Strip build structural strength

Hello All

I put some of Dave Gerr's formulas into an excel-sheet. I am not sure but the results seem very thin and weak (just a gut feeling). Have I don't something wrong?

Thanks any input / J

 

I think it looks reasonable.
And wit strip planking you get more longitudinal stiffness than with other cores.
Whats the estimated totale surface area?
40m2?
So the skin may weigh 350kg?

 

For the noted 22ft hull I have calculated 57m2 surface area including deck, cabin roof, top sides, etc. (very rough).

Best regards / J

 

57 x 8.7 = 500kg, sounds sensible.

 

As a rule, I've found Dave's scantling recommendations a touch on the heavy side, strip included. He openly admits this in the book, as his wish for "healthy" craft. He also offers some possible scantling reductions in several elements of the structure. Have you included scantling proportion increases for speed, service, conditions, etc.?

 

I used formula 4-4 to 4-6 (adjustment for speed and heavy or light displacement) but I have not added or subtracted anything due to off-shore, work boat, etc.

The boat I am considering is a 22ft "wheel house skiff" but with decent deadrise to handle the archapelago chop here on the west coast of Sweden.

Best regards / J

 

Numbers match

I can’t fault your spreadsheet comes out the same as mine. Internal/External heavy sheathed strip uses 12.5mm core.

Mr McNaughton gives a figure of 13mm, though with less glass. Dave Gerr notes that “boats Sn < 1 will not see much benefit” from his heavy sheathing.

Its amusing that Mr McNaughton councils his students not to prepare for his course by reading Dave Gerr. He bases his calculations on displacement while Dave Gerr uses total hull volume. But its uncanny all the way up the scale how the final numbers parody each other.

The ISO 12215 however is “from Venus” . They are focused on bottom pressure (draft) and frame spacing. For your 5 frames they give a very hefty number.

I don’t have any real world experience I’m just running numbers. But for a one ton seagoing vessel planning at 40 knots its going to be taking a smack.