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Old 03-27-2014, 08:07 PM   #1
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Just got a 2006 34 Mainship trawler

Used to be a member of this site about 3 years ago, and are now coming back

I sold my 2005 mainship 30ft Pilot and just bought a 2006 34ft Mainship trawler, twin 240hp .
Loved my 30 but my wife wanted something bigger.

I want to travel slowly to save on fuel. Can anyone tell me optimum speed for fuel economy. keep in mind, I don't think my wife will tolerate anything less than 5 MPH,

It has a flybridge, twin 240 yanmars, weight about 20000#, overall length about 36 ft . not sure about wet length
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Old 03-27-2014, 08:32 PM   #2
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Originally Posted by jann View Post
Used to be a member of this site about 3 years ago, and are now coming back

I sold my 2005 mainship 30ft Pilot and just bought a 2006 34ft Mainship trawler, twin 240hp .
Loved my 30 but my wife wanted something bigger.

I want to travel slowly to save on fuel. Can anyone tell me optimum speed for fuel economy. keep in mind, I don't think my wife will tolerate anything less than 5 MPH,

It has a flybridge, twin 240 yanmars, weight about 20000#, overall length about 36 ft . not sure about wet length
Contact Mastry Engine Center in St. Pete. They supplied the engines, and can give you all the fuel burn info. They had a full time tech at the Mainship factory in St. Augustine that checked out all the engine installs.

Welcome to Mastry Engine Center

That being said, If you stay at least one knot under hull speed you should get good economy.
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Old 03-27-2014, 11:02 PM   #3
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Old 03-28-2014, 08:21 AM   #4
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Congratulations, I just tell the Admiral at 7 knots that I am wide open then when she's not looking I back off to 6 to 6.5 and enjoy the trip. Life is too short to be in a hurry. Enjoy your new boat!!
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Old 03-28-2014, 07:56 PM   #5
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my 390 has a single 370 Yanmar and I normally run 1900 rpm's which gives me about 8 kts. Fuel burn at that speed for me is about 3 gph. Yanmar says to run the engine(s) at 2500-2600 rpm's every few hours for about 15 minutes to get the turbo working. because you have twins your rpm will be different but 8kts should give you a similar fuel burn as the hulls are similar. the old Mainship site used to have fuel burn for each hull with each series of engine. Sadly Marlow did not keep the old info on the site.
Good luck with the boat. The 34's are really nice
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Old 03-28-2014, 10:11 PM   #6
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I have a 2005 40, so obviously a different hull, but the same engines. So far I've noticed that there is very little difference in fuel burn on my boat between 1600 and 1900 RPM. But there is a pretty big difference in speed, about 1.5 to sometimes two knots. So for me, 1900 is actually more efficient in that I get better NMPG. At 1600 I burn slightly less fuel but I'm going slower.

Depends on your perspective, I guess.
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Old 03-29-2014, 06:55 AM   #7
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Interesting
Per the Fuel consumption chart for the 240 yanmar given to me by Masey. 1700 rpm burns ~2.7 gph, while 1900 rpm burns ~ 3.5 gph.
That's a difference of ~ 2X .8 = 1.6 gph ( ie. for two engines )

so if your only picking up 1.5 knots ie. 1.65 mph, your only getting about one 1 mile per gal at 1900 rpm. Not very good

where is the paradox?

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Old 03-29-2014, 09:22 AM   #8
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Dougcole Interesting Per the Fuel consumption chart for the 240 yanmar given to me by Masey. 1700 rpm burns ~2.7 gph, while 1900 rpm burns ~ 3.5 gph. That's a difference of ~ 2X .8 = 1.6 gph ( ie. for two engines ) so if your only picking up 1.5 knots ie. 1.65 mph, your only getting about one 1 mile per gal at 1900 rpm. Not very good where is the paradox? Jann

Jann,

I have an 09 34t, same engines as you. It is relatively new to me as well, but the last couple of months I have been tracking this, and so far around 1600 rpm seems to be the sweet spot. Burn around 4gph and maintain a hair over 7k. I find the upper rpm range to maximize nmpg is almost 2100. There I am usually running 9-9.5k (almost doubles fuel burn though). Still not more efficient than 1600rpm but works when I have people out for short cruises that are rushing to get to the bar/restaurant/sunset, etc.
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Old 03-29-2014, 10:59 AM   #9
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To be totally honest, I haven't scientifically tracked my fuel burn at each speed, so I could be off base especially at the low end around 1600 rpm. Personally, I don't really enjoy going that slow, so I don't run the boat much there.

With no flow scans it is tough to tell exactly, but tracking hours/gallons burned from fill up to fill up we typically burn about 4.5 gph if we mostly cruised at 1850 to 1900 rpm. I realize though that this gets skewed as I don't run at a consistent speed the entire time. I'm quite certain that it is better than 1 nmpg though.

I have tracked my fuel burn pretty closely at higher speeds as I have now done two long (160 miles non stop) gulf crossings 1 long (110 miles non stop) and one short (70 miles) stream crossings all of which I filled up prior to and ran at 2900 rpm, about 15 knots depending on sea state, sometimes a little faster. I filled back up at the end of each of these trips, so my gallons burned are accurate. At that speed I burn almost dead on 15 gph.

Shoot, if I am only getting 1 nmpg at 1900 rpm I might as well run at fast cruise all the time.

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Old 03-29-2014, 11:35 AM   #10
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I will go 1600 rpm at ~7K. I think it is close to true trawling speed of the 34.

If I can get close to 2miles/gal...I'll be happy.

