Extended Transom

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SuperYeti
Posts: 6
Joined: Sat Sep 17, 2016 2:30 pm

Extended Transom

Post by SuperYeti » Sat Sep 17, 2016 2:51 pm

Hey all, first time poster here. I've decided to build an extended transom for my 16' welded aluminum fishing boat, here's a couple pictures of it:

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Here's a picture of the design I like from my buddies boat that I plan to replicate:

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I've decided to go with a 30" extended transom, to give me some extra running length, and also to take up the weight of the new to me 150hp 2 stroke Merc black max I recently picked up. So many question so far. I have some pictures of a design I like from a buddy of mine, have spoken with a local builder, and decided on 5086 3/16 plate for the extension, probably go with some round tube for the swim grid, and then some checkerplate. My main questions are before I start building is, what kind of considerations do I need to make for a frame inside the shell with the length, and width I'm looking at? Is there somewhere someone can point me to some research I can do on super-structure design, to ensure I'm going to make it strong enough, but not too heavy? My new motor is 40lbs heavier than the 90 Yamaha 4-stroke I took off it, so the weight difference isn't much, but would still like to add some flotation to the vessel. Any help or suggestions are appreciated.

Thanks
Warren

kmorin
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Re: Extended Transom

Post by kmorin » Sat Sep 17, 2016 8:56 pm

superY, welcome to the AAB.com Forum, and thanks for bringing your shop project along.

First to mention is; the design of almost all add-on's are generally planned to mate/match/correlate/correspond to the existing hull structural elements inside the hull and against the transom. The idea is an add-on would tie-in to the existing structure in such a way as the main fore and aft elements are simply continued aft/outward/backward into the main structure of the add-on mount.

So, if you have a couple of main longs in the hull? or one big VKB (vertical keel bar) or some other primary structural element? you can be guided by those deepest/longest/heaviest scantling pieces of the boat's interior frame to begin to draw your extension.

My advice is to get a good idea of what's inside the transom, its been designed to integrate into the boat And.... it's been designed to hold the rated outboard engine- as part of the overall structure of the original transom. That should give some serious design information about an add-on stern.

Somewhere I did a post here at the Forum, that goes around the overall design helix for transom extensions/add-on's and you might find it helpful? The purpose of that post was not to show structural dimensions for planning; just the overall design cycle of various decisions the owner goes through to plan adding to his welded metal hull. :popcorn:

Cheers,
Kevin Morin
Kenai, AK
kmorin

SuperYeti
Posts: 6
Joined: Sat Sep 17, 2016 2:30 pm

Re: Extended Transom

Post by SuperYeti » Sun Sep 18, 2016 1:40 am

Hey kmorin, thanks for all the great information. That's funny I came to the same realization today while I was inside the bilge hanging the new 150 on it, and looking at the existing superstructure. I do plan on beefing it up a bit, as I'm going with a 150, but the boat is currently rated for a 90. I know the hull was designed for much higher speed than I previously got with the 90, and 30" extra hull and flotation should mitigate any concerns there. Per the USCG power rating calculations for boats I was reading earlier, as it sits it should be good for 140hp, so with the extra length and reinforcements, the theory is I should be good to go :).

Here's a pic I took tonight with the new beast hanging there, now I just need to finish changing out all the controls, gauges, adding a few new gauges, and the injection oil tank. And then thermostats, poppet, and lower unit water pump rebuild, and change lower unit oil on the engine, and I should be ready for a test voyage as is before I design the transom extension, so I can use how it handles with the new power to help drive the design.

Image

I'm hoping to get out next weekend for a test cruise, and then will get going on the CAD modelling for the extension afterwards, as I progress I'll post renders and hopefully can get some feedback on my design.

Thanks
Warren

Chaps
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Re: Extended Transom

Post by Chaps » Sun Sep 18, 2016 12:36 pm

Welcome to the forum! On your project I would recommend you talk to the designer/engineer at River Hawk to get additional input on your idea. As far as handling goes, the changes you'll notice by hanging the Merc on the transom won't necessarily translate once the engine is set back on a bracket. On that light and small of a boat I predict (after the setback) you'll find that sharp turns at higher speeds will be uncomfortable if not downright scary and porpoising will require tabs full down all the time.
1987 24' LaConner pilothouse workboat, 225 Suzuki
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SuperYeti
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Re: Extended Transom

