Big Boats Faster?

[PaulO]

Chris and I have been tossing this one around for a while. Let's see if anyone can come up with a magic answer:

When I was a young punk with scary outboards I always thought that the object was to get more horsepower into smaller lighter boats to get more speed. That and airing the hulls out to decrease wetted surface and therefore, friction.

I am now convinced that longer boats seem to go faster without proportional increases in horsepower. In fact, I believe that there are times when longer boats (all else being equal) will go faster with the same horsepower.

If you put the same 5.7 litre merc into a 16, 18, and 22 Donzi, which will be the fastest? Just read an article on the new 38 foot Fountain that has a plush interior including air and gen and hits 75 (radar confirmed) with twin 470HP Merc Bravos.

It takes Geoo in excess of 600HP and an arneson drive to push his 18 into the high 80s low 90s. A 38 footer doing 75 with all that weight and only 920HP?

There are plenty of other examples of boats in the 24 to 34 foot range that are fast without excessive HP.

What do you all think? And the answer is not less deadrise as far as I can tell.

PaulO

[roadtrip2]

My guess on the 5.7 question would be the 18 due to the hull weight to hp ratio and from watching them rip across the water at the AOTH!!

The way this more hull, more speed thing has been explained to me is as combination of hull length, dynamics, weight, and environment. Weight being the necessary enemy to get the length needed to keep the props in the water in rough conditions.
Then comes hull dynamics.

Interesting to witness what is happening with the APBA in factory class I and II. In Factory I, the hulls have gotten a little longer, 29' typically, and faster. In real world testing, the 29' hulls have also been faster with identical power due to lessons learned and applied to hull dynamics and set up for racing. In Factory II, the boats are getting shorter. Starting with the 38 and now moving to 35 or less with the entry of the Sunsation at under 32. Comes down to the fact that most races are held in calm, spectator friendly venues and the longer hulls are not needed to stay in the water and win. But once in awhile things get choppy, like Corpus Christi in a few weeks, and then the boys and men are no longer equal.

Sorry for the racing talk, but it is a great place to gather knowledge on the world of hull speed. I can not wait to see the new F2 boat from Donzi. Bring it on!!!!!!!!!!

[woodsy32]

It's all in the hull design. Don't forget, GEOO has no real trim per se... he can't lift his nose out of the water so he ends up pushing alot of water... and he still does 90 and is in control. Amazing actually!! If he could reduce the wetted area of that hull I am sure he could easily hit 100. Look at Franks 22Zx with the procharger, he runs ridiculous speeds with a reasonable amount of HP. Its not the HP as much as it is how efficient the hull is at a given speed. All things being equal, air entrapment catamarans are faster than stepped hulls, stepped hulls are much faster than hulls with a running pad, hulls with a running pad are faster than conventional hulls (no pad early donzi's). Also, the speed/horsepower equation is a curve with diminishing returns. My 22 with a 454 mag (385pshp)runs about 68-70 (factory, not verified yet.. http://www.donzi.net/ubb/smile.gif to get my 22 to 75 mph, all things being equal would have to add almost 100hp. Thats a 25% increase in hp for a 10%-15% gain in speed. At a cost of about $5000 for a (Procharger). To get to 80 mph, I would need approximately 650pshp. That same 385pshp will push a cat close to 80, push a stepped hull close to 75, but my hull only to about 70. Of course there are lots of other variables such as weight, x dimension, etc, but all things being equal it boils down to hull design. Just my .10 worth

Dave

[GEOO]

PaulO,
The speed comes with hull design. A 20' Allison with a 250 Outboard can run close to 100mph. The 24' Bat Boat hits 80-85 with a 360 hp small block. The trick is to reduce drag. The newer Donzi's & Fountains have great hulls, which are much more efficient. Our smaller classics are very wide for their length. Look at the running surface of an Allison, Bat Boat or Fountain. The running surface width is much less then our small boats. GEOO

[PaulO]

Excellent responses. How about boats of the same design? You would think that my 20 Cigarette with identical hull design as an 18 Donzi would be slower with the same HP. I think it is faster (a little tough to verify right now. I am going by prop vs. RPM). I also think that the 22 Donzi is faster yet than my 20. I like the point from Roadtrip about having enough hull to keep the prop in an efficient direction under rough conditions.

