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Old 08-14-2017, 11:37 AM   #1
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City: Madisonville
Vessel Name: Nirvana
Vessel Model: 41' Litton
Join Date: Feb 2011
Posts: 181
Thruster Motor Change

Several people have told me I can decrease my thruster run time by going to a larger motor. There is a 6hp on it now and I can go up to an 8hp by simply swiping the motors. Everything else stays the same. Here's my confusion, if everything remains the same (prop, tube diameter, rpms) how will increasing the hp change anything? More hp should only come into play if the 6hp motor is being overloaded. What am I missing here. I don't mind the cost or labor involved but I'd like to hear from you guys before I make the plunge. Thanks.
Oh, it now takes about 45 sec. to make a 90 deg. turn in the harbor. I've been told it should be about half that.

41' Litton
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Old 08-14-2017, 11:50 AM   #2
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City: Cape May, NJ and Englewood, FL
Vessel Name: Irish Lady
Vessel Model: Monk 36
Join Date: Dec 2007
Posts: 3,897
The answer is "maybe". IF the lower unit can handle the extra power. IF the mounting is the same. IF the power cables were oversized with the old. IF the tunnel is big and deep enough. Get the point? Talk to the manufacturer.

Irish Lady
1984 Monk 36 Hull #46
Currently in Cape May NJ
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Old 08-14-2017, 12:13 PM   #3
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City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,176
More hp motor probably makes more rpm with same prop. Talk to mfr, often they use one gearbox and prop with different hp motors so the upgrade might be a no-brainer.

Before you do that, go put a voltmeter on motor terminals and have a helper run thruster. See what volts are there. If below about 10 (??) batt or cabling is not up to the job and that will have a BIG effect on performance.

Also, barnacles on the prop will kill performance.
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Old 08-14-2017, 12:48 PM   #4
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City: Litchfield, Ct
Join Date: Aug 2012
Posts: 5,859
You probably won't see that 45 second figure drop more than 5 seconds with a higher hp motor. I agree that the higher hp motor will probably turn the prop faster, but 1/3 more power won't result in 1/3 more rpm. Even if it did increase by 1/3, prop performance is non linear with rpm. All of which means I doubt that you will get more than 10% less turning time.

And as Ski notes, check for barnacles and voltage at the motor first. Heavy barnacle growth doubles my turning time.

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