36" Dumas Atlas U76

[peter ostertag]

Hi All

I have an old Dumas Allas U76 I built for nitro years ago but never finished, it's just been collecting dust, so I decided to continue the build and convert it to electric just for fun. Doing some research on this site I learned a few things that would improve the design of the boat like reducing the sponson depth, cutting lightening holes in the old framing and replacing some of the wood with 1/16 aircraft ply. Thinking of using 2x3s in series so i can put them on either side of the motor. I took the bottom sheathing off to gain access to the framing, this is where I'm at with the rebuild. Let me know if guy guys have any other tips for this old hydro.

 

[Hydro Junkie]

I would make three changes before reinstalling the bottom panels:

1. add glue blocks even with the bottom of the cross frames on the inside of both sponson insides to secure the new bottom to. Epoxy alone won't hold up the the flexing and pounding the hull will take without something to secure the bottom to, other than just butting the bottom against the sponson insides
2. add a doubler, either 1/8th thick 6061 aluminum or 3/16th aircraft grade plywood, to the inside of the right sponson transom to secure the turn fin bracket to. If you use the aluminum, you will need to tap threads into the holes you drill while with the plywood, you will need to install blind nuts or threaded inserts to secure the mounting screws into. If the kit had the fin on the face of the sponson inside, it won't work. In that location, the fin will throw water up against the bottom of the hull, making it much more likely to roll in the turns
3. add glue blocks to the sponson lengthwise framing as in my first change. Again, the outer bottom panel won't hold up, especially on the inside edge secured to just epoxy.

Here's some links to pictures I took while building a redesigned 30" version of that kit several years ago. All the glue blocks and stringers are 1/8th inch spruce:
https://www.rcuniverse.com/forum/attachment.php?attachmentid=1647750&d=1375582311
https://www.rcuniverse.com/forum/attachment.php?attachmentid=1647753&d=1375582311

One last thing that you might want to consider is the widen the tunnel. I know people that have tried to race the Dumas 36" blue Atlas, without any luck. One person cut the hull in half down the middle of the engine bay and widened it an inch. That modification made the boat more stable and prevented it from rolling over due to engine torque. I made the same change on my build, as shown in this picture:
https://www.rcuniverse.com/forum/attachment.php?attachmentid=1647744&d=1375582310

 

[peter ostertag]

Hi Hydro Junkie, Great information and detail, Thank you! I actually started adding the 1/8 spruce along the framing



I took about 5/8 off the sponson depth, tried to keep the same profile. What's your thoughts on the outside of the sponson? I'm trying to keep the ride pad the same dimension I was going to add a step or angle the side sponson.




Thanks for your input.

 

[Hydro Junkie]

Going off the full sized boat, the bottom is angled upward at 3 degrees from the sponson inside to the chine at the outside edge of the sponson bottom, as shown in this picture. The outer lower panel is probably more like 10 degrees, the inner is 50 when compared to the sponson inside. Something else of note is where the turn fin bracket is located, right at the joint of the bottom of the sponson side and the sponson bottom. This is where I would locate the turn fin on the right sponson transom.

With that information, it becomes a judgement call on your part. I know that, with my 1/8th scale build, I've redrawn parts to be closer to the full sized boat than the Dumas kits or as shown on my plans. Here's what I did to make my sponson insides more accurate in depth and the shape of the air trap. The picture also shows how I removed a considerable amount of wood to lighten the boat:

 

[Hydro Junkie]

How about a bonus shot? Here's a picture I took in late 2021 outside of the Hydroplane & Raceboat Museum. From what I can tell, the boat is either going on a road trip or had just gotten back. The boat normally doesn't have the cockpit and engine covered while at the HARM

 

[Jerry Dunlap]

If you go with a 6 cell configuration, make sure your ESC/motor are capable of handling the voltage/current draw. If you would like the boat to corner like the full size boats, ABC make reverse pitch propellers. Most 1/10 scale hydroplanes are in the 35"/36" range and use a 4S pack and an ESC rated 150 or higher. A 2,000 kv motor works well in 1/10 scale.
JD

 

[Hydro Junkie]

I forgot about the props spinning clockwise on some electric boats!!!!!!!! Thanks for mentioning it JD
I've been thinking nitro set up, not electric, so he may need to install the turn fin on the left sponson, like on the full sized boat and the 1/10th scale hulls.