[jwright]

I know that in traditional boat design (planing yachts etc...) the beam to length ratio is a big factor. For a given beam, a longer boat will go faster with the same power. Since most performance boats are 7' to 8' wide, the longer models are more efficient. The additional length adds weight though which can offset this gain in hull efficiency. I do not have a strong understanding of the reasons why, but I work at a boat builder that builds planing yachts, and our designers have explained this to me....
I had an 18'2+3 with a 351, and my GT-21 with a 350 has the same top speed....
Obviously there are many other factors as others have brought up. just thought I would throw one more out there!

[CDMA]

Wow Paul more progress than we have made in our 11 pm driveway " why the hell don't these things go faster" sessions!!!

At least my 18 feels fast to me...for now...here we go again....

[BigGrizzly]

PaulO a guy here has a 20 Cig with a Alpha and a 350/300+ hp and SM exhaust. I beat him with the corsican so do most python Vipers with similar engines. I belive same huls need high end torque and hp to go faster

Randy

[Frank Civitano]

Take the bat boat. I was in Chuck Spragues 28 the boat with a 600dbx drive was running full tank 4 people at 93mph gps. I was impressed the way she flew and i mean flew! The boat would take off everytime she hit awave and would float back down. Look at the size of the 38ZX Pretty incredible boat to be running the speed she does with the weight she carries. But the twin step adds to that boats performance. And step hulls came from Aircraft. The pontoons were designed for less drag and better take off from the water.

------------------
Frank Civitano

[RickR]

Although they look similar most 22 Classic hulls are different from a 18 Classic.
The 22s is straight with reverse chines.
The 18s has a rocker with straight chines.

Paul O
What kind of chines are on the Cigarette?

------------------
RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[PaulO]

Straight chines and straight bottom. Had the old straightedge out to make sure while I had it off the trailer. What difference do you think the chines make?
PaulO

[Donzitester]

The answer is the 18 is the fastest with
the same engine. I have driven them all with the same power and it is defiantly the 18.

[GEOO]

And the X-18's are even faster yet, do to the aerodynamics. Right??? GEOO http://www.donzi.net/ubb/wink.gif

[Looped]

How about what is faster, the 18' true "V" bottoms (66-69 give or take)or the rounded bottom 2+3 Classics 70-present? Hmmmm?

Craig

[digitalMan]

It seems the 16, 18, and 22 hulls have less in common than meets the eye. I know my '73 16 has hook built in.

Do they still do this today?

[Forrest]

Yes, both new and old 16s have a hook between the chine and the outer lifting strake. The primary difference between 16 Donzis is the inside or closeset-to-the-keel lifting strakes. On 16 Donzis built in the 1960s, the inside lifting strakes end about four feet or so before the transom, where as the same inside lifting strakes on the 16 Donzis built in the 1970s ends about a foot-and-a-half from the transom. But then guess what happened when the 16 was brought back in the 1980s as the "Sweet 16"? Donzi shortened the inside lifting strake to end about four feet or so from the transom, just like it was back in the 1960s! 16 Donzis built in the 1970's handle and ride much differently than other 16 Donzis. From what I can tell, for the same HP and weight, the 1970's 16 Donzis are the fastest of the 16 Donzis, but are the least stable and most sensitive to trim adjustments when running fast.

------------------
Forrest

[This message has been edited by Forrest (edited 05-23-2001).]

[RickR]

PaulO
The reverse chines give stern lift causing her to ride more level with less wetted surface. They also jar your teeth in the wrong kind of sea.
Thats why some recreational boats (ie.Sting Ray, Sea Ray) have such good top speeds (they also have less dead rise and reverse strakes) but you wouldn't want to take them offshore. http://www.donzi.net/ubb/eek.gif

------------------
RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>