Work Barge Drive Hull Questions

New to the site but very impressed with the quality of technical information. Have spent many hours reading posts on hull design, developable surfaces, mold making, mechanical loading, etc.

I would like some feedback on a work barge design to add a third hull to a standard 24x8' pontoon frame. I've attached two sketches below, here are the operating characteristics I'm working with...

Inboard electric - 5Kw PMAC 48V motor, thin plate pure lead AGM battery pack (C3 185Ah)
Solar - 2s x 2p 1.2Kw array, MPPT control, Vmp 70VDC, Imp 17.8A (not illustrated)
Propulsion/Steering - 1" SS shaft, 12x10 prop, spade rudder, stick steering, mech linkage, single cam
Fiberglass hull - 2 section female mold on CNC ply frames, 2mm gloss PVC planks, 2 planks/section
Displacement 3,000 lbs @ 35% immersion, 6.8" level draft, hull speed 6.8 kts @ 3.7Kw
R/T station transit ~ 3miles, station duty on alternate days, 9 months/year
Inland bay & river location, seas < 1 ft, max current < 2.5 kts, wind < 15 kts

I've spent many hours researching this information and have run into a few spots where I need some help. Your feedback on the mold and related loading would be hugely helpful -

I would like to be able to cut the PVC panels fore and aft for a 14 degree transom and reasonably bluff bow. Both should be developable surfaces, but I don't have the skills or aptitude to design in Rhino. a) Is there anyone who might be able to produce a couple of simple patterns I can send to a CNC to shape the ends of my 2mm PVC panels? b) How best to fillet the corners without sanding the gloss from the panels. I don't need a perfectly round joint, the hull is hidden, but I do need something that can take wax release and gelcoat without failing. c) What layup schedule is appropriate (generally) for the intended load - 600 lbs battery, 150 lbs motor/controller/cabling? How much to reinforce the top flange for this load, which will be through-bolted to new aluminum fame members I'm adding (in orange below).

I have some more questions but maybe this is a good point to pause. Thank you very much for your opinion and feedback.

-Jack

 

I think the closest thing to what you're looking for that you can get is something similar to this

 

Tansl,

Thank you very much for modeling this hull. It is probably a trivial solution for you, but is beyond difficult for me.

There are some additional features needed in order for the layup to mechanically release from the form. The bow will actually need 1" of overhang, and also a 4 degree pitch out from the (normal) sides is required. I accounted for this in the drawing of the tubular section, but could not in the bow shape. I will also need patterns for probably (?) two bow frames, as well as a pattern for a narrow 'batten' in the bow, to run from the keel to the top of the stem. This will allow me to align the first bow plank properly so that once fixed to the frames, the batten is removed and the other plank can butt against the first in perfect position.

I have already arranged CNC for six frames in the tubular section (attached). My intention is to bond each half of hull and bow to the frames, from the keel up, and when dry bond the other half.

I am thinking to first cut the two plank patterns out of cardboard, once I have laid up the frames on my lofting table. This to prove out the design and also uncover modeling discrepancies or artifacts (if any). Does this make sense to you? Can the two surfaces in the model be unrolled, split in half, and the half sections printed on a large plotter? I can probably find an architect such a plotter near me that could do this.

If you would be kind enough to help me further, please PM me. We can iterate to conclusion without filling this thread with detail that others may not find useful.

Again, thank you very much. This part of the project is the most complicated by far for me. Woodworking, fiberglass, repair, electrical, mechanical, I can do. But CAD is black magic Although I did learn how hulls were lofted by direct measure along the way, which is a remarkable process.

Jack

 

Regrets, forgot to upload the frame drawing...rushing too much!

 

Received your mail, I answer in the same way.

 

If I might ask, what is the reason for not letting the third hull be of the same length as the other two?

Also, a conventional shaft and rudder layup will be complicated and I'm also guessing that accessing the shaft seal may not be very easy. In addition to this I suspect that a single traditional rudder will be rather ineffective with lots of hull being aft of it and so to speak working against it (I tow boats and barges for a living so I know how important this is).

A simpler solution would be an electric outboard. No holes in the hull below the water line, better manoeuvrability (especially in reverse), simpler, better rudder authority since it's mounted behind the main bodies, more shallow, easier to put on the dry, easier to upgrade etc. The only benefits that I can see from going inboard (since you're looking at small engines) are that an inboard is unlikely to be stolen and it won't be in the way if you want to push the barge. But then again you could put two electric outboards (and push in the middle) and as an extra benefit not having to mess with adding a third hull at all.

 

Magnus, a smaller hull is much lighter, and I don't need the additional buoyancy. I'm trying to keep dry weight below 2,200 lbs, which is trailable, hopefully without adding another set of bunkers. The trailer saves me haul-out costs.

