Converting our workboat to tugboat.

[Big Builder]

Hello everyone.

We have purchased a used welded aluminum workboat. Detroit diesel 6v-53 with velvet drive. I would like to convert this into a pushtug for our freshwater ( sheltered) barge operations. The barges are about 25 by 60' by 4' deep and we move building materials and materials for septics etc.

I was attracted to this vessel due to the quality of construction and it's excellent condition.

I am well aware that this is not the ideal canvas for a pushboat but I know it is possible.

We are experienced in fabrication and welding so that is not a problem.

However I have no real experience in the design of propellers, ratios, transmissions etc. I would reall appreciate some help.

I have attached pictures of the vessel and can post any others that anyone asks for.

Any help/advice is greatly appreciated.

We need to maintain a shallow draft. I thought I would increase the tunnel size so that I could increas ethe prop diameter.

[eyschulman]

Save money and headache hire a good navel architect to design the conversion.

[Big Builder]

Yes. That would be the easiest and ultimately it may go that way.

I need to keep in mind though that this is a small water boat that is attached to the barge. I am not worried about stability.

I have noted that there is more than 3" or clearance from the current propellor and the tunnel so I could realistically increse the propellor overall diameter by 4" and still have more than 1" to spare.

The only thing with that is the driveshaft is only 1 1/2 " in diamter. If I went to a 26" propellor is that shaft large enough?

Thanks for the help

[Tad]

What is the rating on the engine (HP output and RPM) and reduction ratio on the velvet drive?

[keysdisease]

Why don't you try moving a few barges around with it and see how it works? This would give you a baseline for performance and could help drive decisions.

A minimum rule of thumb for prop tip clearance in a partial tunnel would be 10 to 15% of the prop diameter. You could also change props for one with more blade area which should increase thrust and you can typically change the pitch on a propeller up to 1" without compromising the prop.

I think I agree with your unspoken thoughts that a 1.5" shaft is marginal, but it does look like it's been working for quite some time with the current setup. Sometimes if it ain't broke you shouldn't fix it.

Steev

[Big Builder]

Tad,

I believe the engine is 200 hp @ 2800rpm.

The velvet drive is 2.00 to 1.00 by the looks of it.

I have attached a few more pictures.

Thanks for all the help.

[BATAAN]

Maybe adding a Kort nozzle extension to complete the tunnel around the prop and use a 4 bladed prop would give it more bollard pull.
Here's a modern tugboat setup with a steerable nozzle.
I also agree with the poster who recommended you try it out on a loaded barge in a wind and see if it does or does not do what you require. Do you long tow, hip, or center push or a combo?
From eyeballing it, combined with the proposed barge size and potentially very heavy cargo, I think it's a little marginal for the application unless very protected water and free of strong currents. But if in good shape, this hull could easily be modified for bigger engine, shaft, tunnel and prop it seems.
A qualified and experienced designer saves many headaches down the road if this is a long term business investment.
I once worked on a 100x40' pile driving and salvage barge and we used a 50' too-light wood tug that we put three 6/71 Jimmies in. That was a crowded and noisy engine space but it kept the barge under control most of the time on windy SF bay.
Two big engines is always an advantage if you do a lot of close maneuvering around expensive things like yachts or docks.
Also, if you can rig spuds on your barges it makes everything much less dramatic, as they are so easy to stop and pivot and hold in place and you rarely need anchor or mooring gear.

[Big Builder]

Hello Bataan,

I will try and answer all of your questions.

- Yes very protected waters ( fresh water lakes) and zero current.
- I have seen this vessel with twin diesels. two 6v53's so they will fit but I will try it with this engine and go from there ( I also have a 6v53 turbo that I could install for more power. it comes it at about 275hp.
- We have spud on the barges and I agree with all your comments realating to them
- We need to stay under 25' so we don't require coast guard and TC approvals.
- We center push currently.

[BATAAN]

Sounds like you guys know what you are doing and should just try the vessel in limited service and see how it goes.
At least, that's what I'd do but I'm a cheap SOB.
Possibly some propeller guard would be useful, depending on depths and hazards.
This can be engineered to give Kort effect and more bollard pull, or could just be a dagger type skeg.
When I was involved, which was 1970s and I have not done it since, we center-pushed almost all the time, but hipped when putting the barge in limited space berths.
In 60s in USCG we only towed "boats" as I recall so long-towed and hipped only, and I remember at least one burnt out clutch/reduction gear on the port side 6v53 in a CG 44 foot surfboat trying to wrestle an 80 foot wood dragger into a windy berth in SF.
Another time the starboard wheel of the same 44 footer (CG44347) picked up its own fat nylon towline while revving, jamming it between the strut and wheel, wedging line in and pulling aft so hard on the shaft it sheared the engine bolts off and the engine reared up in the little engine room like a wild pony and thrashed about for a long time, then died in a fountain of various fluids and sparks.
I was on the dock watching the show at the time, but was in the engine room a couple of minutes after the incident and was truly amazed at how much damage could be caused by such a little incident in such a short time.
Kind of like handguns and tequila.

[Tad]

Shaft diameter is a function of the material its made of and the torque being applied by the engine/gear combination. 1.5" may be fine at 2:1 reduction, but if you want a real tug with some thrust you need a bigger reduction and lower shaft speed turning a bigger diameter prop. This means bigger shaft. The velvet drive is a straight through gear (no drop) so any modern gear with larger reduction probably won't work with the existing engine beds and shaft line.

I would look for a bigger reduction gear, 3.5:1 at least, hopefully more, and be aware gears have duty ratings just like engines. So you need one rated for the engines output at a continuous rating. Then a bigger shaft, new strut/tunnel/bearings/seal, and a new larger prop.