dropping pitch for heavy load at altitude - how beneficial?

[93maxum4.3lx]

boat: 93 maxum 18' open bow
motor: 4.3 LX ~ 190 HP
drive: mercruiser alpha 1.81 ratio
current prop: bay pro II 14x20

I am happy with this setup at sea level with 500lb of people - holeshot is effortless and life is great; not much bow rise getting on plane, it's just really nice.

in a few weeks I plan to be running at 3000ft altitude (at a lake I've been at before, and I know it saps some power from the boat); with more people (I know this boat doesn't respond well to extra weight) and possibly even towing a tube or skier.

Can I expect a significant enough difference in holeshot and lower speed handling to make it worth my time to pick up a hustler or amita 4-blade in 19 pitch (1 less than I have now)?

 

[steelespike]

Re: dropping pitch for heavy load at altitude - how beneficial?

Your power will be down about 10% and that would be if you rejetted.
Just my opinion but I think to be safe from your description a Solas Amita 17"or 18" 4 blade.
Your better off under propped than over propped.
To me the Hustler is too agressive and has been unpredictable.

 

[craze1cars]

Re: dropping pitch for heavy load at altitude - how beneficial?

I just saw your other post. Sounds like you are already underpropped...over there you indicated you are over-revving a bit at 5,000 rpm at sea level with your current prop even with a heavy load?

With that built-in cushion, I believe you can probably effectively run what you've got...

Do you recall how many RPMS you lost at top speed last time you visited this lake? That piece of info would be very valuable to know and would probably answer the question with certainty...

And Mercruiser generally recommends the same jetting at 4,999 ft as they do at sea level, so I don't see much reason to rejet...

 

[93maxum4.3lx]

Re: dropping pitch for heavy load at altitude - how beneficial?

yes, by the top rpm method it is slightly under propped at sea level with the 14x20 baypro II, but the prop works too well otherwise to even think about changing it because of the rpm. I just throttle back a touch if I am going to run at top speed for any length of time (which isn't often anyways).

The last good numbers I have at the 3000ft lake with the baypro II are 38mph at 4300rpm with a heavy load (about 1000 lbs of people), but that is before I fixed the timing. The boat struggled to get on plane, had quite a bit of bow rise, etc. Now that the timing is fixed, that will help some; but I doubt it will be enough; especially considering the planned tube pulling etc... I guess I could just be asking too much of the hull, to put that much weight in an 18ft.

 

[craze1cars]

Re: dropping pitch for heavy load at altitude - how beneficial?

How much RPM difference between zero and 3000 ft was there on the same prop before you fixed your timing? Might give another clue...

Either that or how much RPM did you gain at sea level after you fixed your timing problem?

All I'm trying to do is guess what your RPM will be on that trip. Random example: If, when your timing was off, there was a 400 rpm difference with the altitude & load change, then you can also reasonably expect your current prop will run 4,600 rpms with similar altitude and load change...which is right in your acceptable range and should be fine.

But if you had a 700 rpm difference between the two, then you can expect your current setup to turn 4,300....and will be a dog.

Indeed if you intend to load the boat like that, and planing out is more important than top speed, benefit will certainly be had by dropping a couple inches of pitch.

 

[93maxum4.3lx]

Re: dropping pitch for heavy load at altitude - how beneficial?

just an update for everyone:

as mentioned in my first post, I currently have a bay pro II in 20 pitch.

I knew I wanted to get something with lower pitch for tubing/heavier loads at altitude, and I was considering the aluminum stuff...long story short, I know that in the long run I would have always wondered "but how much better would the baypro in 18 pitch have been" so I just went ahead and picked up the 18 pitch bay pro II.

VERY happy with it. Three large adults, 4 kids, full tank of gas, 3000 ft altitude and the 18 pitch pushed the boat all day long with authority. The midrange kick/grip feels nice, such as if you're already on plane at 3500rpm and then nail it. Getting on plane was effortless when not pulling the tube, and and still pretty good when pulling the tube. I also had the same experience as sleeper posted here: http://forums.iboats.com/showthread.php?t=327024&p=2269189&viewfull=1#post2269189 which is that, for whatever reason, the prop doesn't seem very sensitive to weight in the boat. It's performance didn't seem to fall off much from just two people to a boatfull.

So as I said, very happy with it. I don't have any max rpm/MPH numbers to post (didn't take the GPS, this was a tube pulling weekend not a prop test weekend) but the performance while pulling tubes and a boat full of kids is convincing enough that I don't care what the top MPH is (or isn't).

 

[steelespike]

Re: dropping pitch for heavy load at altitude - how beneficial?

Thanks for the update its allways good to see positive results.
Also important to get back with less than hoped for rersults.