Draft depth, in reality?

[cantabrian]

Hi, everyone. I'm new to the group; taking ownership of a 1982 Compac 16 tomorrow! Really excited. I've been looking for a marina slip to keep the boat for the rest of the season, and a marina close to me offered a shallow slip by their bulkhead. I'm going to go by today at low tide to see how shallow it really is, but that made me wonder: I know that the draft of the boat is 18 inches, but how accurate is that in practice? Is it deeper when people (i.e., more weight) are on the boat? Basically, I'm wondering what the minimum depth can be for a slip. At least 20 inches? 25 inches? I suppose I should allow for a tide that's lower than today's low tide.

Thank you! Glad to be joining this community! I've already learned so much from looking around here

[Cats Paw]

Welcome to the group.

A moderate wake in the marina or the waves on open shallow water can bottom the boat out. I prefer my compact to  nest in no less than 3 ft.   

[slode]

3' is very conservative.  I've had my Eclipse in 2' +- a couple inches and it was nothing.  Especially in the protected waters of a marina.  Only downside is you have to maneuver with the rudder up at least part ways.  that's the beauty of a shoal draft keel.  They don't call it that for nothing!

[cantabrian]

Thanks for your replies! The slip I looked at was about 17 inches in low tide. And since people there told me that low tide is even lower sometimes, it's clearly not a great place to be. I did wonder, though, if it would be ok to keep there so long as I didn't go out at low tide. The keel would be in the mud, but it can carry the weight, it seems (since it carries a lot of weight on the trailer). But I'll keep looking rather than test out that theory!

[Renae]

Is it deeper when people (i.e., more weight) are on the boat?

Yes, but not much.  Trusting our old pal Archimedes, in order to displace an additional inch of water, you'd need to add quite a bit of weight.  Let's suppose, for the sake of easy math, that the cross-sectional area of your boat is 6 m².  It might be a little more or a little less, but this should get us close:

6 m² x 0.025m=0.150m³ = 150 cubic decimeters = 150 L

150 L of fresh water weighs 150kg.  150 L of salt water weighs 153.5kg

Your boat will settle 1 inch (2.5 cm) deeper if you add two average-sized adult friends or 1 very big one.

IMO, wind/waves/tide/local water levels etc. are going to make much more difference on clearance than how you load your boat.