Michalak QT powered skiff

[Skunkworkz]

Greetings BD brains,

I have been lurking and absorbing content on this forum and believe I have rubbed enough of my brain cells together to post.


Here’s some basic requirements (SoR):

-2-3 adults mostly sitting. Single person standing at a time

-sheltered waters (harbors) but would like to venture inshore at times when weather permits or inshore Sea of Cortez in Baja

-10hp outboard (I have a sweet lil 9hp two stroke that rips)

-car top, weight target <120 lbs


I’m reasonably handy with tools and laminating practices. I have an aircraft structural engineering background and obviously lack the eye for “looks right floats right” compared to Senior contributors that chime in.


I believe the QT skiff by Jim Michalak fits my design criteria. The QT is approximately 13 ft by 45 inches. I had also considered the Bahía skiff from Spira but the bow contours and v bottom pushed me towards the QT for ease of manufacture.


Based on my background I just started throwing aircraft analysis calculations at it from Roarkes (sparing details but happy to talk through it if anyone is curious) and determined that the original plywood design is capable of approximately 2-6 psi of loading which tracks with the ISO12215 sizing guidance for sheltered waters.


Using the guidance in ISO12215 for loading and maxing out all the giggle factors (seat belts, airborne, etc) yielded 16 psi. I understand that this is wholly unrealistic for a skiff but just figured I’d get another data point. My sizing pressure is somewhere between 2-16psi (haha). The calculation is basically F = m*a where a is a dynamic factor depending on how hard you intend to rally.


Using this pressure range I was able to calculate a suitable arrangement of stiffeners on the floor of the hull. The math says 5 longitudinal stiffeners that are an inch tall across the floor for 12 psi.


The size of the plate assumed in the stiffener calculation determines the spacing of the frames (I believe these are called scantlings?). The frames need to be sufficiently stiff to break up the large flat bottom of the boat into discrete sections.


Anyways, using the scantling and stiffener layout I calculated I was able to hone in a construction method: solid laminate vs. core vs. wood and trade against my arbitrary budget of 1000 dollars for the build materials.


Solid wood would be the most cost effective option, but the least weight competitive option because of the stiffness to weight ratio of wood compared to fiber reinforced composites. Carbon fiber is out of the question due to the astronomical cost compared to glass. The density of carbon is about 20 percent less than glass and the resulting laminate would be 20 percent stiffer for a common laminate orientation. I didn’t feel like these benefits were worth the premium price of the carbon.


I eventually selected a glass over ply construction, and targeted a wood thickness of 4 - 6mm with laminated skins if the following orientation: (9oz 7781/ 18oz 45 biax/ 9oz 7781/ wood/ 9oz 7781/ 18oz 45 biax/ 9oz 7781). This would cover the entire hull surfaces including the scantlings, stiffeners, benches, etc. Everything would be tabbed in with additional glass and resin.


The laminate thickness is approximately .040 inches thick on either side of the wood. So the total thickness is approximately .316 if I used 6mm wood.


The weight breakdown is approximately as follows:

50 pounds of wood

60 pounds of fiberglass and resin

10 pounds of foam (approximate 3 cu ft needed for 3 adults + outboard)


So I hit my weight target and depending on the resin (intending to use vinyl ester) I hit my arbitrary budget as well.


What I am hoping to have answered by the experts is:

-Does the QT design meet my basic SoR?

-any other recommended designs?

-Is the glass over wood construction over doing it? Can/Should we go lighter?

-Are there any other hidden weights that I am not accounting for?

-can anyone chime in on the limitations of the flat bottom design? Instability while turning etc.

-feel free to poke and prod if you’re curious or want to share some lessons learned


QT Skiff Plans PDF https://duckworks.com/qt-skiff-plans/

 

[philSweet]

The 10hp requirement is going to drive the design. The QT is limited to 5hp by the designer, and he states the CG might not approve that much. (It's a very easy calculation for small craft that you can find online from the Coast Guard small craft regs.) You will probably end up at slightly more than your target weight even if pushing the design hard. Tens go on trailers. 130# for a boat that can be registered with a ten hp and three passengers in the US is possible, but not easy. 160# gets you a much more capable boat that I would feel okay with on the Gulf of California. With a ten, it will plane two easily at modest speeds around ten knots. Look at the Lund 1240, for instance, and the jon 14'ers from Tracker and Lowe.

 

[Skunkworkz]

Philsweet, thank you for the reply.

Using the guidance in the backyard boatbuilder manual from USCG yields a rating of 7.5 hp when you bump down on the chart based on the flat bottom design. Adding two inches to either side of transom width (4 inches total) would yield a factor that would allow a 10hp outboard. Doesn’t seem unreasonable.

