# Engineer wanted for preliminary scantlings

Discussion in 'Class Societies' started by Brentmctigue, Jun 16, 2020.

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### AlikSenior Member

What method is used for resistance calcs?

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### fallguySenior Member

You l/b ratio will be much better. Hull speeds better. Aesthetics better. I think all around a much nicer boat.

The ratio I cited is how much does she sink for added mass. So for an inch below waterline; she can take say 200 pounds, or same thing in metric if it is more sensible for you.

It would be a good metric to know to see how weight sensitive she is. And it is probably in the numbers, and these guys can pull it out, but it is above my pay grade.

My opinion worth pric

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You can easily find it from the WPA value:

TPC = WPA x 0.01025
TPC = 2.904 x 0.01025 = 0.0297 = 0.03 roughly!

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### fallguySenior Member

33.67 centimeters per ton

or 33.67 centimeters per 1000 kg

or 3.3367 centimeters per 100 kg

or

8.55 inches per 220 pounds

or

25.73 pounds per inch

She is very weight sensitive if I got my conversions close. I think this number gets doubled for two hulls. 51.5 pounds per inch is pretty low.

If he adds 1.5m to the boat, he gains quite a bit of waterplane area. About 1 full sqm. Call it 3.95 sqm. ..times the 0.01205. Tpc is 0.04 or about a 135% gain or about 70 pounds.

5. Joined: Oct 2008
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Almost...

TPC = Tonnes Per Centimetre

TPC = 0.0297 tonnes per cm or => 29.7kg per cm, roughly 30kg per cm.

or Ton per Inch
TPI = WPA/420

so 2.904 m2 = 31.26 ft2

TPI = 31.26/420 =0.074 ton per inch.

At 2205lbs per ton, 0.074 x 2205 = 163 lbs/inch

Edit
Simple double check...

29.7kg = 1cm. And, 2.54cm = 1 inch. Thus 29.7 x 2.54 = 75.4kg.
1 kg = 2.205 lbs , so 75.4 x 2.205 = 166lbs/inch (rounding errors)

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### fallguySenior Member

166 lbs per inch I like better!

this per hull?

as long as you are here AH, what about his capt n crew too far for'd?

do you have some way of making sure he isn't loaded too far forward of the center of mass?

maybe I am wrong

7. Joined: Oct 2008
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Ahh.. that's where you start getting into the 'subjectivity' of a Design.

Whilst there are "general" rules for "does" and "don'ts" in Design, they are not absolutes.
Some many like the helm position up fwd, some may not.... everyone has their own personal preferences, over and beyond what are considered "industry norms", based upon experience and in-service feedback.
Thus, it then becomes a simple case of - my dog is bigger than your dog. Which we have seen far too many times on this forum too...

Just to be clear.

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### fallguySenior Member

well, I thought his helm position looked odd as far forward as it was for a boat that did not seem to have a counterweight on the stern of say a couple of 500 pound outboards...so I wondered if the forward helm and capt n crew would create an unwanted moment or lever...or if it was all just mostly subjective or if I was full of baloney or some combination...

I didn't want to tell him the boat looked a tad short and be wrong is all. But maybe I was right on aesthetics and l/b and looking to prove it further is all...

When I fish in my 16 foot boat, if I sit in the forward seat and someone is in the middle; the loss of forward trolling motor steering is notable for example. And so to the extent I don't bother and always take the control to the aft station. The boat is out of balance, basically. It tends to operate bow down, stern high and it is like driving a bobber around. Works fine from the aft station.

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### TANSLSenior Member

Perhaps instead of being so concerned about TPCs you should take into account the moment to trim a centimeter (MTC pro illis qui amant acronyms)

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Well, without a full weights & Centres check, and comparing to the hydrostatics - impossible to say!
Thus, i'd prefer not to speculate in the absence of any hard evidence....and so, then it is back to... my dog is bigger than your dog...as many on this forum prefer.

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### TANSLSenior Member

Of course, no data is needed to say that any weight, depending on how and where it is placed, can lead to unwanted moments.

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### rxcompositeSenior Member

I was curious and fed this to my spreadsheet. In post # 7, there is the boat particulars and a hydrostatic report. The numbers don't add up like Lwl, bwl, displacement.
Note also that the moulded volume is greater than displaced volume (demihull). Moulded volume is "internal", minus the skin thickness.

Design displacement of 1.26 tonne, "fully loaded" does not correspond to calculated 1.55 tonne from the data of hydrostatics.

One thing that is very noticeable is the max speed 10 knots at the designed waterline of 6.1 m (6.32 hydrostatics). The corresponding SL # is 2.24 (Fn 0.67). This is a very fast ship/boat in the category of military vessels like destroyer and frigates and definitely not a displacement vessel. The "wish list" of going max out at 10 knots with a 6.1 m vessel is very hard to achieve unless it is a planing boat and I haven't factored in the displacement yet. For this type of boat, SL # should be 1 to 1.34, the onset of planing or a semi displacement hull form.

Brent, can you shed some light into these?

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### BrentmctigueJunior Member

The beam is for two hills overall. The beam of one hull is 0.76. Unsure why Delftship shows moulded volume and displacement differently. I’ve been importing into Mitchlet for resistance calcs and it agrees with the displacement volume of deltship.
Your right about the Fn. I’m counting on the slim catamaran hulls to push past hull speed. Please pardon my ignorance. I’m very new to all this and your comments are very welcome. I’m learning fast (I think).

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### BrentmctigueJunior Member

planning on using duflex strip planks for the hull bottoms. The foam panels are joined end to end to the full length of the hull and laminated both sides. Then the panels are ripped into strips of varying width depending on the degree of curvature in the hull. When the planking is completed the surface is faired and a second laminate is applied both sides to complete the structural requirements. At least that is my plan so far.....

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### AlikSenior Member

Mate, forget about Duflex. This is the most expensive way to build composite boat, they are damned overpriced. And quality of joints is always dubious.
Instead, buy some foam (say, DIAB H80 or similar), slice it into stripes, make the shape, glass over using epoxy.

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