Jet Boat design

[BazzaOz]

Very helpful Ad Hoc thank you. Is it possible to give me some logic behind the dimensions and angles? For instance the height from the bottom of the transom to the top of the transom is 667.5, why? The little flat section at the bottom of the hull is 362.1 why? and the little kick down angle at the outer edges(chine?) of the hull is what measurement and what is the effect? The radius at the front of the boat curves in quite sharply is there a standard radius for that curve etc etc?

 

[BazzaOz]

The starting point is the statement of requirements (SOR). In that you will include constraints (things that are absolutely set in stone,like the jet drive) and a wish list (go fast, shallow water capable, etc.) that don't have an exact number.

Hi Gonzo, good that's a good starting point. Some of those things include as you say jet drive, shallow water capability speed but also include size of motor number of people to carry etc. I would imagine the size and structural requirements of the jet drive will have a major effect. We are going for shallow water so planing hull but there are different shapes of planing hull especially upward angle of the hull. Are there standard accepted measurements for some of the angles and dimensions?

 

[BazzaOz]

Go inspect your son's friend's boats.

You should get all dimensions needed right there.

Or at least one set that works.

Hi Ondarvr, We are looking at a friends boat tomorrow night and that will give us guidelines but it doesn't tell me why those measurements were used. And sure I can copy them but then if I want to improve them I wont know which ones to adjust.

 

[BazzaOz]

Bazza, you said :
"First off I am trying to design a hull for this jet boat. I need to understand what angles for the hull transom etc are suitable and why, what curves for the front are suitable and why. I have been looking at the shape of the underside rear of the hull and they angle back down. So shape and dimensions of the hull are my first objective. I guess then I will need to understand the structural strength and support I need for the hull and so it goes on. If you have a book suggestion or other suggestion I would appreciate it."

These type of boats are complex little beasts to design well, and when you introduce water jet propulsion (rather than a 'simple' outboard motor) they become even more complex, with more chances of getting it wrong.
Rather than a book, my suggestion would be to buy a set of plans re a boat that has been proven to work - maybe even buy two sets of plans for different boats that you like.
And use these as reference for designing your own boat.
Your boat will probably end up very similar to the boat(s) you are using for reference.
It might even be exactly the same.
But you will have got some experience of drawing and figuring out the 'why, what and where' behind it.
Pretty much all boat designs start off with a basis vessel which is then tweaked slightly to suit the SOR (eg make it a bit bigger, or smaller, more or less deadrise etc) - very few boats start off as a completely blank sheet of paper (or a blank computer screen). There is no 'shame' in copying an existing boat - you do not have to re-invent the proverbial wheel by thinking that you have to start from scratch for it to be 'your' design.

If you want to learn the basics of naval architecture or yacht / boat design to help you better understand your boat plans, there are many excellent books available on www.amazon.com - do a search.

Thanks bajansailor. You are right and we will probably start by copying or using very similar dimensions, angles, etc. This enquiry was my first attempt in trying to understand the logic behind the design and I have gathered some good ideas thanks to some of the forums and others replies. Although I am still gathering more.

 

[bajansailor]

Baz, everything in boat design is a compromise - even for a little jet boat for thin rivers there is no 'perfect' design, you just have to try and balance out the (often conflicting) requirements in the SOR.
For instance, you might want to have the smallest engine possible to save weight and cost. And the most basic, simple hull. Which would be a flat dish, with no deadrise - it would give you the most speed.. However it would pound terribly, so you then want to have some deadrise on the hull bottom, to make it more comfortable. But not too much, because you are looking to keep the draft as small as possible.... so you reach a compromise in between.

Re your question to Ad Hoc about dimensions - if that plan had been drawn by hand the dimensions probably wouldn't have been so precise - eg the transom height might have been 670 mm rather than 667.5. It is in this ball park because the designer probably thought this was the minimum requirement for freeboard at the stern to avoid swamping.
If the boat had an outboard engine instead, then the transom height would be dictated by the leg length of the motor chosen.
Re the flat section on the bottom of the hull, this is needed for mounting the intake of the water jet -if you have twin engines and jets then you can mount the jets on the hull bottom even if you have deadrise. But with a single jet and deadrise you need a 'flat' on the centreline. The size is probably the minimum needed for the jet intake. It is very important to ensure that you have good water flow going in to the jet.

 

[Mr Efficiency]

If there was a "right" way to design a particular kind of boat, they would all look the same, but of course there is a great deal of variation, and when you factor in the reasons for the differences, which are not all technical, for example, styling considerations, the mix of practical and aesthetic considerations is impossible to disentangle. You're best advised to pick one you find pleasing to the eye, that is a proven performer, because the chances of your being able to improve on it by basically random "mutations", is minimal.

