Glad I read this synopsis... I think I'll be taking advantage of the Institutional Shares - No Minimum Investment being offered to direct RiverPark investors. It seems opportune to put a small deposit in just to have the I shares. And I emphasis 'small'. I'd like to see current holdings and a little history to see if it matches my temperament before venturing further.
@Derf: Yes, the NAV on RPHYX/RPHIX and RSIVX/RSIIX have dropped, but if HY is yielding 3% and SI is doing 5-7%, I feel I can take a hit on the asset value and offset it with the interest yield while I keep my fingers crossed. My 'Total Return' is still positive and it's better than Money Market or banks.

Thanks for all the great contributions. I've used the MFO risk tool to compare. I've restricted it to funds in the 1st risk group to approximate the risk category of RPHYX and organized them according to Martin Ratio. For the 3 year group, RPHYX is the winner. for five years, NTAUX. For the 20 year group, GSTGX is the winner. (Disclaimer: As clearly explained by others in the thread, an investment in one of these funds is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. You could lose money by investing in the fund.)
Edit-I meant to include FNPIX, but somehow forgot.

Wait long enough, and a lot of what one has in mind will be said. Thanks to Hank for his most recent post above about what cash means. That was some of what I wanted to point out - that if you're thinking about cash as something used for paying bills, you need stable (or very nearly stable) prices. But if you're using "cash" for asset allocation, you can tolerate fluctuations.
In the latter case, these days I wonder about the use of bonds at all (rather than cash) for ballast. Is 1% or so extra return over cash worth the extra risk? While some people are wondering how to get any return on cash, I'm wondering whether 2% on short-intermediate term bond funds is worth the risk. If one does want to take that risk, I'd look at FPNIX - it's always been non-traditional, using derivatives as much to preserve capital as to improve returns.
Back to cash. Regarding I-bonds - which I think are great for cash allocation but not day-to-day cash (since you can't redeem them for a year) - not only can you buy $10K/year/SSN from Treasury Direct, but if you have a refund coming on your 1040, you can buy $5K more using Form 8888.
A similar idea to I-bonds (stable, insured, liquid albeit with penalty) is long term CDs. Many banks offer CDs where you come out ahead of cash even after a year or so (like an I-bond), even after penalty. But there are risks - being able to access your money (early withdrawal may be at the discretion of the bank), and interest rate risk (the bank could increase the early withdrawal penalty). Here's a good post on that. The site (depositaccounts.com) also has a CD calculator showing the net APY after penalty.
Comparing muni bond funds with RPHYX - BobC addressed this to some extent. He likes NEARX. I've been a little uncomfortable about the risk it seems to take (investing heavily in low graded states), but if memory serves, it seems to have cut back significantly on Illinois (lowest graded state), and generally gotten more conservative. Here's a nice graphic on state ratings (you'll need to zoom in to read it well). But NJ's rating (5% of NEARX) has dropped further than the graphic shows.
I tend to look at SEC yield, especially for investments that are not intended to be short term (i.e. used for monthly payrolls and the like) - this is a calculation that's designed to reflect total return (i.e. it accounts for increase/decrease in values of discount/premium bonds). Near the top of short term munis is Vanguard Ltd-Term (VMLUX, VMLTX), a more conservative muni fund that BobC has also suggested in the past as a conservative alternative to NEARX. I feel it offers better risk/reward, in the sense that even though its return is less, its risk is much less. And right now, its SEC yield tops most funds, including NEARX.
It does this, as you'd expect, with low costs. So its portfolio can be shorter term, and higher grade than any of the funds with similar SEC yields. Specifically, its average duration is 2.5 years, and average AA rated (M*). Its SEC yield is 0.86%. There are only two AA rated funds with SEC yields about 0.5% that are comparable - AUNAX (NTF at TDAmeritrade) 2.2 year duration, but high expense and high M* risk (volatile), and DFSMX 2.7 year duration, but 0.52% SEC yield and, well try to buy DFA funds.
Compared to cash (I use 1% as a baseline, since that's about what one can get in FDIC-insured online banks) and a 28% tax bracket, the expected return of 0.86% beats the 072% post tax cash return. Go shorter with munis and you won't beat cash; go longer and you'll be taking on higher interest rate risk that I feel pushes the fund too far away from cash. YMMV.

