Vintage Gentleman’s Cruiser

[CookieOldParticipant @cookieold]

Hi Paul and Bob , Thinking outside the box as i know nothing about boat design , If you are worried about the back of the Gentleman,s cruiser being torn off then could you not tie the bow to the stern with a Ali brace and connect the tugging post to that.

Dave

[Bob Abell 2Participant @bobabell2]

Hello Dave

Thanks for your suggestion, but Paul has already pooh poohed that idea!

And he's pooh poohed my idea too!….However….

My idea is still good…

Imagine a rope fastened to a wall and you pull as hard as you like

Result……Nothing happens apart from a taut rope

This is the clever bit……Thread the rope through the hull …….and repeat the exercise

Result………Nothing happens apart from a taut rope…..With no strain on the hull structure

If the Samson post is out of position, we would get a coupling effect and the hull will try to rise or go under slightly

The motor drive on the central plank is the wall and the hull structure does nothing in the equation

If the going gets tough, I just switch the motors off and get towed along….No harm done

If Lady Joyce puts up any resistance, then Thor will also get the coupling effect

All good fun

Capt Bob the Optimist

[Gareth JonesParticipant @garethjones79649]

Bob,

I am afraid to say it, but your last post makes absolutely no sense to me at all. If you have a rope attached to a wall and pull on the rope, whatever is attaching the rope, and the wall itself, have to resist the load in the rope. If they are not strong enough, something breaks or the wall is pulled over.

Whatever you attach the rope to in your boat has to resist the load resulting from both boats trying to move in opposite directions. If the attachment point is not strong enough, it beaks. It is unlikely to 'pull the back off the boat' but it could pull off your samson post, for example.

Maybe I am just missing your point after a hard day at work for the first time in 5 months.

Gareth

[Bob Abell 2Participant @bobabell2]

Gareth

The wall story is illustrating that the hull is not taking any load

Imagine the motors, propshafts and attachment point were all bolted to a plank and to stop the assembly sinking, it is fastened to a floatation device called a hull

For starters, there's no sense in making the hull extra strong, as Paul wants me to do

I am assuming the Samson post will be strong enough

The plank could be a 3mm steel plate…..No model boat could break that!

Bob

[Gareth JonesParticipant @garethjones79649]

Fine Bob, I understand now, it was the analogy to the wall that confused me.

Gareth

[Dave MilbournParticipant @davemilbourn48782]

Bob

The wall story is illustrating that the hull is not taking any load

Try Newton's Third Law. If there is tension in the rope then it means that there is a force in play. Where there is no resulting movement then that force is being cancelled out by an equal and opposite one. In a mechanical system the application of a force on a body represents a load on it i.e. you can't tie up a hull stern-on to a wall and turn on the motors without applying a load on the hull. In this case it's the tension in the rope pulling against the Samson Post. Putting it simply, the hull is being pulled by the wall just as much as the wall is being pulled by the hull. The fact that they don't move doesn't mean there is no force or load in the system.

I'm sure that you know what you mean; it's just that it doesn't come across in your posting.

(Disclaimer: I only just achieved Grade A in my GCE A-level Physics. I'm sure things have moved on since 1970 but I reckon old Isaac's laws are still pretty much set in stone. I stand to be corrected.)

DM

[Bob Abell 2Participant @bobabell2]

Dave

In the wall story, the hull is simply resting on top of the rope and held in place with chewing gum

Bob

[Dave MilbournParticipant @davemilbourn48782]

OK. I should have known that.

DM

Edited By Dave Milbourn on 29/09/2014 23:30:58

[Amy jane SeptemberParticipant @amyjaneseptember49770]

I agree with Bob. When ever I needed to tow another boat, I would always tie the tow rope around the out board motor, not the boat. I could then give it as much throttle as needed with out fear of pulling the transom out! (ok, so I had a 90hp outboard on a 40 year old 14 footer….)

I've been thinking about this tug of war thing, and might just throw my hat in the ring. Of course being at the other end of the world complicates things a little! So I was thinking of building a virtual boat, how I would do it if I was building the real model, and leaving it to you folk to decide if it was a contender. Any interest, anyone ?

