RAF Fireboat ID

[Dave ReedParticipant @davereed72029]

Hi boaters,

Inspired to post after reading Harry’s Sea Rover post.

I’ve recently acquired an old Fireboat 1:16 scale that appears to have been built from a kit. Out of interest I wondered if anyone can identify which kit it is. Here’s a few photos.

This on back of cabin, can anyone identify it?

B5 stamped on bulkhead near stern (location shown on interior photo)

3A stamped on part near motor mount

Propshaft with aluminium tube and rudder that were fitted when I received it. Tube bearings badly worn. The stringy thing and pins were around the edge of the deck, presumably an addition by a previous owner.

Side view

Interior view

 

Entire boat is nailed and glued together. They don’t show up well in my photo so have marked some of the more obvious ones.

 

The rear deck was much too high as rudder servo mount was up above the keel – I’ve already changed it’s location in the photo so the rear deck can be lowered to a more realistic level.

I did install a motor and give it a brief test run with a light(ish) LiPo velcroed to the existing battery just behind the cabin but it was very unstable on the water. Maybe needs weight further back or additional ballast?

Any info gratefully received.

Dave.

[Richard SimpsonParticipant @richardsimpson88330]

What length is it, that might help?  I think the old Aerokits one was either 46″ or 34.5″ lomg.

[Dave ReedParticipant @davereed72029]

It’s 1:16, 34.5″ long, 10″ beam. I thought there was more than one manufacturer at this size.

It’s just casual interest anyway, I was trying to get a feel for how old it might be. It’s already been through at least one refurbishment, there’s evidence of a number of fittings that have been removed and holes patched.

 

[Richard SimpsonParticipant @richardsimpson88330]

Dave, mine is a 34.5″ Aerokits model, it does appear to be very similar in construction to yours.  Mine was restored from a very sorry state by another member at Kirklees who is sadly no longer with us.

 

[Dave ReedParticipant @davereed72029]

Looks good Richard – gives me something to aim for.

When I tried running mine it was with one battery pack mounted where your receiver pack is. It was the only place there was anything like a battery tray. I can see you have more weight and it’s further back which might help explain why mine was unstable. I’ll rearrange the layout when I get it back together.

[harry smith 1Participant @harrysmith1]

Hi Dave

It looks like the Aerokits kit because of the stamped numbers eg B5.

I have the large 46″ model and ended up with the two 3S lipo batteries( in series for 6S) under the stern deck.

I placed the rudder servo straight behind the rudder post.

Very stable with weight at the stern.

Harry

[Dave ReedParticipant @davereed72029]

Thanks Harry, looks like I need to move the battery/batteries back towards the stern. Maybe what I took to be a battery tray was actually for receiver and speed controller.

Once I get a new prop shaft / tube in, I’ll try it again.

[Dave ReedParticipant @davereed72029]

I’ve cured most of the water leaks and rebalanced it with a battery each side of the keel under the rear cabin. Total weight is now about 3.1Kg, COG at the front wall of the rear cabin.

The next issue to solve is torque roll, there’s plenty of power but once it’s up on the plane it rolls quite badly to starboard. Power is from an Overlander 3536/05 (1500kv) on 4S with a generic 2 blade 40mm prop. I suspect a smaller prop may help, maybe 35mm 3 blade, but hope someone can point me in the right direction.

Thanks

Dave

[Chris FellowsParticipant @chrisfellows72943]

Most models I see with that type of hull tend to be a bit unstable and flop into turns so I’m not surprised you are getting some torque roll. Whilst the prop can be a factor and 3 blade ones I understand are better in this respect the motor itself can have an influence as well but that is difficult to eradicate unless you go for a twin contra-rotating set up. A smaller 3 blade is definitely worth trying.

Chris

[harry smith 1Participant @harrysmith1]

Hi Dave

TRy moving weight to the port side of the boat.

Harry

[Richard SimpsonParticipant @richardsimpson88330]

If it makes you feel any better Dave mine flops over and ‘digs in’ when I turn hard to starboard.  I just trimmed a bit off the rudder movement so now if I want to turn sharply I have to slow down.

As Chris says the real solution is two contra rotating props but I’m certainly not going to rip mine apart for the sake of doing that.  It actually handles beautifully at full speed as long as the turns are gentle so I’m quite happy with that.

[Colin BishopModerator @colinbishop34627]

Dave says in his post that the boat leans over when on the plane. Does it do this when running straight or only when turning? In the latter case then restricting rudder throw is usually the answer. If running straight then it is torque effect and, as Chris says, a smaller diameter 3 blade prop is probably the best option.

Colin

[Dave ReedParticipant @davereed72029]

It does it when running straight. My understanding is that a smaller prop with higher RPM should help reduce this. My original choice of 40mm 2 blade was based on SLEC’s recommendation for their similar kit. I’ll give it a go over the weekend with a smaller prop and see what happens.

[Charles OatesParticipant @charlesoates31738]

The  relative narrowness of the hull doesn’t lend itself to really high speeds without some problems, one cure worth a try is a small wooden wedge on the bottom of the hull near the transom. It gives a bit of lift to the low side at speed. I’d experiment with different sizes just tacked on with a dab of hot glue, then if you like the result do it properly. Start small, perhaps 1.5 cm wide  and a couple of mm high at the end.

