Center of gravity in relationship to center of bouancy and stabilty

[jlapratt]

I am trying to determine the proper placement of a center console in my flat bottom skiff (San Juan Dory). Assuming static weight distrubution (only my fat duff in the boat - no passengers) is there an omptimum placement of my weight, and hence the center console, in relationship to the center of bouancy and the center of gravity?

The easy answer is to steer the boat with a tiller extension and move about until I find the optimal place that provides both bow attitude and stability. I would prefer to install the console et. al. before launching the boat if possible.

The designer couldn't help as this is a deviation from original plans...liability you know. I respect the input from this forum.

Jeff

[Ian McColgin]

Anywhere from your weight a little abaft the CB to further back.

I'm not sure how this design really looks so I'll shoot for two different targets.

If she's really a dory, narrow bottom and narrow at the stern, she won't plane. At displacement speed, you're probably best off with even loading but if you err, you want to trim her down by the stern a tad. You do not want to trim any boat down at the head. So lay it out such that counting motor weight and fuel and all - anchor, lunch, your weight, fishing gear or whatever - it's about even. There's enough small stuff that can be moved around that basicly this comes down to placing your weight enough ahead that you nicely balance the engin's weight. If you err, make it aft. If the error is at all inconvenient, then you can at least add a longer hose to the fuel tank and move that forward. Or something.

If she's really a flat bottomed skiff, she'll get up on a plane and in that dynamic configuration, the effective bouyancy center is very far aft. You need the bow a little light and the stern area can support lots of weight. In this case, put your weight as far aft as make her still ok - a bit stern heavey but ok, at displacement speed. It'll be nice on the plane then and you won't have to scoot forward as you come off the step.

G'luck

[jlapratt]

Ian,

She is indeed a flat bottom skiff, no rocker and essentially no narrowing at the stern. The configuration I had in mind results in my weight being right at the CB. This is just about the fully loaded CG also.

I will look at moving the console closer to the stern. I really appreciate your advice. Thanks.

Jeff

[Ian McColgin]

We have a running gag around the boat shop that starts with "All ya gotta do is . . . "

Well, all ya gotta do is put the console on a track with lines on each side laid out rather like the Haarken traveler control lines and you could adjust the console location in motion.

Just be careful not to kick the jam cleat while advancing the throttle.

G'luck

[TomMcKinney]

Look at the pictures of the glen-l wee-hunk, and see where people have placed their consoles. Its a 16 ft flat bottom dory skiff, much like the San Juan I think. www.glen-l.com (http://www.glen-l.com)

[imported_Conrad]

The CB is usually calculated with the boat at rest, on her lines. As your hull gets up on a plane, the CB will move aft, so a bit further aft than you might originally have thought would probably be a good idea. The penalty for being too far forward is greater than the penalty for being too far aft, so plan accordingly.

[jlapratt]

The Glen-L Wee Hunk lines are very similar to the San Juan Dory and gee wiz some of those consoles are way forward. Looks like the Glen-L plans even show design placement way forward of CB.

I trust the advice from the forum but also know that Glen-L designs have a solid reputation. Hmmmmmm.........looks like I'll just have to test it myself with a tiller extension and shifting my weight. I will post my findings for all to see. I expect to launch her in the next few weeks.....so stay tuned.

Thanks for all of your input.

Jeff

[John E Hardiman]

jlapratt;

Sorry I've been away for a few days...

...Ahhhh When a boat is floating in the water, not moving, the center of gravity is always over the center of buoyancy. And when a boat is on plane, the center of gravity is always over the center of up force which is a combination of center of buoyancy and center of lift.... the boat will trim (up or down) to make it happen...

I assume that the question you're asking is where should you put the console to minimize the trim effects. Ian McColgin is in the right area, common wisdom says the "neutral point" will move aft of her "static" CG by ~ 1/4-1/3 the length to the waterline.

Hope I'm not to late with some help...

[jlapratt]

John,

Is that an aft shift of 1/4 - 1/3 of dynamic or static waterline? The design CB is ~5' forward of the transom. With a static waterline of ~13.5', that would put the dynamic CB a foot or two forward of the transom. Sounds that if I want to place the console forward of this point (which is necessary for effecient deck plan) I should pursue motor trim or trim tabs.

Jeff

[John E Hardiman]

jlapratt ;

Now that I've re-read what I wrote I see I didn't state it well....Never drink and type.

The "neutral point" is the point at which you can place all the lift generated by the hull at the given speed. It is, as you pointed out, usually located 1/4-1/3 aft of the leading edge of the immersed (i.e. dynamic waterline) hull (further aft on deep V hulls due to the delta planform of the waterplane).

A quick approximation of where the cg should be on plane would be to assume that 1/2 the weight of the boat is supported by lift. Draw in a water line that displaces 1/2 the weight of the boat at about a 5-7 degree pitch up angle on the hull. Calculate the lift force at the designed speed (lift force ~ 1/2*rho*V^2*waterplane area*sin pitch) and the buoyancy. Adjust the waterline up or down until the full weight of the boat is supported. Calculate the center of the two up forces, lift and buoyancy. The cg of the boat should be approximately here.

A quick glance at the method above will show that as more of the hull is supported by lift, the immersed hull moves aft, bringing the center of lift and center of buoyancy aft with it. As I was told once; "In a high speed planing boat, you can never have the weight too far aft".