Scale model performance

[small cat]

I'm intrested in building a small powered cat of my own design. I have no boat building or design experience but I am an industrial designer. I want to know if it's worth it to build and test a 1/5 scale R/C model of my design. I started considering this to determine the best CG position for the boat as this will impact on seating and bulkhead placement. I also figured I could evaluate how it handled rough water and determine how dry the boat will ride. Is this possible? If I do build the model (it'll be fun anyway) what other useful information can I glean? Are there formulas to relate the draft, weight and power requirements of the model to the full size boat.

Any input will be greatly appreciated.

[Bill the Cat]

What's the purpose of your power cat? Racing, fishing, pleasure...

I'm building a cat hulled flats boat of my own design - but I would not call it a off-shore or heavy seas vessel in any stretch of the immagination.

www.davesflatcat.com

My boat is designed by my own intuition for the science of power cats visit:

http://www.aeromarineresearch.com/

Good Luck with the project.

[small cat]

The boat will primarily be used for fishing on the western shore of eleuthera in the Bahamas. The water is pretty calm but gets a bit choppy when cold fronts come thru. I hope to have access to the flats and some shallow reefs nearby. I want a little boat with shallow draft that can still handle some chop and keep me dry.

As I said I have no experience in this area but I figure if I keep the boat small enough structural engineering and construction cost will not be big issues. Hey the garage in eleuthera is pretty small to, and I'll be working alone.

My design philosophy goes like this:

small - easy to build low power requirements
low power - wide hulls thick chine
smooth ride with wide hulls - 20 degrees deadrise
low draft with lots of deadrise - wide hulls again
engineering concerns - keep space between hulls small (18")
uncluttered cockpit - external motor bracket - adds effective length to boat

Specs
Length: 12'
Beam: 6'
Hull width: 2.25'
Max Gunnel height: 2.5'
Deadrise: 20 degrees at transom
Power: Max 30hp

I've attached an initial drawing which will be used to build a small model of one of the hulls 12" long .... just to check if the lines look right. If that works out I'll draw plans for the 20% scale model.

Any ideas?

[Bill the Cat]

The 1st model is important - in order to see how your pieces will go together. The 2nd might not be so useful - but will increase the confidence in your design.

For expense control - look into Devlin's Book Book - http://devlinboat.com/order.htm

With the engine to the rear and a small OAL - fuel & supplies will have to go forward - I am assuming tiller control.

What is your anticipated freeboard? My boat has a max of about 4 inches -

Dave

[gonzo]

I am not sure about your lines. The profile and mold sections views don't correspond. Which lines are which? Also, if you want low draft the deadrise should be much less aft. A hull with an equal deep deadrise as you designed will squat untill it gets on a plane. To do that it needs a lot of power. For a low powered shallow draft cat, round sections are much superior. Sharp chines only help performance at high speeds. However lifting spray rails combined with rounded sections are a good compromise. The wight of bulkheads will not affect your trim as much as the engine and fuel tanks. Are you using a single or twin installation? If single, is it offser forward? Another option is a flat bottom aft with a vee section forward. It is easy and economic to build.

[small cat]

Thanks for all the input guys. I'll try to check out the lines on the first model and then decide what direction to take. Gonzo suggested that I consider a flat bottom aft and vee section forward. Question: How flat is flat at the transom 0, 5, 10 degrees?

Thanks again for the input

PS Bill asked about freeboard ..... @20" if floats with the chine line just below the water..

[gonzo]

You can have the bottom completely flat aft. The vee forward can have flat or rounded sections. An easy way to build a bottom is with stich and glue. The flat turns into a vee as you stich it together. If you need to visualize it or build patterns, a cardboard model can help a lot. I have made several; all you need is a razor knife and tape. The best thing is how fast you can modify the design. Also if you lay the model on graph paper it makes instant blueprints.