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SUBJECT: # 17233: Skipjacks' propeller tests; post #17206
Submitted by marinerman from Australia on 9/11/2000 12:09:00 PM
Hey Skipjack, I have placed the results of your prop tests in tabular form as they are easier for me to compare. I am certainly not the prop wizard here but it seems that you have nothing to gain by going to a 20 or 20" pitch prop. If I have done numbers correctly then you are right on the money with the 19" pitch prop. You have better acceleration, less porpoising with less need for drag causing trim tab deployment, better speed and fuel burn at the settings at which you are most likely to operate and most importantly, with the 19" prop you are lugging the engine the least. In fact, the fuel efficiency is better with the 19" pitch prop than it is with the 21" prop. You will soon forget that you got an extra 2 or 3 miles per hour at some particular RPM setting if and when your motors are destroyed due to overstraining them, which is what lugging is doing. The prop guru, the Dunkmeister, has stated this fact time and time again. Please check the numbers as you are showing the same fuel burn at 4000 and 4500 RPM for the 19" pitch prop. Think you have a typo so I corrected my table to be what I think the GPH should have been.
Propeller Efficiency Summary; 21" Pitch versus 19" Pitch.
Prop RPM MPH Gal/Hr MPG.
21" 3000 31 13.8 2.25
19" 3000 26 12.9 2.02
21" 3500 37 17.2 2.15
19" 3500 34 15.9 2.14
21" 4000 42 21.5 1.95
19" 4000 38 19.9 1.90
21" 4500 47 28.1 1.67
19" 4500 44 19.9 1.90
21" 5000 53 42.8 1.24
19" 5000 49 32.9 1.49
- 9/11/2000 2:35:00 PM
Submitted by
Skipjack (207.0.202.146) from FLORIDA says 4500 was 26.4 at 1.6 MPG 44 MPH
I was disappointed that the top end speed fell off 1 MPH and fuel economy was poorer with the 19". Only improvment seemed to be less porposing. My salesman demoed the boat with the trim of the engines up and added in a little trim tab to settle the boat down and did not lose any rpm's, as the whole boat seem lifted up for less drag??Skipjack
- 9/11/2000 5:00:00 PM
Submitted by
dunk (208.243.170.232) from NEW JERSEY says Props
Skipjack, you saw what happened with the 21's when you had 5 heads onboard, 4900. I'd try it with the same load and the 19's. Yams SS wheels are pretty good and before you give up on the 21's you might try lifing the engines a bolt hole or two. Each bolt hole up will give you another 75-100rpms at top end, but props need to be able to hang on and not blow out when you trim them or break loose in rough water. If you can run full trim at WOT now you should try going up a hole.The way she's set up now it sounds to me those 19's are best all around props for her. 5500 is the absolute max you want to turn those Yams, in fact sending those props out and having the cup doubled would probably be better than a set of 20's. Talk with a local prop guy he may have 5 other ideas. I think getting the 19's tweeked is the thing to do. Remember getting those engines as high in the air with props that will hang on is the way to get the most out of her. Dunk
- 9/11/2000 5:59:00 PM
Submitted by
Hawkeye1 (165.247.82.172) from VIRGINIA says I vote for the 19's.
I've been following this one but missed what model yamaha you have. You will enjoy the spunk of the 19's way more than the 1 mph top end of the 21's. The 19's will handle the load, sea's and jump out the hole better along with better one engine operation if one ever went down. Did you every get your 20hr check up strightend out??Hawkeye
- 9/12/2000 1:02:00 PM
Submitted by
Skipjack (207.0.202.82) from FLORIDA says How about the fuel use at same speed, looks like the 21 does better
I am still working on finishing the 20 hour check up. Dunk, if I add cupping to the 19's, will I go back to porposing? Salesman and mechanic(?) thought the props were mounted at the right height, by Contender factory. Skipjack