A Theoretical theory

[Techno]

About balancing the boat and how much set back that is needed without a passel of plates. Doing it with a more scientific method rather than the Edison technique(repeated experiments with small changes instead of figuring it out and doing one experiment)
I think I figured out how to find the center of balance but it seems too simple to work.

With one person in the boat( to replace the weight of the driver) you use your own body as a moving mass. If your forward of the Center of Gravity the bow goes down. If you move aft of the CG the bow goes up stern goes down. If your at the CG then the boat would just sit deeper in the water all around.
This is done just sitting still in the water, not moving. Once you have the CG of the boat, figuring out where the CG should be is(kind of) easy. and how to move it there is easy. I think that the at rest CG is different than the flying CG but is probably pretty close.

Now does anyone have any idea where the CG should be? or moved to? Or could the thrust angle of the OB be indicating it somehow?

[Rickracer]

Hi Techno, first let me say that I really appreciate your inquisitve mind, I have more bookmarks that came from you than anybody else. I think that all of your ideas about how to calculate CG are valid with one exception. How the boat rides in the water is totally different when you are up and riding on the pad, or packing air in a Mod VP or tunnel style hull. The method you have suggested could be useful in determining how much change you have made though, kind of a measuring stick. A ruler won't tell you how much to move the weight, but does allow you to measure how far you moved it, and your method could help determine what you have accomplished by moving (X)lbs (X)number of inches or feet. Please let us know how this works out for you.

[BarryStrawn]

Techno,

Seems to me your method would be affected by changes in the hull displacement caused by the different shapes of the bow and stern. When you move the bow down, it would displace a different volume of water than the stern moving an equal amount. Thus screwing up your data by measuring two things at once.

If you have access to scales, weigh the main trailer wheels and tongue jack seperately. Make sure all the wheels are dead level. No need to have anyone in the boat. Put the motor at neutral trim or wherever makes sense to you. Weigh with and then without the boat on the trailer. Then measure where the boat sets relative to the wheels, subtract the trailer weights and do your baseline arithmetic for total weight and CG location. You can then just add in any payload changes and calculate the change. This only provides the linear fore and aft location. To add side to side, weigh the main wheels individualy to give an (X,Y) location on a plane. To also determine the CG height for a full (X,Y,Z) measurement, you will have to tilt the rig while weighing.

As far as a way to calculate where a CG should be, I have no idea. I suppose you would have to first quantify all the hydrodynamic and aerodynamc forces as they varied with speed and attitude. I don't think that textbook on vehicle dynamics has been written. Cut and try experimentation to find where the boat performs as desired would be my only answer. Careful measurements and record keeping should provide a thinking man with some insight. For specific hull types, lots of folks will probably have good answers.

Barry

[Techno]

The idea is to note when the bow or stern go down, not how much. but I think your idea is easier. I can't actually weigh my trailer because I'd have to leave the boat. I already figured that problem out so I could find the boat weight.
A 4-5' board-lever. Just put the board so 1' is under the trailer and 3-4' sticks out of the side. put a scale under the longer end and figure the weight. A block would be under the other end as a fulcrum. 4' board at 3:1 and 1,000lbs for one trailer wheel = 333lbs on the scale. Under the maximum of 400 for the scale I have. Rig weighs 1 ton now on a truck scale. Tongue about 30 lbs.
I can weigh the whole rig so I just have to compare the lever readings to the actual scale reading. then weigh the empty trailer with the lever method. I can do both, find my boat weight and it's CG at the same time when I do this.

If/when I get my boat running, I want to see how it runs at what jack heights then remove the jack plate and guess the right height and see if performance inproves without the set back. '90 STV.
Thanks.

[BarryStrawn]

Ah yes, the old lever on the scales trick. Can work but hard to set up with precision. I'd just make friends with the person who runs the truck scale. Buy em some donuts or something so they will tolerate your little exercise. Have a friend watch your boat in a nearby lake while you run the trailer to the scales. Or if you know of a decent car race shop, they probably have or know of someone with a set of wheel scales. Either is likely to be a lot more accurate.

Another thing, I would suggest you add some weight to the tongue of your trailer. 30 lbs on a one ton rig is very light. May feel OK but can get real goofy in an emergency.

