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Boat Speed & Fuel Burn
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Geoff Holmes



Joined: 27 Mar 2007
Posts: 2

PostPosted: Tue Mar 27, 2007 11:09 am    Post subject: Boat Speed & Fuel Burn Reply with quote

I am collecting data for the GB42s having twin Ford Lehman 135, CAT 210, CAT 375 & CAT 435 engines. I have fuel consumption as provided by the engine manufactures but need real boat speed information for the following RPMs; 1400, 1500, 1600, 1800, 1900, 2000, 2100, 2200, 2300, 2400 & 2600.
Please provide any information you have. Thanks.
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Davies



Joined: 19 Jan 2007
Posts: 46
Location: Westport Island, Maine

PostPosted: Wed Mar 28, 2007 2:46 pm    Post subject: fuel consumption vs speed Reply with quote

Great topic. Will you please tell us what you learn?
Davies
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BlueChip



Joined: 20 Feb 2006
Posts: 16
Location: Big Torch Key, FL, USA

PostPosted: Fri Mar 30, 2007 6:19 am    Post subject: Reply with quote

The following are the actual speeds that I get out of my 1990 42 CL with twin CAT 3208 NA, 210 HP each. I have Niad Stabilizers that cut the speed down between 0.1 to .03 knots. I cruise the boat at 1600, which seems to be enough to keep the engines clean and still be reasonable fuel economy (1.8 KMPG).

RPM'S SPEED KTS
800 -- 4.5
1000 -- 5.6
1200 -- 6.8
1400 -- 7.9
1500 -- 8.3
1600 -- 8.6
1800 -- 9.2
2000 -- 9.7
2200 -- 10.1
2400 -- 10.7
2600 -- 11.5
2800 -- 12.9
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Mike Kohut
Blue Chip
GB42CL 1163
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Geoff Holmes



Joined: 27 Mar 2007
Posts: 2

PostPosted: Sun Apr 01, 2007 12:14 pm    Post subject: Thanks for the reply, hopefully we will get more data. Reply with quote

BlueChip wrote:
The following are the actual speeds that I get out of my 1990 42 CL with twin CAT 3208 NA, 210 HP each. I have Niad Stabilizers that cut the speed down between 0.1 to .03 knots. I cruise the boat at 1600, which seems to be enough to keep the engines clean and still be reasonable fuel economy (1.8 KMPG).

RPM'S SPEED KTS
800 -- 4.5
1000 -- 5.6
1200 -- 6.8
1400 -- 7.9
1500 -- 8.3
1600 -- 8.6
1800 -- 9.2
2000 -- 9.7
2200 -- 10.1
2400 -- 10.7
2600 -- 11.5
2800 -- 12.9
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Joe



Joined: 03 Dec 2003
Posts: 11

PostPosted: Tue Aug 28, 2007 5:30 pm    Post subject: Reply with quote

I am looking at a GBi42 MY with a single 3208na 210hp and was wondering if this single would work well cruising at 8-9 knot or is this boat under powered?

Thanks
Joe
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Marin Faure



Joined: 13 Jan 2004
Posts: 1124
Location: Sammamish, Washington USA

PostPosted: Tue Aug 28, 2007 6:23 pm    Post subject: Reply with quote

Some people might consider it to be underpowered, but it will depend on how you want to operate the boat. A GB42 with two Ford Lehman 120s has "only" 240 horsepower total. So I would think a single 210 hp engine in a GB42 would get the job done as long as you are happy cruising at hull speed or just slightly higher and don't foresee a need to exceed that speed for any length of time. You'll want to make sure the boat is propped correctly so you aren't lugging the engine on the one hand and losing prop "power" on the other.
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C. Marin Faure
GB36-403 "La Pérouse" (grp)
Bellingham, Washington
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BlueChip



Joined: 20 Feb 2006
Posts: 16
Location: Big Torch Key, FL, USA

PostPosted: Tue Aug 28, 2007 6:30 pm    Post subject: Reply with quote

I would think that you can easily cruise at 8 kts with a single 3208 Cat and it looks like you should be able to hit 9 kts with it running at 2200-2400 the maximum continuous power rating. The only way to know for sure is to talk with someone with a single 210 HP engine or take that one out for a sea trail.

Good luck,
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Joe



Joined: 03 Dec 2003
Posts: 11

PostPosted: Wed Aug 29, 2007 9:17 am    Post subject: Reply with quote

If the boat had a single 3208TA instead of the 3208NA would it cruise at 8-9kn at a lower RPM.

Joe
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BlueChip



Joined: 20 Feb 2006
Posts: 16
Location: Big Torch Key, FL, USA

PostPosted: Wed Aug 29, 2007 10:42 am    Post subject: Reply with quote

If the boat had a 3208TA, you would have 375 HP available and it would definitely cruise at 8-9 kts at a lower RPM. However it might actually be too low to keep the engine producing the power that you need to keep it clean. It takes very little power to push a 42 at 7.9 knots, near its hull speed. The extra power is required to get it faster than that. My twin 3208's will loaf and load up if I run them at 1400 RPM @ 8 knots. I have to run them at at least 1600 RPM @ 8.6 knots to keep them from loading and burning cleanly.

If you are happy running at 8 knots, you do not need the extra power. If you want to run up at 10 knots, you will.
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Joe



Joined: 03 Dec 2003
Posts: 11

PostPosted: Wed Aug 29, 2007 11:07 am    Post subject: Reply with quote

BlueChip,
thanks for the information. What do you think would be the best rpms to run using the single 3208NA and what hull speed would that produce.

