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Don Aronow had this in a notebook...

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Old 03-12-2004, 08:56 PM
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LPA2106
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Default Don Aronow had this in a notebook...

...a friend who I mentioned yesterday was into Don Aronow "tid bits" as he had an Apache' and a former Formula owner...now "looking"

He said an employee who worked with Don said this was written on a poster at Donzi Marine....for the sake of arguement let's say that's true..who knows....but it's interesting to study...after awhile it makes a lot of sense....

What's a formula for top speed?
The answer, verbatim is:

Crouch's formula:
It takes into account the weight and horsepower at the
propeller, and assumes a 50\% to 60\% efficient prop.
Most props fall into this range. Note that it doesn't take
into account the boat length, as that doesn't matter with
planing boats.

Crouch's Formula

V = C/((DISP/HP)**.5)

Where V = boat speed in knots (1 knot=1.15 mph)
C = Constant (depends on boat type)
DISP = Displacement (pounds)
Note that boat manufacturers usually give
innacurate numbers for displacement,
typically on the low side
HP = Horsepower available at the propeller

For comparison sake, here are some average values of C:
150 Typical lightweight, planing cruiser
180 High Speed Runabout
200-230 Race boats, hydroplanes etc.

Basically this means "C" is the most important factor unless you want to run a boat with 10 engines to meet your speed requirements.


...it's interesting that the size of the boat dosen't matter nearly as much as design efficiency per se'...I called a friend who works for the Big Three in the number crunching dept.(chief engineer) and faxed it to him...he looked it up and said that speed was most significantly dependent by this equation on the overall efficiency of the hull/prop combo.

What he calculated mattered most were:

-Deadrise
-Step Placement(s)
-Weight distribution along the hull/angle of running surface when underway.
-Ratio of "undisturbed water" getting to the prop(s)
-Angle of attack of the prop blades
-Blueprinting the hull to be as straight as possible



SO ...put aside brut Horsepower....when designing a boat he said it's in this order...Hull Design ultimately determines Hull/Prop efficiency most notably.
balance keeps the nose there to "cut waves" but not ride the surface needlessly...a "perfect hull would need no trim tabs or engine trim" so a perfect hull dosen't exist.

He called a cad/cam guy to do a mock up and found that as the hull efficiency increases there is a direct relationship with an improvement in propeller efficiency.
Most efficiency is lost at the prop...almost 1/2 of the boats power is lost at the prop...and changing the prop will only yield minor improvements assuming you have a decent prop like a Mirage...the way the water is "fed" into the prop and out like a jet engine determines overall efficiency more than any single factor. The hull "feeds" water to the prop from a qualitative and quantitative standpoint. (Have I lost you yet?)

So the cad/cam showed that there is no substitute for a lesser hull design without having to add more power to compensate...and much of the additional power is wasted in prop inefficiency and harder wearing of the drives. More efficient hulls are "easier" on the engine and drives...we knew that right? They jump on plane with minimal need to trim etc...

We looked at several boats and performance figures...they found using the above equations they fed into their laptop spread sheets....that Cigarettes are way up on the very high level of efficiency with their stepped hulls....what ever they did.. they did really very well....Fountains are well tweaked as well and in showing them the video of the last "speed run" they commented that the hull was very well balanced...as if on rails we said....but interestingly the Formula Hull on the 38'er was deemed a good design by the formula and performance data they felt because of a "blunter" point of entry and diffferent step geometry (one of the guys at the company has one) it's setup is to ride a bit more lower on the bow and give a greater "knifing" action to the oncoming seas sooner if you will and that adds up to less "air time" so a little slower; but great for comfort/speed ratios if that's what you need. So POSSIBLY less of a cantilever like ride..how different? I don't know...you would need to ride and test each...we did this as a meta-analysis based on available test data in magazines and on the web. Donzi's were deemed efficient based on the data we could scrounge....Powerquest showed well....Pantera's.. especially the 24' with moderate power and the 28' too were both great...probably due to a well balanced hull and an overall great design...

....Now some of the older BIG Formulas without steps/older BIG Cig's without steps (except for the 28'er which is very efficient) the older Donzi's/ older Scarabs were not as efficient at the "top end" but said to be very efficient at "knifing" their way through high seas....and good at rear hull re-entry.

....quite frankly it was hard to find a really bad hull using this equation....but we found a few...and I don't want to mention the few we found that were really bad accoring by this equation...no bad mouthing...but the bad one's were not boats we "talk about" much on OSO...they were the lighter "imitation offshore looking boats" that probably have a deck that bounces up and down constantly as it hits and bashes waves rather than "cutting them".

