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Old 02-03-2015, 14:32   #1
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Islander 30 with Volvo MD 7A

..this sailboat have some serious issues when motorizing with current..
We just move at 1/2 knots..
Tried to replace the old 2 blades for a 3 blandes.
The old propeller have this numbers:
Martec (I supposed the builder)
LH (left hand, it's a volvo)
120 (??) X 13 X 3/4
Ok, the 3/4 it's the shaft and the 13 could be the size.. but where are the pitching number (around 10..)
Second question.. a reliable propeller store?? I want a fixed one (I can forget the drag..)...
I was thinking in a 13" X 8 or 9 pitch.. I'm right??
Thanks in advance
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Old 02-03-2015, 14:59   #2
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Re: Islander 30 with Volvo MD 7A

My notes say I have a 16" by 10" prop on a 1"shaft. I think that is right.

She will do 6 knots in flat water, 16,000lb.

Make sure you get a sailboat prop, not a motor boat prop, there is a big difference.
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Old 02-03-2015, 15:39   #3
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Re: Islander 30 with Volvo MD 7A

Quote:
Originally Posted by garrobito View Post
..this sailboat have some serious issues when motorizing with current..
We just move at 1/2 knots..
Tried to replace the old 2 blades for a 3 blandes.
The old propeller have this numbers:
Martec (I supposed the builder)
LH (left hand, it's a volvo)
120 (??) X 13 X 3/4
Ok, the 3/4 it's the shaft and the 13 could be the size.. but where are the pitching number (around 10..)
Second question.. a reliable propeller store?? I want a fixed one (I can forget the drag..)...
I was thinking in a 13" X 8 or 9 pitch.. I'm right??
Thanks in advance
The first number is the propeller diameter, the second is the pitch.
How much current are you motoring into? Maybe the prop is spinning on the shaft or at the shaft coupler. Try calling Michigan Propeller. They should be able to help you select the correct prop.
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Old 02-03-2015, 16:15   #4
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Re: Islander 30 with Volvo MD 7A

Just to be clear, that boat has an MD7 A.
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Old 02-03-2015, 22:07   #5
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Re: Islander 30 with Volvo MD 7A

Garrobito I dont recall the name, but there is a reliable prop shop in the Alameda/Oakland area. They should tell you just what you need, but the first questions that come to mind are "is the present prop clean?" is the boat bottom clean? Is the engine running to proper RPM? It is also possible that the cones in the transmission are worn, and slipping. Is this boat new to you, or is this an on going problem?. If the boat has been sitting for a while, it is probably a good idea to have the bottom/prop cleaned, which is much less expensive than hauling, and a new prop. It may solve your problem. If you dont have a tachometer, you can buy a hand held one for about $100. The MD7A should easily reach hull speed in calm water, but might slow down in a chop. What does your boat do in calm water? Sorry for so many questions, but more information will get you better answers. _____Grant.
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Old 03-03-2015, 09:20   #6
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Re: Islander 30 with Volvo MD 7A

Michigan Prop! Great folks, very reasonable prices. I've had several props re-build by them, they do excellent work. Or rather the same prop fixed several times (I really need a depth sounder on my bow rider) They always go in looking like scrap metal and come out looking brand new! They'll hook you up with exactly what you need. I've bought new ones from them too. Props is all they do!
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Old 03-03-2015, 11:11   #7
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Re: Islander 30 with Volvo MD 7A

Current has nothing to do with speed through the water only over the ground. Your boat probably has a theoretical max speed of 6.5-7 knots and can achieve it with enough power in current or not. If it will do that in calm waters at the rev limit of the engine, messing with the prop probably won't gain you anything for bucking a current. If the engine isn't reaching it's max short term rpm at full throttle, then there could be issues with being over propped which will hurt you bucking into wind and waves as you won't be getting the max hp out of the engine.

Most engine manufacturers state a max sustained rpm and a max rpm for limited periods of time. The lower rpm is a speed the engine can maintain for long periods of time without shortening its operating life. The higher figure is for short term bursts where you need all the oomph the engine will produce but will shorten the engines life if run continuously at that rpm. Exceeding that short term rpm can result in really really short term life of the engine with catastrophic results.
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