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Dumas air boat questions

Old 08-29-2004, 09:52 PM
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Keester
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Default Dumas air boat questions

Hi, I just purchased a beautifully crafted Dumas 31" Air Boat [garage sale]. I think its the Big Swamp model. [no markings]
I have many planes and engines but no experience w/ boats. First I installed 2 servos and Rx, then mounted a LA 40 on it.
First run at the pond was fun, it looks faster than it is. But this is where my questions are.
1. I gather the CG should be 30% from transom fwd?
2. I am whipping water with my prop, should I trim my engine down w/ washers? [seems to plow bow]
3. How do you seal your compartments, Gasket? Silicone?
4. With the 40 LA I am spinning a 10x6 wood prop. [I have no idea about air boats] Ideas?
5. Foam around my Rx and Batt turns to sponge. Wrong idea? should I wrap everthing in plastic?
Sorry for all the questions, but I had to ask. I learned so much on the airplane side you see.
Thanks in advance

Keester

P.S. This seems like a lot of fun!!! Could get in to it!! [my wife will be thrilled Im sure]
Old 08-30-2004, 02:26 AM
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boatman1
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Default RE: Dumas air boat questions

ok sounds like the thrust angle is off
sounds like prop end of motor is to far down makes it plow to much
float the boat & level the motor to the water this should help
plastic container or a ballon will water proof the electronics
check out this web site it has a lot of info on airboats
http://www.airboats.net
good luck boatman1
Old 08-30-2004, 09:23 AM
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dadkins
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Default RE: Dumas air boat questions

Keester, I have the Big swamp buggy too- and your right it does look faster than it is with a 40 engine. Mine tends to plow thru the water rather than getting up on plane. I would try moving the motor further back and angling it up too. IF POSSIBLE..
I have retired my BSB in the summer months and reserve it for the snow where it rocks ! I made a plastic radio box with a waterproof lid for my reciever/servos. and rubber boots/seals for the control rod exits. That thing still takes on water like like crazy. Thats why it is now the snow boat.
MY latest project is to make a scaled up BSB about 1/2 again bigger -something like 35-40" long 18" wide out of foam/glass - with a toro weedeater motor on it. I have the motor close to ready for bench testing
Old 08-30-2004, 10:30 AM
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Selph-Inflicted
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Default RE: Dumas air boat questions

My thoughts....

1) Lay a round dowel on a table, place your boat on top of it. Roll the dowel until you find its "balance point". That should be approximately 30% from the transom towards the bow.

2) Is water coming over the bow and then the prop is poicking it up? If your bow plows, angle the front of your motor up.

3) I use a "Rubbermaid container for my Rx controls.

4) You should never use a wood prop on an airboat. Water and debris will cause it to splinter...possibly hurting you or a by-stander.

Foam around my Rx and Batt turns to sponge.
Not understanding what you mean.

You can check out Rc Airboat World also. There is some good info there also.
www.rcairboats.net
Old 08-30-2004, 11:07 AM
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Keester
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Default RE: Dumas air boat questions

Hi, Thanks for the info, what I meant by the foam statement is it gets wet. The rubbermaid container will work for this. On the angle I was thinking wrong because I thought if you raised the angle at the rear more it would push down in the front since the motor is aft. I am going to check by balance before I change the mtr angle. [it is already angled up quite a bit]
You are absoluteley right about the prop [it already chiped]
How about the servos, how do they deal with moisture? do you just keep them off the floor of the boat and air them out often, replace them often?
Thanks again,
Keester
Old 08-30-2004, 12:23 PM
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Mr Cajun Gator
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Default RE: Dumas air boat questions

Not sure if I'm reading about the angle of the engine correctly. If you was to imagine a line drawn through the center of the engine with a arrow pointing out of the back of the hull. Pointing the same direction as the air flow from the prop. If the arrow is pointed down, then the bow of the boat will be pushed down, if it is level or pointed up, the transome would be pushed down, forcing the bow up.
The CG, 30% forward of the transome is a good number, but with the slower hulls, I would not put a big concern in it, unless its more than about 35%.
I build my radio boxxes water thight, its just a wood box, that is sealed with epoxy inside and out, with a lid that I can tape down with radio box tape, then I use the push rod seals on the linkage. I have used ballons before, it works as long as you fix the opening around the wires so that water does not get in there.
Old 08-30-2004, 01:17 PM
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Keester
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Default RE: Dumas air boat questions

Real quick here on the thrust angle, mine w/out any shims sits naturally about 15% up. prop end. To make it level I would have to add conciderable wedge. Was this built incorrectly? I understand the thrust line coming from the back of the boat. I think thats why it plows so much. I guess a little trial and error never hurts either.
Thanks
Keester
Old 08-30-2004, 03:26 PM
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Mr Cajun Gator
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Default RE: Dumas air boat questions

bring it down level, and run it there, if it bounces, add a washer under the 2 front engine mount screws. 1-3 degrees is all you should need if any.
Old 09-07-2004, 11:56 PM
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pro27
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Default RE: Dumas air boat questions

The BSB was built/designed with a lot of upthrust on the engine. Because this is a tunnel hull, this was done to keep the bow down and not let the hull blow over so easily. With the power available in the days the hull was designed, it worked ok. But by today's engine standards, the more powerful engines will dig the bow into the water.

An option may be to try and reduce the amount of up-angle built into the mount if possible, instead of trying to counteract it by adding shims.

If the pylon mount is bolted to the hull, unbolt it and sand down the front of the pylon feet so the angle of attack is reduced, then bolt it back down.

If the mount is glued in place, try to cut out the plate the engine is bolted to, and re-glue it in place at a lesser angle, or neutral angle.

Just a couple things to try.

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