Boat Ramp Geometry

[stevos]

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[Ocean_Spirit]

I often see people submerging the trailer a lot further than what they should.

Then again, this is also a function of the ramp. On the dual axle trailer, when launching at retrieving at the ramp, I can drive on and off without submersing the front bearings on the trailer.

[trueblue]

As you mentioned, one of my biggest concerns with specifying a ‘design vessel’ is that each hull is so different to the next. At present I am having trouble pinning down a reliable value for draft but am edging towards 0.4-0.5m. This is ultimately something that I would like to consider more after taking a more practical look at the ‘design vessel’.

0.4 to 0.5m is probaly a reasonable draft to be considering. My boat is 0.5m draft when stationary, but the other thing to consider is the depth of the outboar motor skeg below the bottom of the hull. with my motor in the normal position, the bottom of the skeg is about .85m below the waterline. That is why I have to trim my motor right up to not smash the skeg off at Scarborough ramp.

I would probably have the bottom of the skeg about .6 - .65m below the water line when driving the boat onto the trailer, but really would prefer that the ramp was steeper so that I could have it down a bit more to have more control.

Most outboard skegs would be 0.3 to o.4 below the bottom of the hull.

cheers

Mick

[trueblue]

My calcs so far have been based on a worst case scerario where the tow vehicles rear wheels are just above the water line. That is an assumption that I am hoping to check when I find my ‘design vessel’.

That would be by far the norm, where the tow vehicle is backed down till the real wheels are just at the waterline.

cheers

Mick

[BM]

Steve,

A Bertram 28 (borderline trailerable) has a fixed (shaft drive) draft of 0.82m.

A Bertram 25 (trailerable) has a leg up draft of 0.46m. Leg down approx 0.75m.

Its important to consider draft to include the drive setup that the boat has (sterndrive, shaft drive, outboard) and if possible allow for the full draft of the vessel for worst case scenario calcs.

Raising the outboard or sterndrive leg in order to recover reduces directional control to some degree so where possible, leg down retrieval would be the optimum.

Cheers

[Chimo]

When the computations are done and its time to get a few ramps built and some others fixed up maybe this is the way to knock em over in a hurry with plenty of length / depth and with a lot less hassels .

http://www.aquabarrier.com/boatRamps.html

http://www.aquabarrier.com/installRemoval.html

Cheers
Chimo

[whichway]

HI

Also, the draft needs to be when the boat is at an angle going up onto the trailer. It may sit deeper in the water under those conditions. Really need to get a boat half way up the trailer and take some measurements.

Whichway.

[BM]

Just wondering....

Would there be any merit in having a ramp that got steeper in gradient towards the end? This would allow vehicles towing larger boats to not have to go so deep. More of a parabolic ramp shape than a linear gradient.

Batemans Bay used to have such a ramp (I don't think it was intended that way, just how it turned out when they placed it due to the steep bank of the Clyde River at that point).

Such a ramp should allow for a shorter ramp overall with an improved approach depth.

Cheers

[stevos]

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[trueblue]

shouldn’t your trailer be setup up such that you can fully lower your outboard when it is on the trailer (only just clearing the ground of course)? So having half the boat on the trailer (Assuming it is rotating about the waterline on the transom – flotation level) should see a prop clear the deck but not by much. Is this an issue that ramp geometry cant solve? I can design erosion protection for this but it might be a limitation of the trailer setup… I will need to do some calcs and sketch up a section to think about this more.

Boats don't always go on to the trailer all that neatly. My last tailer was quite high at the back and caused the boat to ride up at the bow when the keel first went onto the back rollers, which made the stern go lower in the water (simple lever on a pivot point) till the centre of gravity allowed the nose of the boat to settle downwards onto the trailer and then lifting the stern and the skeg.

A cause for this can be when the towing vehicle has a tow bar height that is too low which in turn makes the back of the trailer sit undesireably high.

Another cause for this is people not submerging their trailers properly which does not let the boat float onto the trailer.

As mentioned, some issues are trailer set up limitations, and others operator limitations and they can't always be included into designs.

Cheers

Mick

[trueblue]

A drop of in the last few metres of a ramp has been proposed by a council in SEQ recently to achieve similar results. Interesting to see how it goes?

I think the secret is to just get a happy medium on the ramp angle. An additional angled drop off on the last few metres will only come into play at the bottom of the tide.

Cheers

Mick

[BM]

Steve,

The Bertram 28's are almost all shaft drive (like the Arvor). A small number of sterndrive 28's were built but it was a very small number.

The 28 is 3.34m wide and the 25 is actually 3.02m wide (where'd you find the 2.7 figure?). Both can be towed on roads with the relevant overwidth permits. Don't forget, the trailer may only be 2.5wide but the load can be wider (as in these 2 examples).

These size boats represent a fairly small section of the market overall. And of those vessels very few would be trailered as most would be moored. The "what size is your boat" (or similar to that) thread showed the majority of boats to be 4-6mtrs. A similar poll on a Vic based site www.mrfisho.com showed pretty similar results.

Perhaps a good approach is to have differing sloped ramps? Patterson Lakes launching area in Vic has about 5 ramps, which face differing directions and of differing angles. Very handy for the changing wind conditions and for the various vessel sizes.

You could google earth patto river to get a look and then perhaps a trip down south to inspect the setup in person?? No doubt there are other well organised facilities around the country.

Cheers

PS. If you google earth "Launching Way, Carrum Vic 3197" that will take the cursor to the entry point of the launching facility and you can see the ramps quite well.

[arvor]

we have an arvor 20 and at rest we need 0.65 but when retrieving i like at least 1mtr to drive on .but i only launch and retrive a few times a year for maintainence.and when i do i have made up a extention drawbar so i can sink the trailer deep and avoid any damage and stress to hull.but with trailer so deep its possible to get stuck and then have to use the boat motor to push the whole car trailer and boat out of the drink.

[stevos]

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[Mr__Bean]

Stevos,

A couple of things to consider, maybe outside your scope but hope you can use some of it:

Visibility when reversing

Some ramps do not blend at the top of the ramp, they just angle off, this makes it difficult to see your empty trailer when backing down.

Reflective markers down the ramp lanes make backing down at night much safer.

Lines down the ramp help people back straight and generally keep the ramp more orderly


Amount of ramp that exists beyond the lowest tide mark

This has been mentioned by some other, but it can be a trap if you reverse over the end of the ramp at lowest tide. Whilst erosion may be the primary cause this may be eased if the concrete did extend further on the initial design of the ramp (in anticipation of the erosion).


The "rigging area".

A rigging area that encourages people to move OFF the ramp to remove their bungs and tie down the boat is good.

It should be well lit and drain well, and be simple drive in drive out.


Amenities

Nearly every boat that comes in has someone that is busting for a pee


Roads

Boat trailers don't like speed humps, try to design the access using curves in the road to control speed not bumps. Much better on the boat and also much better on the neighbours (bouncing trailers are very loud)


Parking

Don't skimp on the width of the parking bays, they can be difficult to back out of on busy days.

Provide some extra long bays that are a bit further away from the ramp than the others, those with bigger boats will gladly walk that little bit extra to get a good long bay.

Provide some grassed area or similar for some overflow parking, whilst it may be infreuently used it is better than pissing off the neighbours when guys have to park right down the local streets.

Don't try to pretty up the parking areas with trees and shrubs, they just provide cover for the thieves, all open and highly visible please.


Signs (some people just don't get it!!!)

Please use the rigging areas provided before approaching the ramp.

Please reduce headlights to parkers when on the ramp.

Have you checked the bungs?


That's about it for now.

- Darren