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Request advice for my first boat build challenge!

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Joe Evans21/01/2020 20:05:40
2 forum posts


I am looking for some advice regarding rudder choice and set-up. My team and I have a project to design and build a fully autonomous boat to compete in the following challenges:

  1. An endurance race around a given course, you will need to complete as many laps as possible in 14 minutes on one battery (no pit stops to change battery are allowed). (can be run anti-clockwise or clockwise)

  2. Speed challenge the fastest vessel to complete an approx. 60m sprint.

  3. To deliver a 2.0kg load of 10mm Pea Gravel to a specified destination within the Lake, The Load must be carried on the deck and be successfully navigated through a prescribed course against the clock. Starting at West Waypoint, to North Waypoint, back to West Waypoint to DELIVER PAYLOAD, Finish at South Waypoint.

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Our initial plan is to design a vessel that will perform average/good in all 3 challenges, rather than specialise into one. Our budget is £250 for mechanical, control and electrical components. We are using a composite “Batwing” Monohull (Of which we do not have to factor into our budget.)

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I am currently looking into rudder size and placement. Could anyone please advise on rudder set up? How beneficial would it be to be offset the rudder to the right, or is this only for a racing setup? Would it be beneficial to have a dual rudder setup or just a single rudder in the centre line? How should rudder area/length be proportioned to the hull dimensions?

Any advice or pointers to reliable sources of information would be much appreciated.

Many thanks!

Richard Simpson21/01/2020 22:28:04
105 forum posts
74 photos

I'll start the ball rolling by adding some preliminary thoughts as I am sure there will be more ideas to come.

First up it is important to realise that a rudder works in exactly the same way as an aircraft propeller by generating 'lift' at the expense of 'drag'. The shape and design of your rudder will be dependant on the use of your model and consequently whether manoeuvrability or speed is more important for you. At one extreme a speedboat will sacrifice manoeuvrability and reduce drag by having a relatively small rudder whereas a tug will sacrifice speed for the best of manoeuvrability by having a large rudder. Have a look at typical speedboat models such as a Club 500 here at:

and compare that with a model tug such as a Mobile Marine Models steam tug or a Model Slipway Envoy Class.

Your next main concern is placement. Looking at your hull you basically have two options, you can either have your prop shaft coming out of the bottom of the hull, with the rudder mounted on the bottom directly behind it, as per the Club 500 arrangement, or you could have the propeller mounted on the transom, again with the rudder mounted behind it. Unfortunately with your flat wide based transom you will have a great deal of difficulty in getting a good flow of water around the rudder so really your only options are either to have the tube exit the hull through the keel and the rudder mounted vertically behind it or an outboard on the transom with the propeller extending below the transom. I would use the Club 500 arrangement for guidance. Offset rudders are only really used in high performance boats, in your case you need the most effect so the rudder may be best mounted directly behind the propeller in the best flow of water. If you use a flat brass rudder, as used by the Club 500, you will have minimum drag and mounting it directly behind the propeller will give you maximum effect.

You haven't said what the dimensions of your model are but taking the proportions of the Club 500 and scaling them up or down should give you an idea of what size and shape of rudder you need. To meet criteria A) you do need performance as well as endurance so you should be looking for as light a construction as possible combined with the 'V' hull. If your model is fairly close to the size of a Club 500 you might want to consider buying the motor, shafting and running gear as well as the rudder unit from the manufacturer.

Anyway just some ideas to start the thinking off. Good luck with the competition!

Empire Parkstone21/01/2020 22:30:49
227 forum posts
8 photos

Might have been more polite to introduce yourself and who you and your Team are in your first post.

Edited By Empire Parkstone on 21/01/2020 22:31:29

Chris Fellows21/01/2020 22:51:36
725 forum posts
446 photos

Whilst some idea of size can be determined from the photographs you need to provide more accurate information i.e. length, width and depth of the boat before anyone can advise you.

I assume that the frames and foam are just temporary to form the hull? Otherwise you won't have enough space for all the gear and battery etc! It will be beneficial of course to have some framing to stiffen the hull, mount the deck and fix the aforementioned gear.

Have you tried the hull in water yet with the 2kg. load in place. Be interested to see how it sits in the water, both loaded and unloaded!


Edit: other posts made by others whilst making mine which have covered some of what I have said.


Edited By Chris Fellows on 21/01/2020 23:27:16

ashley needham22/01/2020 08:59:15
6666 forum posts
160 photos

Hi Joe and welcome to the forum.

Whereas there are loads of theories that could apply to rudder placement, your hull would really dictate that the, rudder sits behind a centrally placed prop on the centreline.

The hull seems to be full of frames and it is difficult to see exactly how you are going to fit a propshaft in the first instance. As the hull, although wide, has a relatively narrow rounded centre section, to my mind there is only the one place to put the shaft/rudder and that’s central. Anything else will be difficult to do. This will mean the prop is very deep in the water giving you a very deep drought.

As for rudder size, get the largest brass rudder (the ones with nylon clamp on tillers) and fit that close behind the prop, allowing space to remove/refit a new prop. These rudders can be cut down easily or enlarged for experimental purposes, and are inexpensive.

You say deliver a you mean tipping from a hopper?

Along with the dimensions of the boat in your next post, do have an idea of how heavy the complete thing will be, and also your ideas (if known) of how you intend to power it.


