not sure if this has been done before..

first i would like to say i am a beginner at boat building and boat designing. i dont have much experience in either but have some ideas i would like to try out on a boat.

Im a teenager and have had the urge to build a small powerboat (11-14ft) lately, because i LOVE boating so much. i know all of the dangers and the time to be consumed in a project like this so i dont feel like getting a speech about that.. i understand fully what im getting myself into.

if you know anything about skis or wakeboards you know they use concave boards rather than flat boards now, giving skiers faster take off, less water resistance getting faster speeds etc... my question is has a boat ever been made with a concave like hull? i know there are tri-hull types and alot of other boats that have a concave like shape going on near the bow. but what about near the rear where the hull may be a little more flat(usually where most of the water resistance is at when on a plane)? i kno some jet skis have characteristics such as this, and was wondering if it has been done on a boat before? if it hasnt been done before how do you think it would ride and if it would be a good idea? any ideas or comments?

i have a jet ski that has some concave curves, giving me the ability to turn on a dime at full speed(50mph)!!! it would be sweet to have a boat that could do the same thing.

sorry im not the best when it comes to the terminology of some things that deal with boats.

 

Concave hulls can and have been done. But a boat with a waterski's cross-section will pound like crazy in any kind of waves. Much more practical is a standard V-hull with slight concavity between keel and chine. The Yamaha and SeaDoo jetboats have some of this.

Your jet-ski's apparent good handling is more the product of being extraordinarily heavy and overpowered for its length, and from having vectored thrust at the business end. But a similar hull form in a larger boat will indeed keep some of the characteristics- the terrible fuel economy, the huge wakes, and the good handling.

For a boat that size you'd probably want to look at hulls made with developable sections; ie. can be built in plywood. Compound curvatures make for nasty plug-and-mold work that can discourage beginners (you essentially need to build 3 hulls to get 1). I would look for something plywood-and-fibreglass that can take a jet inboard.

 

thank you for your advice.

i am planning to use an outboard motor so you wouldnt recommend using this hull shape with this type of engine? im planning on making my boat out of wood/ply reinforced with fiberglass. do you believe it would be a difficult task? say if i had the hull concave but more boxy/diamond shaped rather than a smooth sphere look.. would it do the same task or would there be any other problems involved if i did it this way? this shape would be more possible for me to build rather than the sphere shape.

i dont mind it being choppy(not too much though lol) because i would expect it to be a choppy ride for its small size anyways. my jet ski's ability to turn on a dime(forgot to mention i meant doing a U-turn this fast, and yes theres alot of g-forces hitting ya lol) has a hull built for this kind of manuverability, theres a name for the hull but forgot its name. so its more than just overweight and over powered.

edit- forgot to mention that type of hull on my jet ski helps break through wakes/waves giving somewhat a smoother ride as well (giving me the idea a boat this way wont be so much of a choppy ride)

 

Scaling up a jet-ski hull does indeed work, to some extent. This is what Sea-Doo jetboats are.

When you say 'concave', are you meaning a waterski-type cross-section or a concave region between the keel and chine of a V?

 

i am kind of new to the 'boat lingo' you can say so sorry for my bad explanation of where this concave area is planning on being. yes i am talking about a concave region between the keel and chine, preferably in the rear where when on a plane that is where most of the water resistance is taking place.

edit- doh!- if i payed enough attention i would of realized you said in the beggining a hull with slight concavity between the keel and chine would be more practical, rather than a cross section(which i have no idea what your talking about(showing my n00by side of boat building lol)). my very first post i was talking about a concave ski and wake board, to be more specific i meant they have concave areas on the bottom rather than just a plain flat board. hope that makes more sense.

guess i have to pay more attention lol. if i knew more 'boat lingo' i probably wouldnt of had to type so much, and would have gotten my answer quicker. i believe i have everything clear now and have my question answered

 

Trying to clarify here...are we talking about a transverse concavity or a longitudinal concavity or hook in the bottom?

Steve

 

FWIW -

Weston Farmer's 1958 "Trumpet" design has a fore & aft hollow in the keel. It's an 18' lapstrake outboard runabout. See lines at http://www.dngoodchild.com/5743.htm

Quote: "She is therefore about 25 per cent more efficient than the average—a marked increase in general ability. This is due to decalage in her running lines, particularly her keel, which applies to a hydroplaning surface a portion of the general principles of laminar flow found in airplanes with a “Coke bottle” fuselage. This “decalage” (deck’-a-lage to rhyme with garage) is a French engineering term which refers to different angles of incidence in the lift members of a plane, disposed in such a way as to produce inherent fore and aft trim components." In other words, the keel has a concave arch (easily seen in the drawings at the website listed above).

 

well for i dont have any more miscommunication going on here is a roughly sketched image of a rear of a boat.


all i am wondering is on the flat surface of these 'pads' some say, highlighted in red, if it had a slight concave section to reduce water resistance, would i gain any speed or get anything positive out of it?

highlight in red above is where i would have the small concave sections. this is just a 3d model of ideas i had so if you see imperfections i wasnt trying to make it perfect in the first place. if you say this hull wont work i fully understand. i wasnt planning on using this hull, its just an image that was saved on my hd , so i decided to use this for an example.

 

This is not an easy hull to analyze....
My guess (just a guess, nothing more) is that slight concavity in the centre pad could give it a bit of an advantage in turning ability at the sacrifice of a bit of efficiency. I am not quite sure about the outboard ones; I would suggest either making them extend more towards the sides of the boat and fairly concave, or extending them in the same way but angling upwards a bit. But this is a very complicated hullform and will be hard to predict.

 

hi,
i'm a newby too,im also a teenager with a big interest in steam and vintage launches,but love boats altogether.
i admire the fact you are thinking of a new type of hull to suit an existing well-proved design.
i think perhaps if you took a look at some designs for drag boats you would have some sort of idea of what an efficient hull shape using a transverse concave between the aft' and stern section of the keel is like: using a 'chopped' step hull design on a single chine 'v' hull,meaning when the boat is planing on it's chines a pocket of air is created(actually a very slight vacuum) underneath the hull and this supports the hull on top of the water while halving the inefficiency of the boat due to only the chines and the very stern,be a transome or counter stern,being in the water.
whether this design could be used in conjunction with an outboard is questionable due to the extra weight being over the stern.
have you considerd using a standard HONDA engine as used in ride-on lawn-mowers, available in many sizes and as such perhaps one that would be equal to an outbaoard,but with the weight being distributed in the hull rather than outside it you would have a lot more stability?
greg

 

thanks for the imput..
i havent put much afterthought into these specific designs. because of a tight budget i dont want the risk of wasting money into an unsuccessful built boat. my brother gave me his old boat which needs minor repairs, and so im not thinking too much of building a boat... yet. before i put too much thought into building a small boat from scratch im going to try a kit from www.glen-l.com, and see how it goes from there.