Cobia tower / new top on a 306 Bimini - anyone done it?

[Xavier]

Hello all,

I own a 1999 306 Bimini and I am looking into a full top replacement that would include a cobia tower, and I would be very glad to hear from anyone who has been down this road on a 306.

A bit of context, because it drives the whole problem: the boat lives in Conakry, Guinea, in West Africa. I fish offshore for blue marlin and sailfish. What I am after is height - a standing position above the top so I can watch the trolling lines and spot fish well behind the boat. I do not need a full second helm. Steering alone at the top would be plenty; engine controls up there are not a requirement, and I would be happy to handle throttle from the lower helm or add a wireless autopilot remote.

What I would like to know:

1. Has anyone here put a tower, or a new top with an integrated cobia tower, on a 306 Bimini or a Canyon 306? Photos would be worth a lot to me.

2. Does the factory hardtop work as the floor of the upper station on this hull, the way marlin towers usually do, or does the top have to be replaced or reinforced first?

3. Which shop did the work, and what did it run? I have written to Atlantic Towers and to Bright Light Welding to ask, but first-hand experience beats a brochure.

4. The hard part: the boat cannot come to a US yard. Has anyone had a top or a tower built from measurements and photographs, shipped as a kit, and assembled and welded locally? If there is a measuring procedure or a template that makes this reliable, I will follow it to the letter.

5. Anything I should know before committing - deck attachment points on this hull, added windage, effect on stability or on the ride?

Many thanks for any pointers.

Xavier

 

[DennisG01]

Right up front, I have not done this modification.

While "physics works", you're not going to adding THAT much weight compared to the boat's dimensions that it would be a major concern. I highly doubt you'd notice anything... or very, very minor... when no one is up there. When you and another guy are up there Im sure you will notice a difference in some cases with the amount of boat rock - but since your weight is relatively centered, it won't impact the boat as much. As I said, I haven't done this specific mod - but just going off of years of other projects and being on/around boats my whole life, I think you'll be just fine from the "extra weight" standpoint.

Now... could you use the existing top as your new "floor"? The aluminum structure is MORE than capable of handling this. Concerning the actual top... correct me if I'm wrong, but in your boat it's a beefy structure of reinforced fiberglass? Again, that should be fine - but get on up there and walk around - don't be gentle - walk hard. Going with that assumption...

Now, the fun part... the "thinking/ideas"...

First thought...

-- Fashion an aluminum structure (tubing) that goes around the perimeter of the top. Think of something like the brass bar used a footstep at a bar in a restaurant. Let's call this the "base". To attach it to the top, fashion short, vertical legs (same aluminum tubing) in a numerous places (maybe 6" high). These would be welded to the base. At the bottom of these legs would be 1/4" plates that are maybe 8" long. These would then get thru-bolted to the top (with the same size plate being used as a backer plate .

-- You know have something that you can build upon AND you have significantly increased the rigidity of the existing top.

There must be aluminum welders somewhere around you - search around - I'd bet you can find someone that's being welding their whole life - maybe just working out of their house. But you'll know how good they are once you look at their work (look at the welds). I think the cost of getting something fabricated here in the States, shipping it, then STILL having to weld it together is not the way to go. Plus... what if it's not quite right?

As I'm writing.. I just remembered that there's a nice guy on here that did something similar to his Grady - I believe it was a 22-footer. I think he said he was very happy with it and it didn't alter the performance much. His name is Luckydude - reach out to him or search for his thread on this.

 

[Xavier]

Thanks Dennis, that is genuinely useful. Let me correct one thing straight away, because it changes your first idea.
My top is not reinforced fiberglass. On this boat it is a canvas top on a tubular aluminium frame. Grady-White confirmed to me last week that a 1999 306 Bimini left the factory that way - a soft top on tubes, not a rigid hardtop. Mine is the original structure, re-welded in one spot and reinforced in several, and it shows no weakness today. Measured width: 2.20 m, about 7'3". Photo attached.
So "use the existing top as the floor" is off the table as it stands, and that leaves me with two questions.
1. Is there a sensible way to work out whether that aluminium frame could carry a person standing on it? What would I need to measure - tube outside diameter and wall thickness, span between legs, how the legs land on the deck? I would rather calculate it than find out the hard way.
2. Or is the right answer simply that a cobia tower should not touch the top at all - legs down to the deck and the console, straddling the existing top? That is what I understand the usual architecture to be, and it would make the strength of the top irrelevant.
On fabricating locally rather than shipping a kit: point taken, and you are probably right. One caveat on my side - I have good stainless welders available here, but aluminium is a different trade and I do not yet know what I can find in Conakry. Worth noting too that welding onto 6061-T6 drops the yield strength in the heat affected zone by roughly half, and it does not come back without heat treatment. So if anything gets reinforced, I would want it bolted with backing plates the way you describe, rather than welded onto the existing frame.
I will try to reach Luckydude. Thanks again.
Xavier

 

[DennisG01]

No problem, Xavier. We just need to shift gears. All good!

So, while I know that Al loses strength like you said, I did not know the amount. Good to know. Although, given how strong aluminum tubing is to begin with, it probably is of little significance for the amount of weight we're talking about (even taking into account the stresses the tower takes while operating a boat in "fun" conditions). Yes, Al welding requires a different skill set - but I'm sure there's going to be someone around. Start by asking the SS welders - they'll probably know who to go to.

But.. let's think through, for a bit, how to do this without welding to the existing tubing.