I appreciate your advice on weights. I think 160# is on the upper end of my weight limit but noted that you would consider that to be much more sea worthy than a 130# skiff.

The 30# difference here could be attributed to a couple of design changes like: increasing plywood thickness to .375 to .5 inches or adding another layer of biax on inside and outside. I understand that .5 would be completely bomb proof and worth considering.

 

[philSweet]

Basically, it comes down to length. I've never met a 13'er I really liked. But I cruised quite a few miles in a 14'er when I was young.

 

[Skunkworkz]

Really appreciate the feedback philSweet. 14ft is also reasonable to scale the design to. Whether I stretch the length, or scale all the dimensions by 7.5% seems like a valid approach.

hoping to hear some more feedback on whether the construction is overkill, or if there are any quirks of a flat bottom tiller from a static and dynamic stability standpoint that would warrant a different design.

 

[bajansailor]

hoping to hear some more feedback on whether the construction is overkill,

On the Duckworks site they say this about the QT Skiff :
"This is a simple nailed together job from three sheets of 1/4" plywood. No jigs, no lofting."

Whereas in your specification you mention three layers of glass on each side of 6 mm / 1/4" thick plywood.
And you show two longitudinal timber stringers on the hull sides, and 5 stringers on the bottom - all of which have to be sheathed in 3 layers of glass.

So yes, I think your construction is a bit of an overkill!
I appreciate your aeronautical calculations, and the reasoning there, but sometimes you have to balance the theory with a 'seat of the pants' pragmatic approach.

 

[philSweet]

Nothing earth shattering. I prefer a file-bottom design in general. Say a 6 degree vee. If built in 6mm fir or okoume or meranti, you get narrower bottom panels and you cut all the panels out in pairs. The vee builds in a lot of stiffness. And it's easier to bail and sponge out. When nose up on a trailer, rain water will all run out the one bung hole and not collect in the corners. The narrower panels also help with making a stich and glue build a bit more manageable.

I am also a fan of pram bows in this size. On thing they do is provide a way to get back aboard from deep water. Design so that can be done with or without an outboard attached. That usually means a short foredeck is needed to keep it dry and not be too hard an edge - like 6 inches - and a bridal rope that you can stand on. They also tend to make better use of sheet materials.

The skin thickness is probably more than you need. One drop of 9 oz cloth on the outside and a veil of 6 ounce on the inside will do the job (assuming painted). I'd use Dynel patches on the inside as extra reinforcing and built in nonskid. The Dynel is thirsty and it can be difficult to keep clean if you don't fill most of the weave in. Lastly, run one extra layer of 9 oz tape over all chines. 8" wide on the keel and 3" on the chines inside and out. The ability to do this is one of the nice features of stitch and glue, but you have to be pretty careful with workmanship throughout to get the glass tape to lay down properly after all those steps. All fillets and chine radii need to be well executed to accommodate the fabric.

You will need stringers. For the vee hull, one each side made from 12mm ply on the flat about 4" wide and beveled down to just 3/16 on the edges works well. You can put it inside or outside the hull depending on where you think it will absorb the most abuse. For me, sadly, it's usually on the inside (drops 80# crab trap with concrete base, fills boat full of furniture and drives cross country.)

 

[Skunkworkz]

Welcome to the discussion Bajansailor. I appreciate the pragmatic approach with respect to sending it with minimal modification to the original design structure. I was reviewing some larger boat plans and the glassing schedule I spec’d in my initial post isn’t even recommended for 20+ ft outfits haha. I definitely think that’s a reasonable knob to turn to dial back the weight. I think the chines/joints are the most critical so those will get some pad-ups like what philSweet recommended.

Thanks for another reply philSweet. I had to read it a few times to digest what you were saying and want to make sure I fully understand.

the 6 degree v seems like a no-brainer. Great input. The pram bow is also a great recommendation. I didn’t want to over constrain the SoR but it would be nice to dive from this boat so a bow transom/bridle is a great idea.

I updated the design to reflect your input. The length is now 13.5 ft with a bow transom angled at 35 degrees. You can see the width of the bow transom is sporty at 11 inches but should be do able to climb in with a bridle and internal rails/handles. Math tbd on the stability of climbing in and out.

With the added length and v bottom the calculated factor is 46 and should allow for a 10hp outboard.

I need to do some more work to layout the internal stiffeners, benches and fore deck so standby for another update but I’ll run with this design unless someone else or you have meaningful input.

Looking forward to hearing back.

