32" Imex Cat

[domwilson]

It looks like it's running wet. Where are your batteries placed? Have you tried adjusting the steering trim on your transmitter? It's hooking like it has a steerable outdrive. The hooking usually happens when the prop pushes the transom out of the water. Is the rudder pointing straight down?

[domwilson]

I agree. The rudder may be too short.

[tiqueman]

Lets see if this posts right?

It looks like it's running wet. Where are your batteries placed? One 3s sits in front of the motor, perpindicular across the stringers. Thats why I said, maybe run it down one sponson and just put one of my 2s's in the other sponson just to help balance. Have you tried adjusting the steering trim on your transmitter? If you mean the throw amount (dual rate), yes. And it runs nice and straight as long as you stay on the gas. It's hooking like it has a steerable outdrive. The hooking usually happens when the prop pushes the transom out of the water. Is the rudder pointing straight down? Yes, it actually now has a bit of "toe in" if you will. Maybe a degree.

[tiqueman]

http://www.offshoreelectrics.com/pro...e-502b20-black

I was just going over the specs for that rudder and have some questions. The lead edge is 85mm from the transom. that means that the OSE rudders lead edge would be 22.5 mm (.886 inches) behind where the existing rudders rear edge is. There would be a 41.4mm (about 1.6") difference between the 2 (lead edges). Is that getting too far away??? The lead edge would end up 94mm (about 3.7 inches) away from the trailing edge of the prop.

[domwilson]

How long is your existing rudder?

[Fluid]

The biggest contributor to the hooking problem is, as the others said, the short rudder. In a turn the only thing holding the transom in line is the rudder, and the amount of blade in the water with the rudder on the OP's boat is laughably short. This is a very common problem with cats in particular, and it is better to have too long a rudder than too short. Swapping out for the one linked from OSE will help a lot. I don't think that positioning the blade further back will be a problem, it may increase the steering sensitivity a little, or not. Be sure to use a very stiff pushrod or you will have other handling issues.

The sponson design is a bit different than most FE cats, with no flat ride pads on the bottom. This design can make the hooking issue worse, but it can handle rough water a bit better. The steep bottoms put the sponsons deeper into the water when running, which in turn puts the prop deeper in the water. I see the cat hopping in the video, this is often due to running the prop too deep. I'd raise the strut bottom to 1/4" above the sponson bottoms and try it. Keep rasing it until the prop blows out (looses thrust) or the speed/temperatures/handling improve. Running it higher may allow you to run a bigger diameter prop too.

Where is the CG now? You will want it between 32% and 34% ahead of the sponson transoms, not the extreme back of the hull. Depending on conditions you may need it closer to 30%, but try it in the stated range first. It's a nice looking cat and should be a lot of fun once you have it dialed in.



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[tiqueman]

domwilson, existing is a whopping 75mm. It puts the bottom of the blade about 12mm or .475 inches ABOVE the lowest point of the prop, when the prop is in the verticle position. Obviously its way too high.

Fluid, I did notice, the hopping started after my last strut adjustment which was lowering it. It settles on glass water, but once it crosses its own wake, it takes a bit for it to settle again. The cog, if I did my math right, sitting stock at 34.8.

Whats the recommended depth of the rudder beyond the prop? Is a heavy 20mm (about .8 inches) enough on one of these??? Im asking this because the SV27 uses a 100mm rudder. I can get one of those today. But Im not sure if it would be enough.

[domwilson]

I don't think there is a definitive answer for that. There are a number of considerations here. One being rudder bracket length, hull design, water conditions, etc. But 1 1/2" should be enough. It wouldn't hurt to try the SV rudder. It should be better than what you have now. Too long a rudder will introduce unwanted drag.

[tiqueman]

Thats kinda what I was thinking.

I put the 3s down the right sponson, 2 s down the left (only had the 3s hooked up) and was able to move the cog back to 31 from 34.8. No difference really. Bow raised more on take off. I raised the prop back up and once again lost all control on take off. Its now all in pieces and awaiting a new rudder. I think I may grab a SV27 rudder tonight. It can only be better. If that doesnt work I will replace everything with a new strut/ rudder combo, as 785boats has his rigged. I will get it!!!!

[Flying Scotsman]

I just looked at this thread and that rudder is as many have mentioned way too short. It is very easy to cut down a rudder that is too long.

Douggie

[Fluid]

You will need a minimum of 40mm of rudder blade below the strut, and 50-60 mm would be better with that behind-the-prop rudder. The SV rudder will not be enough IMO but if you want to try it go ahead. It is better to have too much than too little, the drag penalty is less than most people think. The prop aerates the water going to the rudder and reduces its effectiveness substantially, so you need more rudder depth. Once you have enough rudder in the water you will be able to tune the boat with CG and prop depth.



