Snogg the sequel

[Gareth JonesParticipant @garethjones79649]

Some of you may remember an article I wrote for the magazine about 10 years ago, covering a major rebuild of a 1:24 scale model of an ELCO 80 ft PT boat. The article is still on the website and there is a link to it here PT 602

I have now started a second major rebuild and this is the first of an occasional look at what I am doing, As described in the article the original rebuild had a long string of propulsion problems, mainly due to my inexperience at the time, Eventually Snogg did achieve a good performance on a pair of Speed 600 motors as shown below.

Subsequently the model was fitted with a pair of Speed 700 motors (Graupner part no 7307) which ran much cooler.

Here is a picture of the Snogg internals as they were with the later motors and NiMh batteries.

Subsequently there was another weight saving exercise and a 6000 MAh 3 cell LiPo battery was used instead.

However time has moved on and the model was always ripe for a brushless conversion. There are also a large number of other potential improvements I would like to make adding more detailing and hopefully working roll off racks and torpedoes. However the first step has been to strip out all the internals and remove all the deck fittings so Snogg has now been gutted.

Everything came off OK without any damage but removing the propshafts was a bit of a challenge. I made up a wooden block drilled out 8 mm to be clamped around each propshaft with a pair of molegrips.

The prop shaft was then heated with a small blowtorch to soften the araldite holding it in the hull, being careful not to set the hull alight. A bit of twisting with the molegrips and all three propshafts were removed successfully.

A bit of cleaning up is required but so far so good.

Gareth

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[Gareth JonesParticipant @garethjones79649]

[Gareth JonesParticipant @garethjones79649]

Why bother removing the propshafts you may ask? Well there were several reasons. The drive shafts themselves had been lengthened to 420 mm to provide the required clearance to fit the 700 sized motors. This put the motors and overall boat centre of gravity further forward than ideal. The replacement motors are smaller and lighter and I wanted to install them further back.

The boat is fitted with 30 mm diameter 3 bladed propellers which are a good scale approximation to the real things which were 28 inches diameter. I have made many detailed performance measurements on the model and I know that I need around 11000 rpm with the 30 mm props to give good boat performance. However the installation was susceptible to vibration and probably some whirling of the propshafts so shorter shafts were desirable.

Finally the real boat has exposed drive shafts outside the hull, supported on P brackets, and I wanted to make the model more representative of the real thing.

Based on my experience with other brushless motors I had decided that a Turnigy D3548 sized motor should be powerful enough and the 1100 KV variant should give a fast enough prop speed. I might go up a small amount in prop size to 35 mm but even so this motor should perform OK. As a comparison the two motors are shown below. Note the brushless one is a lower KV but the same physical size as the one I plan to use.

As a comparison, the brushed motor weighs 390 grammes, the brushless motor 160 grammes. Both have 5 mm diameter driveshafts.

Gareth

[Ray Wood 3Participant @raywood3]

Hi Gareth,

Please excuse my ignorance what does the thread name mean??, I thought it was what we used to do at the back of the pictures 😀I

Regards Ray

[Gareth JonesParticipant @garethjones79649]

To bring the thread up to date the next step was to carry out a performance test with the proposed brushless motor. For many years we have had two Springers that have served us well as test beds and also have a go boats. One of these was sold at the last Blackpool model show but the other has been retained as a test bed and boat of choice for the Manx model boat club's hooligan competition at their Manannan event. It is normally powered by a D2836/11 motor (750 KV) which drives a 40 mm prop and gives quite a spritely performance but not PT boat speeds.

The chosen motor D3548/4 is a bigger diameter, but fits on the same mount and is an easy swap, just moved forward by about 12 mm. The installation was fitted up with my wattmeter and it was off to the bath to test it.

Driving the existing Springer 3 bladed 40 mm prop and using a 3 cell 6000 MaH LiPo battery, the motor gave me a prop speed of 9540 rpm and took 34 amps. This is above the rating of the speed controller which is a 30 amp Leopard unit. However it was OK for a quick test and survived unscathed. I have not got any photos taken during the test as it takes one hand to hold the boat steady, one to hold the tacho to read the prop speed and another pair of hands to operate the transmitter.

