Is there a formula for running dry?

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Lil Wayne
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Is there a formula for running dry?

Post by Lil Wayne » Fri Jun 09, 2017 4:47 pm

Short story: is there a formula for getting a boat to run dry? Is it just thrust versus weight, or does hull design play a part? Assuming they both have poly on bottom, would a wider hull with more ground contact but lower pounds per square inch against the ground be better than a narrower hull with less contact and higher psi?

Long story: I've got a 12x5 alumitech with a 190 that will easily get stuck in the reeds. Its thick aluminum and a very heavy hull. I have two Doug hamant hulls, one is 14x8 with (supposedly) a 180 and it can go places the alumitech can't. I don't know the weight of each hull, but it seems the wider hull helps. I'm wondering if I had the 190 on a scaled down version of the hamant, maybe 10x6, if I could run dry in it. I know there's lots of variables, the way the engine is setup, the pitch of the prop, etc, but just curious if its expecting too much to run dry with a 190.

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Re: Is there a formula for running dry?

Post by OneBFC » Fri Jun 09, 2017 6:24 pm

Universal is weight. Lower always better due to the fact that friction force is tied to the normal force.

Wide vs narrow introduces something you hit on already wich is weight per unit of area (psi for example), aka, ground pressure.

Consider a hard surface like interstate 95. The resistance to rolling tires produce is less with narrow tires. Wide tires have much higher resistance. The surface of the road means neither tire sinks in any.

Consider a soft surface like edge of lakes. The same two tires would experience very different rolling resistance since the narrow tire will sink in and cause the tire to need to climb over a hump in front of it as it rolls. The wide tire will have a much lower hump to climb over....

Hulls are the same way. Narrow hulls run great on hard surfaces. Wide hulls run better on soft surfaces.

A wide hull on hard surface will have more resistance than a narrow hull. But ask yourself, how often do you get stuck on hard ground?

Theres a dozen other variables to consider in your search for a dry running formula....much too long to type here.

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Re: Is there a formula for running dry?

Post by Whitebear » Fri Jun 09, 2017 6:30 pm

There is no set formula that I have ever heard of. the HP-vs-weight thing is only a part of it. Some of the other things that can be MAJOR players are. What kind of grass, what kind of sand/mud, how wet is the surface, Boat design yes, More HP is always better, Better design is always better. Special running characteristics always come in dollars. More dollars = better dry running. Now all this is in general, but in general airboating is NOT an inexpensive hobby. Stick time has a LOT, VERY LOT to do with it too. Stick time rates right up there with more HP and better designs and in some cases may rate as the single most important qualifier.

The guys you never see stuck probably have spent a LOT of time stuck and eventually learned to read the grounds, grass, mud, sand and anything else on the trail. You can bet they didn't learn it the first time out or even the first year airboating. Its about having fun and its fairly safe to say that if you ain't havin fun, yer doin it wrong. So said my dad about squirrel hunting and trout fishing, and chasing women. Grinn.......
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Re: Is there a formula for running dry?

Post by mojoe » Fri Jun 09, 2017 7:02 pm

You are exactly right Whitebear.

I was riding backseat with a buddy that always seems to get stuck. As we approached a sticky spot the other day I felt him give it some gas and thought "he is finally getting it!"

Knowing how YOUR boat performs is key. My thin Bear style hull runs dry great, as long as I avoid the stuff that eats it and if I have to go through what I know it doesn't like, I STOMP IT BEFORE I get in trouble. On the stuff I know it does well on, I make it look easy.

I often ride with a buddy who has the same motor and prop on a wider hull. His boat does great in the stuff mine doesn't like, but he has a real hard time on other stuff I can ride easily.

Ya gotta just know your boat.
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Re: Is there a formula for running dry?

Post by SWAMPHUNTER45 » Fri Jun 09, 2017 11:49 pm

Hull shape, weight and friction equation are all very relevant with running dry but in the end horsepower to weight ratio is the most critical to success.

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Re: Is there a formula for running dry?

Post by CarMotorBarge » Sat Jun 10, 2017 8:22 am

The secret to running the hill is to get a car motor with a gearbox or belt drive. This solution works every time. :stirpot: :stirpot: :stirpot: :stirpot: :stirpot: :stirpot:

All of the posts above is really good advice. I would also add that the bottom of the hull needs to be stiff. My personal opinion is that all welded and fiberglass hulls should have jacks to stiffin the bottom. It is also a good idea to put a little crown in the bottom using the jacks.
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Re: Is there a formula for running dry?

Post by JB550 » Sat Jun 10, 2017 8:33 am

I agree with JR if you have a little Crown in the bottom and you pull it out and drink it on a regular basis you will run dry with ease.

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Re: Is there a formula for running dry?

Post by rick » Sat Jun 10, 2017 8:42 am

What are you turning that 190 wide open?

Also, is that the O-435 Lycoming 6 cylinder 190, or an O-360 angle valve 4 cylinder...or something else?

Rick

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Re: Is there a formula for running dry?

