Adding keel weight to small houseboat

I'm working on building a small (20ft) camper boat/house boat similar to an old Hobo, but a little longer. This is a photoshopped concept of what it could look like:



I'm going to be building the cabin as I go along. I plan to make it fairly light weight, but I won't know the exact weight or layout as I'm kind of designing on the fly. I have no fear of safety as far as stability goes, the hull is a Bayhawk 200 (similar to a kenner skibarge) has a good beam on it. That said, if I can offset some of the weight to make it more comfortable while sleeping it would be nice.

I'm considering adding concrete filled PVC tubes as low as I can right next to the cabin walkway. They would be set in between the stringers before I pour in the foam. Maybe 50-60lbs worth and they would end up being a few inches below the water line. The dark grey represents the location:


Thoughts? Is it worth it? Is it enough weight to offset some of the cabin that might weigh 150lbs or so? Or should I just skip it?

 

Skip it.
Plan out your build, calculate the weights, then maybe come back here for some advice.
A big cabin on a small boat makes the whole boat a bedroom, with precious little space left for anything other than sleeping.
Bear in mind that Florida is a very warm, humid place, shade is your friend, but being enclosed in that cabin would be hell!

 

Skip it. You will get too heavy. My opinion before I read kapn

 

Have you considered an enclosed bimini top? They are light, fold down for trailering which saves you a lot in gas. Also, they can have opening windows with bug screens.

 

I've lived, fished, and camped in Florida for decades. That said, I'm not changing my plans for the basics of the cabin. I've camped on a Hobo for a weekend and it was some of the most fun we've had. I bought his one, but the hull was to rotted to really save.



I'm working on recreating this in a 20ft version that is laid out a little better for what we want. This is the basic layout, and it will be air conditioned.



I get trying to figure exact weights and such for a righting calc, but as I said I'll be building this on the fly and adjusting as I go. The material weight of the basic cabin will be between 100-150lbs on the high end. That doesn't include possible interior finishes, and adjustments to what windows and such I will use. The hull itself went on an extreme diet when I gutted her. It was originally built with heavy wood stringers, and a 3/4" floor that was carpeted. Using good marine ply and doing an epoxy build I am guessing I will be shaving about 150-200lbs off the boat as it sat. That said I'm not worried about the total weight or it being excessively top heavy. Adding 50+lbs below the water line really won't add to the draft noticably. This boat will not be planing. I'm planning on displacement speeds only with up to a 40hp motor.
All that said, can we circle back to the original question? Do you think it would have any appreciable effect?

 

If your not worried about stability while under way and are only looking to increase stability at anchor, I would put some thought into adding structure to accommodate rocker stoppers. Sailboats will hang them off there booms. You certainly can just hang them off the edge of the boat, but there effectiveness increases if they extend out. So since your doing a custom cabin, maybe think about out a way to extend rocker stoppers wider than the beam of the boat. Just a thought.....

 

I'd be thrilled if we could gain some stability while running to, but at rest is more important to my wife.
The issue with the rocker stoppers is you really need some water depth to use them. I think when I looked them up in the past they recommend a minimum of 2-3 plates plus an anchor weight. All seperated by 16 inches or so. That's not possible when you are camping on Florida's Westcoast and are typically in less then 2ft of water. Thanks though.

 

It doesn't matter where you add the weight. The same submerged volume will increase.

 

I totally get that. I am in a similar situation with my sailboat to trawler conversion. We will be in Florida also, but with a 2.5 ft draft we will be in anchorages with slightly deeper water. I have not settled on a design, but I plan on only using 1 deep per side and take advantage of the leverage of extending the rocker stopper past the beam.

Since you really won't know till the boat is on the water. Is it possible to make the ballast removable? That way if the concrete isn't enough weight per volume, you could invest in a more dense ballast material.

 

Yes. That is how draft works? Like I said 50lbs won't noticeably add to it. Maybe 1/8 to 3/16". I'd have to check my volume notes at home to be sure.

 

It would be difficult. I'm building it in a monocoque sort of fashion so the floor will be bonded down as part of the overall structure.

 

That is how displacement works. Adding weight at any location will increase the submerged volume.

 

Yes, I'm well aware of how displacement and draft work, but your reply makes no sense. It absolutely matters where you put the weight when concerning stability and righting which is what we are discussing.

 

Usually, mass is added to the outside edges to dampen roll, from my limited understanding.

The location you chose would not be ideal.

I believe two saddle tanks on the outside edges that can be emptied would be better.

A keel is offset by a mast to dampen the roll. If you don’t have a mast, a keel mass increases the roll, from my understanding. I believe it becomes a pendulum with no counteracting mass. This is experienced in real life by dismasted vessels or vessels traveling to get their masts added.

So, the better plan is to use ballast tanks, I believe on the sides. When underway, or say in bad weather; you drain them, and you flood them for overnight to dampen under typical chop. Probably something like 20 gallons a side or maybe only 10-15.

Hopefully, someone credentialed will reply. I think if you post the immersion rating of the hull, it would make people more comfortable. Noone wants to help you sink, eh?

Now, I suppose you may argue the hardtop is like a mast, but it isn’t. The mass is too spread out.

You can test the theory I’ve offered easily.

Take the boat to a dock and put 4 sand tubes in the middle of the boat, say 200 pounds. Push the side of the boat down a given amount and release it and count how many times the boat rocks before practical rest. Then take the tubes and put 100 pounds on each outside edge and repeat the test. Pretty sure the outside edge will be less time/less cycles to get to rest.

 

My thoughts, having considered C.G. impacts on my current project. . . .

Make the camper piece as light as possible

Design the boat furnishing and components so that as much weight is as low as possible

Consider water ballast, if needed.

Being wider (beam) will allow you to be taller (camper)