I have a question for all of you guys with experience from offshore racing (or powerboating) in rough waters, and I understand that a specific answer is impossible, so I'm looking for your qualified guesses:

How much longer would a similar hull have to be in order to outperform a heavier hull in really rough waters?

(If you want an example, as a starting point, I would take a 28' boat, weighing in at 5900 lbs: How much longer would a 5000 lbs boat have to be to outperform the 5900 boat? Is it even possible?)

...and yes, I know that there are tons of other variables that make a difference as well, such as bottom shape, deadrise, weight distribution, etc. Also, having a wide, long (big) hull with very low weight can of course make it blow off course from hitting the wind, so there is obviously some sort of breaking point at which the boat will become too light for its big hull as well...

The reason for my question is that I will be looking at buying a new boat this spring, and I have looked at three different models. However, it is impossible for most people here to compare these as one of them is a "one-off build", based on a 27 Magnum hull, but stretched to 30', with a built in vented pad keel. The second boat is a hull copied from a 29' Scarab, stretched to 28,5' in the hull (resulting in an overall length of roughly 31'. The last boat is a Cigarette 28 SS, but converted to a single engine...
(All these boats have single engine drivelines)
You do the math!

Any "guesstimation" is appreciated!

By 'outperform' i assume you mean 'go faster with the same power' or do you mean have a smoother ride and stay in the water better?

Of what you listed, I would only consider the legit Cigarette 28SS. You can't just stretch (or shorten) a hull and expect it to still perform linearly. The lifting strakes in each one of the hulls along with CoG and ALL of the other variables will fock up riding attitude. Go with an original hull that works. There is nothing wrong with a Cigarette 28 that's been converted to a bigger single engine as long as it has the outdrive to support the power and weight.

I meant a combination of these: Go faster with the same power by having a smoother ride in the rough stuff. (I boat in big ocean swells, so the "top speed" of most powerboats will be limited by their ability to handle rough stuff.)

Thanks! That's true: an original design will always be a safe bet. On the other hand, the Cigarette was designed for twin engines and it seems to work fine with a single as well. The Magnum 27 hull is the origin of quite a few quite a bit longer hulls, and most of them have worked well... I agree with your point though...

All those boats should have similar weights and running surface lengths. I think you will need to water test them to see the differences, especially since 2 of them are modified splashed hulls.

If your going to boat in rough stuff and ocean swells I think you will need a larger boat than a 28-30. I was very selective on the days I would venture out in the ocean with my 28.

True, but two of them don't have engines installed, and I have driven the type that does have an engine; the Scarab splash. (It ate the rough stuff with ease.) If I'm going to buy any of the boats without engines, I won't be able to test them beforehand...
I know the Cigarette will perform well, but I also know it will need tons of HP to move...

Thanks for your concern! I see what you mean: I now have a 38' Scarab, but I've been out in 12' waves with a 16' boat: It was no problem, but I really didn't go very fast... 🙂 I know that I could almost have a cup of coffee in my 38" Scarab while going 70 mph in 5' waves, while I would be hanging on for dear life in a 28' boat in the same waters (and still not reach the same speed)... So; Yes, you sure have a point with a bigger boat being better adapted to rough stuff, but I'm willing to sacrifice rough water ride in order to have a smaller boat (with all the benefits that come with it).I just want to try and figure out with which hull I will have the most bang for the buck - a 5900 lbs 28' hull (Cigarette) or a longer (but lighter) hull. 🙂

What’s wrong with a twin engine setup for the type of boating you’re doing?