[CookieOldParticipant @cookieold]

Great to see everyone has a point on this , we all know what we mean ( not easy to put into text sometimes) Bob its a shame its not a natural pond at Haywood you could put down a sneaky anchor then let Paul pull like mad flatten his batterys then raise anchor and drag Thor all over the place HA Ha , but that would be cheating .

Best Regards Dave

[Paul TParticipant @pault84577]

Dear All

We will just have to agree to disagree over the strengthening of hulls.

I could get all technical and add the closed system calculations to the discussion to prove the point but this would only confuse people even further so it might be best if we just get on and do our own thing.

Dave

I like the anchor idea

Paul

[Diede van AbsParticipant @diedevanabs87670]

AJ,

What you could do is make a contender and do a static pull video'ed for the YouTjoopz. If you measure the pull of your boat with a scale (for example such one to weigh your luggage with) and Paul and Bob do the same in the UK, you could compare results and declare a winner.

Ok, I agree it is not by far as exciting as an actual head-to-head pull-off, but it IS some form of getting contestants battle across the globe. And even Ashley could join this way

Re the strengthening discussion: If, and only IF, Bob manages to get his samson post tied rigidly to his motor base (and REALLY close to it as well, not well above deck – that is introducing quite some leverage…), he might be quite right to not have to strengthen all the rest of the hull. The force from the rope ties in to the same point as that from the motors. The hull merely acts as surrounding, only keeping the motors separated from the liquid surrounding them.

You'd need quite a strong motor base and samson post connection, though, so you'd probably need to strengthen everything close to them quite a bit as their connection needs to be friggin' rigid…. And I think that is what Paul is referring to.

Edited By Diede van Abs on 30/09/2014 17:29:53

[mike farrellParticipant @mikefarrell21522]

Now then All . Amazed at the turn of events What started as a lark is now going International . Europe , New Zealand and G.B.

I HOPE WE DO NOT HAVE AN INTERNATIONAL INCIDENT .

Seriously how good it is to have a different slant on model boats ,it certainly created discussion of a different tack.

It will be something better for the public to watch .I for one will be there to enjoy the outcome and may the best tug win (oops) Michael

[CookieOldParticipant @cookieold]

Good shout Mike , But i still think Captain Bob will pull this off after all us Live Steamers stick together.

Dave

[Bob Abell 2Participant @bobabell2]

Thanks Dave

Just don't put your shirt on it!

It's an interesting contest though

Pauls custom made Mauling machine against Bob's Dark Horse!

Bob

[Gareth JonesParticipant @garethjones79649]

Diede and Bob,

I think you are both still misunderstanding a detail aspect of the loading on the boat and I thinks its an important one because it is probably what is driving Paul to say the whole back end needs to be strong, not just your samson post.

There are two major loads being applied, the forward thrust from the propellor and the restraining load from the rope. Its the first of these I think you are misunderstanding. The thrust from the propellor(s) is not transmitted through the motor mount(s). The thrust is generated by the blades and transferred into the hub. Immediately in front of the hub there is usually a locknut, which is tightened against the hub to prevent it unscrewing and effectively becomes part of the hub. Immediately in front of this nut is a thrust washer which transfers the propellor thrust into the tube of the prop shaft. In turn this transfers the thrust into the hull through the point where the tube is fixed into the hull. There should be no significant axial load into the propshaft itself, only the torsional loads which drive the prop. Neither the motor bearings not the typical universal coupling are intended to take any axial load.

The restraining load of the rope is taken by the samson post and transferred down into the hull, maybe directly to the keel itself but more likely partially through the deck as well.

These two principal loads are acting in opposite directions and have to be transferred through the structure which lies between the samson post and the propshaft tube(s). This could be a complicated load path with the structure having to take a combination of compression, tension, shear and bending loads. Hence I think this is the reason Paul wants to strengthen the whole back end structure and there is method in his apparent madness, or rather his attention to detail design.

If you want to minimise the amount of structure exposed to this loading you have to attach the tow rope to the prop shaft tube at the point where it passes through and is attached to the hull. However to do this the rope is going to have to pass through the transom very low down, probably below the waterline which is unlikely to be practical. If you have two propshafts this will make it even more difficult to implement.