Post by SuperYeti » Sun Sep 18, 2016 9:46 pm

Thanks for the response Chaps. Unfortunately I haven't been able to track down the designer yet, he may not be with the company anymore. Still working on that one. Before I took the 90 off, I was running it rather high, and had to put a Laser 2 on it to get enough bow lift for proper trimability, and trimmed up flat out, there was no porpoising. The new engine is only ~35-40 lbs heavier, and I'm interested to see what the CG change will do. I do have Nautilus smart tabs on the boat, and I can turn them a little stiffer to counteract the extra weight if I do find that I'm getting porpoising. The thought was with the extra 30" of running surface, plus the added flotation/displacement from the pod will counteract the additional weight of the engine, and would probably put the CG further forward than it was, or keep it the same. As for turning, it currently turns and STICKS to the point where it will throw you out of the seat, so I would expect that to continue, and yes I agree will probably be scary going much faster, but that is where the gray matter comes in, just because you CAN turn the wheel at full speed, doesn't mean you should :). The main thing I'm still trying to understand is what handling characteristics change, and it what way as the pod gets wider towards the edge of the chines, This would probably help dictate my design. Still searching for more information on that, no luck so far.

Thanks
Warren

Chaps
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Re: Extended Transom

Post by Chaps » Sun Sep 18, 2016 10:32 pm

Well it sounds like you have thought this out. Have you considered what you will do if the plan doesn't work? It looks fairly new and it's likely a valuable boat in its current state . . .
1987 24' LaConner pilothouse workboat, 225 Suzuki
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Fisherman
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Re: Extended Transom

Post by Fisherman » Mon Sep 19, 2016 11:13 am

These projects are always fun to watch...thanks for posting. Just think what you would end up with if you sold your 16' boat and bought a 20'. Think about all that extra deck space...

SuperYeti
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Joined: Sat Sep 17, 2016 2:30 pm

Re: Extended Transom

Post by SuperYeti » Mon Sep 19, 2016 11:40 am

Thanks for the suggestions. I have indeed thought about it. I was able to get the contact information for the original designer of the hull today, he's away hunting until the end of the week, so I'll be picking his brain next week. The only uncertainty currently is my aluminum welding skills, as that's new to me. I've purchased a bunch of 3/16 5086 to get myself up to speed, and won't be burning anything on the build until I'm confident in my skills. The boat is 4 years old, and while AL boats do hold their value, the market is bad up here to sell it right now, and even worse to buy a new one because of the USD exchange. Part of the build is going to be extending the transom to full height, and then I'll be reclaiming the splash well into a small bulkhead for batteries, oil injection tanks, electronics etc in the transom, a small livewell, and reclaiming the rest of the space for deck space. I should be able to gain about 18" in deck space with the conversion, and in essence end up with the equivalent of an 18 footer by the time I'm done. With what I sold my engine for, I was able to buy all the spool gun, and AL welding stuff I needed, the "new" engine and controls, and have enough left over for most if not all of the AL I'll need for the build. I'm also facing a very strict WAF (Wife Acceptance Factor), so that definitely is influencing my decision to go this route. The time is something I'm happy to dedicate, as it's a great project to perfect my AL fabrication skills on IMO. I'm hoping to have some time this week to start CAD'ing up the extension, and existing hull superstructure and skin, so I can start finalizing the design, and getting some feedback from the community.

Thanks for all the great feedback so far, and look forward to much more in the future as I progress. Cheers

Warren

kmorin
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Re: Extended Transom

Post by kmorin » Mon Sep 19, 2016 2:35 pm

SuperYeti,
If you'll follow through on your idea of learning to weld well enough before welding on the boat? then you'll find that MIG is not the final answer- you'll need TIG as well. Welding long seams or even short straight stitches is one level of aluminum welding, and hopefully your practice will get you there in an acceptable time frame? However, welding around pipes as butt joints, brackets and gussets that require close quarters rapid directional movements of the torch? Those are extra complications regarding welding skills- so I do hope you'll consider adequate preparation before working on the stern?

Aluminum MIG's inherent speed of travel, combined with the type of feeder you're using can combine into a difficult series of welds in the project you show in the picture. I realize that you're lining up the scope of work along with the design as you go along- I'm just speaking from my personal experiences welding hulls' long seams versus brackets, tanks, gussets, pipe as trim and structural elements and noting this type of welding with a MIG system (even push pull) is demanding.

Once you begin to weld, you'll begin to understand the idea that wire feeder angle/"lean"/gas cup gap/coverage/and settings all combine (related rates) to allow the MIG system to deposit welds. This is a fairly constant set of variables- that is; once the angles, settings and gaps with flows are all set- moving a uniform speed and depositing MIG beads is pretty easy due to the constant geometry of moving along a linear seam. What may not be as obvious yet? is that once you have to move the gun body around a circle, or a U at the seam weld rate speeds- things become more Olympian in regard depositing welds that are uniform. This is why so many (all fully pro-built boats) have TIG welds in some locations- the slow speed with super high quality control of the weld during those welds allow high enough quality welds to meet standards. Even a high-hour, experienced welder would have a hard time welding all the corner and bracket joints of a typical stern extension purely with MIG and obtain welds of the quality that would happen with TIG combined to control the many "corners".

