CS25 plans and performance

[Lisca1977]

Good morning, I'm writing from Italy.
First of all, sorry for my English, not perfect for sure.
Since I was only a child I dreamt about building a boat on my own.
In the last years I successfully restored two boats maded in fibeglass and plywood, a " Ghibli Senior Zaniboni " now aged 35, and a very old "Fjord 24 DC" now aged 51, modifying tigis last from sterndrive to outboard , addying an outboard on a bracket. I've done every reparation and structural work by myself. I studyied and practice as aeronautical engineer when I was uoung, so I'm able to read drawings, project, bluerint, electric LEM schematics, and I'm skilled enought in simply electric and Hydraulic works. In the last 18 years I worked as industrial

liquid spray painter learning how to work in fiberglass lamination and repairing too.
I' ve an on my own business placed in a +1000 square area.
This long introductuon to introduce myself and my ability.
Now...it's time to do it, I think I'm ready to try! I'm skilled and equipped to do it!
Searching for a boat project that fits my needs I really aprreciate CS25 in jump cabin version with outboard bracket installation.
Before buy plans i had a look on study plans and I found probably problems in okume plywood thickness...
Plans required thikness are 6,9, 13 and 19 mm. Sorry for metric...I'm italian ; it's not a problem to convert it , the possible problem reagards available thikness here in Italy
Local dealers sell 6mm ( ok ) and 19 mm (

ok ) but don't sell 9 and 13 mm
Instead of this last two I found 8-10mm and 12-15 mm instead 13.
Now...before buy the plans I'm asking if I can use a little different thikness okume panels without substancially modify in originale project.
I'd like to use CE, RINA ( European certifyed, Italian Naval Register Approved ) plywood okume panels to build this boat cause I need to have a boat RINA certifyed ( without RINA certification in Italy You can't go as far as 6 NM from the coast line...not enought for my needs) .
Is it possible to use these different thikness without substancially overturns from the original project?
I'm not thinking I can't do that...I'm thinking abput scrupolously follow original plans reaching RINA certification once the boat is finished .

What about that???
That's a really important doubt I got to solve before to order plans!
I just buyed an Honda BF 250 hp for my Fjord 24 ( that's an heavy boat, about 2500 kg ). This motor is about 300 kg weight and 50 hours now.
What I'm wanting to do is to build a CS25 jump cabin version with bracket and use this new engine on it.
Do you know something about the possible performance about this boat with this engine? Max speed acheivable, and most important , consumption when navigate at 4200 rpm, ( my boat move at 24 kts with tis rpm ) consumption 25 L/H.
Thank you all for reading this long writing, I apologise about it and for my english.
Wishing to have answer to my question
Have a Nice day
Daniele

[fallguy1000]

Usually plywood composites are specified within a range of reasonable performance, so 9mm ply versus 10mm ply is 10% or typically within safety margins of most designs.

But, 6mm versus 10mm does not work.

So, I would say operating within 10% is fine. The designer has passed away and so getting some letter from him for an agency is not possible.

However, we can look in the forums and see if he publicly remarked. If you want to build the cs25 to clearf an agency in Europe, I would strongly recommend you find a Naval Architect in Italy who can handhold the process and make sure the scantlings are approved by them as well as any other changes such as the bracket design. Even the designer of the cs25 would not specify brackets because they can be made so many different ways. For example, a bracket can be built by extending the hull stringers, but this is done early in the build.

I do not know any plywood designers in Italy, but have to believe you can find an NA capable of dealing with ply composite structures. That NA may also change the glass specifications to suit your ply changes, etc.

[Lisca1977]

Thank you.
Reading your answer 10 mm thikness would be fine instead of 9mm listed in study plans, and 12 mm instead of 13mm listed in study plans.
Ok, that's a good information, enought for me to understand that very little adjustement can be done.
Yes, first of all, before to buy plans, I have to contact a naval architect for sure to ask him the procedure needed to acheive a RINA certification for this type of work. I only know I have to ask for a "prototype omologation certificate" for a self builted boat, but I don't know anything else regauard the procedure I have to follow.
Regarding the bracket, I know what you mean.
Study plans indicate Armstrong or Stailess marine bracket would be fit. We have italian producer too for certyfied bracket, very similar ( same materials, thikness and construction method ) , so I'm thinking about add one of this certyfied one on the finished stern.
Thank you very much
Daniele

[TomW1]

Jacques Mertens the designer of these boats who has passed away allowed a 1mm allowance in the plywood so go ahead and use what you have available. Remember that in this type of building the plywood only provides the form, it is the fiberglass that provides the strength.

Tom

[Lisca1977]

Thank You for Your answer
Daniele

[fallguy1000]

Jacques Mertens the designer of these boats who has passed away allowed a 1mm allowance in the plywood so go ahead and use what you have available. Remember that in this type of building the plywood only provides the form, it is the fiberglass that provides the strength.

Tom

This is not at alll correct, Tom. The strength of composites occurs to many reasons, including and in large part the distances from the glass to the neutral axis of the core which validates your remark, in part. The core, if it did not matter, could be made of weaker and weaker material. A good example is foam versus ply. A ply composite can be thinner, all else equal. And an all glass composite thinner still and all glass in epoxy thinner still. And all carbon in epoxy, thinner yet. This distinction is important. The cores provide some strength and they provide distance to neutral axis for the skins to perform best.

Tom… Consider, many plywood boats exist and need no glass, so your statement is broad and overreaching. I think the OP realizes this, in general. I have a cold molded 1960 Carver, all ply. The bottom is about 7/8” thick and sides are about 7/16” from memory.

I hate to correct you because I know you mean well, but the statement needs to be clarified.

And, even the direction of the tows is relevant to the overall performance of composites.

All these things matter..

[Lisca1977]

Thank You for Your answer.
I know what You mean, I never built a complete boat, but I know how to build composite panels with PVC or soft core and carbon skin or carbon/kevkar skin for aeronautical reason. I know the final strenght depends from the core thikness also, due to the distance of the internal and external skin from the center line and not only by the skin thikness and core material resistance.
However Tom give me the answer I need about European available okume thikness differences between listed thikness in CS25 study plans.
Thank You

[Aripeka Angler]

Daniele,
Dan/Fallguy gives you great advice. Definitely get a survey of the plans by a marine engineer in your home country before you proceed. Red tape is only red if you don’t take time to figure out how to navigate it.
On the bracket, you’re on the right track with an Armstrong type bracket. A couple of guys in NC built a CS-25 several years ago and the outboard fell off during the sea trial.