PDRacer - towing behind larger boats

Boatmik · #1 25th Nov 2008, 01:08 PM

Howdy,

Towing behind another boat can be one of the highest loads that a small dinghy can face.

Imagine if it fills with water and the mother ship is surging ahead in bad weather.

So how to distribute the load into a lightweight skinned structure like the OZ PDR.

OK ... here is how.

It might be overkill with respect to the two runners that go up the height of the bow transom, but everything else is quite minimal. The load is spread through the mast step structure and the bow transom. That is quite a bit of surface area.

Generally it is good to keep the towing eye down to help keep the bow of the towed dinghy out of the water and hopefully to make it stand on its tail a bit.

Best wishes
Michael

nickpullen · #2 25th Nov 2008, 02:03 PM

Nice one Mik.

Bugger I didnt see this before I stuck the front deck on...

Just wondering, if you make a very short mast section and tie to that for towing, would that be ok? if you catch my drift..

Cheers

Boatmik · #3 25th Nov 2008, 02:26 PM

Howdy Nick,

Are you talking about regular towing - as a tender - or emergency towing because you need it unexpectedly?

nickpullen · #4 25th Nov 2008, 02:33 PM

Well, I assume for emergency, you could just lash around the normal mast... but for the tender, you insert a shorter section into the existing mast partner and tie off to that.. I do see the issue that is is not very low down and could cause the bow to get pulled down...

Other wise it could be fun to try assemble the proper thing through a 150mm inspection port, a bit like building a ship in a bottle...

Boatmik · #5 25th Nov 2008, 04:48 PM

yes .. I have done something similar - fitted reinforcing blocks into tanks.

The drawing is for a real industrial grade solution. For a more normal occasional use you don't need it so bullet proof.

The other way would be to pick up the ends of the mast step from the outside and drill a hole in each of them and do a really good job of gluing two 3/16" (5mm) metal threads in there using them to hold down some fitting. Clean them in acetone dry them fill the hole with epoxy using a pipe cleaner cover the screw as well and glue it in

Another way I have used in the past is to drill at least 4 holes in the surface you need to glue to and drill some similarly spaced holes in the piece you want to put in. Put two pieces of some strong fishing line through the boat panel and pass them out through the inspection port. Pass them through the holes in the pad. Put glue on the pad and then pull the other ends of the lines and pull the pad into position. Then put the drywall/plasterboard screws through the spare holes.

When you need to bolt the fitting to the face of the transom through the block ... you can often hold a nut with a socket taped to a stick or use the end of an adjustable spanner to hold the nut.

The things a boatbuilder has to do!

My biggest feat of this type was the repair of a very thin ply catamaran hull. I put in a whole lot of stringers wrapped in plastic tape to pull everything into alignment. They all had a hole through them with a piece of fishing line threaded through. Did the repair using screws through the ply into the stringers. the pulled out teh screws and pulled all the temp. stringers out through the buoyancy tank hatch about 2 metres away.

Glad it worked!!!

MIK

MIK

seajak · #6 25th Nov 2008, 05:37 PM

G'day Mik,

From the OZ PDR rules:

Quote:

● Towring or fairlead on bow min internal diameter 25mm (1 inch)

Since this towring is for emergency towing I assume it doesn't need to be as industrial strength as this one. However would it be best to at least glue in a plywood reinforcing pad before the deck goes on? If so how big would you suggest.

cheers,
clay

b.o.a.t. · #7 25th Nov 2008, 07:17 PM

Seems to me that the important part is to tie the bolt back to the mast step to
distribute loads away from the bow transom so that the whole transom is not
ripped off should it bury whilst towing....

A ply pad on its own won't achieve that, although it will make the bolt less
inclined to pull out of the transom. Up to the point that the transom fails, or
the pad pulls through the transom, or the bolt pulls out of the pad..

Boatmik · #8 26th Nov 2008, 07:42 AM

Most boats will not need that industrial grade solution above.

For (occasional) emergency towing the crew stays in the boat. you need a guide ring or fairlead on the bow. Club racing in Australia requires that it is 25mm internal diameter as a minimum.

The tow ring on the bow is much simpler!!!

This is because of lower loads. The tow rope goes through the ring - pull about the same length as the boat through. Take a full turn around the base of the mast then crew should sit behind the centrecase and hold the rope.

I have just added it to the plans and put it up on the blog. The attachment can be two 5mm (3/16") metal threads glued in carefully (wash the screws with solvent dry them, put thickened epoxy in the holes with a pipe cleaner dip the metal threads in the glue and screw them home. You can use a plastic fairlead for non club racing or attach the regulation tow ring with a saddle. Put washers underneath the saddle to prevent the deck from being crushed.

For non club racing ... a fairlead like this is neater