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dumas air boat help

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Old 09-21-2017, 06:58 PM
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handyman
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Default dumas air boat help

built a big swamp ...elect power...3 blade 10x6....650 watt output 1100 kv with 3 cell battery..........heres the problem....twin air rudder left and rights both the same throw...wont turn left.......turns right just fine.............motor is set up stright on..nose of motor is piched up a bit...boat sits level in water............HELP
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Old 09-22-2017, 02:34 AM
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Bigger rudders. You want the height covering at least half the prop blast and twice that depth. The deeper the rudder the more responsive they'll be.
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Old 09-22-2017, 08:22 AM
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thanks.....what about the angle of the motor.should that be stright on or angle up....?
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Old 09-22-2017, 06:03 PM
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Originally Posted by handyman View Post
thanks.....what about the angle of the motor.should that be stright on or angle up....?
Have your thrust line parallel to the waters surface on plane. It will take some experimenting to get this part just right. Generally, I set my engines to have he prop shaft parallel to the keel of the boat which generally is parallel to the water when underway.
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Old 09-22-2017, 06:20 PM
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thanks...whats better 3 blade or 2 blade........i started off with 3 blade but with the problem of turning didnt get to far....
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Old 09-22-2017, 09:00 PM
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I only use 2-blade props. Cheaper, more choices, and more efficient.

Dont use wood props on airboats in case you have already. It ends real badly when the prop hits water at rpm and blows apart. Stick to APC, or at the least, Master Airscrew props. The former is the best IMO.
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Old 09-23-2017, 03:14 PM
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Exactly what motor are you using? The “watt output” means little. Regardless, that prop is-probably not the best for the motor unless it is a pretty large one. Too, exactly what ESC and cells are you running?

The reason for poor left turning is usually due to prop torque. As above, larger rudders will help. So will a smaller prop, but we need the above info to make an intelligent recommendation.


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Last edited by Got RPM; 09-23-2017 at 03:16 PM.
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Old 09-24-2017, 01:50 AM
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I have several airboats that turn right as well as they turn left. Prop torque is a thing, but not as influential as it's made to be in a lot of cases. Make the rudders bigger (deeper) and that should fix it up pretty good.
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Old 01-11-2018, 10:46 AM
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I need to increase the size of the rudders on mine as well, right turns are def tighter...
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Old 01-12-2018, 06:38 AM
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Adding some throw to the rudder will help turning as well, but the best performance comes from bigger rudders and equal throws right and left - 45 degrees from centered is really good.
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Old 01-12-2018, 08:59 AM
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Placement of your rudders are very important. Two blade props are the ideal prop and I run a 10/6 with my 3674 1900kv motor using 11.2 volts and 3000Mah. I also use dual rudders BUT, you don't have to,but, it's preferred. The rudders must have as much prop wash as possible to be effective. I've seen too many people build air boats and I have to wonder how that prop wash makes it back there to the rudders. The mounts have to be as thin and strong as possible. Maybe some need to start using 'pushers' instead if the mounts are going to interfere with prop wash. My set-up is all aluminum and my dual rudders are 1/16" thick, I believe. And using a 3 or four blade would probably heat things up if the motor wasn't big enough and the ESC was inadequate for it. Make sure the prop has good line of sight to that or those rudders! Even with the worst case scenario any air boat will steer really really good to the left or right with proper placement and prop wash. One rudder will work just about as good as two rudders if placed right with enough prop wash. BTW, mine is a home made 33" and every function couldn't get any better. We need to know the motor, ESC and batteries you are using and photos do wonders for getting the proper help.
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Old 01-12-2018, 12:33 PM
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Thanks for the tip and the pics, mine is a dumas big swamp buggy, I dont have pics yet just a short vid I can post. I just read about the flat bottom mod and want to check into doing that along with making larger rudders.