I understand the shaft log and strut layup is pretty direct with fiberglass. The shaft log entry is simply cut through a portion of the hull at the keel, a continuous section of dimensional fiberglass tube is fitted through the strut, strut through bolted, shaft log (to be) laminated in place, trimmed and faired...presto, shaft, strut bearing, cutlass bearing precisely aligned, ready to mount/shim motor to match.

Shaft seal access is a bit wonky, but it should (fingers crossed) be a one-time deal prior to placing the batteries. I'm thinking to have the batteries on a thick marine ply stand, bolted and removable, so that I can get at the shaft log in case something goes very wrong and I need to replace the cutlass bearing or seal. One question I have is will I need positive water pressure on the seal to lubricate the cutlass bearing, or will I get enough flow from ambient water pressure in such a shallow draft (<7"). I don't like the idea of a pump inboard, but if thats my only choice so be it. Its one thing to lose a hose and flood to the waterline, quite another to pump water into the bilge and fill it up if there's a pump malfunction!

I decided against outboard for a number of reasons, among them the ability to generate high thrust at low RPM with a large prop. I also want to resolve 'digging' with thrust-based steering, a problem with pontoons generally, which don't maneuver worth a damn anyway. I thought a larger, spade rudder would help in docking, e.g. gaining some sternway without musch steerage, then bumping hard forward (to port or aft) to 'twist' in place. Avoids the issue or prop walk on a large wheel and is a very effective docking maneuver.

Do you really think having the leading edge of a large rudder 8" forward of the outer pontoon transoms will cause big issues? If so, I can easily slide the drive pontoon aft, albeit with a sacrifice in longitudinal COG from the battery pack. I'm trying to keep that lump of lead as far forward as possible.

Some good thoughts, thanks for the input. Also apologies kapn, I do have two threads now, this one in design generally, and one in fiberglass construction specific to hull fabrication and layup.

-Jack

 

In my opinion all through hull fittings should be easily accessible for both maintenance and regular inspection. As far as the actual seal goes there are alternatives to water lubricated units that work very well and are absolutely dry without the need for water. I have a seal from Remmar REXMAR http://www.rexmar.net/english/idrotenute_ingl.html in my keel cooled boat that I installed a few months ago that I now have over 200 hrs on and couldn't be more pleased with the performance.

I don't know how your barge handles (will handle) but I do know that the position of the rudder (or thrust vector) in relation to mass and underwater body is of great significance. But this is only from experience, I'm sure others will have more to say about the actual math and theory behind it.

If you want the inboard and shaft layout you could consider a transom hung rudder. Easy install and easily removable fot trailing as well as simple and about as not heavy as possible.

 

Magnus, I agree with you on through hull fittings. I was not looking forward to installing a small seacock, piping, and pump just for a seal, seems like trading one point of failure for another. I will look at the Remmar fitting, thanks for the lead.

My sketch shows the inboard end of the shaft log just peeking forward of the battery pack. I would be able to inspect the seal and perform maintenance, albeit not terribly convenient. It's a 2' stretch down from the deck through a 2x4' hatch. Could be better but I think its workable (just).

The defunct 2-cycle 70HP that came with the boat occupies a cutout in the deck aft, where the foil above the prop is on level with the bottom of the two pontoons. The prop itself was somewhat inboard of the two pontoon transom(s) but not by much. I had thought to keep the inboard prop about 12" forward of this point and under the drive hull as illustrated in my sketch. The leading edge of the rudder would then be about 6" aft of the prop, then 3" of leading edge, 1" post, and 10" of trailing edge, 12" tall to match prop diameter. The rudder shaft will be hung from the transom, as you suggest. So the trailing edge of the rudder, and the thrust vector (?), will be approx 6" aft of the existing pontoon transom(s), which is further aft of the existing outboard prop. Does this seem workable?

Also, is there standard hardware available for a transom hung rudder? Where might I look this? It would also be good to find a pattern for the rudder foil itself. I'm resigned to fabricate it out of flexible 316 SS plate, welded to the post, unless you have a better idea?

 

I may have missed it but instead of an independent 'power' pod why not use an outboard motor(s)? Keeps it simple, swings up for beaching, steers (fit aux. rudders if needed), off the shelf (Torquedo), less weight, etc. One downside: more exposed to debris so you might have to fit a drop skeg/guard.
ps: I agree with squidly diddly. plus, trailering might be easier without the center hull

 

I'd avoid putting the extra hull into the water.

Have a pod/wave breaker suspended between hulls but below deck, and able to lift prop and shaft clear of water for beaching. See Wharram Cats. Similar to Thai longtail.

Also, adding an actual in the water 3rd hull could create lots of extra drag when so close to existing hulls.