Trim Tuning and Cavitation

[BoatDrinksQ5]

New to big boats/IO (so bear with me) and have been playing with the trim while cruising to find some sweet spots for different speeds/loads.

2010 Merc Alpha1 190hp 4.3L TKS A14
Aluminum prop - stock- 832834A45 14 x 23 RH 3BL (from what i can tell...) clearly says 23p on prop
Boat: 2012 Tahoe Q5SF, 19'5" 2750lbs

I start with drive all the way down until up on plane - then i trim up slowly until ride seems looser and nose is higher. Notice MPH continue to climb as i do this with same RPM (~3000 lets say). Don't notice any porpoising or bouncing. This is the typical procedure for adjusting/learning correct trim?

Once my trim gauge gets to about the half way mark often my prop appears to blow out. Engine races for a few seconds until i trim down and back-off throttle.

To me....seems like the ride should suffer (bounce, high bow, loss of MPH, etc) before prop should cavitate that bad. Normal?

I am still in the break in period so I haven't had a chance to verify properly my WOT rpms/speed. But you would think a builder would have a good prop from the factory... :-/ Do i see a new prop in my future?

 

[steelespike]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

If you get us the details requested in "read this first" at the top of the prop forum
we can tell better about your setup.

 

[BoatDrinksQ5]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

Added the information/details that I have available at the moment. I will get prop info tonight.

Would it be worthwhile to document a couple RPM vs MPH(gps) values? Can't do WOT, since i am limited to 3500 rpm (75% throttle)

I guess i am just surprised that the prop lets go so early... was expecting that to be the last of the worries. Especially just driving straight on a calm lake. Didn't know if that was more a product of hull/motor layout or the prop config...

 

[BoatDrinksQ5]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

Was able to crawl in to get my prop specs - 14" and 23"p from what i can tell from the hard to see numbers. Found the stock prop number on mercpartsexpress - 'black max' model

 

[QC]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

A couple of things. First, just for clarity, what you are experiencing is actually ventilation. Commonly confused, but important to differentiate.

When you say you can only go 75% throttle, is that with trim up? What happens when you run WOT with less up/out trim? All the way down? RPM? ( Just got it, 75% for break-in?) I thought you were allowed some WOT runs during break-in?

Annnnnd finally, I would not worry about, or change anything, until you do run WOT. Low throttle blowout is pretty common at high trim ranges. I agree you should be able to make her porpoise, for the fastest setup. If you can't, then you might want to fiddle with weight balance fore to aft. A little bit of shifting stuff around can make a difference.

 

[BoatDrinksQ5]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

thanks for the clarification on the vent/cav. I'll read more on that....

Yeah i am during the 10hr careful breakin...then second 10hrs is allowed/recommended shorter periods of WOT.

Ok, yeah i guess that makes sense without full power with a large boat porpoising isn't much of a concern. So i am guessing what i am seeing is.... the prop venting because it doesn't have enough power (at <75% throttle) to drive the prop down into the water and raise the bow? Where if at WOT it would drive drive down into the water...and cause porpoising at the medium-high trim?

thanks for the help

 

[steelespike]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

Some wot runs with the motor broken in will be a big help. just because trim goes to a certain degree limit doesn't mean that the max is the right setting.The maximum trim angle is the limit the hardware can handle in normal operation.
I don't know where the black max fits in price/sophistication. Your dealer probably selected your prop I'm sure they selected the size based on recommendations of the builder.They may have selected an inexpensive prop to save a little setup money.
In my opinion your final prop size/type should be selected based on your typical load, activities and performance parameters.

 

[Philster]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

You won't know diddly until you can run faster. As you run faster, the hull will rise out of the water, taking a tremendous load off the prop. The blow out point changes.

Yes, if you keep trimming up, you lose your bite and ventilate. Normal. It' s normal because the trim/tilt ability can raise the prop above a point where it can always hold.

I, personally, think you should continue breaking in without anything but minimal trim ( a moderate range that you stick to). Keep a fair load on the engine and trim within a conservative range. When she is fully broken in, you can get to finding out where the sweet spot is.

Running fast on plane, hull well out, you trim and RPMS/Speed rise. You trim some more and nuttin' happens - darn. You trim more and she ventilates. Dangit! You go back to the point where you were before your trimmed some more and got nuttin', or try a touch less that that. There ya go.

Props fit boats perfectly by pure chance. Therefore, I'd say that 90-99% of the boats out there have a prop that isn't perfect. It might be very close, but it won't be perfect for that boat. And if it is perfect today, tomorrow it isn't when the fuel load and people load change. It won't be perfect next week in high humidity and scorching heat, but the week after that it might be perfect when it's 70 and dry out.

So... you can find the best compromise prop or change props like a mad man. You got one gear. Use it wisely.

 

[BoatDrinksQ5]

Re: Trim Tunning and Cavitation

Re: Trim Tunning and Cavitation

Looks like that BlackMax 14 x 23 is one of the cheapest props you can get (at least on mercpartsexpress) doesn't surprise me with the boat being a Tahoe... haha. I saw that coming. I figure as a new 'big' boater I will get a ding or two and eventually this prop will make an good backup in a year or two. (unless its way off...then would be quicker)

I don't see much in the realm of painted steel....only notice Alum or SS... thinking if my prop is $180 (with a few others in that price point)....others are all $600-750... this was just a quick scan on mercpartsexpr. (of course, i will be getting at iboats someday)

Well I look forward to future testing. Only have 2.5 hrs on her... haha. Surprisingly tough to get hours (i am more of a fisherman, than a 'boater') I suppose its early yet in the year here in MN.

Getting out again today, i'll make sure to keep that prop wet!

thanks again!