Old Motor Torpedo Boat

[ashley needhamParticipant @ashleyneedham69188]

Gareth, I like those sort of numbers. It is nice to have a bit of definitive data to work with rather than all the guesses.

​However, as soon as you move away from the established, then guestimates must become the order of the day.

​Ashley. nb: Grossly overpowered?? I must look that up in the dictionary

[Dave MilbournParticipant @davemilbourn48782]

Ashley

Here is the first lesson **LINK**

and here is the second **LINK**

Some might quibble **LINK** whether or not "overpowered" should be hyphenated **LINK** in this context (i.e. having excess power as opposed to actually exercising it) but I'm sure you know what I mean….

O yes – I nearly forgot! Here's another word for you; note the spelling https://en.oxforddictionaries.com/definition/guesstimate  

Dave M

Edited By Dave Milbourn on 16/11/2016 15:03:59

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by ashley needham on 16/11/2016 14:31:32:

Gareth, I like those sort of numbers. It is nice to have a bit of definitive data to work with rather than all the guesses.

​However, as soon as you move away from the established, then guestimates must become the order of the day.

​Ashley. nb: Grossly overpowered?? I must look that up in the dictionary

 

Actually, I think we are arriving at a reasonable set of outrunner brushless 'rules of thumb'. In round numbers:

 

1 – A 'speedboat' type hull should have a prop speed of around 8-10k revs.

2 – Working with batteries of 7.2 – 12v, that means KVs of 800 – 1400. Say 1000.

3 – For a big (40" ) boat you want 50Amps – for a small light one maybe 5 – 10Amps?

4 – For a prop size, start with a 2-blader the same diameter as the motor. Go to more blades for more speed…

 

I must declare an interest – I also have an old Aerokits 40" Elco PTB, and one day I must get a brushless for it…

Edited By Dodgy Geezer on 16/11/2016 15:15:38

Edited By Dodgy Geezer on 16/11/2016 15:18:45

[ashley needhamParticipant @ashleyneedham69188]

DG. Like that, sounds ok to me.

DM. I like it when you spend that much time trying to further my education. I am still a bit hazy though, underpowered I can handle, but…

​Next time some larger boats appear at the pond I will make note of exactly the equipment fitted. All the big boats go very well, so these equipments would be a good starter for anything large.

​Peter if you read this…your two boats??

Ashley

[John O’CParticipant @johnoc]

Just like to say that it has been worth the £50 paid for the boat to be a part of this thread! Learning things along the way is just an added bonus.

Cheers Guys

J

[Dave MilbournParticipant @davemilbourn48782]

John
There's an invoice in the post…..

Ashley
Why do heads and brick walls suddenly spring to mind?

DG
Looks reasonable to me.

DM

[ashley needhamParticipant @ashleyneedham69188]

DM. Only kidding.

​John. Yes you may even learn something from this thread!

Ashley

[Gareth JonesParticipant @garethjones79649]

I have given myself a slap on the wrist as I have just realised I got my final sum wrong in my last post. It should have read

Taking the battery voltage as 11.9 suggests the motor KV will be 14250 divided by 11.9 which is 1200KV.

It does not affect the overall conclusion though.

Sorry

Gareth

[John O’CParticipant @johnoc]

Gareth

I didn't thank you for your input but I did buy the suggested motor. Thanks.
Don't know what I will do with the other one that arrived this morning. Maybe a tug as suggested.

Found this today and wondered how it translated in to brushless?
http://www.building-model-boats.com/model-boat-motor.html

Anybody help?

J

[Paul TParticipant @pault84577]

Hello John

A conversion website such as link might be useful.

There are probably better conversion sites available but its a starting point.

Paul

[John O’CParticipant @johnoc]

Cheers Paul

How do you put "link" in instead of the web address?

Also I found a place on the Weaver where I can access the water to launch relatively easily so may try Smit and Scrappy out shortly weather permitting.

Almost finished cleaning the hull and getting ready to start the super structure and have ordered the prop and a couple of 35mm 2 blades, one "X" type to test,

I was looking for a good coupling but am not sure what is best and what suffices so bought the common or garden red one with the 4mm and 5mm inserts.

J

[Paul TParticipant @pault84577]

Hi John

Links

The link facility is easy to overlook, when you click on to reply its the little blue and green globe with the chain links right above the message writing section.

Just write link and then highlight it………..click the globe button and enter the web address in the box provided.

Couplings

I seldom use the red couplings on brushed electric motors as I prefer to use short lengths of tight silicone tube (warmed up in hot water and forced over the appropriate ends) probably not to everyone's tastes but it works for me.

Paul

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by John O'C on 17/11/2016 14:28:15:

Found this today and wondered how it translated in to brushless?
http://www.building-model-boats.com/model-boat-motor.html

Anybody help?

J

 

Aha – an opportunity for a wander around the magic world of ratios!

This site uses the well-known Square-Cube law **LINK** to determine power requirements. As you can see from the link, this is an old law – understood by Galileo, and no doubt by the Pyramid builders. Haldane's essay is a classic of its kind – **LINK** and it is also of interest to read this: **LINK** which introduces you to the problem of physical discontinuities as we scale down…

Engineering can be done reliably with reliable data – the problem, as I have mentioned before, is that we don't have reliable data for the scaling effects at our model sizes, which start to predominate as we approach LWSLs of a foot or so, and are quite important up to about 3 or 4 feet. This is why test-tank models are often 10 ft long or more. We often don't have reliable data for a host of other variables either, and so I wouldn't like to suggest that precise mathematical formulae be used to converge on an accurate figure for power requirements.

