mid cabin prop upgrade - video

BobCT · Post by **BobCT** » Tue Jul 26, 2011 3:21 pm

Let me know what you guys think and how it compares. I had 3 blade, 16x17's and will compare to a video I shot last year. I changed the GPS to MPH just to make it easier.

Thanks to Sherman for "taking one for the team" and giving me a starting point. They're Michigan Wheels and not available in 1/2 pitch sizes so I went with 17x14 and a light cup. I believe Sherman has 17x14.5 with no cup.

http://www.youtube.com/watch?v=qcC2b_NSFIw

The biggest surprise is that I hit 4,300 RPM's for the 1st time in my boat. I picked up about 200 RPM's on the top end which is exactly where it's rated according the spec sheet I found - specific to the mid cabin. I have upgraded trim tabs (12x24) and run them all the way down at cruise. That seems to give me the best fuel burn and it's nice not having to see "over" the bow.

Update - just found the video from last year, 16.9MPH was at 2,900 RPM's. Bottom was not in as good a shape and I had two other people on board. Fuel level and conditions were similar.

Bob

tsawyer · Post by **tsawyer** » Tue Jul 26, 2011 6:27 pm

Good info.

So you picked up 1 mph @ 100-200 rpm less.

What was the gph on your original run?

RWS · Post by **RWS** » Tue Jul 26, 2011 6:30 pm

Good info Bob.

The problem, however.....

I must be obsessed with this Trojan International boat $h*%

Wait a minute, here's a better word...

O.K. found it...

It's my . . . . . . . .

PASSION !

Nevertheless,

here is some data on the 1983 Express Cruiser (non-midcabin) for comparision:

4 blade props 18x19
machined exhaust diverters
bigger intake spark arrestors
repaired spark advances
corrected metering rods in the quadrajets
replaced factory 9x24 trim tabs with 12x24 tabs with drop fns
76 gallons of fuel
40 gallons of water (full tank)
one adult & one yellow lab (Hank)
Test data November, 2003

Here's the detail @ 2800 RPM followed by a graph that compared results before the tabs to after the tabs.

Note: I cannot recall ever having to run these tabs full down.

RPM 2800 (digital tach/sync - ENSIGN)
GPH 19.0 (flowscans)
KTS 15.9 (gps)
MPH 18.3 (gps)
NMPG .84 (calculated from GPS & Floscans)

Here's a graph:
Not a midcabin, but hope this helps y'all.

RWS
graph is below

RWS · Post by **RWS** » Tue Jul 26, 2011 6:33 pm

Something else....

These tests with the gas engines were all done with PROBABLY, actually MOST ASSUREDLY with NON-ETHANOL fuel.

That may have a significant bearing on differences from then to now.

RWS

S.A.M. · Post by **S.A.M.** » Wed Jul 27, 2011 9:15 am

Bob,

Congrats on the new numbers. My pitch is 14". The 14.5 is what I should of gone with.

todd brinkerhoff · Post by **todd brinkerhoff** » Tue Aug 02, 2011 7:35 am

Hey Bob,

Do you have some comparison numbers with the 3 blade vs 4 blade with rpm, speed, and gphs. Do you see a significant difference with the 4 blades?

BobCT · Post by **BobCT** » Tue Aug 02, 2011 11:16 am

Todd, the big missing link is GPH because I didn't install my floscans until late last year.

I "think" my GPH is less but can't say for certain. I definately picked up 200 RPM's on the high end and knocked off a couple of hundred at cruise speed compared to the 3 blades. Those two data points I'm certain of so that must translate into a lower GPH. My Stb side floscan gauge hasn't worked since the minute I installed I'm simply doubling the fuel burn from the port side reading which you can see in the video.

Overall, I'm happy with the 4 blade upgrade. There doesn't seem to be any downside. I wanted an extra set of props anyway, and after dinging one of my 3 blades last month it was the perfect time to make the swap during a short haul.

I guess the only minor issue has been my forward speed at idle. It's a little too fast even with the throttles all the way down. I lowered the idle to around 650 in neutral so that'll help.

Bob

todd brinkerhoff · Post by **todd brinkerhoff** » Tue Aug 02, 2011 11:53 am

Thanks Bob,

Did you use someone else's experience to go with that size and pitch or did you consult someone for the calculation? I have 22X22 3 blades running about 4000-4100 max rpm. I would like to try to run a pair 24" 4-Blades and would like to see the top RPMs come closer to the manufacturer recommended 4300-4400. I am unsure on pitch for the desired diameter wheel change.

BobCT · Post by **BobCT** » Wed Aug 03, 2011 7:26 am

Todd,

It was a combination of the two. Sherman and I exchanged emails last year and he gave me his prop sizing. I also gave my engine/boat specs to a couple of prop shops and they came up with pretty much the same thing.

Stating the obvious, but if you could find another 10.8 guy who's already made the upgrade that would be ideal.

Bob

RWS · Post by **RWS** » Wed Aug 03, 2011 7:44 am

My ten meter express curiser (non-midcabin model) came with two sets of props.

She was wearing 4 blade and had a pair of 3 blade nibrals in the box

While we changed the 4 blades from 18x21 to 18x19 we tried out the three blade 17x22 nibrals.

Don't really know how well these were dialed in but w got a better top end with the 3 blades but effeciency at cruise was not as good as with the 4 blade props.

Something to consider: Perhaps the 18x21 4 blades were perfect when she was new, but as she got older, engines had a bit less compresion and the boat weighed more, the reduction in pitch made sense - but this is just a guess.

We really do have a variety of hulls and styles here as well to complicate the issue.

Getting good data and sharing it here is the best thing we can do to help each other out; the goal being to increase everyone's most efficient cruise efficiency.

RWS

todd brinkerhoff · Post by **todd brinkerhoff** » Wed Aug 03, 2011 9:23 am

Thanks guys...great info.

I think I might be one of only a handful of guys with a 10.8 express and I might be the only active viewer of the forum. What I find interesting is that my rpm and speed numbers almost exactly match the 10m midcabins guys with a boat that has a different hull, gearing and props.

I went to boatdiesel.com and did some calculations. The 3 blade recommendation matches my diameter but the pitch recommendation is way too high. I can't get top RPM with the pitch I have now and they're recommending going 3" higher. When I throw a 4 blade prop in the calculation they are recommending going with an 18" with a higher pitch, but I would think with my current my numbers a bigger prop with lower pitch might be better. This science is a little above me. I guess without any other knowledge, I will be favoring diameter over pitch for this boat.

Todd

RWS · Post by **RWS** » Wed Aug 03, 2011 9:54 am

The best you can d is to get RECISE data for what she's wearing right now.

rpm, speed, gph

the prop shop with scanning technology can scan yopur props, then take your data and determine what can be done to increase your efficiency.

The numbers we are providing ffor our Delta Conic hulls and different prop diameters and speeds may not have any realistic bearing on your performance at different RPM's.

Remember this is not a shotgun approach, but a rifle, sharpshooter's game, especially in consideration of the cost of a gallon of fuel.

An increace in efficiency of 10% - 20% would have a significant impact on how you would use the boat over the next 36 months.

RWS