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Kinda Crazy Tunnel Rebuild

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Old 08-08-2017, 10:10 AM
  #1
Jaymanaa@lak
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Default Kinda Crazy Tunnel Rebuild

Hello all, I've got a really fun rebuild going on and thought I'd check in here for some of the good advice I always get (and appreciate very much). The boat is an old Prather "Shot in the Dark" fiberglass job. It's only 27.5 inches long, but the beam in the rear is a whopping 14.5 inches. I stripped everything out including the mildly hopped up 3.5 K&B. I moved the fuel tank all the way to the front, to keep as much weight out of the rear as I can to accommodate the new 7.5 K&B. I'm using a pretty trick Spektrum receiver that has dual throttle and steering outputs, so the steering will get two Futaba S-28 servos, while the carb will get one. I have a third channel that I think is just a 3 position switch. Here's where things get weird; I'd like to use that 3rd channel for a power tilt. I was thinking though, that maybe, a guy could even use the second throttle output for a trim/tilt servo that would pull the motor up as the throttle opens just like you do in a real bass boat. It would take a lot of tweaking I'm sure, but do you all think it's even doable? If so, I wonder what kind of speed it would add. I'm already going with twice the power the boat is designed for, so I'm thinking some sort of wing for sure, but when it comes to wings, I'm totally lost.
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Old 08-08-2017, 03:47 PM
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i used to run a 7.5 size outboard tunnel hull fitted with a K& B 67 [ 11 cc ] this hull planed very well once the balance was set as well as engine trim. now why the 2 x steering servos on a rather small outboard ? i servo will be fine. power trim can be more of a pain than what its worth to set up as once you have the outboard leg set at its correct degress you never need to alter it again unless you change to a different prop that has different lifting caractoristics. if you really want to set up power trim you can remove the 3 rd channel switch and replace it with a dial pot , this will give you proportional movement which is what you want.
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Old 08-09-2017, 03:54 PM
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Quote:
Originally Posted by crapshooter View Post
i used to run a 7.5 size outboard tunnel hull fitted with a K& B 67 [ 11 cc ] this hull planed very well once the balance was set as well as engine trim. now why the 2 x steering servos on a rather small outboard ? i servo will be fine. power trim can be more of a pain than what its worth to set up as once you have the outboard leg set at its correct degress you never need to alter it again unless you change to a different prop that has different lifting caractoristics. if you really want to set up power trim you can remove the 3 rd channel switch and replace it with a dial pot , this will give you proportional movement which is what you want.
Thanks for the input, good stuff. I've been going over the power trim in my mind, and got to thinking maybe a better use of the 3rd channel would be mixture on the fly. It's always a pain for me because the boat isn't moving, so you end up guessing a bit to the rich side. I'm also trying to find out if a cop radar gun will pick these little boats up. I'd like to have the actual speed. I spent half the day today making a velocity stack (a long one). I would think I'm going to be looking for as many RPMs as possible, probably 60 or 70% nitro. I haven't looked to see which con. rod this has, but I'm expecting the round one. It's a gold head air cooled, so I'm thinking the guy would have popped for the good rod. What about a pipe? All it has now is the little tank looking deal with dual outlets, are they any good way up high in the RPMs? I need to figure out how to put a picture up.
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Old 08-09-2017, 05:02 PM
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a 3 rd channel remote mixture conrtol is the way to go as atleast you can run the engine rich in the water and slowly lean it out alittle to get maximum rpm,s . buy your self something like a Garmin GPS , i just grabbed one off ebay for $50 near new., very accurate speeds recorded too. don,t go high on the nitro unless your going to add head shims as if you don,t you will damage the piston crown and head button from detonation, 25% is enough. and that muffler is what you call a box muffler, toss it as there rubbish unless your doing a scale boat. have a look around for a good tuned pipe and header to suit.
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Old 08-10-2017, 01:59 AM
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Cool, I have a fresh gallon of 25% nitro on the shelf. The factory specs on this motor are .010 to .015 head clearance, but that doesn't tell me much as I'm not familiar enough with these things (yet). I hadn't thought about a Garmin, but that will be just the ticket. They have a big plant about 20 minutes from the lake I live on, and I have a friend who works in the office who has offered to get me a good deal before. One more thing on these two strokes; don't the intake runner length, and exhaust pipe length have some sort of correlation? I built the extra long velocity stack as I remember reading something about a longer stack working better for higher rpms? I had a feeling these little box pipes were junk, they sure don't look like any 2 stroke chamber I ever saw. I think I have all radio parts scavenged up, just need to get it all mounted and new cables installed. Got to make a run to the hobby shop for a fresh fuel tank too.
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Old 08-10-2017, 01:20 PM
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Velocity Stacks are a waiste of time so take it off. the carby can collect more air from around the venturi not just through a funnel. this is true fact . just stick with 25% nitro and keep an eye on the glow plug for any signs of the element becoming pushed in deformed, if this occurs you need to increase head clearance by adding a shim and open up the needle valve a couple of clicks richer. a good tune is a glow plug that is abit shinny and in perfect condition , not dull and deformed.
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Old 08-11-2017, 10:36 AM
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I ran across this piece while doing some engine mods on a motorcycle. Kind of interesting, the laminar flow stuff etc......
https://en.wikipedia.org/wiki/Velocity_stack
Dodge did some fun stuff with actual "runner" length in their cross flow wedges and stuff too.

