Should I be concerned about this WOT data? (Pics Inc)

[bsutravis]

I finally got some good data on my boat's speed/RPM so here it goes.

I have a '77 Mercury 115hp Tower of Power pushing a 16' fiberglass Fish n Ski boat. Boat is rated for 1350lbs of gear, motor and up to 6 persons. My prop is a 13 3/4" x 19P (48 65024 A1).

Running at WOT on the lake yesterday with light winds creating a small amount of surface ripples according to my GPS I could only hit 39.6mph with myself and my son onboard (400lbs total). My tach showed just 3200 RPM at that WOT speed, which is my main concern as I know this Merc should push way more RPM's than that.

Prior to having this prop on the boat I had a 21P, and when I hit 2000 RPM the prop would blow out and I would lose all "bite" with the water and I would not accelerate. I went to the 19P and it gets the boat on plane and pushed it to the data I just gave, however when trying to pull an inflatable tube/rider it has a VERY hard time getting on plane even after moving everyone into the bow area to get the back end out of the water to help it plane out. I've tried power-trimming with minimal differences in terms of getting the boat on plane with a tuber. Without pulling anyone it does ok getting on plane and once it hits plane it seems to accelerate much faster.

To be honest I'm happy with the 40mph speed as my wife screams for me to slow down at that speed, but I know it's supposed to be around 5200 WOT/RPM. The motor sounds wonderful at the 3200 RPM it's giving me now, and honestly I don't know if it's got another 2000 RPM in it.

Assuming my tach is correct (How do I know that it is?) what adjustments can I make to help get the boat onto plane faster and help with my seemingly low RPMs?

Here are some pics of the setup:

 

[Georgesalmon]

Re: Should I be concerned about this WOT data? (Pics Inc)

Assuming that your tach is correct. Need to check that out. Another tach, tiny tach etc. Sounds pretty fast at 40mph for only 3200rpm.

 

[steelespike]

Re: Should I be concerned about this WOT data? (Pics Inc)

I suspect your tach. I have your ratio at 1.7 Using a reasonable guess of 10% slip the calculator indicates that at 39.6 your rpm is
about 4200. I don't think its practical to make up 1000 rpm with a prop. So you may want to verify the tach
and check these things.
Your anti vent plate(just above the prop) should be about even with the bottom of the transom to about an inch above.
If you have no venting issues you probably need to raise it.
I would also check that the throttle opens all the way,that the timing is set right and advancing as it should.
Also you wouldn't be the first one to not realize you were missing one cylinder.
Be careful when checking ignition as it may be sensitive to careless handling.
Just checked the pics it does seem the height is pretty good but again if no venting issues won't hurt to raise it a little.

 

[Philster]

Re: Should I be concerned about this WOT data? (Pics Inc)

On a hull like that, the prop seems pretty low to me. Burying a prop would certainly put a ton of drag on it.

Time to do a visual while on plane: Need to look over the transom and get a read on the AV plate. It should be surfacing -- just skimming across the top of the water with some splashes going over it.

My AV plate is 5" above the bottom. My prop centerline is about 3.5 below the bottom. That rig doesn't have a lot of deadrise; I'd be willing to bet higher is better on that.

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