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Old 05-04-2020, 11:51 AM   #1
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Cummins 6BT on a 40 ft crew boat

Hi,

I have a 40ft steel crew boat weighing approximately 18 tons which I use to transport my dive crew out to jobs. I'm thinking about putting 2 cummins 6 BT 210 HP with twin disc 502 2:1 gears with 24'' 3 blade props. Can anyone advise what type of speed I should expect to get cursing with these engines?
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Old 05-04-2020, 11:59 AM   #2
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Google 'Prop Calculator' and you should get some spreadsheets that allow you to enter variables...horsepower, gear reduction, prop pitch and diameter, boat weight, bottom shape ....to determine speed. Boatdiesel.com also has these.
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Old 05-04-2020, 01:03 PM   #3
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Is the 18 ton registered tons or true weight? Numbers can be very different.

36000lb is a lot of weight for two B210's. Probably won't go much over hull speed, guessing about 10kts top. Faster if true weight is less.
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Old 05-04-2020, 01:59 PM   #4
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Welcome aboard. I think that Ski is pretty much on the mark.
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Old 05-04-2020, 02:48 PM   #5
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I also agree with Ski. I have these engines and you won't see much better than hull speed. With 2:1 gears, I doubt that they would turn up to the rated 2600 rpm, unless the props' pitch was on the low side.
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Old 05-04-2020, 04:11 PM   #6
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What is wrong with the engines you presently have in it? If the combined hp is anywhere between 200 and 400 hp the new engines will not change the top speed.

Just for conversation, I would guess that if it presently has 300 hp you will not improve the top speed until you get close to 900hp.

If it is a full displacement hull, for all practical purposes you will never get it much above hull speed until you can run it over a waterfall.

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Old 05-04-2020, 10:43 PM   #7
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Quote:
Originally Posted by Andrew Farah View Post
Hi,

I have a 40ft steel crew boat weighing approximately 18 tons which I use to transport my dive crew out to jobs. I'm thinking about putting 2 cummins 6 BT 210 HP with twin disc 502 2:1 gears with 24'' 3 blade props. Can anyone advise what type of speed I should expect to get cursing with these engines?
Welcome to the forum. However, you won't get far just cursing at any engine.
However, so you may get more help re this specific issue, I've taken the liberty of moving your post to the 'Power Systems' area, from 'Welcome Mat'.
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Old 05-04-2020, 11:04 PM   #8
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One set of documents says the net weight is 10 tons and another says 15 tons so I just used 18 as a worst case scenario. So I really dont know the true weight. She is a pretty old vessel build in 1966 by breaux bay craft. I currently have 2 cummins 903s but one just gave out on me and parts are no longer easy to find so I am switching out. Plus each 903 engine weighs approximately 2700 lbs. If i had to give a rough estimate of her weight I would say between 12 - 15 tons.
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Old 05-04-2020, 11:17 PM   #9
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Were the 903's turbo or natural? Natural I think around 300hp. What speed did you get when they were both up and running?
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Old 05-05-2020, 05:18 AM   #10
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One set of documents says the net weight is 10 tons and another says 15 tons so I just used 18 as a worst case scenario. So I really dont know the true weight. She is a pretty old vessel build in 1966 by breaux bay craft. I currently have 2 cummins 903s but one just gave out on me and parts are no longer easy to find so I am switching out. Plus each 903 engine weighs approximately 2700 lbs. If i had to give a rough estimate of her weight I would say between 12 - 15 tons.
I think your looking at documentation numbers which have nothing to do with actual weight of the boat.

As others have said, whatever HP the boat had before, you would expect similar performance with similar HP.

Crew boats were designed for speed. Lots of workers need to be transported as quickly and safely as possible to the rig or jobsite. 210 HP per side doesn't seem enough without more info on the boat.
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Old 05-05-2020, 05:32 AM   #11
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Crew boats are generally made to plane and fairly flat. He’s got 10,000 lb of running gear so boat could easily weigh 16 ton. 800 hp would be minimum to get out of the hole and that might only get you to 13kn.
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Old 05-05-2020, 06:32 AM   #12
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The 903 were natural. They were some really old engines so I never use to push them hard. I would normally run at about 10 kn but at max i am sure I could get 14 - 16 kn. My options are two Detroit 671 inline natural or 2 Cummins BT. However I was considering the Cummins over the Detroit due to the weight difference, noise etc. Which pair of engines would you recommend to go with.
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Old 05-05-2020, 07:10 AM   #13
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I'd definitely pick the Cummins over a 6-71. Any chance you can get a higher HP rated version of the Cummins 6BT / 6BTA?
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Old 05-05-2020, 07:21 AM   #14
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Not at the moment. I am getting a really good deal for the pair of cummins 6 BT with gears so i'm thinking this is my best option.
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Old 05-05-2020, 07:24 AM   #15
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If you're already used to running at 10 kts, I'd say go for it. It certainly won't be any slower.
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Old 05-05-2020, 10:22 AM   #16
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Very familiar with these boats. Owned a 24ft offshore fishing boat made by Breaux Bay Craft. Is this the look of your boat?Click image for larger version

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I believe this 40ft has 671 turbos...will plane this hull loaded up.
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Old 05-05-2020, 10:57 AM   #17
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Quote:
Originally Posted by Andrew Farah View Post
One set of documents says the net weight is 10 tons and another says 15 tons so I just used 18 as a worst case scenario. So I really dont know the true weight. She is a pretty old vessel build in 1966 by breaux bay craft. I currently have 2 cummins 903s but one just gave out on me and parts are no longer easy to find so I am switching out. Plus each 903 engine weighs approximately 2700 lbs. If i had to give a rough estimate of her weight I would say between 12 - 15 tons.
Net tons are a volume measurement for documentation purposes. A net ton is 100 cubic feet of cargo space. It came from colonial times when a ton of cargo took up about 100 cubic feet. Net tons are a volume measurement of cargo spaces, basically the volume of the boat minus the engine spaces since cargo could not be carried in the engine spaces. Net tons have nothing to do with actual weight.
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Old 05-05-2020, 01:10 PM   #18
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yes that is the same type of boat.
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Old 05-05-2020, 01:12 PM   #19
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Do you have any idea what my 40ft would weigh?
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Old 05-05-2020, 01:39 PM   #20
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Use to do dive charters on a 39' Lafco with a pair of 6-71 nationals. Depending on how many people and diver gear, I think there's a good chance you will get it up on plane. It's not going to go fast. You may need to keep the weight forward to get it on top as the weight of fuel is in the stern and you're losing engine weight mid ship.

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