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Surface drive design/tuning help

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Surface drive design/tuning help

Old 10-01-2010, 12:03 AM
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Default Surface drive design/tuning help

Hello all, been a little while since i poked my face around the speedboat forums, Im asking for a tad bit of help with a micro boat im designing,
This is boat number 3 for me, surface drive attempt #2 with the first one being a total failure for many reasons, my subsurface drive on boat #2 worked great and the little sucker downright flew AND performed.
These boats im building are all 10" long hulls, made from a few peices of balsa, and for the most part every single part of it is small and started out as some form of scrap i had sitting around.
this last boat ive built is a 10" long cat, 3 and 5/8th wide, running weight 4.95 oz (without rudder, servo, hatch) with a 12-30-4100K ammo in runner from electricfly, 8amp silver series speedo, 3S 730 MAH lipo, props from the kyosho mini z speedboats. currently on a 2/1 gear reduction drive train because i had no other resistance free options.
all hardware custom made on my mill and lathe.

My question with surface drives (pics i post later) is once setup, and you get cavitation before the boat is able to get up onto step, what is the next step in surface drive setup ajusting and such? how much does weight balance have to do with its ability to plane? prop size, depth of prop etc have to do with overall setup?
my current prop is also a 3 blade, would a 2 blade be less prone to cavitation in a situation? i have about 70% of my prop showing below the hull, prop is entirely under water at rest in the water and at light speed. the prop is also about 1 inch behind the stern.
so, where to next? thanks for any help
Old 10-01-2010, 06:00 AM
Von Ohain
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Default RE: Surface drive design/tuning help

First of all, surface drives doesnt cavitate.
They prevent cavitation by sucking air with them, wich causes a phenomena that sounds and looks a bit like cavitation.
That is ventilation. But there is a very important difference. Ventilation doesnt hurt the propeller, while cavitation hurts it badly.

That said, I think its hard to prevent the excessive ventilation that occurs in surface drives before the boat planes.
I tried using excessive large props, wich does reduce it, but causes problems with amp draw.
I also tried moving the prop further down for a more submerged-like characteristic.
Doesnt seem to help that much, and it reduces top speed.
So I think its better to just live with it. Keep the boat on a plane most of the time.

Shifting weight backwards seems to improve top speed and the ease of getting the boat up to a plane.
While shifting it forwards seems to aid stability.

Much the same for propeller depth (surface drive). Raising the prop, up until a reasonable degree, seems to aid top speed and amp draw, while lowering it seems to aid acceleration, cornering and stability, at the cost of top speed and amp draw.
Neutral height is when the water line goes straight through the propeller hub, at speed. Not when standing still.

Im no expert, but thats what I think from my own experience and what I read.
Old 10-02-2010, 04:02 AM
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Default RE: Surface drive design/tuning help

I'll wait for the pictures, but it sounds like there's a flaw in your set-up somewhere...

Regards, Jan.

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