Driving the 30 mm PT boat propeller the brushless motor gave 11600 rpm with a current of 17.5 amps. By comparison with the original 700 sized brushed motor this is about 2% increase in prop speed but with only 73% of the current consumption and only 40% of the weight. Overall it looks a promising combination and I can now go ahead and design the mounts for the motor and the new propshaft and P bracket installation. I will probably replace the 30 mm props, which I think are standard Raboesch units, with some similar sized Prop Shop propellers which are a more accurate representation of the full sized ones. If necessary I could go up to 35 mm diameter without greatly compromising the scale appearance and still be within the 30 am speed controller rating.

Gareth

Edited By Gareth Jones on 17/12/2019 18:03:00

[Ray Wood 3Participant @raywood3]

Hi Gareth,

Thread name still a mystery??

Regards Ray

[Gareth JonesParticipant @garethjones79649]

Ray,

You are not paying attention, I explained this to Dave Milbourn a short time ago, admittedly in another thread.

The model is finished as a Royal Norwegian Navy vessel, originally build number PT602, which was built in 1945 for the USN or possibly the Marines, but did not see service in WW2. After the war it was sold/transferred to the Royal Norwegian Navy under a weapons aid program and was named Snogg, which is Norwegian for fast. Snogg was the first boat in that batch which were known to the RNoN as the Elco class.

Subsequently the RNoN have had several other batches of PT boats, several individuals named Snogg and from 1970 there was a batch known as the Snogg class.

The thread is called 'Snogg the sequel' because it follows on from the original article.

Gareth

[Dave MilbournParticipant @davemilbourn48782]

Gareth

I'm also still trying to get used to a model called Snogg. I thought those days had long since passed!

If it's of any help my Huntress 23 @ 1/8 scale is about 35" long and uses a single Turnigy outrunner of 3542/6 1000kv on a 3S LiPo. It spins a Propshop General Scale 3-blade prop of 1.8" diameter and goes beautifully. As the prop tube is 15" long I chose a 5mm dia shaft along with an SHG rubber coupling.

Seasonal things to you and Mrs J.

Dave M

[Ray Wood 3Participant @raywood3]

Hi Gareth,

I must have missed that dialogue with DM 😮,

Regards Ray

[Gareth JonesParticipant @garethjones79649]

I should perhaps have added that although the original PT boats had three engines, Snogg the model will just have two motors driving the outboard shafts. The centre shaft and propeller will just freewheel for scale purposes. In the models latest incarnation the centre shaft will be much shorter and not extend fully into the hull as before, This will give me the option of laying the battery along the centreline of the hull rather than across as before.

Dave, thanks for the information about your Huntress.  I think Snogg will be quite a bit heavier and draggier than the Huntress so it is likely to need more power.

Alas my snogging sessions have also suffered a marked decline over the years.

Gareth

Edited By Gareth Jones on 17/12/2019 19:28:48

[Tim RoweParticipant @timrowe83142]

Hi Gareth

Looks like authentic wartime damage in the engine room!

Just an observation but a trailing prop will produce a lot more drag than a fixed one. Simpler too but seeing as the props will not be visible while underway, my choice would be to have a dummy prop pushed onto the shaft for display purposes and take it off while running.

Tim R

[Gareth JonesParticipant @garethjones79649]

Hi Tim,

That's an interesting point about the drag from the un-driven centre propeller. Intuitively I would have thought that a fixed pitch propeller that was allowed to spin and 'freewheel' would create less drag than one that was held static. However I could be wrong. On another forum that I often frequent (PPRuNe) there was recently a similar debate about whether a seized turbofan engine would create more or less drag than a windmilling one. Opinions were mixed but there was some old flight test evidence that suggested a seized turbojet engine did create more drag than a windmilling one at high speeds. Since I spent over 40 years as an aircraft flight systems engineer, much of it working on the aircraft in question in the PPRuNe debate, I suppose I should know the answer. Unfortunately I don't. I think I know a man who worked at Rolls Royce Bristol who would know the answer but I suspect that he, like me, has been retired for some time – John Bewick, where are you now?