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[tiqueman]

LOL, so heres what I did, because Im bored and this boat is getting on my nerves... actually, its not having the right parts thats getting to me. So, First, I decided, put the rudder hrdwr back on and play with the strut depth and angle some more. I adjusted it, walked down to the pond and had quite a struggle to get it to launch. Oh, and by the way, its trying to go right on the take off and with my stubby, I cant get it to respond to full left rudder. So it took 2 or 3 tries to get it to go. Up the ponds and back, adjusted the strut, up the pond and back. came back inside, took the shaft and strut out, tweaked on the stuffing tube to get it to do what I wanted, got it up about 3/16 to maybe a 1/4 above the lowest part of the sponsons. Took it back out, couldnt get it to launch at all. It would just do right hand circles and when it started to go fast enough to hook up, I was on a full left turn and it would spin out to the left. So then my 3s 4500 dies... again. And I only have that 3s for that boat. So now Im ready to leave it in the drink, get a BB gun and start shooting. Then the "I dont give a hoot" attitude got the best of me. I took the series harness I made for it to run both my 2s 5000mah and when I finished fabricating it, I was too worried Id blow it up and then really be starting from scratch. I hooked it up. 2 - 2s 5000s took that boat from "mmmmm thats pretty cool" to "Holy #$%* thats insane." I had to lower the strut to where its maybe a 1/16 below the lowest part of the sponsons and it has almost a zero degree pitch. Maybe up a half. It spun out going in a straight line, but.. it was fun to watch. 90% uncontrolable. I only turned about 3 passes before I decided to quit while I was still having fun. Obviously I couldnt turn it around without it spinning or coming to a SLOW turn around, I only bumped WOT for less than 2 or 3 seconds at a time. Then I noticed my water exit was only trickling. Theres a lot of small algae or maybe its duckweed, about 1/8 inch little balls, floating all around and under just the surface. I disconnected the line from the top of the rudder and blew thru it, it wouldnt go and then it burst and started letting air pass, but it seemed restricted still. I probably ran it for 3, maybe 4 minutes. Brought it inside, batts at 91.6F, motor at 103.4F. I gotta figure out whats going on now w/ my water, BUT.... What happens if its over volted? Does it just quit, smoke, burst into flames? All of the above, depending on what mood its in? If, once I get it handling correctly, can I continue to run it like this, just monitor heat after every minute or so on the first couple runs? Or am I just looking to blow stuff up??? I wish I had the camera w/ me. It seemed as if it would take my BJ26... and hey, now neither boat turns for ish!!!! I have 2 great drag boats. I have no idea what ESC in in this, Imex hasnt responded to me about the info I requested on it. Oh and the ESC temp was about 104F. Im thinking things will run a bit cooler once I get the water going on a good steam again...

[785boats]

Hi tiqueman.
Just noticed a couple of questions you asked a few posts back.
With regards to running 2 x 3s packs in parallel, the voltage will remain the same at 11.1V. You are right in your assumption that the capacity doubles. The ESC actually would prefer that. I picture it like this. The ESC/motor don't have to 'drag' the current out of the battery so hard because the battery has more to give & lets it go more freely. A bit simplistic I know, but hey, I'm a simple kinda guy.
Another benefit is cooler running batteries because each pack only has to deliver half the current draw. Longer run times are also a plus.
Of course the big benefit is being able to get the boat balanced correctly.
I Just Velcro mine in with a long strip in the hull to allow adjustment.
When you move the balance point back you can drop the angle of the prop shaft a bit. This will stop the tendency to hop, as will raising the strut up a bit.
With regards to the rudder, if you look at the pic I posted earlier, that is a 110mm long rudder with 45mm below the bottom blade of the prop.
As fluid has stated you are going to need all of that & maybe even more.
Don't forget to put a gap between your drive dog & the strut. The pics you posted at the start shows it way too tight. It will be robbing you of power & may even break a flex.
Cheers.
Paul.

Seems you posted while I was typing. Now you really need that bigger rudder.

[domwilson]

With the transmitter on and batteries connected, is the rudder still straight? Can I get a pic of the transom from behind? We're going to figure this out.

[Fluid]

The main cause of your troubles is still the short rudder. It hooks right due to prop walk like most every boat does, you just don't have enough rudder side area to keep the transom from moving rapidly left. Once you have enough rudder in the water you can begin to make some headway. Until then you are wasting your time trying to tune the boat - although if you're having fun that way so much the better as you wait for parts.

You have no worries about the temperatures, your components are still what I'd call 'cool'. How warm is the pond water you are runing on? You can't cool anything colder than that. The top water on our ponds is about 90F so we don't worry until any component exceeds 135F.


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