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  #1   IP: 192.186.122.174
Old 10-29-2018, 09:53 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, 10:20 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 10:45 AM.
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Dave Neptune (10-29-2018)
  #3   IP: 24.152.132.140
Old 10-29-2018, 10:46 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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  #4   IP: 137.200.1.109
Old 10-29-2018, 11:14 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, 11:24 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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Old 10-29-2018, 11:36 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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Last edited by ndutton; 10-29-2018 at 01:02 PM.
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Old 10-29-2018, 11:52 AM
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I have no direct experience, but a look at the prop page shows Alberg 30s, Alberg 35s, and other similar boats using this prop. You might be able to find another Alberg owner for a review.
If you don't want the 3 blade, you might consider a 12x6.
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  #8   IP: 76.7.129.62
Old 10-29-2018, 11:56 AM
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Neil:Cool. I think you are correct that it doesn't matter. My notes from years ago are a bit hard to follow.
The number I have for the space between keel and prop is 30% of prop diameter. I do not think I have info on the thickness of the keel/deadwood but I know that was a consideration with my choice to go with a three blade instead of a two blade.
Dan S/V Marian Claire
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Old 10-29-2018, 12:21 PM
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At Islander we offered both three blade and two blade props for the Freeport 41, the two blade being the recommended prop for the following reason. An option was an indexed prop shaft brake that would align the two blade prop vertically in the shadow of the deadwood. On paper it was pretty cool, in practice it was a disaster.

The problem was it did not incorporate any disengagement safety. It required the skipper to remember to disengage the shaft brake manually before shifting into gear. With a mechanically indexed brake, the result was a number of mangled mechanisms returned to the factory. This could have been easily avoided with a hydraulically disengaged index supplied by engine oil pressure accompanied by a neutral safety switch. It would have eliminated the need for any skipper input.

Perhaps you'll remember my unflattering stories of the in-house Islander chief engineer at the time.
http://www.moyermarineforum.com/foru...ad.php?p=46926
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Last edited by ndutton; 10-29-2018 at 12:29 PM.
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