Jet Boat design

[BazzaOz]

Hi I am looking for advice on designing a jet boat. My son lives on a shallow river and has been introduced to the idea of building a 12/13 ft Jet Boat. He has a couple of friends who race them so wants to make and race his own. We have purchased a 3D CAD package Solidworks. I have experience with 2D drawing software and am at the moment learning Solidworks. I am looking help on shapes angles, dimensions, structural requirements etc, etc when doing the drawings.

 

[Ad Hoc]

Welcome to the forum BazzaOz.

What you're describing is the difference between... drawing a boat - I have Solidworks and I can draw, with that of - Design - which is the the actual number crunching of the weights, hydrostatics, stability, powering etc.
The 2 are poles apart.
There are many existing designs that you could buy off the shelf as such.

But if you wish to "Design" your own (as apposed to just drawing it).... it is not an overly difficult endeavour, given the size of the boat you want, but you need to learn some basics about design/naval architecture first to make sure you get what you want and within your budget. If you but a ready made plan, you of course do not need to do this.

 

[bajansailor]

+1 re Ad Hoc's comments above.
I did a quick google, looking for 'plans for a small jet boat' and I was amazed by how many links came up, most of which do appear to be pretty relevant.
Here are a few :
MiniJet INC https://www.minijetboatbuilder.com

14' Mini Jet boat build 200hp https://www.bdoutdoors.com/forums/threads/14-mini-jet-boat-build-200hp.669523/

Mini jet boat build http://www.jbnz.co.nz/yabbse/index.php?topic=28250.0

Bazza, what is your preferred construction material?
I am rather hoping that you will say 'aluminium', as most of the links I have found are for ally boats.
And they make a lot of sense really, for small boats travelling fast on very skinny rivers where there must be a good chance of coming into hard contact with a lump of granite or similar under the surface.

 

[BazzaOz]

Welcome to the forum BazzaOz.

What you're describing is the difference between... drawing a boat - I have Solidworks and I can draw, with that of - Design - which is the the actual number crunching of the weights, hydrostatics, stability, powering etc.
The 2 are poles apart.
There are many existing designs that you could buy off the shelf as such.

But if you wish to "Design" your own (as apposed to just drawing it).... it is not an overly difficult endeavour, given the size of the boat you want, but you need to learn some basics about design/naval architecture first to make sure you get what you want and within your budget. If you but a ready made plan, you of course do not need to do this.

Hi Ad Hoc,
Thank you for welcoming me.

And yes I was trying to do both ie 1, learn to draw a boat and 2, understand the design/naval architecture theories that are used to effectively design a small jet boat. Without the second understanding I could not fully understand what I was doing for the first.

I was looking for help with both whatever form that takes. Referral to websites, advice on where I might start, for me even the jargon is unfamiliar. My son is keen on diving in and if we make a mistake correct it in the second version. I am trying to understand the theories first so I don't have to deal with the mistakes.

If you can help it would be appreciated.
Regards
BazzaOz

 

[BazzaOz]

+1 re Ad Hoc's comments above.
I did a quick google, looking for 'plans for a small jet boat' and I was amazed by how many links came up, most of which do appear to be pretty relevant.
Here are a few :
MiniJet INC https://www.minijetboatbuilder.com

14' Mini Jet boat build 200hp https://www.bdoutdoors.com/forums/threads/14-mini-jet-boat-build-200hp.669523/

Mini jet boat build http://www.jbnz.co.nz/yabbse/index.php?topic=28250.0

Bazza, what is your preferred construction material?
I am rather hoping that you will say 'aluminium', as most of the links I have found are for ally boats.
And they make a lot of sense really, for small boats travelling fast on very skinny rivers where there must be a good chance of coming into hard contact with a lump of granite or similar under the surface.

Hi Bajansailor.
We were looking to design our own version. We have had contact with the NZ suppliers/manufacturers and have looked at those websites as well as other similar sites. No offense but they give some info but not all that I need obviously they have a product to sell. They definitely help but I am looking towards the theory about the angles/dimensions etc so I understand why I am using them.

My son has a small jet boat that is fairly well worn. It is made of aluminium and he has been advised that its the most suitable material. The river that he travels on is actually what you describe.

Appreciate your help and if you find anything else that might be of use please forward it.
Regards
BazzaOz

 

[Mr Efficiency]

What is the intended area of use, bazza ? Meaning, rivers, sheltered areas, bays, open water, etc. That would, or should guide the thought process.

 

[BazzaOz]

Hi Mr Efficiency,
The intended use is shallow rivers.

 

[Mr Efficiency]

That should give you an abundance of choices, with existing plans and designs, it is quite difficult to find a good hull for a jet boat that ventures off shore, where a conflict exists between hull shapes that handle rough water well, and ones that suit jet drives best.

 

[Ad Hoc]

...I am trying to understand the theories first so I don't have to deal with the mistakes...

Understood.
The issue is... how deep into the subject do you wish to go?
It is like an onion.. the more layers you peel away, in knowledge and understanding, leads you into the more you need to learn.

 

[BazzaOz]

You are right Ad Hoc but I need to get started with something and then as I gather the info at some point I will say "that's all I need". 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.

 

[ondarvr]

Go inspect your son's friend's boats.

You should get all dimensions needed right there.

Or at least one set that works.

 

[Ad Hoc]

... but I need to get started with something and then as I gather the info at some point I will say "that's all I need".....

Agreed. But don't get blind sided by absolutes!
There are basic parameters the help naval architects to define a hull. Armatures incorrectly tend to use these "absolutes" as their starting point. That is incorrect.
These ratios are helpful and provide an understanding, but it is not the starting point. In fact, is it more the 'end point'. Since the hull shape ends up whatever it is, to suit your requirements (The SOR). What the ratios end up being... are just that and nothing else. So you can sue them as a metric of comparison, in that sense.

So you really need to focus more on the generalisation of what and why.. and not the exact angles or Cb or Cps etc (you'll learn about those later).

For the type of boat you nee, it is going to be a flat bottom boat and there are many examples - notably the flat bottom high speed boats of NZ and the means to power them, the Hamilton jets, - which is how they first started.

As for generalisations... you could try watching this overly simplified general comments on high speed hulls.
Then perhaps read up on the concepts noted here too.

Don't get bogged down in overly technical stuff to start with... just understand the basic of how's and why's.

And you'll probably end up with a boat looking not dissimilar to this one:

 

[Dejay]

That does look like fun. Crazy irresponsible fun

I'd start with the theory behind planing. It's good so you know what you're looking for. But it's probably just an incredible balancing act to design a boat like this well and figure out all the little details and things that make it work in certain situations.

And to design it to be safe. At those speeds the biggest danger might not be drowning. Although that doesn't really seem to be a concern haha. Shouldn't they have a roll-over bar and wear helmets?

You might just need a lot of experience from trial and error, and the ultimate expression of that experience is a boat plan validated by customers. If you build out of aluminium you could probably modify it later.

 

[bajansailor]

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.

 

[gonzo]

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.