Now if I can only get the admiral to agree to only go 7K

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Old 03-29-2014, 12:25 PM   #11
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Thanks for the input on this guys, it is very interesting to me. I guess there is some efficiency gained in my boat with the longer waterline, but likely some lost in the greater weight and windage as well.

Our idea when we moved up to the 40T from our old Gulfstar 36 was to get a boat that could happily cruise at 15 knots when we wanted it/needed it. I'm very happy with the boat in that regard. When we bought the boat we looked at A LOT of 40's and 390's. Almost every owner had a good idea of how the boats performed at hull speed, no one that we talked to knew much about how they did at 15 kts plus. We took an educated gamble and went with the 40 over the 390 as we felt the new hull shape would be better at speed than the old 350/390 hull, which has a rep of being inefficient at faster speeds.
Ten months and 300 hours in I can say so far so good.

I wasn't so much worried about the fuel economy at less than 10 knots. Given our current lifestyle (busy with two teens and own our own business) we really need the ability to eat up a long crossing. For me, this outweighed the value of 5 gph at 8 knots compared to 3.5 gph at 6.5 knots, though I realize that for others that fuel savings may be very important.
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Old 03-29-2014, 03:50 PM   #12
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Congrats Jann... I sure like my 34T... I have the single 370. I called Mastry and they send me the data sheet for my 34 with the 370... it's a table - RPM's ~ burn rate..

please post photos of your new MS...
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Old 04-19-2014, 05:51 PM   #13
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congrats
we are looking at a 34t with twins and are wondering about access to the engine room. The curve where the shaft is seems awkward to step on . Any good ideas?
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Old 04-20-2014, 02:23 AM   #14
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Floor for 34T

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I know what your talking about.

Fortunately for me, the prior owner had a raised floor put in in that area. Makes access a whole lot easier.

My friend, who also has a 34T. Had a another friend ( who was a carpenter) put in a floor for him.

You got to find someone who likes to work with wood.
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Old 04-20-2014, 03:21 AM   #15
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Thanks for the input on this guys, it is very interesting to me. I guess there is some efficiency gained in my boat with the longer waterline, but likely some lost in the greater weight and windage as well.
.
You're right as to the gain at the greater length. While this doesn't mean your nmpg will necessarily be better, it definitely means your most economical cruising speed will be higher. I don't know the waterline lengths of either of your boats. But just for purposes of comparison and lets say we're talking 36' versus 30'. Then hull speed would be 7.8 knots vs. 7.1 knots. Doesn't sound like much but it's about 10%. There are so many factors with hull design being a huge one.

The ideal economical setting however, as you related, isn't always the most fuel efficient. Unless or until we're crossing an ocean or otherwise must extend range to it's maximum we'll never run for long at hull speed. I've seen a lot of boats recently use terms like "Range at Cruise" and "Range at Economic Cruise". Honestly, I think most of us have several different cruises. They're based on some combination of comfort, conditions, and fuel usage. That's where design really comes into play and match to engine.

I looked at a 63' Riva (using a non trawler just to show the difference) usage chart that has three distinctive ranges. From 11 knots to 15 knots is loses 20% of it's range. But from 15 knots to 28 knots nothing changes. The nmpg is flat. Then from 28 knots to 39 knots it loses nearly 20% again. So three good levels to run for combination of speed and efficiency. So 11 knots and 28 knots are the cruising speeds.

Meanwhile I just looked at tests on a Nordic Tug 39 which has a 37' waterline length so a hull speed of around 7 knots. Yet it's design gives an odd chart. At 4 knots it gets 10 nmpg. At 6 knots, it drops to 7. At 7.5 knots it drops to 5. By 8.1 knots it's down to 3.1 nmpg. By 9 knots it's 2 nmpg. 11 knots is 1.2 nmpg. Then between 11 knots and 15 knots the drop is less dramatic to 1.0 nmpg. And from 15 knots to 17.5 knots then it only drops to 0.9 nmpg. So from 11 knots down to 4 knots you save huge amounts of fuel with every drop you make. But then from 11 knots to WOT of 17.5 knots the change in nmpg is relatively small. It seems that it's a single engine that really plows up to 11 knots or so and then it starts rising just enough to do better from there up. I guess I'd then say that boat has idling speed of 4 knots, cruising of 6, cruising of 7, cruising of 8, cruising of 9, cruising of 11. Significant differences with minor changes but 6 knots to 11 knots is an increase from 1000 to 2250 rpm. So it's obvious that the efficiency is dropping. From 11 to 17.5 only increases to 3050 rpm so it takes much less to accelerate through that range.

I did find a 41' Mainship Expedition with a 380 hp Yanmar. At 4.8 knots 6.4 nmpg. From there to 8.9 knots, rapid drops. 5.7 knots is 4.5 nmpg, 6.5 knots is 3.2 nmpg, 7.3 knots is 2.3 nmpg, 8 knots is 1.4, and 8.9 knots is .9 nmpg.

But then a strange thing happens. From 8.9 knots at 0.9, at 16.7 knots is still gets 0.9 nmpg. In fact around 10-11 knots it is worse at 0.7. So just looking at fuel on that boat, you lose all the way up to 8.9 knots but once there might as well it any speed you want up to WOT. Much like the Tug, hard to get it going but once you do it has little change.

I recommend metering to find out your real flow and learn those ranges for your specific boat, not just one like it. You may be propped differently or have some other difference. And as you can see from just two examples it's not a linear drop as you get faster. Most boats have areas over their operating range of rapid drops and areas where the fuel usage doesn't change much.

I'll bet there are very few Mainship 41 owners who realize how little things change from 8.9 to 16.7 knots. The value of really knowing the flow you're experiencing.
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