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Thread: CG Calculations, Trim, Bouncing, Driver Weights etc.

  1. #11
    Tomtall
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    Does this mean you shift your weight to the rear for straight-a-way attitude near or at the end of the turn for optimum acceleration and top end.
    The Ideal balance point will allow the driver to control the ride attitude of the boat by shifting body weight front to rear at ANY given point thru out the course. If you watch most veterian drivers they will pull UP (only up, without rocking) on the steering wheel when exiting a corner to help POP the front of the boat.(boats that are bow heavy will not allow the driver to do this). Then they are at that back of the boat all the way down the straights. You need enough saftey factor to your weight placement however to hopefully bring the bow down if you get in trouble down the straights by moving up on the wheel to bring in down, but not slam it down. We rig our boats with motor and fuel race ready with no added weight and then balance it on a saw horse with a broom handle mounted to the top of it. Find your balance point. Mark this point with a magic marker. Know place any weight over that balance point to start out with when first testing. Adjust accordingly from there to drivers feel of the boats handling thru out the race course. Remember however that different race courses will sometimes need weight moved around depending on the number of bouys,wind conditions/direction and how tight the turns are. If you notice the fast drivers thru the turns they will carry the bow high thru out the turn with just enough turn fin grabing in the turn to keep the boat locked in place. The following picture is of a Sorenson runabout. The boat is entering a turn (as noted by the turned engine). The boat is riding high with just a small amount of turn fin in the water. This set up had 15 pounds total weight added to make weight. It was placed at the centerline of the throttle. 10 poinds under the throttle and 5 pounds on the right side at the same centerline. The boat is 1 pound over weight at the scales when checked after the heat.The driver scrubs very little speed thru the corners balancing the boat like this.(on a flat turning runabout). This boat has raced in "J" class,AXSR class,ASR class and FAR class. It has run well with both a cleaver and a tulip blade style props. Weight has always been at drivers knees /forward depending on the class raced and speeds.
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  2. #12
    Tomtall
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    Does this mean that for runabouts you want to shift some of the weight load towards the front of the boat and have a flatter angle of attack to the water? More wetted surface on the water and/ allow more lift from the rear by the cleaver prop and up thrust? Doesn't this get squirrly?
    We have found with our runabouts (both marathon and closed course) that when running in the slop (waves) they tend to POP the bow up easy when hitting a roller. By placing the weight midship/forward it tends to calm this tendency down. I have seen some runabounds run 5 lbs. of lead right in the nose of the boat to make them carry well and lay down. You still want to be running only on the back trailing edge however down the straights so you have to experiment. Props can make a huge difference on how a runabout handles. But you still see people running both styles of props. The prop that pushes the boat STRAIGHT ahead from a thrust standpoint is what I prefer when setting up a boat for balance. This tends to give you a neutral set up that you can always go back to if things start acting strange. To much tail lift or bow lift from a prop can just make set up changes difficult. Yes to much tail lift will make a boat squirrly and can also cause them to spin out in the corners in sever cases. This is one of the reasons people tuck the gearcase so they can get hydrodynamic lift from the gearcase rather than the prop. Most Kelo record set ups run alot of tuck to get stern lift and relie of steeper tunnel angles to pack air to lift the bow thus turning the boat into a wing. I'm talking hydro class here. However trying to get this kind of set up thru a corner is a whole nother story. This is one reason the prop walkers that were so fast in the 1960's never fared overlly well in cuircut racing. They couldn't get them thru the corners. Fast as stink down the straights thou.

    It's all a balancing act and unfortunetly I do not feel personally that formulating out C/G will always be an exact science in boat racing as there are so many varibles.You may get close but in the end the driver is the biggest factor of what he/she likes for his or her set up. Just my two cents. Everyone has their own ways of doing set up. This is what I have learned from trial and error and from others and am just passing it your way.

    Some more pictures of ride attitude of different set ups.

    Boat #1 - "J" class. slower speeds. Restriced to class prop.Weight on transom. Body weight on transom thru out course. Bow drops in turn when speed is scrubed of going thru corner (lose of speed) thus putting fin down to grab thru corner.