Gareth

[Bob Abell 2Participant @bobabell2]

Thank you Gareth

I think you are seeing forces in the hull which don't exist?

The hull is not in any way involved with the towing forces…..Apart from the up or down forces created by the tow line causing the hull to rise or settle when the slack is taken up

The motors and prop shaft are bolted to a 3mm steel plate

At the rear end of the plate is a drilled hole for the tow line…..This is the Samson post in disguise

The props push against the prop tubes, which may have thrust bearings, the prop tubes are bolted to the plate

The towing forces are thrust on the prop tube bearings, into the prop tube retaining bolts and into the base plate and the baseplate tow hole

The hull is fastened to the base plate with a piece of string……So to speak

There are no forces taken by the hull

Bob

[Gareth JonesParticipant @garethjones79649]

Bob ,

I would be interested to see a picture or drawing of this motor/propshaft installation because it does not sound like anything I have ever seen before in a model.

Gareth

[Bob Abell 2Participant @bobabell2]

I don't think it's been done before, Gareth…..There's not been a need for such a set up

I will do a simple sketch tomorrow……But, Paul may twig it then!

It doesn't matter really, because I'm on a hiding to nothing!

Bob

[Colin BishopModerator @colinbishop34627]

I don't think we should get too hung up on this. Model boats are not the same as their full size counterparts and the model structure is proportionately a lot stronger than in full size vessels.

As Gareth says, the thrust from the prop is normally taken on the trailing end of the prop tube in a model boat. The tube itself is firmly attached to either the keel, or, in the case of twin screw boats, the internal structure of the model – usually the bottom skin of the hull and probably one or more internal frames or bulkheads through which the tube passes.

The Samson post therefore needs to connect with with the keel or internal framework of the model to distribute towing loads across the model structure. Job done! It is a similar situation in a sailing boat where the mast will pass through the deck and be stepped on the keel to transmit the load of the sails.

On a full size ship you will have thrust blocks which transfer the thrust of the propellers to the structure of the ship. Again, as Gareth says, the motors and couplings play no part in this, their job is simply to transmit rotational forces to the props.

Colin

[lnvisiblemanParticipant @lnvisibleman]

Have any of you have a clue what you are talking about ????

Mike ( My Phd means nothing to me. Why should it mean anything to anyone if they don't want to listen ????????&nbsp

[lnvisiblemanParticipant @lnvisibleman]

Bob is the only one of you talking any sense !! Why do you all insist on arguing when you have no basis to work from ????

[Bob Abell 2Participant @bobabell2]

Hello Mike

I'm much relieved that you have confirmed my idea has some logic after all!……..I was beginning to have doubts when the cream of the land couldn't grasp it

Welcome back to the forum, your blunt snippets of wisdom are very welcome

All the best Mike

Bob

[Colin BishopModerator @colinbishop34627]

Bob, we are all basically saying the same thing here. The driving force from the prop is taken by the end of the tube and then transferred to the engine mounting plate by the bolts holding the tube. The engine mounting plate is then fixed firmly to the hull structure which in turn transmits the driving force to the hull itself and thus moves the boat along which is what we all want. If you install a Samson post to take a towing load then the post must be attached to a part of the hull structure (and I include the engine mounting plate in this) sufficiently strong to take the force generated by the towing load.

It's all just basic mechanics really. For example, I was taught that if you need to tow a yacht in anything other than very calm conditions you do not attach the towing rope to the fairlead on the bow as you would probably just pull the fitting out of the deck. Even a small Samson post mounted a bit further back bay not be able to take the load of the whole boat as it is usually intended for mooring purposes rather than towing. So the correct practice is to take lines back to the sail winches at the sides of the cockpit and bring them forward via the bow fairlead so the load is taken by the winches which are in reinforced mountings designed to take the pressure from the sails and, together with the mast and other sail winches, transmit their thrust to the hull.

Colin

[Bob Abell 2Participant @bobabell2]

Thank you, Colin

You are correct in what you are saying

But I think I've said enough on this subject?

Bob

[Author]

Posts