3.5 tunnel props

[alfredo colon]

I've been trying to get my old tunnel back into heat racing, but feel a bit clueless why the boat won;t handle a relatively small prop (x437/3). If memory serves me right, the prop center is level with the rear sponsons

The hull is a Prather Lap Cat III / K&B 3.5 air cooled head / K&B gold carb / Garcia pipe / 30% nitro at sea level

I found my dad's old prop box, and before cutting the pipe, I want to test some old props, to see if any of them will make the boat run without having to make mods to the pipe.

• JG F22
  
• JG F25
  
• JG E20
  
• JG 3122
  
• Dumas 3106
  
• Octura 1938
  
• Octura X437/3
  
• Octura X438
  
• Octura X440
  
• Octura 1240/3
  
• Octura 1740/3
  
• Octura 1.6/3
  
• Octura 1.8
  
• Octura 1938
  
• Prather 215
  
• Prather 220ss
  

and last but not least, a rather small prop called texas cut.

I've been out of boat racing for over 10 years, and vaguely recall setting up the boat... and since most

any suggestions would be greatly welcome

 

[RodneyPierce]

yank the 3 blade off there, and toss in the x438 or x440. Set the bottom of the bullet either level with, or slightly above (1/8 inch) the bottom of the sponsons. Set the motor trim at neutral and start there. Should let it unload abit more.

 

[Tim leyde]

My VS1 was a rocket with a 440 ( 3 blade ) powered by a K&B SX.

TL

 

[LarryConrad]

The engine won't pull the prop? You're not achieving the performance you expect? What do you mean when you say the boat won't handle the 437/3? If you have the canister muffler that the engine came with try it. 30% nitro is pretty tame for even a K&B .21. The engine will make much more power on higher nitro fuel. I wouldn't cut the pipe. I ran the same model of K&B on a 28" Woodstuff on 60% nitro and the best performing prop for that engine, hull combination was an Octura X437/3.

 

[alfredo colon]

The engine won't pull the prop? You're not achieving the performance you expect? What do you mean when you say the boat won't handle the 437/3? If you have the canister muffler that the engine came with try it. 30% nitro is pretty tame for even a K&B .21. The engine will make much more power on higher nitro fuel. I wouldn't cut the pipe. I ran the same model of K&B on a 28" Woodstuff on 60% nitro and the best performing prop for that engine, hull combination was an Octura X437/3.

that's correct, I'm not getting the performance I expect from it. unfortunately the canister is long gone, altho I do have an other o/b tunned pipe i could try.

as far as nitro %, I ran 21's for over 10 years, and didn;t use anything less than 50% Getting anything over that here is a bit hard since most racers are running petrol. I wouldn't hesitate using 60% on it, the issue would be getting the higher nitro.

I'll try the x438 and raising the engine above the sponsons as a first step. Worse comes to worse, I'd trade the boat for a .21 rigger h34r: those JAE look pretty interesting

 

[LarryConrad]

The engine won't pull the prop? You're not achieving the performance you expect? What do you mean when you say the boat won't handle the 437/3? If you have the canister muffler that the engine came with try it. 30% nitro is pretty tame for even a K&B .21. The engine will make much more power on higher nitro fuel. I wouldn't cut the pipe. I ran the same model of K&B on a 28" Woodstuff on 60% nitro and the best performing prop for that engine, hull combination was an Octura X437/3.

that's correct, I'm not getting the performance I expect from it. unfortunately the canister is long gone, altho I do have an other o/b tunned pipe i could try.

as far as nitro %, I ran 21's for over 10 years, and didn;t use anything less than 50% Getting anything over that here is a bit hard since most racers are running petrol. I wouldn't hesitate using 60% on it, the issue would be getting the higher nitro.

I'll try the x438 and raising the engine above the sponsons as a first step. Worse comes to worse, I'd trade the boat for a .21 rigger h34r: those JAE look pretty interesting

Try the 215, and the x440 at that setting too. Is the engine reving well with the 437/3 but running slow?

 

[alfredo colon]

The engine won't pull the prop? You're not achieving the performance you expect? What do you mean when you say the boat won't handle the 437/3? If you have the canister muffler that the engine came with try it. 30% nitro is pretty tame for even a K&B .21. The engine will make much more power on higher nitro fuel. I wouldn't cut the pipe. I ran the same model of K&B on a 28" Woodstuff on 60% nitro and the best performing prop for that engine, hull combination was an Octura X437/3.

that's correct, I'm not getting the performance I expect from it. unfortunately the canister is long gone, altho I do have an other o/b tunned pipe i could try.

as far as nitro %, I ran 21's for over 10 years, and didn;t use anything less than 50% Getting anything over that here is a bit hard since most racers are running petrol. I wouldn't hesitate using 60% on it, the issue would be getting the higher nitro.

I'll try the x438 and raising the engine above the sponsons as a first step. Worse comes to worse, I'd trade the boat for a .21 rigger h34r: those JAE look pretty interesting

Try the 215, and the x440 at that setting too. Is the engine reving well with the 437/3 but running slow?

It feels as if the engine doesn;t have the power to properly run with the stock 437/3. The engine sounds about right on the bench, it'll idle fine, and rev up fine without load. I've got a hunch I could lean it a bit more

 

[John Knight]

When I owned my Lap Cat 1, I ran a Prather 215 prop. It was by far the best one for the boat & engine. I tried many different ones and none performed better. Give that one a good test.

 

[alfredo colon]

Thank you all for the advice!

The prop was originally 1/2" under the sponsons level :blink: I rotated the L piece on the motor mount and gained anywhere between 1/2" to 3/4" that did the trick. The boat launched nice, was very responsive (idling to full speed) and ran as fast as I had expected it to If the wind hadn't been as strong as it was, It would have been an amazing day

None the less, thank you for the replies