Great stuff from everyone.
Cash seems to have widely varying definitions and widely varying purposes among investors. And it's natural to climb the ladder a little in search of something better than 0%. Even 2 or 3% on RPHYX looks good by comparison. No objection to that. But, it's not "cash" in the most basic traditional sense of the word.
I'm not sure one's definition of cash (or "enhanced cash" as someone put it) really matters a whole lot. I guess if you're running your household budget under really tight constraints, or meeting a weekly business payroll, or running a tax-funded government entity, than precise dollar amounts are critical to the operation and you best keep the necessary money in insured deposits or TBills.
For most of us however, fluxuations of 2, 3 or even 5% on our short term reserves (cash and cash substitutes) really isn't all that worrisome. We can take a bit more risk and accept those fluxuations. MSF makes a good point about some of the ultra-shorts that took a big hit during the '08 meltdown. Yes, they didn't appear all that risky before the crisis imploded. I have no answer to that one. A valid point.
I earn 0% on my Prime Reserve fund at Price and 0% on a couple checking accounts. But wait a second ... I'm actually receiving compensation through a wide range of services. These include free check-writing and electronic fund transfers. Free 24-hour online access from any device. Free phone numbers and live agents able to respond to account related questions or provide guidance. Also, live tellers at their branches, convenient credit cards and debit cards linked to these accounts and many other free or discounted services. That's a lot of "free" stuff available to me to in return for those cash deposits.
In the case of Price, keeping cash with them allows free and immediate access to any of their 100+ quality funds at literally the touch of a keypad. And exchanges between these great funds and my Prime Reserve account are conveniently exempted from their excessive trading restrictions. If I need some of the cash, they quickly transfer it to to my bank of record. Again at no fee. So, besides the higher stability of these low yielding cash accounts, I'm actually receiving quite a bit in return.
Advice to the original poster. Don't put all your eggs in one basket. As compelling as RPHYX may appear, spread your short term holdings (assuming they are significant) around a little.

I just looked at RPHYX's chart on M*. It's performing exactly as advertised: a very gentle but nearly straight line up, delivering a bit more than 3% better than MMFs, which are delivering just about zero. The only time it ever had a negative quarter was Q3 2011, when it fell only 0.07% and made up for it the following quarter.
Unless the manager is having trouble finding investments, and the 3.5% cash stake does not indicate that, why look elsewhere?
But I'm not putting all my cash here. I like to have a chunk of cash that is truly cash, with a government guarantee if possible.

I've always stayed away from this one because despite what I've heard here, it *is* dependent upon the performance and health of the junk bond market. Thus, since its inception has never been tested. As expatsp mentions above, its worst performance was Q3 2011 which just happens to correspond to the worst performance of junk bonds since its last bear market in 2008. Then again, bear markets in junk bonds are few and far between.

I agree with Expatsp. So far as I can tell, the fund has done exactly what the manager said it would do. This "money good" stuff, so far as I understand it, means that even if the bond issuing company goes bankrupt, the manager is confident that the bonds held by RPHYX will be paid in full as a result of the bankruptcy proceedings. The general state of the junk bond market would have only a temporary effect on the fund's NAV.
It's not a true cash substitute since you are dependent upon the manager's judgment that these holdings really are "money good" (and there is that temporary effect on the fund's NAV from the junk bond market even if things work out as planned), but I believe that the manager has always made this clear.

I just looked at RPHYX's chart on M*. It's performing exactly as advertised: a very gentle but nearly straight line up, delivering a bit more than 3% better than MMFs, which are delivering just about zero. The only time it ever had a negative quarter was Q3 2011, when it fell only 0.07% and made up for it the following quarter.
Unless the manager is having trouble finding investments, and the 3.5% cash stake does not indicate that, why look elsewhere?
But I'm not putting all my cash here. I like to have a chunk of cash that is truly cash, with a government guarantee if possible.

I've always stayed away from this one because despite what I've heard here, it *is* dependent upon the performance and health of the junk bond market. Thus, since its inception has never been tested. As expatsp mentions above, its worst performance was Q3 2011 which just happens to correspond to the worst performance of junk bonds since its last bear market in 2008. Then again, bear markets in junk bonds are few and far between.