On Snogg in its previous incarnation the centre propshaft and propeller were allowed to freewheel and the overall performance was OK. Whether, and how fast the propeller rotated I have no idea. On the rebuilt model it would be relatively easy to try all three options, fixed, free and no centre propeller and see if there is a discernible difference. It would be easy to leave the centre propeller loosely threaded on the shaft for display purposes and remove it when sailing and that should clearly give the lowest drag. However it would be only a matter of time before I forgot to remove it or tighten it before sailing and lost it in the oggin. I will report back with some test results when the model is tested some time next year.

Gareth

[Chris EParticipant @chrise]

Posted by Gareth Jones on 17/12/2019 18:02:39:

…………………………………The chosen motor D3548/4 is a bigger diameter, but fits on the same mount and is an easy swap, just moved forward by about 12 mm. The installation was fitted up with my wattmeter and it was off to the bath to test it.

Driving the existing Springer 3 bladed 40 mm prop and using a 3 cell 6000 MaH LiPo battery, the motor gave me a prop speed of 9540 rpm and took 34 amps……………………………….

As a useful comparison do you have the numbers for the D2836/11 (750kv) originally fitted in the Springer using the same prop etc?

[Gareth JonesParticipant @garethjones79649]

Chris

Its difficult to provide the whole picture on the forum as there is no easy way I know of to display an Excel spreadsheet. If you would like a copy of the complete sheet with all the data I have collected, send me a PM with your email address and I will forward it to you. However a summary of the Springer data is as follows.

Graupner Speed 500E brushed motor, 7.2V NiMh 40 mm 3 blade prop driven at 3250 rpm (45% of unloaded speed) max load current 4.1 amps, This was the original configuration I used for the model, worked OK but a bit slow and the motor got hot with prolonged running at high power.

Turnigy D2836-8 1100KV brushless motor, 7.2 V NiMh 40 mm 3 blade prop driven at 5900 rpm (70% of unloaded speed) max current 12.2 amps. Not sailed in this configuration

Turnigy D2836-11 750 KV brushless motor 7,2 V NiMh 40 mm 3 blade prop driven at 4685 rpm (82% of unloaded speed) max current 5.5 amps.

Turnigy D3536-9 910 KV brushless motor 7.2 V NiMh 40 mm 3 blade prop driven at 5710 rpm (80% of unloaded speed) max current 9.9 amps. Standard working configuration now, also used with 11.1 V LiPo occasionally

Turnigy D3536-9 910 KV brushless motor 11.1 V LiPo 30 mm 3 blade prop driven at 10740 rpm (89% of unloaded speed) max current 9.4 amps Not sailed in this configuration

Turnigy D3548-4 1100 KV brushless motor 11.1 V LiPo, 40 mm 3 blade prop driven at 9540 rpm (76% of unloaded speed) max current 35.1 amps Not sailed in this configuration

Turnigy D3548-4 1100 KV brushless motor 11.1 V LiPo, 30 mm 3 bladed prop driven at 11630 rpm (89% pf unloaded speed) max current 17.5 amps Not sailed in this configuration

As a guide I work on the basis that provided the max loaded speed is at least 75% of the unloaded speed there is a good match of motor and propeller. Between 45 and 75% the combination is usable with care, i.e. provided the model does not need to spend long periods at high powers, eg a model narrow boat. Below 45% the motor is running too inefficiently and will quickly overheat if high powers are used.

I have sailed the Springer with a D3548-6 790 KV brushless motor on an 11.1 V LiPo but not measured its performance. However I did produce a rather blurred video of its performance and there is a link here supercharged springer

Gareth

[Gareth JonesParticipant @garethjones79649]

Chris,

Sorry but I made a mistake in my post on the 17th, the normal motor in the Springer is now D3536-9 which is the same diameter as the proposed PT boat motor but about 12 mm shorter.

I did use a D2836-11 for a while and it was OK and safer as a have a go boat but a bit underpowered for fun sailing.

Sorry for the confusion, I had too many motors sitting on the bench when I was swapping things around.

