Any thoughts from the I/O experts???

[wca_tim]

I've done some searching and have read a number of threads on this topic already - thanks loads for the info guys!

current set-up: 1988 18.5' shallow v, new reman 4.3 that I would guess is turning somewhere from 225-250 hp? (little higher compression pistons, factory marine spec marine roller cam, Vortec heads, electric fuel pump, edelbrock carb and intake, careful tuning / jetting, relatively open exhaust, etc...). It really seems to run very well overall and has great power from off idle through wot. Currently pulling between 53 and 56 MPH (GPS) depending on wind, chop loading in boat, etc... with 23" Laser II at WOT of ca. 4800 rpm. Gear ratio is 1.84 per the side of the outdrive. Best speed is when trimmed to where can just start to see a evidence of "rooster tail" (talking a few inches here...) in the center of wake. Note that even a little below WOT, this is where it's at, and the last bit of throttle increases noise a little but no change in engine rpm or speed. Currently has a decent holeshot, pulls good size skiers right out ot the water, etc... Note that I'm not thinking that I'm going to make a huge difference, mainly playing / learning. I'm planning on getting something a little bigger (say 22-27) with a lot more engine in the next year or two and honestly bought this boat to play with and learn on before stepping up to much larger dollar mistakes. I bought it with a trashed factory 2bbl in it, had a local mechanic drop the engine in (had a borken shoulder at the time) and have done the majority of the work since then myself.

Questions:
1. Does the fact that I have more throttle left with no corresponding increase in rpm or speed, indicate that I'm coming out of the power band for the engine as it's set-up? (though about trying a step up on main jets, but spot checked plugs from wot shutdown look about right as far as I can tell - med tan, etc...).

2. If so, what prop might be best to try? Seems like many of the 25" or higher props I see out there are designed for heavier / hogher horsepower boats and have thicker blades, etc... might not be as efficient in transmitting power to the water? Any place to get good information on blade shape, etc...? Also, not sure if more or less bowlift is desirable for a little more top end in this kind of an aplication.

3. If I changed the gear ratio (would likely just change out the upper half of the outdrive - replaced the lower a month or so ago when sheared the lower drive shaft) say to 1.65 or even 1.5 and went with lower pitch propo until wot in the right place, how much of a gain might one see from that difference? I've read that around 19" is probably optimum in getting power to the water in blade speed ranges that are seen with typical I/O applications, and also have the impression that lower less reduction in gear ratio (all other things being equal) corresponds to less power loss in the drive system.

4. What if any effect might trim tabs have. I get a little porpoising now and it planes at about 16-17 mph. Note that I'm asking the question not in terms of improved handling, time to plane out, etc... but in terms of tweaking top end.

4. Any other factors that I'm not considering that I should? In the context of trying to wring as much top end out of current set-up as I can...

Thanks in advance for any thoughts you might have!

 

[John_S]

Re: Any thoughts from the I/O experts???

You have exceptional slip numbers. If you give the lazer 1" extra pitch for cupping and use the 53mph = 10% slip. If you use the 57mph and 23" the slip is the holy grail of zero. Since you used gps, I'd do a revolution test to verify the ratio.

If you are porpusing at wot, you probably could gain 1 maybe 2mph using smart tabs.

 

[Don S]

Re: Any thoughts from the I/O experts???

Moving to Prop Topics and Questions.

 

[wca_tim]

Re: Any thoughts from the I/O experts???

So should I be adding the 1" for cupping? I'll double check the drive ratio next time I have it off. I did just replace the lower half (in hindsight, should have thought about gear ration then - seems now like I would have been better served with 1.62), and was pretty sure then... but worth double checking I spose.

I noticed that the torque / horsepower curve I have for that engine peaks at about 4500 rpm. It is possible that I'll pick up a little bit by putting the WOT rpm at the beginning of the peak HP plateau putting WOT RPM at both greatest torque and hp possible?

On porpoising, only get at like 3/4 throttle with trimmed out pretty far. Am thinking about trim tabs to be able to plane at wakeboarding speeds, but that's a separate issue.

I'm kind of looking around for a good 25" or 26" prop to try as well.

