2000 MonArk Knight 160 structural

[Skiffing]

If anyone has structural plans of this boat it's appreciated. There are a couple of things going on with the transom and a detailed plan / cross section would be helpful before I start talking to a repair shop or cutting things apart. I've got a bunch of photos that I can upload if there is interest.

Thanks - and be kind.

[welderbob]

I don't think anyone is going to have a set of plans. If you post some sort of pictures,I'm sure you will get some feed back. If your north of Boston you could talk to the guys at Viking marine in southern NH.

welderbob

[kmorin]

Skiffing, actually speaking I'm not sure if anyone had those 'plans' they'd be able to post them! You did identify the boat as a MonArk so I have a feeling the only plans around are with that manufacturing company and I'm not sure I've actually heard of their plans being for sale? ( )

And like welderbob, I'm pretty sure images of your own boat would draw some suggestions in reply to your questions. Often pics inside boats are pretty dark, so if you can post process them and brighten them; it helps.

If you're able to use some drawing tools to show the problems (some are very subtle in a photo, like hairline cracks) it sure helps low rez photos to pass more information, and ... if possible blow up the areas that need discussion then use the Crop tool to reduce a 2nd or even 3rd - 'save as' version of the original, and by cropping the area of focus; can fill the entire photo size limit? Just some suggestions so that the images you post are most effective in communicating the answers you're seeking.

Cheers,
Kevin Morin
Kenai, AK

[Skiffing]

Thanks guys.

I'm having problems posting images.

ETA - I have an appointment @ Viking 11.10 & have heard they are good guys. Just want more info for me.

[Skiffing]

Try this:

Interior view of transom:



Stern view:



Port leak - exterior - appears to be stress related, not impact, not corrosion.....



Port leak interior - at transom brace. Looks like transom below deck is single skin:



Starboard leak exterior:



Starboard interior - at transom brace:










ETA - Bingo - I'm a slow learner

[Skiffing]

Kevin - Thanks for the info.

As far as plans are concerned - I wasn't looking for proprietary information. There are many instances where design plans are legitimately available to the public [and required to be so] - but not readily available on the internet. There can be many reasons for this, such as they were part of an unclassified government procurement - therefore public record - because we all paid for them.

S

[kmorin]

skiffing, great job with a picture being worth countless words! I see from your high quality documents what you're discussing and have some suggestions to give some context.

First and foremost, you suggest the two hull leaks are sites on the transom where 'stress' has happened? That is; your idea is, the inner press folded channel shaped transom stiffener, has been pulled away from the transom with enough force to pull the weld out of the transom material? I agree that does seem to have happened.

I think a very large engine (oversize) was tried (?) or collision at some time loaded the transom top aft- that force was enough to flex the entire (too thin) transom structure and the result was these two 'MIG weld origins' were torn out at the HAZ (heat affected zone) edges. These edges worked as 'notches' to provide the failure point of the tears, as the transom material appears to be less than 1/8" (perhaps 0.090" or so?).

Note: its odd the transom seems folded, drilled and riveted then welded outboard aft??? that seems like the rivets may have been just a construction tool? but anyway we look at this design, it is flawed: the knees/channels/longs should all reach the bottom transom and deck in order to provide adequate stiffness.

To confirm this, I will assume, but would like be sure that the paint chipping and scraping around the opening in the aft of transom shot was done by you?

The bilge access is not reasonable for repair, which is very simple. The transom channels need to tied to the hull bottom panel on both sides (inboard and outboard) like they are on one side! Notice the bottom stiffening longitudinal in the background of the (inside) photos of the leaks? these should be installed on both sides- so one should be added. In other words, if you tie the unsupported side of the transom channel to the bottom with a long, as was done on the side which did not tear out- then neither side will tear out.

The bilge is a complete mess, so I'd expect to remove a foot or so of back deck (and replace it). I'd think this will be 3' wide or so and is mainly required to get access to clean the goop out. The goop and paint all have to go in the welded areas in order to weld, and the new longs welded to the hull bottom, the channel and transom too.

