Choosing a towing line

[Pascal]

Hi all,

I have a Zodiac Pro 4,7 with a 40hp and would like to be able to tow other boats occasionally, up to 1000 kg.

I’m thinking of getting floating PP rope that I will tie to a loop attached to chainplates each side of the engine.

How should I choose the breaking load of the towing line compared to the weight of the towed boat ? Does it have to be greater ?

Thanks

[spartacus]

Any reason you’re likely to be towing boats up to 1 tonne? Only reason is your boat seems far from suitable. Zodiac Pro 4.7 is lightweight leisure rib and your main power unit is just 40hp.

In anything but relatively calm waters you’re going to get shock snaps on the towing line, and that’s assuming you can find anchor plates. Those anchor plates are going to have to be through transom bolted, and then I have my doubts as you could compromise the transom.

[willk]

Originally Posted by Pascal

How should I choose the breaking load of the towing line compared to the weight of the towed boat ? Does it have to be greater ?

When you have to tow, you have to tow.

Go with a relatively light, stretchy rope. If it floats, good, but likely bulkier. You want a rope that can take some 'snatch' from waves without snapping. You are a light boat with 40hp, so you can't apply much force, the big boat will use a heavy rope to damage you - better use a light rope and let it snap if conditions are poor.

Slow and steady for the tow.

[ShinyShoe]

Originally Posted by spartacus

your boat seems far from suitable. Zodiac Pro 4.7 is lightweight leisure rib and your main power unit is just 40hp.

I'm not convinced I agree AT ALL.

1. 40hp is huge for a slow and steady chug. A 26ft yacht likely carries a 10-20hp diesel and weighs in at 3T+. But a 4m RIB with 40HP would happily pull it in conditions up to F6.

In anything but relatively calm waters you’re going to get shock snaps on the towing line, and that’s assuming you can find anchor plates.

Longer line. Keeping underway GENTLY

Those anchor plates are going to have to be through transom bolted, and then I have my doubts as you could compromise the transom.

Because of poor drilling etc, or because you think the load on the transom is too much?

If you tow a car behind another car - the weight of the other car doesn't go through the tow rope, just the force to get it moving. You can push a small car with 2 people. I'm not under any illusion that my arms come with half a ton of pushing power.

The same applies to a 3T Yacht. You can move a yacht on a marina berth using just your arms.

Your alternative is to tow alongside. But good fixings for that on the rib can be harder to find.

As an example where I can give numbers and rope thicknesses. An Optimist Dinghy weights 35kg. It's sailor even if light 35kg and add in spars, foils and no doubt some water and you have to be 80-100kg per dinghy. They are towed 8 in a line routinely. So perhaps 800kg. Done it with 30hp. The tow rope on an optimist is 8m of 6mm floating polypropylene. That's a class rule - specified to allow that daisy chained tow.

I've seen Poppies towed 2 miles like that in conditions up to F6. Never seen a line snap

[Pascal]

ShinyShoe makes the point perfectly.

The towing I will most likely do will be for becalmed dinghy or smallish traditional sailboats, up to 600-700 kg really. And that will be gentle towing at 3-4 kn max in flat water conditions.

I'm thinking of getting 20m of 10mm PP rope with a breaking load of 642 kg, so that should be plenty in most cases.

[spartacus]

Originally Posted by ShinyShoe

I'm not convinced I agree AT ALL.

Just my opinion Shiny Shoe. I used to own a Zodiac Pro 4.2 with 40hp so I wouldn't have put it on the list of suitable tow vessels in anything other than flat calm. I see Pascal has updated details to confirm that.

1. 40hp is huge for a slow and steady chug. A 26ft yacht likely carries a 10-20hp diesel and weighs in at 3T+. But a 4m RIB with 40HP would happily pull it in conditions up to F6.

Have you been in a Zodiac Pro in F6? It's a shallow hull rib and slaps when the wind gets up. Better get the the waterproofs on as you'll be shipping sea-spray!

Longer line. Keeping underway GENTLY

Agree with that, and have a knife handy if you need to part company quickly under tension.

