Novice Modeller

[Robert MillsParticipant @robertmills13867]

So I kept the original design I had at the bow, I quite liked it, but have redesigned the stern significantly. For now I've removed the topside at this point to show the proposed motor position.

I think the design would almost certainly benefit from spray rails, despite the flatter stern. I also can't help but think that a twin prop / rudder combo would look better as well as giving better acceleration

As before comments welcome!

[Dave MilbournParticipant @davemilbourn48782]

I would say from my experience with deep-Vee hulls that spray rails are mandatory unless you like watching large bow waves wash over the foredeck. Twin motors just double the chances for problems, and a small model doesn't really need twin props. You can increase acceleration by fitting a different pitch prop or increase the voltage to the motor, and you can't see the prop(s) when the boat is in the water anyway!

I will, of course, defer in technical matters to the more learned and qualified contributors to this forum. I just draw 'em to look pretty and they mostly work OK, innit?

Dave M

[Robert MillsParticipant @robertmills13867]

Morning Dave

Addition of spray rails it is then. I can see where you are coming from with the complexity introduced with twin motors, a bigger prop would provide more thrust for a given pitch and RPM at the cost of top speed due to more load on the motor.

I too am a subscriber to the theory of if it looks right it often in, unless it is horribly wrong.

[Robert MillsParticipant @robertmills13867]

So I've taken this design through to the 'Frame' stage where I now have a frame design onto which thin sheets of balsa will be glued. To make it easier to give a fine entry to the bow I will laminate and shape several layers of 6mm balsa which will also be sheeted over.

Using a tool like sketchup makes this stage relatively easy once you've practised it:

The first photo shows the hull encased in a rectangular block divided into smaller blocks the thickness of the frame material with the larger ones the gaps between.

Then you can carefully remove the extra lines supporting the surfaces covering the hull and do the same thing for any longitudinal frames.

Once that is done you can tailor the inside of the frame to fit around the hardware in the hull. With this keel the cutout has to be very low beneath the motor due where I chose to mount it.

With the frame I have here there are three longitudinal frames. The idea being that the outer two provide most of the structural rigidity while the centre one supports the prop shaft and rudder.

[Dodgy Geezer 1Participant @dodgygeezer1]

How are you going to ensure that the hull is not built with a twist in it?

The design is quite similar to the 'eggbox' technique of Les Rowell's Aerokits – though it uses a lot more material.

The shape you have shown is also similar to the 'internal box' technique that Phil Smith of Veron used for his RTTL launch – he created a rectangular box of the same size as your internal space, and then added individual side and base formers to create the hull shape…

[Robert MillsParticipant @robertmills13867]

My intention was to build it the 'wrong way up' as the top is flat so I could ensure the top of the sections remained in the correct places during construction. Additionally, I haven't decided quite how the frames will intersect yet and this will help avoid twist too.

Edited By Robert Mills on 21/04/2016 22:14:14

[Dave MilbournParticipant @davemilbourn48782]

Robert

Sorry to say it but I think you've put too many frames in there. Not that I'm the oracle when it comes to small powerboat models, but my 600mm-ish models have no more than six frames plus the transom. Your 520mm hull has fifteen plus the bow section. These add weight and complications. Do you really need all of those to maintain the shape?

[Dodgy Geezer 1Participant @dodgygeezer1]

I'm also interested in the skinning – are you expecting to skin the hull with the triangular shapes shown on the drawing? if so, the joints are unsupported at many points. If they are to be butt-joined, I expect a lot of flexing will occur, which will make the jointing a difficult business and leave weak points along the hull. Your multitude of frames will make it hard to reinforce inside with fibreglass or similar material.

I get the impression that you have not examined a lot of the traditional model boat-building techniques, which have been developed over the years to deliver successful hulls…?

[Robert MillsParticipant @robertmills13867]

Hi Dave,

Seeing your photo I agree with, I think it would be a better idea to reduce the frame count significantly and add support along the chines.

