Prop Test advice
I had an opportunity to do some prop testing on the new motor for my boat on the weekend and keen to get some expert advice from the knowledgeable folk on here.
I figured before the trial that the standard 3 blade 14 inch pitch I picked up was going to be the one after some initial tests in the Cairns inlet of three other props.
I took those with me to test as well. The testing was done at altitude and fresh water in Tinaroo dam so the numbers are skewed to what I will normally be doing. The good news is I do have some information from the salt water tests to do a bit of a comparison.
My thoughts are the 3 blade 14 inch is still the pick but keen to see what others think, especially since I didn't achieve the recommended WOT range (5500 to 6000 and I got 5350). Considering the salt water data showed a massive increase of at least 300 rpm in the other props I figured I would be fine going with the 14 X 3 blade as it will come up within range. The fresh water tests were done with a relatively full tank and two adults on board. I would say the load was on the light side with no esky, ice or fish on board. Having said that, WOT testing in the salt with the 13 X 3 showed no decline in WOT RPM between light and heavy loads.
The final prop is to be stainless too and I've heard the rpm might pick up a bit more due to the thinner blades?
Anyway some observations not shown in the pictures are that the 4 blade props do plane the boat faster and allow slower planing but in the fresh, not by much. I figure since my rev range/speed will be around the 4000 to 4500 mark and 20 knots, this was the sweet spot for the 14 X 3 although it was almost the pick all the way through. I also figured that in the salt when it gets another 300 rpm plus it will outperform the others in the higher range as well being able to generate more power?
This boat rides so well and planes easy enough I figured the few times I would need to plane slower would not outweigh the extra speed/economy the 3 blade offers in my cruising range. Hole shot is not an issue.
I still plan to do a final run in the salt to confirm but keen to get peoples thoughts on the attached information and what the stainless version might do in comparison to the alloy.
Props tested were all Alloy, the 4 blades were the spitfires. Boat is a modified Hartley Scamp design around 16 feet long, weight unknown.
Doooma
Posts: 791
Date Joined: 05/12/09
Going just on your
Going just on your graphs....
ld say you were spot on!
Stratos-Fear
Posts: 61
Date Joined: 27/03/13
I was always led to belive
I was always led to belive you should always prop your motor to manufactory recommendation WOT. Over propping is loading the motor too much like driving up hill in 6th gear. Rasing the motor can increase RPM. For a few extra knots in speed i would get it in the RPM range if it was me.
Tim
Posts: 2497
Date Joined: 26/09/06
13 x 4
My choice would be the 13 x 3 or 13 x 4. Closest to manufacturer spec and both sets of numbers look ok.
I went from an ally 3 blade to a stainless 3 blade on a Suzuki 140 and dropped top end speed. Blade was also thicker on the stainless. Revs dropped a little as well as the stainless doesn't flex like the ally so you get better drive but a little more load
Bunny
Posts: 678
Date Joined: 05/08/10
Thanks guys. That's very
Thanks guys. That's very ineresting Tim, I'll have to consider that. On the subject of the WOT issue and maximum rpm, these tests were done in freshwater at altitude so worse case scenario and I will be usng this boat in the salt 99% of the time. I'm hoping to test the 14 X 3 again tomorrow in the salt and expect to see at least a 300rpm increase at WOT. I'll post some results. I've also just heard most outboards pick up a hundred or so RPM after break in. Has anybody experienced this?
z00m
Posts: 1086
Date Joined: 10/05/14
Other things to consider is
Other things to consider is the type of 4 blade as well. I just put Mercury Spitfire's on my twin v8's and they work a treat. Blade configuration has been tuned to provide similar drag figures to the traditional 3 blade.
Also, have you calculated prop slip for each of your tests?
Bunny
Posts: 678
Date Joined: 05/08/10
Here is the latest with the
Here is the latest with the 14 X 3 blade in the salt. Big improvement in the lower rev range plus a slight improvement in the higher range. WOT yielded around 5840 rpm, almost 500 rpm more than the fresh water at altitude test. Very interesting. zoom the 4 blades I'm testing are the Spitfires.
z00m
Posts: 1086
Date Joined: 10/05/14
Be interesting to see the
Be interesting to see the rest of the results from the salt.
bushbeaver
Posts: 159
Date Joined: 08/07/13
You are probably spot on with
You are probably spot on with the 10.1x14P. That's the one which the Mercury prop selection calculator recommends as well. Nice set of data you have there.