Last edited by Jaymanaa@lak; 08-11-2017 at 10:38 AM.
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Old 08-11-2017, 02:32 PM
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Quote:
Originally Posted by crapshooter View Post
Velocity Stacks are a waiste of time so take it off. the carby can collect more air from around the venturi not just through a funnel. this is true fact . just stick with 25% nitro and keep an eye on the glow plug for any signs of the element becoming pushed in deformed, if this occurs you need to increase head clearance by adding a shim and open up the needle valve a couple of clicks richer. a good tune is a glow plug that is abit shinny and in perfect condition , not dull and deformed.
On the subject of glow plugs, how do you feel about idle bars, and with my 25% nitro, is a #5 ok for boats? Thanks.
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Old 08-12-2017, 12:34 PM
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Idle bar plugs are not a good idea for marine engines , firstly you never need to have a very low idle on the marine engine and if the bar should come away from the plug you will destroy the piston and or liner. just use good quality normal glow plugs.
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Old 08-14-2017, 09:33 PM
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Putting a 7.5 on a Prather 3.5 boat is like putting a space shuttle engine on a model-T. Can it be done? Sure, Will it work? Doubtful. Prather's were designed in 70's and still run like they are from the 70's. Even the Lap Cat II wouldn't work well over 40 mph. S-28 servos, wow, haven't heard/seen one of those since the mid 80's. If those are to be used for steering, hope you have a box full. First blow over and those gears will be stripped. Get a metal geared servo with high torque. Don't need high speed, but high torque is very much needed. Get the boat to run before you start doing any mods. Even stock, I doubt the 27.5" boat will handle a stock 7.5, let alone a mod'd one.

BTW, radar guns will pick up these little boats.
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Old 08-15-2017, 04:48 PM
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Okay, one of the internet experts(not any way tongue-in-cheek) has spoken so, needless to say, I'd listen. Shelve the 7.5 and get yourself a 3.5(.21) if you want to run that hull. If you want to run that engine, pick up a 36" hull or kit and use it on that.
I was researching servos and found the following on the S-28:
It weighs 1.8 ounces
It is only rated at 4.8 volts
It has a .24 second speed to spin the arm 60 degrees
It can provide 48.7 oz/in of torque
This is comparable to the Futaba S3003 and S-3004 servos that are available now, both of which are considered under-powered when it comes to turning an outboard. You might be better off upgrading to an S-3305MG high-torque servo to handle the steering. It's the same basic size but kicks out about twice the torque of the S-28 at the same voltage

Last edited by Hydro Junkie; 08-15-2017 at 04:52 PM.
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Old 08-17-2017, 12:43 PM
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I think it was my first or second post that I said I was using dual servos for steering, with the Spektrum MR3000 which has dual steering and throttle outputs. I was just trying to use up some stuff out of the junk box. This old hull seems to heavy for a 3.5, and it's built like a tank. I imagine it will get kind of interesting up around 40 or better. Almost done with the motor. I got rid of that funky box muffler and so far have a 180 degree header that's water cooled. Got plenty of water for cooling as the gold head is air cooled. I'm going to take it real easy with this thing, and it has a fail safe that closes the throttle if it gets out of range. I'm also adding a mercury switch to close the throttle if the front of the boat gets a certain hight. I lost a con rod once from hydraulicing when a boat flipped and the throttle was wide open, oops.I got a bit side tracked working on a real pontoon boat for a guy here at the lake I live at. Also a little balsa wood boat got a new Thunder Tigre .18 that for the life of me I can't buy a header and pipe for. I've found pictures but no prices, crazy. Thanks for all the tips guys, much appreciated and I'll keep ya in the loop, even if this ends in disaster.
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Old 08-17-2017, 12:55 PM
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Oh, BTW, I am going to ditch the plastic gear servos, for some reason I was thinking they were metal. Good catch OB nut, thanks.
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Old 08-19-2017, 04:37 PM
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Dual servos work great to steer a 7.5, use them in my current 7.5 setup - Tactic TSX57.

My 11cc, using a single RC Gear Shop DMC300 (Torque: 396oz-in (28.5kg/cm) 425oz-in (30.6kg/cm))

I have seen a fellow club member, back in the late 80's, that did what you are looking to do. The motor used was a non-piped K&B 7.5. He could bearly move the throttle past 1/4 open and it would be point sky high.

Tommy Lee's record setting 3.5 tunnel, which held the record for over 20 years, was over 6 lbs. Weight has nothing to do with it.. Setup...

Good luck in your quest..
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