Hi DG,

The small triangles in the bows are an artefact of the CAD program I've been using, these would would be a much longer strips of triangles with the longer edges together butted together

[Dodgy Geezer 1Participant @dodgygeezer1]

Elsewhere on this forum you can see an example of the 'internal box' technique that I mentioned earlier:

 

One problem with the ''criss-cross' frames that you have is that cutting the joining slots must be dead-on precise. Get one of them slightly out and things fail to fit properly – then the hull gets a skew. Are you going to cut these on a numerically-controlled cutter?

 

Edited By Dodgy Geezer on 22/04/2016 20:04:44

[Robert MillsParticipant @robertmills13867]

I'm thinking it would be worth going back and redesigning the frame from scratch.

Unfortunately I don't have a numerically controlled cutter, although a cnc router is on my list of things to build.

My other thought had been to 3d print the hull sections, a friend of mine has access to a 3D printer. Although I can't help but feel like this is cheating.

[mark69Participant @mark69]

Quote,,,, A bulkhead by design is a means of holding shape ,Where the direction or shape changes another may added to maintain structural integrity in full or part , looking at your pics I think you might have to many !!! The outer shell will add significant structure ,Dave's model is a typical example of where bulkheads need to be to !! Mark

Edited By mark69 on 22/04/2016 21:07:10

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Robert Mills on 22/04/2016 20:15:34:

I'm thinking it would be worth going back and redesigning the frame from scratch.

Unfortunately I don't have a numerically controlled cutter, although a cnc router is on my list of things to build.

My other thought had been to 3d print the hull sections, a friend of mine has access to a 3D printer. Although I can't help but feel like this is cheating.

It's a good idea to have some idea of the way you are going to build any item before you design it – the construction technique you are going to use will often influence the final design…

If you are wondering about a self-build cnc router, take a look at the eShapeoko – **LINK**

I suspect that 3-D printed sections would be quite heavy – wood is a marvellous composite medium if you use it correctly.

[Dave MilbournParticipant @davemilbourn48782]

There are a number of model boat suppliers who now offer CNC routing or laser cutting for plywood. Examples from memory are SLEC & Model Boat Bits; I'm sure there are others. All you need do is submit the shapes accurately drawn in a vector drawing program (CAD package) and saved as closed polylines with nil thickness. They can then be translated into the correct format for cutting.

Dave M

[Robert MillsParticipant @robertmills13867]

So it's been a while since I posted, work has been a bit busy recently. I've redesigned the hull slightly: removing the rudder and moving the prop further back. The intention is to use a transom hung rudder, hence the double thickness transom.

The frames are designed to fit into a deck which will provide a flat base for manufacture.

As for materials I was considering 3mm ply for the frames and 1.6 mm (1/16" ) balsa for the skin which I will then coat with polyester resin inside and out to waterproof it.

What it not in this design but I would likely include are some supports to run along the edges of the chines and some spray rails to go along the outside.

Edited By Robert Mills on 13/05/2016 13:15:25

[Dave MilbournParticipant @davemilbourn48782]

Robert

Avoid balsa – it's too soft, even using a hard grade with glassfibre cloth and resin. I use 2mm liteply, available from SLEC (there is a link in the RH column of this page).

You will need external spray rails along the chine line – I'd suggest 3mm x 3mm. The photo above shows the strips I used to support the chine internally in my Huntsman. They are two laminations of 3mm x 6mm bass (only one is shown in the photo).

Dave M

[Robert MillsParticipant @robertmills13867]

Hi David, thanks for the heads up on the LitePly their costs / postage don't look too bad!

[Dave MilbournParticipant @davemilbourn48782]

SLEC are a great bunch of folk to deal with. I've known most of them for over 40 years!