We can see this in the calculations made on the site. Though they appear to work, you will notice that the first example adds a 50% 'fudge factor', justified by the assumption that the model drive-train must be less efficient than the full-size. I am not at all sure about that – model drive-trains are usually straight-through – but it does allow a wide enough variation of answers to include the actual motor that was used.

The second illustration magically forgets to add the fudge-factor – but picks a full-scale motor range where the high end is 100% more than the low end. Again, the calculation has such a wide spread that it's not difficult to arrive at the chosen figure.

This is why I would stay with the rule-of-thumb – giving a starting point which we can experiment with. In fact, I find, using different propellers makes by far the largest difference to a model and so I would recommend varying them first.

I was hoping to arrive at an easy brushless rule that went, for speedboats:

Prop revs: 10k

Volts: 10v

KV: 1000

Amps: 10 – 100 depending on boat size

and allow 10% variation of any figure – but I'm not sure that it's perfect yet…

 

We can end by mentioning Froude – the doyen of model testing. This little excerpt **LINK** provides an introduction to his work. Perhaps Model Boats should commission an article on him one day…!

 

 

Edited By Dodgy Geezer on 17/11/2016 18:20:34

[John O’CParticipant @johnoc]

Found this and thought it useful but I would guess most of you have read it.

Couldn't make the link work but will try again sometime

**LINK**

J

[John O’CParticipant @johnoc]

Oh!!! It worked. Cheers Paul

J

[Paul TParticipant @pault84577]

Nice one John and thank you for keeping on trying to enlighten us.

All the best

Paul

[Ray Wood 3Participant @raywood3]

Thanks for that link John, I'm going to throw all my old Lipo's away and start the new year with new one's

Regards Ray

[John O’CParticipant @johnoc]

Glad to be of some use Ray.

J

[John O’CParticipant @johnoc]

OK Guys

Got the motor, the controller, battery and charger so where can I best get replacement guns?

J

[Dodgy Geezer 1Participant @dodgygeezer1]

I don't think the Vintage Model Boat company, which produces many of the old Aerokits, has got round to doing that model, so there are currently no matching guns being made specifically for that kit. In any case, the original kit guns were not at all like the original boat fittings, and could have been considerably improved. As could the superstructure, radar, etc….

Look at this site for a good view of the sort of weaponry the Elco boats had towards the end of the war – **LINK** There were roughly three major upgrades to the boats:

The earliest boats had the two twin 0.50 gun tubs, Mk18 steel torpedo tube launchers, no radar (just a bare mast) and a single 20mm Oerlikon on the stern. A few optional depth-charges were also carried

This got upgraded, typically with a quite distinctive 37mm Oldsmobile cannon on the bow, Mk 13 aircraft type torpedo racks, and an SO radar set on the mast. The depth charges tended to disappear

Finally, we have a 40mm Bofors on the stern, the SO-3 radar set on the mast, and several Oerlikons joining the 37mm on the bow. Twin rocket packs were also added.

The 40" boat is 1:24 scale. I would decide which variant you want to model (and there were lots of odd special additions as well, like the Thunderbolt stern gun set) and then go looking for the relevant 1:24th model weapons. If you find a decent cheap 1:24 40mm Bofors, tell me!

Alternatively, make up the weapons yourself from brass. You can easily do a better job than the original plastic mouldings! Here is a page where I'm making some (very simple) Elco PT Boat weapons in 1:48 scale for the small eezebilt: **LINK**

You'll find the plans on the eezebilt site, and they are easy to scale up or down. You might want to copy the torpedoes and mast as well…

 

Edited By Dodgy Geezer on 19/11/2016 13:13:26

[ashley needhamParticipant @ashleyneedham69188]

Like the brass oilies DG.

​However, exactly the same thing can be made from styrene rod/tube/sheet/extruded sections for those not wanting to solder, or indeed not having the facility to. I am not suggesting any method is better, and from the point of damage resistance the brass wins hands down,

​The guns on my LCM`s are made from plastic tube, bbq sticks, wood dowel, 0.8 and 1.5mm ply and card. They look ok at a few paces, as yet another alternative.

​Just be glad they didn't fit 8 barrelled pom-pom`s !!!

​Ashley

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by ashley needham on 19/11/2016 13:38:51:

​Just be glad they didn't fit 8 barrelled pom-pom`s !!!

​Ashley

They would have done so if they could! As it is, they managed 6-barrelled systems – here are some Thunderbolt images: **LINK**

 

2 .50 machine guns for tracer mounted with 4 20mm Oerlikons…

Edited By Dodgy Geezer on 19/11/2016 13:51:33

[John O’CParticipant @johnoc]

DG

How about this?

**LINK**

J

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by John O'C on 19/11/2016 14:01:55:

DG

How about this?

**LINK**

J

Ah, but not the right mount.

Weight was always an issue on the PT boats, and they often dropped the armour shields on their guns to save it. My Oerlikon plans are a bit non-scale in this respect. The Bofors they used was the simple gun mount with a lightweight frame around it – like this **LINK**

And I would be looking for something a bit cheaper, being the skinflint that I am. I suspect there's a plastic kit of the right size around somewhere…

[ashley needhamParticipant @ashleyneedham69188]

Not ever seen a Thunderbolt. Learn sumthing every day. Does not look quite so difficult as a pop-pom….. Ashley

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