Bow roller design tips?

[prv]

Not the whole assembly, just the roller itself.

Both of Ariam's are knackered, and need replacing before I do much more anchoring. I'm going to go along to the boat tomorrow evening to measure up, but I thought I'd see whether the forum has any ideas I should incorporate or pitfalls to avoid.

The starboard one is pretty much only used for the snubber (the anchor obstructs it when stowed, so can't be used for mooring pendants). So the required profile seems pretty simple, just a roughly semi-circular dip from one side to the other. It's the port roller, where the chain goes in and out and the anchor is stowed, that's more tricky.

It was originally flat (ie, cylindrical) with a narrow groove in the centre. This is obviously designed to accommodate chain by having the vertical links fit into the groove while the horizontal ones rest on the sides. Unfortunately, the wedge-shaped profile of the Spade anchor shank has been forced down into this groove, pushing the sides of it outwards and splitting the roller apart (this is why it needs replacing, big lumps are missing and the chain jams in the gaps). I need to come up with a new profile that will carry the chain, and also fit the shank of the anchor without strain.

I can really only design this with the parts in front of me, but some questions occur:

How big a difference does roller diameter make to the effort of hauling chain; is there a minimum to avoid going below? The roller assembly has a closed top, so I can't end up with the anchor sitting significantly higher than it already does, which limits the diameter I can have.

Any reason not to just have a relatively steep-sided V form, or will chain jam in this?

Similarly, is the narrow groove design really that much better than a flat roller?

I'm going to get it made by Versatile Marine in Penryn; they can do a range of materials but I guess acetal is best?

(Or if someone can suggest a better maker I'm open to suggestions.)

Cheers,

Pete

 

[Andrew G]

Hi Pete. The groove in the roller is, in my view, very important. It needs to be deep & wide enough that the chain sits there and doesn’t skip out. This prevents, and unwinds, twists in the chain. I made mine from a standard “cotton reel” bow roller (flat main profile with a steep rise to the max dia at the ends). I just routed the groove myself (router table with roller constrained side to side and rotate carefully by hand, a small cut at a time. I used my best 12mm router bit when a 10 would have been better (8mm chain). The other roller is a plain cotton reel, no groove. The diameter of the roller needs enough meat to retain strength even/especially at the bottom of the groove. Cheers, Andrew

 

[prv]

Hi Pete. The groove in the roller is, in my view, very important. It needs to be deep & wide enough that the chain sits there and doesn’t skip out. This prevents, and unwinds, twists in the chain.

Mmm. Preventing twisting would be good, since I have a windlass close behind the roller which doesn't like twists, and an anchor shackled on without a swivel. The current roller doesn't seem to work well at this, but perhaps that's due to the missing chunks.

I can see it's worth having a groove if possible, so I'll see if I can design one compatible with both chain and anchor.

Pete

 

[RobbieW]

Sounds like I have a very similar set up to yours, Pete - spade on 10mm chain, no swivel. With the help of a friend with a lathe, I had a new roller made up using a piece of Delrin bar a couple of years ago. The diameter of the roller is very constrained by the design of the bow fitting, I didnt put a groove in because I felt the remaining diameter was insufficient for the weight, 25kg, of the anchor when resting on the roller. We used a 'bobbin' design where the centre is a little over the width of a chain link, 35mm, then slopes up at about 45 degrees to 20mm lips.

The chain does twist, that doesnt really cause a problem but without a groove the anchor doesnt 'locate' when home. I'm sure you already know this but one thing to consider is that the Spade shank is quite narrow around the point its going to sit on the roller, around 10mm. That puts quite a point load on where the anchor sits. When doing the job I upped the pin from the 10mm threaded bar (which had fractured) to 12mm solid. The old rollers had a stainless sleeve which actually sits on the pin, the hardest part of the job was drilling those out by 2mm to re-use for the new pin.

 

[garvellachs]

I designed some new rollers for our 8mm chain recently and had them turned up - they seem to work ok. I drew the roller in Word with some chain links and moved them around until the links would fit the roller snugly. I can send you the Word doc if you PM me an email address? I used the company that advertise on eBay (in Cornwall I think). The central groove for the chain links in my rollers leaves a weak spot between the two halves of the roller - with hindsight I would have gone for slightly more "meat" there I think. Our anchor (Fortress) lives on the pulpit so no strain there.

 

[RobbieW]

I've put (I hope) some drawings into my dropbox account; here - https://dl.dropboxusercontent.com/u/43708122/Anchor Roller Design V1.PDF & here - https://dl.dropboxusercontent.com/u/43708122/Anchor Roller Design V2.PDF. V1 was the old roller and initial thoughts, V2 was more or less what we made.

Looking at the drawings, I realise that the V1 thought didnt work when drawn out to scale - the shoulders just didnt seem high enough to stop the chain running off to the sides. So I ditched the groove to improve that but retain some meat in the centre. So far its had one seasons use, mostly at anchor for 5-6 months; theres a little wear round the centre line but its good for a few years yet. Having just checked, we left the min dia at 60mm as the drawing is confusing.

 

[Marsupial]

The question of diameter, last year I assisted a yacht trying to recover an anchor. The problem was that the "nylon" roller had disintegrated leaving only the stainless axel which was about 1" dia. In practical terms was impossible to pull the 10mm chain over the axel even with a large electric windlass - hence the need for assistance (two chain hooks and a dinghy). So my list of design considerations would include and easily removable axel and replaceable roller - OR at least a 3" diameter axel so you would have a fighting chance if the plastic bit breaks up.

The next requirement would be for the cheeks of the fitting to be fettled to remove all sharp edges so in the event of having to use warp or chain hook and warp from that roller the warp wont chaff through

 

[Versatilemarine]

Hi all,

There are some good suggestions here. Although I would not advise cutting grooves in plastic bars with a router! I have visions of finger tips being mangled. Better (safer and quicker) to get someone with a lathe to make them up.

Some simple notes on bow roller design:

Firstly material: Metal will undoubtedly last longer but will be more expensive due to material costs and longer machining times. There may also be metal on metal noise issues (chain on roller) which may or may not be important. Plastic rollers will be cheaper and quieter but will not last for ever. If taking this option it might be better to order spares if you intend to keep the boat for a number of years. If you are going to get a engineering company to make plastic bow rollers please do not ask for 'Nylon' as generally it is not used in the marine environment. Nylon absorbs water causing swelling and jamming. Better to ask for Acteal / Delrin or if you are old school Tufnol.

Shape: In some situations it may be a requirement of the roller to guide the chain through a windlass or similar system with out the chain twisting. In this case the roller may require some form of stepped profile to maintain the alignment of the chain. If this is not a requirement it is better to opt for a simple V or U shaped profile as this will be stronger and the roller will last longer (this also should be cheaper to produce). Where a stepped profile is required it would be best to give the engineer a sketch of what is required or an old bow roller to copy as there are many different forms of roller profile depending on the set up.

Size: Generally Bigger is better. As a larger roller will reduce the effort on the winch, windlass or sailor. Usually the size of roller is constrained by the metal work around it. One point to bear in mind (for plastic rollers). If the edges of the roller are thin and they extend past the metal cheeks of the bow roller assembly the roller can be broken if the chain pulls at an angle to the bow.