"Best" hull for a 13hp Airboat?

Hi everyone, I am relatively new to all this. me and my brother are about to build an airboat with a 13hp(not much, I know). as I understand, the hull is the most important part and there are all kinds of hulls, from planning, conventional to catamarns and hydrofoils etc..


I know that most people use plain simple "planning hulls" for airboats but I wonder if they are really good. isnt there huge drag at "some speed range" I am planning of a small geared 13hp that is going to drive around a 60inch.

it all depends on the speed right? and the speed depends on the hull or vice versa? I am not sure how fast this is going to go but i would love to have as little drag as possible. any body that can direct me?

I think I can hope for speeds up to 15knts?


/Kalle

 

speed potential with an air prop has been discussed on this forum before. Use the search function and I suspect that you can get some good information.

Generally speaking the air prop is not as efficient as a prop immersed in water. I suspect that the 13 horse engine is something like the ones used for riding mowers or hopefully one of the 13 HP Hondas as on pressure washers small welders and such. (They seem to be bullet proof).

If you are to use small power then total weight of the loaded boat will become a critical matter. Keep it as light as you can reasonably build it. My first thought would be a perfectly flat or very nearly flat bottomed sled on the same pattern as most of the existing airboats. If you can keep the total weight low the boat may plane a 15 or so MPH. If it is too heavy it will bog and the flat sled will not be the best way to go. If you mean to carry more than a few hundred pounds you may need to consider a long boat with some rocker in the aft bottom. If you do it that way it will not go very fast but will slip along easily at a moderate speed. All very well except for the noise any air boat is going to make.

Do get some more input on this matter. With props you must be careful of tip velocities. There is a practical limit of RPM for props of this sort. Maybe someone who knows this subject well will jump in with better help.

 

That diameter prop would suit half that power at twice the proposed boat speed. Even then the efficiency will only be around 65%.

With the proposed power and target speed you could expect the prop to achieve 43% efficiency. Hence the drag would be 550N. This is what you could expect from a 200kg planing boat at 15 to 20 knots.

To determine the most suitable hull form you need to have a target weight in mind.

You would also be better off considering a larger diameter prop.

The horsepower will get impressive speed if you swing a large prop on a light boat. Whether planning or long slender hull depends on what you want to do and how much weight you want to move. It is likely the planing will be the fastest if you were just after outright speed with minimum weight.

A human powered hydrofoil has achieved over 18kts with an air prop. This required about 1HP:
http://www.youtube.com/watch?v=l2UOH65QOI4
So you can do plenty with 13HP if it is applied well.

Rick W

 

Just curious, how does a Human produce 1Hp ? Or was that related to the video? I cannot watch youtube..

 

thank you for the information

my plan was to to get the boat as light as possible, and it will be designed to l take one man only. so how much weight can I expect? I weigh 80kg and the hull with the motor should weight no more than 80-100kg? ot is this farfetched. I was thinking also about a larger prop diamtere as it is always offcourse much more efficient and the mach number is always lower.

concerning that video in youtube.. wow! it looks really sped up but I hope it aint , .. He is fast because of the hydrofoil, why cant I just make some hydrofoils instead of the planning hull and fly the boat


Kalle

 

It is not sped up. There was a DuPont sponsored prize back in the 1990s for the first human powered boat to exceed 20kts. The MIT team did not quite make that. I think it was 18.7kts but as they were the fastest close to the record they got the prize. There is a 2 man boat that has since gone faster but I do not know if they got over 20kts.

With your proposed lightweight boat you should aim for around 20kts. The best for this speed and faster will be a planing hull.

A hull 2m wide with a very shallow "V" or even flat with a total weight, including the pilot, of 300kg will have a drag around 500N at 20kts. Weight needs to be towards the aft.

Ideally you would mount the motor low and use a belt drive to get the prop at the best speed. The attached JavaProp printout shows what you could expect. It requires a little under the 13HP but there would be drive losses and air drag to consider. The more streamline all of this is the better.

Making a hydrofoil boat work well takes a lot of design effort. The foils are prone to damage as well. They would not be consistent with shallow water operation, which is the normal domain of an airboat.


Rick W

 

wowo, so the human powered boat is as fast as a 13 hp!!,


I think that the human powered is ultra efficient (1hp) because of using a hydrofoil. Also I think that the weight of the boat will be more like 200 kg

is there no other hull design that is more efficient than "plan hull" and less complex than hydrofoil?

thanx for the help
/Kalle

 

That depends on your definition of "efficient". A long slim hull with round bottom , rocker and minimum drag (read skull etc) is extremely efficient...requiring almost no effort or power to drive the hull up to and even beyond hull speed...but that hull will never go faster than a certain point because it doesn't have the lifting surface to get it out of the hole it makes in the water. If you want the boat to plane then it simply HAS to have the surface area to lift onto. The amount of area required is a function of the amount of weight it has to lift and the available hp/thrust to lift it. A flat bottomed boat with no rocker aft has the greatest amount of lift vs surface area and will require the least amount of hp to plane. It will not do so well at sub planing speeds because the transom bottom is submerged and drags and so are and do the chines. Chine design is also something to be considered because the water must break cleanly from the hull so as to be of minimal drag. A slight mod of the bottom to a very shallow Vee (5-8 deg) will help in handling the boat in turns. A flat bottom with a sharp chine tends to "trip" on the chines in turns...sometimes flipping the boat. The Vee lets the boat rock over onto the flat and lift the outside chine out of the water a bit more. Of course you would lose a little bit of lift but the trade off is minimal and in your favor...saving yourself and the motor from taking an unnecessary bath. My recommendation would be a wedge shaped boat of about 5-6 ft beam aft to about 3.5-4 ft forward with wide flare to the side panels, a shallow Vee of about 5 degrees and about 18-19" deep at the transom tapering to 13-14" at the bow.

