F1 Tunnel boat hooks and spins out.

[SlickZERO]

I built an old Dumas 29” Hot Shot IV mostly for fun. ProBoat outboard with Aquacraft 36-59-1800 on 4 cells with X442 prop. Balance 29% and prop level. No turn fin. It is much faster than I expected and airs out well but doesn’t blow off.
If I try to apply even slight rudder at anything over about 1/3 throttle it hooks, does an insta-stop turning 180 degrees, and comes to rest facing the opposite direction. Any advise on CG? prop angle? turn fin?
I am not planning on racing it but I run with some guys in a fairly narrow lake and would like to turn around without having to slow to nearly off plane.
Finished LF.jpgFinished LR.jpg

[Doby]

need close up pick from the side showing you prop height and angle.

[SlickZERO]

need close up pick from the side showing you prop height and angle.

Hard to tell from the picture but the center of the prop shaft is even with the lowest point of the rear sponsons. It is angled down a couple degrees to push the front down. It rides fairly high in the front with about 4" of sponson touching and the prop.Motor height.jpg

[trigger]

I'm no expert by any means on these (I have a Topspeed 3). However mine used to do the same thing, the biggest improvement was found using a decent digital hi torque servo, and a thick gauge linkage for stiffness. As for prop depth, I found I had to go higher than I thought because the prop is so far back from the boat with it being an outboard. This length while the boat is on plane pushes the prop down, mine ended up being quite high before I got good temps and a nice rooster tail.

[Shooter]

I had a hotshot sprint many years ago. I can't remember much, but I definitely had a turn fin on the inside of the right sponson.

I remember my tunnels being pretty sensitive in the turns due to the steerable outboard and massive dihedral on each sponson.

[SlickZERO]

Servo

I'm no expert by any means on these (I have a Topspeed 3). However mine used to do the same thing, the biggest improvement was found using a decent digital hi torque servo, and a thick gauge linkage for stiffness. As for prop depth, I found I had to go higher than I thought because the prop is so far back from the boat with it being an outboard. This length while the boat is on plane pushes the prop down, mine ended up being quite high before I got good temps and a nice rooster tail.

Thanks for the information. I have a nice strong MG servo with solid linkage. However there is some play in the gears of the servo that translates into slop at the motor. I will look into a servo with less slop.

[SlickZERO]

Thanks for the information. The blue prints do show a turn fin and they supply an aluminum one. I looked at the "modern" ones at our lake and didn't see any with turn fins so I left it off. I will put it on and give that a try.

I had a hotshot sprint many years ago. I can't remember much, but I definitely had a turn fin on the inside of the right sponson.

I remember my tunnels being pretty sensitive in the turns due to the steerable outboard and massive dihedral on each sponson.

[SlickZERO]

I tried it again before I put the turn fin on. Lowered the motor a little to push the front down. Watching closely it seems to side slip when I start to turn. Then as the front comes down it suddenly grabs and hooks. Maybe the turn fin would make it start turning as soon as steering is applied and avoid the side slip and sudden grab? Looks like the pads would also help from the front dropping in the water suddenly. I saw a picture of a VS-1 and it looked like it had a long wedge on the inside of the right sponson only. Is that so? Thanks, Bret

[electric]

I tried it again before I put the turn fin on. Lowered the motor a little to push the front down. Watching closely it seems to side slip when I start to turn. Then as the front comes down it suddenly grabs and hooks. Maybe the turn fin would make it start turning as soon as steering is applied and avoid the side slip and sudden grab? Looks like the pads would also help from the front dropping in the water suddenly. I saw a picture of a VS-1 and it looked like it had a long wedge on the inside of the right sponson only. Is that so? Thanks, Bret

This might help a little. Dial the steering down or use the..(forgot the name for it) percentage adjustment that cause causes the servo to move very little at first and progressively more later in the turn process. They use this with cars a lot to dial down the sensitivity. I had a tunnel once and when you turned it tended to hook in tighter and spin out. Dialing out some steering helped keep it from doing that. Might not solve it, but could help a bit.

[SlickZERO]

Thanks for the advise. I have the exponential set for slight center movement. Linkage is great but the metal gear servo has some play. May install a better servo?

This might help a little. Dial the steering down or use the..(forgot the name for it) percentage adjustment that cause causes the servo to move very little at first and progressively more later in the turn process. They use this with cars a lot to dial down the sensitivity. I had a tunnel once and when you turned it tended to hook in tighter and spin out. Dialing out some steering helped keep it from doing that. Might not solve it, but could help a bit.

[trigger]

I would try a different servo, maybe try the traxxas 2075.

[grsboats]

Definetively a good and precise servo with decent torque is a must in a tunnel.I also would say that your balance of 29% might be causing this reaction....32% can be an improvement.Gill

[SlickZERO]

I would like to thank everyone for all the advise. I installed the turn fin that came with the kit at the location shown on the blue print. That fixed it! It will carve a nice turn at any speed. I even tried turning sharper than the buoys at full speed and nothing bad happened. As it scrubbed off speed and the right front sponson came down you could see some spray coming over it but it just kept on its path.

[SlickZERO]

GPS speeds

I have the motor height and angle (about 0 deg) set and it airs out good with the nose up and "flying" on the prop with about 2" of the sponson touching. Two runs with GPS were 40.8 mph and 41.5 mph. Very happy with it and "fun to run".