Offset Low Water Pickup Holes in Nosecone

[rgsauger]

Is it an optical illusion or is the whole skeg offset to starboard?

[CI STV]

Is it an optical illusion or is the whole skeg offset to starboard?

Neither. Just a bad camera angle, lol. Here’s a better overhead shot of the whole L/U:

[rgsauger]

Thanks. Good looking case

[CI STV]

Thanks. Good looking case

Thanks. It was a real piece of junk when I got it. I did a whole thread on here about it. Bought it from a S&F member as a “freshly-built” unit and when I went to install it, I found it was leaking from a crack in the case near the lower pinion bearing area. The oil inside was littered with metal shavings (steel and aluminum). Real sh!tshow.
Pulled it down about a month ago and found that the driveshaft was junk as well, since it was cut too short (looks like he just guessed what it needed to be and cut it, forgetting to add 5/8” for the adaptor plate thickness) and the the shaft had a groove and was pitted badly in the seal area. The gears (SM 2:1 gears cut down) didn’t look the best and neither did the propshaft. So we cleaned it out, got the crack welded, changed the driveshaft and bearings and we’re going to test it to see how long it holds up. I might as well get some use out if it, since I have well over $5K in it.
Going to use it only for 800ft drag racing, so hoping it will hold up.
I have another XR6 unit that I’m rebuilding now with brand new 1.78 gears and all new bearings, clutch etc. so, if/when this one blows, I’ll have a backup.

[Greg G]

If you all keep talking about performance boating stuff on a boating forum I will have to report you

[transomstand]

If you all keep talking about performance boating stuff on a boating forum I will have to report you

We could all start yelling at each other if it makes everyone more comfortable

[rgsauger]

Everybody just SHUT UP!!!

oops, wrong forum. As you were…

[CI STV]

You guys are hilarious.
Now, how about we start talking politics?

[rock]

hell no

[CI STV]

Okay, now that we got that out of the way…

Once I get the other case together, I’m going to do some testing to see which works best/is more efficient: the 2:1 XR6 with a 24P Yamaha, or the 1.78 XR6 with a 20P Yamaha.

I’ll probably start a separate thread to document what I find out, and I’ll include datalogs to verify the results.
To try to “level the playing field”, I’m probably going to plug and fill the outer 2 of the 5 holes in the Bob”s nosecone on the 1.78 case, since I’ve already enlarged the 3 middle ones, like this:

The weird thing is that this setup is pretty damn efficient, with prop slip at only 7%, as you can see from the screen shots of a datalog and the prop slip calculator posted below. This was using a Ronnie Gilbert 20P drag prop:

[CI STV]

Sorry, just realized that I put 158.9kph instead of 158.5kph in the prop slip calculator, so the correct amount of slip calculated should be 7.26%…

[CI STV]

Couple of things to note regarding the above test data:

(1) I think the prop slip is that low because I’m running the case a little deeper than optimum.

I say that because when I tried running the gearcase a little higher, the water pressure dropped quite a bit, to like 14-15psi, IIRC. I had a discussion with some really competent folks on these forums about it and the consensus was that I should plug the 2 outer holes on the side of the nosecone, as they may be sucking air when the gearcase is run “where it needs to be”: @ ~1 3/8” above the pad on my STV.

(2) The data point shown on that screen shot is somewhere beyond 700’ mark and approaching the finish line on a marked 800’ course, and that shows what the boat was running right before it hit the 10K limiter that I didn’t know the ACU had in it. I’ve fixed the rev limiter issue now, so I should be see what each gearcase/prop combo can do in the 800’ when I get it going again.

[Greg G]

You are doing some top shelf work, thank for posting it up

[CI STV]

You are doing some top shelf work, thank for posting it up

Thanks. I couldn’t have done this build without the good advice of several members of these forums.
Despite the occasional bickering, and despite the fact that we have lost a number of our icons, this place still rocks…

[transomstand]

Couple of things to note regarding the above test data:

(1) I think the prop slip is that low because I’m running the case a little deeper than optimum.

I say that because when I tried running the gearcase a little higher, the water pressure dropped quite a bit, to like 14-15psi, IIRC. I had a discussion with some really competent folks on these forums about it and the consensus was that I should plug the 2 outer holes on the side of the nosecone, as they may be sucking air when the gearcase is run “where it needs to be”: @ ~1 3/8” above the pad on my STV.

(2) The data point shown on that screen shot is somewhere beyond 700’ mark and approaching the finish line on a marked 800’ course, and that shows what the boat was running right before it hit the 10K limiter that I didn’t know the ACU had in it. I’ve fixed the rev limiter issue now, so I should be see what each gearcase/prop combo can do in the 800’ when I get it going again.

I'm convinced Bob's used 5 holes in order to satisfy a very wide range of conditions and avoid lawsuits from overheated powerheads. 5 holes in most cases are completely unnecessary, so go ahead and plug em.

As far as prop slip goes, it's something to be used for amusement and nothing else. Unless your tach and prop are laboratory quality, the numbers you insert in the calculator are just guesses. I've seen boats set best top speeds with terrible slip numbers (over 15%) so slip calculations are a waste of time to me. If you put a prop on that runs 3 MPH faster but the slip is "bad", do we throw it in the dumpster?