The trick is to buy an off the shelf small pulley. I think that Leece-Neville only do the one I bought, because I looked at the Pulley options on their website. The shaft on the 175 A alternator is imperial 7/8 inch so you cannot just buy any old pulley, and they have a funny keyway. Anyhow they are easy to buy on eBay USA.
The small pulley nut can be undone with a ring spanner and an 8mm alan key that slots into the alternator shaft end. I cut off a section of alan key and slotted it into a ratchet handle as it requires considerable force to remove. Like 100 ft lbs!
The big pulley is easy to take off, just undo the four bolts and gently prise it off. It has a male flange/hub that sets into the crank pulley about 3mm. If you actually have the small pulley to give to the machine shop it sure makes it easy to explain what you want.
If you are going Lithium, you will clearly need a special regulator for the alternator with a heat sensor so it does not overheat, and you should maybe think about ducting cooling air directly at the alternator.
I wish I could go back to the boat and fit it all and make sure it all works well. My original set up had two idler pulleys one top and one bottom. This improves the wrap, however the belt has a slack side and a tight side, and I was always told to only add the idler to the slack side, if possible. So I experimented with the old set up removing the idler on the tight side ( bottom one), to no real improvement. My current view is that without any idler the wrap is 160 degrees, and with the top idler only, which is the slack side, it will be a bit better at say 170 degrees. These are both very good wraps.
How to decide on the pulley you need
I went about it by seeing what pulley Leece-Neville could supply. This pretty much nailed it as the biggest they had was the K section 8 rib one.
Then I looked at various websites that sell high output alternator kits and conversions and cribbed the Micro v belts that were used, and looked at the size of drive pulleys they used.
Many of these alternators are running very small pulleys like 50mm, the drive pulleys are typically 150mm.
So they get the 3:1ratio from 150mm drive pulley and 50mm alt pulley.
We are much better off as the 230mm pulley machines down to 200mm, and we need 3:1 so 67mm would do it. The one I bought is 69mm max diameter but 63.5mm across the grooves. If making from new one could have 230mm drive pulley and a 75mm alt pulley. But that would be expensive and frankly unnecessary.
There are two forces to consider; We are think the small pulley as there is never a problem with the big one.
1. Belt wrap, i.e. friction. This is how much belt is in contact with the pulley. So the circumference of the pulley x the proportion of the pulley wrap. So 160 degrees wrap is 160/360 x 2 x 3.14 x radius
2. then there is the lever effect which is also the radius. Imagine the belt being your arm and the radius being the length of the wrench, a short wrench needs a stronger arm.
3. then there is the width of the belt, i.e. number of ribs. 6,8, 10 or even 12.
Clearly by going for smaller pulleys you add load on the belt. Not only from less surface area in contact with the pulley but also the shorter turning moment.
Looking at these web sites they all use serpentine (micro v belts) and either 6, 8 or 10 ribbed belts.
Having decided that we are in the right ball park, and seeing as Leece-Neville sell that pulley a their top one I was happy with the engineering choice.
Then I thought it would be interesting to calculate how many KW such a belt would support;
Now I am not 100% sure, so any engineer who knows better please correct me but
by the tables I think that the 8 rib belt should be able to run 8 X 0.41KW assuming 120 degree wrap and a belt tension of 10kg per rib on a 45mm pulley.
That is 3280 Watts with a belt tension of 80Kg. However we have better wrap so could add 40% = 4592 watts.
This is 170A at 27v for a 45mm pulley, but we are running a 63.5mm pulley so the radius is increased from 22.5 to 31.5 which is another 40%. Which works out to 238A at 27v.
So in conclusion we could reduce the belt tension slightly by experiment. You want it so that it does not slip at maximum output. I intend to watch the belt, run the engine at say 1500 rpm and engage the bow thruster. If it does not slip then, reduce the tension slightly until you get slip then tighten it a bit. I have a Kriket tension gauge. I reckon it will be at about 75kg. Which is pretty tight, but the belt gates specs about 100kg.
It has been a fun project.
Nick (in the UK)
Amelia AML 54-019 in Leros.
Begin forwarded message:
Hi again Nick,
Just wanted to add.
In my project changing to Lithium with Oliver on “Vela Nautica” help and guidance I was going to disconnect the engine alternator from the charging system.
Thanks to you I will now add the engine alternator back into the system with a charge controller for the Lithium batteries.
I have added the pulley and belt you showed to my Ebay buy list. Just got to find a machine shop here in very expensive Le Marin to machine the engine pulley.
Thanks and Very Best Regards
Barry and Penny
“SV Lady Penelope II”
Amel 54. #17
Sainte Anne anchorage Martinique