What keeps a boat upright?

[Mark42]

What keeps our boats upright, and prevents them from just rolling over in the water? There is a lot of pressure pushing up on the hull to compensate for the displacement. Why doesn't it roll over and sink?

 

[tommays]

Re: What keeps a boat upright?

The C/G (center of gravity) is below that point or it would as happened to the tour boat at Lake George last summer when they made some upgrades like changeing plastic and fabric to Glass and Fiberglass

 

[45Auto]

Re: What keeps a boat upright?

Actually, the CG is ABOVE the Center of Bouyancy (CB) on about 99% of the boats and ships out there. To make it real simple, as the boat tips there is a "righting moment" due to the movement of the CG and CB that pulls them back into line with the CG over the CB.

Here's a link to a basic powerpoint explanation:

http://navsci.berkeley.edu/ns10/Spring06/Lesson 24 - Buoyancy & Stability.ppt#256,1,Stability & Buoyancy

 

[Woodnaut]

Re: What keeps a boat upright?

-42, In addition to the slide show that 45- posted a link to, here is another link to a USCG document that is fairly informative. Pages 16, 17 & 18 get right to the point.

http://www.uscg.mil/hq/g-m/cfvs/Stability%20Book%202nd%20Ed%202004.pdf

Hope this helps. - Woodnaut

 

[Mark42]

Re: What keeps a boat upright?

Thanks for those links. I really enjoyed reading them and got a much better idea of what is going on in my boat.

One of the reasons I asked is because of the hard top I'm working on for the Bayliner. I don't want to raise the center of gravity or boyency by adding a very heavy hard top. I'll keep it light, except for the windows will be laminated safety glass and not plexy, that will add weight, but probably not as much as all the oem safety glass that I'm removing. I was surprised how heavy the factory windshield and side windows are.

I'm not too worried now about making the boat top heavy, I'm adding fixed weight, and not enough to make a difference (I hope).

Another change is to install a centrally located fixed permanent fuel tank rather than two 6 gal portables. That will reduce shiftable weight that can lead to rollover.

Thanks again for those links!

Mark

 

[dodgeramsst2003]

Re: What keeps a boat upright?

Mark

I just finished raising my hardtop. I have a 74 century with factory hardtop that was to low to stand under. I raised it nearly 17". I had it out in lake erie twice now, first time was calm, second time had 3-5ft chop. handled fine. I used 1" square aluminum tube, plexiglass for the windshield portion, and I will be making clear isinglass windows for the sides. My hardtop was heavy too, took two people to lift it on and off. I'm 5'10 and can stand under it with room to spare now.

Chris

 

[Mark42]

Re: What keeps a boat upright?

Mark

I just finished raising my hardtop. I have a 74 century with factory hardtop that was to low to stand under. I raised it nearly 17". I had it out in lake erie twice now, first time was calm, second time had 3-5ft chop. handled fine. I used 1" square aluminum tube, plexiglass for the windshield portion, and I will be making clear isinglass windows for the sides. My hardtop was heavy too, took two people to lift it on and off. I'm 5'10 and can stand under it with room to spare now.

Chris

Thanks for that info, Chris. Gives me more confidence in what I'm doing. I do realize that the biggest impact to the boat will probably be handling due to greater area for the wind to push against.

When I added a navy top to my MFG, it would move around with the wind more, especially when trying to dock. When I installed the navy top that came with the Bayliner, I had a real hard time docking on a breezy day. Ended up reaching out with the boat hook to catch a dock cleat and pull the boat up to the dock that way. Every time I would shift to neutral the wind would just blow it away from the dock. I'm going to get more and longer boat hooks.

 

[dodgeramsst2003]

Re: What keeps a boat upright?

Yeah, the added height is a problem in wind, but luckily I'm in a protected harbor. (reasonably so anyways). I take it you are adding a hardtop to a boat that didn't have one originally. Do you have any plans? How high are you going to make it from the waterline? How heavy is all the material added together? Do you have any fabrication skills? I started with some plans that I sketched together, and then changed them as I went along to make things work. My original idea had flaws as I found out as I went along. I will try and gather some before and after pictures and start a new thread in boat building/restoration section when I get the chance.

Chris

 

[Mark42]

Re: What keeps a boat upright?

This is what I have...

This photoshop photo is what I want to build.

 

[dodgeramsst2003]

Re: What keeps a boat upright?

Ok, I looked at your two photos, and from experience this is what I have, take it for what its worth, or disregard it. First, it looks as though you are taking the top winshield railing out, and making it one big piece of glass. This is going to add a lot and I mean a lot of flex to the hardtop. What kind of cross-bracing, reinforcement are you planning to add? What kind of materials are you planning to use? You might have to add diagonal think "X" across the boat width wise just under the hardtop. This would be to help stabilize. I didnt' do that, and mines sturdy but flexes a bit. I'm trying to get pics togehter to show you what I did, and give you and idea of what I did.

I will either post to this post, providing no objections from you, or post in the boat restoration section and PM you.

Chris

 

[Mark42]

Re: What keeps a boat upright?

Shoot me a pm. Thanks!