TIGging up the two tiny weld pull outs is not much work and would seal the hull. but... getting the backing area clean enough to do that much seal welding may be a real challenge unless you can open the entire area to get the tools and cleaners to the glue removal area!

One last thing to consider- in my view. If the deck area immediately forward the engine's clamps is not super critical to some operation in your boat, I'd run some struts to the hull longs about 12" to 18" forward the transom/deck line. These should be welded to the deck where the key hole welds of the deck to bottom stiffening longitudinals are, in fact in this skiff the deck doubler plate (twice the size of the pipe struts) should be welded over the nearest keyhole weld on the inner most hull longs as they meet the deck.

These eliminate the original over stress that allowed the transom to flex so much it could tear the lower sheeting out. With these (1" sched 40- 6061-T6) struts triangulating the deck & transom there is not way to load the press formed channels' lower weld end shown in your images.

Hope to help in your consideration of the work implied by this somewhat light design's poor performance and the repairs needed?

Cheers,
Kevin Morin

[Skiffing]

Thanks Kevin. You confirmed my feelings. I think the transom is poorly designed and poorly constructed. There's a 2000 40hp Merc on the boat. The boat is rated for 40hp. I was told it was originally purchased this way. I don't see any evidence of impact. I scratched up the leak locations to get a better look at them. Starcraft purchased MonArk a few years ago. I hope they're not still using this design.

The transom braces appear to go up above the deck height - inside the upper transom assembly. I have no idea how high. The area where the motor is hung is just that single skin below deck. It's ~ 1 1/4 inches thick above deck - but I have no idea what's inside to hold it together. If those braces are IT for structural support I fear the repair will be pretty involved.

If I raise the engine full tilt and pull up on the lower unit the transom will flex better than 1/4". I should have put the camera below deck in movie mode when I did this - but didn't think of it.

Thanks again

[Skiffing]

Thanks again for your help on this.

The boat has always been trailered and the bunks are short of the transom by several inches. I think the bouncing around over the past 15 years broke those welds.

The shop I brought it to took a conservative [and appropriate I think ] approach to the fix. Here's a pic:



The removed deck got replaced with .125 - 5200 and cap rivets due to concern of a foam bonfire if welding was tried. The remaining deck is fully welded w/ cap rivets.

Before it got closed up I cut some foam out and examined the hull interior and it was bright with no corrosion. I also cut in a deck hatch in the front quarterdeck and examined the hull from there and it looks good to me.

I have read that foam in place will hold moisture against the hull and lead to corrosion.

Besides keeping the bilge dry is there anything I should / can do to keep it in good shape?

Thanks

[kmorin]

skiffiing, the gusset looks like it will keep the two vertical bent channels from flexing the thin transom and deliver the bending moment to thrust on the two main longs? I can't necessarily see the hull well, and in most photos it seemed little covered by 'bilge wash' not sure of what kind? But the look of the bilges seems to me a bit on the unclean side so, I'd want to rinse her out bow to stern if she were mine.

Not sure how much air circulation you can generate bow to stern but I'd want to etch her bilges then neutralize that acid wash with a base, then rinse with the hose for a few days. Then put a fan on her bow port PULLING from the hatch while the stern was open and the bow tilted up on the trailer in a sloped driveway.

If you've found clean, shiny and unpitted metal below the bilge coating?? (Then) what I'm suggesting may be unnecessary but - the photos near the transom sure do show a lot of some sort of bilge wash coating???

Foam does trap water against the hull, the images show a dirty bilge, the two items combined would imply to me there was a good chance that is happening where you can't see it? If you look at some other threads here you'll see what I'm talking about, where foam filled voids that were not air tight, fostered corrosion.

The mechanics of how this works is also covered in detail in other threads' discussions as well as pictures of some boats that may be 'near losses' because of the combination.

Best of luck with the more reinforced transom,

Cheers,
Kevin Morin
Kenai, AK

[Skiffing]

Thank you for your time and advice. I've researched the site and gleaned good information.

