Power rating on the hull

I see boats that have a max power rating by the manufacturer being exceeded
for example max power @ 500hp outboards & some run an extra 250 motor, totaling 750hp

My?? is will the craft loose the structural integrity in the transom
or will it benefit in fuel consumption running a lower rpm @ the same speed

 

As a legal matter in the US boats are rated to their horsepower based upon the calculations and table found in the Code of Federal Regulations Title 33, Volume 2 revised July 1 2002 (there may be a newer revision, but this is the latest I have). http://edocket.access.gpo.gov/cfr_2002/julqtr/33cfr183.53.htm

As a practical matter the general rule of thumb is that you need 1hp for every 25 pounds of gross weight, so a 4k pound hull will need about 160 horsepower. Since almost everyone seems to overpower outboards anyway being underpowered is rarely an issue on a used boat. Another general rule of thumb is that for every 10% increase in horsepower you will gain 4mph. So to increase that 4K pound boats top speed by 15mph would require going from 160horsepower to 235hp.

As far as fuel efficiency goes no there is no gain by running multiple oversized engines at a lower RPM. In fact just the oposite. More engines means more weight, more stuff in the water, more complicated stearing and throttle controls (again more weight), and of course all of this comes at a major premium in terms of cost. Add to this that adding an extra couple hundred pounds of engines hanging from a transom and yes of course you are going to overload the structual design of the transom.

There are also some legal and insurance issues related to overpowering a boat most importantly a lot of insurance companies either won't insure or will charge a premium if a boat is overpowered by more than 10%. The Coast Guard can ticket you as an unsafe boat and escort you back to the dock. And god forbid you ever get in an accident the fact that you have overpowered the boat is going to significantly increase the likelyhood that you will be found at fault even if you didn't do anything wrong (same as haveing a non-strret legal car operating on a public road).

Of course people do it, and I am sure someone here will disagree with me, but if you really feel the desire to run a boat that fast get one that is designed to take the loads and the impact forces of running that far up the MPH scale.

 

....Thanks for that information. That can be quite useful. The main reason I'm here is a coastal freighter idea, and a pair of small craft to run cargo for ports of call not requiring the freighter itself to put in. It's just mindless bouncing of the ball on the wall at this point. But if I need good advice, this seems to be the right place. I hope my ideas are not to so nutty as to embarrass myself though....

 

Danny,

I doubt that a general rule of thumb for outboard sizing on small planing hulls is really applicable to a costal freighter, or any heavy load cargo boats. For that type of thing I would really ask an engineer to take a look at the design specs to determin proper power requirements.

For a displacement boat the requirements are vastly different. For instance I know a 72,000lbs boat pushed at hull speed with a 170hp diesel. A far cry from the 2,880 hp that the thumb i gave above would sugest.

 

...So true. I was referring to a trawler yacht type cargo carrier. Something you could winch up to the freighter. The idea is to have a fair size enclosed cabin with bench seats for passengers or loose freight and an area to mount a standard Unit Load Device Container like aircraft use. That way you could roll off the load quickly, pick up the outgoing ULD and cast off fairly quickly. I'd like to use ULD's to use smaller then 20" standard container loads. Plus the same ULD could also be used on delivery aircraft in inventory.

I was thinking about a good rugged offshore type hull. Top speed maybe 16 knots tops. Something to handle a 3500lb ULD and maybe 6-8 passengers. Incidentally the Freighter would be something akin to a modern US Army FS/FP. A bit larger, maybe 200 foot. But as you pointed out, that is a whole nother kettle of fish.I noticed the FS ships also utilized a pair of 25 foot launches for deliveries too. I'm trying to see if a modern version could service remote coastal towns cheaply.....

 

It is similar to the car exhaust: the smaller your ***** ...
There are cases where boats were so much over-powered, that the extra weight and trimming issues had rather lead to a reverse effect.
Transom strength - you are right - should not be ignored. Usually there is a safety margin included, but who knows the static condition of an older boat?

 

here is a boat that could possibly fit the role that you are looking at
http://www.boatdesigns.com/products.asp?dept=540
24' long small enough to be loaded and unloaded form the freighter and has a good cargo capacity

 

"Freighters" don't have crew who can load or unload anything. They can get the boat from A to B and tie it up...that's it.

Your requirements for small coastal freight are already being met by tug-and-barge with a ramp to drive trucks aboard hauling standard containers. Also smaller landing craft, also with ramps to quickly load pallets or small containers via forklift.

http://www.billmunsonboats.com/