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[mudway]

Looking at some Faireys for sale on FleaBay and I noticed something odd about the shafts and rudders of several of the boats on offer. Well odd to me anyway.

The shaft comes out of the hull at what seems to me to be a rather steep angle halfway along the hull. The rudder varies from being just behind the prop on these boats or right up near the transom a fair way from any prop wash.

Was this a 1980s thing, a Fairey thing or is/was their some logic in this placement of shafts mid way along the hull?

[DaveM]

The Precedent/Modav 46" Huntsman was more or less a straight copy of Vic Smeed's 1/12 scale plan, the logic being that if it's good enough for Vic then the rest of the herd should have little probem. I think it was designed as a "steering boat", which is a class which I don't think exists any more but which apparently benefitted from either a short, fixed skeg mounted halfway down the keel or the rudder/shaft layout show, From a scale point of view (quite forgetting for a moment that H31 has twin props and rudders) it looks better and performs at least as well with the trailing edge of the rudder in line with the transom and the prop clearing the rudder by just enough to allow ease of removal and replacement. I think it looks better, too, even though it spends most of its time underwater.
Dave M

[rickles23]

Hi,


In years past I have seen the prop shaft up near the bow and the boat still performed well as a conventionaly built boat.


Regards

[Plastic - RIP]

It's one of my biggest gripes too - it's almost like the propshaft is going to be so leaky that the inner bearing has to be above the waterline.     It also puts the weight of the motor high up too so stability is compromised.

I always go for longer shafts to get the angle to be more scale-like - and it puts the motors nearer the middle of the boat so more accessible without having to destroy the decks to swap a motor out.

[DaveM]

Before anyone blows a fuse or starts a flame-war about "scale-like" and "Shaft Angles" this is a snip from Fairey's own GA drawing. You can measure the angles by whatever means you have and you'll also see that the two engines are fitted under the cockpit floor. The reasons why the model is the way it is have already been given, so if you have any huge "gripes" about it then either a) modify it, or b) don't build one.

Life's too short to get wound up by a model boat...

DM

[Colin Bishop]

Now that is what I call an educational drawing! For models of this semi scale type the reality is, as Dave says, that the shaft angle is really not critical at all. Having built both his 1:16 SLEC Huntsman and Swordsman the performance is as much as anyone could reasonably want. I actually downrated my Huntsman to a 2S Lipo as the 3S was faster than I was comfortable with, especially in a turn.

It is also very interesting to see the small size of the full size version props. Probably these might be a bit impractical for a model version as you can't scale down water.

Colin

[malcolmfrary]

It is also very interesting to see the small size of the full size version props. Probably these might be a bit impractical for a model version as you can't scale down water.

Water remains water, no scaling required.  What does scale is the volume of water moved, and how fast.  Thats why models in test tanks have been so useful over the last century and a half.
A perfect scale prop, turned at scale speed, will move water at the correct scale rate.  Model props tend to not have the perfect scale profile.  As a result they need to either spin faster or be a bit bigger to compensate.  Both options require more than scale power, and if electric motors are involved, it pays to remember that electric motors are rated by power IN, combustion motors by power OUT.

[mudway]

The Precedent/Modav 46" Huntsman was more or less a straight copy of Vic Smeed's 1/12 scale plan, the logic being that if it's good enough for Vic then the rest of the herd should have little probem. I think it was designed as a "steering boat", which is a class which I don't think exists any more but which apparently benefitted from either a short, fixed skeg mounted halfway down the keel or the rudder/shaft layout show, From a scale point of view (quite forgetting for a moment that H31 has twin props and rudders) it looks better and performs at least as well with the trailing edge of the rudder in line with the transom and the prop clearing the rudder by just enough to allow ease of removal and replacement. I think it looks better, too, even though it spends most of its time underwater.
Dave M

Thanks, I did wonder that as many of them were petrol powered with heaps of thrust if the placement and shaft angle were some sort of effort to keep the bows down at speed. Obviously not.