Good to know that that's a GW top... they're not known for under-building things... unless we're talking about some of those transoms Case in point... that tower is way overbuilt - good!

Yes, fabbing a Cobia tower to be independent is an option. But let's focus on adding onto the existing top as that, in the end, is a nicer setup. The following is going to be, possibly, a little jumbled in how it's written - but if you're taking on the is project, I suspect you don't really need a step-by-step - but instead just bouncing ideas around. So, thinking through how you can do this with minimal expense and miminal compromising to the existing tubing...

-- That top has a LOT of support
-- Hang yourself on the side of the top? How does it feel?
-- But, you don't need to utilize the entire square footage and likely don't want to have something that large... plus it helps to keep your weight (and another guy) more centered. Meaning, the part/sides of the existing top that extend sideways don't really matter.

-- Before we go to far... how is the top anchored to the boat? If it's down to the floor - perfect. If it's "feet" are mounted onto a vertical surface... more investigation needs to be done. BUT... two extra vertical legs per side could be added for support - and they could be formed such that they don't impede the walkaround nature of a CC and mount close to the CC. These could even

-- You can easily figure out the curvature of that top by transferring the radius to a sheet of 1/4" plywood (or similar)... or even 1/2" foamboard insulation... this will provide the shape you need to allow for a flat walking surface.

-- You've got so much lateral tubing going on there (and, again, realizing you don't need to utilize the entire top square surface), I don't see any issues with sandwiching the existing top/tubes between the new top and backer plates that run on the bottom side of the existing top.

-- What if you uses Al I-beam for the structure of the new floor (meaning, the "rim joist" on the port/stbd side, so to say) of enough height to overcome the radius. This would then get attached as above and then build the rest of the Cobia on that.

--After that, can there be a flooring that can be strucurally solid enough to span between the two I-beams without adding a lot of weight? First thought... aluminum trench grate (google that).

-- Maybe 1/2" thick neoprene would be good between the I-beams and existing tubing. Maybe something better than neoprene? I don't know - that was just the first thing that popped in my head.

-- Or... instead of "sandwiching"... maybe use aluminum tubing clamps (with tabs) on the exisitng top tubes to mount the I-beams to.

Well, anyways, there's some more food for thought! Bounce back with thoughts and ideas and maybe we'll shift gears again!

 

[DennisG01]

You know... I'm kinda liking the idea of clamps with tabs that allow for mounting the I-beams. And, there's no reason that the I-beams have to run fore/aft. They could run port-stbd, allowing for the clamps to be secured a little close to the centerline of the boat... requiring slightly shorter I-beams... and there's no reason the port/stbd running I-beams couldn't overhang/cantilever the clamps by at least a foot, if not more.

I think once you get your mounting of the "floor" figured out, the rest is easy. Other than possibly having to vertically reinforce the existing tower if it turns out it's not adequate enough to your liking. But, again, adding some extra "legs" is not hard.

 

[Xavier]

Answer to your question: all four legs land on the deck. None of them sits on a vertical surface, and none of them lands on the seat module. So we are in your "perfect" case.
Two things that may help size this up: the top is 2.20 m wide overall, so there is plenty of tube to clamp onto well inboard of the outer edges, and the aft edge already carries a six-tube rocket launcher, so the frame is used to some load out there.
I like the athwartships I-beams on clamps a lot. Load stays near the centreline, nothing is welded to the original frame, and the whole thing is reversible if it turns out to be a bad idea.
Two questions before I start measuring:
1. How would you size the I-beam? I can measure tube outside diameter and wall thickness, the span between legs, and where the clamps would land. Is there a rule of thumb you apply, or do you simply overbuild and stop worrying?
2. Access. On a 306 the console is narrow and there is a leaning post immediately behind it. Where would you put the ladder so it does not kill the walkaround - on the aft face of the top, or angled off one side?
Xavier

 

[DennisG01]

Athwartship... oh, good! I don't have to over explain things

I-beam size. There are calculations that can be done to figure out load capacity based on span. But the reality is that the span we're talking about is extremely minimal based on the load they are carrying... especially if you end up adding a third I-beam, depending on the flooring choice. But I-beams, by nature, are extremely strong as I have no doubt you already know. If it were me... I'd might break down and do some calculations as it would be easy enough to do and, well, kinda fun! But to answer your question... a 2" I-beam is going to be stronger than a 2" tube... at least in the dimension we are discussing. Meaning, I'd use 3" I-beam as the weight difference is going to be inconsequential vs the strength gain. So, yeah, overkill - but it looks better and I'd be MORE than comfortable with it without doing any calculations The other factor is that depending on the floor you choose, that could easily ADD to the structural integrity of the whole thing as well.

I thought about your ladder dilemma, as well. While I'm not overly familiar with Cobia towers, I believe they are typically narrower than your top, which means a side mount ladder works fine - but it won't in your case.

Off the top of my head...

-- How do you feel about a front mount ladder?
-- How do you feel about cutting a notch on the side of the existing top to allow for a side mount ladder?
-- You know the boat better than me, but could you work with a ladder that starts going "up" along say, the port side of the bait tank, and then transitions 90* CCW?
-- Maybe the ladder could, again, start going up along the side of the bait tank (again with the rungs running fore/aft) but continues in that fashion - simply requiring you to turn as you step onto the new platform (and maybe that platfrom needs a short gangway plank)?
-- How about loosing one rocket launcher and making a folding ladder?