1990 wood Hot Shot Sprint back to life

[xfactor]

Way back in 1990, I built my first RC boat, a dumas hot shot sprint 7.5cc/11cc wood kit. I went with the 11cc K&B completely stock. To make a long story short, the boat was difficult to drive and blew over half a dozen times so it got put on a shelf in the garage untill about 6 months ago. The motor had only ran about a quart of fuel through it but turned over hard when i dug it back out. I ended up tearing the motor down and rebuilding it with ceramic bearings, raised exhaust ports, raised compression, 707 header, silver bullit pipe, and Nitro water sports prop shaft kit.. I am running the old style carb with velocity stack. The motor now seems to run great and is very responsive. The motor was mounted with the stock K&B mount before and now is hung on a "High-Jack" mount with about 2 1/4" of set-back from the transom with prop shaft even with sponson bottom. I have a few final questions before I get it back in the water. 1- What is a good starting point for the pipe length ? 2- I have 3 nice props that I have picked up on flea-bay -- Ballanced and polished Octura X455 brass, Ballanced and polished Prather 240 SS, and a ballanced and polished Prather 255 SS . Which would be the best prop to try ? The 255 barely clears the cavitation plate but the aftermarket prop shaft bushings are wiggle free so should be ok. Help ! I'm dying to get it wet again after all these years. Also what kind of speeds will it see ? What % nitro will be best ? Thanks, Byron

 

[Jerry Dunlap]

If the boat was difficult to drive with a stock K&B 11cc, it could be even worse with a mod version.

1. I would start with running the pipe full length. You say it's running well now in testing.

2. I think you should try the smallest diameter prop first. It would have less troque.

3. 40% fuel should be adequate.

4. If the boat will stay on the water, it should run over 50.

However, more power is going to create more handling issues.

JD

 

[xfactor]

If the boat was difficult to drive with a stock K&B 11cc, it could be even worse with a mod version.

1. I would start with running the pipe full length. You say it's running well now in testing.

2. I think you should try the smallest diameter prop first. It would have less troque.

3. 40% fuel should be adequate.

4. If the boat will stay on the water, it should run over 50.

However, more power is going to create more handling issues.

JD

I was assuming that the added engine set-back would make it handle a little easier, maybe I'm wrong on a tunnel. The main thing that sticks in my mind from before was spinning out in turns. It seemed that the nose would dig in and it would swap ends.

 

[Jerry Dunlap]

Did the boat have the front "stumble blocks" installed?

JD

 

[xfactor]

Did the boat have the front "stumble blocks" installed?

JD

No, I don't think so, it was built straight from the plans in the box. And, as a the repeat tunnel beginner, I have to ask--What are stumble blocks ? The other question that has not been answered yet is what effects does engine set-back have on a tunnel hull ? I know of the effects of engine set-back on bass boats from experience as bass fishing has been my #1 hobby for many years but not sure on RC tunnels.

 

[Jerry Dunlap]

"Stumble Blocks" or recovery pads are door stopper(wedge like) shaped pieces of wood that are glued to the inside front of the tunnel boat floor. They assist the tunnel boat in a turn by keep the sponsons from digging into the water too deep.

Moving the engine back should help "free" up the ride of the boat.

JD

If you go to aquacraftmodels.com website, click on Manuals, click on TS2 Manual, and scroll down to page 13 you will see photos of recovery pads installed on a TS2.

Similar pads can be made from balsa wood and glued to your boat.

JD

 

[xfactor]

"Stumble Blocks" or recovery pads are door stopper(wedge like) shaped pieces of wood that are glued to the inside front of the tunnel boat floor. They assist the tunnel boat in a turn by keep the sponsons from digging into the water too deep.

Moving the engine back should help "free" up the ride of the boat.

JD

If you go to aquacraftmodels.com website, click on Manuals, click on TS2 Manual, and scroll down to page 13 you will see photos of recovery pads installed on a TS2.

Similar pads can be made from balsa wood and glued to your boat.

JD

Thanks Jerry, I'll check them out.

 

[xfactor]

One more very important question. When measuring the center of gravity with the engine set-back -- do I measure from the rear of the sponsons, the transom, or from the new mounting surface of the motor ?? What is a good starting point for the CG on this 36" tunnel with an 11cc ?? Oh yea -- I found the stumble blocks in the original text, they are calling them "auxillary sponsons" and they give dimensions in the plans. Looks like I didn't read to well back in 1990. I still have the blueprints and all paperwork in the original box.

 

[Jerry Dunlap]

1. For determining C/G, measure from the back of the sponsons.

2. On a 36" boat, 12" forward of the back of the sponson is a good starting point.

JD

 

[xfactor]

Thanks jerry, hopefully I'll get it out in the water in the next couple of weeks and see how it does.