Jack plate and prop testing

I just installed a new jack plate and completed initial testing yesterday.

So straight to the facts:
- 19' Sprint Fish and Ski v-hull
- Mercury XR6 150, runs great. 115 psi across all cylinders.
- 6" setback jack plate
- 25P Merc Trophy Plus prop in great condition
- Live wells empty
- We started with 16 gallons of gas and ended with 2 gallons.
- 300 lbs total with driver and passenger
- I started at 2.75" prop-to-pad (p-p) distance and raised it in 1/4" increments from there.
- I have a water pressure gauge which I monitored while testing. I never had below 15 psi at WOT.
- Water conditions were flat with ripples to very slight chop. I used coves to keep the wind and waves off of the boat.
- Prop has an exhaust ring on it; vent holes are partially plugged.

Here are the results:

2.5" p-p
55 mph GPS
5500 rpm

2.25" p-p
55mph GPS
5500rpm

2" p-p
55 mph GPS
5500rpm

1.75" p-p
55mph GPS
5600rpm
note: prop blowout during holeshot - had to trim up slowly. Rooster tail at about cowling height.

1.5" p-p
55mph GPS
5800rpm
note: more prop blowout during holeshot - had to trim up more slowly. Rooster tail above cowling height.

Comments:
- Prop slip is at about 21% for all 5500 rpm tests using the Merc calculator
- It was harder to steer the boat to port so I need to add more torque tab to offset that; just FYI
- Speed kept topping out at 55 mph in all cases. We gave the boat plenty of time to reach top GPS measured speed for all runs.
- I am surprised I did not get a little more speed than 55 mph. Based on similar Sprint hulls with 150 HP outboards others have reported 60+ mph (gps).
- Holeshot feels just as good (if not better) as it was with a 21P Trophy Plus. Weird I know, but there it is.
- Speed seemed to climb faster with reduced p-to-p, especially 1.75" and 1.5"
- Goal is to hit 60 mph if reasonably achievable. If not, I'll live with it.
- If I trimmed down from full up trim the boat would start slowing down; once the boat hit top speed I played with the trim a bit for all tests.

Questions:
- As mentioned I expected a little more speed based on other reports. Any suggestions here? Could I be getting a lot of drag from the F&S style windshield and that is creating the brick wall? We kept the step through glass door closed for all tests.
- So what is next? If everything is dialed I am tempted to go down to a 23P 4-blade to cover the extra weight with fishing gear and the live well, and maybe (?) get a little more efficiency. I hardly ever use the boat for towing tubes/skiers but I have a 21P for that anyway.

Try some different props. Sounds like the hull maybe is not breaking free. Something with more bow lift. See if you can find a prop shop to work with and test some out. Trophys are popular, but they dont work for everyone

So try a 3-blade 25P like a Tempest?

Tempest run well, lots of bow lift. Maybe not as good out of the hole. I'd try every prop you can it really makes a huge difference when you get one that really works for you. Your process of easing into engine height is solid, repeat that with each and you'll get that rig moving

Try prop deeper. Easy test and costs nothing . You started relativity high for that speed. Did the bow move freely with trim? Did it react to trim at all?
In the past, boats with many different changes showing the same speed often had a hook in running surface. Put a straight edge on the boat.
I agree try as many props as you can. However I like the trophy plus on your engine many of the newer props are for 3 liter engines.

Agreed on the height. I already have it set to 3" p-to-p so I just need to get back out to the lake to test it. I can go as low as 4" p-to-p without repositioning the motor on the jack plate.

The bow definitely responds to trim during launch. At speed it is hard to say as I was focusing on speed vs trim (and boat handling, and watching the water). I'll have to watch more closely for the bow reaction vs trim at speed.

Try the 23 too. Don't be afraid to prop it to turn 5800 or 6000. I suspect you'll find the height to make the boat happy without repositioning the plate.

I find it more fun to run a boat set up for all around performance and not just top speed. Prop it to 6000 light so when you have her loaded she still turns 56 or 5700.

The 2001 XR6 I have (carbureted) will not spin up to 6000 AFAIK. The Merc manual specifies WOT RPM as 5000-5500. Max RPM is 5800 and after that the ignition controller starts retarding timing and then dropping spark altogether.

Then 5800 it is.

I have not been able to find a straight answer on where the peak power is on the 2001 XR6. Not sure if it is at 5500 or 5800, seen both answers here and there.

I have only ever seen that happen when the boat was way heavy (waterlogged in that case). You can weigh the boat on the trailer if you measure carefully: https://www.boatsnews.com/story/3867...a-simple-scale

There are different ways to reason top RPM. Higher R's gets more prop speed and prop action. If you prop is to top of or slightly over light it leaves you high in the range with loaded boat. Its always more fun to run a short propped boat; more acceleration, carries loaded better, easier on powertrain. Over propping bogs engine, is harder on gearcase, is harder on motor, often runs hotter. When you load the boat you may fall under recommended range.

Prop it for how your gonna use it.

Probably need more set back. Try a set of Bob machine shop 2" set back spacers. Cheap and easy test, possibly in the right direction. Trophy props work good

Let me back up. Check the last 3ft of running surface to make sure your hull doesn't have a hook

Quote:

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Originally Posted by Jim Speros

Prop it for how your gonna use it.

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Yep. Like I mentioned in the OP I almost never pull skiers and I have a 21P Trophy+ for that anyway. This boat is mostly used for solo fishing, which means light load, and running faster (to cover more ground) rather than cruising.

Quote:

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Originally Posted by Baker343

Let me back up. Check the last 3ft of running surface to make sure your hull doesn't have a hook

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I'll check it again but when I used a long straightedge to measure the prop to pad it looked good.

Have you done a black box delete? Oil injection delete? If not do both

Oil injection delete is done. I do not think this gen XR6 can have a black box delete like the earlier ones; the only black box on the motor is for the ignition system.

So as an update there will be more testing tomorrow:
- I am going to try the same Trophy Plus 25P at 3.25" prop-to-pad which is as low as the jackplate will go without remounting the engine. That's lower than the lowest from the first round of testing.
- Then I am going to switch to a practically brand-new Tempest Plus 24P I just scored and incrementally raise the jackplate from there.
- Test conditions will be close to the same as before. Lake, temperature, wind, boat load.

Then I'll come home and crunch the data, think about it, pick the prop, and sell off either the Trophy or the Tempest. Again I already have a 21P Trophy+ for slower and heavier work; this testing is for the go-fast, fishing-solo prop.

Well... That prop test did not happen today. A testing trip with two ended up being a fun trip for four so I reinstalled the 21P Trophy+ and we cruised and tubed around on that all day.

What I did confirm is that with four adults in the boat, 30 gallons of gas, and miscellaneous gear that the 21P Trophy+ will definitely work as the heavy load all-purpose prop. It topped out at 50 MPH (gps) at 5800 rpm with four in the boat, and at 45 mph with three in the boat and one being towed on the tube.

Again this XR6 is set up by Mercury not to run above 5800 rpm so I believe that 50 mph wall was an rpm limitation and not a drag limitation; in other words, 21P was not too much prop. With a light load it would be too little (but I knew that already).

I'll report back when I complete testing with the other two props.