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Dumas Hots shot sprint help....With video

Old 08-12-2007, 09:08 PM
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Default Dumas Hots shot sprint help....With video

I have a Dumas Hot Shot Sprint 3.5 Kit 24" ABS with a K&B 3.5 on it. The boat was taken out for its first run today. After the boat was set launched in the pond it began to jump out of the water. It only got worse with speed. I tried to stop and start slowly but the jumping got only worse. The boat is jumping almost a foot out of the water. I assumed with my airplane background the boat was tail heavy. I added a ton of weight that made the front of the boat very nose heavy but the boat still continued to jump bad. I also tried to raise and lower the prop, and also added negative pitch to the motor to try to plant the nose. No luck. So what the hell is wrong with her?
Old 08-12-2007, 09:27 PM
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Default RE: Dumas Hots shot sprint help....

hmmp, kinda weird situation.....is it literally jumping or bouncing like the bow flapping back n' forth? what prop size ur using? looks to me lot of torque. how far from the centerline(tip of sponson) is the prop angle and height.
Old 08-12-2007, 09:36 PM
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Default RE: Dumas Hots shot sprint help....

I mean the boat is jumping nose towards the sky then belly flopping down. The doing all over again. Only quicker with speed. It gets so bad the boat trys to back flip. I am running the prop my old KB came with. Motor is running great. Just can't get the damn boat to stay in the water. Looks like a bull frog jumping from lilly pad to lilly pad...
Old 08-12-2007, 10:01 PM
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Default RE: Dumas Hots shot sprint help....


ORIGINAL: baxternick

I mean the boat is jumping nose towards the sky then belly flopping down. The doing all over again. Only quicker with speed. It gets so bad the boat trys to back flip. I am running the prop my old KB came with. Motor is running great. Just can't get the damn boat to stay in the water. Looks like a bull frog jumping from lilly pad to lilly pad...
If you were using the old aluminum prop that K&B used to include with their engines, that's a terrible prop. Try an Octura X440, c/l of prop 1/8" above the bottom of the sponsons, neutral prop thrust, and probably about 2 oz. of lead on the sponson on each of the sponson tips.

JD
Old 08-12-2007, 10:06 PM
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Default RE: Dumas Hots shot sprint help....

I will try a new prop. We added over a pound of weight to the nose already in the radio compartment. All lead shot. The instructions said 8 inches from the transon is the balance point. So that is where I started. Now I am like 15 or more inches form that point. Still jumping..
Old 08-12-2007, 10:42 PM
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Default RE: Dumas Hots shot sprint help....

My boss has a boat just like that, and had a similar problem with it jumping, and hooking. He never fully solved the problem but he got it much better. He put wood recovery pads on his sponsons to add a little height and to make the edges sharp.

I'm not sure how true it is, but he was told that the ABS kit had different shaped sponsons than the wood kit, and that the ABS was a poor design. I guess alot of people with that boat have similar problems. Maybe someone else with one will have some tunning tips
Old 08-12-2007, 11:54 PM
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Default RE: Dumas Hots shot sprint help....

ORIGINAL: baxternick

I mean the boat is jumping nose towards the sky then belly flopping down. The doing all over again. Only quicker with speed. It gets so bad the boat trys to back flip. I am running the prop my old KB came with. Motor is running great. Just can't get the damn boat to stay in the water. Looks like a bull frog jumping from lilly pad to lilly pad...
LOL!!!
It really does look like a bull frog jumping from lilly pad to lilly pad.LOL

All of us were laughing so hard at how it was hopping around the pond... including baxternick.
But seriously, if it will stop hopping around, I believe it will be FAST!!!
Old 08-13-2007, 01:06 AM
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Default RE: Dumas Hots shot sprint help....

I have seen several Dumas ABS Sprint 3.5 that were warpped badly. Place the boat on a flat surface and check the sponson bottoms. Both sponsons should touch the flat surface evenly. If you have a twisted hull, there isn't much you can do to make it work correctly.[]

JD
Old 08-13-2007, 09:40 AM
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Default RE: Dumas Hots shot sprint help....

No warping. sits nice and flat. Could I need some positive angle instead? I did not do the recovery mod, but figured that only helped in the turns. The boat you can tell is going to be fast, but only if I get it level in the water. I don't know how much more weight to add or if that even helps. Where is the correct balance point on theses babys?
Old 08-13-2007, 10:02 AM
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Default RE: Dumas Hots shot sprint help....

You should not have to add weight to the boat, well not that much.

Giving positive angle to your engine could make it much worse, do you have any pics of your setup?

Dan.
Old 08-13-2007, 11:25 AM
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Default RE: Dumas Hots shot sprint help....

Guys... all I can say is... this boat is super light...

"It's kinda like bolting a K&B 3.5 outboard to a feather." [X(]

My tunnel is heavy compared to Nick's and when I throw mine in, it cavatates at first just like his, and when the prop grabs, mine jumps out of the water too, but not a foot high!!! It then levels out and gets up on top of the water, like it is supposed too.
His boat, just doesn't get to the leveling point... it just keeps jumping and jumping and jumping...............
Old 08-13-2007, 11:31 AM
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Default RE: Dumas Hots shot sprint help....

I hear ya,

the Hot Shot Sprint was design for the K&B 3.5, there must be something in the setup thats being missed, some pics may help us help him.

Do you have the exact same boat?

Dan.
Old 08-13-2007, 12:18 PM
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Default RE: Dumas Hots shot sprint help....

