Hello Steve
I too would like to replace the fixed two blade prop on my P27 for next season, and have been doing some research...
I now know that whatever prop I buy, it should be able to be fitted without need to withdraw or change my existing prop shaft, but I will need to specify my shaft dims [dia, taper, keyway etc] when I place my order.
And whilst I now know from the PSSA forum that my 1989 P27 #146 almost certainly had a 1" dia, 1in12 taper shaft with 1/4" keyway when it was built, with two previous owners, the last of whom told me that it had been renewed in 2007, I have no way of knowing for certain that it's not been changed for metric at some stage...unlikely perhaps, but I've just discovered that what I was led to believe was the original 1989 engine is in fact a 2004 replacement! No wonder the paintwork looked so well cared for...so I feel that I need to remove my existing prop and check the shaft dims before I commit my money...and until I run some speed/rpm checks with a clean bottom over a measured mile, I've no way of knowing whether the pitch of my existing fixed two-bladed 14" diameter prop is any where near correct for my 2GM20 or whether it is a hangover from the original engine which might even have been a 1GM10 for all I know...[ah, the joys of buying a used boat]
In my case I also happen to have a non-standard Yanmar 2GM20F, 18 crankshaft HP at 3600 rpm for 2 hours intermittent, 16 crankshaft HP at 3400 continuous rating, with a 2.62/1 reduction gearbox = approx 15 prop shaft HP at 1300 shaft rpm.
You'll need to have similar data ready from your own engine & gearbox.
The most comprehensive evaluation of folding and feathering propellors that I know of was the 10 page article in the May 2009 edition of Yachting Monthly - I would suggest that the cost of purchasing a download of it from the YM website will be money well spent, since everybody has their own priorities as between cost and performance.
For example, a three bladed feathering auto-pitch prop would probably be the choice of the 'money-no-object' cruising man [maximum efficiency under all sea states with maximum economy], whilst a two bladed folding racing propellor [eg Gori Race series] would be the sort of prop that a 'money-no-object' racing man [minimum drag, sod the efficiency...] might choose.
At this stage, the easy answer is to go along to the Boat Show armed with the above info and quiz the prop manufacturers' sales staff and just accept one of their many and varying "recommendations"...
On the other hand...
A good propellor manufacturer will ask you more than just which model of yacht and engine you have before recommending pitch and diameter.
Maximising efficiency comes from first maximising diameter. An ordinary two bladed fixed propellor on a P27 will be 14" diameter, leaving the requisite minimum 2" clearance between blade tip and hull. However if you decide to fit a two bladed folding propellor, many - if not all - will have an extended boss that effectively moves the blade tip position about 3" further aft - the relative angles of the shaft to the hull means that there is then enough increased clearance to safely fit a 15" dia prop [ eg I have seen and photographed a 15" dia x 10" Brunton Varifold fitted on the identically hulled SS26].
In my case, having decided on a two bladed folding prop because of cost considerations [1st choice Darglow Flexfold, 2nd choice Brunton Varifold] , I will certainly be buying a 15" dia one.
Having established max dia, then pitch will follow, dependent primarily on the shaft horsepower and rpm delivered by your engine and reduction gearbox, waterline length, and displacement. However some boat show salesmen will simply base their recommendation on their experience, perhaps claiming previous experience of your boat model ["A Parker 27, you say Sir? - Oh yes, you mean a Seal 26..., with 1GM10?... we usually recomend a X dia x Y pitch brand Z..."]
And they will invariably recommend a size that only absorbs full power at max permitted intermittent rpm...So for 99% of the time, when you want to hear yourself speak and you don't want your teeth to chatter, you will probably limit your engine rpm to about 2500, and hence will cruise well below your theoretical max speed, albeit that you have more horsepower and speed available in extremis.
However, my understanding is that you can choose to ask them to specify a larger pitch that will absorb full power at less than max rpm, which is equivalent to selecting a higher gear ratio. You will accelerate a little slower [just as in a higher gear in your car], but you should reach maximum speed at lower rpm = less wear and tear on both engine, eardrums, teeth plus lower fuel consumption for a given speed! The downside is that if you carelessly bang the throttle wide open, you will feed the engine with more fuel than it can burn, and you will trail clouds of unburnt diesel [£££] behind you... On the other hand, when the speed falls off because you are punching into a head sea, you will still have available throttle to open, feeding more fuel, and increasing HP to counter the additional load and hopefully maintain rpm and hence speed.
The 'Propellor Handbook' by Dave Gerr is widely regarded as the definitive reference source for all matters relating to propellors - I certainly feel that it was well worth the £15 I paid for a new copy from Amazon...
Waterline length means that the maximum speed potential of a Seal/Parker 26/27 is 6.56knots [ SL [Speed-length ratio] for displacement sailing yachts hulls = 1.34, and theoretical max hull speed = SL x#8730;WL = 1.34 x #8730;24.0 =6.56kts ], provided that you have sufficient power available.
In my case, using David Gerr Chapt 2, and given my guesstimate of fully laden displacement of 6,500lbs with full tanks & cruising gear aboard [Parker’s P27 brochure weight of 5000lbs is a net weight, excluding engine, gearbox, shaft, batteries, etc,etc!] I have calculated that if I want to reach max hull speed, I only need 12.9 horsepower at the shaft, whereas I actually have about 15HP available, at 1300 shaft rpm [the fact that I should ideally fit a 17" dia prop, which is physically impossible, is another matter that means increasing the pitch to partially compensate for the undersize pitch].
Similarly, I have calculated that if one only has 7.5HP available at the shaft [eg 1GM10], then max theoretical speed will be lower at 5.46kts.
http://www.electricboats.co.uk/surfprop/ - don't be put off by the electric boats bit, this is just the first site on Google that leads you to Surfbaud], which lets you play around with different permutations.
If nothing else, you will get a better feel for the way in which the prop converts engine rpm and HP into forward motion, and you will be able to hold your own with the prop salesperson!
sincerely
Geoff Sheddick
Parker 27/146 "Stroller'
Geoff Sheddick
Parker 27/146 "Stroller'