Help! Sport Hydro Lost Speed with new CG

[Shooter]

Hi Everyone. Looking for a little help again. Last year I had this thing absolutely blazing in the low/mid 50’s (~30 runs, gps range 52.6-55.8) with an ABC1717 prop and UL-1 electronics and some improved cooling. I was really happy with it....it was movin!!!!

Video: http://www.youtube.com/watch?v=CMh8u6fhBrI

Since then, I moved the CG forward to prevent the occasional blowover and I lost an easy 5-8mph!!! Has anyone experienced this much of a speed loss from CG alone? I even tried a brand new motor and esc with no improvement. Mike helped me out last week a little with some strut adjustments and it rides better, but still not even close to the speeds I had last year.

Some Potential Root Causes:

1. CG moved considerably forward via motor and batt position
(WAS: 2” from sponson edge, IS NOW: 7/8” from sponson edge)

2. LONGER flex cable <- Nitro guys run long cables all the time!
(WAS: 7”, IS NOW: 15”)

3. Tired batts?? <---would be quite a coincidence.

Thanks!

[longballlumber]

Your packs were puffing on Wednesday night... My money is on batteries...

Later,
Mike

[T.S.Davis]

Could also be flex whip in the stuff tube. I had that problem with my whiplash. Wasn't enough bend in there.

[Shooter]

Thanks guys. I'm going to try new batts tomorrow. They were definitely puffing and my esc was stuttering which could be a sign of the batts going bad. Probably about 80 hard runs on them! However, the speed loss started right when I changed the configuration.....late last year....well before the puffing started.

Terry - If you get a sec tomorrow, please take a look at the bend. I've redone it twice! Lol! First it was straight with a late 's' bend. Now it is more of a gradual angle with a 'u'. No difference observed.

[785boats]

Shooter.
Has moving the COG forward introduced that tail hopping?. What I mean is, the prop is blowing out of the water a bit.
I Think the sponsons are now running a bit wet. The loss of speed & the extra amp draw that may be puffing your packs indicate this to me.
I would raise the strut as bit. This will effectively drop the transom a bit which will increase the angle of attack of the sponsons.This should air the boat out a bit more & give you back some speed with less amp draw.
But also, is the strut at the same angle as it was before you changed everything. Is there positive or negative angle to the strut? Or is it neutral.
If it has negative angle, then reducing that angle a bit will also free up the sponsons & reduce the hopping. But one thing at a time Eh!
That hopping also puts a lot of extra load on the ESC, motor, & batteries.

[Shooter]

785 - Thanks for the reply! Angle is the same. Neutral. No, the tail hopping seen in the video seems to be inherent to the 1717 prop. I tamed it a bit with raising the strut (as you said), but it still does a slight bunny hop. Just to be clear though, the video was taken BEFORE the CG change. The video is actually when I thought I had it running well! LOL!

Interesting point though...... When I give it a slight 'left' rudder input on the straight, the tail stops hopping and it sings along real nice and smooth. You can see it on some of the straights in the vid.... WHAT IS THE RUDDER DOING TO CAUSE THIS??? IS IT LIFTING THE TAIL?? ....OR IS IT LOWERING IT?? Curious to know what you guys think. I assume it's lowering it.

[785boats]

Hi Shooter.
In general it is accepted that the rudder will generate a bit of lift when turning. But if the toe is angled forward it can create down force. If the toe is angled back it can create lift.

I would be inclined to introduce a bit of positive angle into the strut.
Although you have it set at neutral with the hull, if the tail is running slightly lower than the front of the boat, then the strut is actually running at a negative angle to the waterline, or direction of travel. This has a tendency to drive the prop out of the water. About half a degree to start, & see what effect that has. If that makes it better try a bit more. But you might even need to raise the strut a bit more yet to increase the angle of attack of the sponsons.
The COG might just be too far forward at 7/8" behind the heel of the sponson. But I've no knowledge of what hull that is, so I can't really say.
I would have thought someone else would have offered a few suggestions by now.
Cheers.
Paul.

