Weight Setup?

[Mark]

Would it be better to run less weight in the rear of the tub and have the prop push harder with less lift, or would it be better for the rear to be heavy and use the weight at the rear so the prop has something to keep it in the water as the blades attempt to lift the back out of the water at the expense of forward drive, or is there a happy medium that most good rigger designs are already close to when they are setup as per the instructions? I'm assuming cup gives more forward thrust than lift and pitch creates more lift but still increases forward thrust but not as efficiently as cup. I obviously don't dial my props, I have a friend tweak them for me and it makes one hell of a difference when they're almost spot on.

 

[J Solinger]

It seems that keeping the COG about 1 to 3 inches behind the front sponson ride pad always gives the best balance.

Just as a thought, IMO the tighter and more focused you can run the thrust cone from the prop the better. In other words, it should be good if the the prop can do more thrusting than lifting. I want to hear from the more experienced racers about this subject.

 

[topfuel443]

good topic guys.... Ive always wondered the same thing about prop weight. lets hear it expert dudes....

~James

 

[Mark]

Must be a bit of a mystery, not much response.

Another question, are the props that the tunnel guys run a surfacing type?

 

[J Solinger]

Yes. Generaly the "X" series props is what gets used for tunnel hulls.

I don't know if the COG vs prop lift issue is as much a mystery as it is going to be an issue of preference. Things like sponson width and length come to mind. If you prefer a sponson that is wide and will lift a lot of weight you can shift your COG further forward, allowing you to use a more efficient drive set up, a low lift prop with a smaller, tighter thrust cone with a thrust line that is more parrallel to the line of travel. The down side to a wide sponson is it will react badly to rough water. On the other hand you can go with a narrow sponson that will work well in rough racing water but you will need to shift the COG back to keep them from dragging. Then you will need to change your thrust line to lift the extra weight in the back and or use a prop with more lift which in turn will slightly deminish your efficiency and cost some speed.

This is one of thoughs subjects that you can realy bang your head on. I usually end up over thinking it. I settled on a set up that is solid and handles well, I let the other guy push it and blow over.

I've seen a few people that will have a couple different sponson sets for the same boat. I assume that this is for the varying water conditions. Not a bad idea.

 

[user 115]

A lot of guys run lifting props on tunnels.. The 1440 with some work done is very popular..

 

[J Solinger]

I haven't been around tunnels for about 15 years and am out of touch with them. At that time the X props where popular.

What kind of work is being done with the 1440's on tunnels?

 

[Russell Bear]

This is what Grim sells. Backcut 1440 with 3.3 pitch and cupped.

http://www.grimracer.com/Hardware/146octura1440prop.htm

 

[Mark]

Thanks guys, I see that it's a setup/balance thing, I now understand what everyone is trying to achive to go fast.

J Solinger,

When you talked about sponson sizes I'm assuming you thought I was talking about the weight balance point of the boat. I was more thinking of tanks in sponsons and radio in where the tank usually is and creating a lighter rear end because of the radio not being in the back half of the tub. If you didn't change the prop would it create hop in the rear.

Thanks for explaining what effect sponsons have on the boat when it comes to smooth and rough water.

 

[J Solinger]

Ya, I kind of ran off there.

If I remember right, I was day dreaming about the ramifications of moving the weight up and it's effect on sponson size and shape as well as thrust lines and prop sellection.

I don't think you will have much hopping trouble. I'm certain that strut adjustment will take any of that out.

 

[Nigtmare]

Hi mark You know that i have the tanks in the sponsons and when i was trying to see were the c/g will be with the engine at the same place and the radio at the back the c/g was at gap were the stuffing tube ends then i took all the stuff from the radio and placed them at the front of the engine to my surprise the c/g din't changed very much it was at the flywheel.I think if you go with most of the weight on the front the boat it will hop. there is a magic number that you must have as weight at the prop i was told is 12oz.The weight on the prop on the Nightmare is .368 grams so far i found that the x props work the best and the new abc with the prop surface drive 3/16 above the rear sponsons works also.By the way the width of my rear sponsons is 1/2" at the riding surface and the boat comes on plane immediately B) My problem now is that we have 6"of snow so i'm done till spring :angry: Nick

 

[Mark]

Nigtmare,

I think the 12oz you are talking about would be more of a percentage thing of the total weight of the boat. I found with race cars and superkarts that weight balance was all about percentages and how you distributed them. Sometimes a small change made a big difference. Just a thought.