 

[bajansailor]

She is now starting to look a lot like a garvey with her pram bow.
Although I think you might need some more curvature (in side profile view) of your hull bottom - perhaps similar to this 14' garvey?
Jon Boat / Garvey 14 Boat Plans (GF14) - Boat Builder Central https://boatbuildercentral.com/product/jon-boat-garvey-14-boat-plans-gf14/

Although she does not have any deadrise - this smaller garvey has 11 degrees of deadrise in the hull bottom panels, but she is a bit smaller, at 11' length.
Jon Boat / Garvey 11 Boat Plans (GV11) - Boat Builder Central https://boatbuildercentral.com/product/garvey-11-boat-plans-gv11/

 

[Skunkworkz]

Thanks for the info Bajansailor. Super helpful since I’m not familiar with the many different types of hull shapes. I went ahead and added more curvature to the shape and proportioned the dimensions to Spira’s Galveston Garvey.

the total curvature ended up being ~5 inches over the 12ft of the hull bottom. Let me know what you think, editing is easy at this stage. Also, I’m not aware of any of the quirks of too much or too little curvature or other hull dimensions, so Id appreciate if anyone is able to chime in on how they anticipate this hull might handle.

basic dimensions are:
Transom width: 40.8 inches
OAL: 13.5 inches
6 degree v
Bow transom width:~11inches
Bow transom angle: 35 degrees
Curvature: ~5 inches
Freeboard: estimated to be 12-14 inches
Factor: 40.8/12 * 13.5 =45.9

cheers. I appreciate the feedback.

 

[bajansailor]

I think that I would like to have increasing deadrise as you go forward from amidships to the bow, in similar fashion to the GV11 garvey in my previous post.

Have you calculated what the draft might be when static, in a typical loaded condition, say with 3 people, outboard motor, fuel, fishing equipment(?) and supplies?
If you cannot get the bow up in the loaded condition, the bow shown in your previous post might behave a bit like a bulldozer?

Even if you are in calm water most of the time, it would still be useful to have increasing deadrise forward if you occasionally meet slightly choppy conditions in the Sea of Cortez?

 

[DogCavalry]

How labour intense do you want it? For strength to weight ratio it is hard to beat stripper canoe type construction. More formally called strip plank composite. Unfortunately it would be a lot of work. Far more than plywood.

For comparison re scantlings, my boat is 26'x10', carries 4000#, weighs 4800# empty, planes at 33knots with 275hp on rough water. The hull is ⅝" wood with 34oz biax in epoxy outside and 28oz inside. No stringers. ½" ply in glass would be massive overkill.

 

[Skunkworkz]

Bajansailor, good point.

I calculated the volume displacement for 900 lbs of payload which accommodates an ample load of 3 adults, hull weight, fishing gear, outboard and gas. The resulting draft is 5ish inches which is right about at the waterline where the bow ramp begins. Based on how the gunnels curve upwards, the draft would increase as you move towards the bow.

5 inch draft corresponds to a minimum freeboard of 11 inches located about midship at 6.5 ft from the transom.

welcome to the thread DogCavalry. I appreciate the data point on your hull. Sounds like a sweet outfit. As far as labor goes, I’d prefer to keep it light. I’ll occasionally have a friend help but I’m not nearly crafty enough with wood working to make a strip plank craft in a timely manner. I’m confident in ply wood and composites but not necessarily strip plank haha. The resulting crafts are beautiful but outside of my wheelhouse.

cheers. I’ll post another update when I layout the benches and stiffeners.

 

[Skunkworkz]

Here’s an update of the final layout.

I scaled the width and draft dimensions to accommodate a factor of 52 and facilitate a 15 hp rating. Doing so required me to grow the transom and walls by an inch or so. The curves still look alright so I figure this is it. 52 = 13.5 ft (length) x 46 in (transom width).

The fit and finish on the inside is important, so I intend for rub rails and hand rails, as well as marine deck hatches. The large hatches ended up sizing the length of the benches. Who knows maybe the front bench could be a live well someday. 10 gallons would fill the front bench to half capacity.

With the price of the hatches, I think I’m going to spend almost 2,000 dollars. Considered buying a dinghy at this price point but I’m going to persevere. Having my own layout and running through the entire process will be immensely satisfying.

Going to document my build. If anyone is interested in that content let me know and I will post periodic updates. Otherwise, thanks for the help and feedback.

 

[bajansailor]

I still think it would be useful to have a bit more deadrise forward, tending more like a garvey, while ensuring that the point where the bow transom joins the keel is always above the waterline, even in the maximum anticipated load condition.

Going to document my build. If anyone is interested in that content let me know and I will post periodic updates.

Yes please - everybody on here loves build threads, so please do carry on with this current thread and give us updates (with photos! Lots of photos ) - including the sea trials when you launch.