Hoping you'll have patience to fully develop the skills needed to produce a final project that will retain the boat's original value? I'm not disregarding a WAF "WTHeck" factor if you begin planning to buy ANotheR welder???? "You just got one!!! and you don't even know how to run it!~~??" Just noting the facts from building some skiffs myself where I used MIG for most welding but moved to TIG to do the corners- for a reason.

I'm not trying discouraging your project, just suggesting you want to make sure your practice is adequate, and you set up exercises to do the joints that are implied with the equipment you're providing yourself. In fact; this relationship between bead types and joint location and types- is often the source of serious discouragement among new aluminum welders because no one has mentioned the truth of the MIG systems' positional limitations.

Cheers,
Kevin Morin
Kenai, AK
kmorin

SuperYeti
Posts: 6
Joined: Sat Sep 17, 2016 2:30 pm

Re: Extended Transom

Post by SuperYeti » Mon Sep 19, 2016 3:09 pm

Thanks for the reality check Kevin, much appreciated. While not a pro welder yet, I have been welding hobby level on and off for nearly a decade, and the last 6 months have been focusing on improving my tig and mig skills. My welder is a multi-process, so I can do DC tig, mig, now DC AL with the spool gun, and stick. If i have to get an AC tig welder to finish up the project, I'm ok with that, I've been eyeing up the Everlast macines for a bit now, and could probably justify that with the bigger picture in mind. I've been focusing my learning/practice on building some products, and finally being able to build many of the ideas/things I've dreamed up over the years.

This is all part of a larger plan for me to transition out of IT fully/partially over the next couple years, I've been doing it for nearly 20 years, and am ready for a change. I also have a few friends who are quite experienced AL welders, having worked at shipyards around where I live before, and there are a few local shops I could lean on to finish up the TIG stuff if I feel my experience isn't going to be sufficient.

When I spoke with a builder who's located pretty close to me about my idea to do the transom extension, they said that they do them all the time here on the west coast, and shouldn't be a problem, and they usually build them separately, and then attach after it's built, is there any reason besides possibly adding some complexity that I should not consider building it in a similar matter? I figure that still gives me a golden chute to pull if I'm unhappy with the extension before I make permanent modifications to the hull. My thoughts were build extension, attach it to hull and reinforce before I start cutting out the splash well, and increasing the height of the existing transom to minimize any movement of the hull from where it sits right now. Does that seem like a reasonable plan of action? Again thanks for all the knowledge you're imparting, and time you're providing me, I will definitely owe you some beers in the future :).

kmorin
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Re: Extended Transom

Post by kmorin » Tue Sep 20, 2016 1:52 am

SuperY, sounds good, at least you're not coming to welding without any time under the hood and that's usually positive. I've had some difficulty getting some steel-only welders to run at aluminum MIG speeds in the past (training new hires) but generally most people who have run steel MIG will transition to Al MIG if they'll just practice.

As you mentioned, with AC TIG hands and facilities available you could (worst case) leave those ends and weld corners to those guys' if you had too.

The test for me as to building on or off the hull is the final design, lots of times I've used the hull as the primary fixture or jig to do the layout and fitting of a hull extension. But I've sure built them off the hull before- as well and it works fine if the transom is reasonably fair? One sequence I've used several times was to tack flat bars on edge, under the existing hull for 4' and extending aft for the length of the bottom's extension. This allowed me to be sure the planing surface was a common set of planes. So in those cases is was just easier to tack directly to the hull and add the bottom plates, then sides and any interior longs, butted to the hull and tacked to the bottom, finally adding the new transom all on the existing boat. To me, the build method depends on your design approach.

I was involved in building a hull extension where it was a huge box (2 ea. X 250hp Honda 4strokes) that bolted to a plastique boat, and we had to build it off the hull, make it a fully sided box and bolt through to mount. But if the hull had been metal it sure would have saved time to just tack and weld on the hull as the fixture to set up the entire fabrication.

Cheers,
Kevin Morin
Kenai, AK
kmorin

SuperYeti
Posts: 6
Joined: Sat Sep 17, 2016 2:30 pm

Re: Extended Transom

Post by SuperYeti » Wed Sep 21, 2016 12:25 am

kmorin, again awesome advice, thank you. After I speak with the hull designer next week hopefully, I think I'm ready to start CADing the project up. Stay tuned!

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