Repair/Redesign of transom on 4.8m RIB

[DanAzWa]

Trying to clean up a mess. Transom is fully rotted, so it will be replaced.
This was originally a 2010 Bombard(Zodiac), designed to support 60hp.
I don't understand the design of the transom, doesn't look like enough support. Although it rotted out, didn't crack anywhere.
While I'm repairing, I'd like to design it so the transom is fully flat, ~ 30 degrees from vertical.
I guess I'm seeking validation/advice on whether this would be a good idea. If so, I will extend the floor out the 6-8 inches to meet the the lower edge of the transom. And knee braces.
The original design looks cool, but I don't see how it was strong enough to support the transom under full power. It also resulted in wasted space, and on a vessel this small space is valuable.

Last pic is painted just to show contrast. First and second pics include the side I've already cut out. It opens into large open space. For reference, the lowest part is 90 degrees vertical, the part above that is ~30 degrees, and then the wing is angled in even further (I'd guess 45 degrees in from the upper part of the transom.

 

[fallguy]

Wow. That is a mess. You are going to need to use some aircraft methodologies here I’d say. And you cannot arbitrarily change things. The design is done for load paths I’d say.

This repair is a little past my desire to foray into, but I can ask some real important questions.

1. Was there tabbing on the inside of the angled section? Where? It must be replaced, access or not. This is done using pre-release methods, but may require removal of some of what is there. Hard to tell. You may consider epoxy due to the access issues; it will be more forgiving of minor lamination issues.

2. The area the motor hangs is not 30% versus the waterline. The engine rigging specs dictate the angle. Maybe you mean the side piece.

3. Forget redesign. The transom is designed this way so all of the pieces function together. Your notion to make it your way is very, very bad. Not a question. But can you offer more pics of the inside and the piece removed?

4. Were you going to remove more? I think this is perhaps a bad idea until you have the repair plan draft. A composites guy really would help. Do you have access?

5. What building materials?

 

[fallguy]

Pre-release is a method where you build the tabbing out to a false and releasable transom section open in the middle so you can get your hands inside. Then you bond the transom section to that tabbing. Using epoxy allows for some bonding errors which are easier to occur tabbing from the opposite side. Perhaps I am ahead of myself here, but perhaps not and just wanted to give you a heads up.

In order to tab from the wrong side; you need to see to make fillets and good lay down of tapes, so tabbing to a piece of clear material is good. Like a piece of plexiglass covered in shipping tape, etc.

 

[fallguy]

You cannot have any loose stuff on the inside. Add some pics and closeups and answer my list above. There are some guys who post here who may know this three panel method better, but pretty sure I’m on the right track. Also, let us know the prior transom thickness.

 

[DanAzWa]

Thank you!
I was a bit short in the info, since it was my initial post.
The structural mess is all mine. It was a whole boat I started taking apart when I found the transom and floor rotted. Transom is so bad I have literally pulled all wood out of the bilge plug that I could reach with my finger.
The structure you see in front of the transom was actually molded as part of the floor and will be removed. I decided to quit removing pieces until I came up with a more comprehensive restoration plan. The short of it is I will have full access to the front of the transom and inside of the hull for any tabbing needed. I won't need to consider the pre-release idea.
The cutaway part is simply a flat piece that, while thick, wasn't attached to anything other than what can be seen in the original photos. No knee braces to be found anywhere. So while I see that the "wings" would be somewhat supportive due to their angle, It doesn't seem that it was supported all that well.
A lot of the smaller Bombard boats were known for leaning a little too far toward form while ignoring function. IE, it was nicely designed with a bench seat/locker in the back and integral anchor locker in the front, but the entire fuel system was inaccessible without cutting the floor.

Honestly, my design mod ideas were based on:
1) The desire for more floor space, the bench/locker in the back took up a lot of floor space.

2) Wanting to redistribute weight, battery, fuel filter, main seating, and 75% of the overall weight was in the last 1.5m of the boat. Leading to a problem getting on plane and constant nose up attitude.

3) Compensating for what I perceived as a poorly supported transom by design. It looks to be the standard laminated 2x 3/4" marine ply. But is simply an inverted triangle, that goes no wider than the area I cut out, on either side. I'm still baffled as to how it was supported, but found no cracking in the transom area or the attached wings. I have assumed they just overbuilt everything around it. My plan was to build a transom that went the full width of the boat, attached full width to the hull, and add 4 knee braces.

I am going to lay it up in vinylester and composite (likely Coosa) for a couple reasons:
a) The whole tear-down adventure began when I realized the hull had been wet on the inside for a period and the gelcoat has become osmotic nearly everywhere.
b) Compatibility with other laminate systems (however; I believe it's all poly).
c) My desire to avoid any osmosis or decay problems ever again.

