Boat Lift Design: Building 8'x12' Platform

[FrankH272727]

TL;DR Summary

Choosing the right motor and gearbox

Building a 8' x 12' platform that will hold a small boat next to my dock. The platform needs to be able to be lowered below the surface of the water (launch boat) and lift above the water (store boat). Four pilings (one each corner) that each have an electric motor & reduction gearbox that has a spool to wind up & extend the wire cable (to raise and lower platform). Each motor/gearbox will need to handle 1000 lbs weight verticle lift (4000 pound total weight of platform and boat). The speed of the lifting/lowering process is insignificant (assume 3-5 feet plus/minus per minute). How do I choose the motor HP and the gearbox speed and torque capacity? The spool that winds the cable will be typical of what is found on a truck winch (3"-5" diameter?). I could add a pulley at the platform to double the wire cable. I need to use AC so either 110V or 220V motor. Thx!

 

[anorlunda]

That's a good project. Power and torque are not the only questions. Have a look at this website for the types and sizes of motors available.

https://boatliftwarehouse.com/shop/motors/

Most of those motors are 3/4 hp or 1 hp.

You could also search vendors of boat lifts, and find one able to handle 4000 pounds.

I would trust info about commercially available ones, rather than trying to calculate it yourself. It's hard for you to know what margin to add to the calculations. Weather, lubrication, alignment and other factors that are hard to calculate could influence performance.

 

[Baluncore]

Welcome to PF.
A floating dry dock platform would use less cables and winches, and could be operated by pumping water or air. Why does the boat need to be lifted out of the water?

 

[anorlunda]

Why does the boat need to be lifted out of the water?

Usually because of barnacles and other bio fouling creatures.

Edit: Also to eliminate wear and tear on a docked boat because of wave action and/or boat wakes .

 

[FrankH272727]

It needs to be lifted due to barnacle growth, waves (saltwater bay) and I want it to be an extension of the main dock when we have company (raise it to the same height as dock). I looked at some of the website products and nothing really dials in my specific needs. It seems simple to me in theory (wind up a cable that carries 1000 pounds) but I'm not clear on what to buy. I've had a floating dock (JetDock) ... it gets NASTY after 3 months. My big boat is on a typical 10,000 pound lift. I don't want the transverse beam on the new platform lift ... thus the 4 pilings with dedicated motor/gearbox on each piling.

 

[DaveC426913]

Do you need a motor?

I had a sailboat that I lifted off the ground manually with a chain winch so I could scrape and paint it.

You may find that doing it by hand is more reliable and less in need of maintenance.

 

[jack action]

I don't want the transverse beam on the new platform lift ... thus the 4 pilings with dedicated motor/gearbox on each piling.

Having four motors is a really bad idea; nearly impossible to synch. Plus it will probably cost a fortune. Any of your designs should have only one motor.

Suggested designs:

Side drive cradle

Although the design does have transverse beams.

Cantilever lift

This one has no top transverse beams, but the larger ones seem to use hydraulics instead of cables. What I like about this design is that it has one cable, so if it breaks, the whole boat falls straight instead of having one end (or three corners) dangling in the air.

Four-post parking lift

This one is actually for cars but I think the design could apply to your need. No top transverse beams. The inside is as follow:

There is one hydraulic cylinder (I guess it could be a single cable connected to a winch) that is pulling 4 cables. The ends of each cable are connected to the top of each post. As the cables are pulled, the whole assembly raises. For safety, when the lift is at the correct height, it is locked by 4 pins (one in each post) instead of relying on the cables for support.

But I'm not sure if cables & pulleys resting underwater would be appropriate.

 

[FrankH272727]

Gentlemen, all good input that I truly appreciate. I'm on west coast of Florida. ANYTHING that stays in the water for a week becomes part of the ecosystem. The solution is to position everything in air while not being used, NOT in the saltwater. Even the treated structural pilings are wrapped with a thick vinyl sleeve (from the sea bottom to above the high water line) to keep the worms from destroying the wood. No part of this lift can reside in the salt water and I do not want to duck under any beams (platform has to function as a deck) ... thus the choice to mount a motor/gearbox/pulley/wire cable on each piling. Yes it will cost more than a conventional lift that typically uses 2 motors and beams. So again, I'm asking for assistance from y'all on how to know how much torque I need out of a reduction gearbox to raise 1,000 pounds at 3-5 feet per minute using a spool that is winding up a wire cable. Thanks :)

 

[Baluncore]

I think there are some assumptions that must be made.
1. The platform is like a boat trailer in that it will only be dipped during launch and recovery of the boat.
2. All cables needed to keep the platform level will be above the water at the piles, or cross between piles inside the rigid platform structure.
3. As a lifting device that supports people, insurance will require that all relevant design codes be met. Mind the gap.
4. There will need to be an automatic brake.
5. To initially install or replace cables it will be necessary to lower the platform onto a pontoon or floats.

The way the cables are used to support and keep the platform level will determine cable diameter, and the number of parallel cables on the winch drum. The diameter of the spool drum, and full diameter of the cable spool, will determine the torque required to support the weight of the platform.

