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Trim tabs question

Old 06-28-2004, 04:29 PM
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cmho2
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Default Trim tabs question

Anyone have a good explanation about how to properly adjust trim tabs?
Presently my boat has 2" aluminum tabs on each side. They are hard to bend, especially when I'm at the pond trying to enjoy my boat. Anyhow, I was playing around with the strut and rudder positions yesterday, and I found a bit more speed (especially at take-off, much less cavitation)... BUT this also caused the boat to spin out when turning even the slightest bit. So I began bending the tabs, and I think I improved the handling some.

My next step is to purchase some new adjustable tabs, probably nylon (mine look crappy too), and I wondered does anyone have any "guidelines" about making adjustments? i.e.- "boat spins out left - adjust inner part of left tab down". "Boat doesn't get on plane - adjust both tabs down"

thanks
Old 06-28-2004, 04:59 PM
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pompebled
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Default RE: Trim tabs question

What boat are we talking about?

Do you have some pics?

Regards, Jan.
Old 06-28-2004, 05:37 PM
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cmho2
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Default RE: Trim tabs question

Jan it's my "Aries" boat. Kind of an odd boat really. The pics of it BEFORE I removed the subsurface drive and put on a strut & rudder assembly, added wood stringers, and re-inforced the transom are here:
http://www.rcflix.com/photopost/show...500&ppuser=275
I still don't have pics of the boat as it's fitted now. The strut/rudder combo and wire drive came from Steve at www.offshoreelectrics.com.

The trim tabs I'm talking about are the ones shown in the pictures.
Old 06-29-2004, 09:54 AM
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pompebled
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Default RE: Trim tabs question

Trimtabs should be avoided if possible, have you tried shifting the CoG forward? What happened?

The place where the trimtabs are positioned now, at the outside of the transom, is the place to adjust turning properties.

If you are looking to eliminate porpoising, the trimtabs need to be closer (next) to the propshaft.

Dont mount them flush with the bottom, but 1-2 mm higher to minimize drag.

Please describe, why you need trimtabs.

Regards, Jan.

P.S.: The spinning out indicates the need of a turning fin, not trimtabs....
Old 06-29-2004, 10:17 AM
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Aaronie
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Default RE: Trim tabs question

Trim tabs will also help with prop walk. If you need to give the boat some left rudder to go straight trim tabs will help.
Old 06-29-2004, 04:17 PM
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cmho2
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Default RE: Trim tabs question

The boat will porpoise a bit at full throttle. In fact it will actually become airborne, rudder completely out of the water, when negotiating small waves. This is easy to fix by moving the batteries up a bit in the hull, changing the COG as you mentioned. But it also "flattens" out the boat... it's kinda boring that way. lol

As I said in my original post the reason I wanted to adjust the tabs in the first place was to fix the spin-out problem after I adjusted the prop shaft and rudder to gain a bit more speed.
Old 06-29-2004, 04:41 PM
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pompebled
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Default RE: Trim tabs question

Changing to surface drive will do that (spinning out, I mean), as the propwalk becomes more prominent.

Using a longer rudderblade and/or adding a turning fin will counteract the propwalk more and reduce the spinning out, if the boat leaves the water completely, there's no telling what it will do upon landing of course, but a longer rudderblade will help (make it thin to reduce added drag).

Regards, Jan.
Old 06-29-2004, 07:22 PM
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MikDee
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Default RE: Trim tabs question

I concur with Jan, turn fin needed,,, by the way Jan, I just setup a Synchron stepped mono, with a 700 8.4V motor, 12cells, center rudder offshore hdwre (offshoreelectrics), and an X442 prop,,, the boat runs fine, but struggles a bit to get on plane???, (no cavitation?) especially as the batts wear down,,, Is this characteristic of a double stepped hull???,,, COG is about 30% from the transom. Do ya think trim tabs would help???,,,
Old 06-30-2004, 11:32 AM
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pompebled
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Default RE: Trim tabs question

Taking the risk to go off topic, I assume a X442 has a 42mm diameter?

The 700 motor should be able to handle a 43,5mm and a 45mm prop easily, the increased thrust will get the boat on the plane with more ease.
My mono2 (MHZ Flash, 700 neodym, 12 cells, 45K prop) jumps on the plane in a matter of feet, when the batteries are nearly empty after eight minutes, I must stay on the plane, the remaining power is not enough to get on the plane again, but this is in the last 30 seconds before the ESC cuts the power, in order to be able to get the boat ashore half throttle, on the remaining mAh.

What cells are you running?
How is the boat set up?
Got any pics?

In general a stepped monohull should not need trimtabs.

Regards, Jan.
Old 06-30-2004, 11:58 AM
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pompebled
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Default RE: Synchron

http://www.dirk-kramer.de/syncron/syncron.htm

This is the hull we're talking about right?

Dirk Kramer is a well known boater in the European scene, look around at his homepage.

His complaint about the Synchron hull is, that the sawtooth pattern is far too deep/steep.

He has flattened the steps, so that they run more parallel, he also raised the first step to avoid "sticking" to the water, after a few testruns he's much happier with the hull.

Download the video from his Synchron, it screams!

His CoG lies at 22%, 165mm from the transom.

If you have more questions, and your German isn't up to speed, just mail Dirk in english, or run them by me, I'll ask Dirk and translate his reply for you .

