Another Prop Question

[ids987]

Here in the UK, we have an offshore racing association (actually two now). Races are mostly endurance - approximately 1/2 hour heats - hence a lot of fuel is needed. Because of the fuel needs (eg 2 litres plus for a .45 engine), the boats tend to be very big for the engine sizes - compared to oval racing boats for example. Many are running a straight drive .45 in a Challenger 48 for example. From what I can see, we have a conflict between the prop which the boat needs to run efficiently, and the prop which the engine can drive without going too far out of its powerband. We seem to get reasonable results with similar prop sizes to the circuit boats (eg X447/3, sometimes X450/? for the .45s), but speeds are much less - presumably, largely due to prop slip. Wait times for the prop to bite can also be very long. I know that this kind of setup will never acheive the same top speeds etc as circuit boats, but I think the available props are the biggest limiting factor at the moment. In an ideal world, the answer would probably be a lower Pitch to Diameter ratio, but 1.4:1 is the lowest commonly available. Gearboxes are one answer, but add complexity, cost etc. Wondering if anyone has any useful info on props I may not have heard of, custom modded props, or what kind of prop mods may work. In terms of prop mods, I guess the way forward would be to take a bigger prop, and mod it to load the engine less.

Thanks in advance for any info:

Ian

 

[Terry Keeley]

What about a 12 series Octura prop? Haven't seen one in a long time and not sure if they were made in BeCu tho.

 

[ids987]

What about a 12 series Octura prop? Haven't seen one in a long time and not sure if they were made in BeCu tho.

I did wonder about those, but can only find a very limited selection. I found some very small ones in BeCu (<= 45mm) - then just 50, 55 and 70mm in GRP. Also wondered about the lifting characteristics for a mono......

 

[ids987]

What about a 12 series Octura prop? Haven't seen one in a long time and not sure if they were made in BeCu tho.

I did wonder about those, but can only find a very limited selection. I found some very small ones in BeCu (<= 45mm) - then just 50, 55 and 70mm in GRP. Also wondered about the lifting characteristics for a mono......

In GRP, they might just be too cheap to ignore. The 55mm should have just slightly less pitch than a X450.

All suggestions welcomed - thanks.

 

[lohring]

A lot of the pitch in X series props is at the trailing edge. I'm not sure what the pitch of the body of the blade is, but back cutting the blade will remove a lot of pitch.

Lohring Miller

 

[ids987]

A lot of the pitch in X series props is at the trailing edge. I'm not sure what the pitch of the body of the blade is, but back cutting the blade will remove a lot of pitch.

Lohring Miller

Thanks Lohring,

I've never done any prop mods yet, but thought from what I've read that the trailing edge should probably be the focus for reducing pitch. Wasn't sure what to expect in terms of lifting characteristics, or whether to back cut the whole leading edge, or just the centre area. I guess that removing material only towards the middle will (maybe) tend to make the boat run lighter, and will have less effect on loading than back cutting the whole length of the blades. Back cutting the whole length will presumably (maybe) reduce lift by reducing pitch.

I'll have to try and find some X series props in Ali or Stainless. I don't fancy messing with BeCu - the main reason I haven't yet done any prop mods or even balancing.

Ian

 

[HendricX]

Prather 235 and 240 in steel .. or PropShop 50-52-55/14 2 and 3 blades

PropShop has them in Aluminium too .. but for the price-difference I would take the SS ones.

A Dremel / flapping-wheel , mask and gloves , and a steady-hand is all it takes to tweak your own props.

(Hmmmmmmmm forget about the balancer)

Buy the PS-props give them a try , and if tweaking is needed ask for help in your club/organisation.

If you can't find someone to help you in GB , your rescue is on the other side of the pond.

 

[dwilfong]

Ian

I have bin messing with Octura X series props this month. I have found that if you remove the tongue of the prop that the leading edge pitch is larger and more in tune with the trailing edge pitch. I go with a larger prop and reduce the diameter. Then I use a large bar cut to unload the prop and reduce lift. Then start cupping the tip till the eng comes off pipe and back up a little.

Attached are picks of a stock X452/3 and my mod X457/3. So far it is showing great promise on my RS45 in a mono. This is a surface drive boat.

David

 

[ids987]

Thanks for the info so far guys. Lots to think about.

Dave, Nice work on that modded X457/3 !