Naval Architect Needed (paid?) - I Cut My Houseboat In Half

Hi all,

Am searching for some feedback and would also like to explore having an NA officially do a writeup of some sort to provide some real data back on my houseboat restoration. I have a 1976 Nautaline 43' houseboat that I have owned for 15 years. The stringers failed, the hull hogged to the point where doors wouldn't close any longer, and I had it brought to my house to restore.

An opportunity came up to scavenge windows, trim, and other parts from another exact Nautaline 43' houseboat as long as we chopped the boat up and disposed of it. We kept an 8' section chopped my houseboat in half, and are inserting the 8' section into the hull right in the middle. The idea was to give us more room to have a small bedroom and more kitchen space.

This was a terrible idea, but we're past that point now. The boat has an enormous amount of sentimental value, has been on the news multiple times over the years for surviving a tornado, being in a flood where the boat docks broke free and careened across the lake, being in a dock fire, etc.

Here was my current plan to extend the hull by using this 8' center section:

• New section lines up on both sides (fore and aft) because it's from the same exact model boat
• Scarfing being done on old hull and new hull piece using a router and sled 10" on both the top and bottom side of the hull to give a minimum 20:1 scarf ratio for the hull in the 1" thick sections, and 40:1 scarf ratio in the 1/2" thick sections, and 80:1 in the 1/4" thick sections like the sides of the hull
• Plan is to use 1708 bi-axial cloth with 3:1 epoxy from USComposites
• First 50 gallon batch of epoxy and 100 yards of 1708 already on-site
• Bonding the hull sections on the inside of the boat where a higher build of fiberglass is not an issue across the scarfed area would be the full 20" (10" scarf on each side) and then layers over the top using the full 32" 1708 roll width
• Bonding the hull sections on the outside of the boat would be the full 20" width and then sanded fair and smooth
• Replicate stringers but with thicker layup as the factory ones seemed pretty inadequate and flimsy
• Add new transverse bulkheads over the bonded seam areas and at other points down the hull, still curious how often to add bulkheads. The original hull had NO bulkheads other than the engine room wall and the very front by the helm. This is probably a big contributor to why these houseboat hulls hogged so bad.

I have done 3 point flexural testing on piece of the original hull to verify that a double scarf seems like the strongest way to bond the hull pieces together. I CNCed some strips of the original hull all to the same thickness, then did the 3 point test on the following.

• Original hull until breaking
• Cut hull pieces bonded with a scarf on the top only
• Cut hull pieces bonded with a scarf on the top AND bottom (to see if double sided scarf is stronger)

After doing this test I measured that the original hull pieces broke around 170 pounds of force, the 6:1 top only scarf pieces broke around 100 pounds of force, so these were weaker. However, the 6:1 scarfed pieces on the top and bottom sides broke at over 300 pounds of pressure, being stronger than the original hull pieces in this particular test. (Understood that this was only a few samples, not a totally scientific test)

I have the hull drawn up in Sketchup based on the exact measurements of the hull that I can provide. Since the boat is in my driveway I can gather any other relevant data pretty easy.

What I am looking for:

• Advice from the community on if I am going about this the right way.
• Scarcastic comments welcome on the idiocy of cutting the houseboat in half instead of just looking for a bigger houseboat.
• An NA who would be willing to work with me on getting an actual writeup on some of the processes to validate the structural integrity will be the same or greater than the original boat. Would also be great if the NA would be willing to do a short video call as we are documenting all of this build on YouTube to hopefully inspire future generations of boat enthusiasts.
• This would be a paid engagement, although I am just a weekend warrior boat builder so I am not sure what a good rate for consulting on something like this would be.

Pictures below! Thanks all!
-Kyle

 

I'm a boat builder with no c.v.

The layup on the scarf needs to be done differently than you imply.

You are implying laying up over the entire scarf, but this is not matching my intuition. Keep in mind I'm not a composites engineer.

I would start the layup by filling the scarf in. This means a calculation to determine the thickness for each of the ten inches. Let's say the hull is an inch thick. A ten inch scarf is changing at a rate of 0.100" per inch. 1708 is about 0.035-0.050 per layer, so for 10" each piece at an inch bigger is 10 pieces and 0.5". To build to an inch means a progression of 1/2" per side or one inch steps at 20 pieces to match hull thickness; then some layers continuing over the full hull thickness. This means something on the order of 20-40 pieces of glass; not 5-6.

The most important thing is the quality of the work. Staggering all these layers must be done with thickened resins to avoid air at the steps, and a slow approach to avoid heating and failure of the bond.

Another builder intuition is the stringers were designed for the original hull length. You would need to either make the existing stringers thicker or add more to be at par.

You cannot be flippant about the stringers or the boat will break that way.