Gareth

[Gareth JonesParticipant @garethjones79649]

Since Colin has updated his show and tell thread on Brenda's upgrade, I thought it was time I updated the story of Snogg the sequel. Snogg has been sleeping in the shed for the last three years while I was diverted into the more genteel world of restoring and building vintage model yachts. However a visit to the Blackpool Model Show a couple of weeks ago has reawakened my interest in scale models so Snogg has been transferred to the workshop with a view to getting it at least to the stage where it can be run again.

The brushless motors have been fitted and the propshafts aligned as best I can. I used a piece of 4 mm stainless steel rod through the centre of each motor mount position and a ply plate fixed to the rudder post holes. The mounting plate was set as square as possible to the shafts by sanding the face which mounts to the existing model bulkhead. The motors were then fitted to the bulkhead and the propshafts aligned to the centre of the motor drive shaft before the propshaft tubes were glued into the bottom of the hull. The original full length propshaft tubes have been replaced by shorter tubes and the shafts will be supported by a P bracket just in front of each propeller.

The bottom of the hull has been rubbed down and will require some patching over the original rudder mounts as these will be replaced with lengths of brass tube araldited into the hull. Originally the rudders were mounted too far aft. They projected beyond the transom which was not the correct scale position and also gave very limited space for the servo arms inside the hull. The rudders will be reshaped to be more accurately to the correct scale shape by cutting and filing the original commercial fittings down to size.

[Colin BishopModerator @colinbishop34627]

Sometimes it's nice to come back to an old project Gareth, especially when in the intervening period you have had the opportunity to have a bit of a rethink about aspects of the model.

Keep us updated.

Colin

[Gareth JonesParticipant @garethjones79649]

Some progress towards getting Snogg in the water again.

The modified prop shafts with exposed drive shafts and support brackets have been made and test fitted. The final propeller standard is not yet decided but in principle, I plan to fix the propellers with a lock nut on the end of the shaft, as on the centre unit shown. While the hut will have to be bigger than scale size, it will look more scale like. Only the two outer shafts will be driven, the centre one will be a.dummy. Whether the propeller will be fixed or free wheel is yet to be decided.

The rudders have been reshaped and the brass bearing tubes cut to length,

[Gareth JonesParticipant @garethjones79649]

Snogg went for its first test sailing with brushless motors last weekend. It started off very promisingly with the model planing beautifully, but came to a rather premature end when it slowed down dramatically and was obviously suffering a major problem. After cautiously bringing the boat to the side of the pond and recovering it to a work bench examination showed that one of the prop shafts had seized in the upper bearing. Fortunately the only damage was a blown 25 amp blade fuse and not a £35 speed controller..

After removing the prop shaft, rechecking the alignment and re-lubricating it Snogg went for a second test sail this weekend which went much better and all seems to be well. There is a short video of the event here:- Snogg test sailing

As you can see this is going to be a very wet model

[Colin BishopModerator @colinbishop34627]

Looks very impressive Gareth. From what I can make out, once the water level gets above the chine strake which acts as a sprayrail, the flare of the bow is insufficient to throw it away and it climbs up over the deck, particularly in a turn. Dave Milbourn was always very insistent that his Fairey models have a sharp edged chine strake to help prevent this.

With regard to the drag from a freewheeling prop, it is possible that there is some confusion when relating full size practice to your situation. In full size ships and boats the shaft is usually connected to some form of transmission component at the inboard end which won't take kindly to being turned over in reverse causing wear to parts and bearings. Even if the prop can be declutched it will still be exerting pressure on whatever bearings are stopping it falling off which they are not designed to take. Locking the shaft is therefore the preferred option despite the additional drag.

The recent damage to the carrier HMS Prince of Wales was caused by the last section of the propshaft becoming detached whereupon the prop pulled partially free from the hull hitting the rudder and damaging both. The prop had to be removed underwater at Portsmouth before the ship was able to proceed to Rosyth for repairs.

With your model, the middle prop is a dummy and will simply pull against the inboard end of the tube where presumably you have fitted a collet. Logically therefore it should create less drag free running than if fixed.

Colin

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