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Old 01-12-2018, 04:29 PM
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Originally Posted by Prohack01 View Post
Thanks for the tip and the pics, mine is a dumas big swamp buggy, I dont have pics yet just a short vid I can post. I just read about the flat bottom mod and want to check into doing that along with making larger rudders.
Yeh, that's cool. I have three so-called runners under my boat. One on each side and one down the middle and they are none other than the roll of car/automotive sticky trim you can buy at any automotive store. It works wonders and of course is water proof. It also protects the bottom skin of the boat pretty well. They help keep the boat straight and yet allow it to slide in the turns. I'm happy with the idea. I plan on building a 72" as well pretty soon and I am doing a lot of research first. I don't need any hiccups while building it. I may go with 3/16" plywood from Home Depot but I will make sure I have all my bases covered first. Then I may use the sticky trim again if I don't build some tiny aluminum ribs under the hull. Thanks.
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Old 01-12-2018, 04:51 PM
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I still have my Dumas BSB from '76 with the original ST60 glow engine. Painted it with K&B epoxy paint (do they still make that ?). Looks as good today as it did when I built it.
Mine always turned better to the right and I just lived with it. I always used wooden props and the only time they wore out was when I was following my buddy around ingesting the water spray from his boat. Trimmed the prop down nicely !
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Old 01-14-2018, 08:37 AM
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I threw my BSB in the trash. A friend built it from the kit using the wood provided. The kit wood is heavy and often warped (much of it was mahogany which is a heavy but hard and dense wood. The design of the BSB aid basically a tunnel
hull which is often regarded as a high speed straight line design and doesn’t lend well to being an airboat without converting the bottom to a flat bottom. Mine was converted to a flat bottom with a sheet of Lexan across the bottom. Due to my
boat hull alone weighing close to 10 pounds, I decided it wasn’t worth refurbing and making water worthy. If I were to do another BSB, I’d use spruce or basswood and plywood instead of the kit wood and either use an outboard on the tunnel hull and do it wet prop style or make it a true flat bottom and use an air prop.

And I would strongly discourage using wood props on airboats going over water. Bad juju.
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Old 01-14-2018, 08:36 PM
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I eventually tossed mine after many years of use, although apparently I had more luck than many. I had modified the boat by sheeting over that big deck opening and building a pod for the engine, placing the tank directly behind the engine. Zero of the fuel delivery issues which plagued many. I also increased the rudder sizes and set them as low as possible. The Fox .45 and a 10x7 Master Airscrew prop (no APC back then) gave speeds in the upper 30s and as long as it was on step handling was decent. Off plane, not so much.

Actually, tunnel boats are noted for their excellent turning, anyone who has watched F1 tunnels race knows they can literally turn on a dime. I oval raced R/C catamarans for 30 years and know that turning is their strongpoint, but being air compression hulls maintaining high straightline speed is a challenge. I should know, Iíve held a lot of SAW records with cats.

The biggest issues with the BSB were its high weight, poor motor thrust angle, sketchy fuel delivery and too-forward CG. The biggest improvements would be losing a pound and moving the CG back to 22-25%. Covering the tunnel making it a flatly would help turning, but top speed would be dramatically reduced. The biggest contributing factor to low speeds with airboats is excess drag/large wetted area.

Bottom line - there are better hulls but if you like the boat thatís what matters.



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Old 01-15-2018, 06:46 AM
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I should have clarified - a tunnel hull doesn’t turn as well as a flat bottom boat will when you consider the airboat will want to slide in a turn. Many of the BSBs I’ve seen run had a tendency to flip over in a turn because the sponson (maybe not the right term?) on the outside of the turn would bite and dig in and cause a flip. A flat bottom typically will not exhibit this behavior provided the setup is good.
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Old 01-17-2018, 07:34 PM
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Thanks for the info guys, I think for now I will just add 2 trim tabs to the back, sounds like filling the tunnel will add weight and if I do something wrong could cause an issue. Its still a fun boat, just didnt slide in the turns as I expected... it does in grass and snow though.. I guess Ill just have to build a smaller flatty in the future!
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Old 01-18-2018, 04:16 AM
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If you heat and form a sheet of lexan to the bottom, glue it, screw it on, and let dry... You will have a flat bottom boat that will slide in turns.
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Old 03-01-2018, 08:48 AM
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Scrap the Dumas boat and start fresh with something lighter.
I agree with Got RPM. I have looked at those kits and they don't lend themselves well to someone who wants the whole airboat experience. Most of those kits were stamped out over 30 years ago and have sat on shelves in shops for that long.
This is the reason i made my design. The dumas boats will not stand up to the running that i have done over the years.
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Old 03-02-2018, 04:19 AM
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Originally Posted by SWAMP_RAT View Post
Scrap the Dumas boat and start fresh with something lighter.
I agree with Got RPM. I have looked at those kits and they don't lend themselves well to someone who wants the whole airboat experience. Most of those kits were stamped out over 30 years ago and have sat on shelves in shops for that long.
This is the reason i made my design. The dumas boats will not stand up to the running that i have done over the years.
I wish you the best of luck selling your boats. Chris Selph’s Selph Inflicted boats were some of the very best airboat kits made; sadly not made anymore due to lack of demand. The 3-point hydro and tapered flat bottom boats run stellar. I built two hydros and one flat bottom. The little 20-series hydro has run 55.1mph and I’m gonna try for 60. Need to find a tuned pipe header to fit my JETT .35.
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