Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Abandon Ship]

Hi All,
My very first boat build is near completion, a small (16”long, 5” beam) cabin cruiser. My intention was to learn from my mistakes and check my build skills. I made mistakes and ended up with a boat that floats, goes, stops and turns but is much heavier than I expected.
Bulkheads and keel are 3mm ply, hull skin is 2mm balsa, deck and superstructure 2mm ply.
I realise now that 3mm ply bulkheads could be thinner (inexperienced modeller following the plan).
So to my question: How do I build as light as possible?

[Tug Fanatic]

Do you have any pictures?

Is the model an own design or a plan - if  plan then what?

Details of drive train & battery?

What does hull weigh & weight of superstructure.

Building light is a skill. Weigh eveything and ask if you can build it lighter as you go including adhesives etc.

[ChrisF]

As said, more information needed. But from what said I don't think you've built that heavy. 3mm bulkheads are Ok, did you, were you, able to cut the centres out of the bulkheads to save weight? Unless there were lots of them!

As TF is alluding to I think it is probably all the non-timber components that are the problem. Particularly if a big heavy battery.

Chris

[Abandon Ship]

Hi TF,
Thanks four your response.
I built from plans that I was given many years ago, but wasn’t interested in boats at the time. Found them in a recent clear out and thought “Why not?”. There was no name on the plan and no instructions but there was a materials list I used to get ply thickness.
 
My scales aren’t that precise but approximate weights are:
 
All up   590g
Hull with rudder and shaft (glued in) 230g
Motor, servo, receiver, esc, battery     310g
Superstructure 50g
 
Motor: MFA Torpedo 400
ESC: Mtroniks Marine 15A
Battery: 3 x 18650 Li-ion cells in plastic holder
Receiver: (for testing) Acoms 2 ch 27Mhz
 
This model is not really my interest, it was just to make sure I could build something that floats! I really asked the question for any future builds to make sure I wasn’t building heavier than necessary. In the back of my mind there might be a winter project of a fast Vosper MTB or Rescue Launch.

[Abandon Ship]

Thanks Chris,

I did cut out the bulkheads, had to, to fit the electrics in. Does 9 + transom sound a lot for such a small build? I thought I could have got away with thinner ply with that many but I’ve never built a boat before.

You can see weights on my reply to TF – I tried to add photos but failed miserably. Will have another go

[Abandon Ship]

Unfinished and unpainted but floats and moves

[ScottW]

Unfinished and unpainted but floats and moves

While "floats and moves" is good for certain; "Floats, moves, and returns" is even better.

[RST]

Hi,
Before I saw your pics your description of weights sounded about right.  I build allot of models about this size so know what it's like and sadly still I often end-up on the wrong side still!  Your project catches my interest though so I offer some feedback whih might not be right for everyone but based on experience:

• "Bulkheads" are technically solid items.  Sections or stations as you have done are cut-out inside. from your pic from the top there's no harm in lots of them but generally I would space them out much further where there is less change in shape aft, compared to up forwards where you want more control of the shape around the bow.  Nonetheless it looks a good job from your pic anyway
• To save a bit of weight if needed, your 15A viper is probably an over kill.  Their 10A ESC is pretty small and lightwight for motors needed in this size of model, either that or Component Shop have some very good ESC's
• For the drivetrain, might I be right recognising that's a cut-down 2mm shaft with something like a very shallow pitch 30mm propeller from Radio-Active?  If I'm not wrong, the MFA torpedo 400 is a 5 pole motor (a385) so turns allot slower than a 380.  That coupled with the shallow pitched prop might make it a little lack-lustre.  Also, if you greased the shaft before, have you checked it turns freely -often too much grease is a problem.  Best way to find a happy medium for the motor is to lightly grease, connect a single AA battery to the motor and feel the "sweet spot" where the motor runs most freely before fixing it in place. In a model this size with that prop I would have thought a 280 motor might give reasonable results also (would save a bit of weight compared to a 380)
• Why the bend in the rudder linkage rather than straignt.  Maybe makes no difference but why?
• I would not worry about using 27mHZ radio -if it works it works and will still do so.  Just make sure your motor is suppressed to prevent noise not just for you but anyone else in the area, nothing has changed for this requirement even in digital radio days except for folk running brushless motors
• If you want to shave some weights off the next model you can try:  Using Obeche sheets (I get them from Hobbycraft) or by using thin Balsa with a skin of very light glass woven cloth with either z-poxy (on balsa) or Deluxe Eze Kote on something harder like obeche or ply.  I'm not a complete convert to Eze Kote yet as I think it's just a water based DIY varnish in a fancy bottle but it's very popular

...I can't help you on batteries.  I'm still using Ni-Mh in all my models.  I can't convert to Li-Po but I have looked at Li-Ion like you but I can't seem to get the parts together for an order to try yet as there's always something out of stock these days.
Not sure if these comments help any,
Rich

[Abandon Ship]

Hi Rich,
Your comments are greatly appreciated and you’ve raised several points, some of which I knew were wrong, some I didn’t. I’m new to the hobby and this was intended to be the steep bit of my learning curve before things started to get more expensive.
 
·        Bulkheads – I now know the difference between bulkheads and sections (more learning, the whole point of this build!) thanks. I followed the plan but your advice on bigger gaps where there’s less shape change makes sense.
·        I intend to reuse the 15A ESC in a future build, so didn’t want to splash out on a smaller one just for this proof of concept build (bit of a skinflint),
·        When I purchased the motor and prop I really didn’t know what I was doing. In fact this was one of the few areas I tried to research before the build but there seems to be so many different opinions that in the end I just guessed, I didn’t even consider propeller pitch, just got a cheap 30mm plastic propeller. You’re correct that the MFA Torpedo is 5 pole, I didn’t know the difference between 3 and 5 pole, I do now (more learning – this is what I wanted)! All that said, when I tried it in the bath it made a valiant effort to escape out of the other end!
·        I didn’t pay enough attention to rudder and servo locations. A straight linkage fouled the rudder horn(?) before getting to full right rudder. The bend solved this.
·        I’ll stick with 27 mhz then.
·        I’ve not used Obeche before, I’ll have to look into that. I have been considering glass cloth with epoxy resin over balsa in the future – is it really lighter than ply once you have enough epoxy on it to give strength? From what I’ve read about Eze Kote, it needs a lot more applications but I do prefer the idea of a water based product.
In the past I’ve had occasion to dispose of a few laptop batteries, at the time they mostly used 18650 Li-ion cells. Before recycling, I got myself a suitable charger and charged / tested the individual cells, recycling the duds keeping the rest for redeployment. The charger came with some new cells (the ones just showing in the internal photo).
However none of the cells, new or redeployed can deliver more than about 3A without rapidly dropping voltage and having the ESC cut out. I’m not saying this is the case for all 18650 Li-ion’s, mine were all heavily used or cheap Chinese ones.
 
Thanks again Rich, I really do appreciate the time you’ve put in to help me.

[Fastelectrics]

The 18650 cells can be very variable in performance and the battery holder can add unwanted resistance into the circuit. You weight does not appear to be too bad, so I would suggest changing the batteries to a 3 cell RC battery pack for better perfomance.