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Old 08-06-2020, 09:20 PM   #1
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Is this speed realistic

https://www.yachtworld.co.uk/boats/1...assic-3662666/
Listed speed for twin 120 hp Lehmans on a 38000 lb boat, 10-13 mph.
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Old 08-06-2020, 09:25 PM   #2
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https://www.yachtworld.co.uk/boats/1...assic-3662666/
Listed speed for twin 120 hp Lehmans on a 38000 lb boat, 10-13 mph.
Sure, does it seem too slow?
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Old 08-06-2020, 09:42 PM   #3
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It seems a bit fast to me. But also it says MPH not Knots. So 10 mph is a bit over 8.5 knots which is about right with 120s.
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Old 08-06-2020, 09:49 PM   #4
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I too thought it a bit fast. My limited knowledge of trawlers based on reading posts on this forum and looking at listings seems to me 8.5 would be it's top speed. Hoping some GB 42 owners would post here as well. Both of your posts are appreciated. Thanks.
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Old 08-06-2020, 10:00 PM   #5
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I think that they could go faster than 8.5 knots but you would be pushing the engines more than I would like.
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Old 08-06-2020, 10:42 PM   #6
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WFO.....yes.....but realistically that is an 8 knot boat(9.5mph)
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Old 08-06-2020, 10:59 PM   #7
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Maximum displacement speed is square root of water line times 1.34. So I assumed water line to be 38 feet and square rooted that and multiplied by the factor to get 8.3 knots.
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Old 08-06-2020, 11:46 PM   #8
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No people, no gear, no water, very low fuel and throttle wide open on a dead calm day. Then just maybe you will see 13 MPH. Reality, it’s an 8 knot boat.
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Old 08-07-2020, 05:21 AM   #9
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No people, no gear, no water, very low fuel and throttle wide open on a dead calm day. Then just maybe you will see 13 MPH. Reality, itís an 8 knot boat.
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Old 08-07-2020, 08:37 AM   #10
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Thanks to you all. So many on this forum talk about running at 6-8 knots. The ones who run normally at 6 are just babying their gear and/or watching gph flow.
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Old 08-07-2020, 08:57 AM   #11
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No people, no gear, no water, very low fuel and throttle wide open on a dead calm day. Then just maybe you will see 13 MPH. Reality, it’s an 8 knot boat.

Agreed.



We have a GB 42 with twin Lehman 135s. We cruise between 8.3 and 8.5 knots (10 MPH) at 1800 RPM regardless of weight. It's very comfortable there. We push it every once in a while to 2400 to stretch the legs, at which point we go 10.5 knots (12MPH). 2600 is max loaded RPM for our engines.
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Old 08-07-2020, 09:05 AM   #12
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Quote:
Originally Posted by Swfla View Post
https://www.yachtworld.co.uk/boats/1...assic-3662666/
Listed speed for twin 120 hp Lehmans on a 38000 lb boat, 10-13 mph.
My twin Lehman 120s in my own 1972 Grand Banks 42 at 1750 RPM would get it to 8.6 Knots (9.89 MPH). What I thought of as a more comfortable RPM of 1600 would do about 8.3 knots (.54 MPH). 10-13 MPH is inflated sales speak..
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Old 08-07-2020, 09:31 AM   #13
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Thanks to you all. So many on this forum talk about running at 6-8 knots. The ones who run normally at 6 are just babying their gear and/or watching gph flow.
If it’s fuel burn you’re worried about (you seem to be like many others) just keep shopping until you find a single engined GB36. Why do you need such a big boat?

Once you buy a big twin you’ll be reading fuel burn posts on and on. The problem is that the new buyers had a lot of money and fuel burn wasn’t an issue for them. But for normal people fuel economy is. You probably can’t even count all the threads on fuel burn. But now you’re in a position now to do something about it.

Some say fuel burn is only a small part of boating expenses. Depends on how you boat. Where you moor your boat, where you spend the night like anchored out eating grub from home or at a marina eating out upper end and head’in for other places after ect ect. But if you’re a low-buck boater fuel is a big issue. Why else would all these TF members stress and worry about fuel? And they do.
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Old 08-07-2020, 09:39 AM   #14
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GB hulls are not displacement boats. They can be pushed faster with more horsepower

A GB is considered a semi planning hull. With more horsepower you can get them to semi-plane along with a lot more fuel consumption. The old 1.3 times the waterline length was never a scientific formula. Sort of like the definition of a good investment is one that turns out that way.
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Old 08-07-2020, 09:44 AM   #15
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Why do you need such a big boat?
My god, Eric! That's what the man wants!
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Old 08-07-2020, 10:20 AM   #16
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Why else would all these TF members stress and worry about fuel? And they do.
Not here anyway. I did not care about it with the 3.35 GPH at 9.5 MPH on the GB, thinking that it was fairly low, and I don't care about it at about 9 GPH at 16 MPH now because the last thing I want to do is spoil my fun on the water worrying of fuel burn rate.
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Old 08-07-2020, 10:29 AM   #17
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If itís fuel burn youíre worried about (you seem to be like many others) just keep shopping until you find a single engined GB36. Why do you need such a big boat?

Once you buy a big twin youíll be reading fuel burn posts on and on. The problem is that the new buyers had a lot of money and fuel burn wasnít an issue for them. But for normal people fuel economy is. You probably canít even count all the threads on fuel burn. But now youíre in a position now to do something about it.

Some say fuel burn is only a small part of boating expenses. Depends on how you boat. Where you moor your boat, where you spend the night like anchored out eating grub from home or at a marina eating out upper end and headíin for other places after ect ect. But if youíre a low-buck boater fuel is a big issue. Why else would all these TF members stress and worry about fuel? And they do.
You misread me. I'm not worried about fuel burn rate. I just wanted to know what the realistic cruise speed is. Sales postings, exaggerate too often. I gave a size range because I'd like to buy in my geographical area and need to be flexible to match availability. My boating skills are honed on twin engines. A single would be fine with a bow thruster. But again, it's availability or going into the yard for a bow thruster. I'm old enough to know my abilities on a single screw. Thanks for your post.
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Old 08-07-2020, 10:32 AM   #18
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Thanks to a few GB 42 owners for sharing their experience.
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Old 08-07-2020, 10:42 AM   #19
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13 mph is 11.3 knots, way too fast for that boat. If it actually did go that fast it would be plowing waves and burning a lot of unnecessary fuel. Figure 8 Kts or so for reasonable cruising.
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Old 08-07-2020, 10:46 AM   #20
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I think a good setup for me would be 36' with bow cabin and second cabin (aft likely). Twin engines like FL 80 or 120 hp or single cummins with bow thruster. Cruising at 8 knots is perfect. A genset is preferred but would consider an inverter setup with ample ah batteries for two days on the hook. Not likely to be more than 48 hours port to port. Don't need huge tanks. Still have a few life situations to get squared away but see myself ready in 12 months. Should give me time to read, look in person, join the power squadron once Covid is behind us. Then actively shop. In the meantime thanks for all the great members who share opinions and or knowledge.
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