"Best" hull for a 13hp Airboat?

here are som pics of jet boat, ski boat, and bayliner, ..they all are planning hull designs but dont look "flat" like the "airboat hulls" because of the smoothness of the ride as you said right? I feel though that the "vee " is too much on some designs

 

Where the tale is told is at the transom almost all except the bottom right one...that has a rather deeper Vee than the others. What you see at the bow is called Forefoot. Look at the transom area of the one catching air...it is almost flat and that is what it rides on when it is on plane. It takes more power to get it up on plane than a simple flat bottom but is more comfortable in waves due to the Vee forward and the forefoot.

 

the forfoot of all boats posted are "inclined" at a certain angle.

Furthermore the forfoot view from above is tapered at some certain degree.

are these two "properties" related to be smooth on waves?


I once saw a special designed hull where the flat underside of the hull had some

" sticking-outlines" 90 degrees o the length of the boat. the designer claimed it increased the speed of the boat by somehow creating a low pressure or high presure I dont remember. I know that on aircraft wings there are thing called vortex generators that produce low pressure vortices above the wing to enhance the lifting performance of the wing


Kalle

 

To do 20kts with a planing hull you need about 4kW with the best design and minimum weight for one person driving a water prop.

With an airboat you would need to swing a larger prop than you propose to get the best from the engine. The structure to support this also adds weight. There are stability issues as well because the thrust is generated a long way above the resistance.

Building to keep total weight under 200kg will take careful design. For safety reasons powered air props on boats are guarded. That adds weight and a little extra drag.

Decavitator was an optimised design for its intended purpose. If you want to go as fats as you can on water with 13HP then it will be faster than 20kts - maybe 40kts. However that will take considerable design effort and you will have a boat that is impractical for anything other than tearing down a straight course in good weather.

You could target 30kts with 13HP using thuoghtful design. It would be quite narrow. About 1.2m beam. So you need to keep the weight low. The deck would need to be aerodynamic to keep windage low. The faster you go the better efficiency from the relatively small air prop. A 2m prop will get 71%. If you use a shroud for the guard then the efficiency gain would offset the drag on the guard.

So it becomes a design challenge. Not something you throw together over the weekend. It becomes more specialised for just going fast for the installed power.

Rick W

 

yeah I know,, it is a realy pity everything must be a compromise

Is 2m prop small? , the boat is going to be around 2mx4 and yeah the weight.. hmm I never built any boat before, it can be blue core foam, fibre glass, or aluminum or even wood.


Kalle

 

A 2m airboat prop is quite large. There are many advertised in the US. They typically have quite wide blades so are not designed for high efficiency rather for high power and low centre of gravity.

There are also ultra light props that might be suitable.

Blue foam is not really suitable for building a boat like this. Sheathed wood is possibly the best.

If you want to get anywhere near the best speed you need to design for it. Where will it be operated. Do you need special permission.

Rick W

 

I will use it in the lakes and rivers here, well,.. I wont go for the maximum speed design since I will lose other properties. but it will be a 2m belt/geared prop.

 

You've received a lot of good suggestions thus far. I will add two more, FWIW.

As Rick stated, you want more weight aft; however, you do not want too much weight, as that may make it harder for the boat to jump the bow wave and get on plane. With the boat in full operating condition and the operator (you) seated at the helm, the back of the boat should be slightly lower than the front - this would give you best ability to get on plane given the power/weight ratio you are proposing. This may necessitate moving the helm forward in the boat.

It has been my experience with airboats that it is easier to get on plane when in very shallow water. This is because the bottom (of the lake, river, etc.) provides resistance and as the water is trying to escape from under the weight of the boat, it is forced out from the sides of the boat rather than downward - anyway, net result in more uplifting force on the hull of the boat. Thus, if 14hp would plane the boat and 13hp would not, perhaps the 13hp in 2-4 inches of water would do the trick to get it on plane.
HTH

 

aha? so are airboats made for shallow waters of 4 inches? the lakes and rivers I have here are way much deeper but there is never or hardly the slightest wave.. the water is very calm

 

Yes and that is good. Airboats are purpose designed for swamp and especially what they call swail....basically everglades conditions of tall grasses growing in shallow still waters.