Because of poor drilling etc, or because you think the load on the transom is too much?

Yes, I'd question the forces on the transom if the tow-rope shocked, depends on the weight of the vessel being towed, and weather conditions especially if you're advocating up to F6. Personally I'd say calmer. The Zodiac Pro does have towing eyes on it, usually for strapping down on the trailer, or potentially water-skiing using a bridle set-up. If it were me then I would reinforce them with stainless penny washers to spread the load.

If you tow a car behind another car - the weight of the other car doesn't go through the tow rope, just the force to get it moving.

Sorry - what happens when you go up a hill?

Your alternative is to tow alongside. But good fixings for that on the rib can be harder to find.

Agreed, fine in flat calm water in a marina to manoeuvre, but then you get into issue of having suitable fenders and tow points.

[Salty Pete]

"Sorry what happens when you go up a hill?"

This reminds me of the country that couldn't hold the International Waterski Championship because they couldn't get the water to stay on a slope.

[spartacus]

Originally Posted by Salty Pete

"Sorry what happens when you go up a hill?"

This reminds me of the country that couldn't hold the International Waterski Championship because they couldn't get the water to stay on a slope.

I was being pedantic Pete!

The breaking strain of the tow line will come into play given weather conditions, currents, weight of boat being towed, etc.

[ShinyShoe]

Originally Posted by spartacus

Have you been in a Zodiac Pro in F6? It's a shallow hull rib and slaps when the wind gets up. Better get the the waterproofs on as you'll be shipping sea-spray!

Doesn't have to be a fun ride.

Originally Posted by spartacus

Agree with that, and have a knife handy if you need to part company quickly under tension.

A knife is a given on a safety boat.
But we'd also often not tie the lead boat just take 2 turns round the towing bridle and bring the loose end onboard and the crew holds it, so you can drop a tow instantly.

Or two turns and a slip hitch.
https://sailingsimplicity.com/the-knot-you-never-thought-you-needed/

Yes, I'd question the forces on the transom if the tow-rope shocked, depends on the weight of the vessel being towed, and weather conditions especially if you're advocating up to F6.

Longer line, not trying to tow too fast will make a big difference. I'm not physicist but I wonder how the forces compare to being hammered at WOT...

Zodiac Pro does have towing eyes on it, usually for strapping down on the trailer, or potentially water-skiing using a bridle set-up.

Which makes me think - if you are doing 4kts towing or 20kts pulling a skiier... Is that 5 times the force with the skiier - I genuinely have no idea!

If it were me then I would reinforce them with stainless penny washers to spread the load.

Oh I would absolutely have assumed that was standard!

Sorry - what happens when you go up a hill?

Physics! Physics happens.

https://en.m.wikipedia.org/wiki/Inclined_plane

Genuinely this is why two people can push a 1 ton can but can't lie under it and lift it up.


It sounds like the loads involved in the OP's proposed tow will be not massively different from an optimist daisy chain tow. I've never known a 6mm floating poly prop break in one of those (rigged badly - the boats break!) So his proposed 10mm sounds plenty. But I'm seeing higher breaking strains too:
https://www.lomo.co.uk/products/10mm-floating-rope-braided-polypropylene-20-metre/

[Salty Pete]

Originally Posted by spartacus

I was being pedantic Pete!

The breaking strain of the tow line will come into play given weather conditions, currents, weight of boat being towed, etc.

I was being stupid Sparty.

Boats that tow other boats are designed to tow boats. They have much more power than they need just for their boat and a prop pitch also suitable for the extra load.

My advice (absolutely worthless by the way) provide suport for the strickened vessel and contact sea rescue to despatch a tow boat. You may have to assist in towing it off a reef but you could damage your own vessel by towing it over a distance. That's my two shillings worth, I'm going now.

[Pascal]

Just to re-emphasize, I'm talking about leisurely towing, not this:

https://www.youtube.com/watch?v=mN51tVp0tRs

[Salty Pete]

Hell Pascal, do you want to tow or not? Either all the way or nothing.

