Gas Scale Info

[Dave Larocque]

I'm about half way through building an ML 171 gas scale hull. I'm hearing some folks are having issues getting the CG to 1.5" to 2" behind the sponson rears. I'm wondering where builders are setting their engines? Are you guys leaving the manual starter on the engine or opting for a geezer pulley? Are you staying behind the solid bulkhead (#7??) with motor placement? Adding weight to the tips? Any reinforcing of the hulls? Where do most of the builds weigh in at? I see on the vids that the boats seem to run and steer well. It's a beautiful kit that I'd like to get right. We are trying to get a class going up here in Canada District 1 and there are four more boats being built. All Ml kits, three extreme builds, one 8255 and my 171.

Any info would be greatly appreciated.

Thanks, Dave.

 

[harvknox]

Dave, I built a Gas Scale T-6 from a ML Boatworks kit. Your CG is about right. I had to add weight in the nose of the left sponson to get it to balance. My hull is not the same as yours, and I don't know if the frame numbers are the same. I moved my engine forward until I just cleared frame #7 with the engine mounts. I did use a Speedmaster collet and starter pulley. The engine is as low as I could get it in the hull and still move the starter belt off the pulley. I'm not sure that your cowl is the same as mine , but check that the sparkplug clears everything. Yes, I did remove the rope starter. I am using a modded K-30 and it is tough to pull on the rope, plus I had interference with the starter and the hull. My boat is 23+ lbs. The youngest and strongest guy in the Knoxville Gang throws it in for me. Yes, there is reinforcement in the hull. The tub is long and wide with no bracing. I created an extra frame extending just across tub. I doubled the frame portion which the supports the turn fin. I also built a carbon fiber tube into the spar at the front of the cockpit. Your boat is different, but may still benefit from the reinforcement. The boat will want to build tail heavy. Don't do anything that will add rear weight. I used 3/32 ply for the entire bottom. Some of that could have been 1/16 ply. Mike's kit and Roger's drawings gave no detail about the rear ride pads. I built them from photos.

Any other questions, just ask.

Harvey

 

[Dave Larocque]

Harvey,

Thanks. That's exactly the type of info I'm looking for. Much appreciated and I may hit you up for more as I progress. Again, Thanks.

Dave.

 

[Rick Larocque]

I am Dave's brother and one of the ones building the extreme 173 kit. It will be the 95-96 Smokin Joe's. Made a lot of changes to the build adding strength where needed. Used carbon fibre 2" tape on all the seams and the bottom of the motor mount to stiffen it and strengthen it for the full mod K30 rear exhaust. Placed the motor behind the rear of the sponson rib and loaded everything in the boat to get a feel for the CG. Would have needed 3 1/2 lbs of weight in the nose to get it to 2 1/2 ". Cut out that frame and tripled it for strength and moved the motor way up under the canopy. Loaded the boat up again and with no weight added ended up at 3" back. Lots of filling and sanding to do but it's coming along. It will weigh in at close to 22-23 lbs wet with paint on it. Tried to bring it in under 20 but no chance. This is an awesome class but if we want to make it grow we have to start sharing building tips, set up tips and help each other out.

Rick

 

[Blackout]

I am Dave's brother and one of the ones building the extreme 173 kit. It will be the 95-96 Smokin Joe's. Made a lot of changes to the build adding strength where needed. Used carbon fibre 2" tape on all the seams and the bottom of the motor mount to stiffen it and strengthen it for the full mod K30 rear exhaust. Placed the motor behind the rear of the sponson rib and loaded everything in the boat to get a feel for the CG. Would have needed 3 1/2 lbs of weight in the nose to get it to 2 1/2 ". Cut out that frame and tripled it for strength and moved the motor way up under the canopy. Loaded the boat up again and with no weight added ended up at 3" back. Lots of filling and sanding to do but it's coming along. It will weigh in at close to 22-23 lbs wet with paint on it. Tried to bring it in under 20 but no chance. This is an awesome class but if we want to make it grow we have to start sharing building tips, set up tips and help each other out.

