Advice on fixing a ski pylon/pole to floor

jago23 · 10 December 2020, 14:47

Hey all

I'm keen to put a ski pylon/pole into a 6.5m RIB to get a better height for towing wakeboarders than using a bridle onto the transom.

The pole would be placed vertically onto the boat's floor just forward of the engine (lots of space), and then two diagonal struts go aft to the transom.
For fixing the pole to the floor, options would be either large self-tapping screws, or bolts. But the bolts will need nuts under the floor and hence the requirement to cut a small access hatch.

What method are most people using to solve this?

I'm thinking of something like this:
https://galaxyleisureandlifestyle.co...al-boat-mount/

Cheers
J

spartacus · 10 December 2020, 19:30

I've never fitted a ski-pole, but even from a lay perspective, then I'd rule out self-tappers.

This needs to be rock-sold, so that means having an understanding of the forces if will be asked to withstand. The telescopic arms look straight-forward enough, so 316 marine grade bolts through the transom. You can use Sikaflex 291 to coat the bolts and form a waterproof seal, penny washers and the nut.

For the main strut, then what's underneath? If you can speak to the manufacturer of the rib, then you can determine where the stringers are. You will need access underneath to add a plate(s) to spread the load, again stainless steel, fastened into position with bolts. Any waterproof access hatch added then deck areas need to be sealed after cutting the deck. This adds a bit of bling over the conventional Plastimo hatches. 316 SS Deck Plate

Last Tango · 10 December 2020, 20:02

An alternative.....rope end anchored anywhere in the back half of the boat. (mines is on a pillar on front of the well) but would work fixed near the back of the jockey seats.
There is very little load on the "A" frame and it's kept above the engine.....
Check from 7 mins >

https://www.youtube.com/watch?v=WcZ3EShm8JY

jago23 · 10 December 2020, 20:49

Cheers guys, both great feedback and taken on board.
LastTango - that's a neat idea! Very clever, i like that.

Limecc · 10 December 2020, 21:45

If you go with the pylon I'd use some ~120mm x 3mm stainless flat bar to spread the load as suggested but on top, not underneath, and this could be self-tappered down to the deck. The underside of the plate can be drilled and countersunk to take Allen screws that fix the pylon. Being 3mm it will not present a tripping hazard and it's a reversible mod, you won't have to cut into the deck.

HDAV · 10 December 2020, 23:16

Quote:

Originally Posted by jago23

What method are most people using to solve this?

I'm thinking of something like this:
https://galaxyleisureandlifestyle.co...al-boat-mount/

Looks similar but cheaper https://boatworld.co.uk/t-top-towers...ater-ski-pylon

gtflash · 11 December 2020, 23:56

What’s the deck made of? Is there backside access? Rather important before choosing in my opinion

Salty Pete · 12 December 2020, 23:10

Another option are "Nutserts." They are nuts that can be fitted into sheet metal and are fixed something like a Pop Rivet are fixed, but of course a lot stronger.

Also metal is better in tension rather than compression so having the support going aft, the metal is in compression, rather than the support going forward where it is in tension.

Also the forces on an aft support will have a pulling effect on the base of the mast.

Supports going forward, the force on the mast would be down on to the deck, much more desirable.

spartacus · 13 December 2020, 08:26

Quote:

Originally Posted by Salty Pete

Another option are "Nutserts." They are nuts that can be fitted into sheet metal and are fixed something like a Pop Rivet are fixed, but of course a lot stronger.

Also metal is better in tension rather than compression so having the support going aft, the metal is in compression, rather than the support going forward where it is in tension.

Also the forces on an aft support will have a pulling effect on the base of the mast.

Supports going forward, the force on the mast would be down on to the deck, much more desirable.

Not sure why you would want the supports going forward. I understand the logic, however you're just going to create a trip hazard and eat into usable deck space I'd have thought? Everything I've seen with these types of installation, then the supports go to the transom.

GuyC · 13 December 2020, 10:33

Most of the load will be longitudinal i.e. fore and aft rather than up and down so however you fasten it to the deck will be in shear. Someone mentioned a plate self tappered to the deck which would probably work. It’s about maximising the surface area of the connection.

The stays that go to the transom will be in compression but unless they are really flimsy or you bend them putting them in you should be OK. It will try and make your transom bend but as it is designed to resist the loads from an outboard I don’t think it will be too onerous. From a belt and braces approach I’d land them on a pad just to minimise any load concentration.

GuyC · 13 December 2020, 16:29

Please ignore the twaddle directly above.

I've looked at the pictures properly now and the stays go almost to the top of the pole. I incorrectly assumed they would start about a third of the way up the pole or something.

The arrangement shown does mean the fixings at the bottom of the pole will mainly be in tension with only a lttle bit of shear.

As a result and if it was me, I would be putting a chunky pad under the deck to a) spread the load and b) transfer it towards some underdeck stiffening.

For the stays, the load is in line with the stays so possibly more shear on the transom than bending which is good / easier to deal with.

I did briefly Google tow pole loads and whilst I couldn't find much info, it did suggest towing inflatables applies a much higher load than 'just' a skier.

Apologies for any confusion I may have caused.

Salty Pete · 13 December 2020, 23:21

Hi GuyC

In my twaddle I also thought that the stay would be at the bottom of the pole.

With the stay at the top of the pole then there is very little upward force as I previously suggested.

To twaddle even further maybe two stays going to either side of the transom to reduce the load in the centre.

That was interesting about the difference between a skier and an inflatable but, I would have thought (happens occasionally) that the skier would take less force to ski but more force to pull them out of the water.

butty · 15 December 2020, 08:01

For the base fitting you could laminate a new piece on the deck that has either a tapped plate inside it or bolts going up for the base flange of the ski pole to bolt down onto. It could be a section of 19mm marine ply that is twice the size of the base flange that is glued down to a prepared section of deck and then laminated over, this can then be flow coated to match the rest of your boat. I