Take care,

Barry

[crowfly]

I think when you've found C.G. you're still going to do the same thing you would have done before making this discovery. Move everything to the rear, keep adding set-back 'till performance falls off, keep jacking up motor 'till performance falls off or you run out of water pressure.

[Techno]

The idea stems from the problem that inches of setback don't mean anything. Look on this board and you will find a bunch of different setbacks on what should be similar rides. If someone is running a 450lb. OB and someone else a 350lb. OB then they really can't help each other.

If you go by CG you now have a reference point so you know whats happening. What I mean is you know what the inches change means on the boat balance. Without the boat reference you've got nothing.
If you know what a good CG for your boat is then adding setback becomes a snap because it remains the same no matter how the boat is rigged.
If enough boats where listed with CG instead of setback this is something that anyone could use to tune their own rig.

If you add setback to a boat its complete guessing.
If you relocate the CG then it should only take three tries to achieve the correct set back. that is assuming you do the math and don't move in 2" increments.

I think that the trim angle of the OB really make a difference with all this but I don't think anyone is taking advantage of that either. Reading the angle of thrust for the changes made and factoring this in.
Using the trim and putting probably a substantial wieght in the back to act as a setback, you could do away with swapping setbacks and just add or subtract wieght. Then re-setback the OB to achieve this balance figure. this is really where the known CG comes in to it's own

[crowfly]

Some props have good stern lift, some have good bow lift, some have both, some have niether. To list a good c.g. point for a particular hull would actually require several c.g. points according to the prop & what you intend to do with the boat. Bow lifting props can stand to have their c.g. further foward & stern lifting props can tolerate a more rearward C.g.

[W. Tripp]

For a good starting point as to the balance of weight I usually have to decide if I want to launch well or if top end speed is what I am after. In general terms, more setback will add to top end speed - to a point. Less set back improves the launch by reducing the amount of leverage the prop can provide and will prevent the prop from digging in deeper before rising to the surface - to a point.

If I am trying to balance the boat, I will run the hull with as much weight as I can move centered 1/3 of the hull length from the transom. Although I have found that %27 of the way is usually a better starting point. I then make a video of the boat launching and running at speed. Then adjust the weight fore and aft to get the attitude you are after.

Then you can move a small amount of the weight to corner points to correct a problem with the way a given hull rides at speed. The farther the weight is moved to the corners, the less weight that is needed.

Then work with propshaft height. find a good low starting point and move upwards until you no longer see improvement. Going higher will then only reduce the control of the boat.

Each prop will require a little work to find what works best. Also remember that the distance a prop sits from the nose of the gearcase can make a difference as well. The rear of the hub can be trimmed and washers can be added between the hub and the thrust washer to move the prop forwards and back. Just watch the amount of nylon on the front of the prop nut.

Then look at blueprinting the pad/running surface.

This is what works for me, I hope it helps.

[Delta]

That one was definately worthy of being put in my "save" folder!Much appreciated!
Delta

[Dewey]

How will the prop distance in relation to the nose cone effect the boats launch or top speed? I am going to test a Chopper and a Mirage prop this weekend and I notice the chopper sits closer to the hub than the cleaver I am taking off. The cleaver is all the way at the end of the propshaft and barely catches the nylon.

How do the Mirage props work on heavier boats? Do they have much rake/bowlift?

Thanks

[W. Tripp]

Unless you can change the diameter of your gearcase, the nose angle and length of a gearcase in front of a prop has everything to do with how much water a prop gets and when it gets it. Especially when it is totally submerged when launching the boat. The nose of the gearcase forms a compressive wave and the water will tend to flow out away from the sides of the case before coming back in towards the prop. In bad cases, a bubble of air will be trapped against the side of a case in a low pressure area and allow cavitation to blister the case or even the prop.

The theoretical top speed of sail boats and large cruisers (displacement hulls) can be figured from the total wetted length of the hull. The longer the hull the higher the speed. This is why nosecones help produce less drag on a gearcase and allow the water to come back against the sides of the case and feed the prop better.

By slightly moving the prop forwards and back you can find the sweet spot that a given prop design likes better. Will it make a difference on a lake boat? Probably not. Having a good prop worked by a pro WILL.