Help with design

I'm currently drawing up plans for a 30' sportfish express style boat but need some help. I have built several smaller boats and moving to something bigger. I want a Single diesel inboard due to costs. my problem is trying to figure out where to start for HP, Prop size, Gear ratios, and spacing for the plywood forms for cold molding. I really like the 30' Gamefisherman so I've used that as sort of a base line. I was thinking 400-500hp. The gamefishermans displacement is 10k lbs so id say this boat would be similar. All the boats I've built have been under 20ft so this is the largest project to date and the first inboard I've ever thought about doing. Any help would be greatly appreciated.

 

I understand there are too many variables to factor and get a straight answer and maybe its a shot in the dark but if someone's got a similar boat or knows of one I'm just trying to get approximate prop size so I can design motor placement and shaft angles. HP rating so I can get weights and figure COG. Im not looking for exact answers just a nudge in the right direction. LOA 30', water line length 28', beam 10', draft without prop or rudder 18" +/- From what I could find id say prop would be 16-20 inches. Do these numbers sound realistic or am I off in left field.

 

For building I was planning on doing plywood forms every 3' and 2' toward the bow where there more curve.

 

Dryfly, you must be a seriously affected gambler. You are going to bet some real money in the belief that you can design and build a boat that will work as well as a similar one, designed by experienced professionals. Yes, you can build the boat with the craftsman skills that you probably have. Doing the design and specifications are quite another thing.

Many of us have done that sort of thing, including me. But I am not nearly as brave because I have only designed and built cheap little boats, like 16 foot fishing skiffs and a few ragboats.

May the force be with you. I admire your courage and wish you the best of outcome.

You might get some useful information from Dave Gerrs book; The Nature of Boats, or his other book; The propeller handbook.

 

I will wholeheartedly second Messabout's thoughts above.
There are so many excellent and well proven boat designs out there, and there are plans available for many of them. Do a bit of googling.
Here is one for starters - ok, she is a bit smaller than what you want, and she has O/B motor propulsion, rather than a single shaft drive, but you might find some useful design ideas here?
Build a Boat - Carolina 25 Boat Design - Salt Boatworks https://saltboatworks.com/product/carolina-25/

Or how about this 30' flying bridge motor cruiser from Paul Gartside - she has a single inboard shaft drive engine. Ok, she is probably too slow for you, re your power of 400+ hp, but a glance at just the sample plans in the link will give you an idea as to the level of detail required for designing a boat like this.
Gartside Boats | 30 ft Motor Cruiser, Design #157 https://store.gartsideboats.com/collections/power-boats/products/30ft-motor-cruiser-design-157

Or a slightly larger version of this boat, and adapt her a bit for inboard propulsion?
Powerboats - Cape Lookout 28 - B&B Yacht Designs https://bandbyachtdesigns.com/cl28

 

DryFly,
I will wholeheartedly third the above thoughts and encourage you to build from a proven design that you like.
Can't find one? Look harder or adjust your expectations.
Best of luck.

 

This is what I have so far. She's got about a 9 degree deadrise in the stern that goes to 20+ in the bow

 

Sorry, but just because it looks like a boat, it's not a boat. A boat is much more than two surfaces laid at any specified angle. Choose a proven design.
JS

 

I guess I'm in the wrong place for help. Thanks all.

 

Awesome!
Wicked good numbers.
She'll be a dream.

What's the completion date?

 

Not at all re the wrong place - I think you are actually in a good place here, as you will receive a lot of help as you go forward with your design.
Can you re-post please the sketches that you had posted earlier in post #7?
I think I can summarise the advice here as saying don't try to re-invent this type of boat - very few boats are designed starting off from a blank sheet of paper, they invariably have a basis vessel that is similar for reference and to start with.
Try to collect as much information and data that you can on other similar vessels, even if you have to pay for study plans, and even pay for a full plan if you find something that you do rather like - you can still design your own boat, using this as useful reference.

 

I'm still working on the flair. This is without any. Trying to make it look good is the tricky part. I have designed and built 4 boats but nothing this large or complex. Im not trying to make a boat to outperform anything besides my expectations. If I could cruise at 22 knots and have a wide open of 26 I would be thrilled.

 

Flare

 

Better picture

 

@Dryfly,

When you done with your drawings, get a copy of Arthur Edmonds' "Designing Power and Sail".
It will step you though the process of designing and building your boat.
If you still have questions, they will be more focused and you would be more likely to get good answers.

Regards AlanX