No, I have a Steve Muck Littl' Light'n... I have had it at least 22 years. Mine is ancient, compared to Nick's.
That's why we are a little frustrated. We figured... newer designed boat, better handling, and easier set-up.
Old 08-13-2007, 01:16 PM
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Default RE: Dumas Hots shot sprint help....

A Dumas Sprint isnt much newer (if any?) than your boat. Like Dan said, if you post a picture of your boat on a set-up board, im sure we could get you going in the right direction. I also agree with Jerry, get a good prop on it, make sure its sharp and well balanced, and see what happens.

I personally would set it up with the bottom of the prop shaft 1/8" above the bottom of the sponsons, with zero to 1 degree of negative. Set the balance point roughly 32-33% (about 7.7"-8.0" ) from the back edge of the ride pad. These boats are no lighter than the PSRP Villain kits, so they should balance out easily, and maybe by only moving the radio equipment around. You should have to put no more than 2 ounces on the boat. You can try a X440, M440, X437/3, X438, X640, or P-215. Most of these props will get you very close to where you need to be with a stock 3.5 K&B engine

~James
Old 08-13-2007, 02:15 PM
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Default RE: Dumas Hots shot sprint help....

Nick... they need some pics.
Old 08-13-2007, 02:34 PM
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Default RE: Dumas Hots shot sprint help....

Here are the setup pics. I didnt bother taking pics of the motor angle because we adjusted it every possible way just about yesterday and same with the depth. The servos are both standard hitec 78 ounce servos. The tank is 8 oz. The battery is a 4 cell AAA Hidrimax pack, and the Rx is a futaba 2 channel AM. Here are two pics...
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Old 08-13-2007, 02:35 PM
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Default RE: Dumas Hots shot sprint help....

Someone suggested moving the engine back more with spacers. Will this help any?
Old 08-13-2007, 03:42 PM
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Default RE: Dumas Hots shot sprint help....

well, thats not exactly what I was looking for, but its a start. Do you have a set-up board you can put it on and take picture of it from the side, showing the angle/height of the engine? I know you said you adjusted it in every way imaginable....

What height is the motor at now? what is the highest position it went when you were testing?

Can you take a picture of the bottom of the hull, from the back and level with the ride pads?

I would still stay the prop has a lot to do with it.

A longer motor mount will make the boat a little more stable in the straights, but will negatively effect turning ability. It also makes fine tuning a must. Looking at the mount you have now (dubro?), it should work just fine like it is.

In my opinion, you should set up the boat the way I described above, and get one of those props. That will give you a basline to start with. Once you get to that point, you will need to make fine adjustments to get the boat properly set-up, but it should NOT hop like you say its doing. If it still does it, then you have other issues. Either the hull is wrong, or the prop is cavitating, then hooking up, cavitating, then hooking up etc. etc.

One last question, why do you have the tank turned around backawards? wouldnt it be much easier to turn it facing the other direction so the lines arent so long? Just curious.

James
Old 08-13-2007, 03:44 PM
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Default RE: Dumas Hots shot sprint help....

oops.... I missed the fact that this is the ABS version, and not the wood. That may be the problem. Those boats had a few "bad habits". Dotn give up just yet, try out the suggestions.

~James
Old 08-13-2007, 04:08 PM
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Default RE: Dumas Hots shot sprint help....

I ordered the prop from TLR. I will shoot some video today of it. I will also get some pics later tonight from behind the boat like you said. The tank is that way because it is a airplane tank I just put in it so the clunk is in the back!
Old 08-13-2007, 08:02 PM
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Default RE: Dumas Hots shot sprint help....

Heres a short video. It only gets worse with speed. I even have tried to start giving throttle very slowly with hopes of it just building up without breaking water. The motor took in water and shut off this time...

http://www.rcuvideos.com/item/QV25T340NMSBN4XG
Old 08-13-2007, 09:30 PM
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Default RE: Dumas Hots shot sprint help....

It's the "rocker" bottom I bet that is causing it. I had experienced it with mine, but I was able to tame it with nose weight and engine trim and prop. I now use an X438-3 Octura and their mount. My setup the propshaft sits with the bottom of the propshaft even with the bottom of the sponsons, and it seems about parallel - no up or down trim.

I forgot to mention also, get it going fast as soon as possible - anything slower and it starts to bow slap with mine. I have to keep it half throttle at least...anything below that and its a joy ride.

Some people have taken up the practice of sheeting the sponson bottoms with 1/16" balsa, sanding it till the rocker bottom doesnt show up, and then skinning the balsa after its been sanded to shape with 1/64 ply. This will leave a valley inside the sponson insides which can be filled with epoxy and micro-balloons and then sanded down sharp so it also adds cornering benifits.
Old 08-13-2007, 09:48 PM
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Default RE: Dumas Hots shot sprint help....

we need those pics,

direct shots from the rear, angled and sides.

need to see exactly where that motor is sitting.

Dan.
Old 08-13-2007, 10:34 PM
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Default RE: Dumas Hots shot sprint help....

Hey Nick...

They need some more pics!!!
Old 08-14-2007, 12:57 AM
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Default RE: Dumas Hots shot sprint help....

Never did like my ABS Sprint. So I went back to the wood version. As with all the boats I build, this was done in the WOF style. Don't know what the prop line/depth nor angle is (motor is no long mounted). Prop in use is a props-4-u x1440 b/c. Boat was radar'd at 46.3 using Wildcat 65%. Boat balanced out sweet, no added weight anywhere.

Thought I had a vid out on RCFLIX, can't find it...

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