Post by JAMES » Sat Jun 10, 2017 1:27 pm

Of course there is it's called money money and more money
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Re: Is there a formula for running dry?

Post by terrible ted » Sat Jun 10, 2017 2:18 pm

Just fallow Tim the tool mans advise

MORE POWER

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Re: Is there a formula for running dry?

Post by helicsher » Sat Jun 10, 2017 3:29 pm

Car motor your advice makes me throw up in my mouth !!!!!
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Re: Is there a formula for running dry?

Post by dave1971 » Sat Jun 10, 2017 9:38 pm

Super Light or Big Power!!!! Thats the formula!!!! :stirpot: :violent1:

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Re: RE: Re: Is there a formula for running dry?

Post by Lil Wayne » Sun Jun 11, 2017 10:44 am

rick wrote:What are you turning that 190 wide open?

Also, is that the O-435 Lycoming 6 cylinder 190, or an O-360 angle valve 4 cylinder...or something else?

Rick
From what I can gather from our engine builder (its one of five boats in our work fleet) its a lycoming o-360 that's been modified to make 190 hp. If the angle valve is common and makes that hp, that's probably it.

The last time I ran it I think it would turn around 2700 on the top end.

Thanks for all the help guys!

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Re: Is there a formula for running dry?

Post by CarMotorBarge » Sun Jun 11, 2017 1:23 pm

What prop are you turning and what is it pitched at?
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Re: RE: Re: Is there a formula for running dry?

Post by Lil Wayne » Sun Jun 11, 2017 6:17 pm

CarMotorBarge wrote:What prop are you turning and what is it pitched at?
I'm not sure the manufacturer and pitch, but its wooden. My dad swore by wooden props, probably because he had done this for the last 35-40 years. We're transitioning to composites now, and if we decide to replace the hull we'll put an adjustable composite on it.

I was just curious more than anything, I had mentioned to our engine builder how easy it gets stuck in floating mats of grass, since that's where we spray its kind of a problem. He said something to the tune of "with the right hull it'll (that engine) run across the parking lot and over the railroad tracks with both of us in it." I'm sure that's true, I'm just trying to figure out what hull that would be and see if it'll work for us.

There are a couple jobs we have that are a series of roughly 2 acre ponds separated by earthen dikes. It would be awesome to run over the dike versus load and unload for each pond.

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Re: Is there a formula for running dry?

Post by rick » Sun Jun 11, 2017 8:52 pm

Years ago (before adjustable composites), we tried to pick a stick prop that would turn up to about 3000 rpm's wide open. 2700 would have been a little low for us back then. However, I have seen more than one wood prop come apart at high rpm's so don't take my opinions as prop advice lol. We were also running hunt/fish boats and not work (spraying) boats.

IMO "floating mats of grass" may take the mechanics out of the formula. We've got some stuff like that around here and if your boat gets set down in it, it has to push the whole floating island to move. That's a lot different than running dry ground.

Rick

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Re: Is there a formula for running dry?

Post by CarMotorBarge » Sun Jun 11, 2017 8:52 pm

The wider composite props push better on the hill.
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Re: Is there a formula for running dry?

Post by Sniper » Mon Jun 12, 2017 6:51 am

Best formula I have found is lots of HP and low weight, seems to work real good for WaterThunder :lol:
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Re: RE: Re: Is there a formula for running dry?

Post by Prototype » Mon Jun 12, 2017 10:41 pm

Lil Wayne wrote:
CarMotorBarge wrote:What prop are you turning and what is it pitched at?
I'm not sure the manufacturer and pitch, but its wooden. My dad swore by wooden props, probably because he had done this for the last 35-40 years. We're transitioning to composites now, and if we decide to replace the hull we'll put an adjustable composite on it.

I was just curious more than anything, I had mentioned to our engine builder how easy it gets stuck in floating mats of grass, since that's where we spray its kind of a problem. He said something to the tune of "with the right hull it'll (that engine) run across the parking lot and over the railroad tracks with both of us in it." I'm sure that's true, I'm just trying to figure out what hull that would be and see if it'll work for us.

There are a couple jobs we have that are a series of roughly 2 acre ponds separated by earthen dikes. It would be awesome to run over the dike versus load and unload for each pond.

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Wayne,
The best thing you could do for that motor is roast weenies with that wood prop, after you de glaze it.
Honestly haven't seen a wood prop on a spray boat in a decade at least. It's fine for the 90's spray boats and maybe even a few early 2000's but it really sounds like your missing the top end tweek that you don't have with wood.
Spray boats take a ton of tuning of what they can and can't do. Running dry is normally the minimum empty! Running the hill empty is like stopping a few times on the way up full. It really does not relate to rec boat use unless the rec users dump and pick up passengers along the way?

If you spray you will have a system that can pump poly and that alone can turn your 190 into a 0540 at 2mph! All of the above is true but my 190 hull has 800 horses on top of it and it still gets stuck on occasion! If it's floating mats only then just roll them under your rig!

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