I power washed the accessible bilge and am not happy with the looks so want to proceed with Kevin's recommendation above.

Zep a-lume isn't available in less than 4 gallons with high shipping. I can get ALUMA_BRITE for around $100 gallon. Based on the manufacturer's dilution schedule it seems one gallon would be plenty.

I also found a product by Bio-Kleen Products, Inc. called Step 1 which is a solution of Ammonium Bifluoride of unknown concentration but the MSDS Section 3 indicates non-hazardous - so I'm guessing pretty dilute. It also contains some surfacant blend which purports to inhibit corrosion - so that may be a coating of some type when dry? This is less than $30 a gallon.

Does anyone have experience with Step 1?

To recap - this is a 16' skiff w/ welded deck with expanded urethane foam below. There is bow access to the hull at the quarterdeck. There is a deck plate at the stern.

I intend to level the boat and flood below deck with the acid solution [concentration to be determined after experimenting and observing foaming action] and allow to sit for XX? amount of time. Considering the foam displacement I expect this will take 15 - 20 gallons of solution. Concurrent with draining hull admit clean water and continue to flush & fill for XX? [a couple of hours?]

After clean water flush flood with base solution [probably ammonia] then flush. High tilt boat & force ventilation at bow drying for a few days.

I can get some Alodine 1201 solution and rig up a collection / containment system. Even though the hull would not be "like new" after acid washing would there be reasonable benefit to applying this?

[welderbob]

Try looking for a local truck supply. The acid etch is used to clean the aluminum dump trucks. We buy it here in NY for $20/ gallon.

Bob

[Skiffing]

^^^
Under what brand name?

[welderbob]

Local guy has Zep-a- lume and a no name aluminum brite..
Bob

[Skiffing]

I've tried all the major truck supplies around Boston. Nada. No acid at all.

[Skiffing]

Finally found a product called WAB Aluminum Trailer Cleaner & Restorer. It's concentrated sulfuric acid w/ hydrogen fluoride. had to travel to Concord, NH to get it. $20 a gallon.

[Skiffing]

After playing with a few concentrations I decided on 20:1. This stuff is strong and reminded me of my college inorganic chem course so many years ago.

After leveling the boat I degreased below deck and flushed. Fifteen years of spent wire ties, beach sand and some organics came out. Then poured 20 gallons of dilute WAB into the deck hatch. The liquid came about 2/3 way up to the stern deck plate - just below the new transom brace. The water was cold - 50* and ambient is 65* so I let it sit for 20 minutes then added another 20 gallons of fresh water and let that site for 30 minutes.

Then I drained it and continued to add fresh water at the deck hatch oscillating the hose to wash edges as best as possible. After it was drained and flushed for an hour I put the plug back in and added 4 gallons of ammonia and filled it up to the deck plate again and let it sit for an hour. Then I drained and flushed.

Before:


After:










Now to flush it for a while then ventilate.

Thanks AAB and especially KMorin.

[kmorin]

Skiffing, you're welcome to what little we know about aluminum skiffs, and those are some really nice pictures inside the bilge- nice job.

The etch shows that there is some surface pitting going on, but not extensive and the few areas of corrosion pitting seem to be all open and not the 'cancer' type seen in some other boats' bottoms.

Interesting the boat bottom panel appears to be supported with 'top hat' or 'bowler hat' bent (or roll formed) material where the flange is welded to the hull with 0.023" wire in a miniature MIG process we don't often see in the heavier scantlings of the bigger plate boats. The transom joint is interesting in that it appears to have a flange bent on the transom and only welded outside where the bottom panel overlaps the flanged joint? That is not totally clear but is what I see in the photos.

I'd be concerned about crevice corrosion in those laps, but the acid and base and rinse will clean those cell out as well as can be done with that original joint design.

Glad you got the structural issues addressed and now have the bilge washed, that process; "acid then base and rinse forever" is good maintenance for the boat and should help keep the pitting from developing further.

Cheers,
Kevin Morin
Kenai, AK