Homemade Foam Boat - Can’t turn left?

[handsomtransom]

I’m new to building RC boats. I built a custom foamboard monohull using some very rough guidelines. This is the 6th prototype — I tweak something a little every time and I’m learning a lot.

My problem is: when I trim my rudder perfectly straight, the boat takes left turns very slowly and right turns very fast. See this video for a demonstration of the problem:

Even when I trim my rudder hard-left, the boat still tracks a bit to the right, and has the same underpowered left turn.

ALSO, it seems like it’s not getting on plane when I run it on a straight path at max power, because it occasionally will and the difference in speed is huge.

Here are the specs:

• 27” long, 7” wide, 15° dead rise at transom
• 980g (2.15 lbs), CG is 10” from transom
• 3S LiPo battery on a 21T 550 motor
• Straight shaft at 13° to keel, exits right at keel line from transom.
• 1.5” dia. CCW 2-bladed surface piercing propeller, 2” pitch
• Rudder is centered right behind the prop

Bottom-line I need this boat to go at least 20mph.

For the more experienced folks on here — is there an easy answer to why I’m experiencing this problem? (My thought is prop walk).

Also, does anything jump out as obviously hindering performance? What would you change about the design?

 

[WickedFog]

I’m new to building RC boats. I built a custom foamboard monohull using some very rough guidelines. This is the 6th prototype — I tweak something a little every time and I’m learning a lot.

My problem is: when I trim my rudder perfectly straight, the boat takes left turns very slowly and right turns very fast. See this video for a demonstration of the problem:

Even when I trim my rudder hard-left, the boat still tracks a bit to the right, and has the same underpowered left turn.

ALSO, it seems like it’s not getting on plane when I run it on a straight path at max power, because it occasionally will and the difference in speed is huge.

Here are the specs:

• 27” long, 7” wide, 15° dead rise at transom
• 980g (2.15 lbs), CG is 10” from transom
• 3S LiPo battery on a 21T 550 motor
• Straight shaft at 13° to keel, exits right at keel line from transom.
• 1.5” dia. CCW 2-bladed surface piercing propeller, 2” pitch
• Rudder is centered right behind the prop

Bottom-line I need this boat to go at least 20mph.

For the more experienced folks on here — is there an easy answer to why I’m experiencing this problem? (My thought is prop walk).

Also, does anything jump out as obviously hindering performance? What would you change about the design?

View attachment 243253

View attachment 243254

View attachment 243255

Look at the angle of your steering rod. It should be parallel with the centerline.

 

[9x19]

Welcome to RCT

 

[Guest D]

Welcome to RCT. Maybe torq is to blame for the difference in right to left turning.

 

[WickedFog]

It would work a lot better if the servo were rotated 180° so the steering horn exactly mimics the steering arm on the rudder.

 

[handsomtransom]

Look at the angle of your steering rod. It should be parallel with the centerline.
View attachment 243256

Unfortunately I don’t think that’s it. The small misalignment of the control rod, the rudder arm and the servo horn had no appreciable influence on the ultimate angular throw of the rudder. The rudder travels the same angle left and right regardless. I even tried biasing the rudder one way and the other using transmitter trim and it didn’t affect the problem

 

[handsomtransom]

It would work a lot better if the servo were rotated 180° so the steering horn exactly mimics the steering arm on the rudder.

I think you’re right that having the servo horn and rudder control arm rotating the same direction instead of how I have it would solve control issues down the line. But as I mentioned above the rudder rotated the same small angle clockwise as counterclockwise, so it doesn’t explain the huge power difference between left turns and right turns.

 

[handsomtransom]

Welcome to RCT. Maybe torq is to blame for the difference in right to left turning.

I think this is it… the paddle wheel effect of propellers, especially surface piercing propellers where only one half of the rotation is imparting sideways push, is probably to blame.

One idea I have is to put the rudder to the right of the prop, so it blocks some of the sideways thrust coming off the prop. That might introduce problems too.

The next idea is just to upgrade to a twin prop setup, with CCW and CW props either side of the rudder. That way their paddle wheel effects cancel out. Lots to do…

 

[WickedFog]

I think this is it… the paddle wheel effect of propellers, especially surface piercing propellers where only one half of the rotation is imparting sideways push, is probably to blame.

One idea I have is to put the rudder to the right of the prop, so it blocks some of the sideways thrust coming off the prop. That might introduce problems too.

The next idea is just to upgrade to a twin prop setup, with CCW and CW props either side of the rudder. That way their paddle wheel effects cancel out. Lots to do…

Maybe try lengthening the rudder.

 

[handsomtransom]

I think I’ve found a possible culprit.

One of these things is not like the other…
The top image is of the older build that worked really well, got on plane, went fast and didn’t have trouble turning either way. The bottom photo is of the build that had the issues turning left. I accidentally installed the rudder wrong — it was facing backwards. See the axis it pivots on is closer to the straight edge than the curved edge. If the curved edge is the leading edge, then the hydrodynamic center is in front of the rotational axis, making it unstable. It probably fluttered or snapped to one side, causing more drag on a left turn. That’s my working theory right now, I’ll test it by flipping the rudder back tomorrow and trying it out.

Thank you all for your suggestions and theories!

 

[1/48 fleet]

A lot of good advice given in this thread and I am glad to see that you recognized the issue with your rudder. I hope that it improves the handling for you. Remember to change only one thing at a time and test the results. Otherwise you will not know which "fix" solved the issue. And you will not learn from it. Most of us have been there and learned from experience.
Last comment. If you make your push rod adjustable in length, you will not be restricted to your trim switch on your transmitter to make adjustments.
Good luck and keep plugging.

 

[WickedFog]

I think I’ve found a possible culprit.

One of these things is not like the other…
The top image is of the older build that worked really well, got on plane, went fast and didn’t have trouble turning either way. The bottom photo is of the build that had the issues turning left. I accidentally installed the rudder wrong — it was facing backwards. See the axis it pivots on is closer to the straight edge than the curved edge. If the curved edge is the leading edge, then the hydrodynamic center is in front of the rotational axis, making it unstable. It probably fluttered or snapped to one side, causing more drag on a left turn. That’s my working theory right now, I’ll test it by flipping the rudder back tomorrow and trying it out.


View attachment 243346View attachment 243344
Thank you all for your suggestions and theories!

It's hard to see in the pics, but have you created a bit of an airfoil shape in the profile of your rudder?

 

[1/48 fleet]

I meant to add one more thought. Most high speed mono hulls off set the rudder to compensate for the torque effects of the prop. Something to consider.