GB 42 freshwater draft

[Mac G]

Our GB 42 draws 4.5 feet in saltwater.
What can we expect her draft to be in freshwater, when we cruise the Great Lakes?
Please advise
Thank you

 

[Caballero II]

Well, can you crack the front windscreen window 1", and the aft salon window 1"? That's what we do in So Cal. Maybe not enough for you, but is more than enough for us. And does'nt bother the boat washers. You know we're not humid here, so maybe you need more circulation.

 

[Caballero II]

OK. Apparently I posted that response to a different thread. Sorry 'bout that.

 

[psneeld]

I think I remember the max is around 2.5%. Temperature also affects draft so cold North Atlantic to tropical fresh I guess would be the extremes.

Unfortunately it's not linear as the volume on many small boat hulls also changes (usually increases) as the boat is further immersed.

 

[Nick F]

Very rough calc:

water plane area 422 sq ft
Water density 62.4 lb / cu ft

gives 2194 lb per inch of draft change


Change in water density 2.5%
Displacement 45,000 lb

gives (same draft) buoyancy change of 45000 x 2.5% = 1125 lb

implies draft change of 1125 / 2194 = 0.5 inch

 

[Mac G]

Very rough calc:
water plane area 422 sq ft
Water density 62.4 lb / cu ft
gives 2194 lb per inch of draft change
Change in water density 2.5%
Displacement 45,000 lb
gives (same draft) buoyancy change of 45000 x 2.5% = 1125 lb
implies draft change of 1125 / 2194 = 0.5 inch

Wow! Very impressive.
Thank you!

 

[psneeld]

Wow! Very impressive.
Thank you!

You can also just multiply the draft by the percentage and get a rough figure if you fudge the greater or lesser immersion volume.

4.5 foot draft times 2% is about one inch. Not sure a 1/2 inch difference when only guessing exact draft based on "current" loaded displacement should change your navigational draft worries.

Now if you own a VLCC.....