Regarding PFF, which we used from early 2009 through 2012, this is NOT a safe investment. Consider the 23% loss in 2008, when there was not market for these securities. Yes, it bounced back in 2009, but it is not SAFE. We currently use KIFYX in our income portfolios, but like PFF, it is NOT safe. Cash, individual short-term Treasuries, CDs are safe in that there is a guarantee of return of principal (of course you lose to inflation and taxes). For short-term, non-cash holding, we have used THIIX, LTUIX, NEARX, LLDYX. But these are not safe, in terms of principal guarantee. And that is what most investors expect in a 'cash-substitute' holding, I think. And while I admire how RPHYX is run, a sudden credit crunch would likely be a problem for it, too. Just keep that in mind.

I just looked at RPHYX's chart on M*. It's performing exactly as advertised: a very gentle but nearly straight line up, delivering a bit more than 3% better than MMFs, which are delivering just about zero. The only time it ever had a negative quarter was Q3 2011, when it fell only 0.07% and made up for it the following quarter.
Unless the manager is having trouble finding investments, and the 3.5% cash stake does not indicate that, why look elsewhere?
But I'm not putting all my cash here. I like to have a chunk of cash that is truly cash, with a government guarantee if possible.

The holy grail that you're looking for used to be called "enhanced cash". The idea was to go a very little up the risk ladder (not conform to Rule 2a-7), but be pretty steady in price (albeit floating). They differentiated themselves from MMFs in both not complying with the MMF SEC rule, and in making clear that they were not MMFs. At least they were supposed to make that latter point clear.
Part of the problem with them was overzealous marketing - TDAmeritrade settled a suit because it had marketed Reserve Yield Plus as a MMF, which it was not. Part of the problem was miserable risk management by some of these funds. From Kiplinger, Are Money-Market Funds as Good as Cash (a prescient article from March 2008):

Many sponsors, analysts and investors used to think that ultra-short bond funds and short-term floating bank-loan funds were nearly as [safe as] money-market funds, but that thinking is in shreds after big losses at such funds as Fidelity Ultra-Short Bond (FUSFX) and Schwab Yield Plus (SWYPX).

Despite the notoriety that Reserve got (much of it deserved, for the way it went after Lehman bonds), I'm one of the very few who feel that Reserve Yield Plus was a fine fund, likely the best enhanced cash fund. It maintained an extremely stable NAV (unlike ultra short bond funds), and provided decent return. Exactly what an enhanced cash fund should do.
When the fund collapsed, its NAV dropped only 3% - compare that to the above named ultra short funds from Fidelity and Schwab. And even that was in part due to a foolish (IMHO) run on the fund by investors. As mentioned above, TDA had sold this fund as a MMF-equivalent, thus when its sibling Reserve Prime, a true MMF broke a buck, people panicked.
Despite its holdings in Lehman, despite the demise of this fund, I think it shows that enhanced cash funds can work (if not oversold to people expecting MMFs). For the most part, this type of fund doesn't seem to exist any more. RPHYX might be considered a version 2.0. But I think it is unique, and as OJ commented, not reproducible (limited supply). I'll offer thoughts on a couple of other alternatives in another post tomorrow or Thurs.

How about a hard close for RPHYX , if the cabbage patch is getting lean ?
Derf

"Does that sound like a "cash account substitute"?"
@heezsafe: Well, it seems to me that it does have some attributes of a cash-equivalent account, if you consider MF MMKT funds to be that. Money-market funds use their capital to make relatively short-term loans, and, in the old days, their so-called "dividends" were really just a cut of the interest that the MF company obtained while lending those funds. While they also had disclaimers similar to the one that you mention, it was generally believed to be with "a wink and a nod", as the MF companies tried very hard to maintain the $1.00 NAV, and generally succeeded.
If you recall, to prevent a massive run on those accounts, which would have reverberated through the entire economic structure, the government quickly "guaranteed" the safety of those funds for quite a while. It was that necessity which scared the government into trying later to pressure the MFs to cut loose the $1.00 NAV, and make clear the possibility of "without limitation, the loss of principal." But so far, not much change there.
As I tried to say above, a fund like RPHYX looks to me to be structured in the way that the government would now like MMKT funds to be: low risk (with RPHYX due to the ultra-ST commercial paper that you mention), no guarantee implied, and no artificial maintenance of the NAV.
It seems to me that the main problem with the governments MMF wish-list is that no one wants to borrow the money that's sitting there in all of those MMKT funds. Why would a company bother borrowing short-term when they can borrow long term at incredibly great rates? So the MMKT fund money just sits there, shrinking a bit as the theoretically non-existent inflation termites actually eat away at the pile, slowly but surely.
The only reason that RPHYX works at all is because they don't loan new money, but rather buy up the tail-ends of old loans that are just about to mature. Unfortunately, there is a limited supply of that kind of stuff.

