My Powerboat plan (Take II)

I am not trying to make a fast boat. Speed costs money.

The regular use of this boat will be in a range of 30 miles. So for this range I think 12kn is enough.

But I am not happy with this process to take decisions.

There must be a process to quantify the different options. How much will it cost to increase the cruising speed from 12kn to 15kn or to 20kn?

The same happens with the beam and the length of the boat. I don’t have a scientific reason that supports the dimensions that the boat has now.

I will be able to take decisions alter quantify the options.

That’s what I am trying to find now.

As soon as I came to some conclusion I will let you know.

If you have some information that help me with this questions send it to me please.

My main source of information is the book “Principles of Yacht Design”.

 

This process makes many people unhappy. It makes you face reality. I will post a system of charts that makes decision making easier later today. It is based on assigning weights or percentage of importance to each item or characteristic.

 

I am pretty sure you too will find that there's an evident logical incoherence in these two quotes.

But I do understand your desire, so good luck with this project.

 

Hello Adrocha,

If I say something that offends you, just remember you started the thread first

I like people who are stubborn enough to do it themselves. I'm like that myself, don't tell me I cannot do something...

Since you have an idea what you want, there are so many options. I started designing a specific boat years ago, and still make changes. I hope I get the bloody thing done one day before I run out of change options

I assume it's a monohull you're building ? Have you considered the other types also ?
All have advantages and disadvantages, it's a matter of deciding what you can live with and what you cannot live without. Many start of with an idea, then a ways down the path they change their minds and then want something else.

I see many guys wanting a boat, the trouble always start with the it must be slow. This is bad, because you crate many problems for yourself. You want a fast boat first, and from there on you negotiate what you want. Trust me, there are enough that you have to give away to achieve what you want at the end. Fast boats are fun, efficient and enjoyable. It's almost like making the choice between do you want to be rich or do you want to be poor.

There are thousands and thousands of boats out there, you can see many on youtube video's and pictures and comments on websites. If I was you, starting with the boat thing, every day look at boats sailing, motoring, sinking , and what ever there is about them. Even the ones that are too big and too small. This will help you form a better understanding of your own needs and what you can expect, but also how boats behave in different types of water and weather.

Don't rush yourself into something unless you're 96 7/16 years old. Better spend some time researching and then when you realize which and what it is what you really are after, the reward will be what you expect. MOST first time builders build a boat, and right after they're done, or even worse, half way through, they realize they want something else. Spend the time, it's worth it.

While you're in the ripening stage, do attempt a small project, just something that will give you a better understanding of the materials, the amount of work and of course the results.

The only thing you have to watch out for are the Aussies.... they're from down under

 

That's very re-assuring Par

I didn't go into the trouble of actually finding someone to agree to take on a one off project, to actually reply to an email, let alone provide a firm quote.

When I win Lotto and have enough money to continue with my 8 metre project, I will use this estimate to negotiate the deal.

 

The OP is right about speed costing money, if comfort is high on the list of requirements. And it should be, if it can be called a pleasure boat. I think 15/16 knots cruise would be a lot easier to design for than 12 knots, which is an "orphan" speed few people want, or many (monohull) boats of this size can operate effectively at.

 

"Extra" speed cost money, like if you cheat and go carbon to save weight to gain an advantage over others. Nothing keeps you from having a relative fast boat in what ever you design, it is one of the comforts you seek.
If you want a pleasure boat (or is this a pure pleasure boat ?), then it is something completely different, all it has to do then is get the nekked wiemen crew drunk while it slowly sails to some secluded spot. Mind you, even they are surprisingly quick when officials are spotted. It's only legal when the authorities are involved

 

The easy way to "cheat" is to decrease the beam. You gain in most every regard, except elbow room.

The problem with this speed target, with a boat of these general dimensions is you're dragging a huge wave train, which just eats up fuel, for nor good reason. It would be easier to take these hull dimensions up to S/L 3.0 and you'd probably save fuel over the 2.4 S/L requirement, again given these dimensions.

Do a GA, then work up a rough weight estimate and see what you're working with. Unless you have a need for this much beam, you'd be best advised to reduce it from nearly 9' to below 8', which will dramatically reduced wetted surface friction, wave train size and weight and would be easier to propel with less power, making it much more efficient.

 

Decrease the beam as Par says... although the new fast mono hulls have almost a triangular shape and flat bottom with lots of beam, and they are light, so they sit on the water.

On catamarans you can retain your narrow beam, and widen it above the water line for more space. Here the one hull will keep the other upright, if you do the same with a mono it's going to become top heavy.

 

May I ask why is the beam set to 2.7m in your SOR?

A catamaran might be a good option, if the overall beam is allowed to increase.

 

I followed PAR suggestion and decrease the beam. Now it has Beam: 2,47 and Beam WL: 1,93.

The main reason for the initial beam was related with the marine fees. This boat is being designed to fit on the 8x2,7 class. 2,7m is the maximum beam.

I used freeship to create the first lines of the boat.

In attachment I send some images of the different plans and a file with the hydrostatic data generated by freeship.

 

and the profile view

 

The speed:

I don´t have nothing against speed. I think the faster we can go the better.
But I like comfort. I prefer comfort instead of speed.

Why?
What is the big difference in the design that can make one boat best adapted for 12 kn and any other for 15 kn?

I am asking this because we are talking in differences of 3 knots. I understand that there is a big difference in the design that runs at displacement speed and an other that runs at semi-displacement or planing speeds. But from 12 to 15 Kn? What is the big change?

Is at this speed that this boat moves from semi-displacement to planing?

I used freeship to try to find some clues about this question.

I generated the resistance chart for this boat.

The result was a curve much less exponential than I was expecting. I found an almost linear line for the resistance.

I used Fung-Leibman 1995 method. This method is applicable for this type of boat?

 

12 knots doesn't produce any dynamic lift normally worth considering, but 15/16 knots will if the boat is sufficiently light and has enough bottom area, and a suitable shape.

 

The more efficiency is comfort. Less fuel is more comforting, capable of more speed is comforting. There are times you need (more of) what you can get.

You just decreased the beam for the discomfort it will cause at the mooring.

If you really want it slower when it is fast, you can either trim back on the power, or drag a drogue.

Sometimes things enhance one another and you gain, sometimes it doesn't, then you compensate.