[Chilli]

I'm no expert in hydro set up but I want to learn more and I think this is s cool discussion. I think your 1717 might just have a little too much lift in it. Solutions... talk to a prop guy and get ideas on how to take some lift out of the prop. Also, as Paul suggested try adding a little bit of positive in the strut angle (a degree or two). This will slightly raise the angle of the thrust cone (reducing prop lift) while loosening up the front sponsons. I think when you give a little left rudder, you're increasing drag which may be keeping the stern down a little, reducing the hop. I've seen many porpoising mono whose set up was a little off also settle down with a little rudder input. No two boats are the same. Keep playing with CG and strut settings. It might take a couple more trips to the pond but you will get her dialed in.

[Shooter]

Paul – Thanks for the insight. I didn’t think about the fact that if the tail is dragging, it affects the prop angle. I had it set neutral to the bottom of the hull, not the ride attitude.

Chilli – I agree with you regarding the prop might have too much lift. I think converting some of the lift to push would be a good thing. I was considering cupping it the other day, but want to play with strut depths, angles, and CG before I go that route.


Another thing I noticed after watching a video from our last race: Judging by the sound it was making, it could be that the flex cable is winding up and releasing like a spring. This certainly isn’t helping anything…and could actually be putting some severe impact loading on the motor when the prop pops in and out. The longer cable is more susceptible to this spring behavior since it has more energy stored in it and the freq is lower. I think I’m going to stick with shorter cables from now on. My bend is somewhat subtle as well and (2) folks now have told me it could be causing some resonance.

[Chilli]

Sounds like a plan. A little cup at the tips might be all you need to settle down the rear. A neutral strut is almost always best as far as putting the power to the water. I was kind of surprised you are running the 1717 successfully on a UL1 set up. I would think that would be too much prop for that motor but you seem to be happy with it and that's cool. I wouldn't worry too much about the compression of the flex cable. The prop loading and unloading from the water as the tail hops is what can put the hurt on your electronics. You using .150 or 3/16" flex?

[Shooter]

You are right, the 1717 is pretty aggressive, but the UL-1 electronics have stood strong. I'm still using my original ESC from the UL-1 I bought 4 years ago, with two caps in parallel! The 40°F water up here in Michigan helps, but later in the year doohickey's like this (photo) come in handy to keep endbell temps down. I have over 40 runs with big props at 80+ degree F water with no signs of the motor wire insulation melting.

SAM_0838.jpgSAM_0832.jpg

[Chilli]

That's a neat cooler. I cant even run a 445 for three laps in the summer on my Insane FE30. The tell tale sign a UL1 motor is running too hot is an odorless wisp of white smoke when you pull the hatch. I guess that's the coating for the winding's slowly burning up. I had about 15 runs on my motor with that condition before it finally died. Anyway, keep us posted.

[Shooter]

Gave up. Moved the motor back to the original position. I was getting ready to glass the insides of the motor mounts tonight, but then I thought it might be OK as-is. There is only glass on the outsides. Anyone try this before?

Looking forward to getting the 'hop' tuned out with the 1717.

BTW - Is there a battery capacity limit to P-spec in MMEU? I'm going to need to add weight up front with the new motor position, so I'm going to try to add it in battery capacity, not lead!

SAM_1373_R.jpg

[detox]

IMO the heavier Whiplash FE needs slightly wider and higher sponson ride pads.

I balance hull with my finger. I let hull hang with finger centered on inside edge of turnfin. Slightly heavy stearn works good for me.

You can add weight to lefthand sponson tip to prevent blow overs.

You will need atleast 5000mah capacity. No capacity limit that i am aware of? More sponson angle (1 degree) will help with added weight.

[Shooter]

Thanks! I've heard a lot of people mentioning adding weight to the left sponson. I think it's in Phil's instructions as well. I've always tried to bias weight to the right to help in turns. Do you know the reason for the addition to the left?