[Richard SimpsonParticipant @richardsimpson88330]

Dave, that is definitely torque reaction, as suggested.  Basically two approaches, either reduce the torque, which can have performance effects as well, or stabilise the hull.  The challenges there are that you tend to be able to improve the stability for one specific speed, usually the fastest, but the model remains unstable at other speeds.

Always a bit hit and miss but the solution might be a combination of the suggestions made above.  Good luck with the experimenting!  I’ll measure up the prop I have on mine later and let you know the size.  As I said I’m happy with the stability at speed and just have to remember to slow a little for starboard turns.

[Dave ReedParticipant @davereed72029]

I thought about the wedge idea as well, but have never heard of anyone doing it before so sort of assumed it wouldn’t work. My head keeps telling me “if it worked, everyone would know”. Might give it a go tomorrow, what could possibly go wrong…….

[Colin BishopModerator @colinbishop34627]

Adding a wedge is an accepted solution but, as Charles says, you may have to experiment. Simply adding weight on one side will make the boat slump over slightly at rest and low speed. This may or may not matter to you. My SLEC Fairey Huntsman kit model adopts a nose down attitude at rest but is fine once it gets going. Dave Milbourn’s prototype did not exhibit that characteristic and we had a discussion about that. It is possible that I had carved a bit too much away from the bow block to get the Fairey shape which reduced forward buoyancy.

Dave also suggested a 3S LiPo pack to power the model and restrict rudder movement but I still found that the boat turned on its side in a turn so I substituted a 2S pack which made things more civilised. The wider Swordsman was more stable and coped OK with the 3S pack.

In an ideal world, twin screws are the way to go but for practical reasons many boats rely on one and you have to accommodate the consequences of that. Solve one problem and create another! All good fun!

Colin

[John W EParticipant @johnwe]

hi there,

I think what has been suggested regarding a smaller prop will solve your problem to a degree.  Basically, you have too much power for your model – we all tend to do this with our models i.e. over power them.

One of the fixes you may try is to put a set of gears in between your motor and prop shaft – so the propeller shaft is driven in the opposite direction to your motor.   Glynn Guest does this in his P.T. boat plans – where Glynn uses a 2-1 gear ratio which allows you to use a larger prop and a slower rpm motor.     Adding to trim tabs or wedges to the bottom of the boat will force the bow of the boat down to a small degree which, will very slightly, improve it.   If you have a look at Surfury, which was a single drive prop, she had hydraulically operated trim tabs on the stern to be adjusted at varying  speeds.   Permanent wedges which were fitted to the RTTL launches in the 1950s were a permanent fixture under the stern of the boat.   I will put a picture on of one of the RTTLs which was being cut up and you will see the wedge just above the rudder (this was a scrapped RTTL).   The other pic is of the gear box which I made for the Glynn Guest model which counteracts the tork problem.

If you want to know the ins and outs as to why this is happening – like everything else in marine technology it is a very sophisticated subject.   There are several good books which cover this subject.   One of those books which I have is Commander Peter Ducane’s book – High speed small craft.   In there, there is a good explanation.

John

[Dave ReedParticipant @davereed72029]

So I made a wedge from 1/8″ balsa about 1 1/2″ square, sanded to a wedge shape. sealed with Eze-Kote ready to superglue on after trying different props.

At the pond this morning I tried various props, 2 /3 blade, 40mm to 30mm. It turns out that for this boat (not necessarily other fire boats) 35mm 3 blade is the sweet spot. It has no significant roll running straight and speed is only slightly reduced. As with Richard’s boat, hard turns to starboard need to be done at reduced speed.

I’m quite happy with the way it runs now, wedge not required in the end. Just need to tweak motor alignment, reduce slop in rudder linkage, sand and repaint hull, add spray bars, repair/remake warped roof panels, fix leaks in low rear deck, etc, etc,………

[Colin BishopModerator @colinbishop34627]

All good then!

Colin

[Richard SimpsonParticipant @richardsimpson88330]

Glad you got there Dave.  When you get these just right they do put a smile on your face when you put them through their paces.

[Dave ReedParticipant @davereed72029]

Hi John, Interesting to hear about the wedges on the RTTL launches, I believe the shafts turned in the same direction?

I have the Vic Smeed plans for the boat but intend to cheat and build it as contra-rotating two shaft (if I ever get round to it). I’m not too far from Hendon so plan to go and have a good look at the one at the RAF museum before I start.

Dave

[John W EParticipant @johnwe]

Hi there Dave

Yes, you are quite correct about the RTTL shafts driving the same direction.  I built my model several years ago.  I used Vic Smeed’s plans as the basics with several other pieces of information I received from Christian Sheppard – he was such a great help in building my model.

Couple of pics to look at.

Going back to where I mentioned the Surfury, what I meant to mention as well (and it completely slipped my mind, as things generally do these days).  Surfury power boat’s engine configuration was 2 engines in line facing each other; driving through a specially adapted gearbox to a single propeller shaft.   The reason for this is that one engine rotates clockwise and the other engine will be rotating anti-clockwise so, in theory, cancelling out a lot of the torque affect.  So that will clear that up.

john

[Author]

Posts