 

[David Krienke]

I would like to know where they wanted it to balance. The one I restored I was never able to get the balance point that far forward .I thought to make my turn fin have a long bracket and just bring it back to the balance point . Would that work or just let it be tail heavy?

 

[Jerry Dunlap]

I would like to know where they wanted it to balance. The one I restored I was never able to get the balance point that far forward .I thought to make my turn fin have a long bracket and just bring it back to the balance point . Would that work or just let it be tail heavy?

One nice thing about FE powered hulls is often the batteries can be moved forward to allow the balance point to move forward. I will take a photo of my 35" 1/10 scale Coors Light using 2 2S packs in series and post them.

 

[Hydro Junkie]

OH NO!!! NOT THE COORS LIGHT!!!!!
A long bracket will add a little weight to the back but that's not the biggest issue. The bigger issue is that a longer bracket can flex under load or it can damage the sponson transom's wood, letting the bracket move even more, causing more damage. You'd be better off using the batteries or motor & ESC location to balance the hull

 

[David Krienke]

OH NO!!! NOT THE COORS LIGHT!!!!!
A long bracket will add a little weight to the back but that's not the biggest issue. The bigger issue is that a longer bracket can flex under load or it can damage the sponson transom's wood, letting the bracket move even more, causing more damage. You'd be better off using the batteries or motor & ESC location to balance the hull

Mine is a old sport 40 with a k&b 45 in it. I would have to add 15 oz to the nose to get it to balance at the fin. that was why I wanted to either cut the back off 6 inches or make the fin sit further back.

 

[peter ostertag]

If you go with a 6 cell configuration, make sure your ESC/motor are capable of handling the voltage/current draw. If you would like the boat to corner like the full size boats, ABC make reverse pitch propellers. Most 1/10 scale hydroplanes are in the 35"/36" range and use a 4S pack and an ESC rated 150 or higher. A 2,000 kv motor works well in 1/10 scale.

Hi Jerry, thanks for the info on 6s vs 4s. I was thinking 6s because of the original weight of the boat but it’s gotten much lighter as I cut away at it so maybe 4s is the way to go.

 

[peter ostertag]

Going off the full sized boat, the bottom is angled upward at 3 degrees from the sponson inside to the chine at the outside edge of the sponson bottom, as shown in this picture. The outer lower panel is probably more like 10 degrees, the inner is 50 when compared to the sponson inside. Something else of note is where the turn fin bracket is located, right at the joint of the bottom of the sponson side and the sponson bottom. This is where I would locate the turn fin on the right sponson transom.
View attachment 333448
With that information, it becomes a judgement call on your part. I know that, with my 1/8th scale build, I've redrawn parts to be closer to the full sized boat than the Dumas kits or as shown on my plans. Here's what I did to make my sponson insides more accurate in depth and the shape of the air trap. The picture also shows how I removed a considerable amount of wood to lighten the boat:
View attachment 333449

Gray stuff Hydro Junkie, I’ll go with the angle to the bottom. I don’t think I’ll be at scale at this point but will get it close. Thanks

 

[Jerry Dunlap]

Here's the battery placement for my 1/10 Scale Coors Light. Well, Mark the Coors Light is a little more colorful than BOAT. And, with an FE you can use rattle can paints. I would try running the boat with the turn fin attached to the back of the sponson and move the batteries to adjust the balance. I would recommend flex cable drive as this will allow you to use the prop depth/angle to adjust the ride angle.
JD

 

[peter ostertag]

Mine is a old sport 40 with a k&b 45 in it. I would have to add 15 oz to the nose to get it to balance at the fin. that was why I wanted to either cut the back off 6 inches or make the fin sit further back.

I’ve also been thinking of the balance point, is it at the end of the sponson transom or mid point of the turn fin?

 

[David Krienke]

I think its supposed to be at the leading edge of the turn fin Ideally. But I am unsure on these old hulls that was why I was curious what others thought on it.

 

[David Krienke]

OH NO!!! NOT THE COORS LIGHT!!!!!
A long bracket will add a little weight to the back but that's not the biggest issue. The bigger issue is that a longer bracket can flex under load or it can damage the sponson transom's wood, letting the bracket move even more, causing more damage. You'd be better off using the batteries or motor & ESC location to balance the hull

I totally get you how it would tear up that rear mount surface on the sponson. I was thinking of maybe attaching it to the side of the hull too for more support if I tried it. . I haven't even ran it after I finished it and saw it wouldn't balance. I figured it would just be a flop if I tried it.