Pitch, Diameter, Water type and Altitude

[saffer]

Hi

I recently got myself a used 5.2m semi rigid with 2 x 40Hp 2 stroke Mercurys on.

The boat currently has 3 blade 11.6X11 Solas props on and is only revving to about 3100 RPM. Admittedly I am at an altitude of about 1700 meters above sea-level which will likely rob me of about 20% of my power, but I would like to get them revving up a bit closer to the factory's max RPM of 5500 RPM.

I also plan on using the boat at sea which is where I really need to have decent performance, so here my question is, should I have different props for using in the dams at altitude to what I would use at Sea, and what props would be my best option for this boat to achieve best all-round performance?

BTW I am going to be testing a pair of 4 blade 10.5X13 props in a few weeks, but for now I would like to know what you guys recon?

Thanks

[Searider]

Is the bottom clean and what speed are you getting?

Generally you'll need slightly less pitch when operating in salt water in comparison with fresh water - although this is tinkering with the last couple of % performance.

I guess the altitude is the main factor in this case though.

How does it run at sea level?

[saffer]

I haven't checked it out at sea level as yet, figured if I can get it running 80% up here I can "risk" launching at Sea.

Surf can be quite rough here. ;-)

[jyasaki]

Are you sure your throttles are adjusted correctly? 3100 rpm with 11" pitch props sounds way low to me.

I run a single F115; what gave me about 5.9K rpm in salt at sea level yielded about 5200 at 6000' in fresh (and really lengthened the hole shot.) That's injected, though; if you're carbed you can probably expect a bit less in rpm.

You will most likely need 2 sets of props if you're changing between altitude and sea level. Or if you tune for a single altitude, just accept reduced performance at the other.

Ideally, you'd change jets when changing altitudes, but that sounds like a pain.


Luck;

jky

[Locozodiac]

You have some tech issues here :

-Engine has a power loss of 15% at each 1K meter altitude.
-Can be compensated by buying a high altitude jet kit, available with different altitudes up to 3K meter heights.
-Will need to go down for lower pitch props to gain more lost RPM
-Ideal would be have 2 carbs and 2 props.
-Standard carb with standard prop for sea level use providing that actual set up delivers near max rpm stated for that engine.
-A second carb jetted for high altitude + lower pitch prop.
-Down issues: Have 2 engines which are 1 or 3 carbed OB's ?
-For high altutude use, will need to multiply carbs by 2 or worst scenario by 6 (Bad Music)

Happy Boating

[9D280]

+1 for the altitude.

Is the boat new to you? - if so I assume you bought it form someone at sea level - ish?

The only engines I now that did "easy mixture adjust" were the 1960s Johnson - Evenrudes. I guess a new fangled fuel injected machine should be able to "auto adjust", but as said above you are looking at mixture changes.

OTher thing to note (depends on your Merc - so check this out before changing) is that some of the 3 cyl mercs (certainly the 50/60Hp Clamshells - but no idea how old yours is) had a different jet on the top carb......

Do you have anywhere nearer sea level you could give it a blast without having to play in the surf? - It would remove a lot of the guestimating!

[saffer]

Thanks guys

Its quite a drive to get to Sea level, but I will try and make a trip sometime soon to check it out. I feel the altitude difference should affect a 20-30% drop, but 3100rpm from 5500rpm is drastic.

These motors are 2005 2cyl 2 strokes.
I have checked the throttles and am pretty sure I am achieving WOT.
I bought it used, and have to try and figure out what the previous owner did to it.
I have had it fully serviced, the tech tested and said everything is fine, but that was without going onto the dam.
This whole pitch / diameter thing is quite a thing. As a layman I would say to get more rpm you need less resistance, so you need a "smaller" prop, hence the diameter needs to drop. Now to get the same "drive" from a smaller prop surely I need more pitch. Hence why I want to try to go from the current 11.6X11's to the 10.5X13's I have.
Will test it out and see.

[Searider]

To increase the rpm you need to reduce the pitch.
Reducing the diameter will have a smaller effect but will also increase the rpm.

[9D280]

Don't underestimate the effects of diameter.

Pitch is relatively easy to guestimate - if you have two nominally similar props (diametrer, blade shape etc) then you will see approx 200rpm difference per inch of pitch.

Diameter also affects things -a bigger prop needs more torque to spin it, so again a bigger one will drop RPM. BUT it's nothing like as linear as the pitch - RPM ratio, so an experiment is all you can do.

I went up a shade under 1" in duiameter (I had way too much RPM - the engine used to be on a much heavier boat before I got it) and I stayed at 14" pitch but went from 10&1/8" to 11" diameter. Dropped my WOT RPM by about 1100. Difference was that the prop has "let go" once since. It used to do it on every other wave, which was at best tiresome!


So yes, if you have a super light boat, you can drop diameter & load the engine back up with a higher pitch thus getting more speed, althoug hslip wioll increase as the diameter drops.... (another reason why it's anything but a linear relationship). Yam used to do a series of props that were interchangeable with the "K" series that were sold as something like "speed prop".

The theory is all well & good, BUT there comes a point where it needs blade area to "grip" the water. The lighter your boat & the flatter the water you boat in, the less grip & therefore blade area it will need.

And all that before you start changing between prop genres!