DM

[Robert MillsParticipant @robertmills13867]

Before I finalise my plans and start cutting I wanted to get some thoughts on power plants, I was thinking along the lines of a 2000 kV brushless such as this one **LINK** running on a 2 cell LiPo. I realise that I would be well within the motors limits but it gives me a lot of room to play with RPM / prop pitch / prop diameter.

[Dave MilbournParticipant @davemilbourn48782]

Inrunners are very fast and there's precious little low-speed control with them. You might be better advised to use a 1000-1200kv outrunner for 2S. A 30mm 3-blade prop is a good place to start with that size motor. I have several of these and can recommend them **LINK**

Dave M

[harry smith 1Participant @harrysmith1]

Hi Robert

I am using a B28-57-17L 1900kv (750watts) on a 2650mah 3S with a car ESC (HK-60A)( handles 4S as well) with a 2 blade 32mm metal prop.

My boat is a 640mm Sea Hornet and my other boat is a scratch built 620mm shallow vee with a Hobbyking outboard which has a 28mm 3000kv on 3S with the same prop size.

The problem with the outboard is the 3mm flex drives(no spares).

As for speed the Sea Hornet is fast and the outboard one is dam fast.

The outboard will handle 4S but I am not going to try.

I like the motor placement and the shallow drive angle, more room to use the battery yo get the balance point of the boat right.

As for the XK3665-2100kv(2030watt) motor , this would be too much power in that size boat even on 2S.

It require a larger boat by far!!!

Harry Smith

[Robert MillsParticipant @robertmills13867]

Thanks both for your input, the motor in the model shared here is very similar to the Turnigy one suggested by you Dave. By biggest concern with Outrunners is the ability to cool them but at 100W it shouldn't be a problem if I'm careful. Which ESC would you pair with that motor? There are a large number of 'cheap' ESCs available on hobbyking and other site but finding one capable of reverse is quite difficult.

The watercooled inrunner would have a peak power output of about 500W with 2S, so not too far off the motor in your Sea Hornet

Edit: there appears to be a large number of discussions on this subject so I'll have a dig through these for en ESC

Edited By Robert Mills on 18/05/2016 08:38:41

[Dave MilbournParticipant @davemilbourn48782]

I've just ordered two more of these **LINK** which brings my stock of them to five. I use them with the Turnigy motors in my Huntsman and Huntress models, both of which are slightly larger than your model. BTW it is described as a flight ESC but it does forward and reverse as one of its programmed options.

On 2S – or even 3S – you will not need to water-cool either the speed controller or the motor. You can also swap the 2817 motor for a 2822 or even a 2830 if you use a standard glass-nylon 400-size mount and arrange to allow for the different motor lengths. Incidentally 750 watts is the same power that the original Fairey Huntsman 31 had – and that was 47" long – while 2030watts is almost 3BHP… Torque roll with motors of that power in your model would be significant.

Dave M

[harry smith 1Participant @harrysmith1]

Hi Robert

Hobbyking have a few 28mm water-cooled motor mounts (WTCMT01 and BP22-MMWC).

Tubing I use Silicon fuel pipe 2.5mm (OR005-00601) a metre lengths.

As for ESC's the Hobbyking car ones(HK-30A,HK-45A,HK-60ASL and HK-100).

All have reverse, I have not used the 30A, to lower Amps.

The 45A handles 2 and 3S batteries, so good for all 28mm motors.

I weight the 45A at 69 grams, 60A at 91grams and 100A at 106 grams.

Using the same program card for all, very easy to setup and very good low speed control.

These are the only ESC I use!!!

I tested a XK2845B-2000kv(280watts) in my boat, it would not pull skin of a custard.

The high kv motors maybe OK with a surface hydro prop setup which a number of boats run.

[Robert MillsParticipant @robertmills13867]

Hi Harry, could you give your definition of pulling a skin off a rice pudding? I'm not necessarily looking for something to do 50KM/H as my nearest boating lake (Eastrop Park in Basingstoke) isn't exactly massive so comfortably planing would do me nicely.

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