Low Top End WOT

[I/O New Guy]

Hey guys, I/O neophyte looking for advice! Have a (new to me) 1990 18' Sea Ray w/ 4.3 liter/alpha one. It starts fine, idles fine, runs smooth and accelerates and planes well. It is also running at about the right temp. However, it does not have the top-end power I'd expect. It accelerates well to a point and then just levels out. I ran two new props with these results:

- 19 pitch merc - 37 mph/4200 rpm
- 17 pitch merc - 34 mph/4500 rpm

I understand WOT should range be 4400-4800 for that engine, and I would think the 4.3 should reach that range with the 19 pitch prop. I read through this topic in the Adults Only section and have eliminated most of the potential reasons. I even weighed the boat, and there doesn't seem to be any water intrusion. It was at the shop last week for a checkup and fluid change. They gave it a green light relative to compression and overall running condition, but that wasn't done in the water under load.

I know this doesn't tell enough to get at "the" answer, but I'm hoping for some direction. With everything described above, what are the likely culprits that are left? I appreciate any assistance.

 

[billy4hp]

Re: Low Top End WOT

Hey guys, I/O neophyte looking for advice! Have a (new to me) 1990 18' Sea Ray w/ 4.3 liter/alpha one. It starts fine, idles fine, runs smooth and accelerates and planes well. It is also running at about the right temp. However, it does not have the top-end power I'd expect. It accelerates well to a point and then just levels out. I ran two new props with these results:

- 19 pitch merc - 37 mph/4200 rpm
- 17 pitch merc - 34 mph/4500 rpm

I understand WOT should range be 4400-4800 for that engine, and I would think the 4.3 should reach that range with the 19 pitch prop. I read through this topic in the Adults Only section and have eliminated most of the potential reasons. I even weighed the boat, and there doesn't seem to be any water intrusion. It was at the shop last week for a checkup and fluid change. They gave it a green light relative to compression and overall running condition, but that wasn't done in the water under load.

I know this doesn't tell enough to get at "the" answer, but I'm hoping for some direction. With everything described above, what are the likely culprits that are left? I appreciate any assistance.

I would ask a moderator to move this thread to the prop section and see what Steelspike and Hwsiii can come up with... Read their stickies at the top of the prop forum and make sure you have all the information they request necesssary to come up with a solid answer...

In the meantime here is a link to a prop calculator that I have found to be quite accurate...

http://go-fast.com/Prop_Slip_Calculator.htm

Good luck...

 

[QC]

Re: Low Top End WOT

I always start with speed and tach accuracy. Your numbers add up OK, but the 19 numbers show only 10% slip which seems too good.

How you getting the speed numbers and do you know your tach is reading right? Most of these tachs are adjustable for 4, 6 or 8 cyls. Again, your numbers compute OK, but the top speed seems low to me as well.

Also, is this the 4.3L or LX? The L only shows 155 propshaft hp which is not a whole lot, and Sea Rays are notoriously heavy . . . still seems like low top speeds to me though.

Edit: I was posting same time as billy. May move to prop forum, but not sure that's where it belongs. Props seem right with these numbers. I am always suspect of low WOT RPM when the owner says that it planes well.

Also, tell us about how you're using power trim.

 

[I/O New Guy]

Re: Low Top End WOT

The tach is set to 6. Is it common for a tach to be off? Also, I measured the speed with GPS, and it matched the speedometer exactly - go figure!

The boat as I was running it probably weighed in at about 2600# (gas, two people and some gear,) which I agree is fairly heavy but not excessively so. The engine is not the LX and is rated at 175hp in the service manual.

I accelerated trimmed fully down, then brought it up to about mid-way between Down and Up on the gauge. That lifted up the nose nicely, but if I continued Up, I started to get some ventilation and backed it down again.

I understand that's all she may have, but I hope not!

 

[QC]

Re: Low Top End WOT

The tach is set to 6. Is it common for a tach to be off? Also, I measured the speed with GPS, and it matched the speedometer exactly - go figure!

My GPS and Speedo are dang near perfect too. Maybe you and I are the only ones . . .

So I think your numbers are all correct then. Numbers do add up, propped close to correctly with the 17, but getting carp top end with it.

The boat as I was running it probably weighed in at about 2600# (gas, two people and some gear,) which I agree is fairly heavy but not excessively so. The engine is not the LX and is rated at 175hp in the service manual.

I would bet she's substantially heavier as is.

Yes, rated 175 at the crank and 155 at the Prop. Merc transitioned everything in 1990 from Crank to Prop horsepower So it is equal to a 155 today if there was one.

I accelerated trimmed fully down, then brought it up to about mid-way between Down and Up on the gauge. That lifted up the nose nicely, but if I continued Up, I started to get some ventilation and backed it down again.

I understand that's all she may have, but I hope not!