I do not understand how anyone can equate ultra-ST commercial paper, having a duration less than 60d (MMF, svg accts), with non-investment grade HY bonds, having durations of 1-3 yrs, or less? To get there, what kind of mental gymnastics do you guys use? I guess, when one wants something badly enough, belief and desire can override rationality.(?)
From the website of Cohanzick Management (the subadvisor for RPHYX and RPIVX):
http://www.cohanzick.com/our-story/strategies
Note the sentence at the bottom of this page:
"All of our investment strategies are speculative and involve significant risk including, without limitation, the loss of principal."
Does that sound like a "cash account substitute"? I think these funds were intended to play an enhancing/supportive player role in a short duration/ST strategy [and for that I'm very grateful; I'm in RPIVX]; I don't think they were intended as replacement for everything else in this space. Just my take.

ER for RSIVX is 1.24%, and 1.18% for RPHYX. Seems they trade a lot, if I understand the strategy.

I guess that with respect to RPHYX, it is behaving the way that the SEC would like all mutual fund money-market accounts to behave: no suggestions, implied or otherwise, regarding "breaking the buck". No suggestions, implied or otherwise, regarding any government protection. Value can fluctuate, according to the market.
That being said, the relatively small amount that RPHYX pays is certainly better than my remaining MF MMKT accounts, which have paid NOTHING for at least five years now, but have maintained their $1.00 share value. The only thing supporting those funds is the good faith of the MF families, American Funds and American Century.

Well, with RPHYX I'm up some $5600 over my basis. I can't give actual percentage gains because I've added to that account a number of different times, and while I do know the methodology to approximate the percentage gain it's just more trouble than I want to go to, so I just keep the YTD data.
RSIVX is up 1.3% YTD, not too bad, but it's also a different animal than RPHYX.
One reason that I like those two is that they're NTF at Schwab, so easy to get into and out of cheaply.

I have parked cash in RPHYX and RSIVX for about the last 15 months. My basis is actually more than the market values of these funds given the reinvested dividends and paltry increases in NAV. The money seems safe, but I almost feel as though I'm paying someone a parking fee. Like Whakamole, I would never have known about these funds without MFO.

RPHYX nav dropped .1 % last year, returning 2.6 according to Old_Joe Averaging 3.7 % return over the last 3 years maybe it's time to find a greener pasture? Is the manager running out of places to invest in? Total assets 896,000,000 as of 2/28/15. FWIW ( It seems to me the manage at one time stated the amount of assets he thought the fund could handle). More digging to do.
Derf

Thanks, Old_Joe. The return on RPHYX is nice so far (and I never would have heard for it if not for this site), though it's quite another thing to invest ~10% of my net worth into it!
Which is why now I'm thinking that short or intermediate-term munis may be a good idea, though PFF sounds like an interesting strategy (I remember learning about preferred stock back in the day as "these are out there but really look into stocks or bonds.") Which term do you like better for munis?

Well, with that situation, you're obviously only concerned about the Federal bite. You know, there just are not any easy answers at this particular point in the economic playout (I hesitate to use the word "cycle", because I don't think that any of us have been here before).
Like you, we are also heavily into RPHYX. In very round numbers, last year, it did 2.6%, after the IRS, 1.9%. Subject to some risk, completely uninsured, of course.
So far this year, 0.6% / 0.4%. Wow.
We also have some cash stashed in various bank accounts, earning next to nothing.
I just don't know. One thing about RPHYX- it wouldn't take much in the way of NAV decline to pretty well offset the small dividend return.

You could go with FLTMX or @Ted might remind you about PFF. I think RPHYX is fine though. Why are you looking for an alternative? If you are looking to limit tax liabilities, there are plenty of muni options.