;

Thought I would start a new thread about the prop I have as I wasn't sure about its size due to being able to go 2-3kn at idle, plus vacuum readings - but the engine needs a good tune up in the spring before I can feel good about those numbers.

The pics below of the prop. it's a 13x7 2 blade in an Alberg 30.

- The gap between blade tip and top of notch is about 1/4"
- nothing is bent or loose for the prop to ride that high in the space. it is angle of the shaft log as built from what I can tell.

As some have mentioned - maybe going with a 12x 7 or the indigo 3 blade (I only cruise about) 10x7

The small gap at the top worries me, but I have no experience with this and so looking at discussion/advice

Sorry for the out of focus....
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  #2   IP: 76.7.129.62
Old 10-29-2018, 09:20 AM
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My understanding is that the space between the prop and the rudder should be 15% of the props diameter. Almost 2" for a 13" prop.
Dan S/V Marian Claire

Edit: If I am doing my math correctly you would need to drop down to a 10" prop to comply with this space suggestion.

Last edited by Marian Claire; 10-29-2018 at 09:45 AM.
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Dave Neptune (10-29-2018)
  #3   IP: 24.152.132.140
Old 10-29-2018, 09:46 AM
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General industry specifications were 1 ~ 1½ X shaft diameter for clearance between the prop tip and the hull. Any closer risks a water jetting effect on the hull.
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1977 Catalina 30
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prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others
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Dave Neptune (10-29-2018)
  #4   IP: 137.200.1.109
Old 10-29-2018, 10:14 AM
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I had a similar issue with a 13x7. Getting the Indigo 3 blade was a large improvement in motoring ability and made the boat much quieter. Maybe not so much with the Alberg, but for my boat the prop is under a flatish section that acts as an amplifier.
Seriously, if you are out of the water, get the Indigo. Way cheaper and easier than trying to redo the engine mounts and shaft log!
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  #5   IP: 76.7.129.62
Old 10-29-2018, 10:24 AM
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Neil. Is that spec for a prop in the open or in a aperture? IIRC the info I used was for a prop in a aperture. I should have added that in the post.
Dan S/V Marian Claire
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  #6   IP: 24.152.132.140
Old 10-29-2018, 10:36 AM
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Other than my own Westsail I never built a boat with a prop in an aperture. However, I'm not sure that matters. Prop tip proximity/water jetting would apply in either case. I'm not advocating for one standard over another, only providing what the standard was at several boat manufacturers. One thing is for sure, ¼" clearance is way too close by any standard so a reduction in Greg's prop diameter will be a significant improvement. Also, I should have mentioned in my earlier post that the dimension was a minimum.

Deadwood is a factor though. Greg's only thrust comes from the outer half of the prop's diameter so a size reduction hits him where it matters most.

edit:
Correction, the Freeport 41 and Gulf 32 both had aperture props and both were built during my involvement at Islander and Capital respectively. Tip clearances applied the same.

Here's another little oddity that comes to mind. If you ever get a chance to see a Gulf 32 out of the water, even a picture, look for the two hernias in the hull underbody a little aft of amidships, port and stbd. They are there to allow interior space for the engine to fit low in the hull.
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prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others

Last edited by ndutton; 10-29-2018 at 12:02 PM.
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