 

[ondarvr]

Hi Ondarvr, We are looking at a friends boat tomorrow night and that will give us guidelines but it doesn't tell me why those measurements were used. And sure I can copy them but then if I want to improve them I wont know which ones to adjust.

Even if you understood every aspect of hull design for a small jet boat it wouldn't help much. The design can be perfect on paper, but as soon as it hits the water and used in the way you want to use it, it needs to be modified.

On a small craft like this, where you put the cooler full of drinks and snacks will change the balance and handling.

If you like how the friends boat works, copy it, if not look at one you like better and use some features of that design.

 

[Dejay]

Now THIS is pond racing!

 

[Lazar]

Hello everyone,
I am planning to build a water jet boat for private use for 2-3 people. I am considering the option of buying plans for cutting and bending and then modifying them as needed. I am also interested in the motors and pumps used as well as the accompanying parts. Where do you get these accompanying parts? Is it perhaps best to get some used motors? Which motors, what power of the motor, I am interested in everything? Where can I get used ones?

@BazzaOz , I saw that you had similar questions before and were planning to build your own boat. I would like to know how far you got with that project, did you manage to realize it and what are your experiences so far?

Any information would be welcome.

 

[bajansailor]

@Lazar what size boat are you thinking of, re being able to carry 2 - 3 people?

Have a look at the Yamaha jet boats webpage for some ideas perhaps?
Yamaha Boats – The Worldwide Leader in Jet Boats | Yamaha Boats https://www.yamahaboats.com/

If you want to build a fairly 'small' boat (say 18' or so), and you want to source engines and jet drives secondhand, there must be a lot of 'old' Yamaha Waverunner jetskis in Europe that are being scrapped (or even offered for sale fairly cheap, compared to a new model).
You could probably put two secondhand Yamaha motors and jets in your boat for a relatively modest sum if you shop around for old jet skis? Are they popular in Serbia? If not, then you would probably have to look a bit further afield.
However be aware that by the time you have paid for the materials to build your boat, and bought a pair of engines and jets for it, it might be easier and cheaper in the long run to simply buy a secondhand boat that is ready to go.

 

[gonzo]

We are going for shallow water so planing hull but there are different shapes of planing hull especially upward angle of the hull. Are there standard accepted measurements for some of the angles and dimensions?

You are mixing and matching constraints with things you would like. Start with a properly written SOR, and jusify each of your choices individually. Also, what is you level of math, dynamics, and properties of materials? That is the foundation to understand what you are asking for.

 

[Lazar]

@bajansailor thank you for your answer.

I work in a manufacturing company and it is not a problem for us to build a boat for our private needs, which is why we want to do as many things as possible in-house, of course, that we know and can do.
We plan to build a small jet boat for 2 people, 3-4m long. I looked in Serbia, but they are really hard to find because they are not popular, neither Yamaha nor other manufacturers, and therefore I can't find used engines either. Do you have information on where used engines can be found abroad? I am also interested in what recommendations are there, what power engines are suitable for that size of boat, where to get the intake, adapters, grille and other parts.

 

[montero]

@bajansailor thank you for your answer.

I work in a manufacturing company and it is not a problem for us to build a boat for our private needs, which is why we want to do as many things as possible in-house, of course, that we know and can do.
We plan to build a small jet boat for 2 people, 3-4m long. I looked in Serbia, but they are really hard to find because they are not popular, neither Yamaha nor other manufacturers, and therefore I can't find used engines either. Do you have information on where used engines can be found abroad? I am also interested in what recommendations are there, what power engines are suitable for that size of boat, where to get the intake, adapters, grille and other parts.

Why you need jetboat ? What will the sailing areas be like? Rivers? E.g. on the Danube you don't need a jetboat.

 

[Lazar]

@montero We need a jet boat because of the possibility of using it on very shallow rivers, so we would use it on such rivers in Serbia. We've seen footage, so we'd like to build one in our factory.

 

[montero]

Balkan rivers are one of wonders of this world. I wish to sail there . I know Danaube from Orsova till Golubac.

There many types o jetboats . I thik best idea is use Jetski drive . Kiwis are most experienced of that kind of mini jets . Using more modern 4-stroke drives you are able to propell bigger boats like 3 person . Also use of bi 2- stroke is possible but they are older units.

Another way is use jet pump with car engine . You can choose engine from whole automotive brands . Jet pump manufactureres are :
Hamilton , Castoldi , Scott, Berkeley , American turbine etc.