Rick

Looks great, thanks for sharing. At 3", it sounds like it may run good with no weight added. That would be nice for sure. No way a side exhaust engine will work. Are the rear exhaust K-30's still available in the states or Canada?

 

[Rick Larocque]

Redline has the K30 rear exhaust but they are stock motors. I would imagine they could modify one for you.

Rick

 

[Blackout]

Redline has the K30 rear exhaust but they are stock motors. I would imagine they could modify one for you.

Rick

Good to know, thanks. Was wondering since Gizmo no longer imports as far as I know.

The rear exhaust packages great in riggers as well.

 

[Dasboatman]

Hey Rick,

Who's motor mount is that on your K-30 in the Smokin' Joes hull? CHEERS !!! Bob

 

[harvknox]

Rick, the photo of your engine installation seems to show the RCMK exhaust ball joint at its maximum deflection. I am using the same header on my modded K-30 RZ in my T-6 and at maximum deflection. At that angle there is an o ring water leak that allows water to enter the engine. Mine is older than yours and RCMK may have fixed the problem. I solved my problem by cutting a slight wedge out of the header, then welding up the pipe. This reduced the deflection on the ball joint, and keeps everything dry. You may want to check your CG some more. I believe most turbine style 1/8 scale and gas scale boats have CG's about 1/2" to 1 1/2" behind the sponson transom.

Harvey

 

[Rick Larocque]

Bob it is an RCMK catamaran motor mount. Got mine from Giz but he does not sell them anymore. Check the RCMK site.

Rick

 

[Rick Larocque]

Harvey I would love to get my CG 1/2 to 1 1/2" behind the sponson rears but without adding weight to the nose that is as close as I can get it. Dave's boat won't allow him to move his motor up as far as I did . Anyone else building this style of gas scale?

Rick

 

[mark couty]

What pipe are you using and where did you get it from ? cost ? Can you post larger pics of the engine bay, the header to pipe connection. Thanks.

 

[Rick Larocque]

Marc

That is a Zippkits pipe on there. I think they are less than $100. Good performing pipe and easy to tune in my experience. The header and connection are RCMK made for their rear exhaust motors. Again Redline probably carries them,

. Have not fired up the motor yet so can't comment on performance. Will try later to get some more pictures up. Pretty busy right now. Maybe my brother Dave can get some pics up of his build. He is an amazing builder. Much better than I am.

Rick

 

[Dasboatman]

Hey Dave,

Has anyone considered building the sponsons for these hulls out of heavier material to add structural strength, and weight forward, rather than just adding lead after the fact to balance the hull to CG?

I have built 1/8th Scales from scratch for many years and have tried to build the front ends heavier and very sturdy, while keeping the after planes as light as I could except for transom and strut support. My sponsons are Dow Chemical 'Styrofoam' covered with birch ply. I have never had to add much lead forward to balance to CG.

To support the starboard sponson transom and attached turn fin, I use a 3/4" X 38" piece of maple hardwood stock, and actually add another piece to the bottom of this "spar" to support the turn fin bracket. The sponson transom glues to this spar - the full width of the sponson rear plate, across the bottom of the hull, and through the longitudinal stringers to help distribute the load to the structural portions of the hull. CHEERS !!! Bob

 

[Blackout]

Hey Dave,

Has anyone considered building the sponsons for these hulls out of heavier material to add structural strength, and weight forward, rather than just adding lead after the fact to balance the hull to CG?

I have built 1/8th Scales from scratch for many years and have tried to build the front ends heavier and very sturdy, while keeping the after planes as light as I could except for transom and strut support. My sponsons are Dow Chemical 'Styrofoam' covered with birch ply. I have never had to add much lead forward to balance to CG.

