Can't get my boat on plane

[Dave - M]

1990 270 DA 7.4. When I first put in, around May 4, I had a long no wake area, followed by a short run on plane, to get to my dock. The boat got up on plane with no problem. About two weeks later, I put 50 gallons of gas in it and it struggled to plane, although it did, but i couldn't get over 4000 rpms. It ran about 30 mph. Next time I went out, it just wouldnt plane. Tabs are down, drive down, I get 4000 rpms max but i just plow water.

Next time out, same thing. I did put 2 bottles of Lucas carb cleaner in. I max out at 4000 rpms but just plow water. The guys with me agree that the motor sounds great, no missing, no knocking. We checked the tabs and they are operating properly. Drive also appears to be fully down although one person feels there's too much of a rooster tail for the drive to be fully down. My bottom is painted however I dont know with what. It was done by a prior owner. It's clean looking at the sides though.

One thing new that happened the last attempt though, while at full throttle, it suddenly dropped from 4000 rpm to 3500, then back up.

I don't know what prop is on there but wondering if it got hit during the first run. May have to consider having the boat pulled to look at the bottom and drive.

 

[Lazy Daze]

Don't need to pay for a haul out to check the bottom and prop condition. Trim the drive up and/or just get in the water. Take a beath and dive under the boat - or reach under and feel for slime.

I don't think the problem is your drive. If it was, you'd have a problem with hole shot OR top end... not both. I assume you've physically verified that it's going up and down, too.

If you're certain that the bottom is clean and the prop is good... The first thing I'd look at is dist cap/wires/plugs. Maybe it's as simple as a wire being loose. But look at the dist cap/rotor, give each wire a good looksee and then pull each plug and inspect. Make sure to put each plug back in the hole it came out of. Google for info on reading a plug. You can also check for spark at each plug.

 

[Jimmy Buoy]

It's possible that the rubber hub inside your prop has spun. It would provide enough grip to spin the prop at lower RPMs but when more torque is applied the shaft/prop hub could be spinning within the prop, failing to provide nominal thrust. Just a thought.

 

[Golfman25]

It's possible that the rubber hub inside your prop has spun. It would provide enough grip to spin the prop at lower RPMs but when more torque is applied the shaft/prop hub could be spinning within the prop, failing to provide nominal thrust. Just a thought.

That's what I was thinking.

 

[Lazy Daze]

Typically what happens with a failing hub, though, is that the RPM's skyrocket. When the hub slips, the propshaft still spins... but now it can spin faster since there is essentially no load on it.

 

[Lazy Daze]

Just wanted to add something to my first post... a V8 can run with one cylinder down and it can sometimes be nearly imperceptible (the sound of the running engine, that is).

 

[Dave - M]

Thanks for the replies. Forgot to add that I checked the spark arrestor and made sure it wasnt dirty. Someone did lay on the platform and ensure that the drive was going up and down.

I'll have to go out and anchor to check the bottom and prop. I have a platform extension that makes it hard to see and getting in the water at the dock is forbidden.

 

[WV 320 Dancer]

With a 7.4 you should be able to spin between 4400 and 4800.

 

[Wanderer 290]

Outdrive position is good.
Prop is good.
Engine sounds good and has top rpm intermittently?
I would check fuel flow and/or fuel filter?

 

[Thornton69]

Fuel filters

 

[aerobat77]

is your tachometer correct at all ? assuming your driveratio and prop are in range 4000 rpm should be easily enough for a 7.4 to plane out this boat.

that you are not able to get over 4000 might be another issue .

 

[Dave - M]

Got a new fuel filter, plugs, wires, cap and rotor. Got 4500 rpms. Got up on plane and saw 25 mph at first. Cruised at about 3600. It doesnt get on plane fast but still fun again!

Wondering how a 4 blade prop would do.....

 

[rpasquarello]

Got a new fuel filter, plugs, wires, cap and rotor. Got 4500 rpms. Got up on plane and saw 25 mph at first. Cruised at about 3600. It doesnt get on plane fast but still fun again!

Wondering how a 4 blade prop would do.....

1. Sounds like a spun hub.
2. I had issues getting on plane as well for years as my boat is way underpowered. I eventually changed to a four blade prop (thanks to hitting a rock) with a smaller pitch and it made a big difference. Had to switch to a “hi-torc” hub, though.
3. Check out and call these guys: https://www.ptprop.com/

 

[Dave - M]

As I said above, I got 4500 rpms after a tune up. That was with a 1/4 tank of gas. A week later i put 25 gallons of gas in and again only got 4000 rpms, but I did get on plane.

There is only one place at my end of the lake to buy gas so if the gas was bad, there would be other folks suffering as well.

Still wondering, even though my prop looks ok, maybe I'm propped wrong. I'm going to see what happens as I burn some of this gas off and lighten the load

 

[Dave - M]

Boat is out of the water and winterized. Bottom was not particularly dirty.

Any attempt at planing this summer was pretty much a dismal failure. I got my prop taken off while winterizing; it is an aluminum 14 1/4 D x 17 P (Turning Point).

There happened to be an identical boat to mine getting winterized at the same time. He runs a stainless mirage 15 1/2 x 17 and reported not having any planing issues. Shop has one on hand so Im trying that for spring launch.

 

[Handsome Transom]

Fuel seems to be the thing for me and not bad gas but delivery because when you put in with original gas it ran well then fresh gas stirred up flakes sludge whatever and tour pickup could be blocked. Fuel filter for sure water separator.

 

[Dave - M]

Hit the water yesterday with my new Mirage stainless 15.5 x 17 prop. Last year's gas. Was able to cruise about 27 mph at 3400 rpms. Had a little under half a tank and 3 adults aboard. Got up to 30 but didnt try for anymore. It was really breezy and chilly so I had windscreens and front side curtains up. I didnt really play with the trim. I was just happy to be finally up on plane!