Absent a good water line. I'm using the keel as my zero angle.
The small section where the bilge drains are is 90 degrees from my zero.
Above that, transom, is 115 degrees from zero.
My idea was to build the full width transom at 115 degrees, which leaves the motor mounting at the original designed angle, while deleting the "wings". If they truly offer minimal support, I'd rather reclaim the floor space and knee brace for better support.

More Pics:
It occurs to me that the items circled in red were probably intended to serve as some sort of bracing system. However; they are hollow and are just part of the bench/locker/floor, the only place it touched the transom was at the very top, and was only a 1/4" piece.
Worth noting also. The original bench/locker/floor had begun to crack, so I'd assume any support they provided the transom had, at least partially, failed.

 

[DanAzWa]

Another mention:
While I respect the laws of design related to seaworthiness' this a RIB, so buoyancy is less a concern than structural strength.
I don't believe RIBs are unsinkable, but as long as the tubes stay intact, there's always something to hold onto in emergency.

 

[wet feet]

Unless you have a particular attachment to that boat,throw it away and find one that suits your needs.The amount of work it would take to return it to standard is huge and the standard version doesn't seem to be what you want in any case.The cost of the materials alone for a repair exceeds the value of what you have and you will lose a chunk of your life in the process.I've known people undertake fairly mammoth rebuilds and at the conclusion they realised that if they had spent the same time at a second job stacking shelves,or similar,they would have earned enough to buy a nicer boat and wouldn't have itched so much.

 

[fallguy]

I’m gonna opt out. You are way beyond my ability to help.

 

[DanAzWa]

Based on other input. I'm going to retain the angles and overall design of the original transom.
However; I'm considering using a full width transom or laminated Coosa (or similar composite).
Open the hearing any ideas on this part.

 

[montero]

Unless you have a particular attachment to that boat,throw it away and find one that suits your needs.The amount of work it would take to return it to standard is huge and the standard version doesn't seem to be what you want in any case.The cost of the materials alone for a repair exceeds the value of what you have and you will lose a chunk of your life in the process.I've known people undertake fairly mammoth rebuilds and at the conclusion they realised that if they had spent the same time at a second job stacking shelves,or similar,they would have earned enough to buy a nicer boat and wouldn't have itched so much.

This is a forum about building boats, not buying them.
Itchy skin. Yes.

 

[wet feet]

A sensible boatbuilder should be able to recognise when an endeavour may be technically possible,but not financially sensible.It will be a characteristic that saves him,or his customers,money and avoids a reputation as a bandit who preys on the lack of knowledge of those who need help.

In this particular instance,we have no idea if there is a record of the shape of the hull portion that bonds to the tubes,so recreating that will be a challenge-or new tubes required- and the owner/builder initially stated that the hull wasn't quite what he wanted.Hard to see a way out of that situation.

 

[HelmutSheina]

My soggy hoofed friend boat ownership has a big emotional component, while industry professionals are all about practicality and safety.

They think we are boringly cautious and frequently digress; while we cannot understand why they spend thousands on shiny metal, foam filled fabric and oiled teak when the engine is on its last legs.

Never the twain shall meet, except at bill settling time.

 

[fallguy]

Honestly, the boat is way beyond economic wisdom to repair.

This is a hard pill for all of us.

About 15 years ago or so, I bought a twin engine Starfire in winter. After the boat thawed, I discovered a rotten core with solid mahogany stringers and a good transom at least up at the keyholes. Replacing the core was possible, but after all the work was done; the boat’s final value was maybe 5-10 grand.

Boats don’t magically take all the hours of work and add them to their value. I figured 3000 hours to repair all the issues. Was gonna need 3k in parts and I was in at 3k with the boat and tandem trailers. That meant, on the high, my time was worth 4000 and the low -1000 in dollars. 4000/3000 hours is less than $2/hr. And on the low, I was working for someone else for free. I took the engines out and fuel tank and it went to the landfill.

Do yourself a favor. Accept first that the purchase, even free, was a mistake. Then, do a fair evaluation of the same craft in good condition and what it would sell for. Then decide your money in and subtract it. No idea what the tubes cost, but those are surely not cheap.

And tons of glasswork in money and time.

I think you’ll at least have a financial awakening.

Most boats, when major repaired, are easily spotted by buyers and they do not exceed the value of the same boat in good or only fair value. Wise buyers walk and the value drops.

Cut your losses, friend. Noone will blame you for it.

 

[montero]

In general, building or repairing anything these days becomes an overly expensive undertaking if you lack experience.
In the US, given the massive market, you could likely get something similar for a tenth of the price.