# Help with planing hull design

Discussion in 'Boat Design' started by ringoo83, Dec 1, 2008.

1. Joined: Sep 2007
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### ringoo83Junior Member

hello everybody

here I present lines plan of my first design (Aluminum planing boat with 2 outboard engines), I hope you can share me your opinios about the lines, especially those located on the forward part.

I read many articles about planing boats, my focous was on savitsky's calculations but I still have the many questions:

1- what basis to specify:
- chine width
- entrance angle
- transom width

2- what is the best location of LCG percentage of LWL or LBP or LOA)?

3- For stability calculations: what is the upper limit of the freeboard; is it the deck or the lowest point of the bulwark(aft), any samples/reference for stability calculations of a planing hull?

4- Any samples for aluminum structure of a planing hull with outboard engines, drawing, photos..?

5- when should I use the trim tabs?

6- Is there any calculations for cathodic protection of aluminum hull ? what is the amount of zinc needed for a given hull, and how should I distribute the anodes?

7- ABS rules for HSC specify the speed/lenth ratio of high speed craft with 2.36; any one knows the reason?

I used Rhino marine to create the hull, import it to Autoship as iges file so I can use the whole package to calculate power, stability, plating, and structure.

Any help will be highly appreciated

Thanks

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2. Joined: Apr 2006
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Location: SW Florida

### bexman86Junior Member

it looks like the boat has a lot of curvature in the topside near the bow. keep in mind that if this is to be an aluminum boat, adding complex curves to the hull surface will increase cost and production difficulty considerably. most aluminum boats are fairly boxy looking because of this.

As far as CG, it depends on a number of factors. If you're satisfied with the hull bottom as it is now, find a reasonable waterline (based on weight), and sit the boat level in the water. The center of the underwater volume (CB) should be in line (longitudinally) with your desired CG.

trim tabs are to be used when necessary. since you're installing outboards, the running trim of the boat can be adjusted through the trim on the outboard. the tabs would be used one at a time to offset any transverse imbalances.

i know i didn't answer all of your questions, but i figured some is better than none

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### amolitorJunior Member

I am NOT an expert at all, but here's my thinking.

Your chine rises fairly far forward, and the sections forward look fairly deep. It looks like you're going to have more buoyancy forward than you'd like - you want the center of gravity something like 40-45 percent of the waterline forward of the transom, which means this is where your center of buoyancy ought to be as well, as bexman86 suggests.

Your boat is also pretty narrow for the type, which is fine for efficiency, but not typical.

4.7 tons also seems incredibly heavy. Between 1 and 2 tons is more typical.

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### ringoo83Junior Member

Developable surfaces

Thank you for guiding me, Any comment on using developable surfaces?

5. Joined: Sep 2007
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### ringoo83Junior Member

change the underwater part shape to fit LCB & LCG

Appreciate your feedback. Do you suggest to raise up the forward part of the chine a little bit , or to increase the deadrise angle, which means more resitance (much comfortable) and and less lifting?

what you meant with "fine for efficiency, but not typical", may you identify the typical planing hull?

what do you think of changing the shape of underwater volume to fit the LCB at 40-45% LWL and later to arrange CG to be on top of CB?

Thanks

6. Joined: Jan 2005
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Location: San Francisco

### amolitorJunior Member

Your deadrise angle is fine. As you've learned, the flatter the hull, the more efficient the lifting! On the other side, the more deadrise, the more seakindly the hull, so it's a tradeoff. I don't know what your intended use is, so I can't tell you if your deadrise is "right" or not, but certainly there are plenty of boats with degrees of deadrise.

Check out other boats about the same size as yours, for whatever use you're thinking of, and see what they use! What I do is cruise boats.com for boats that look about right, and then I got to manufacturer's web sites to get detailed specs.

I think you'll find, if you see what other boats are out there, that other boats of this length tend to have more beam. Narrower is more efficient (to a point) but less roomy, and your design seems narrower than "typical" designs use. I'm just comparing your boat to similar boats, really. See, for instance, hewescraft.com.

Yes, that's exactly how you want to approach this.

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