Scale Scarab 38KV scratch build

[70skid]

Hi everybody,
I'm usually not one to start build threads, but I've been finding so many great tips and suggestions from other builds on this sub-forum that I thought I'd make an exception.

The inspiration this time around is to build an RC version of the Miami Vice Scarab 38KV. As a kid this boat was an aspirational obsession, and a recent Ebay posting for the old Dumas 55" model threw me over the edge.

The Dumas model is not very scale, about 10" too short for its beam, and there are no scale outdrives that I've come across that match the model's ~1/8 scale. I'm sure it was optimized for performance, but I'm more interested in accuracy.

The nicest looking outdrives I've come across are the Arrowshark 1/5 scale M8 outdrives.
M8 Collector Edition-Twin_M8 II Collector_Scale Drives_Arrow Shark RC Marine
However, to make this work on a scale hull, I'd be looking at a 90"x20" hull. That's too big for me right now.

For a more realistic scale, there are these TFL outdrives from OFE that look really nice. TFL P1 Drive System with 2960 Motor | Offshore Electrics
From what I've gathered, these are about 1/12 scale, which puts me in a 38"x8.5" hull. This is a much more realistic size for me right now.

As such, I've decided on 1/12 scale for size.

[70skid]

Next challenge was finding plans. In summary, they don't exist, so I was tasked with drafting them myself. I found two helpful tools. The first was a 1/36 scale plastic model of the 38KV on eBay. The kit is from 1988, and if you want the Miami Vice version, it'll cost you $200-$700. However, there were other variants that were made, and these are more reasonable. I picked one up for $85. Still overpriced, but it was a sealed kit and the dimensions are those of the 38KV.

The second tool is the free software, Freeship. (FREE!ship download | SourceForge.net) My plan was to transfer the measurements of the plastic model into the software and then create the electronic version of the boat. I could then scale it to my final size for the RC model and print out the full-sized plans.

[70skid]

The first actual task was getting critical measurements off the plastic model. To do this I needed away to hold the hull perfectly level while pulling measurements. I made a wooden brace that captured the hull chines at the transom. This leveled the hull as well as I could hope for. Next step was then pulling measurements at specific distances along the side. I chose 50 mm increments and penciled a straight line down the hull side. With a micrometer I pulled the key measurements off the model.

[70skid]

Second task was uploading this data into Freeship. The learning curve was at first steep, but there's a collection of YouTube tutorials that got me to a working level of competence. The videos from Army Lake Boat Co. resonated with me. Those videos allowed me to draft the hull along with the keel and bulkheads. After many tweaks, I believe I have a pretty accurate representation of the plastic model's hull. An overlay of the plastic hull against the computer monitor allowed me to get the final shape really close.

[Xrayted]

Very cool project! Interested to see your progress on this one

Good luck with the build!

[70skid]

Here are some screen shots of the hull shape in the Freeship software. I've added several bulkheads as potential ones to go in the actual model. Additional ones were added in the software to achieve better curvature of the lines.

[70skid]

The current stage is deciding on a final scale. I started with 1/12 and managed to find a way to print out the actual sized keel and bulkheads. From the measurements included in the TFL outdrive pictures, I started to work out where they would go on the transom. OSE has some really close to 1/12 scale trim tabs Adjustable Triangle Trim Tabs | Offshore Electrics, so I added those measurements onto the transom.
Along the way I realized that the transom blueprint did not accommodate for material thickness. With components being really tight, I added 1/8 to the outer dimensions. This gives me a little more wiggle room. Adding this additional area to the transom required a re-scale. The adjustment brought the final scale to 11.6, with a 39" length.
Before making any cuts, I wanted to have the actual outdrives and trim tabs in hand. These have been ordered and should arrive in a week or so.
The current biggest burning question is what to make the hull out of. As an initial leap, I ordered some 1/8 and 1/16 Finnish ply from Aircraft Spruce, Finnish Birch Aircraft Plywood (Metric) | Aircraft Spruce. An old school wooden boat is the intended plan.
I figured on making the bulkheads and keel from the 1/8 ply doubled up for 1/4 parts. For the hull and deck, I'm leaning towards the 1/8 ply, but with some reservations. I'm a bit apprehensive about weight and want to be mindful of not overbuilding this thing. Going with the 1/16 ply would save weight, but I'm not certain of how durable it would be. Adding some 1-ounce fiberglass cloth is a hesitant option, but that gets messy and heavy fast.

[70skid]

There are a growing number of tricky details to address.
For one the chine will make it difficult to oversize the ply. Notice the lower hull is recessed inboard on the chine. I've got to get the plywood shape pretty close prior to install.
Also, I can't figure out how to create the chine rail in Freeship. I think I may have to start with a new file to create this part as the existing hull may be getting in the way of the program's ability to see just this one component. Having this shape means I can create it on plans to cut it out.
Another detail I'm thinking through is how to determine battery placement. I'd like to keep the cockpit floor to scale depth, but that means moving the batteries pretty far forward under the instrument console.
I'm wondering if there's a good way to figure out where the hull's center of gravity is. This would allow me to optimize the battery placement.
Another question is whether to go with a fiberglass outer skin over the thinner 1/16 ply. I'd be interested to hear what others have to say about this.
As you can see, I've got lot's of thoughts running around in my head at the moment...

