Thanks everyone. According to the dealer, Volvo recommends the following: 3 blades, 18x12 if folding, 17x12 if fixed. I'm assuming diameter comes first. If this is correct, why the difference? As I mentioned I won't know what I have until I haul in the spring, but the factory prop sounds too small.
Doug
As I said in post#2 the actual size will depend on the specific design of prop because they vary in things like blade shape and efficiency, blade area, BAR (which is the proportion of blade area to total prop area). The size recommended by the manufacturer of the prop is the one that they know works for the boat/engine combination. The Volvo folder for example is fundamentally different in blade design from the FlexoFold or the Featherstream. The prop calculators such as Propcalc and Vicprop are based on a middle of the road fixed 3 blade (such as the standard Volvo fixed prop), so a 17*12 is a good base recommendation for your boat.
Just to give you an idea how different props can be different sizes, my 2015 B33 with a D1-30 came with a fixed 2 blade 16*13, but the FlexoFold I have is a 16*11 and works perfectly - achieving the required revs and speed specified by Volvo. I have never run it with the OE prop, but the engineer who commissioned the boat said it achieved the same performance with the folder as a standard boat.
So, if you are considering buying a non standard prop take the recommendation of the supplier. However, one thing to bear in mind that changes to the basic boat, particularly added weight may require a deviation. Many cruising boats are marginal on engine power - for example 38s with 30hp engines or 40/41/42s with 40hp. Once they are loaded with full liveaboard cruising gear (rather than kitted out for short term charters in gentle locations like the eastern Med) they struggle to maintain good speed, particularly in adverse conditions. In these circumstances, reducing pitch by say 1" will increase revs at any given speed by around 250-300 so allowing the use of more of the limited power. Alternatively you could consider buying a variable pitch prop such as a Bruntons Autoprop which automatically adjusts pitch to the load placed on the engine. These are popular with liveaboards who spend long periods motoring or motorsailing.
If you have the luxury of buying a new boat, consider the larger engine option, particularly if you intend adding significant weight for cruising. Displacement is much more important than length of boat. As examples, the J&J designed 32/33/34 models from the late 1990s to 2010 displaced between 3800 and 4500kgs. My Farr designed 33 displaces 5500kgs, the same as my previous J&J 37. The earlier boats are fine with their standard 20hp engines, but I chose the 30hp for my new boat against the standard 20. Design displacement of 5000kgs is the break point for going up to 25-30hp, 7000kgs for 40hp and 8500kgs for 50hp+. Liveaboard gear on a 42' can easily exceed 2500kgs, plus all the extra windage of davits, sprayhood, bimini, radar arch etc.
As with most things to do with boats, propellers are a compromise. For many people the standard prop is fine, but a folder/featherer really does give significant benefits in terms of sailing performance, particularly in light airs and most (but not all) perform as well if not better under power than the standard.
BTW if your boat is still in commission and has a reasonably clean bottom, carry out some structured speed trials, plotting speed through water against revs in 200 rpm increments from say 1600 rpm up to maximum. A key measure is being able to achieve within 150 rpm of the rated maximum. If your prop is indeed too small you will exceed this and the governor will cut in at the rated maximum. This is essentially the commissioning test on new installations.
Hope this helps.
