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1967 ericson 30 propeller question

meemee

Junior Member
Hi I have an ericson 30 with a yanmar sb 12 My present prop is 12 x 12 two blade.
The boat weighs 7800#

I motor alot on the Hudson river which being as estuary has two tides per day

The current is 2 mph

in the current the boat does 5 knots and when current is minuimum it does 6 knots

I have 4 inches from the tip of the prop to the hull.

I need more power in the current and wanted recommendations for a new prop. 2 blade ? three would be fixed as my budget is too. Is it possible to add to my existing prop?

lol 30 lwl 23 max rpm 3000, continuous the same
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
I have read that the clearance between blade tip and hull should be about 10% (min.) and better at 15% of the propeller diameter. Some also express this as multiple of the shaft diameter. Your boat might have a 1" shaft, and I would guess that the minimum clearance needs to be 2".
I have found that more blades gives more "push" for a given amount of HP, i.e. a three-blade prop will power a boat better in a chop than a two-blade.
It worked that way for our boat. We have used a two-blade fixed, two-blade featherer, and currently for several years a three blade fixed (narrow blade "sailor" prop). The current one gives us almost a half knot more in speed over the others, and of course costs us a half knot of speed under sail.
Everything's a compromise, as the old saying goes. :)

I am not a mechanic or a boat builder, so others here may have better information.
 

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
I think Loren said it: a 3-blade like mine will really help, but the cost is slower sailing, especially in light air.

I am often tempted by a folding 3- or -4 bladed prop, which for my boat pushes $4000 but does everything better. Even a new fixed prop is expensive.

Another way to look at it is that motoring at five knots doesn't seem bad for a 30-footer. I generally plan on only 6 with an E38.
 

meemee

Junior Member
e30

I have read that the clearance between blade tip and hull should be about 10% (min.) and better at 15% of the propeller diameter. Some also express this as multiple of the shaft diameter. Your boat might have a 1" shaft, and I would guess that the minimum clearance needs to be 2".
I have found that more blades gives more "push" for a given amount of HP, i.e. a three-blade prop will power a boat better in a chop than a two-blade.
It worked that way for our boat. We have used a two-blade fixed, two-blade featherer, and currently for several years a three blade fixed (narrow blade "sailor" prop). The current one gives us almost a half knot more in speed over the others, and of course costs us a half knot of speed under sail.
Everything's a compromise, as the old saying goes. :)

I am not a mechanic or a boat builder, so others here may have better information.

thanks Loren. How do you arrive at 2 inches with a 12 inch prop. I have a 3/4 shaft .Does that enter into the calculation or did you estimate 15 percent of 12 and round. A L S O...today i measured prop to rudder distance and thats NOT 4 inches. Perhaps that is why the 12 inch blade. I could modify the rudder edge by about two inches to allow a larger prop. Your take on thatis appreciared
Regards Meemee
 
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