15.5" diameter prop & low RPM, less diameter & more pitch?

[fishrdan]

Should I get a prop with less diameter and more pitch? Here's what I have;

- 18' Crestliner Superhawk
- 24-2600# loaded without people (200-700# people and extra gear)
- 140HP Mercruiser, recent rebuilt, good mechanical condition
- 15.5 x 19P Michigan Match prop, no nicks/dings
- 4200 WOT RPM, should be 4200-4600 WOT
- 33 MPH off speedo (no GPS available)
- no watersports
- fishing, trolling

With 200-300# of people on board it jumps out of the hole, but tops out early at 33MPH 4200RPM. With 800# of people & extra gear on board it's a tad slow to plane, but still very close to 33MPH at 4200RPM, maybe 32MPH at 4100RPM.

From doing some online research (guesstimate) about prop diameter and pitch:
+ 200 RPM for every 1/4" less prop diameter
- 200 RPM for every 1" of added pitch

I don't have a good baseline to go by since I'm running a large diameter prop when I should be using a 14+"diameter prop. The props I have are 15.75x17, 15.5x19, and a high altitude 15.5x15 (Iboat's clearance props a few years ago). Mercury's prop selector suggests a 14.5 x 19P prop, but with the online "guesstimates" that could put me well past the engines 4600 WOT RPM if I use a 14.5 x 19P prop. I'm thinking I should go with a 14.25 x 21P prop

So what do you think, 14.5x19 that could over rev the engine or 14.25x21?

One other question, would there be much of a difference trolling with those ^ 2 props, 1 has more pitch, other more diameter?

Thanks! Dan

 

[steelespike]

Re: 15.5" diameter prop & low RPM, less diameter & more pitch?

Don't worry about the diameter it generally simply follows the design.Generally as pitch goes down diameter goes up.It would be a little easier to tell where your at with a test with one aboard and be a little more specific about the weight.Simple answer is if you run with a lot of people and gear go with a 17" pitch.Should put you at "about" 4600.But watch the rpm with a light load.

 

[fishrdan]

Re: 15.5" diameter prop & low RPM, less diameter & more pitch?

Crestliner says it weighs 2100# with the engine/drive. I added extra 3 batteries, bow mount trolling motor, anchors, etc so I figure an extra 400# there.

With the 15.5 x 19P prop at 1000' elevation:
- By myself 225#, WOT was 4200RPM at 33MPH (2700#). It would immediately pop up on plane.
- I took out my 6 & 9 YO kids with a 2 coolers, a little camping gear and WOT was 4200RPM at 33MPH (3000#). It would immediately pop on plane.
- I took out the family, 4 people, coolers, etc and WOT was 4100RPM at 32MPH (3300#). It was a bit slow to plane, but not like it was lugging the engine for a long time.

We took it up to 5000' with the 15.75 x 17P prop and it was very sluggish planing and WOT RPM was around 3800RPM, not sure of the MPH as we were quickly chased off the lake by bad weather. It planed slowly with just myself or the family on board.

Mercury suggests a 14.5 x 19P prop and I'm not getting WOT RPM with a 15.5 x 19P prop, though other performance is good. For 5000' Mercury suggests a 15 x 17P prop and I'm not getting good performance at all with a 15.75 x 17P prop. The only prop that I have that fits Mercury's suggestion is 15.5 x 15P for 9000', but I haven't had a chance to try it yet. I think I'm using props that have too much diameter (heavy load props, wrong application) since they are all 15.5"+ diameter. Probably a mistake on my part when I bought them on clearance.

I'm either not getting the performance from the engine or my prop selection is wrong. I'm not sure which at this point since I'm not using the exact prop Mercury suggests.

 

[steelespike]

Re: 15.5" diameter prop & low RPM, less diameter & more pitch?

Again forget the diameter Ill grant you that extreme diameter can affect rpm
And if these props are special purpose props then perhaps that accounts for the large diameter.In 5 years playing around on iboats I have only seen a few prop discussions where diameter was even partially considered in prop selection.
generally as pitch goes up diameter goes down and I think if you observre the prop listings you will see that.I feel your motor is under performing perhaps it is the
large diameter and I think that typically 3.0 props run in the 14"+ demension.
If you go to the iboats prop store and select a 17" prop for your motor it will be around 14+ inches. And no doubt if you select a 19 it will be in the 14+" area prop it will likely rev higher than the 15.75 X 19. That is an extreme.I can't help think a 15.75 is for much larger engine with an alpha 1 outdrive. No doubt an odd application and as you suggested why on special.I would suggest you select an 19' aluminum Mercruiser prop
it will no doubt be under 15" and you could use it to establish a base line performance.at sea level. It may be right on with a relatively normal load.
Any final selection will come from its numbers.
As far as elevation the ideal is to re jet for that altitude then select a prop.
A prop alone isn't likely to solve extreme altitude problems.
It occured to me that someone may have messed with your outdrive and changed it to a lower numerical gear ratio intended for more powerful motors.

 

[fishrdan]

Re: 15.5" diameter prop & low RPM, less diameter & more pitch?

Thanks SS,

The outdrive is the correct 2.0 ratio, checked it. The engine is a fresh rebuild with 15-20 hours and everything is running fine, carb, timing, etc set correctly. When I go up to elevation I re-jet the carb -2# for 5000' and -4# for 9000'. I haven't been playing with timing at elevation, but going to do that on the next trip since I found a Mercruiser bulletin discussing high altitude timing.

I do think the 15.75x17 and 15.5x19 are special application props. I just picked up a nice looking used MW 14 x 19 prop off Ebay and an experiment, $40. Once I see how this prop acts on the boat I'll have a better idea of which new prop to buy. Heck, if the Ebay prop runs nice (vibration, holeshot, WOT) I'll probably just run it, though I'm not banking on it.