So again, I'm asking for assistance from y'all on how to know how much torque I need out of a reduction gearbox to raise 1,000 pounds at 3-5 feet per minute using a spool that is winding up a wire cable. Thanks :)

First find the path that the cables will take.
Motor torque, gear ratio, RPM and power will follow from that.

 

[sophiecentaur]

So again, I'm asking for assistance from y'all on how to know how much torque I need out of a reduction gearbox to raise 1,000 pounds at 3-5 feet per minute using a spool that is winding up a wire cable. Thanks :)

I'm going to sound a bit Mr. Negative here but, if you have the confidence and ability to design and build this structure (any version of it) well enough to address all the safety aspects (well enough for an insurance company to consider the risk) then you would already be capable of working out all the tensions and other forces involved. Stresses of each part of the system would be bread and butter stuff for you.

You would also have added to this, the fact that it's a marine environment with possible waves and wind all the time the boat is suspended up there.

That implies that you would not be asking what could be seen as a very basic mechanics question about Forces times Distances (=Work) and the time to do the lift (=Power) etc.. This worries me.

PF is usually very safety conscious and, if you were to have asked an equivalent EE question about such a beefy system, I'm sure the Mods would have registered how high risk this project is.

 

[Baluncore]

m = 1000 lbs = 453.6 kg
Force = m * g = 453.6 * 9.8 = 4445.2 Newton.
4 ft / min = 1.2192 metre / min.
Energy = force * distance = 4445.2 * 1.2192 = 5420 joules / min.
5420 / 60 = 90.33 joule / sec = 90.33 watt.
Say, about 100 watt.

 

[sophiecentaur]

Say, about 100 watt.

That's a good starting point and sort of agrees with a hand operated screw jack for a car. Add the friction all round after the effects of salt in all the pulleys, slides and fouling (even after only an hour's immersion during lift in and out) and you can multiply that by ten. Surprise surprise, you arrive at 1hp+. No pointing in avoiding a large motor - it would be a tiny fraction of the whole project - reliable and a known quantity
Good idea to use a screw drive to avoid the back-drive problem.

 

[Baluncore]

Good idea to use a screw drive to avoid the back-drive problem.

I would use a 1 HP motor with a worm gear reduction box.

 

[sophiecentaur]

My big boat is on a typical 10,000 pound lift. I don't want the transverse beam on the new platform lift ... thus the 4 pilings with dedicated motor/gearbox on each piling.

I had a sailboat that I lifted off the ground manually with a chain winch so I could scrape and paint it.

But I'm not sure if cables & pulleys resting underwater would be appropriate.

Some very conflicting requirements and points in this thread. If there is enough room below and/or to the side, a floating dock would seem to solve most of the requirements - and also it would be very much a fail-safe system. Even a major leak would result in the boat settling down evenly. (A mud bottom would only be the same as in most marinas). It's something I would trust myself to make - even with a 10klb boat. Pontoon floats last for years without being taken out and scraped and vertical rails are self-cleaning. Four good timber posts would look after themselves 'till the OP has finished his boating life. A pump would be the only moving part and that could be well out of harm's way.

 

[FrankH272727]

Again, I'm appreciative of the creative input, ideas and comments. My intention of posting this thread was to gain insight from higher-level minds than mine on the "hardware specifications" that would allow this project to work (example: 110v 1/2HP motor coupled with a 25 ft-lb worm gear reduction gearbox). I am a 30-year state licensed Contractor ... I get liability, risk, fail safes, safety factors, etc. Anything I build will be overkill. I'm just unsure how to "dial in" the "calculated" design hardware needs. With that knowledge, I could add overkill to the hardware. I am attaching photos. You will see my next door neighbor's floating platform ... it requires a gang-plank to access it as the tides rise & fall. One of my "must-haves" is a platform that can become a continuation of my stationary dock (same level). Thus, no floating docks. You'll see my existing boat lift. It can be raised/lowered to avoid the waves or allow passengers to board from the dock level. The area on the left side of the stationary dock is where I plan on putting the powered-lift platform. In closing, I appreciate the opportunity given to me to gain knowledge from your Forum. I do apologize if the information I'm seeking is "outside" the scope of typical discussion of your group. Enjoy the Holidays and thank you.

 

[Baluncore]

Anything I build will be overkill.

I think you really mean "quite safe".

Avoid multiple motors, so you can avoid synchronisation and tracking. The path taken by the cables to support and keep the platform level will pose an interesting design challenge.

 

[jack action]

Pushed the research a little bit and I found this:

Isn't that what you want (a little smaller)? For the bigger boats, they seem to use the transverse beams you disapprove. But with a little roof on top, as below, it looks fine to me:

Or for a lower budget, a nice little canopy to hide everything (not the same boat lift, but you get the idea):

Best of all, these systems are from a business in Safety Harbor, Florida! Even if you want to do it yourself, at least discuss with them to see how they do it.

 

[FrankH272727]

Thank you Jack that PWC lift platform is EXACTLY what I have in mind. I will definitely get in touch with that company. We have many many docks and lifts in my area and I have not seen one of those. It might just be a new business venture for me

Our neighborhood/county does not allow canopies or roofs over docks or lifts (blocks neighbors' water view).

Much appreciated THUMBS UP !