[SteamboatPhil]

The steering boat ( as Mr M suggests) was designed by Vic for straight running competitions (which is sadly disappearing ) is was 4 ft long and 5inch wide, it was called the B Liner it had an electric motor about 8 inches from the bow with about a 6 inch prop shaft.....and the fin (rudder ) was around 4 inch from the transom. A crazy design but it worked and did in fact run very straight. I ran one at Mayhem 2 years ago. The long Build (Larry) is now the proud owner   

[ChrisF]

I asked a similar question elsewhere before I started my build and went down the rudder near transom and prop close to it route because it looks better and similar to full size boat (even though only one prop and rudder in my model) and lowers the prop shaft angle which I prefer whether or not it has any discernable bearing on performance.


With regards to proximity of prop to rudder it seems that race type boats benefit from some distance between them, probably to make the steering less sensitive at high speed with many of them having the rudder to one side so out of the prop wash anyway.


Prop and rudder when close probably benefits very slow speed manoeuvrability when a quick burst of throttle and prop wash  will give a quick change of direction and so essential for slow, displacement craft. For models like the Huntsman apart from when coming into the bank etc. it isn't so important and why you see such different setups.

Chris

[rickles23]

Hi,


With regards to proximity of prop to rudder it seems that race type boats benefit from some distance between them, the big petrol boats have the rudder off to one side and some of the faster ones have only one blade of the propellor in the water. I think all these problems occur due to not being able to scale water.

Regards

[SteamboatPhil]

A lot of the racing boats (in the UK) tend to run circuit, ie only one direction (anti clockwise )and they use propeller torque and the off set rudder to help the sharp turns.

[RST]

Most small craft have relatively "sharp" propshaft angles nesessary for the practical design -it's only when it comes to models we wax lyrical about theory.  There's not really any secrets behind it -look at the fairey owners club sites for example (people don't mention or quote it it much but talk allot about the boats).  Full-scale you're trying to hide a big engine and gearbox amongst a practical working inside, not the same as in a model -duuh!

Personally I find it more intriguing why propshafts are fitted on models with no outer support whether it is a P or A bracket, even a skeg.  It makes little difference for me except when I see a small shaft poking out completely unsupported which is just wrong in so many respects.

the big petrol boats have the rudder off to one side and some of the faster ones have only one blade of the propellor in the water

...I think you're getting mixed up with full-on racing craft (big and small-scale).  These tend to have very shallow prop shaft angles and typically very shallow exits after the stern with surface peircing / cleaver props which as you say are designed (in real life) to actually have only a few square inches of the prop in the water at a time and the prop iself is designed to lift the back of the boat out of the water as well as the hydrodynamic effect of the hull.  There is no room for a rudder here and on a single screw boat, it's often off-set to the side, particularly with boats designed to be optmised for one turn direction only, which might have a turn fin up font anyway.  This isn't the case for the likes of the boats we are discussing here.  As far as I can tell anyway no 30' odd deep vee boat design was fitted with a V12 supercharged lamborghini engine and fired towards a marker buoy in calm water with a surface drive set-up hoping to achieve 150mph+?

I'm leaving this comment with a video link I think most of us know who like fairey boats, but I still like watching it again every now and again.  This is why we build the little ones really, it would take an awfully big lottery win to fully enjoy one full-scale these days...

https://youtu.be/PoU1CdobKIE

[ChrisF]

Rich

I thought the link was going to be for the usual White Horses round Britain power boat race.

I've seen B&W photos of the construction process but to see it in film and in colour as well as the other stuff is great. Thanks for that.

Chris

[tonyH]

Found this while on amble round stuff. www.scribd.com/document/419849508/High-Speed-Small-Craft-Peter-Du-Cane

[Martin (Admin)]

I always assumed the shaft was that angle because it was the shortest 'standard' shaft available the would fulfill;
 Diesel engine flywheel size,
 Engine position,
 Prop size,

 ... but what do I know?    

[Andy M]

Lots of interesting stuff in the link/book above 😁