5.7 with low wot - Maxum Boat Owners Club

hooked514 · 08-08-2011, 12:09 AM

I just purchased a 1993 scr2300 with a 5.7 mercruiser alpha 1. The boat idles at 700 rpms. Drives great at slower speeds. At 2200rpms the boat will plane and drive nice. When I give it more throttle it will only go to 3500 rpms and it losses plane. I don't feel a miss or anything and my oil pressure and temp. are good. The boat has been kept in the water the bottom has some growth. Plugs, wires etc are not new. any ideas were I should start. Thanks

dronthelake · 08-08-2011, 01:05 AM

What size prop? Aluminum or stainless steel? Number of blades?

Also, does the tach seem to be operating properly? Does it stick or bounce around?

seapuppy · 08-08-2011, 03:09 AM

first off..welcome to the zoo....2nd..try telling us a bit about where you are......might be someone close by that can drop by and willing to help..
3rdly sounds like the growth is keeping you from wot.....now keep in mind it doesn't take alot to keep you from reaching wot...if you clean the hull...and still can't reach wot...then try a full up tune up...and also check the gas or vent to the gas tank.....

SP

ss3964spd · 08-08-2011, 01:14 PM

Quote:

Originally Posted by hooked514

When I give it more throttle it will only go to 3500 rpms and it losses plane.

Can you explain further? When it goes to 3500 and "losses plane" are you saying the boat falls off plane?

shrew · 08-08-2011, 01:34 PM

Quote:

Originally Posted by ss3964spd

Can you explain further? When it goes to 3500 and "losses plane" are you saying the boat falls off plane?

I was trying to wrap my head around the same thing. Technically, the RPM's aren't what get you on plane, it's the speed. If you disagree, then put the motor in neutral and crank her up to any RPM and see how fast you're going. LOL

. More important would be the speeds and the RPM relationship. For example, the slower my boat is going, the more tab IN I need to keep the boat on plane. Below a certain speed, she starts to drop off plane. The wind, tide, current, and sea state will dictate the amount of RPM's required to maintain a given speed. So, all things are a bit relative.

What you describe is the boat is on plane 2,200 RPM's, which in itself seems very low. Then at 3,500 rpm's she drops off of plane? Where are the speeds at 2,200 and 3,500 respectively? Does the boat rpm's drop dramatically once they hit 3,500 rpm's?

The only thing I can think of is a spun prop hub. Theoretically, if the hub is toast, you 'might' be able to maintain bite at lower RPM's, but at higher rom's the hub can no longer maintain grip and the shaft spins inside of the hub. This would reduce the amount of usable torque being transferred to the prop. The engine RPM's are spinning away, but the prop is not rotating at the relative RPMs's to maintain bite in the water.

I'd have the prop pulled and taken to a prop place to have them inspect the prop hub and replace if necessary.

mcoffey · 08-08-2011, 06:34 PM

I 2nd Shrew here. I have seen a spun hub cause this exact issue. Boat seemed to run great until WOT holeshot or WOT for top speed. When you did this, RPM would go up but boat would slow down

Easy to tell too. Raise outdrive and with paint pen (or nail polish) mark a spot on the prop nut AND the inner ring of prop (have them lined up too). Then go test drive and duplicate issue. If it spun the hub you will find the marks no longer line up.

shrew · 08-08-2011, 07:11 PM

Oh ya........Welcome to the site.

hooked514 · 08-08-2011, 08:05 PM

Thanks I will try checking for a spun hub. At 2200rpms the boat rides more level. when I give it full throttle the rpms are at about 3500 and stay even. The wake changes and the bow raises up. I am sorry I don't have alot of experience in boating. My speedo has not been working I am hooking up a gps to get my speed. I can tell that the prop speed does increase with the throttle I thought that my problem was in my wot which should be at least 4200 rpm. I noticed that my spark arrestor is dirty and I thought that I would try a new fuel filter and a good tune up as well.

ss3964spd · 08-08-2011, 08:49 PM

Are you starting out with the drive all the way down and then trimming it up a little once you get on plane?

Still though, you should be able to make more than 3500.

hooked514 · 08-09-2011, 11:44 AM

Yes I have made sure that my engine is all the way down. I just think that I am not geting enough rpms to keep the boat on plane.

ss3964spd · 08-09-2011, 04:42 PM

In basic terms getting on plane is the moment when the bow of the boat gets lower relative to the stern. In other words, when it levels out as it builds speed. The hull design mostly determines at what speed it will achive plane. It should remain fairly level at any any speed above the hulls planning speed. If I am interpreting correctly you are saying the boat starts to level out around 2200 RPM and will continue to run at that level attitude as long as you do not go to WOT. Then, when you go to WOT, the RPM's max out at 3500, the bow rises, the stern lowers, and the wake changes. The only reason the boat will come off plane is a reduction in speed to something below the hulls planning speed. When you go to WOT does the engine sound any different? Does it start to vibrate?

So, despite an apparent RPM increase, it does sound like the boat has slowed and is coming off plane. I suspect the wake has become larger because the boat is plowing which is when it is running in a bow up/stern down attitude.

Once on plane, and instead of simply going to WOT, have you tried to just gradually increase RPM's past 2200 to see what happens? 2300? 2500? 3000? 3200?

Quite simply if the RPM's are increasing - even if just to 3500, so too should the boats speed. There really arn't many things that can cause this scenario. 1) As already suggested, the prop has spun on it's hub. 2) The prop is cavitating due to bent, broken or otherwise screwed up blades, or ventilating because the drive is too high.