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Building a scale 1973 Pay'N Pak from plans

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Building a scale 1973 Pay'N Pak from plans

Old 05-29-2021, 09:21 PM
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Hydro Junkie
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Default Building a scale 1973 Pay'N Pak from plans

Years ago, I built a sport 20 Pay'N Pak, starting with a Dumas kit. Since I didn't want to build it using the die "crushed" mahogany framing, I spent several weeks reworking parts and changing into a hull that would work. I even documented the process in a thread linked below:
Re-Engineering A Dumas Pay'N Pak - RCU Forums (rcuniverse.com)
At the request of a new builder from the land down under, I've agreed to do a thread on building a 1/8 scale 1973 Pay'N Pak. Like the sport 20 from last time, I will be making several changes from the plans based on measurements taken from the full sized boat, presently on display at the Hydroplane & Raceboat Museum in Kent Washington.
Before I actually get started on the build, I'm going to give a bit of information on the plans. The plan set was originally drawn by Roger Newton as the 1974 Miss Cotts Beverage. The full sized boat was originally the 1972 Notre Dame, so changing the plans to make any one of 39 similar boats was basically changing the color sheet that showed the paint scheme and the configuration of the cockpit, cowling and tail arrangement. The "Pak" was the fourth plan set drawn and was, however, different than the rest. The designer, Ron Jones, made FIVE major changes when he started working on the "Pak" from the previously built Notre Dame:
  • The transom at the back of the boat was flat topped while every other boat was arched upward a few inches in the center
  • The hull and all of the framing was built primarily from 1" thick aluminum honeycomb instead of wood. Only the deck and curved sponson skins were made from plywood with fiberglass on top
  • The cockpit, cowling and tails were welded aluminum instead of the normal molded fiberglass
  • Instead of a single centerline vertical tail faired into the cockpit, he installed twin vertical tails with a horizontal wing mounted on top
  • The cockpit was going to be in front of the engine in the now common "cabover" configuration. This last part was vetoed by the person buying the boat, Dave Heerensperger, due to the fact that it didn't work on the previous twin "Hemi" powered 1970 Pak, but that's another story
Due to these changes, Roger modified the plans to make the parts usable to build the Pak. He did, however, make some compromises that I will be fixing as I go.
Now, it's time to start the build.
The first change is the bottom of the sponson insides. Using measurements taken off of the full sized boat, the depth of the sponsons has been reduced from the plan depth and the air traps reshaped to match the full sized boat.
Change number 2 is the height of the transom and the deck shape of the rear of the boat. The plans have the transom shorter than the scale height. The transom on the full sized boat is 13" high, thus making the scale transom 1.625"(41.275mm), .25"(6.35mm) taller than shown on the plans. This requires redrawing the top of the engine bay walls and the sponson insides to make the corrected transom fit properly.
Change number three is I have to move a full width cross frame forward roughly an inch(25.4mm) to get the firewall at the rear of the engine bay into it's correct scale location and add a second bulkhead at the front of the engine bay where there is one on the full sized boat that supports a large oil tank used for the lubrication system in the Rolls Royce Merlin that powered the boat.
One change that I didn't have to make was the bottom of the engine bay walls. I went with a 4 degree rise at the rear of the airtrap section of the sponson insides which, surprisingly, matched the plans. It was also very close to the full sized boat which has a 5 degree rise.
Now that I've bored you all to death, it's time to cut out the lengthwise frames and add stringers to the bottom of them so there will be more to glue the bottom to. Pictures to come within the next few days

Last edited by Hydro Junkie; 05-29-2021 at 09:30 PM.
Old 05-30-2021, 10:15 PM
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I like it, 91 views and no comments. Considering there's nothing shown yet, that's not surprising.
Anyhow, I have the lengthwise frame cut out and took some shots to show the changes I've made. This first shot shows how much I removed from the sponson inside bottom. The straight line at the bottom of the picture is how the plan was drawn. Since this didn't match the full sized boat, I redrew it as can be seen below the cut out part. Something else that is very visible is the areas where I removed plywood from the middle of the part. This was done for two reasons. The first being to reduce the weight of the boat. With the sponson insides, much of the wood is inside the boat and not subject to being exposed to the water and, therefore, it's not really needed. The second reason is that if water does somehow get inside the boat, having sealed areas will trap water inside, making the boat heavier and eventually rotting the wood so this is a way to get water out.
The second picture shows how I notched the bottom of the sponson. This was done to align the bottom of the sponson to the bottom of the airtrap. The inner part of the sponson bottom is angled at 45 degrees, giving the 1/16th(1.5875mm) ply a thickness of 3/32" (2.38mm). How this all works will be shown later in the build


This shot shows the top of the engine bay wall. The cut away area is where the flash pan will be, something not shown anywhere except on the color sheet. For those not familiar with piston powered hydroplanes, it was not uncommon for an exhaust header to fail. When that happened, hot exhaust and still burning fuel would be sent at the deck of the boats and, since the boats were made out of wood, this would tend to set the boat on fire. Metal flash pans were used to prevent the exhaust from hitting the wooden structure.

