Cummins 6BT on a 40 ft crew boat

[Andrew Farah]

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?

 

[Ken E.]

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.

 

[Ski in NC]

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.

 

[Comodave]

Welcome aboard. I think that Ski is pretty much on the mark.

 

[Ken E.]

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.

 

[PM]

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.

pete

 

[Peter B]

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'.

 

[Andrew Farah]

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.

 

[Ski in NC]

Were the 903's turbo or natural? Natural I think around 300hp. What speed did you get when they were both up and running?

 

[Sailor of Fortune]

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.

 

[Arc]

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.

 

[Andrew Farah]

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.

 

[rslifkin]

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?

 

[Andrew Farah]

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.

 

[rslifkin]

If you're already used to running at 10 kts, I'd say go for it. It certainly won't be any slower.

 

[ragin cajun]

Very familiar with these boats. Owned a 24ft offshore fishing boat made by Breaux Bay Craft. Is this the look of your boat?

I believe this 40ft has 671 turbos...will plane this hull loaded up.

 

[Comodave]

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.

 

[Andrew Farah]

yes that is the same type of boat.

 

[Andrew Farah]

Do you have any idea what my 40ft would weigh?

 

[O C Diver]

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.

Ted

 

[ragin cajun]

Do you have any idea what my 40ft would weigh?

Call Breaux Brothers. I'm sure they can give you any info needed on your hull. They made 100s of these crew boats. 1-337-229-4232, Loreauville LA.

 

[Lepke]

About 9 knots, tops. Unless your hull can plane and then you'll need more hp.

 

[Transaxial]

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?

I have the greatest respect for the 6BT210 but it is 359 cu in replacing a 903 cu in engine so just thinking a little more discussion might be in order like on what rpm was your 10 kt cruise at? You are talking about changing the transmissions to a 2.0:1 ratio and using the existing props. The big displacement 903 NA could chug along at mid teens for rpm and make good hp. What rpm is this new gear setup going to allow the 6BTs to run at? Max torque on a 6BT is 14-1600 so will probably be fine to run there. They will get good fuel economy there. Just remember that you are probably way over propped. No problem if you drive it like the old 903s, take her easy at 8-10 kts.

 

[FF]

I would invest about $125 for an EGT gauge, the newer units operate with std wiring and do not require a special cap tube to the exhaust.


This will easily show if you are asking too much from the smaller engine at any operating speed.

 

[timb7734]

I owned a trucking company and used 100’s of 903 Cummins, which we found pretty much indestructible to over 10,000 hours and 500,000 miles. The original naturals were rated at 320 horsepower if memory serves me. Rebuild the 903 would be my choice, as the 6BT does not produce the h.p., and the 903 was fairly light weight to horsepower.

 

[WaArete]

Nfn

That Cherubini is a good looking vessel....

 

[Lepke]

Launch services on the Columbia had a 40' crew boat with twin 12V71 twin turbo. Around 900 hp. It did about 18 knots and used about 50gph.