[Pascal]

Next question is where to fit the chainplates on the transom to attach the bridle to.

I've got two options : either two chainplates wide apart, next to the existing mooring chainplates, or two chainplates fixed to the engine mount.

There are 4 stacked holes on either side of the engine mount. I could fix chainplates through them and through the outboard bracket.

I like the second option as it will avoid having to drill through the transom, but the bridle will be quite narrow and may touch the engine while turning. Do you think that could be a problem?

[Orwell boy]

Originally Posted by Pascal

Just to re-emphasize, I'm talking about leisurely towing, not this:

Yup definitely go for the penny washers for that tow

[Salty Pete]

So not being to bright I am dabbling in the dark here.

The OP wants to tow boats and unless he accidentaly and continuously comes across boats in trouble all the time that he intends to do it for a business. Of course I could be wrong but that is the feeling I am getting.

Due to my lack of knowledge in boat towing I summise the following.

The owner of a boat has a choice as to what HP outboard they can have for there boat by being given the max and min HP. The min HP is the least they can have on their boat to drive it safely and not overload the engine and the max is normally the physical weight the boat can handle on the transome.

So, the difference between the min and max HP is the extra power that you have for towing. Also the bigger the boat the more difference there is between the min and the max HP.

Why tow boats tend to be at least 6 metres long.

I can't find any details on the Zodiac boat in question but Highfield specs on their 4.6 Metre Patrol is a HP between 60 and 80 HP. NB it is a lighter boat being aluminium.

The OP only has a 40 HP for I assume a 4.7 metre boat just enough power to run his own boat and not enough for towing.

If he is going tow on a regular basis I can't see his outboard lasting long.

What do you think? Am I accurate?

[Pascal]

Just to clarify: I do NOT intend to tow other boats on a regular basis or make a business out of it.

I am also into sailing and where I sail it is customary for motorboats to help out becalmed sailing dinghies or slightly larger traditional wooden boats. That is just solidarity and if you can, you may want to help others back and not always be on the receiving end.

Towing a smallish boat gently at 3-5 knots doesn’t take much power, and as far as I am concerned 40 hp is plenty. Don’t forget that many sailing yachts well over a ton only have a 5 or 6 hp outboard to get going.

[beamishken]

Originally Posted by Pascal

Just to clarify: I do NOT intend to tow other boats on a regular basis or make a business out of it.

I am also into sailing and where I sail it is customary for motorboats to help out becalmed sailing dinghies or slightly larger traditional wooden boats. That is just solidarity and if you can, you may want to help others back and not always be on the receiving end.

Towing a smallish boat gently at 3-5 knots doesn’t take much power, and as far as I am concerned 40 hp is plenty. Don’t forget that many sailing yachts well over a ton only have a 5 or 6 hp outboard to get going.

Theres an awful lot of overthinking going on here a 10 or 12mm polyprop rope will be fine for emergency /occasional towing with your boat. You will effectivley move surprisingly large boats with your rib. You will know on the day whether its safe or wise to go ahead or call reinforcements.
Tow eyes either side of the engine somewhere convenient where the rope bridal wont get in the way & you have easy access to it. Above the waterline but not too close to the top edge of the transom, no hard & fast rules . Unless your transom is rotten & your engine is ready to fall off it will be plenty strong enough.
What your planning isnt a huge deal folk just love to overcomplicate things

[Salty Pete]

Originally Posted by Pascal

Just to clarify: I do NOT intend to tow other boats on a regular basis or make a business out of it.

I am also into sailing and where I sail it is customary for motorboats to help out becalmed sailing dinghies or slightly larger traditional wooden boats. That is just solidarity and if you can, you may want to help others back and not always be on the receiving end.

Towing a smallish boat gently at 3-5 knots doesn’t take much power, and as far as I am concerned 40 hp is plenty. Don’t forget that many sailing yachts well over a ton only have a 5 or 6 hp outboard to get going.

Thank you Pascal, now you have described the circumstances under which you will be towing, it puts an entirely new perspective on the requirements.

You are to be commended for assisting the sailors and especially the young ones that don't have much experience.