This last shot shows the slot for the firewall and where the plan shows it to go. When Roger drew up the plan for the Cotts Beverage, the engine bay was covered. With the Pak, it normally ran without an engine cover so, to get the location of the firewall and the front of the cockpit correct, that frame had to be moved forward

Last edited by Hydro Junkie; 05-31-2021 at 12:00 AM.
Old 06-04-2021, 12:01 PM
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Now that I have a bit of time to work on the boat again, it's time for a quick update. I've (barely)had a chance to epoxy on the bottom stringers on two of the lengthwise frames, a major achievement considering how crazy it's been at work(been called in several times over the past couple of weeks). Now is where things get fun. I know that bass, spruce and fir are fairly soft woods that can be damaged easily. Due to how I'm planning to build the sponsons, I decided it might be better to go with harder wood. Since the boat is being built using birch plywood, I felt the obvious choice would be to use birch for the bulnoses and blocking in the front of the sponsons where it will be exposed. What I failed to anticipate is that birch isn't readably available so it's taken me two days to find some without having to order on line. I know some are going to say "Why use birch?" The most easily to understand answer is that birch is twice as hard as the readily available firs. Using the Janka Hardness Scale, the typical fir has a rating of 660, that being the softest of the normally used domestic woods. Yellow birch, on the other hand, has a rating of 1260. Now, for those like me that have never heard of the Janka Scale, it's a measurement of the force(in pounds in the US) needed to embed a .444"(11.277mm) ball half way into the wood. Getting back to the finding the birch, I ended up finding only one place that carried any birch(other than plywood) and they only sell it in 2ft(61cm) lengths. Needless to say, I figure it's better to have too much with costs skyrocketing, so I bit the bullet and bought a 4ft(122cm) piece, spending $20 in the process. At this point, I think I have all the wood I'll need to build the boat so it's time to start making sawdust

Last edited by Hydro Junkie; 06-04-2021 at 12:04 PM.
Old 06-06-2021, 07:30 PM
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Not much got done over the past two days due to other things happening. Today, it was an R/C Unlimiteds boat race that I went to part of, as a spectator. My race boat has had a couple of issues that I've been trying to get worked out, just didn't make it for today. Between rain showers and before running a few errands, I got to see four different classes of hydroplanes race:
  • 1:10th scale electric
  • 1:8th scale electric
  • 1:8th scale nitro
  • 1:6.7 scale gas
What made the trip worth it was not the boats or racing but, rather, who showed up. Part way through the second set of heats, Jimmy Shane(driver of the full sized Homestreet Bank/Miss Madison unlimited showed up with his wife to watch a few heats and pick up a scale unassembled fiberglass 0706 hull, the first one he drove for the Madison team. I met Jimmy at the HARM a few years back so it was nice to be able to say HELLO.
Getting back to the boats, I should have the rest of the stringers added to the lengthwise frames tonight and will follow that up with getting the three transoms laid out. I'll be doing things slightly different than what the plans show with these parts. To keep the weight down and strength up, I'll be making these frames out of two pieces of plywood, laminated together. The plans show all three parts being cut from 1/4"(6mm) plywood and, in previous builds, that is what I would have done. This time, all three parts will have a 1/16"(1.5mm) outer skin with 3/16(4.5mm) inner framing on the right sponson and rear transom while the left sponson transom will be framed with a second piece of 1/16"(1.5mm) plywood for a total thickness of 1/8"(3mm). I'm sure this will have people wondering "WHY?" The answer is weight. This inner frame will have a lot of the material removed where it's not needed so I'll be cutting lightening holes in the inner frames. As far as using thinner material on the left side, I won't need as much strength since the turn fin will be on the rear of the right sponson. Once I get the three transom part sets cut out, I'll weigh them and then weigh them again after the excess material is removed and they are laminated together. I'll try to keep a running total of how much the boat weighs and how much I've removed as I go along through the build.
As for my present racer, here's a shot of it running at slow speed:

As can be seen in this picture, there isn't a turn fin on the back of the sponson, hence the lighter construction of the sponson transom while the recent picture of the Pak being lowered into the Columbia River shows the turn fin, drive gear and rudder.

Last edited by Hydro Junkie; 06-06-2021 at 09:29 PM.
Old 06-06-2021, 09:15 PM
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Just got to thinking. I have one sponson inside and one engine bay wall ready to go. Assuming both sides come out about the same, I can take the weights off of the first two and double them for the time being and, if they aren't that close, update the weight later. What I measured the engine bay sidewall at is 4.85 ounces or 136 grams. The sponson inside weighs 5.3 ounces or 150 grams. That gives me a total of 10.15 ounces or 286 grams so, when I double it, I get 20.3 ounces or 572 grams. To keep this simple, I'm going to set up an Excel spread sheet so that I can keep track of the weight of wood, epoxy and hardware as I go. Should be interesting to see how it all breaks down when the build is complete. Since my goal is to be under 12 lbs (5.45Kg), it should be fun to see how close I get. One thing I do need to be very aware of is the back half of the boat has a lot of what can be called "dead weight" This includes the cockpit with all it's details and driver, tails and wing structures, all but the driver being visible in the picture in the previous post. To run properly, the boat needs to balance 1-2 inches(2.54-5.08cm) behind the sponsons so the weight at the back must be kept to a minimum or extra ballast will be needed up front to balance the boat

Last edited by Hydro Junkie; 06-06-2021 at 09:28 PM.
Old 06-14-2021, 05:08 AM
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I have finally had a chance to get the other two lengthwise frame bottom stringers installed. With that done, the boat weight is 20.3 ounces or 576 grams. My next task will be to get the three transoms made. HOPEFULLY, that will be within a couple of days as this first step has taken way to long due to work and the wife's "Honey Do" list

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