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Build 6: SI3 Hydro .60/.90 size

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Old 06-22-2015, 03:19 AM
  #26  
1QwkSport2.5r
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Through the help of crispy, I decided to use wood for my deck stiffening strut of which I glued right to the deck. It made a HUGE difference. Used white oak which is what the hull is bordered with. I sanded it all down and recoated it with epoxy finishing resin after the deck struts cured. Now to sand the boogers out of that coat (damn dust nibs) and put a coat of polyurethane on it and clean up the bottom and it's going back together and getting re-maidened next weekend.
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Old 06-22-2015, 09:31 PM
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Hoping to get some real good video of it along with lots of other boats.

Taking some scale boats, and some fast electric too. The swordfish 300a ESC I got for my 1/4 scale SI electric was DOA so I won't be trying to break 70 with it anytime soon.
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Old 06-23-2015, 03:59 AM
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Looking great, watching this with interest
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Old 06-23-2015, 04:03 AM
  #29  
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She's just about done and RTR. All that's left is the turn fin bracket and she's ready to go. The deck stiffeners made a HUGE difference in making the deck more rigid. Massive improvement. I'll weigh it when I get the turn fin mounted - should be about 8.5-8.7lbs without fuel. Hoping she goes like stink.
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Old 06-23-2015, 11:50 AM
  #30  
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She's done and RTR. 8#9oz minus fuel. Not bad, but could be lighter. Oh well - I'll see how it runs as-is.
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Old 01-16-2016, 08:00 PM
  #31  
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After the last outing, we determined that the trailing edge of my sponsons was a bit too flat which caused extra drag. So I'm going back to the drawing board and remodeling the sponsons. What I'm not going to do is tear the sponsons off and rebuilt from scratch - I don't want to disturb the integrity of the plywood deck and risk delaminating the plies. The sponson sides are epoxied and fiberglassed to the deck pretty well, so I'm not going to risk it. If this next trial doesn't work, then I'll go back to square one.