    Boat #2 - AXS class - faster speeds. Any style prop. This prop was a cleaver. 10 lbs of lead on transom. 10 pounds under drivers knees. Small sponsons (shoes) up front (lighter bow weight). Great lift but sacrifice some turning ability thru corners in the rough stuff.

    Boat #3 - Someone from Wisconsin and from the time period, more likely alky than Mod. Prop walking.

    Boats #4 - "A" class Runabout pack coming down for the start. Note hanging attitude of hulls. Nice!

    Boats #5 - Ron Hill and Frank Zorkan. Note body positions. "All out, don't give a damn what happens, boat racing". Love it!
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  3. #13
    Sam Cullis Mark75H's Avatar
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    Boat #3 is someone from Wisconsin ... Washington State's letter is R, and from the time period, more likely alky than Mod
    Since 1925, about 150 different racing outboards have been made.


  4. #14
    Tomtall
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    Sam - My bad. Thanks for the correction.

  5. #15
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    Tom,

    You have made my day with your explainations and especially the fotos of the boats. Gets your heart thumping. A very good primer for newbys and my grandkids will understand it.


    I plan to print out these responses in booklets for a reference manual.

    I saw Hubert Enthrops cabover with his six banger mercury in he late 50's early 60's. It was a sight to see daylight under that hull just like the foto you have here. We were all mesmerized. He tested his boat in the boeing wind tunnel. That was the beginning. Classic.

    I understand the physics of sitting upright.. You impart a negative moment with your body in the wind to pop the bow up and break it loose. And Yes I agree that you need to adjust the CG for all conditions and expected conditions.

    Thanks.

    Smiley

  6. #16
    Tomtall
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    Default Hu

    Smiley - Your more than welcome.

    Here are some pictures of Hu in Mexico 1962 running his 4 cylinder OMC.

    Photos from Frank Zorkan picture archives.
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  7. #17
    Sam Cullis Mark75H's Avatar
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    I personally specifically asked Entrop about wind tunnel testing ... He said, no, he was not allowed to use the wind tunnel, he just built airplane parts tested in it.
    Since 1925, about 150 different racing outboards have been made.


  8. #18
    Tomtall
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    Bob - Something to consider with your grandkids is to hook up with your local club and work with them on mentoring new racers. The club we belong to MHRA (Michigan Hydroplane Racing Association) have put some great programs in place to do just that. The programs have had great suckcess so far (three new racer families in one year) and are growing (club owned enclosed trailer with three fully riged race boats). Having the veteran drivers work with new comers not only grows the sport but hopefully makes the drivers safer on the race course.

    View our clubs programs at this link.

    http://www.michiganhydroplane.com/NewRacers.html

  9. #19
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    Mark 75

    Amazing, how rumors become legends. Sometimes I prefer legends. More interesting and adds color to the memory. Oh well, back down to earth. At least we can be aware that Hu certainly had an advantage with his knowledge of aerodynamics in the early years.

    Smiley

  10. #20
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    Tom,

    That sounds fantastic. A club owned setup with mentors. Sure beats investing in setups only to find the kids don't have the desire to participate.

    I have been in model control-line precision aerobatics competition for the past seven years and newcomers are rare. The building time, commitment is too much for them. You spend many hours building a plane and may crash the plane in 30 seconds. A steep learning curve for many to achieve continued success. My grandkids each built a plane and we flew them last year. Much fun but no takers for continued participation. When approached about boat racing, they jumped on the idea. Now I am getting them involved with the shop end of the sport. Next spring we get onto the water.

    A senior racer here in Puget Sound has sponsored two youth over the years with a trailer outfitted with two hydros and a runabout, two motors and all equipment. He retired the sport this year He and the young man are now going to build a sportplane with a Rotax motor. Sounds like great fun.

    I was forutunate to acquire all his equipment to get a jump start and will add to it to support our six kids and myself.

    The Seattle Outboard Association has sponsored building Sorensen hydros each winter for newcomers. There were six Sorensons built last winter at the hydroplane museum here in the the NW. I guess there will be more built this year. The APBA and clubs have done a great job bringing in new blood. What other ideas around the country have been tried?

    Smiley

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