Boat, Engine, or Prop?

[Mike220]

I am posting this in the engine section because it "could" be engine related.....

I am chasing down a WOT low rpm issue. My boat is a 1998 Stingray 220DS (Deck) boat. It is about 4300lbs dry. I have a 1997 5.7 LX with an 1.47 ratio Alpha 1. I am currently running a Quicksilver 4 blade 17p prop. I can get about 4200-4400 rpm's at about 37mph with this setup. According to Stingray I should be able to get at least 46mph. And I should be at 4400-4800 rpm.

Some more history...I bought the boat used 6 years ago and it has always been this way. A new shortblock had been put in prior to my purchase due to previous owner not winterizing.

I've been through the adults only section and have done everything EXCEPT the following - compression check and fuel system pressure check. The engine runs great and has good acceleration - really no noticeable issues other than lack of top-end. The boat is recently tuned up, has a new Edelbrock 1409 carb (the Weber was shot), new manifolds and risers, and a new Tbolt 5 ICU (I had previous posting on why this was done). This week I changed the fuel filter and noticed the gas line from the filter to fuel pump was 5/16. I changed that out to 3/8 and ended up replacing the anti-siphon valve.

The boat is wet slipped in fresh water and does have bottom paint that is not in great shape. Other than that the bottom is clean. The outdrive does get some algae like growth on it, although the low WOT rpm's occur when I put the boat in when everything is clean. I don't have much gear in the boat, maybe 150lbs.

I know my WOT rpm's are close to spec but I should be able to swing a 19p prop with this boat.

 

[HT32BSX115]

Re: Boat, Engine, or Prop?

Howdy,


Well it appears that you are on the right track with what you have checked so far.....

Did it EVER (while you have owned it) make "correct" RPM/speed for you?

A new shortblock had been put in prior to my purchase due to previous owner not winterizing.

Hard to know what that means though....(since you cannot EVER believe what the PO says.....) ...... It might not be what you think it is (unless it did make correct WOT RPM and speed before....FOR YOU)


I might try a 3 blade 19' prop and see what happens.... while 4 blade props will improve hole-shot, more blades mean more drag and slightly lower top end.

The other thing to consider before you start changing things...... is you MUST measure your speed with GPS and use a known tach.......that means you should verify your tach before you make decisions on a few hundred RPM (the tach can easily be that far off...........using an inaccurate tach can have you chasing your tail as it were.....)

 

[Mike220]

Re: Boat, Engine, or Prop?

The tach is definitely questionable but I think it is in the ballpark - I have measured it with a timing light/tach up to 2000rpm. I am measuring speed via gps and the speed matches the rpm on the tach. This boat has never hit over 4400 rpms with a 16/17p prop. I noticed in my above post I said 17p, it is actually a 16p 4-blade. I have tried 19p and 21p three blades and they perform slightly lower rpm as would be expected.

The boat was purchased from a marina and the marina did the short block swap. But I have also been fixing stuff from short cuts taken during the swap - i.e. different fuel line sized, putting back on shot manifold/risers, wiring electric fuel pump to ignition coil - etc.

 

[Don S]

Re: Boat, Engine, or Prop?

You REALLY need to get the tach checked since that is what all your information is based on. Use a good shop tack. The induction timing lights with tach built in are usually very accurate.

Next, I see no mention of trim. Do you use the trim at all, or just use full down?

 

[Mike220]

Re: Boat, Engine, or Prop?

I wish my inductive timing light had a long enough cord so I could read it while driving. Maybe I'll get one of my kid's to help me.

Don - you question about trim has me thinking. Yes - I do trim, but maybe not enough... I did some testing today and as speeds start to increase the boat gets a very nose down attitude. I will try trimming up more until I start to hear ventilation.

 

[Howard Sterndrive]

Re: Boat, Engine, or Prop?

the marina did the short block swap. But I have also been fixing stuff from short cuts taken during the swap - i.e. different fuel line sized, putting back on shot manifold/risers, wiring electric fuel pump to ignition coil - etc.

could be a 175HP car block with a smog era cam instead of a 260HP marine engine
could be a 305, too.
Maybe google up your block casting number
could also have 350 heads on a 305.. that sort of thing

 

[Don S]

Re: Boat, Engine, or Prop?

You don't have to be on the water to check your tach, In neutral set the rpm at 3000 on the helm tach, and look at the shop tach.

You trim up till you have max rpm and max speed, a bit too high, and the speed goes down and the rpm's up. Once you get to best rpm/speed if the boat starts to porpoise, just bring the bow down just a touch till it settles down.
If you have trim tabs, try to not use them at first. It takes a bit of getting used to to use both. Just use the tabs to prevent it leaning to one side or the other.

 

[HT32BSX115]

Re: Boat, Engine, or Prop?

If you have trim tabs, try to not use them at first. It takes a bit of getting used to to use both. Just use the tabs to prevent it leaning to one side or the other.

Yes. This is fairly important too.....Tabs can help with holeshot but having them down at high speed WILL reduce top end. At high speed, they're more drag than benefit.

If you're "leaning" at high speed, have the fatty move over a little!! Or move that cooler to the other side!

Don't even try to use the trim gage.......do it all by feel/sound etc. use the trim switch in small "bumps" up/down.

 

[Mike220]

Re: Boat, Engine, or Prop?

Don - I have checked the tach with the method you mentioned and it proved to be accurate.

Howard - I am concerned that you may be correct. I can't get to the casting number but I can see the engine suffix - V02O1XJA. Googling this gives me :

According to the MSA-1 engine identification code guide,
the XJA suffix code identifies a 1996 350ci 210 hp L31 engine attached to a 700R4 trans in either a 1/2 ton body or a 3/4 ton body.

Looks like the L31 is the being sold as marine engine.

 

[Don S]

Re: Boat, Engine, or Prop?

I wouldn't worry to much about the engine until you test run it and play with the trim and trim tabs, because your problem does not sound engine related at all.

 

[Mike220]

Re: Boat, Engine, or Prop?

I don't have trim tabs (though I probabally need them). I'll do some more testing with the trim when the lake is calmer. I think the hull design of this boat really pushes the nose down once above 30mph and I probabally have more trim to play with. Also my trim guage doesn't work - so I don't need to worry about that

 

[HT32BSX115]

Re: Boat, Engine, or Prop?

Just start out with the drive all the way -IN.

once you're on-plane, "bump" the trim up/out a little at a time until it starts to increase speed....and until speed decreases to find the "sweet spot"

It will be different for different speeds.....