Joe
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Marin Faure



Joined: 13 Jan 2004
Posts: 1124
Location: Sammamish, Washington USA

PostPosted: Wed Aug 29, 2007 11:38 am    Post subject: Reply with quote

FWIW, I believe a boat has only one hull speed, and that's the maximum speed the hull can go through the water without inducing a huge amount of drag. It's based on a formula (don't remember what it is) and it has to do with the speed a wave that's the same length as the waterline of the boat travels through the water.

The GB has a semi-displacement hull (which I prefer to call a semi-planing hull), with a deep forefoot but a flatter aft section and hard chines. So this hull can be driven faster than hull speed without incurring the huge drag penalty of a displacement hull. However, the faster you want to make a GB hull go over its hull speed, the more power you'll need and the rise in fuel consumption can be pretty dramatic.

210 horsepower is more than enough to move a GB42 along at hull speed. To find out how much faster you can cruise, I think you first need find out the optimum maximum cruising rpm of the engine in question. This is different than the maximum rpm. You don't want to be overstressing, overheating, etc. the engine.

As an example of what I mean, the old Ford Lehman 120 is "happiest" running between 1500 and 1800 rpm. Its redline is (if I recall correctly) 2400 rpm. But you don't want to be running the engine up around there all day. So if you want to know how fast a boat powered with this engine will go on a run-all-day, "be nice to the engine" basis, you would want to find out how fast the boat will go with the engine(s) running at 1800 rpm.

I have no clue what the high end of the "normal" cruise rpm envelope is for the Cat 3208na, but other people on this list will.

Once you know what this rpm is, you (or someone) can figure out how much power is being generated, and after factoring in the propeller characteristics can figure out how fast the boat will be going at that power setting. And you'll have your "max cruise" speed.

The boat will go faster, of course, as an engine can develop more power than its "max cruise" power, but you're entering the "increased temperature, increased stress, increased wear" section of the engine's operating envelope, and you probably don't want to stay there very long.
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C. Marin Faure
GB36-403 "La Pérouse" (grp)
Bellingham, Washington
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Mike Negley



Joined: 22 Jun 1999
Posts: 4659
Location: Safety Harbor, FL USA

PostPosted: Wed Aug 29, 2007 6:48 pm    Post subject: Reply with quote

The 120 HP Lehman at 1600 RPM's puts out 55 HP with a torque rating of 150. The Caterpillar 3208 NA at 2200 RPM's puts out 102 HP and a torque rating of 243 (fuel consumption will be 5.5 GPH). You can easily reach your expected cruising speed with this engine and altho no longer in production, there are thousands of them in use and parts are still available including remanufactured short blocks. A single 3208TA (the same engine as the 3208NA, but with a turbo and aftercooling which boosts the HP to 375) will cruise a 42' GB at 11-12 knots effortlessly. The NA version at 210 HP is way under-rated, but can be run at sustained high RPM's for hours on end. It was one of Cat's best engines and very popular for stationary power (pumps, generators, commerical and pleasure marine).
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Joe



Joined: 03 Dec 2003
Posts: 11

PostPosted: Thu Aug 30, 2007 10:14 am    Post subject: Reply with quote

Thanks to all for the help.
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frank@qg.com



Joined: 16 Apr 2004
Posts: 83
Location: Milwaukee, WI

PostPosted: Mon Sep 03, 2007 5:51 am    Post subject: Hull Speed. Reply with quote

The calculation to determine a boat's hull speed is:

Square root of actual length at waterline multiplied by 1.34
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EXW



Joined: 31 Jul 2007
Posts: 10
Location: Seattle, WA

PostPosted: Wed Sep 19, 2007 10:27 pm    Post subject: Reply with quote

Frank - so that means that all GB 42's have a hull speed of approximately 8.68 knots? I read through this topic several times again tonight. I am interested in a 1982 GB 42 that has been repowered with twin Volvo TAMD71's at 375 HP each. It sounds like what Marin and others are saying is that while the hull cruising speed might be just above 8.6 knots the larger engines might have a cruising sweet spot (burning clean) that gets a comfortable cruise of say 11 or 12 knots. I am just concerned about fuel consumption at cruise but it also sounds like if I run the boat sensibly (like I would) that I can also find a "fuel economy sweet spot". Is that true or do many of you with larger engines (3208t, for example) see a conflict between the spped the hull was designed to run and the speed larger engines want to cruise at?

FYI - the latest Sea Boating Magazine (October) has an article by a naval architect on the evolution of the GB 42 and he seems to cast some doubt on whether the older glass hulls ('74 to '91) were designed to take the increased horsepower. Below is a copy of a response he sent me when I asked him about this in an email... Sorry about the long posting here - apparently I have been thinking about this alot!
ERIK,
Seattle, WA

COPY OF EMAIL SENT TO ME>
Dear Erik:

Thanks for the inquiry. The hull design is capable of the speed range
you indicate assuming the weight (displacement) is "reasonable". The
problem is the structure of the hull and whether or not the panel spans,
coupled with the longitudinal and transverse frames, can take the impact
loading that comes with the increased speed. I don't know for sure but
suspect that the structures of the older GB's were not engineered in any
great detail. In today's world much greater care is given to the
structure and most builders use the services of naval architects to
provide structural laminate engineering to insure there are no potential
problems with hull failure.

I think I would ask GB what speed their hulls were designed for before I
proceeded in any re-powering project.

Best regards,
Jack W. Sarin, P.E.
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ERIK
Seattle, WA
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