Car top boat loader: What is the lifting weight?

[HRG]

BACKGROUND:
I'm thinking of building a flat bottom Jon boat. We don't have room for a boat trailer so I want to load the boat on top of our 2019 Honda Odyssey van. Without a trailer, the boat can be stored on a side against our hollow tile wall. The dimensions of the boat will be:

12 feet long (length can be 11 feet long)
04 feet wide at the back transom
03 feet wide at the front
17 inches high
(Bottom = 3/8" plywood) (sides = 1/4" plywood) (transom = 3/4" plywood) (flat front = 3/4" plywood)
I think the weight will be about 125 lbs.

The boat loader will just be two 2x4's at about a 42 degree angle from the ground (call them the 2x4's) to two cross bars on the roof rack. The right triangle formed will be: (vertical = 6 feet) (hypotenuse = 9 feet) so the (horizontal = 6.7 feet) See this youtube video for an example of the boat loader.

In the youtube video, the boat is already at the top of the 2x4's but for my loader the 2x4's will have pegs spaced about 8 inches apart. The boat will be "walked" up the 2x4's using the pegs. When the upper side of the boat reaches the top of the 2x4's, the ground end of the 2x4's will be lifted up so that the 2x4's are level. The ground end of the 2x4's will then be supported by two 6 foot 1x4's.

SO FINALLY MY QUESTION:
When I lift the 2x4's up as shown in the video, what will the lifting weight be given the statistics above?

 

[hutchphd]

Worst case about half the weight of the boat. Flopping it onto the carrier before lifting may be the most difficult part.

 

[HRG]

Worst case about half the weight of the boat. Flopping it onto the carrier before lifting may be the most difficult part.

Thanks for your reply.
I was wondering whether the weight would be more than half the weight of the boat because of the angle of the 2x4's. Is there a mathematical formula to calculate the weight of the initial lifting of the 2x4's given the angle?

Flopping the boat on to the 2x4's should be easy since there will always be two people. So one on each end of the boat to flop it on to the 2x4's.

 

[hutchphd]

The worst case is when the carrying beams are horizontal and you have a two to one lever arm (hence half the weight). You supply a force F of rotation on those 2x4 arms perpendicular to the arms and acting at their full length $L$ . The boat opposes the rotation because of its weight W acting through a distance $(\frac L 2) sin \theta$. Therefore $$FL=(W\frac L 2) sin \theta$$ which is worst for $\theta=90$ and therefore $F=\frac W 2$

 

[HRG]

hutchphd,

My math skills are limited to algebra so the "sin theta" lost me.

But your comment about the worst case being when the beams are horizontal got me thinking.
If the beams were vertical, that would be the best case since the beams and boat would be hanging on the pivot points. So there would be very little force needed to push the beams off vertical.

My initial thinking was wrong since I thought the weight would be worse with the beams at a 42 degree angle and diminish as the beams were lifted to the horizontal positions. I could manage lifting half the weight of the boat for the loader and if I needed help the 2nd person (my wife) could assist.

Thanks for enlightening me,
HRG

 

[Lnewqban]

Those two 6-foot 1x4's may be a little flimsy for the pushing/pulling over the roof effort, especially in cases the supporting ground is not perfectly flat.
When in horizontal position, the hinges of the 2x4’s may allow some freedom for the wood to turn or rotate some on the horizontal plane.
Perhaps you could brace the 1x4’s to the 2x4’s or add a cross member/foot at the bottom end of each.
Just a safety suggestion.

 

[HRG]

Those two 6-foot 1x4's may be a little flimsy for the pushing/pulling over the roof effort, especially in cases the supporting ground is not perfectly flat.
When in horizontal position, the hinges of the 2x4’s may allow some freedom for the wood to turn or rotate some on the horizontal plane.
Perhaps you could brace the 1x4’s to the 2x4’s or add a cross member/foot at the bottom end of each.
Just a safety suggestion.

I'll try the 6-foot 1x4's first with testing for worst case loading of the boat. If too flimsy I'll try adding a 1x4 horizontal piece at the outer end of the 2x4 beams to tie the outer ends of the beams together. If still too flimsy, I'll change the vertical 1x4's to 2x4's . It will be a work in progress.

Thanks for the support,
HRG

 

[berkeman]

Summary:: What is the lifting weight for a 125 lb boat using 2x4's as levers?

BACKGROUND:
I'm thinking of building a flat bottom Jon boat. We don't have room for a boat trailer so I want to load the boat on top of our 2019 Honda Odyssey van.

12 feet long (length can be 11 feet long)

I think the weight will be about 125 lbs.

SO FINALLY MY QUESTION:
When I lift the 2x4's up as shown in the video, what will the lifting weight be given the statistics above?

I've only skimmed the thread, so apologies if my questions have already been answered.

What roof rack do you have on your Odyssey van? What are the spacings, and how flat is it? (You say your boat will have a flat bottom?)