...we didn't have much data on OL with stock engines to figure a constant out...like a 40'er with 525's....etc so we couldn't do much there...

...so the good news is just about every boat that is a ture offshore... is "variably" efficient....but there are those outstanding one's...like the Pantera without steps does really well...we all commented them to be great values too!

But Cig's blew us away as did Fountain's...in terms of efficiency they hit their mark well...speed wise and otherwise as really well....what we are talking about is "safe high speed".....

...so on a usually boring Friday..we found something fun to do...hope it's helpful...make my job of finding a new boat easier...
 
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Old 03-12-2004, 09:13 PM
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The correct answer is....................Pantera!
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Old 03-12-2004, 09:40 PM
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QUOTE "a "perfect hull would need no trim tabs or engine trim" so a perfect hull dosen't exist"


When was the last time You seen a perfect wave
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Old 03-12-2004, 10:00 PM
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if we tested boats to prove or disprove this equation the seas would have to be ideal....it's all on paper...but the cool thing is we showed is that there's a lot of outstanding boats out there and the task is more to match one to one's own needs rather than to look to see which is "best". So for me the comments I have read and interactions I have had here have been very useful...it's like having a bunch of people who have tested the products by their own experiences that tell you what to expect...an informed customer is the best customer...the bottom line of our calculations were to avoid jumping into something that is not tried and true...and if you buy a 40+ footer without steps that is heavey...cool....just do the math and add in some extra horsepower...

...also...efficiency in design equals less expense in purchasing and less expense in repair bills...

....and don't get too worried about the trim tab thing....I was in a boat that needed constant trim adjustment and had no business being so overpowered....if the boat needs some trim fine...but if you are all over the buttons constantly..then you have to ask why....
 
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Old 03-12-2004, 10:49 PM
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I see you've been doing some homework. Very well done.
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Old 03-13-2004, 06:09 AM
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LPA2106- Great post, that is some great info
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Old 03-13-2004, 06:52 AM
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that's to much reading for me at 5:50 in the morning great post
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Old 03-13-2004, 08:51 AM
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awesome post. 2 questions: any info on the long island boats like superboat, kryptonite, activator, progression? also, what possible solutions are there to increasing prop efficiency? (surfce drives for cleaner water, etc)
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Old 03-13-2004, 12:09 PM
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we speculated that extension boxes improve the props ability to get more "sticky" water....cupping...and additional blades have a place...prop flex was a problem with aluminum materials that were run with high HP engines...Stainless and Nibral fixed that...

...the neat thing is that we have some great products to choose from due to lots of R and D. Racing is a great test grounds. When a manufacturer wins on Sunday" we win on "Monday".

...building a FAST and SAFE boat is a challenge...it amazes me that looking at the equation that we now have big boats that run in the 80's with powerplants that are "moderate" and reliable. That means less expense and more fun!

....in addition to prop inefficiency we also have engine to outdrive power losses...the introduction of the Bravo drive for higher HP engines and what IMCO offers too was/is a big step in the right direction....in loose terms the drives are part of the prop inefficiency equation. So when trying to improve prop efficiency try to look at ways of improving drive efficiency....the bigger older drives offered a lot of "drag co-efficient" to the eqaution...#6's are very low in terms of drag as are Arneson's. A surface prop has it's pros and cons but less drag..I would imagine maybe a bit harder to dock?

...so we crunched the numbers...and if you are not happy with how your boats is running...look at the whole picture...and a lot of times it's something like sheer displacement while "planed" that is the cause. Too much boat in the water and not enough air under the hull...if you play with stepping the boat balance is critical...staggering engines a consideration as are other manufacturing consderations to balance everything out from stem to stern and from port to starboard.

....I rode in a Pantera 24' years ago with a 330HP engine and it was with a new modfied pre-Bravo drive in it. The boat was well balanced, narrow for slicing waves like a "razor blade" and it was put together perfectly. You could run at 70 all day in some rough stuff and be very safe. It's an incredible 24' boat...safer than some 26-29'ers I have driven at those speeds....so hull length dosen't matter as much as you think if the boat is designed perfectly....

.....this forum gives you the edge because the folks here look out for each other and keep things in perspective...that's a real plus!
 
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Old 03-13-2004, 12:17 PM
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Thanks, that was some good reading!!
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