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May 21, 2012, 04:11 PM
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Thread OP
Help!

Hydro/ racing propeller expert needed!!


Hi all

Today i have just finished my first home made rc boat, got it down to the lake for its first run, i started to pick up the speed, it was going pretty fast with very low rpm( all of the propeller was in the water), but not yet planning. I gave it some more throttle and it started to produce lots of bubbles and bubbly noises but not go any faster (the propeller was comming out the water as i beleive hydro props are supposed to). Finally i gave it full throttle it produced lots of bubbles and didn't go very quickly . I think the prop could be too small but i was wondfering if anyone could spot any problems with my design or setup. So basically all seems well until the prop starts comming at the water. The boat never picks up enough speed to plane. Attached are a few photos of the setup but here is a brief breakdown:

Any help or advice greatly appreaciated!

grauper 600 size motor (so more than powerfull enough)
15A mtroniks speed controller
graupner prop shaft and 30mm 'hydro' propellar

Cheers Sam
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May 21, 2012, 06:48 PM
Thermite + ice = Big boom.
boaterguy's Avatar
What is the size of the boat? weight?
A 30mm prop won't move a very big boat (especially a mono) very quickly, and will cavitate and ventilate (the bubbly stuff) a ton until it gets on plane.
Worst case, rev it up and have a friend give it a forward toss into the water to get it up on plane.
I like the wood deck, very well done.
May 22, 2012, 05:31 AM
Boaters are nice people.
Hi Sam,

Nice work on the deck of your boat!

The set-up however doesn't look too promising...

The propshaft angle is too steep, once you've sorted out the 'bubbling', this will cause running attitude issues, as the nose will be pushed down, making the hull run too wet, causing the ampdraw to surge, overloading the motor.

When sitting on a flat surface, you want the stuffing tube to be 1,5 - 2 mm clear of that surface like in the picture below:


I don't know the size of the hull (yet), but the prop also seems to be too close to the transom; the distance between the transom and prop should be between 10 - 13% of the hull length.

Getting the propshaft angle as shallow as possible is acheived by using a long propshaft, so the motor can sit way up front, like in this picture:

The hull in question is 31,5"long, the propshaft is 17" long.

Another way is to use a flexshaft, or a wire drive, which allows for a bend in the shaft, which in turn allows the motor to sit closer to the transom.

As you'll have to do some reconstructive surgery anyway, while you're at it, offset the entire drivetrain (motor and shaft) to the right, about 4 -5 mm, to counteract propwalk.

Leaving the propshaft in the center of a single prop boat will cause the boat to veer off to the right, which means you'll have to steer left to make it track straight.
This causes considerable drag, slowing you down and with the speed increasing, the rudder will act as a lever, lifting up the right side of the hull, resulting in a violent flip if the boat gets fast enough.

Your prop isn't the right one; for a Speed 600 you'll want a Graupner 2318.36K, or 2318.37,5K, you guessed it, these are 'real' hydroprops with 36 and 37,5mm diameter.
Get a few of those and you'll notice the difference

Regards, Jan.
May 22, 2012, 07:47 AM
I feel the need for speed!
scorpion1's Avatar
Great looking boat! As mentioned try a larger prop 33-37mm range and see what she does. The angle may possibly be too steep but you can sort that out once you get it on plane.
May 22, 2012, 12:19 PM
Registered User
wparsons's Avatar
As it's already been said, you definitely need a bigger prop. Once you're up to speed you can see what tweaks need to be made, but until then don't change anything except the prop.
May 23, 2012, 05:30 AM
Registered User
Thread OP
Thanks for such quick and detailed responces. The boat is about 60cm (24 inchs) long the propshaft is about 11 inches. I think i may have found the problem. When i tested the boat first time out it was very low at the back so i placed some foam on the rudder fixing to lift the back up and prevent water getting in until i could seal it properly. The foam was very close to the top side of the prop and could have prevented a good flow onto the blade. I have attached a picture to show where the foam was. I have tested it in the bath ( havn't had time to go down to the lake yet) and i can give the boat a lot more rpm without it cavitating or ventilating without the foam than with the foam. However later this week i should be able to give it another run and i shall report back. If i doesn't work i shall look at the propellar first then go from there as suggested.

Thank you for the compliments on the deck! the boat has been a school project that has taken me about 8 months to make, since i have only worked on it about an hour a week. It was built from scratch.
May 23, 2012, 11:36 AM
Boaters are nice people.
Hi Sam,

As you may, or may not know, a speedboat at rest always sinks in rather deep, so taping the hatch shut is mandatory (or settle for a really short maiden trip...)

What powersource do you use?
If NiMH cells, these should be in a split pack, with four cells on the left and three cells om the right (yes, 7 cells NiMH, on 6 the boat will barely get on the plane with that small prop).
If it's a 2S Lipo is should be on the left side of the stuffing tube.

The boat should balance at about 28 - 30% of the hull length, for a 24"hull the would be between 17 and 18 cm measured from the transom.

Motorcooling:
The Speed 600 has a built in fan, which draws air through the armature, but this will only happen, if the cooling slots on the front end of the motor are duplicated on the motormount!
Closing these slots makes for a very hot motor very quick.

On top of that, the aircooling alone isn nearly enough to keep your motor from overheating, you'll need to add at least watercooling on the brushtabs (where the wires are soldered onto) preferably combined with cancooling.

In the picture is a 700 motor, but the principle is the same: a spiral with a tight(!) fit around the can ant two brass tubes solderen onto the terminals:





If space is an issue, it's also possible to solder two (or more) brass tubes onto the (removable) fluxring, this will be enough cooling for 6 minutes runtime full throttle.
The tubing on the rear end, goes to the brushtabcooling (not visible in the picture):



Top of the line would be a custombuild jacket like on this 700 Neodym motor:



Regards, Jan.
Last edited by pompebled; May 23, 2012 at 12:24 PM.


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