Transverse bulkheads are generally not placed over seams.

So, as a boat builder, I see a lot of problems with your plan as stated. And I've only touched on a few items.

 

Here is the basic idea. I can't spec it for you, but you'll need to grt back to the original thickness plus something.

Technically, the existing hull may be too light a skin as well, I can't delve into that detail, or much, but at least point out potential issues.

Here is a basic idea for the scarf. You ought to be getting an engineer to specify because a two sides scarf is something entirely not intuitive to me. It may be that you need to do one scarf and then do the next one over the epoxy bonds and not over thin poly, etc,

 

Thanks Fallguy! My explanation on the layup wasn't very clear, but the idea is exactly how you just drew it out. Below is a picture of what I was reading about regarding a double sided scarf. This would mean grinding a bevel on the outside of the hull in addition to the inside. The main reason it sounded good was since I have access to both sides of the hull and in theory would provide a stronger joint. It seemed to be that way in my small testing of breaking a few test pieces. There's not a ton of information out there about fiberglass/composite repair with a double sided scarf joint though (at least that I can find).

For the stringers I am definitely curious about increasing the wood dimensions, laminate thickness, and like you mentioned even the possibility of additional stringers. It took about 3 months of the houseboat being supported by the outer hull to get the center keel to sag back down to normal. It's amazing how bad the hull hogged up in the middle once the stringers started to disintegrate.

 

A good general principle is that each glass ply should extend a couple of inches beyond the previous piece.I would begin by taking the initial laminate a couple of inches at each end beyond the tapered zone and not attempt to fit it within that zone.Which means that you may wind up a foot or more beyond the edges of the taper.The stringers should be treated the same way as they are important structural elements and the new sections should be inserted a good distance beyond the skin laminates.I would be sure to use a couple of plies of biaxial over the entire length of the additional stringer with a 45/45 orientation and a few longitudinal plies of unidirectional tape along their top edges between the other plies.Obviously the glass over the stringers should overlap in the same manner as in other situations.Its probably worth laying up a few sample pieces to get an idea of what it will take to reach the thickness of the original hull and to use no less for the repairs.In terms of beam stiffness of the hull,the same principles will apply to the surperstructure and a messy and unpleasant job is in store when you laminate overhead.It can't be skimped or there may be a nasty surprise when the boat is supported in slings.A band of unidirectional tape along the corners of the cabin sides/coachroof angle will be helpful,it can be supplemented by further strips extending down the glass sections between the windows.Some elements,such as bulkheads or columns will inevitably have to be installed to keep the top and bottom linked at the correct distance apart and they will need to be diligently attached at top and bottom.

 

Hi Kyle, is the bottom sample in the cutting mat picture a 6:1 scarf from both sides sample? What is the actual thickness of the pieces used for testing (I just re-read above and saw the various hull thicknesses)?

 

In order to get to the original hull thickness of 1", if we use 0.050" per layer; he needs 20 layers. If you start by going over the entire scarf, and you add 2" to each layer; he starts at 22" and ends at 42" wide and the hull is 2" thick then over the unscarfed section and 1" thick at the scarf. Is this what you propose? My gut tells me this results in stress risors on each two sides or 4 sides of the 2" thick area, or an area of superstrength at the 2" thickness with area of less strength at the regular hull thickness. As you mention, when in slings; this is the scary bit because I'd not want and epoxy bond 2" thick next to a poly bond 1" thick. But I'm not a composites engineer either and have never seen this scale of repair/mod attempted. And this is why I think he needs to hire a composites engineer.

OP - your attached drawings refer to plies of wood. I'd be cautious about mixing apples and bananas. The scarf shown does not look strong enough to me and something like what @wet feet suggests will be needed. I just have a hard time with two sets of 2" thick areas. But I also have a hard time with ending up neat and clean at only 1" thick when done and this was not my suggestion ever to be clear. I said you'll need more than 20 layers. My gut also tells me that 1708 alone is not wise either.

An engineer or NA may also advise more longitudinals over the repair; this would require precision to fit over thickened areas. Another reason to hire a professional.

The outside repair cannot be the same either or the hull would have two bumps..maybe this is passe' for all, but should at least be noted. So, there are not two scarfs at 1/2" deep each. The outside repair would not be able to extend out past the hull thickness; so is somewhat cosmetic. But an engineer is needed to tell you how best to do it. If you find no NA; your testing needs to reflect less glass on the outside as you would build. For example; it may be that you grind 3/4" on the inside and leave 1/4" for grinding the cosmetic repair or some such. And then the outside scarf cannot be the same width as the other side because you have no overlap area for cosmetic reasons.

