Weight distribution in an RC yacht hull

[Peter CrowParticipant @petercrow25030]

My first-time scratch build of a yacht hull. It is of RG65 dimensions based on Bryn Heveldt's Racing Sparrow RG65 plans. Construction has progressed to the point of the balsa plank-on-frame hull having been glassed and epoxied and my thoughts are now turning to fitting out.

Having built some electric powered boats, my head is being messed about with the layout of all the RC paraphernalia in a sailing boat.

In a propeller-driven hull all the weight sits above the keel and must be set out with careful regard to lateral balance of the hull , both fore and aft, and side to side.

With a yacht, particularly with a fin keel, there is a ruddy great pendulum weight dangling underneath! The dynamics are totally different.

The greatest mass is represented by the battery pack. I'm aiming to install this next to the fin keel box on the port side, keeping it as close to the centreline as possible. The rest of the equipment I'm hoping will be installed on the starboard side.

Having weighed all the components, there will be an imbalance between the two sides. Will this be compensated for by the effect of the keel bulb? Any tips and wrinkles on layout will be gratefully received!

[Peter CrowParticipant @petercrow25030]

[ashley needhamParticipant @ashleyneedham69188]

Peter. Not that I am a yot man, but in theory you would want an even balance of weights either side.

It might be that a bit of lead might be needed after a pond tryout.

Ashley

[Tim RoweParticipant @timrowe83142]

Hello Peter

The deep ballasted fin will have the greatest affect by far. The cg position of the ballast is important to get the right fore and aft trim but hopefully the designer will have got that sorted out.

As long as the weight of the battery is not too different from the other components you should be ok with your plan to keep the battery as close to the centreline. If you had the chance to install the other components a bit further outboard this would help a bit. Try that first.

The chances are you won't need any lead due to your ruddy great pendulum but if you do, as Ashley suggests, put the smallest possible lump as far outboard as possible to keep the overall weight down.

If it is only a couple of degrees I wouldn't bother because when the boat is on the water it won't sit still and the sails, keel and wind combination will dominate.

Tim R

[Peter CrowParticipant @petercrow25030]

Thanks for the replies, Ashley and Tim.

Tim. Your suggestion re: mounting components a bit further outboard on the side opposite the battery is the key. The next largest piece of kit is the sail servo. As it is of the lever arm type (and pretty lengthy at that) the pivot point will be well outboard. I'll now consider whether to mount the servo athwartships (as per the 'thrown together' arrangement in the photo) or fore and aft as close to the outside of the hull as possible, thus pushing the mass even further out.

I'm just loving the discovery learning on this project. I've made my first lead casting ever and am having to improvise on a number of fronts. I'm cheating on the power unit by using a Dragon Force mast and sails, but maybe next time I'll try the whole thing from scratch.

[Kev.WParticipant @kev-w]

May I suggest (as a long term rag & stick man), that you consider mounting your sail servo in 'fore & aft' , with a double sided arm, this will keep the balance of pull between the main & jib, with less strain on the servo.

I speak with experience as a former Marblehead & current IOM & Micro magic racer..

I can only offer what I have learned by time on the water.

[Kev.WParticipant @kev-w]

My preference today is for 'drum winches', they are quicker when it comes to hauling in when rounding a mark in heavy weather.

[Malcolm FraryParticipant @malcolmfrary95515]

What Kip said in both his posts.

When arranging the line run(s) on a sail arm, it is best to arrange things so that when hauled in, the line passes as near as possible to the fixing screw of the servo arm from its last guide point. You get the best mechanical advantage when it is needed that way.

The new breed of drum winches are much easier to mount and rig. When mounting a sail arm, you need to remember that all of the forces from the sail contribute to a twisting effort at the servo mount, and nature is always looking for the weakest link. A drum servo just offers a pull in one direction. Also, they don't need anything like the same space to allow for the swing of the arm not hitting anything. Beware of mounting the switch anywhere where a slack line (from letting the sail out in light airs) can snag it. A yacht that has managed to switch itself off is a PITA.

The layout looks fine, if you are going for competitive racing, fine tuning the balance "might" make a difference, but sailing for pleasure, you will not notice any difference at all. Obviously, if there is another sailboat on the water, a race is happening. That is just the way that it is.

[Tim RoweParticipant @timrowe83142]

Hello Peter

You have made a very nice job of the hull and planking. Did you enjoy the build?

I am using a Lever type sail servo in Galileo. It is the big Hitec one and packs some punch. Maclom's point about having the line go across the servo arm screw is important like he says and also enables better fine adjustment of the sheeting angle when close hauled when it is most useful. The other important point is that in this position there will be the most tension in the sheets and because the servo arm will be at or near "top dead centre" it will be using little or no current to keep the sheet in that position. Firstly this will make you battery last a lot longer between charges and secondly your servo will last longer as it is doing less work.

The other thing is to make it impossible for the servo arm to hit anything over its full range of travel. Sometimes when using a tester or when switching on, a servo moves more that the normal travel before settling down. Mine would very easily punch a hole in the side of the boat.

Looking forward to seeing your progress

Tim R

[Peter CrowParticipant @petercrow25030]

Kip, Malcolm, Tim. Many thanks; lots of valuable advice to inwardly digest. It's great to be able to have access to such a wealth of experience.

Maybe next time for a drum winch setup.

I've nailed my trousers to the mast for this project. The core of my inspiration for this boat, apart from Bryn Heveldt's Racing Sparrow, includes John Goodyear's 'Varmint' (Model Boats September 2017) and his 'Alpha' (MB May, June, July 2018). Add to this that my 'taster' yacht is a Joysway 'Orion', I'm pretty much committed to a lever arm system this time round.

Tim. My sail servo is a 55g metal gear King Max from Component-Shop. It is rated as 11.75 kg/cm at 4.8V and 13.06 kg/cm at 6V. From my researches I reckon that this should be enough, bearing in mind that I'm using a DF65 'B' rig.

And why am I using a 'B' rig? 'Cos I'm using a shorter fin keel to suit my club's water.

(Isn't it strange that when one sees one's thought processes in writing they take on a totally different perspective? Sod it! As I said, my trousers are committed!)

 

Edited By Peter Crow on 14/10/2019 22:44:50

[Malcolm FraryParticipant @malcolmfrary95515]

Since it is a new build, this probably dosen't apply.

Several years ago I got a second hand Victoria at a B&B. Downloaded the instruction book and sorted the previous owners more obvious mistakes which were the probable reason for the boat being disposed of, then went sailing. The makers blurb claimed that a normal servo would work the sails. They lied. Sent off for a high torque one. Worked great, but soon showed up the wiring, which needed replacing.

Hi-torque servos need a good power supply, they do draw more current than a "normal" servo.

To get the required travel, the line was doubled back at the servo arm, when it was eventually replaced by a drum winch, I opted for a 4 turn one which wasn't noticably slower than a 2 turn, but had more travel than needed. I did the doubling on deck, which increased the torque at the sail boom. This allowed sailing in stronger winds, which resulted in new sails. The drum winch does let the batteries last longer, they are not constantly trying to correct for being pulled off point.

[Chris FellowsParticipant @chrisfellows72943]

Worth noting the DF65 uses a drum winch so it might be a better match for the sails and how they are controlled.

Can you get the nails out!

Chris

[Ray Wood 3Participant @raywood3]

Hi all,

The DF65 needs to use the drum winch as the hull beam is so narrow, the heavy duty Hitec lever servos have taken 5 years of sailing barge racing without a problem but no space problems.

Regards Ray

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