Designing a Portable Lift

Hi Everyone. I’m new.

I’m trying to design a portable lift to lift a hull out of the water on my catamaran. i’ve about had it with marinas/yards in the USA:

*narrow beams only
*unreturned calls, emails
*ridiculous prices
*we might be able to fit you in the schedule 6 months from now
*we can haul your boat for $2000 plus 2x our yard storage fees since a catamaran, but you can’t work on it

It’s time to cut them out of the picture.

I need to design a lifting system that will lift one hull of my boat up by 24+ inches (less than a meter) to allow for the work one would do on haul out.

I need to:

*fix cosmetic hurricane damage on topsides
*clean bottom and apply bottom paint
*clean the bottom from time to time

Boat is 50ft x 25ft
max displacement of 18,000lbs to waterline
current displacement 14,000lbs or so

Goal: lift one hull at a time up 30 inches (less than 1M) from the waterline rest point

Important features:
1) cheap - it should be the most economical method possible
2) lightweight- this is a performance cat and i’ll have to drag this around with me
3) demountable - has to be able to stow in a locker when not in use

Progress so far:

i have looked at a pair of 4400 lbs pillow bags from Subsalve to create 8800lbs of lift.
was thinking of just tying a pair of them together but they won’t get the boat high enough if i do that.
probably need a demountable aluminum frame. 6” aluminum i beams?

Open to other ideas since this is getting complex. i just want to blow something up and lift the hull clear, then deflate it and put it away

Thoughts?

 

note: cannot simply dry out as water/land cops will hassle me and these are multi day projects

 

here are my current thoughts but it’s getting heavy/complicated/more expensive

 

Bags or barrels just make sure that they won’t slip while you’re under it.
Wakes and waves may make it unstable.
Bottom paint residue is not allowed to go back into the water, how will you contain it?

 

thank you for the response. I don’t think I can use barrels. They don’t provide enough buoyancy. I read somewhere that each barrel is about 450 pounds of buoyancy. i need 8000lbs. that’s 18 barrels and barrels don’t go into the locker when you are done with them. So I don’t think I can use those. Unless there’s something I’m missing.

this is ablative bottom paint. there is no sanding involved. Just putting more on after cleaning the bottom.

 

Spring tide, wait a few days, do the work, refloat.. That's how it's done in my whereabouts.

 

I wish. I’m not in a remote location like you are. People are very very uptight in the Northeast USA. There will be somebody over there within hours. First SeaTow trying to tow you off, then the marine patrol. Then probably the land police. I have done it before decades ago, but times have changed

 

Sorry to hear that.. Not even on private shore property?

 

on a side note: maybe Teddy is right.

anyone know of a spot to dry out in the northeast usa? willing to travel

 

Sounds better than buoyancy bags.
I bet you wont want buoyancy bags that can hold 8 ton in your "lockers". I doubt you have that much room, and weight allowance.

 

doing it on private property is the first place to where people would actually call the police on you. The only possibility is a sandbar in a remote location and hope no one notices.

 

it’s true I don’t want any of that. But I’m finding in a necessity. I have plenty of room for them. They’re not big at all when they are deflated. they weigh 90 pounds each. they fold up into a suitcase size. i have 1250 square feet here and completely unused bow lockers so plenty of room. though i hate to add 200lbs to the boat for nothing.

BUT I also need to get this work done and there are no marinas that are compatible. Everything is awful in that department nowadays. So I have to find someway to do this on my own without marinas.

You are suggesting it would be better to do it by drying out also. Where can I do that? Any ideas?

I’m not dead set on doing the bags. In fact I would rather not. I would rather dry out. I just can’t find anywhere to do it

 

Can you get into shallow water? In that case you can make two spuds and use rope and tackle to lift the hull.

 

this is a really good idea I like this.

24 inch draft steering by motors alone.
36 inches if i want to steer by rudder (they kick up)

do you have any thoughts on how to go about doing the spuds?

this would be the most ideal way to do it but I always figured the spuds were going to be too heavy and I didn’t know how to attach them to the hull.

might have been good to have built extra extra strong dagger boards. They would have been heavy but I could probably have used it as a spud

 

There's this great unit of measure called pounds per square inch. When you want to support weight the object it rests on will have to resist deforming. The load is proportional to the area of contact, that's why you can easily drive a nail into a board, but if you miss the nails head the hammer doesn't dive into the board.
For an inflatable object to not deform significantly under the applied load the pressure inside must be greater then the pressure from the outside,

Half a catamaran at full displacement is 9000lbs. To obtain the typical 3.5 psi maximum load an inflatable boat would use (in order to use the same construction methods and materials) you need at least 2571.4 sq in of contact area, presuming high quality fabric and construction, but you actually want a lot more because you don't want your float bending like a banana.
To support this weight in water you need a submerged volume of 141cubic feet, let's say a rectangle 8x8x2.2ft. Add an additional 30in of height above the water, so your bag is now 8x8x5ft.
If you don't want to take a crash course into building inflatable structures your only choice is to have it custom made, just make sure to sit down when they tell you the price and weight.

The same psi stuff applies for the spuds, if you don't want to turn your boat into a pile driver you need to distribute the load over a large enough area. How much that is depends entirely on the sea floor composition.

Since the above doesn't change the fact that what you want to do is illegal in the USA, my advice is to ask commercial fishing yards for haulout and storage, they shouldn't have a problem with your beam, and the rates should be reasonable once the local fishing season starts.