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Scratch build help needed, please

Old 07-01-2010, 01:23 AM
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john4vario
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Default Scratch build help needed, please

Hi, I am building a bait boat for my carp fishing hobby.
So far i build the bottom part of the (semi) mono hull out of fibreglass (from a plug shaped as i wanted out of foam)
Now i did have a old boat that had a unknown motor in it with drive shaft and a 3:1 gear reduction.
Never was a quick boat, but i am NOT looking to race..
I tried this setup in my new hull, but am afraid now that my prop (3 blade) is not deep enough in the water...Is there some sort of minimum rule for that, maybe?
Alsom prolly because of that almost no movement, let alone some speed. (I'll be tickled to death with 4 miles an hour)
My 3 side 'weedguard' enclosure of the prop area (mesh) doesn't help but i figured it would cost me about 40% speed to have a weedguard. (really need some sort of weedguard)
My main question would be...is it safe to say that with a 550 motor and one 3 blade prop it should be possible to reach that 4 m/ph?
(I tried a new Multiplex 7.2 vented motor with this prop setup as direct drive and in notime the wiring kinda gets hot and that motor seems to have a problem with this prop as soon as i put enough weight in the boat to make the prop "bite" water..... Wrong motor?)

It turned out almost impossible to get advice on the phone and it seems i talked to everybody that i thought knew something about boats, but very unsuccesful!
My main goal is working with a 6 Volt / 12 AH battery, direct drive and 3 blade prop (roughly 60 mm circle) that i all already have and go from there.
I'd love to stick with at least the 6 volt battery for multiple reasons....mainly having the amphours.
Am i asking the impossible or....?

I was always under the impression you can get a square box to reach some sort of speed and my hull is quite smooth, actually and shaped.
Can anybody shine a light on this subject, please?
I am aware that most people here race, but i really am not looking for such speeds..

Would dual motors and props do a MUCH better job or could one prop do the same, giving the fact i don't need a racer?

Any help is seriously appreciated and even if you can direct me to a knowledgeable store/webshop i'd love that too!!!

Thank you very much upfront..
John
Old 07-01-2010, 06:36 AM
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nick_75au
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Default RE: Scratch build help needed, please

For a displacement boat 4 Mph might be somewhat optimistic, I run a Springer (truck-able tow-boat, a square box essentially) which weighs 8Lbs and a 40 mm prop, I gps'ed it at 2.9 Kph. To get a 60 mm prop working you need about 3000-5000 RPM under load so you either need a largish designed for this RPM at 6 volts or a somewhat smaller but faster motor with gear reduction. 60 mm prop is too big to be direct driven by most hobby brushed motors, A graupner speed 720 torque will work but at 6 volt it may not have enough rpm.

A higher volt battery with the correct setup will have lower amps than a 6 volt setup and still have the same or better power

As long as the prop is far enough under the water that it doesn't suck air then its deep enough.

Nick
Old 07-01-2010, 12:32 PM
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SV27dayton
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Default RE: Scratch build help needed, please

Seems like something else is going on though. Any idea what pitch this prop is. How long / wide is the hull? Load some pictures. I'd try to run it w/o the weed guard and see what it does. Also unknown motor does not help. I think we should be able to fine a brushed motor with a good wind for this app for cheap. Current shouln't be a big problem as I don't expect this will run full out for long periods of time right?

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