 

I am astonished that a 40 foot hull is an inch thick.That is the sort of thickness I would expect on something over 60 feet.I also wouldn't bother with scarfing a section on the outside,a near feather edge and all glass on the inside would be my choice and it would leave just a small cosmetic repair on the outer surfaces.The same applies to the section above the sheer.The tapering of the plies of glass would mean that the full thickness would only occur at the point where the new and old sections meet an there ought to be only a single ply at the extremity.

The level of precision needed to attach the stingers to the hull isn't all that great as the glass over the top will provide the strength.Given the massive thickness of the hull,it would be wise to ask a specialist for a detailed laminate specification.Don't overlook the need to taper the exisitng glass on the longitudinals as the strength is replaced.

 

One starts to wonder what the inch is made of.. a half inch of chop isn't much to bond to..though the pictures look solid and less than inch thick

The double sided taper makes me very nervous. And I was wrong, there are actually 4 sections that would be 2" thick the way we described the repair. So, in the slings; the boat has four areas of super high strength with an inch of polyester from the original hull in the extension. The boat almost surely would require multiple longitudinals over these areas to deal with stresses.

If nothing else, hopefully the OP will he willing to hunt for NA and engineering after all my speculation.

 

The outside of the hull will need a ship tape or such mould to deal with that span. That detail will he important to iron with the architect because if you feather it to zero; there is no space for any cosmetic work. The exterior will need to be prepped first and then a releasable moulding placed,

Get professional help on this work.

 

Thanks for more feedback guys. I'm adding a couple pictures because I think I clouded things with the different hull thickness part. In the picture below you can see where in the keel section the hull is 1.0" thick, but in other areas it's just 0.5". The sides of the hull and the roof are only 0.25" thick. What I envisioned was doing the same width scarf to plan for the 1" thick sections of the hull, and by doing the same width scarf everywhere would actually be a better scarf ratio for the thinner sections of the hull.

I also drew up the two ideas for the scarfs, one being the normal single sided scarf with a 20:1 ratio (20" scarf width for a 1" thick hull) and the other being the double sided scarf where you are only scarfing through half of the hull thickness.

I definitely would like to enlist a professional NA to get their input. I wasn't sure if this forum is the best place to find one or if there is some other good website for hiring an NA for ad-hoc type of work.

Thanks!

 

@Kyle Motiv8Labs Your data so far supports the hypothesis that the repair material is stronger and stiffer than existing. Love the whiteboard pics!

I think I would consider a 1:10 or 1:12 scarf from the inside, not 1:20, so you are not working against gravity and you can use a backer from the outside to bridge gaps. Like @fallguy said it would be about 20 layers for 1". I don't think you'd want to lay these all in at the same time, maybe groups of 5 layers and let them set up so they don't overheat and then attach the next group before the epoxy cures too much for re-bonding w/o sanding. Make sure to understand any directions with your resin concerning amine blush removal before subsequent layers.

Somewhere along the way I picked up the idea of doing large patch piece first, so as to get more long fibers in contact with the old hull at the joint.

The one picture with the green dimensions shows a need for a system to keep the hull sections in line and planar to each other before glassing. You don't want to stick it together crooked...

Naval architect advise about keel, stringers, and any other 'scantlings' needed for this would be good to sort out before glassing.

 

Yes, use bigger pieces first like tops suggests. USCG used to require that, probably still does.

 

The additional pictures are useful;for practical purposes the 1/2 inch laminate thickness is what you might work with and then add an additional strip of mat or several to build up the thickness along the keel and chines.It would be very unwise to attempt to scarf a section on the external surface.Not only would the alignment be difficult but the finishing would be an epic undertaking.With a simple scarf from the inside it is a simple matter to use numerous battens, which can be attached with self tapping screws, to pull the parts into alignment.You will also need a large collection of props and sundry blocks to support the hull correctly.Start at the keel-then the chines and then use lengths of batten between as needed.

I wouldn't go near epoxy for the job as polyester gel and resin are simpler to work with and identical to the original materials.The notion of attempting to arrive at a quality finish with a cocktail of epoxy and old polyester is scary.Fear not,the rest of the polyester boat hasn't disintegrated yet and tens of thousands get launched each year.The bonus is that with no epoxy there is no risk of epoxy sensitisation.You ought to aim for a balanced laminate and a composite engineer is the best source of information.

Don't overlook the need to incorporate limber holes in any longitudinals and a simple solution is to use lengths of plastic pipe that has been split down the middle and laminated to the hull prior to adding the main longitudinal.The ends of the split pipe can be trimmed back to avoid water traps when all the laminating has been done.

 

I agree on the polyester. Maybe vynilester. As far as the laminate schedule, the longer pieces go first and the smaller ones are added until it is flush or a bit protruding.