Ideal longitudinal center

Hi, new member in the process of widening and stretching a Novi skiff that will be my trailerable commercial dive boat. Curious what a rule of thumb is for locating an ideal longitudinal COB? Its a planing hull with a mostly flat bottom with a small keel. Power will be 140-150 hp outboard. It will not be a fast boat but expecting 25 knot cruise in calm water.

It's a 4' extension (2' hull and 2' bracket) and 1' widen for most of the now 23' length. I have some ability to change the long center of buoyancy now by reducing or increasing the width of the full flotation bracket, or possibly making it a partial flotation bracket. I have the design in Rhino so it's easy to analyze changes to LCOB. It looks like I can move it aft by 1' or more just by modifications to the bracket.

 

The image depicts the LCB at the water plane 54% from the aft end. I'm finding as diplacment increase, the LCB moves aft, which I think is good as additional load will happen mostly on the aft deck. I'm building the boat to to place the LCG over the LCB and provding ballast areas throughout the bilge to make up for my inaccuracy in calculations.

 

Hi,
the 54% part of the waterline lenght aft ist shorter than the 46% forward in your sketch. I don't understand this.

 

Thanks, I corrected it.

 

“On almost any planing hull you can just assume that the center of gravity and buoyancy is 60 to 65 percent aft of the bow,” explained Dave Gerr,
noted naval architect and dean of the Westlawn Institute of Marine Technology

The LCG and the LCB will only coincide when the boat is not moving.
The Center of Lift for dynamic forces and buoyancy moves rearward when the boat is on step. The LCG of the boat will remain in the same position. This could cause the boat to travel with an
inefficient attitude. ie nose down.

A quick look at similar sized boats on trailers will reveal that the axles are not at the 54% from aft as you are trying to design. Not that you would design around this but it
shows that the COG is quite aft on the majoritiy of planing hulls. ( 10% prox of the weight of a boat and trailer is on the hitch)

 

Doesn't that depend on whether the boat has outboard or inboard power?I do understand that the total displacement of the inboard boats is greater and the chances of them sitting on a trailer are smaller.

 

Thanks, that's helpfull. Hadn't considered the trailer but moving the bow stop aft or the axles forward should help.

I'm worried about bow steer and porposing if I try to push the boat too fast. Similar boats that I've seen are known to be slow and additional HP doesn't achieve more speed.

The image is the current state of the design. I'm going to try adjust the angle of water plane to move the LCB/LCG aft.

 

I think that you have missed my point wrt to the trailer. I mentioned it as it is an indicator that the CG should be further back than than your post below:
"LCB at the water plane 54% from the aft end." (Gerr below suggests 35% to 40% forward of the aft end)


“On almost any planing hull you can just assume that the center of gravity and buoyancy is 60 to 65 percent aft of the bow,explained Dave Gerr
noted naval architect and dean of the Westlawn Institute of Marine Technology"

You are trying to locate the center of buoancy at the LCG --- for a planing boat---- and ignoring the effect of the many additional forces that prevail when a planing hull is planing.
You want the boat to cruise at 25 knots, with your shallow deadrise the optimum attitude may be around 3 to 4 degrees up. At 25 knots the dynamic forces are significantly more than
buoyant forces yet you appear to be ignoring these major forces.
You appear to be ignoring the fact that your waterplane volume will be greatly reduced when planing.

Thrust from the engine, live load (people dive tanks etc) boundary layer friction, wind drag the list goes on.

The buoyant and the lift provided by dynamic forces create a center of lift for the hull. A reduced buoyant force and an increased dynamic force.

If the CG and the Center of Lift coincide, you will more than likely create porpoising. You would not want the CG to be ahead of the CL or the bow will drop. Remembering that the CL moves rearward as speed increases.

On our aluminum boats up to 24 feet, normally 21, we had an 11oo pound engine/pump pack at the back of the boat (I am guessing here but would think that the CG of the power pack
was 3 feet forward of the transom) and approximately 45-55 gallons in twin tanks within 4 inches of the transom.

In summary, it is my opinion that you need to consider that:
1) the dynamic forces are more significant than buoyant forces for the boat that you are designing when planing
2) the combination of the buoyant forces and dynamic forces create a center of lift, CL, and that you want the CG to be behind the center of lift
3) to mitigate porpoising you do not want the CG (and the other external forces) to be close to the CL.
4) as speed increases the buoyant forces diminish as the hull lifts

(To repeat there are other forces which affect the boat trim)

An aside: Porpoising has been discussed in several threads over the years and occassionally contributors have suggested that moving the CG will stop this from
occuring. (and it will) When you take a free body diagram, the boat, (2 dimensional within the the x-y plane) with forces in addition to the CG and CB and the only way that
you can balance the forces is to create a couple. Thrust, wind and water drag ( there are others of less significance).

An easy example is when porpoising is just beginning to occur and you can often trim the boat up or down with flaps or with an outboard with trim, and the porpoising will stop.
In this case, you have not changed the center of gravity but rather the CL.

 

If this boat is to be used out of flat water, and the bow indicates you expect rough conditions, you will save some hefty bills by not leaving the outboard way out back to be drowned. If you are a boat person you will know about wakes and following seas, particularly stopping suddenly, at rest or when heading astern, and the motor has to swallow huge gulps of that salt water laden slop zone it is forced to operate in.

 

I dont think that this is much of an issue anymore with the new outboards. The twin Yamaha 300's and the twin Merc 250's air intake is right at the top of the cowling. And we fish big water. (though never for fish that we had to back down on)
If you take in water that high up you have much bigger problems as the tops of our cowling were above the transom height
That being said, both of these setups were mounted on swim grid not a bracket.

Some images, not our boats but show the height of the engine over the transom and the mounting off the swim grid

 

Barry what I've often found with backing up is that waves slap against the outboard and throw water up around the cowling. And then there's catamaran spray.

Yamaha two stroke V6 outboards were breathing through the top of the cowling and dropping like flies on Kevlacats before the boat manufacturer installed a ducting system from within the boat.

I suspect that four stroke outboards take longer to exhibit water entry entry issues due to their shell bearings handling crankshaft pitting better than roller bearings, which destroy engines after failure. Also diminishing engine oil pressure is likely to warn of an issue before it becomes a catastrophic failure.