Malcolm Frary22/01/2020 11:22:23
895 forum posts

The flat transom will kill any performance* unless the boat goes fast enough to break the water flow around it. Any propeller would need to be below the hull, along with its rudder. If the prop was mounted behind the transom, it would never get clean water flow.

Racing boats generally have an offset rudder because they are only intended to turn one way. A rudder generally sits in the waterflow caused by the prop and redirects it, it doesn't need to be any bigger than the flow.

Any boat weight detracts fromits payload. I think most boat modellers would conside 5 or 6 of the frames to be surplus to requirements, although without knowing the size of the hull, it is guesswork. Guessing that a 2Kg payload + weight of whatever holds it will remove any likelyhood of going fast enough to break the transom waterflow. Probably mess with any hopes of stability, as well.

*both speed and efficiency (ie run time) will suffer.

Ray Wood 222/01/2020 11:39:13
2047 forum posts
720 photos

Hi Joe,

Shame your boat won't have a sail, it is incredibly similar to a RS300 racing sailing dinghy, which is fast when sailed upright, but as soon as the wings go in the water it slows down dramatically.

The payload of 2 Kg on the deck is a very hard goal to achieve and will require loads of ballast in the hull low down , a single chine hull would have been a better option for both speed being able to plane, and with sufficient displacement to carry the load required.

Look at fullsize hulls which go fast for a guide

Good luck, keep us posted

Regards Ray

Dave Cooper 622/01/2020 19:58:00
171 forum posts
16 photos

Hi Joe

I'm intrigued by your phrase "fully autonomous boat". Presumably, this means a 'pre-programmed' motor run with rudder inputs at the predetermined turning points (approx. 90 degrees) on the lake ?

Whilst you're solving your propulsion /steering methods, it would be a good idea to get a team member up to speed on control systems eg any radio control experience would be useful....of course, you can't use radio as such but you could use Arduino (or, another microcontroller) to take the place of the radio transmitter and receiver. Arduinos can drive the rudder servo and, the motor's speed controller, and they are cheap !

You don't say if you're allowed to use GPS ? If so, this could tell the Arduino when to change course etc. You can write some simple sketches (programs) to achieve this using the 'C' programming language.

I think you'd better allow plenty of time for testing and fine tuning !!

Hope this helps,


Joe Evans22/01/2020 21:37:52
2 forum posts

Thanks to everyone that has replied, really appreciate the interest and advice this has generated, a lot of useful information here to consider already!

A little bit more info on the project:

My team and I are Foundation degree students studying a range of engineering-based topics: Naval Architecture, Mechatronics, Mechanical Engineering and Marine Engineering. We are all a bunch of novices at model boat building but all work in the marine engineering industry.

The previous photos of the bat-wing hull in my original post are photos of the plug that was used to create the male mould. This will then be used in a vacuum resin infusion to create the composite hull. (Therefore those wooden frames will not exist in the composite product.)

Length: 900mm Width: 370mm Depth:140mm

Displacement (Hullform only – unladen): 3.359kg

The vessel will be fully autonomous: to include propulslion, steering and load delivery. The race lake we are using has full wifi coverage to support this. The mechatronics member of our team has drawn the short straw on this one because I admittedly have very little knowledge on that specific subject. I will keep this thread updated as to how we are to go about programming this once I have spoken to him on Friday!

We have short listed the load delivery system to:

  • A centre-line actuator that will push two hoppers to tip overboard (1 to port and 1 to starboard.)
  • A lightweight frame construction that will sit on top of the deck, with a pully system to pull two hoppers overboard.

These ideas were shortlisted as the actuator/pulleys will be positioned on the centre line and will allow for even distribution of the payload weight (1kg per side). We are leaning toward the frame design as it will be easier to remove for the endurance and sprint challenges!

We are to choose between 7 different speed-controllers, 4 motors and 4 batteries . We must also use a 3/16” Drive shaft.Aside from these limitations, we must everything else through research.

Regarding rudder selection it seems that I should write off the idea of an offset right rudder. I was reading other forums suggesting that centre-line rudders can sometimes make the vessel unstable in a straight line due to the rudder being in turbulent rather than “clean water”. Not sure how much of this is true, but maybe that theory is just reserved for high performance models.

I have began to look at the above mentioned examples of model boats and I can definitely see some similarities. Thanks for all the info again, ill keep this thread posted on how we are getting on. Race Day is in May. We order all parts for a fortnights time where building and testing will commence.

Fingers cross it works haha!

Dave Cooper 623/01/2020 11:04:06
171 forum posts
16 photos

Hi again Joe

Get your 'mechatronics' person to have a look at Arduino Starter Kits - I think it will have a lot of the bits you'll need for the fully automated bit : eg micro servo, stepper motor, breadboards, jumper leads etc. Mine cost around £29 - good value and much quicker than buying all the parts separately ! It comes with a tutorial on CD as well.

As you have 'wi-fi' coverage you could use a mobile phone for a GPS signal, although, a dedicated chip (eg an Arduino "Shield" would probably be lighter, more compact and easier to program.

In the Arduino IDE (development environment) he /she will find lots of pre-written code examples. You could probably pluck these out as they are and 'tailor' them for your application. I should think some of the "robotics" code (turn left /turn right etc) would give a starting point.

Good luck,


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