There are MANY different forces involved in on-board
power generation and this might help some people understand it better.
Transfer of
power from the driving sheave (crankshaft pulley) to driven sheave (alternator and
water pump sheaves (pulleys) depends on friction between the belt faces and two (or more) sheaves, so the more surface area contact between them, the better. As well, the more pressure between them to limit slippage, the better - within limits. Bigger diameter sheaves, with greater contact area and greater torque capability, will transfer better than smaller diameter ones, but generally there are space limitations and the smallest are cheapest and so they are used.
Also the greater the greater the "wrap-around" of each sheave the better, which is why most automotive serpentine belts travel around idler sheave/s in the opposite direction. This is not usually possible on V-belts. Without this, the total perimeter of the contact area of the whole belt on two or three sheaves cannot exceed 360 degrees, so 120 degrees each on average. This means that, due to physical limitations, some will likely get much less than 120 degrees. As the
water pump usually requires the least transfer of power to circulate the
cooling water, it would be best if that sheave receives less than the crank and the alternator.
On a V-belt system especially, it would be preferable to have a dedicated belt for the water
pump, and a dedicated belt to the alternator, which allows the two sheaves in each case to have an average of 180 degrees of contact, the larger having more and the smaller alternator sheave having (usually) far less, and therefore most slippage occurs there on that tight alternator radius that the belt must travel around. But two separate belts requires a double-groove driver at the crank.
Most of the wear of a v-belt occurs at that small alternator sheave, as the two belt faces are forced to bulge or flex around the sheave faces and so they have difficulty maintaining proper contact. They will also be forced to bulge far more than at the driver sheave so they can never perfectly match the two dissimilar sheaves. Only if the driver and driven sheaves are the same diameter could that occur, and even them not perfectly, as there are transitional stages in the flexing.
Also, the best angle to transfer power of a large diameter sheave is not the same as a smaller diameter sheave, due to more or less distortion of the belt. That is why there are differences in the manufactured angle of belts depending on their application. There is not one standard v-belt angle for all sheaves, even withing the automotive industry!
The radius (and diameter) of the effective transfer of power between belt and sheave is generally regarded as halfway up the side of the belt and that should be slightly higher than about halfway up the sheave face. It certainly is not so low that it will allow the belt to bottom out in the V groove of the sheave, and not so high that the top (outer face) is higher than the sheave face.
This effective diameter of the sheaves can have a marked impact on the speeds of the sheaves. Not so much of a difference on the large driver but a big impact as a percentage change of the driven alternator sheave. But the serpentine belt can also flex around a much smaller diameter sheave, so if you reduce the size of the alternator sheave from say 3" to 2" you have a 50% increase of the speed of the alt. Conversely, if you can increase the diameter of the crank sheave, the belt speed to increase, further increasing the alt speed.
On most boats, the matching surfaces of the contact faces of the belt and sheave will be about 3/8 to 5/8" wide (high) and that is a large surface to have to flex. Compare that to the approx 1/10th inch of contact that each V of a serpentine belt has with its sheave, and you will see that there is a huge reduction of flexing having to take place, and that the overall reduction of contact area is compensated for by having many grooves to do the job. So the belt does transfers power better and wears less as well, producing less dust.
Many people do not know that while some industrial and
marine alternators are designed to produce high amps at lower
RPM, automotive alternators are not - and that is one of the biggest reasons that those alternators, and their derivatives, when coupled up to a slower
diesel with a small driving sheave, burn out. They just do not get enough air through them to keep them cool while struggling to produce higher amps. Automotive alternators are moved by a huge crank sheave of about 8" to 10" diameter and an alternator sheave of about 3" which means an increase in speed of 3 times. When you are in your car doing 3,000
RPM on the freeway, your alt is doing about 9,000 RPM and keeping cool at the low demand asked of it. With my typical, brand-new, Universal M35B marine
diesel engine running at 2,000 RPM, my factory supplied automotive alternator(!) was only doing about 3,500 RPM, yet trying to fill those deep-cycle
batteries. Yes, I have changed it to a Delco CS144 etc).
It is a very smart idea to make the kits of the "slip-over" driver sheaves to install on the crankshaft of a marine diesel, as not only is it a very simple device to install by bolting onto the front face of the original sheave (no worries about removing the old sheave and dealing with the
oil seals) but the diameter jumps by about 20% to 40% - increasing the belt speed, due to the change to a serpentine. The kit should also supply a larger sheave for the water pump so that it does not increase in speed so much that aeration takes place, and so reduces the
coolant effectiveness.
So the mystery to me is why do marine engine manufacturers or modifying companies not automatically install larger diameter driving sheaves?
Being an ornery type (my wife tells me) and needing a challenge (I tell her!) I am in the process of machining up my own kit - but I already admit that to do it right is a time-consuming job and it would have been smarter to simply buy the kit. On the other hand, my changes will be custom-made of heavier materials for optimum performance on my particular engine and alternator.
Cheers, RR.