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KenD 05-28-2002 09:00 AM

three blade prop
 
what are the advantages or disadvantages of a three blade prop can they make backing in a boat less of a thrill and how will it affect the moter ie wear,rpm,fuel comcusumption and other things that i can''t even imagine.also I have read somewhere that you should have a ratio of 1 hp to 1 ton of boat this doesn''t seem enough what are the thoughts of the experts

928frenzy 05-28-2002 10:47 AM

three blade prop
 
I''ve read that the weight/HP ratio for a displacement boat is about 500 pounds/HP.

There are advantages and disadvantages to a three blade prop vs. a two blade prop. The advantages are most apparent when backing down and during initial forward thrust. Some say (and claim) a three blade tends to exhibit less (or minimal) prop walk. The disadvantage is that no matter where the engine is stopped, at least two blades are in the direct stream of water flow. With a two blade prop, it is possible to get the blades to be in line with the rudder, thus providing minimal additional resistance to water flow. However, if this is a major concern, you should consider installing a folding prop.

Whatever you do, make sure the prop you install will fit in the space provided with sufficient clearance from the hull, and the rudder. Also, do not exceed the recommended pitch for the engine''s HP and its optimum operating RPM.

sailor582002 05-29-2002 07:09 AM

three blade prop
 
Ken,
From my limited experience with a two blade prop they have less ability to provide thrust in both forward as well as reverse thus less manuvering ability. I would imagine that to fully realize the desired effects of 2 vs. 3 blade, one would have to actually experience the use of both props on one specific boat as boats all handle somewhat differently. A 2 blade on a light displacement boat may be sufficient for that boat and provide desired manuverability. The 3 blade will cause some drag, however unless you have a race boat I believe the drag created would not be noteworthy. With sailing, almost everything is a compromise.
As far as the HP rated per ton let me say this... If you are in a headwind, a heavy sea, trying to buck the tide the more HP available the better off you''ll be. On my 17000 displacement 38'' cruising boat with a 50hp engine and 3 blade prop I can make 6.5 knots under power in a flat sea. In a headwind and heavy chop I do well to make 3 knots. If I had a 27hp engine (for example) I doubt if I would make much headway at all. For the formula you mention of 1hp per ton of boat my boat then would have an 8.5 hp engine which may get me underway from the slip with no wind at all, but that''s about it. Then on the other hand, there are many sailboats that have circumnavigated with no engine at all. So, I guess it boils down to what one expects from their boat. For me, I want the power and I would personally would not want any less HP than I currently have.
In addition, in a crusing boat one is likely to have a high output alternator of 125 amps or more maintain charged batteries and operate power hungry "electronics of the day" and refrigeration. A larger engine (than 8.5 hp)is required to provide the hp necessary to turn the alternator (I''m likely to get hits on this one). But then again, the purists with no engines typically don''t or can''t have refrig., radar, cold beer and color TV. :^)
Fair Winds
Fred


gnorbury 05-29-2002 12:57 PM

three blade prop
 
There is a noticable change in my 35hp engine''s rpm when the LectraSan MSD is activated. Short term power draw is in the region of 30+ amps depending on water salinity.

Under calm conditions, the same 35hp propels my Nicholsom 35 at a comfortable 5.5kt or a "pushing it" 6.2kt.

In adverse wind and short chop (as found in a 40kt summer squall on the Chesapeake), I feel lucky when I maintain 4kt through the water.

Must say I didn''t notice much difference when I switched from the original 2 blader to the current 3 blade Maxprop a few seasons back. Backing down is less wild, but things can still get pretty hairy if you don''t pay attention!

Graham

KenD 05-30-2002 08:39 AM

three blade prop
 
thanks all for input on T.B.P. At the moment I am sailing a 35 foot slug with a 45hp volvo tons of power but oh what a feeling in reverse I could prop walk across the bay before it straightens out if a three blade prop would help that it has to be worth it I can moter all day at 2000 rpm and 5.5 kts it''s just threading the neddle backing in that makes me shiver singlr handing with a center cockpit can give youa real rush once again thanks and fiar winds to all.

KenD 05-30-2002 08:42 AM

three blade prop
 
thanks all for input on T.B.P. At the moment I am sailing a 35 foot slug with a 45hp volvo tons of power but oh what a feeling in reverse I could prop walk across the bay before it straightens out if a three blade prop would help that it has to be worth it I can motor all day at 2000 rpm and 5.5 kts it''s just threading the needle backing in that makes me shiver. single handing with a center cockpit can give you a real rush. once again thanks and fair winds to all.

bsfree 05-30-2002 09:09 AM

three blade prop
 
For what it''s worth, I have a 35'' full keel 18,000# boat with a 36hp diesel. I changed from the 3 blade to the 2 blade to be able to increase rev''s to a range that brings rated horsepower into being. I have no problem with "walking" in reverse if I let the blades bite at low rev''s then increase power slightly to maintain control. I have wheel steering and it''s the same principal as driving a car in reverse (turn the wheel in the opposite direction to go the way you want)also, my gps says I make 7kt''s in calm waters. Bottom line is engines run best if they are run in the rev range they are designed for.
Happy Sailing

sailor582002 05-30-2002 09:55 AM

three blade prop
 
Ken
Does your boat have a full keel? If so, I don''t think the prop change is going to make much difference. From talking to full keel boat owners, common opinion is they all back down like a drunken buffalo, just the nature of the beast I guess.

harvh 06-18-2002 02:23 PM

three blade prop
 
I went from a two bladed folding prop to a three bald feathering prop on my C&C 34+. Backing was no longer a major concern. Boat vibration was much less with the three balded prop, and stopping was almost instintanious.

I did loose about 1/2 knot of boat speed with the three blade, but the trade offs were worth it. I used to dread backing out of a slip in a tite marina. I truly had no idea where the boat was going. The three blade feathering prop solved the problem with almost no drag under sail.


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