So... What I'm going to do is build up the keel of each sponson in incremental steps with a layer of plywood each step and sand the leading edge of each piece of plywood to taper it down to essentially step the trailing edge of each sponson so there is no flat spot. It will probably take a few layers to get it built up enough to where I'll be happy. So the first step was to remove the old skin, glue new balsa support strips and add plywood.
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Old 01-16-2016, 11:08 PM
  #32  
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Here is a better idea of what I'm doing and where the progress is going. Once I get all the "steps" epoxied onto the sponsons, I will block sand the bottoms until the rake is smooth and then add one more layer of 1/32" plywood to the whole length of the sponsons and final sand/shape so I can get some epoxy back down. Once the sponsons are done, I am going to re-work the rudder. It's all fubar after the maiden runs with the animal engine on it. First "step" is 1/16" and covers half the sponson (first pic). You can see the rake angle is actually okay I think - this is how she rides in the water pretty much (second pic). Added the next "step" (third pic). The sponson rake is about 10" long, so the first step is 5" long, second 4" long, and so on. I figure that should be spaced out enough to still get a nice taper on the rake plus give me the height I need to ensure she isn't riding on any flat spots. I know I won't hit 65mph with this beast, but if I can just get better than the 45mph readings I was getting last summer, I'll be happy. Maybe. Who knows - I may just burn the damn thing if it still sucks. I did build it from actual plans for crying out loud...
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Old 01-18-2016, 03:40 PM
  #33  
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I've got all the "steps" glued on, now to sand it flat and get the new skin glued on.
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Old 01-20-2016, 06:21 AM
  #34  
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Looks to me the steps are going in the wrong direction?
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Old 01-20-2016, 06:45 AM
  #35  
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Originally Posted by asmithnc View Post
Looks to me the steps are going in the wrong direction?
Instead of destroying the deck trying to remove the sponsons and rebuild them, I'm building up the trailing edge of the sponsons with plywood. What I'm doing is sanding down the edges of those steps so it's all smooth and then glue on a single piece over the whole length of the sponsons. Even if I had the edges going the opposite direction and left it like that, each edge would possibly create a vacuum under the sponsons causing drag.
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Old 01-20-2016, 07:17 AM
  #36  
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Understood! Good luck with the build!
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Old 01-20-2016, 09:45 AM
  #37  
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It's just an experiment. This hull may end up at firewood if it doesn't run any faster. I can put the "animal engine" on my outrigger if need be.
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Old 01-20-2016, 11:33 AM
  #38  
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You're going to put a Briggs and Stratton Animal on it. Woo-oh, slow down! That a big engine on a tiny boat...... Sorry, the Animal is an engine produced by Briggs. From my go cart racing days!
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Old 01-20-2016, 05:09 PM
  #39  
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Sorry - Dub Jett once called the 56LX the "animal engine". I got the 56LX for the outrigger, but figured I'd try it on the big SI3 to see what she'd do. Turns out it wasn't that great. At least first run around anyway.
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Old 01-20-2016, 08:06 PM
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Originally Posted by 1QwkSport2.5r View Post
Sorry - Dub Jett once called the 56LX the "animal engine". I got the 56LX for the outrigger, but figured I'd try it on the big SI3 to see what she'd do. Turns out it wasn't that great. At least first run around anyway.
It's funny how you refer to a 60 size as "big". LOL
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Old 01-20-2016, 09:16 PM
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Originally Posted by crispyspa View Post
It's funny how you refer to a 60 size as "big". LOL
Well, it's my biggest boat, and bigger of the two SI3s I have... So it's fitting. LoL. But yeah - I get what you're saying.
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Old 01-23-2016, 07:54 AM
  #42  
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I sanded my stepped sponson modification, and I put a coat of 5min epoxy over it all and sanded it smooth. Ready for final sheeting. After the sponsons are fully sheeted, I'm gonna re-sand the bottom of the deck and re-seal it. I noticed some cracks in the epoxy around the base of one of the sponsons that I want to repair, but in order to repair it, I need to get back to bare wood. So anyway, a little more work, but should make it more sturdy and maybe lose a little tiny bit of weight.
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Old 02-01-2016, 07:25 AM
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Originally Posted by 1QwkSport2.5r View Post
Instead of destroying the deck trying to remove the sponsons and rebuild them, I'm building up the trailing edge of the sponsons with plywood. What I'm doing is sanding down the edges of those steps so it's all smooth and then glue on a single piece over the whole length of the sponsons. Even if I had the edges going the opposite direction and left it like that, each edge would possibly create a vacuum under the sponsons causing drag.
I don't know how much you know about hydroplanes but many of the boats have steps or recesses in the running surfaces to reduce the affect of water along the skin. These areas break the water's hold and allow the boat to plane much easier. These can be seen on the picture of the Oberto unlimited hydroplane shown below:
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Old 02-01-2016, 09:23 AM
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[QUOTE=Hydro Junkie;12170721]I don't know how much you know about hydroplanes but many of the boats have steps or recesses in the running surfaces to reduce the affect of water along the skin. These areas break the water's hold and allow the boat to plane much easier. These can be seen on the picture of the Oberto unlimited hydroplane shown below:
/QUOTE]
Some hydros have stepped sponsons and/or center tubs, but this design is proven to be fast without any steps in the keel (65.1mph). In reality, given the weight of the hull, I'm not sure I would want it that much faster anyway. They tend to go airborne.

Just to add; the plywood "step" sheets are 1/32" thick and have been sanded smooth and is awaiting a final full sheet of skin sheeting. Unfortunately this project is on hold due to having to move to a new house.
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Old 02-03-2016, 01:31 PM
  #45  
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Since I'll be moving in the next few months, I needed to get my works in progress more put together so stuff doesn't get lost or broken in the move. Ugh - I hate moving. Anyway, I put the final sheeting back onto the sponsons on this pig. First pic is self explanatory, second pic shows how much I raised the trailing edge of the sponsons. This should make the actual contact area with the water as minimal as possible. I'm crossing my fingers this works. Needs to be sanded and coated yet.
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Old 06-30-2018, 03:34 AM
  #46  
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Hey Tim. Long time no see. I've been out of the RC world for a few years because I lost EVERYTHING to a house fire and the thought of how much hobby related stuff I lost and the thought of starting over again was too much. But, once a glowhead, always a glowhead. Lol. I just HAD to smell that nitro again. I figured the easiest way to get back in was with airboats. Well, I'm hooked again. Lol. I wish I found this thread before I scratch built a SI3. Anyway,I'm building one and have a few questions. How do you balance one? How important is the turn tab on the front sponcen? Looking forward to hearing from ya.
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Old 07-01-2018, 01:25 PM
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Aw man Steve!! I’m so sorry to hear about your fire!! I’ve thought about you more recently. Hopefully insurance covered most of it? if you want to chat via email, PM me your email address.

Balance an SI3 3-pointer so the bow hangs about 20-30 degrees below “level”. This is a starting point and should be adjustable via moving the engine back and forth a little bit.

The turn fin isnt necessary if you make wide turns, but it is necessary for sharp high speed turns. It keeps the inside sponson planted (left side is the turn fin side).
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