I am curious. I hear so much about getting the lowest prop slip, but intuitively I'm not fully understanding whether or not some prop slip is a good or bad thing. Prop slip indicates essentially the difference between the theoretical speed of the boat based on the pitch of the prop, but doesn't take into account how effiiently the power is transfered into forward motion. Aren't there cases where slip could be significantly higher, but the loss of usable power that is translated into forward motion is lower? As an extreme, I very large bladed prop on a light boat that was turned relatively slowly would presumably show a very slow slip rate, but would waste a huge amount of the usable torque / horsepower in doing so. I realize that it really gets even uglier than that when we consider the viscoelastic properties of the water as well.

In a round about way, I guess I'm asking if there is something to be gained by running a higher pitched prop, with lower blade surface area at higher rpms. and is so under what kinds of circumstances is this the case.

Just thinking out loud and trying to learn a bit from some of the members' experience. I'm having a ball with this, but am kind of new at it...

Thanks!

 

[180shabah]

Re: Any thoughts from the I/O experts???

First, I think you power estimates are a little high. With that intake, vortecs, and a stock cam you are realistically looking at 205-230hp depending on the presence of a balance shaft. That thing makes the 4.3 nice and smooth, but it does suck some power.

Second, you are running a 23" Laser II. It is cupped, making it effectively a 24" prop. It has youtr WOT at 4800, right where you want it.

Third, the last little bit of throttle makes no difference, other than sound. This means that the throttle is no longer the limiting factor. With decent heads and a good IM, most likely you have hit the flow limit of your exhaust.

Increasing pitch, will lower your WOT and most likey your top speed as well. It will also make a huge difference in your hole shot(in a bad way).

You don't want to increase pitch much, if any. There are some really good props out there(enertia, turbo...., but I will have to defer to others for specific recomendations.

 

[QC]

Re: Any thoughts from the I/O experts???

First off I think 180shabah is almost dead on, especially about the volumetric efficiency stuff i.e. exhaust, air limited. However . . .

I noticed that the torque / horsepower curve I have for that engine peaks at about 4500 rpm. It is possible that I'll pick up a little bit by putting the WOT rpm at the beginning of the peak HP plateau putting WOT RPM at both greatest torque and hp possible

I think you actually could potentially pick up maybe 1 or 2 MPH by pitching up slightly. Almost all test data I have read shows that boats propped towards the bottom of their WOT range are slightly faster. Not always for sure, but usually. You are a little confused though about the relationship between torque and horsepower. Peak torque is never at the same RPM as peak horsepower in high RPM engines. (It is sometimes in low speed diesels, but a totally different animal as we are talking below 400 RPM type stuff). For speed you theoretically want to be at Peak horsepower. hp = torque x RPM / 5252 . . . Stare at it, noodle on it. Torque begets horsepower, but it is not helpful alone, ya gotta have RPM.

On porpoising, only get at like 3/4 throttle with trimmed out pretty far. Am thinking about trim tabs to be able to plane at wakeboarding speeds, but that's a separate issue.

Also, generally speaking, boats are fastest just at the point that they begin to porpoise. If you cannot make her porpoise at WOT and full trim, I would shift a little weight aft. This may get the bow up a little teeny bit more. Trim tabs are also part of this discussion, but I am not 100% convinced that Smart Tabs will allow you to keep them out of the way enough not to scrub a little speed (I mean very little like 1/2 MPH). The nice thing about helm adjustable tabs is you can bring them above your hull line and theoretically have zero tab in the water. Then a teeeeeny tiny bump down on the tab can control that teeeeny tiny bit of porpoise we're looking for but can't stand . . .

Prop slip indicates essentially the difference between the theoretical speed of the boat based on the pitch of the prop, but doesn't take into account how effiiently the power is transfered into forward motion.

This is a contradiction kinda. The theoretical statement is correct, but slip absolutely indicates how efficiently you are transferring power to the water. It is impossible to have zero slip, but accurately calculated slip below 10% indicates that things are going really well . . .

In a round about way, I guess I'm asking if there is something to be gained by running a higher pitched prop,

As I noted above, I think maybe . . .

Just thinking out loud and trying to learn a bit from some of the members' experience. I'm having a ball with this, but am kind of new at it...

Good noodling, you're getting very close