Trim use sounds right, so maybe you pick up a little with a decent stainless prop, so I'll move this over there.

You need to confirm your assessment that the engine is right. Assuming that, then propping and weight removal and placement are about all you can do. Tabs don't seem indicated although I want them on all V hulls, I just don't think they will help you run any faster if she's not porpoising as is. I would immediately shift some weight aft as you are ventilating now at mid trim so maybe you pick up a little there. You're not going fast enough for full trim now IMHO, but you may get another degree or two and that may be faster.

 

[I/O New Guy]

Re: Low Top End WOT

QC, I'm not ready to completely abandon the possibility that something is amiss with the engine, so I'll chase that down here. What engine conditions could cause everything to operate seemingly fine but limit the top end?

Also, I did weigh the boat, and I estimated about 600# for the trailer - it is pretty heavy duty for an 18'. It did come in a bit heavy (about 250# over what I expected) but I attributed that to probably underestimating the trailer weight. Is it possible I do have water intrusion, and if so, how can I tell?

Thanks again for all the advice.

 

[QC]

Re: Low Top End WOT

You could spend forever chasing a phantom engine problem which is very frustrating. I think I'd play a little with props. What type and condition are these props? Do try shifting some weight to the stern. Could make a couple of MPH difference.

I'll bet that 19 has some cup to it, and it still has decent hole shot . . .

I stand corrected on the weight, good to know. You said in your first post that you had eliminated most of the WOT things . . . what's left to verify/eliminate?

 

[I/O New Guy]

Re: Low Top End WOT

Admittedly not knowing much on this topic, the only thing I can think of that is engine-related is #15 - throttle plates not opening fully. I would think the shop would have checked that, but running the motor out of the water it proabably jumped up to an acceptable WOT when they tested. Is that a possibility? I will stop chasing phantoms after this, and again, I appreciate the help!

 

[QC]

Re: Low Top End WOT

Yes, not getting full throttle is a distinct possibility. Doesn't need to be running or in the water to see if the linkage has any farther to go after the throttle is fully depressed.

I think it's the 155 thing the most, but she's not particularly heavy, so still seems a little slow to me. I'd expect 40, and you're close with the 19, so a couple of tweaks here and there should get you something. Perfect prop (hard to find), perfect trim, perfect balance, etc.

 

[I/O New Guy]

Re: Low Top End WOT

Ok prop experts, QC has helped with engine questions and it seems "I got what I got" when it comes to power/weight ratio. Based on the info included in this thread, any recommendations on props? The 17p Merc (aluminum) put me in the WOT range I need, but a little disappointed in the top end. Any suggestions, or am I just stuck driving her like an old lady?

 

[steelespike]

Re: Low Top End WOT

You might confirm the timing is advancing as it should.
Is the bottom clean and smooth? If the 19 is cupped I've seen them take a little cup out with very good results.It does seem to me too that it should do about 40.
According the the NADA site the 180 comes with a 125 as a base motor. 2150 lbs.
That must be a real bow wow.

 

[I/O New Guy]

Re: Low Top End WOT

I cleaned and buffed this boat thoroughly (I thought) when I brought it home last month. However, after reading your reply and taking a look, I never cleaned the BOTTOM of hull! It has some dried on veggies that are probably producing significant drag. So, first order of business is to clean that up.

Now I have a really dumb question. How do you clean the bottom of the boat? Can't get to it in the water, and sitting on the trailer a good bit of it is on the bunks. I also don't have access to a lift. Any suggestions?

 

[fossill]

Re: Low Top End WOT

Maybe try a stainless 4 blade like a Stilletto Bay Pro as they are more efficient than an aluminum if you're not happy with its performance.

 

[QC]

Re: Low Top End WOT

How do you clean the bottom of the boat?

You should repost this in the Boat Topics section ^^^^^ They'll give you some good ideas up there.

 

[45Auto]

Re: Low Top End WOT

I would do a compression check (get the actual numbers, not just "it's OK"), a leak-down check, then a tune up (cap, rotor, plugs) before spending money on props. Performance wise, one low cylinder would give you exactly what you're seeing .....

 

[I/O New Guy]

Re: Low Top End WOT

How about two low cylinders? After almost four years of driving a doggy boat, the problem turned out to be burned exhaust valves on two cylinders. They are both about 20% low, which I know means a big drop in HP. Expensive repair, but will be worth it this season. Should be able to pull my fat brother-in-law up on one ski!

I made the mistake of assuming the largest marine shop in my area was also the best. It turns out the two issues I've had with the boat over the past four years were both misdiagnosed by them. I have been using a different one for the past year, and am very satsfied. Thanks for all the advice, and just wanted to pass along to tie up this thread.

I/O Not-So-New Guy

 

[MikDee]

Re: Low Top End WOT

Thanks for the feedback, I thought it might be engine related. That boat should fly now, 40+ once all is done right.