There are 2 main types of drag associated with boat hulls. Frictional and wave making. When the boat first starts moving the main type of drag in effect is frictional. Here is where the displacement hull shines because of its slippery shape it is easy to get going accelerates quickly up to the point where the wave making drag overcomes the amount of power available to push it THROUGH the water. I emphasize through because the hull does not have the bearing surface to lift itself onto the bow wave. As the hull accelerates through the water a wave train is set up. Once the boat gets to the point where the bow is supported by one wave and the stern is supported by the next following wave you are very close to the max speed of that hull or Hull Speed. As speed increases beyond this point the following wave moves further aft and the stern starts to settle into the hollow between the two waves. With a displacement hull shape the amount of power needed to make it climb the bow wave is not possible to provide. The shape of the hull here is the determining factor because an easily driven hull shapes the water flow to go around the boat and meet back together as smoothly as possible but it also doesn't let the boat rise up on the crest of the bow wave. Semi-displacement or Semi-Planing hulls (same thing) have enough bearing surface to available hp to allow it to partially climb the bow wave but are not very efficient in any mode. The Planing hull is most inefficient when moving through the water at a little over hull speed. Here the frictional resistance and the wave making resistance are conspiring to stick the nose up in the air and suck as much water along as is possible. However...the power of the boat and the bearing surface of the bottom allow the boat to ride up the back of the bow wave until its weight is supported on the crest of the wave (which actually reduces in height the faster you go). Here is where the planing hull is most efficient...skimming the top of the water. There is almost no wave making resistance and the frictional resistance is reduce with the amount of wetted surface that is raised out of the water. At very slow speeds (below hull speed) the planing boat sucks a lot of water along with it causing lots of frictional resistance.

So...Airboats are designed the way they are because that is the most efficient form for them to be for their intended use...up on plane.

Steve

 

Steve, I appreciate your effort alot!


so if I understand correctly, a planning hull with a water propeller should be most efficient and ride "on the back of wave" as you said.. but it makes me wonder why no water prop boats have planning hulls

Kalle

 

You lost me there. A planning hull rides on "top" of the wave and there are far more water prop planing hulls than any other out there. Any old aluminum jonboat, bass boat, ski boat, Bayliner even some pontoon boats are all water prop planing hulls. Even a jet boat is a water prop although it has more blades and is enclosed. Virtually all of those are planning hulls.

 

aha ok, than it makes sense, but still there are many water prop jet, ski boats that have a deep "vee" and look like displacement hulls,..hmmm why? let me guess, it is because :


1. they need to cross over waves in open seas and displacement boats are more suited?
2.these boats are made to turn easier in expense of lost of "lift"?


/Kalle

 

Ah...I think I know what you are talking about...Ships! The reason you don't see planing Ships is because you need a high hp to weight ratio to plane...big ships are too heavy and too much hp would be needed. Don't get me wrong...planning boats are not the most efficient hull form...simply the most efficient when you need to go fast. Ships derive higher speeds from the length of the waterline. The wider apart the crests of the waves the faster you are going. The longer the hull the wider apart the bow and stern waves are. A Nuclear Aircraft Carrier can do almost 50 miles an hour but isn't planing...maybe a bit on the semi planing side but no where near full plane. This is due to the 900 or so ft waterline length.

 

Mostly the reason for the Vee is to soften what is called "Pounding". A flat bottom or very shallow Vee slaps the water when there are waves and can be very uncomfortable (watch bass boats in a tournament). Vee bottoms part the water as they come down and decelerate smoother, kind of smoothing out the ride. They aren't as efficient at planing but make it up by having more horse power. A lot of craft have what are called "Warped" bottoms which mean more vee forward and less aft. Another compromise with its own associated inefficiencies.

 

I guess the most important thing about boats is that every single stinking one is a set of compromises that most suits your needs...not all but most. To go fast you put up with inefficiency, to be efficient you give up speed. To be comfortable in one way you give up something in another way. Think of a boat as a record balanced on a marble...if you put something on one edge you have to put something on the oposite edge. If you want speed you have to balance it with high Hp/weight. If you want lots of carrying capacity you need to balance it with a displacement hull or an inordinate amount of horsepower....etc.

 

Another part of the equation is rocker. Rocker is the curve of the bottom lengthwise along the hull. planing boats do not have rocker whereas displacement hulls do. it helps the water return to itself smoothly. The hull pushes water to the sides and down around the hull, the rocker aft allows the water to come back up with the least amount of effort and thus drag. A planning hull leaves a "hole" in the water which only disappears well aft of the hull as it passes. More drag at slower speeds but reduced drag at planing speed. Vee bottom speed boats have a straight run aft ...no rocker. The vee helps smooth the ride and the straight run is necessary to plane properly. They still use more hp to plane than a flat bottomed boat though.