Excellent project Howard.  I look forward to seeing more.

I totally agree with your comments regarding speed of the Petrel Play.  It is a wonderfull hull, but at about 5mph it hits a brick wall.  For comparison, the Ch17LT is about 0.5mph faster and the Mystery is about 1.5 mph faster at full effort.

Howard,

Had you considered keeping the bottom 3/16" and adding stiffness with a layer of unidirectional carbon fiber?

Second, where are you launching in Spa Creek? Is Truxton Park safe for kayaks again?

Laszlo

 Thanks Mark,

Yes brick wall is a great description.  When we  are paddling along with a group and we need to take it up a couple notches, the folks in standard 17 foot sea kayaks just march away.   It’s a very sudden onset of resistance on the petrel play hull which for most of the rec paddling  (65%) is only moving at about 4 mph….as is everybody else.

I do also want to discuss for folks a little bit more of using dual core 3/16 and ¼ inch and why that is important on this boat as well as some other bits to get to the tageted weight of 25 lbs.

One of the things that makes a petrel or petrel play hull sublime in its handling qualities is the blended hull form.   It is rounded on the bottom fore and aft of the cockpit (e.g., rounded bilges).   But there is approximately a 3 to 4 foot area under your butt and extending aft that is hard-chined and the floor is flat absent a slight v in cross-sectional shape.   This hull form, if you haven’t experienced it gives an amazing combination of properties i value and is also very efficient.  It  can turn easily, go straight easily, respond to leaning and is a great blend of performance and stability.   Romany's, which are renowned for their paddling properties use a similar hull form.

With respect to construction and stiffness, where the hull is rounded, it is self bracing similar to rolling up a piece of paper into a tube.  Without being rolled up, paper flops around, rolled up into a tube it becomes stiff and rigid.

So for the curved sections of the hull, the hull layup recipe  (the combination of glass and core  (e.g., the wood) and epoxy)  can be less less stiff (stiffness measuremennts for various hull layup recipes are always measured on flat plates) yet perform stiffer because it is curved.

For the flat sections, you don’t get the curve/self reinforcing effect so you simply have to have a stiffer recipe.   From a recipe perspective, a thicker core (vs adding more glass and resin) dramatically increases stiffness. So a ¼ inch core is almost twice as stiff as 3/16 inch core with the same skins (epoxy and glass).   The key thing in this build is I will only go with the stiffer recipe on the flat sections so my increase in weight is limited to less than10 square feet.

Each recipe has a predicted ounces per square foot (weight of resin, glass and core for a 1 square foot plate).     So paying attention to total square feet of hull is important too.   And  bigger boats have more skin.   So if you have two boats of the same length but one has high sides vs the same design with less freeboard, you can also accrue the weight savings of less square feet of hull.  So the other element of the modification to the project is reducing the boats  freedboard by 1.5 inches resulting in reducing the square feet of the boat skin by 4 square feet or 10% compared to its baseline.    

Finally paulownia is about 18 lbs per cubic foot vs 20 lbs per cubic foot for western red cedar….or about 10% lighter.

Take all of the factors above and the ‘recipe’ book, the boat calculates out to about 23.5 lbs before the rigging, seat, and misc fitting.

I also want to acknowledge Laszlo’s question on carbon fibre and keeping the bottom 3/16, and spa creek.

I have looked at carbon fibre (e.g., change the recipe to have skins with less stetch (i.e. more tensile strength) and like everything, in addition to the physics and the recipes, there is something I call workability – can I duplicate the lab results in the real world with the tools I have. 

The challenge I have had with carbon fibre (and i have a couple square yards that i have played with) is most of the layups I have researched and their predicted weight and stiffness are based on applying some kind of vacume bagging technology with the carbon fibre to minimize the resin (and resin weight).  

In my personal experience with carbon fibre, and as somebody who does not have nor wants to have vacume bagging technology given all the extra stuff involved) I can’t get the weight performance on the hull because I can’t easily control the resin through the remaining means I have available.   

I do use carbon fibre for my fins and skegs and small parts,  but on these bits, I can typically pull saran wrap over the piece to compact the resin/fibre onto the pieces to keep the resin weight low or the piece is so small the extra expoxy doesn’t amount to anything significant.      

I guess what I am saying is I am more comfortable, given my skills and tooling, that for a hull build that I will get the expected results of stiffness and weight targets with a thicker core than with carbon fibre.  

The other change I have in my builds is I build with S glass (vs E-glass) which is between e glass and carbon fibre in terms of tensile strength but has the working properties of e-glass which I can easily control.

On spa creek, I see folks launching from truxton park all the time.  They have the signs up saying its dangerous or something like that due to electricity in the water.  I don’t get it.   They certainly are not stopping anybody which I expect they would if it was really dangerous.    I launch from a dock a hundred yards away from it but routinely paddle there.   Call me and happy to meet you over there for a paddle.

h

   i was away on holiday for a week of sailing and finally back to my micro-bootlegger sport project.

this evening finished stripping the bottom of the hull.  top picture is the hull and the next picture are the tools.

the strips, as mentioned before, are paulownia with rectangular sections so no cove and bead.   as usual the tool that got 75% of it done was the stanley low angle block plane.   the robo-bevel tool from nick shade got some good use as this particular hull form has a lot of nice, straight runs.   a new addition this year to the tool box is a vintage stanley #79 side rabbet plane that i picked up on e-bay and did a nice job doing the center line.  i had seen a video of nick using one and was hooked on it.  while not used a lot, if you have to do a center line, it makes it very easy ....and lastly, an exacto knife for some hard to get into places to take a shave off.  really not a lot of tools at the end....and combined with paulownia that is very easy to work with, was very pleased how easily the bottom of the hull stripped up.

next step is to flip it over and start on the deck....but a glass of whiskey  first - or half a glass....and we will start that after celebrating the little victory.

h