Portager swim step and davit

[Portager]

Lately, between too much work and business travel, I’ve been struggling with the design of a deployable gangway concept for Portager (my transportable Trawler design) but it hasn’t been going very well. I decided that I was trying to make it do too much. I was trying to use it to board the boat both in the water and on the trailer and to act as a davit for the tender. It also needs to stow itself and the tender lower than 13’6” height for bridge clearance on the road. So I decided to try and separate some of the requirements. I can provide a separate means of boarding the boat when it is on the trailer and I can separate the boarding function from the davit.

As you may have read in some of my previous posts, I am not fond of swim steps on “Passagemakers”. Swim steps have a tendency to invite following waves aboard and the sea dislikes appendages. That is why I wanted a deployable gangway in the first place. The other reason is the Admirable and the Great Pyrenees don’t like ladders and some day I’ll be too old to lift 120lb dogs aboard. Then it occurred to me that if I made a conventional swim step raise and lower like the lift gates on some trucks, I could solve both the boarding ladder and boarding wave problems.

Attached is a conceptual design for a lift gate type swim step or “lift step” for short. The second attachment shows the lift step on Portager in plan and profile views. The mechanism for the lift step would be built into the lazerette to port and starboard of the rudder mechanism and would consist of two parallel linkages on each side to keep the platform level as it rises and lowers. The upper linkage on each side would be interconnected by a shaft and actuated by a hydraulic ram. The lower linkages can be disengaged to allow the swim step to rotated for transit and reduce the overhang. The bottom side of the swim step would be painted yellow with “OVERSIZED LOAD” in black block letters. The slots in the stern for the linkage arms would be enclosed with hull plating and a circular water-tight seal would be provided around the interconnect shaft so that the linkages would be outside the hull and the hydraulic ram would be inside the hull. The lift step is shown with a depth of 30”.

The remaining attachments show the davit concept. My plan is to have two davits located outboard of the lift step mechanism. The extended orientation is designed to allow the tender to be raised and lowered without impacting the lift step in either position. I’m thinking the perimeter of the lift step should be covered with closed cell foam bumper material. By the way, my current tender plans are leaning towards the OCEAN Craft http://www.oceancraft.org/ 3.3 meter Yacht Tender which is 11’ long with a 5’ beam and weighs 106 lbs empty. Power for the tender is still up in the air. I am hoping there will be a lightweight diesel outboard (that might be an oxymoron) available by the time I need it, but just in case, I’m adding a gasoline fuel locker to Portager. One significant advantage of this davit design is the tender remains upright, so I can use a conventional engine without having to offload the engine.

The next attachment shows the davit retraction process, which would be actuated by a hydraulic ram(s). The first step is to lower the tip of the davit, which prevents the tender from banging into the davit during retraction and aligns the tender with the cradle. An interlock switch would prevent the davit from retracting unless the davit tip is in the lowered position.

Once the davit is retracted the cradle arm would be extended and the tender would be lowered onto the cradle. Finally the telescopic davit arm would be lowered to fit within the 13’ 6” height limitation. As a simplification, the davit arm could be non-telescoping and removed for transit, but removable parts tend to get misplaced or left behind. In the stowed position the tender would overhand the stern of Portager by ~1’ 1.5”.

I would appreciate any comments or suggestions that anyone might have. Just keep in mind that this is only a conceptual design and does not include any detailed structural analysis yet.

Regards;
Mike Schooley

[gfrisbey]

There is a web page that has a platform lift that does what you want done. I think the lift is a TNT lift and in fact I think that is the name of there web page www.tntlifts.com. Something you could look at any way.

Frisbey

[Willallison]

I don't think that the TNT (and others like them) will have the extent of travel that you're after.

How about a variation on Yipsters folding hardtop idea.... http://www.boatdesign.net/forums/att...achmentid=2649
Only, you wouldn't have the 'lid' flip over as he has it... more like this (apologies for drawing quality! )

[PowerTech]

Nice drawings and neat boat.I think the old boy knows what he is doing.I enjoy reading your posts and there dry humor.

[mattotoole]

Originally Posted by PowerTech

Nice drawings and neat boat.I think the old boy knows what he is doing.I enjoy reading your posts and there dry humor.

Ditto. Nice looking boat! The step and davit look good too, but that's a whole lotta custom machinery.

[Willallison]

This is the 1st chance I've had to take a good look at your pics Mike. Like the others, I like the boat - though for me, I simply couldn't be without an aft cockpit However, I understand your swamping concerns, so we won't go there.
As far as the lift step is concerned, where do the actuating arms exit the transom? As you've shown them, it looks like you would require two vertical slits. This would allow the ingress of water into the lazarette, meaning no storage and serious drama for your steering gear.... I think the only way to do this is to have a variation on the TNT or similar lifts, as suggested by gfrisbey. Unfortunately, I think that means the hydraulics are fitted outside where they are vulnerable to corrosion etc.
The simplest solution of all - and for me simple is almost always best - is to have a flip-up platform. It could be engineered in such a way that it was very firmly locked in the up position whilst underway. Similar, flip down stepd could be incorporated to provide trailered and diver access...

[ErikG]

Perhaps a goofy idea and possibly not structuraly sound but...

What about a "cut-out" of the stern and "rail" (notthe hand rail but the part that extends from deck level upwards, don't know the proper name). It would look neater, remove some height when decending and all linkages and hydrualics would be stored further away from the sea making it less attacked by corrosion.
Also having a less complicated system makes for longer MTBF.

Choosing at what level the cutout starts will obviously depend on what ellse you have in the back of your trawler, steering and what not.
The lower you could make it the less complicated the linkages, but the larger the risk for water intruding at the join. But as it should be quite a bit above dwl I don't think that would be a problem.

You could also have a fold-out platform extension (but then it starts to get complicated again).
I have no drawing program here on this computer so I can't show you.

Also all that custom made mechanics will be expensive AND heavy, and I don't like weights high up aft. But that might just be me

BTW if I ever wanted to have a trawler style boat this one would do more than just fine

[asathor]

I think with your skill you can get the swim platform to do both.

If you raise the swinplatform partially and sail the dinghy under it - kind of like a lid - then attach the outside of the dinghy (under the platform) to the outside edge of the platform all you have to do is swing it up to bring the dingy along.

Of course you got a bit of work left designing suitable fastenets and locks if you don't like bungecords and ducttape.

[FAST FRED]

The design is great but very complex.

Since the hight requirement is only for road work , why have that complexity for somthing not done often?

A simple setup would plonk the dink on the aft cabin roof or pilot house top with little construction effort.

The high weight might even help slow the roll motions.

If the mast for flopper stoppers was also a kingpost ( had crane arm), only ONE item needs to be built.

FAST FRED