How will you get your 125 pound boat from your vehicle to the water and back once it's unloaded? Will you have a partner to help, or will you be on your own with a wheeled cart?

Are you comfortable lifting half of the 125 pound weight in a controlled manner without any extra leverage?

I'm asking these questions because my wife recently got back into kayaking, and we figured out a way that she could lift her 10 foot 50 pound kayak onto the roof of my Jeep Grand Cherokee to head out on weekends on her own to the local kayaking places. We came up with a routine where she uses her kayak cart thingy to get it out to the Jeep, and lifts half the weight of the kayak from behind the Jeep to put the front of the kayak on the back top edge of the roof of the Jeep (with appropriate low-friction blankets placed on the roof rack).

She is then able to lift the back of the kayak and push it up onto the top of the Jeep roof rack and then secure it with ropes. There are lots of intermediate tricks that we've figured out along the way, but that's the main idea of lifting and pushing the kayak up onto the roof of the Jeep. A similar procedure is used in reverse for unloading.

So would something like this work for you? If you can lift half of the 125 pound weight and you have a good roof rack on the van, this might be a lot simpler than what you were asking about. It gets even simpler if you were counting on a partner to help you get the boat from the van to and from the water.

 

[HRG]

What roof rack do you have on your Odyssey van? What are the spacings, and how flat is it? (You say your boat will have a flat bottom?)

Our ody van has the OEM roof rack on it. The spacing of the OEM cross bars are too close together but that will be a non issue since I will install two 2x4's at the ends of the roof rails. The 2x4's will be about 55" apart. This will work out well for my proposed boat loader since I can extend the 2x4's beyond the roof rails with the ends even with the width of the van's body.

How will you get your 125 pound boat from your vehicle to the water and back once it's unloaded? Will you have a partner to help, or will you be on your own with a wheeled cart?

I'm planning to have a wheeled cart, though I'll always have a 2nd person, mostly my wife.

Are you comfortable lifting half of the 125 pound weight in a controlled manner without any extra leverage?

Assuming that the boat will weigh even 150 lbs, that will mean lifting a little less than half the weight so a guesstimate is about 70 lbs. Since I will only go boating with a partner, that means lifting 35 lbs each which will be easily manageable. That's only lifting the end of the beams to the horizontal position.

Walking the boat up the beams using pegs on the beams will be appreciably less than 70 lbs since the boat will be about 11.5 feet long (138 inches) and the beams will be 55 inches apart. So when walking the boat up each peg on the beam, the opposite side of the boat will extend beyond that beam which will counter-balance the lifting weight.

Same with walking the boat horizontally across the beam on to the roof rack 2x4 cross bars. The boat extending beyond the cross bars will counter-balance the lifting weight on the opposite side.

Should be very manageable for us. BTW, the OEM roof rack is rated to support 165 lbs so a 150 lb boat should be fine.

I'm asking these questions because my wife recently got back into kayaking, and we figured out a way that she could lift her 10 foot 50 pound kayak onto the roof of my Jeep Grand Cherokee to head out on weekends on her own to the local kayaking places. We came up with a routine where she uses her kayak cart thingy to get it out to the Jeep, and lifts half the weight of the kayak from behind the Jeep to put the front of the kayak on the back top edge of the roof of the Jeep (with appropriate low-friction blankets placed on the roof rack).

She is then able to lift the back of the kayak and push it up onto the top of the Jeep roof rack and then secure it with ropes. There are lots of intermediate tricks that we've figured out along the way, but that's the main idea of lifting and pushing the kayak up onto the roof of the Jeep. A similar procedure is used in reverse for unloading.

So would something like this work for you? If you can lift half of the 125 pound weight and you have a good roof rack on the van, this might be a lot simpler than what you were asking about. It gets even simpler if you were counting on a partner to help you get the boat from the van to and from the water.

I think my proposed boat loader will much less strenuous to load a 150 lb boat on to the roof rack, than doing it the way your wife loads her kayak since her kayak weighs 50 lbs.

Thanks much for your suggestion. I appreciate all input.
HRG

 

[pbuk]

How are you getting on? I notice that there are a couple of confusions in the answers above.

I think you are saying that the lifting 4x2s are 9' long? And that the boat which is 4' wide will be 'shuffled' to the top of the 4x2s before lifting, so that the centre line of the boat will be 2' from the top and 7' from the bottom of the 4x2s?

In this case you will yave a mechanical advantage of 7:2 so you will only have to lift $\dfrac 2 7$, a bit less than a third of the weight of the boat, not a half. Even better, if you could 'shuffle' the boat a foot higher before lifting you would only have to lift one eighth of the weight.

Starting with the 4x2s vertical is a bad idea, because

1. you only get the benefit of an easy lift at the start, you still have to bring it to horizontal when you experience the maximum load
2. the mechanical advantage for lifting will be limited by the height of the rack from the ground, in this case 6' reducing the advantage from 7:2 to 2:1
3. it will be much harder to shuffle the boat up the beams because you will be lifting it vertically instead of at 42°