CG Calculations, Trim, Bouncing, Driver Weights etc.

[smiley]

I' am getting back into boat racing after a respite since 1963 with the grandkids. These six kids come in all sizes and weights thus CG becomes a consideration in trimming. Recent experiences with aviation taught me how to calculate CG for various components and from that experience I make the following assumptions:

Once a boat (runabout) is properly trimmed for smooth water you calculate the CG for that condition. Then you trim a boat for rough water and calculate that CG.

Assumption Rough water optimum CG is forward of smooth water CG

Assumption, regardless of driver weight etc. That CG point goes with the particular boat, prop and setup configuration. This is the optimum trim setup for the boat.

Thus, if you are going to have multiple drivers of varying weights in a boat; you will have a wide envelope range of CGs for the drivers. Performing multiple calcuations, I have discovered that lighter drivers with multiple weight distribution anchor points on the boat have the widest fixed CG range.

Larger drivers have a narrower fixed CG range but a wider CG range in live action. They can move their weight around inside the boat and affect performance to a greater degree in real action.
Lighter drivers need to have their boat more closely trimmed for the existing conditions.

I have seen this play out to heavier drivers who are 30-40 lbs overweight in A class handling the boat with authority in varying water conditions while the lighter drivers need to throttle back in rough water.

From the above I surmise that a more closely optimally balanced runabout is more beneficial to the lighter drivers.

Also given an optimum CG point for each boat and water condition. You can calculate weight distribution on paper (ie computer) for each driver and not need to real time test each setup for each driver and still get the same results.

What are your thoughts on these assumptions and am I missing anything here in this thought process?

[Mark75H]

That would assume all props are actually identical and boats likewise


Knowing that these 2 variables are not controllable, you are still obligated to test, test and test. Assume nothing.

[smiley]

You are correct.

I assumed the same boat, prop, setup, envirionmental conditions. When you change drivers you need to adjust weights to maintain the same CG to repeat the same trimmed ride. That was my premise. That can be mathamaticly calculated thus eliminating the need to test each driver for the same conditions.

Now you change prop, setup, weather conditions etc. and it is test, test, test. I don't know of any way compensate for that. If you properly docucument all these tests and know the optimum CG for each condition, you can then develop a body of knowledge to proactively adjust the CG for anticipated conditions.

Smiley

[Mark75H]

In J you do not get to use your own props, you use APBA provided props. Doesn't that put you back at square one?

[smiley]

You are right again with respect to J. In fact everyone has the same disadvantage in that everyone must use a prop provided by the racing club sponsoring the race. According to Craig Dewald, he can make a J copy from his J pattern that we can use for testing but of course we all know that it is near impossible to make an identical prop with the same characteristics. Hence everyone has that problem. I was told that there ia about a 2 mph variance in the club props so it is the luck of the draw.

Now with AXS and A that is a different story. We have control of our props and setups.

In private aviation, as a pilot, we are required to obtain all necessary information for a flight. One of those items is CG calculations for fuel, baggage, pilot and passengers to insure we are not over gross and within the operating CG envelope. For optimum speed, you want a rear CG loading. This gives you the fastest speed but also the least stability and the most difficulty in landing with adequate control at slow speeds. Everything comes at a cost.

Forward CG loadings require down force on the horizontal stabalizer which increases drag because you must now lift that downforce plus the weight of the plane and the increased angle of attack on the wing increases drag.
This also becomes a dangerous situation when landing at low speeds. There may not be enough downforce on the stab and elevator to prevent a nose wheel first landing which can be very dangerous. I assume we should have our boats properly trimmed as well for similar reasons.

Smiley

[Skoontz]

Don't get too confident with J props and duplications. We have always had difficultied planing using J props from the club boxes, and never had an issue using our own J prop. 99% of this problem was using an AX/A boat to race J, when a decicated J boat needs to be used to run optimally in J class.

The inset transom of the A/AX boats is the largest issue, also the dedicated J boats are a tad shorter, like 8" or something.

[smiley]

I have heard that from other sources as well. As I understand it, and please correct me if I am wrong, but the J setups need more distance from the bottom trailing edge to the prop. This is so, when stepping onto the plane, the water flowing from the trailing edge has a chance to rise, or stated another way the prop has more water to bite and increase thrust to push over the top. The A and AX setups prefer closer to the transon so the rear can be lifted up and create air under the boat!!!! Is this a correct assumption?

Smiley

[Tomtall]

The A and AX setups prefer closer to the transon so the rear can be lifted up and create air under the boat!!!! Is this a correct assumption?

Correct - Some things to consider and it's only my observation setting up my sons boats.

1. You must get thru the corners and pop the bow as fast as possible coming out of the corners. Weight placement plays into this.
2. Body weight does play a roll in acceleration out of the corners.
3. Hydros like weight midship/back. Runabouts like weight midship/forward.
(on our boats)
4. Every prop change usally requires a little different set-up.
5. No two drivers drive the same or always like other peoples set-up.
It's a personal feel thing.
6. I have seen many people fight weight placement with no luck in changing boat handling problems when the real problem is the hull itself.

Good Luck and keep good records when testing. :)

[smiley]

"1. You must get thru the corners and pop the bow as fast as possible coming out of the corners. Weight placement plays into this."

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.

"3. Hydros like weight midship/back. Runabouts like weight midship/forward.
(on our boats)" 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?

4. Every prop change usally requires a little different set-up.
5. No two drivers drive the same or always like other peoples set-up.
It's a personal feel thing.
6. I have seen many people fight weight placement with no luck in changing boat handling problems when the real problem is the hull itself.

I can see that. You must have a true bottom else any contact intermittant or constant with the water will create erratic responses. This is the same in high precision aerobatic performance of real and model airplanes. All lifting surfaces in both aviation and boating must be straight, true and rigid for predictable and efficient performance.

