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Old 10-14-2011, 10:06 AM   #1
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1978 34' Mainship trawler "propellar woes"

Have recently started upgrading a 34' mainship trawler single screw. Having problems getting the hull speed that was originally advertised. I believe a 4 blade 20" x 17 was original. Anybody with thoughts about this. I have no problem with RPM, bottom cleanliness or pitch of hull. It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM.

Thanks
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Old 10-14-2011, 10:49 AM   #2
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RE: 1978 34' Mainship trawler "propellar woes"

I*owned a 78 Mainship for 14 years. Are you running the original Perkins T6.354 at 160 hp?

Assuming that engine, your OEM propeller should have been a 4 blade 20 x 20.* That would be with either the BW at 1.91 to 1 or 2.10 to 1.* You should be able to reach a minumum of 2450 rpm at WOT (with a calibrated tachometer).

I had a very heavily loaded boat and the 2.1 tranny ratio. I could get about 10.5 knots at WOT (2500 rpm) with the original engine.*

Fairing the keel (by adding fairing blocks ahead of the prop) I gained 250 rpm so then added one inch of pitch to my prop then could get 11 knots at 2550 rpm. Installing Big A$$ trim tabs (42 x 12) will get the arse end of the boat out of the water at 7.5 knots and really help the boat.

If you want to contact me privately try

jleonard@usa.norgren.com during "normal"* working hours (east coast time).

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Old 10-02-2016, 07:12 PM   #3
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My 1986 M34 MK III with the 200 hp. Perkins with a clean bottom and a reconditioned 23"x21" 4 blade does 16 knots (gps) @ 2500 rpm.
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Old 10-02-2016, 07:25 PM   #4
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Quote:
Originally Posted by harry hammett View Post
Have recently started upgrading a 34' mainship trawler single screw. Having problems getting the hull speed that was originally advertised. I believe a 4 blade 20" x 17 was original. Anybody with thoughts about this. I have no problem with RPM, bottom cleanliness or pitch of hull. It gets to about 8mph at 1700rpm and then just sucks aft downward to 2350RPM.

Thanks
That's about right for hull speed for a 34 footer given the old rule of thumb formula. I'm guessing you're really talking about top speed.

You mention .... 4 blade 20X17 was original but you don't say that's what is on it now ..... so what is on it now ?

What engine, what horsepower ? What gear reduction ?
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Old 10-02-2016, 07:58 PM   #5
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This post is 5 years old.....just FYI
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