|Topic Review (Newest First)|
|09-19-2003 11:43 PM|
I just installed a Kiwi Feathering Prop on Tuesday.
In really light winds Wednesday we gained between 0.4 and 0.9 knots. Wind speeds started at 5 knots and dropped quickly to 2knots. On other nights we would have quit long ago.
At night we were also able to see the phosporescent trail of the rudder and offset prop with the old fixed prop. With the feathering prop we could see the trail disapear as the blades feather.
Real world motoring speed added .4 to .7 over most of the range. Top cruising speed climbed both from a reduction in vibration and increased power (5.4 to 6.7). Reverse is much better.
Prop has un-geared composite blades on bronze hub. Boat is 41 ft fin keel, spade rudder, about 21,000 lbs
|05-27-2003 12:28 PM|
We had this discussion at our handicapping meetings in Nova Scotia. Under our system 1 point ASPN is roughly 6 sec/mile. Previously we did not differentiate ratings between two identical boats one with a folding prop and the other with a fixed two bladed prop. The main reasoning was that anything less than 6 sec/mile would be a partial point under our system and we did not wish to go there.
Using IMS numbers and a modelling program one of our members who is a marine engineer ran various scenarios against a C&C 30 and a C&C 40 (cannot remember length other than greater than 40) under various wind strengths and directions. This was done with an exposed shaft with no propellor, an exposed shaft with folding or feathering propellor, the same with 2 bladed fixed and then with three bladed fixed.
Under lighter wind conditions the difference between 2 bladed fixed and a folding or feathering prop was never less than 5 sec/mile in this experiment and as high as 7 sec/mile. As the wind speed was increased to over 15 knots (true) the difference lessened.
As a result of this process it was decided that 1 point ASPN (6 secs/mile) credit be given for fixed two bladed prop over a folding or feathering prop on the same boat.
There were many sidebar discussions but this was the basic outcome. Please be warned that this is all from memory and if specifics are desired I would have to look them up.
|05-09-2003 05:52 PM|
Take a look at the January 1, 1995 and October 1, 1993 issues of Practical Sailor. There are detailed results of propeller tests conducted at M.I.T. The articles are quite technical.
The 1995 article provides detailed graphs of prop drag. The folding and feathering props have perhaps 2 pounds of drag, while a 2 blade fixed prop has 6 pounds of drag at 3 knots, 12 pounds at 5 knots, and 34 pounds at 8 knots. Of course this doesn''t mean a lot, what you want to know ''s what is the speed loss with a fixed prop.
The article doesn''t address that directly, but says that "a computer model for one boat showed that while drag could slow a boat up to about .8 knot, over a wide range of wind speeds and angles the average speed loss is only about 1/3 knot."
By some back of the envelope calculations, it appears that a two bladed fixed prop increases frictional resistance by 20-30%. At 3.5 knots on a boat with a 25 foot waterline, frictional resistance is about 75% of total drag. At 5 knots it''s about 50% (the rest is wave making drag). The biggest impact on speed will thus be in light airs when the boat is moving slowly.
If the Practical Sailor tests are accurate, it appears that, in light air, a folding prop could provide a 10% speed increase. That means getting into port an hour later on a 10 hour 40-50 mile sail. This is more than 60 seconds per mile (albeit in light airs). Even the 1/3 knot average cited in the article would be 6-7% of a 5 knot boat speed, or about 45 seconds per mile.
|02-11-2003 06:16 PM|
To determine the worth of a folding prop, would it be valid to experiment by dragging another prop behind the boat? It would have to be on a fixed strut, so it wouldn''t spin, and roughly the same size and pitch. Watching the speedo drop when you did this would give you an idea of the effect of a nonfolding prop on boatspeed. Going to a folding prop would likely increase the speed of the boat by about the same amount as it was observed decreased by the drag of the second one, no?
This wouldn''t work well for a boat whose prop was in an aperture, but for one that''s just stuck out there in front of the rudder, it might work better than guessing. It''s certainly a good excuse to go out sailing.
|02-11-2003 02:54 PM|
By modern standards that is a pretty heavy boat. (Moderate displacement is usually quoted as being closer to L/D= 160 to somwhere between 180 and 200 these days) My initial reaction would be to mark your prop shaft so that you can lock the prop in the vertical position and try setting the prop in that position when racing for a season. I doubt that you will save the 3 sec (or 6 secs a mile) that the folding prop will cost yoru rating.
|02-11-2003 08:15 AM|
Thanks Jeff. My boat is a Cal 28-8 that I sail out of Marblehead, MA, where there are notoriously light winds in July and August. It''s a moderate displacement boat (7200 lbs)with a displacement/length ratio of 253. It''s got a fin keel and a spade rudder and a 152% genoa. Sounds like it would be worth it?
|02-11-2003 04:29 AM|
It depends on the boat and where you are sailing. The speed differences between a fixed prop and a folding prop, in moderate winds can be very small but in light air and in marginal surfing conditions the speed gains can be huge. So sailing on a lake, or in an area like the Chesapeake or Long Island Sound, folding props are really helpful.
Folding props have their greatest impact on compartively light weight boats, boats with small sail area to displacement, and boats with fin keels and spade rudders where the prop is hanging out in the free flowing water stream. For boats in those categories going to a folding prop can easily be worth it. The question gets dicier with a heavy cruising boat, where the prop is a proportionately small amount of the boat''s over all drag.
|02-10-2003 07:19 PM|
Does anyone out there have an opinion about folding props? I have a standard two-bladed prop now...can anyone even make a wild guess as to how much speed I could pick up? Quarter knot? Half knot? Is it worth giving up the PHRF points to get it?
Thanks for any help.