[70skid]

Yet another detail to iron out is how low to mount the outdrives. Using a clean transom at 11.6 scale, I repositioned the trim tab and outdrive. The outdrive mount is square and quite large. This limits how low the outdrive can be mounted. The "S.P." notation in the drawing is where the outdrive would be surface piercing (prop hub is in line with the keel). The "full depth" notation is really how low I can go in the transom before the mounting hardware hits the physical floor of the hull. As you can see there's not much room to play without modifying the outdrive mount. I've been reading up on issues with twin surface drives, and they include the outboard prop lifting out of the water during turns to name one. I'm not sure if this would be noticeable with this model given its rather slow speed setup. Nonetheless, I'm curious if anyone has any thoughts on the benefits of surface mounting these outdrives. I'm wondering if it would reduce the overall load on the motors/batteries.

[70skid]

The OSE order just showed up, so I've got some great pictures of the key hardware.
First picture set is the outdrives. These units are really amazing, and they look better in person compared to the on-line photos. Their construction and the overall quality and finish is very high. The way the motor is mounted to the outdrive unit is also really clever. These units utilize a drive dog, which I've seen have some issues from others on this forum. I'll be inspecting mine carefully, so we shall see...

[70skid]

The next picture set is for the trim tabs. These are also really nice, and they look very much like the ones from the Scarab plastic model kit. For this scale they are just a little oversized at 1/9th scale, which translates to 1.38" tall instead of 1.09" (for 1/11.6th scale) for the mounting bracket. I don't think it'll be noticeable.

[70skid]

The final picture set today is the hardware mockup on the paper transom (SB side). As mentioned before, the spacing is cramped. I've got the outdrive positioned in a surface piercing position, where the prop hub is in line with the hull bottom. I've got 3/16 movement up and down, but that's about it. A few observations:
1-The bellcrank barely misses the trimtab at full lock. I'll have to consider this when designing the pushrod system for steering. I should be able to move the outdrives inboard if necessary.
2-As positioned from the transom centerline, the outdrives are consistent with where they are on the plastic model. In real life, the outdrive spacing was dictated by the width of the motors. I believe they pushed them as close together as practically as possible, but I can push them in a little further if necessary.
3-Flush mounting. I had visions of flush mounting the outdrive mounting bracket into the transom since it's a big obtrusive block. However, I quickly realized the bellcrank goes into the transom if flush mounted. To avoid cutting out sizable clearance holes in the transom, these will have to be mounted against it. The mount is 3mm thick, so this is one small ding against a scale appearance. I think I'll be fine:)
4-drive angle adjustment: OK so this could be a significant limitation. As designed, these outdrives have a +/-3 degree angle adjustment. The transom is significantly angled down more than this, so at best, these outdrives will operate with a downward angled bias. For reference, the trim tab is adjusted to be in line with the hull bottom. Does anyone have any predictions or thoughts on how this will affect performance?
5-Final thoughts: If this model never sees water, but looks the part, I'll still be happy. The hardware quality is so nice that a display-only model will still be a success. If on the other hand, I can get this to operate such that it doesn't overamp or have any seriously bad performance issues then I'll be thrilled.

[paulejr]

Looking good there skid. But, somebody forgot the popcorn..... waiting for this build to hit the water, and hopefully not sink.. stern gonna be a tad heavy ya? lots of floatation . Battery size a concern. when I converted a Dumas gas 1/10 AVL to FE , I made cardboard blanks of the three battery sizes i wanted to run. trying to determine what type of use, race or recreational running... times and prop will dictate which you use , resizing the bulkheads to fit the batteries I chose was perplexing ... since no clubs near so no racing perse' , but SAW is always a throb... last minute changes for battery placement could mean ripping out bh's and reworking that strategy.

[Returntohome]

You have taken a lot on , so many variables, any of which could trip you up. I ran the tfl outboard for a while, it has exactly the same gears as the P1. I was servicing it every few runs the bearings fall apart, so a big improvement was replacing them with bronze bushes, no loss in performance. The gears don't last long with the supplied motor, a much smaller motor helps no end and still gives good speed. I sold a rib with my last tfl outboard on to a club mate and he's having a lot of fun with it , with the mods it's quite reliable.

[70skid]

You have taken a lot on , so many variables, any of which could trip you up. I ran the tfl outboard for a while, it has exactly the same gears as the P1. I was servicing it every few runs the bearings fall apart, so a big improvement was replacing them with bronze bushes, no loss in performance. The gears don't last long with the supplied motor, a much smaller motor helps no end and still gives good speed. I sold a rib with my last tfl outboard on to a club mate and he's having a lot of fun with it , with the mods it's quite reliable.

thanks for the heads up on bearings. I was pricing some out from Fast Eddy earlier today. I noticed the lower unit bearings are press fit, so pulling them out looks like a small project. The gears are brass, so I suppose they’re too soft for the task? I’m going to see about adjusting the mesh in hopes this gives them a longer life. TFL sells the gear set for $35, so maybe I should start thinking about a spare or maybe two.
for the smaller motor, what would you recommend as a starting point?