View Single Post
Old 12-26-2012, 12:28 PM   #16
twiisted71
Guru
 
Join Date: Aug 2011
Posts: 577
Quote:
Originally Posted by psneeld View Post
I have always understood that 3 blade was more efficient at your top RPMs but a 4 blade would be at your lower cruise ranges.
'tis true. The higher the speed the fewer blades preferred. Fewer leading edges that have to "cut" through. Way back in the day of early OBs you could get a 3 blade "power" prop or a two blade "speed" prop on the same engine. Now-a-days manufacturers have had to dumb down the selection and give a prop that allows the engine to reach max revs. That has become a sticking point with surveyors and others who buy into it. This is especially true for planing hulls. On a displacement hull or any hull that is used at displacement speeds I prefer a slightly larger than "spec'd" engine and I'm going to set it up to give the speed I want at lower (but still reasonable RPMs), it can do this as it has more rpm "flexibility" due to producing more power at a lower range.

On our commercial trawlers a four blade was preferred for more "bite" when pulling trawls or pushing nets. This was true on the low speed boats, if the boat was expected to plane, to and from working grounds, they usually got a 3 blade esp. with gassers. But there were some who had 350HP 6 cyl Caterpillar powered 27' skiffs. They had the oomph to swing a 4 blade, and needed it to put all that power to use. It was an amazing sight to see a Lafitte skiff with skimmer nets sticking 16' in the air "loafing" down the pass at 35 kts! This was unheard of until the mid-late '80's when the lightweight high HP diesels began to hit the market. Before that you had a 6-71 DD, or a turbo/aftercooled 3208 Cat (bad rep as they didn't hold up putting out over 210 HP under the demands of a commercial shrimper who wanted the grunt/economy of a diesel AND the speed of a BB chevy). So while it has become regurgitated gospel that a boat HAS to reach max RPM fully loaded, remember that is so the 4 weekend/year Capt. Bligh can have a warranty. My boat is a 1983, I have no delusions about taking her out and holding her on the pins to head for the Dry Tortugas for the weekend. Common sense will play a big role in the longevity of just about anything. If you run your boat hard and it has marginal HP for the performance you want stick with the 3 blade, your engine will thank you. If you have an engine that the market has demanded the HP be doubled or more from its original design/ratings and you intend to use at its new rating, do not be alarmed when it goes KaBlooey with relatively few hours compared to what "they used to get"! The 250 HP 4BT Cummins is a prime example, as is the 375-425 3208 Cat. Just because it can be done doesn't make it a good proposition. A 40 yr old cypress Lugger with its original 4-71 DD was nothing rare. A planing skiff with a hot 3208 more than 3 yrs old without major repairs was an anomaly.

What I'm trying to say is that on low speed boats, while important its not as critical to have the "perfect" prop. I would much rather my trawler speed boat be at its Tq peak or PDC at the top of my cruising speeds. I.E. 1600 RPM at 8.5-9 kts. I don't care if it will turn 2800 with that prop/gearing configuration, its not where I operate my boat.

Let the flaming begin.....its my boat and my opinion based on my experience, not some industry concocted blanket rule.
__________________
TIME well wasted
1984 34' Mainship III
Arkan'tsaw
twiisted71 is offline   Reply With Quote