Over built flat deck power cat

[Scopic]

Hello,
I am going to build a flat deck cat with a wheelhouse to look along the lines of a wind cat service boat.
8.5 x 3 metres so not massive. I intend to use it in coastal waters and our local Humber river estuary.
The build will be out of marine ply and sheathed in fibreglass. One of many questions I have is can this type build be built too rigid. Obviously weight is always an issue but is flexibility a prerequisite. I've no doubt this will be the first of may questions as this is my first "big" boat build.

Grateful for any advise.

 

[DogCavalry]

Hi @Scopic , welcome to the forum. The single most popular thread on thd forum is @oldmulti 's Multihull Structure Thoughts, which is,as I write this, at the top of the page. Start there. If you're going to spend time and money to build a boat, reading the entire thread is a pretty small investment.

 

[BlueBell]

One of many questions I have is can this type build be built too rigid. Obviously weight is always an issue but is flexibility a prerequisite.

No, it cannot be built too rigid. But it can be built too heavy.
Don't confuse the two. Weight is the enemy.

What does a "wind cat service boat" look like?

 

[bajansailor]

@Scopic are you designing this vessel yourself, perhaps using a larger wind turbine crew transfer vessel (CTV) as a basis?

If so, do you have any sketches that you can post on here of your design?
Or have you purchased a plan for an 8.5 metre power cat?
Please be aware that while you can scale things like hull shape down from a typical 16 - 17 metre (or larger) CTV, you can't really scale down the wheelhouse - it will still need a minimum of 1.8 metres headroom, and it might look a bit large in relation to the rest of the boat.

Here is some background info for reference on wind turbine CTV's -
An Introduction to Crew Transfer Vessels https://www.4coffshore.com/support/an-introduction-to-crew-transfer-vessels-aid2.html

What type of engines do you intend to install on your cat, and what speed are you hoping to achieve?

@BlueBell the attached PDF is a typical example of an 'older' wind cat service boat from 12 years ago, built by Alnmaritec (who are sadly no longer in business )

 

[bajansailor]

Hi @Scopic , welcome to the forum. The single most popular thread on thd forum is @oldmulti 's Multihull Structure Thoughts, which is,as I write this, at the top of the page. Start there. If you're going to spend time and money to build a boat, reading the entire thread is a pretty small investment.

It will take time to go through the entire thread, but @oldmulti has provided a very useful index near the beginning, and you can perhaps look through it to find references to 'small' power cats - he has mentioned these quite a few times in this epic thread.

 

[Scopic]

No, it cannot be built too rigid. But it can be built too heavy.
Don't confuse the two. Weight is the enemy.

What does a "wind cat service boat" look like?

Hi BlueBell,
Wind cats come in so many different shapes and sizes but the inspiration for my design is this.

 

[Scopic]

It will take time to go through the entire thread, but @oldmulti has provided a very useful index near the beginning, and you can perhaps look through it to find references to 'small' power cats - he has mentioned these quite a few times in this epic thread.

Hi bajansailor,
looks like I've got some seriouss reading to do thanks for the info.

 

[Scopic]

Hi banjansailor

Hi @Scopic , welcome to the forum. The single most popular thread on thd forum is @oldmulti 's Multihull Structure Thoughts, which is,as I write this, at the top of the page. Start there. If you're going to spend time and money to build a boat, reading the entire thread is a pretty small investment.

Hi,
as you can probably tell I'm useless with a key board and a mouse, however I can knock nails straight and lift heavy things!

 

[bajansailor]

Hi BlueBell,
Wind cats come in so many different shapes and sizes but the inspiration for my design is this.

The type of craft shown in your photos is called a SWATH - I think it would be rather difficult to scale down the SWATH shown to an overall length of 8.5 metres.
@Ad Hoc of this forum is an authority on SWATH vessels, and I am sure that he could offer you some useful advice about them.

But I think that for a 'small' power cat for the Humber estuary you would be better off just going for a more 'conventional' type of hull form - there are many plans available online for 'small' power cats.

Have you drawn any sketches showing what you have in mind?

 

[gonzo]

Before you get too far, there is some important terminology. Rigid is a theoretical absolute where a material does not deform regardless of the force applied. To understand engineering formulas and values, the property is stifness (K). For a body, that depends not only on the elasticity modulus of the material (E), but also on the size and shape.

 

[Scopic]

Before you get too far, there is some important terminology. Rigid is a theoretical absolute where a material does not deform regardless of the force applied. To understand engineering formulas and values, the property is stifness (K). For a body, that depends not only on the elasticity modulus of the material (E), but also on the size and shape.

Thank you point taken.

 

[Burger]

Richard Woods has a 30' power cat design (Jazz 30) which will suit your needs. Many different deck/cabin arrangements possible.
He has 40-50 years experience as a multihull designer/builder, and is one the most respected in the industry.
Buy the plans and start building.
You will save a huge amount of time, and get a very good boat.

 

[Ad Hoc]

Hi BlueBell,
Wind cats come in so many different shapes and sizes but the inspiration for my design is this.

I recognise that design

For the length/size of cat you want/need, I wouldn't worry what CTVs or any other boats looks like, in that sense.
You need to focus on what is your SOR (requirements) and go from there.