To support the starboard sponson transom and attached turn fin, I use a 3/4" X 38" piece of maple hardwood stock, and actually add another piece to the bottom of this "spar" to support the turn fin bracket. The sponson transom glues to this spar - the full width of the sponson rear plate, across the bottom of the hull, and through the longitudinal stringers to help distribute the load to the structural portions of the hull. CHEERS !!! Bob

If the strength is needed then great idea! Otherwise, adding lead to the very tips is the best way to keep the total weight of the boat at a minimum. I would think anyways....

 

[Blackout]

If the nitro 1/8 scale boats need a CG no farther than 2" behind the sponson backs, I would think the 1/6.67 scale boats could, in theory, go farther back, maybe 3". All the dimensions are scaled up for gas scale, even the CG location. Just saying not to get too bent out of shape on an exact CG number yet. But, getting the weight as far forward as possible is still a good build goal.

 

[Dasboatman]

My 1/8th Scale 1983 U-40 Miss Houston weighs 12 pounds 7 ounces ready to race , less fuel.

What does your 1/8th Scale Pak weigh, Craig? Is it a glass or framed up hull?

1. Did you note the amount of lead Wes was adding to the sponson tips of his framed up Winston Lobster hull? In Gas Forum.

2. Harvey says his T-6 Bud from MLB kit weighs 23+ pounds with a 3/32 hull bottom. Hummmmm ???

John Olson, Aaron Olson and Marty, and others presently running Gas Scales should be observing - - What do your U-95, Miss US, and Coors Light, and others weigh in at if you don't mind. May well be generic to this thread.

Looks like the 5 East Coast owners of racing Gas Scale may well be in Evansville for IMPBA Nats. Hopefully they'll chime in when they get the chance. I'm sure Phil will. CHEERS !!! Bob

 

[jwo]

The U-95 is at 21lbs race ready w/ fuel. CG at 2" behind sponson backs. pictures are on the PNWTA.org web site. I could of built it a lot lighter but wanted it to last. Windowing out all the frame to the maximum and using carbon fiber reinforcement will save on weight. Also using Balsa instead of hard wood for stringers will save a lot.

 

[Blackout]

My 1/8th Scale 1983 U-40 Miss Houston weighs 12 pounds 7 ounces ready to race , less fuel.

What does your 1/8th Scale Pak weigh, Craig? Is it a glass or framed up hull?

1. Did you note the amount of lead Wes was adding to the sponson tips of his framed up Winston Lobster hull? In Gas Forum.

2. Harvey says his T-6 Bud from MLB kit weighs 23+ pounds with a 3/32 hull bottom. Hummmmm ???

John Olson, Aaron Olson and Marty, and others presently running Gas Scales should be observing - - What do your U-95, Miss US, and Coors Light, and others weigh in at if you don't mind. May well be generic to this thread.

Looks like the 5 East Coast owners of racing Gas Scale may well be in Evansville for IMPBA Nats. Hopefully they'll chime in when they get the chance. I'm sure Phil will. CHEERS !!! Bob

Not done yet. I'm just thinking out loud on this thread to keep the conversation going.

On another note, if you are adding a lot of lead to the front sponsons, you will need a beefier front sponson to carry the weight. So your suggestion is good for sure.

I have a friend who has a 1/8 scale with a CG 4 7/8" behind the sponson backs. The back runs real wet like this, but it goes around the course pretty good and at 50 mph with an old OPS. Once the CG is fixed, it should run better. It sounds like the gas scales are less forgiving because the total weight is so much higher than 1/8 scale. And I have to wonder whether a gas scale motor should be modded differently than the typical modded gas motor.....

Again, just thinking out loud....

Edit: It's a glass hull from RC Boat Company. I already moved the motor 1.5" farther forward than the stock tub allows. Even with this tub mod, I'll probably end up at 3" behind the sponson backs for the CG. (With no weight added.)

 

[Samuel Hagan JR]

How much can you lower the motor to help with installation. I guess you would have to look at the belly pan rule and see how low you can make it?