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View Full Version : Highest practical hull speed?



Riley
07-11-2001, 11:51 AM
There have been several posts on here about hull speed vs. chine walking in the Classic hulls. Brownie had an informative post on another thread about the limitations of the 16' hull back in the 60's. I'm curious as to what the highest practical hull speeds would be for the Classic hulls, without using trim tabs. I don't have tabs on my 18', and with a high lift prop and a bit of "out" trim it can become a handful if you aren't paying attention, especially above 55 mph. Any thoughts?

blackhawk
07-11-2001, 01:14 PM
That's a good question. Maybe GEOO could answer this one! http://www.donzi.net/ubb/biggrin.gif

Jamesbon
07-11-2001, 02:41 PM
I'll let ya know when the Arneson and 502 are installed....if I live to tell http://www.donzi.net/ubb/wink.gif

RedDog
07-11-2001, 03:00 PM
Even with trim tabs, I highly recommend paying a lot of attention above 55 http://www.donzi.net/ubb/eek.gif.

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Reddog
mailto:tsfloyd@bigfoot.comtsfloyd@bigfoot.com</A>

Forrest
07-11-2001, 03:56 PM
Another way to look at this question is to ask why is a Classic 18 Donzi so stable at high speeds when pushed by a Arneson Surface Drive (ASD) when compared to the same boat being pushed by a sterndrive with its prop buried deep in the water.

In the case of the sterndrive powered hull, as the boat increases, there is progressively less support for the hull. As the prop turns (action), there is an equal and opposite reaction. The force of the rotating prop's reaction acts on the sterndrive unit as a lever arm, which is sequentially transmitted to the boat. Since there is less equilibrium force holding the hull level to the water than there is prop reaction force, the hull is pushed down on one side. As the hull is driven down on one side to a point where the force of the water acting against the hull on that side is equal to, and then greater than, the reaction force of the prop, the hull moves back towards a level running condition. At this point the whole process repeats itself and the oscillations become more frequent as a function of time. To make matters worse, any slop in the drive or steering amplifies this effect. Props with less blade area tend to reduce this effect since there is less reaction torque applied to the sterndrive unit form the prop.

As the prop is moved towards the surface of the water, as is with the case of a raised X-dimension, the torque reaction effect is further reduced since the prop draws some air, effectively reducing its blade area. In the case of the Alpha SS, the lever arm effect is reduced as well. To go one step further, with the ASD there are a number of factors working here to reduce prop torque reaction. First is the fact that the prop is only 1/2 in the water creating the effect of very little prop-blade area, and second, there is little if any lever-arm effect since prop shaft is close to being in line with boat's center of gravity. Finally, ASDs don't usually have any slop in their mounting or steering system.

Since it impractical to do anything about your X-dimension, I would first concentrate on making the steering and drive bushings "tighter" and after that, try some different props.

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Forrest

[This message has been edited by Forrest (edited 07-11-2001).]

blackhawk
07-11-2001, 04:44 PM
A friend of mine has a Four Winns Unlimited 19 (Classic copy) with a 285hp 5.8L EFI and a Volvo Duoprop. It chine walks big-time in flat water. My first time in it it scared the hell out of me. I have driven it quite a few times and I just steer "into" it quickly or tap the trim button down then back up. The steering way is quicker and you don't lose speed, but it takes some practice. Or sometimes, if he has someone new in the boat for the first ride, we just let her go and scare the hell out them. http://www.donzi.net/ubb/biggrin.gif

Flash
07-11-2001, 05:56 PM
Power Boat mag issued a comsumer alert on that boat because of its chinewalking problem. They said that it was the first boat that they felt they needed the public to be aware of how bad it handled!

jaroot
07-11-2001, 09:13 PM
went 55.5 mph (GPS) on just a tad less than flat water tonight (4 or 5 inch chop).. mid 70's and 85% relative humidity, 1010 ft above sea level.. water surface about the same temperature... slight breeze quarting you... trimmed out 3 or 4 taps of the button (no where near the first line on the gauge)... no chine walk what so ever, was an extremely enjoyable ride through the middle of all the fisherman on their pontoon boats. i think she has more in her... well i know she does since my prop still looks like someone took a hammer and chisel to the edges, err i mean sand bar with a little gravel... guess i SHOULD get that repaired eh... i just can't bare to go a week or two without an after work ride.. it's my relaxation! think i need to step the secondary jets up some too, plugs don't have a lick of color to em.. so i'll pop 80's in and try it again... ordered a set of accu-comp autometer gauges from Rex Marine today, those have the be some of the best folks i've ever dealt with. Said they hope to be making the S/S riser again by winter, guess they had tooling and jig problems. anyone out there think 60 is attainable by tweaking? 4 1/2 mph doesn't seem like much BUT... I also agree that playing with the trim is the best way to control chinewalk... i only get it when waves are around and i am trimmed out too far and pouring way too much HP to it... but boy does trimming it way out ever make the girl feel like she is floating on air.. probably cause she is eh...

Jamie

Surrounded by water on 3 sides...

blackhawk
07-11-2001, 10:33 PM
Flash, you are correct about powerboat trashing the U-boats. But it wasn't for the chine walk. It was for other handling quirks. Four Winns made the mistake of setting the boat up for a "performance" driver. Basically, they just did a few things to the trim switches (which are awesome on a Volvo). This is also the way my friends is setup. Basically it allows the drive to be "tucked under" for wicked holeshots. But you have to be very careful in turns if the drive is under too far. You can basically put the boat on it's side!!! Which is what powerboat did, so they called it unsafe. One off of the showroom is not setup this way and will not do this. It's not a Donzi, but it is one bad-ass boat. The mid-range on it is unbelievable.

blackhawk
07-11-2001, 10:40 PM
What kind of speeds are you guys getting with the 5.7 in the 18'? I'm just curious.

GEOO
07-13-2001, 12:41 PM
Riley,
Most boat's, if not all older boat's. Where not designed with a top speed in mind. The top speed was limited by the available engine and drives.
Set up is the key to stability and the more horsepower, the faster you can go with out chimewalk. (in most cases)
An 18 with 260hp will go 55mph with out chime walk and with little trim. If you air out the hull by trimming it out she'll go 60 but will start to chimewalk. An 18 with 400hp will go 65 with little trim and won't start chimewalking until you trim her out to go 70.
The Arneson Drive itself does not create a great ride. It takes time to balance the boat and add fins to create bow lift and stability. I'm sure a Bravo could come close in speed and stability with the proper setup.
A full hydraulic external steering system is a must for speeds over 65!!!
By the way I hit 90.2 and 90.6 on the gps. She'll run 83-85 normally but I hit the right combinations of wind and wave size, and frequency to have the Max speed on the GPS to record these two speeds on Monday!!!

I plan on going to a radar shoot out this weekend I hope to hit 80 do to the limited space. GEOO

Tom on Lanier
07-14-2001, 07:20 AM
Holy smokes George - be careful with that thing!! http://www.donzi.net/ubb/eek.gif

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Tom
"Tomahawk"