Regards, Jan.
Old 06-30-2004, 08:39 PM
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MikDee
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Default RE: Synchron

Thanks for the reply Jan, I was also told trim tabs don't work on a stepped hull,,, I am American of German Descent, but can't speak much German, much less read any,,, but the pictures say it all,,, and yes, the steps are pretty radical on this hull, I have an Octura 42mm prop on the boat, and the motor gets abit warm, because it is direct drive & 28.5" long & 8.5" wide. A 45mm prop may run too hot,,, I'm running 12-2400mah matched Sanyo's. I also have the no step version of this hull, with the same setup, that gets right up on plane always, No Problem. I might like to modify mine like Dirk did, if I knew how. http://www.rcflix.com/photopost/data...hron_2-med.jpg
Old 07-01-2004, 06:34 AM
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pompebled
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Default Synchron.

If I'm not mistaken Dirk has used epoxy filler, mixed with microballoons to keep the weight down.

I could mail and ask him if you like.

Just let me know.

Regards, Jan.
Old 07-01-2004, 08:58 AM
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MikDee
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Default RE: Synchron.

Hope u were able to see the picture?,,, don't know what microballoons are?,,, anyway, I was thinking of maybe just adding a narrow ride pad over the steps instead, something like what you did.
Old 07-01-2004, 11:58 AM
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pompebled
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Default RE: Trim tabs question

Yes, I have seen the picture and I must say that the steps don't look that steep to me.

Have you tried shifting the CoG back towards the transom?

If you try a ride pad, limit it to the last step first, otherwise the lifting of the rear end may result in the nose going down too much and you'll need to counteract that by raising the first step a mm or two with filler.

Micro balloons are just that: tiny glass balloons you can mix with epoxy resin or filler, the air in the balloons reduce the weight of the mixture.

Did you see Dirks video?

Regards, Jan.
Old 07-01-2004, 01:37 PM
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MikDee
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Default RE: Trim tabs question

The motor is kinda heavy & it's located jut in front of the first step, but with the batts against the transom, COG is about 7" from the transom, or about 25%. Yes, I saw the video, there is no boat in the water, just a prop,,, LOL,,, I really have no complaints about how the boat goes, just that it's sluggish to get on plane, in racing this is detrimental, especially coming outta the hole, and in turns.
Old 07-01-2004, 02:52 PM
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pompebled
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Default RE: Synchron

Just came back from scaring myself by nearly running my mono2 into the wall of the canals of the fair town I live, man, a small mistake really makes the brick wall get close very fast.....

With your boat being a 8cm longer than my Flash, with roughly the same ingredients to propel it, I'd go for a larger prop, a 43,5 or a 45mm.

The extra thrust should get it on the plane faster.

It's either that, or your strut may be a little on the high side, bring it down a mm at a time to see what happens in terms of acceleration from a stand.

Regards, Jan.
Old 07-01-2004, 03:30 PM
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MikDee
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Default RE: Synchron

Sorry to hear your misfortune, hope u didn't sustain any serious damage,,, coincidence u should say that, I just got back myself from trying my boat with temporary homemade trim tabs on it, it was much better,,, in fact, it helped the handling all around, made it more stabile?,,, I will take off the trim tabs, & try a 45mm prop I happen to have, on it next chance I get, shoulda done this first,,, lol,,, I'll let u know how it works. By the way, my strut is dead straight back as best as I can tell.
Old 07-01-2004, 04:57 PM
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pompebled
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Default RE: Trim tabs question

No, just a scratched paint job.

In general, a ridepad does the same job as the trimtabs, but in a more subtile way, and without the extra drag.

Let me know how the prop testing goes. See if you can find a prop with the same diameter but with different pitch, as the proplift plays a role aswell.

Regards, Jan.
Old 07-02-2004, 03:43 PM
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MikDee
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Default RE: Trim tabs question

Ok, just tried a 45mm prop on the Synchron, it worked much better!,,, got up on plane much better & it was faster too, of course. It was fun, but before the batts dumped I pulled it in & the motor was abit hot, according to the 5 sec rule, even tho I also set up prop wash cooling which looked like it was working fine. Another thing I noticed was, it did not respond well to left rudder at speed, better at low speeds, I figure it's prop torque due to the large prop.
Old 07-03-2004, 05:40 AM
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pompebled
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Default RE: Prop size and pitch

Good to hear that the bigger prop works, and yes, the bigger the prop, the more outspoken the propwalk becomes.
Left turns at full speed become very wide, narrow ones tend to provoke flips.

Do you have only can cooling, or brushcooling as well?

Regards, Jan.
Old 07-03-2004, 06:33 AM
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MikDee
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Default RE: Prop size and pitch

Just can cooling, working on adding brush cooling. I have the 700 8.4Vbb motorin there now, also have a 700 Neo I'm gonna try, to see which one is faster.
Old 07-03-2004, 08:10 AM
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LtDoc
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Default RE: Trim tabs question

MikDee,
If the motor is getting hot then it means you are getting close to the drive system's limit (if you haven't already reached them). A motor is most efficient at it's highest speed (rpm). When the speed of the motor is lowered (size of prop, or pitch) it means it has to do more work, and is going to get hotter than 'normal'. Cooling can extend the system's limits a bit. Or, the 'other' option is to increase the abilities of the system by making 'things' larger, motor, ESC, batteries, etc. 'Larger' depends on the size of the boat, naturally. After some point there's just no more room (also depends on your bank account! LOL). Juggling the size and pitch of the prop is one way to 'tune' your drive system, and probably the most cost effective. At some point you just don't have any more options and that's it...
- 'Doc
Old 07-03-2004, 09:33 AM
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pompebled
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Default RE: Trim tabs question

I'm interested to hear which motor is faster in your set up.

Regards, Jan.

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