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 Post subject: W50 - What RPMs Do You Use When Motoring
PostPosted: 19 Mar 2013 10:07 
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Midshipman

Joined: 05 Apr 2011 17:27
Posts: 66
I expect this answer is somewhere in the forum but after a lengthy search, I could not find it. When I bought my 1981 T37, the previous owner had placed some "hints" in a few places on the boat with a tape gun (those old kind where you can press white letters onto a colored or black tape). And he had put next to the tach either 1800 or 2000 rpms for crusing/motoring. I find that around 2000 rpms pushes my boat along on 0-2 ft seas at about 6.5 knots, so that's how I motor. Everything thing I have read says that a diesel runs better at higher rpms than at idle or low rpms for long periods. So my question is whether the rest of you run around 2000 rpms, or if you run higher than that? At 2000, there is a lot of throttle left but I figure I'm about at hull speed and don't want to cavitate, or over-tax the engine.

Thanks for any advice or input.

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Mark
Celtic Lady, Hull #315
Muskegon, MI


 
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 Post subject: Re: W50 - What RPMs Do You Use When Motoring
PostPosted: 19 Mar 2013 12:48 
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Skipper

Joined: 21 Oct 2006 15:36
Posts: 268
It seems that most if not all T37s with W50s were over-propped as originally supplied from the factory. Mine clearly was. I had a 2 blade 16"D prop with a 14" pitch. I had to cruise at 2000-2200 RPM. At that RPM I was clearly at hull speed. If I tried to run at ~2500 RPM or greater my engine would overheat, the engine was clearly straining and I would set up up stern wave you could surf on. Like you, I was concerned about 'lugging' my engine at these lower RPMs. About 5 years ago, I changed the prop pitch to 11 in. Now I can cruise at 2400-2500 RPM, but I still can't get to the max RPM of 3200, without again overpowering the engine. My guess is that about 9 in. is about the correct pitch for a 2 blade 16" D prop. I have chosen to not repitch mine again for no real good reason other than it's OK where it is now.

I have heard somewhere that Tartan over-propped their boats so they would back down better in reverse. I can't confirm or deny that premise. I find that the boat is equally poor behaved at both the 11" and 14" pitch.

As a point or reference, a friend of mine has an Erickson 38 with a Universal Engine with about the same HP as the W50. His boat was alos over-propped to the same extent so it wasn't just a Tartan thing.

Hope this helps.

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Jim Voelxen
Odyssey #191
Home Port: Osterville, MA


 
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 Post subject: Re: W50 - What RPMs Do You Use When Motoring
PostPosted: 19 Mar 2013 18:54 
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Skipper

Joined: 27 Oct 2006 10:32
Posts: 826
I think there's a discussion on props in another thread, but I'll give you what I found on Tortuga's Lie. We bought the boat with a 16" X 12" folding Martec prop. It would power along in forward fine at around 6.2 knots at 2200 RPM's, Westerbeke 40. The reverse sucked on this prop but it was an old design. A year ago, I decided to switch out props. I went with a Varifold 16" X 11" (as per manufacturer recommendation). I now am about the same in forward and can reach a little under hull speed at 3000 RPM's, so I think it is about right for this brand. The reverse is NOTICEABLY improved Very Happy I was real tempted to go with Flex-o-fold but they never returned calls or e-mails after several attempts. Varifold was very, very responsive in my book. Rick Steadman is the US rep for them.


 
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 Post subject: Re: W50 - What RPMs Do You Use When Motoring
PostPosted: 19 Mar 2013 19:32 
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Skipper

Joined: 29 Dec 2006 09:38
Posts: 544
I will second what Jim said about the stock prop being overpitched. Lots of production boats have this affliction in my experience. It makes the engine sound quieter but leads to a much shorter engine life because you have incomplete combustion by lugging the engine at all speeds. Most diesels are best run around 80% of max and should be pitched to max rpm at max hull speed in quiet water ( source is David Gerr's "Propeller Hanbook".) This would suggest that, with a properly pitched propeller (the stock ones are not) you would want to cruise about 2500 rpm and about 6.5 kts assuming hull speed of 7.x kts. . Properly pitching a fixed propeller can be a pain in the butt, but an externally adjustable feathering prop can work with some tweaking. As someone who facilitated the early death of my Westerbeke I commend your attention to this matter.


 
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 Post subject: Re: W50 - What RPMs Do You Use When Motoring
PostPosted: 24 Mar 2013 18:23 
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Skipper

Joined: 14 Jul 2012 20:36
Posts: 380
I run at 1850 with a light boat and a clean/smooth bottom at 6.1kts. It's a smooth rpm for me. When I go over 2200 rpm the fuel usage goes by a factor of two, so I pull it back a little. I'm using a Flex-a-fold, I think it's a 16X12. The worst backing prop I've tried.

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 Post subject: Re: W50 - What RPMs Do You Use When Motoring
PostPosted: 22 Apr 2013 20:11 
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Midshipman

Joined: 05 Apr 2011 17:27
Posts: 66
Thanks for the responses guys. I neglected to mention that the boat came with a Martec folding prop, but it was causing some vibration and kept loosening up the fairing compound where the strut attaches to the hull. I figured it needed to be re-balanced so I put the original Michigan prop back on. The boat seems to have better performance backing up now, but I think I've lost 1/2 knot of speed as the trade-off. A new 3-blade folding prop is not in the budget right now. But I might send the Martec in for re-balancing (if it hasn't been re-balanced yet - I think Martec only allows one re-balance), and then put it back on the boat.

Anyone have a thought on the Martec, RPMs, and motoring?

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Mark
Celtic Lady, Hull #315
Muskegon, MI


 
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