Smiley
Good Luck and keep good records when testing.
__________________

[smiley]

"1. You must get thru the corners and pop the bow as fast as possible coming out of the corners. Weight placement plays into this."

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.

"3. Hydros like weight midship/back. Runabouts like weight midship/forward.
(on our boats)" 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?

4. Every prop change usally requires a little different set-up.
5. No two drivers drive the same or always like other peoples set-up.
It's a personal feel thing.
6. I have seen many people fight weight placement with no luck in changing boat handling problems when the real problem is the hull itself.

I can see that. You must have a true bottom else any contact intermittant or constant with the water will create erratic responses. This is the same in high precision aerobatic performance of real and model airplanes. All lifting surfaces in both aviation and boating must be straight, true and rigid for predictable and efficient performance.

Smiley
Good Luck and keep good records when testing.
__________________

[Tomtall]

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.

[Tomtall]

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!

[Mark75H]

Boat #3 is someone from Wisconsin ... Washington State's letter is R, and from the time period, more likely alky than Mod

[Tomtall]

Sam - My bad. Thanks for the correction.

[smiley]

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

[Tomtall]

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.

[Mark75H]

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.

[Tomtall]

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

[smiley]

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

[smiley]

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

[smiley]

Tom,

Question, You stated that CG changes with the classes. Does that mean the CG must move forward for the faster speeds to compensate for the increased bow lift?

In turning, how do you test for optimum cornering speeds off a course? Also similarly, how do you test acceleration for different setups? In the old days I just tested for top end.

[mercguy]

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



you buy Norm's stuff???

[smiley]

Daren,

You are correct. We seem to have a connection with boat racing and collegiate wrestling. The neat thing is Norm is going to build a plane. I have been there and done that. I am returning to boat racing and he is a machinist. So we will probably be able to support each other in these new ventures.

Do you know of any guys in SOA that can help me train these six kids. Stuff like pit ettiquete, race strategy and tactics, how to time the starts, and how to turn wide and tight corners etc.?

We live in Kitsap County so crossing the ferry to the Seattle side gets expensive and takes a lot of time. The hydroplane museum is too far for us on weeknights.

Smiley

[mercguy]

Daren,

You are correct. We seem to have a connection with boat racing and collegiate wrestling. The neat thing is Norm is going to build a plane. I have been there and done that. I am returning to boat racing and he is a machinist. So we will probably be able to support each other in these new ventures.

Do you know of any guys in SOA that can help me train these six kids. Stuff like pit ettiquete, race strategy and tactics, how to time the starts, and how to turn wide and tight corners etc.?

We live in Kitsap County so crossing the ferry to the Seattle side gets expensive and takes a lot of time. The hydroplane museum is too far for us on weeknights.

Smiley

I am sorry to see Norm go, he was a great guy, excellant mentor for new drivers and all in all GREAT PERSON for local boat racing.

as for helping your mission on that side, your best bet is Dwight and Zach Malhiot (Bremerton area)and John Erickson (Bainbridge Island)...........you should really attempt to attend an SOA meeting. They are held the last Tuesday of each month and you will find all the answers and help you need there............

[smiley]

Daren,

I called John tonight and he was enroute from Arlington and stopped by. We had a great visit.

Yes, my opinion of Norm is the same. I hope we can keep him involved somehow. Perhaps I can get him to come to Kitsap lake and help us out. That way he can still influence the group.

Shucks, I just missed the meeting this last Tuesday.

Smiley

[mercguy]

Daren,

I called John tonight and he was enroute from Arlington and stopped by. We had a great visit.

Yes, my opinion of Norm is the same. I hope we can keep him involved somehow. Perhaps I can get him to come to Kitsap lake and help us out. That way he can still influence the group.

Shucks, I just missed the meeting this last Tuesday.

Smiley



FYI............I would not be so concerned about CG, etc right now with new comers..............they need to learn how to make starts, turn and get a "feeling" for the boat...........only way to do that is to get on the race course or go testing.........BTW.....Reg10 has the largest J participation in the country and LOTS of people there to help and give guidance......several of our races in SOA, J hydro had the biggest turnout!!!!!! Yep, there have been elims in J at local races..........

[smiley]

Eliminations in J? That is awesome. That has got to be a rare occurance these years in racing. In the 50's, I believe it was more common.

Right on about boat familiarity etc. I have disscovered that the closest lakes are Kitsap Lake in Bremerton, WA. 20 miles away.with speed limit under 45 mph and underwater exhaust. They have a bunch of bouys in the lake and require a left turn circuit. Perfect set up for training. We will need to use KG7 standard lower units with transom jacks to keep the exhaust underwater. (Training with training wheels and save the racing equipment!!) Mason Lake in Belfair, WA. No limits there to we can use the Evinrudes and Mercury. About 30 miles. Lake Pleasant near Forks, WA . UGH about 2 hours drive. Not a real option.

What are the lakes in Pierce and King co that the SOA guys test on? They might be a better option. Is Spanaway Lake an option?

Smiley

[smiley]

Eliminations in J? That is awesome. That has got to be a rare occurance these years in racing. In the 50's, I believe it was more common.

Right on about boat familiarity etc. I have disscovered that the closest lakes are Kitsap Lake in Bremerton, WA. 20 miles away.with speed limit under 45 mph and underwater exhaust. They have a bunch of bouys in the lake and require a left turn circuit. Perfect set up for what you mentioned Daren I can use the KG7s for that. Mason Lake in Belfair, WA. No limits there. About 30 miles. Lake Pleasant near Forks, WA . UGH about 2 hours drive.

What are the lakes in Pierce and King co that the SOA guys test on?

Smiley