Mounting components to the hull of a Highfield RIB

[HeadedToTexas]

Our dinghy is a circa 2019 Highfield 310CL with the narrow bow. I looked hard at adding their FCT helm and seat option and converting my tiller steered Yamaha 15 to cable steering, but it was both too heavy and too expensive. Easy to envision; hard to implement. And heavier than I want given the height of our Dinghy Butler davit system.

Most Highfield models come pre-fitted with threaded inserts in the flat section of the hull that accept mounting bolts for the various helm and seating options they offer. The threaded holes are hidden by the non-skid mats stuck to the sole. This flat section creates a tunnel for the fuel line and any wiring that needs to get from bow to stern.

My idea is to have a pair of aluminum boxes fabricated and mount folding swivel seats to the boxes. That not only creates far better seating than the tubes or Highfield's plastic bench seat, but it also better balances the load, opens up the floor space, offers securable storage in the boxes, and weighs a lot less than the FCT option. It will still be tiller steering, but far more comfortable than sitting on the tube. The blue boxes represent the location of each seat box and the insert shows my cardboard mockup of the rear seat box.

The inside of the boxes will be accessible through the hatch, but the inside of the Highfield tunnel is not accessible beyond the first few inches on each end. That means bolting through the hull. Four bolts per box should be fine and 5200 should keep the water out. What am I missing?

 

[DDW]

Seems like it would work. I'd use round "elevator" head bolts as the head is larger, lower in profile, and will give more bearing on the aluminum. Another possibility is Rivnut blind fasteners in the double floor. You might also have to watch for chafe where the seat box rubs on the tube. But could glue some patches on in anticipation.

I had an Apex 10' with the console and seat and bow locker - and sold it. Way too much furniture in a 10' boat, no room left for people. Works better on a 12'.

 

[HeadedToTexas]

Interesting idea with the elevator head bolt. I like it and they seem ideal for this application. I have had the Rivnut recommended for another project, but the point where these seat boxes cross onto the flat double floor is narrow and therefore thin and I think the Rivnut would either not fit or bind on the outer hull. At the moment, my plan is to not drill through the double floor at all and rely entirely on bolting through the outer hull.

Clearance between the curved or hex shaped outer sides of the boxes where they live along side the radius of the tube does make me wonder about tube flex. It's one thing to build boxes with sufficient clearance for no load inflation levels, but those tubes flex a lot under load and repeated chafing against the edge of the seat box can't be good. Have you seen slow motion video of RIB tube flex? I have talked to a Highfield dealer about their spec for tube flex and he did not know if there was such a thing.

 

[DDW]

Well the good news is that the boxes you make would be removable to service the tubes. But if it were mine I might get some matching Hypalon from a dinghy repair shop and glue on some chafe patches prophylactically.

I think McMaster will have elevator head or round flange head machine screws in 316.

 

[Dougcole]

Interesting idea. I have a 340 Classic, and as DDW said above even in our 11' there is very little room for furniture. We use our RIB for everything, a tender, fishing, diving, taking the dog to the beach etc. It needs interior space and though the little console ribs are more comfortable to drive, they just aren't practical for our usage. I even took out the center seat and use a YETI storage box in the center.

That said, I get tired of sitting on the tube all of the time and my back isn't what it used to be, so I have been kicking around adding in a removable stand up grab bar like a lot of microskiff flats boats have. It would allow me to run the dinghy standing up. Basically just a "T" about 4' tall that slips in and out of a female tube base mounted on the floor. I'd hold the stand up bar in the tube with either through bolts or pins, so I could take it out for storage.

The biggest issue I would have would be mounting the base to the floor. The bar would put a lot of leverage on the base.

You guys think those Rivnuts would work for that?

Doug

 

[DDW]

The Rivnuts are pretty good, especially the SS ones, but a tube cantilevers off the floor is asking a lot of the 3mm aluminum floor. You'd need a wide base made out of something to make that work.

 

[Dougcole]

The Rivnuts are pretty good, especially the SS ones, but a tube cantilevers off the floor is asking a lot of the 3mm aluminum floor. You'd need a wide base made out of something to make that work.

That's what I am thinking will be the issue as well. I'd be ok with a base that spans the entire flat section of the floor athwart ships.

 

[HeadedToTexas]

So my aluminum fabricator came back with an estimate of $2000 to build the two seat risers. I have not adjusted well to the last several years of shocking inflation, but that feels too high for what on the surface is a very simple project. Am I expecting too much (or too little) for a pair of aluminum boxes?

I have gone from the curved outer surfaces of my cardboard mockups above to three easier to cut and weld flat surfaces. Having sat on my share of aluminum fishing boat seats, I believe that ⅛" sheet stock is adequate for these seat risers. What am I missing?

 

[DDW]

Probably 0.080 is adequate for those, certainly 0.100. But it is a 4 x 8 sheet of aluminum, that is going to be ~$600 plus transport and markup, then around a day of cutting and welding, 8 hrs at $150? So $2K these days isn't beyond the far end of reasonable. $1K would be a bargain. Or buy a tig torch and have at it. It would be a pretty good beginning welding project, not too challenging.

 

[HeadedToTexas]

Probably 0.080 is adequate for those, certainly 0.100.

That would make a big difference. 4x8 sheets are definitely not cheap, but I don’t need that much material and ironically buying smaller sizes is much cheaper.

Or buy a tig torch and have at it. It would be a pretty good beginning welding project.

Hmmm, never thought of that. Very interesting. I’ll look into it.

 

[HeadedToTexas]

Still working on this. My original designs kept the seats as close to the tubes as possible, but after adding a seat to my cardboard mockup I saw that the lateral seat placement put the seat cushion and riders quite high in the little boat. The newest versions have the seats closer to the centerline. No problem for tiller steering and better balance with just one aboard.

I also went from a riser design with parallel sides to the latest design with a trapezoid shape that will be much stronger, perhaps enough so that 1/16" aluminum will be sufficient for the loads.

The question of the moment is welding vs. bolted assembly. SendCutSend.com (an amazing resource I found here, by the way) doesn't do welding, but they do have press in fittings that would allow the risers to be bolted together before being bolted into the boat. Each riser will have a waterproof hatch to allow storage inside the riser. The seat swivel will be bolted to the top and the bottom of the riser will be bolted to the boat.

Any benefits to welding that bolting can not match structurally? I believe welded risers would be water tight. While that is generally good, it might also mean that water leaking into the riser through top bolt holes would have nowhere to go and eventually add enough water weight to the dinghy to compromise my davits. The less than water tight bolted seams would seem to prevent that.

 

[DDW]

Of course with welded you can always drill limber holes at the bottom (which I would do). Welding theoretically weakens aluminum, though not very much in 5000 series. Bolting with 1/16 you better have a lot of bolts, or it will leave the edges with insufficient support and the panels may buckle. That is one thing about welding, the entire edge is supported. I don't see the trapeziodal shape as being stronger than square or rectangular, in similar dimensions, but if you like it go for it.