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GB36 with Twin 210 Cummins

 
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Bob Greenhaus
Summerhaus
New York, NY
Shopping for another GB
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Joined: 14 Dec 2003
Posts: 1079
New York, NY, USA

PostPosted: Thu Aug 03, 2017 6:29 pm    Post subject: GB36 with Twin 210 Cummins Reply with quote

Can someone with a GB36 with twin 210 Cummins tell me what the real-world fast cruising speed is?
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Orin Guidry
Pelican
Charleston
1989 GB36
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Joined: 27 Apr 1999
Posts: 464
Charleston

PostPosted: Thu Aug 03, 2017 7:19 pm    Post subject: Reply with quote

Define "fast cruising speed ".
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Bob Greenhaus
Summerhaus
New York, NY
Shopping for another GB
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Posts: 1079
New York, NY, USA

PostPosted: Thu Aug 03, 2017 7:58 pm    Post subject: Reply with quote

Cruising speed at or near the engine manufacturers maximum recommended causing RPM. Not necessarily the most fuel efficient causing speed. I assume one can always cruise at 8 or 8-1/2 knots to save fuel.
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Orin Guidry
Pelican
Charleston
1989 GB36
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Joined: 27 Apr 1999
Posts: 464
Charleston

PostPosted: Fri Aug 04, 2017 7:36 am    Post subject: Reply with quote

It is hard for me to give you precise numbers. As boat speeds increase, the condition of the bottom and running gear is a big factor. Additionally I boat in an area with swift currents so its a little hard to know what actual speed through the water is. I usually run around 1400 rpm and 8.0 to 8.5 knots.

Attached are two pages. One is from Cummins for the 6BT5.9M 210 and gives fuel consumption versus rpm. The second was from Grand Banks with single and twin Cummins in GB 32, 36, 42 and 49. Unfortunately the curves start at 2000 rpm.

The maximum speed of any displacement hull (hull speed) is governed by a formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). For a GB 36 that works out to 8.04 knots. Going significantly faster than that requires much more power. In my experience, (if my bottom and running gear are pristine), full throttle puts the boat on what I would call a “semi-plane” and it will get to 16 knots as noted in the attached curves. Any reduction in power seems to drop the boat back into the water and the speed dramatically falls. The two plots for the single and twin GB 36s are a virtual straight line between 2000 and 2600 rpm and makes me wonder if the points in between were actually measured.

I use wide open throttle for about five minutes on each use to make sure that the engines get up to temperature and to ensure that the cooling system is fully functional. The engines don’t seem to like full throttle so I don’t use it often.

Sorry this is not much of an answer but after owning the boat for 23 years, going faster than 8 – 9 knots doesn’t often seem to be important.
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bill.read
Pea Pod
Clinton, WA
1987 36 Classic

Joined: 18 Nov 2011
Posts: 154

PostPosted: Fri Aug 04, 2017 10:47 am    Post subject: Reply with quote

I have reached the 16 kt speed as shown on the curve with almost the same specifications. Yesterday, with 260 gals fuel, 190 gals. water, and 4 people aboard, running abeam of an ending incoming tide, 11 kts for a short term at 2100 rpm....cruising for me is best at 1400 - 1500 rpm...
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Bob Greenhaus
Summerhaus
New York, NY
Shopping for another GB
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New York, NY, USA

PostPosted: Sat Aug 05, 2017 7:37 pm    Post subject: Reply with quote

I used to like to cruise my '88 CL with twin Lehman 135's at 1850 RPM for about 8.5 knots. A marina neighbor had a '90 CL with twin 210 Cummins. My memory is rough but I believe he told me he cruised at 10 or perhaps 12 knots.

It sounds as if you guys are choosing to cruise at the same speed I used to cruise at, albeit at lower RPMs, which I assume is quieter and puts less wear on the engines.

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bill.read
Pea Pod
Clinton, WA
1987 36 Classic

Joined: 18 Nov 2011
Posts: 154

PostPosted: Sat Aug 05, 2017 10:49 pm    Post subject: Reply with quote

In the PNW the numerous current challenges coupled with the ever changing wind direction can hinder a steady cruise. With 1400 - 1500 rpm, my cruising speed in an average 6 hour day will vary from a low of 6 kts to a high of 11+ kts as read on the MFD. A perfect day is blue skies, flat conditions, 1400 rpm presenting a muffled hum, cutting through the water at 8.5 - 9 kts, burning less than 5 gals. per hour combined for both engines. Under these conditions, the admiral has a tendency to fall asleep, so I have to be extra vigilant for bad things in the water.
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