5.8 ltr Mercruiser not fast enought

1986_Horizon

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1997 SeaRay 215 Express Cruiser with Merc 5.8 ltr engine. Can't seem to run it over 32 mph. RPM is around 3500 at top speed. I have a SS prop 3 fin, i believe 23 degree pitch? Engine runs great, no compression issues either. Any suggestions?
 

alldodge

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My guess is the 23 pitch prop is a bit much, need to go to a 19 pitch. What is the drive serial number or ratio?
 

Bondo

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1997 SeaRay 215 Express Cruiser with Merc 5.8 ltr engine. Can't seem to run it over 32 mph. RPM is around 3500 at top speed. I have a SS prop 3 fin, i believe 23 degree pitch? Engine runs great, no compression issues either. Any suggestions?

Ayuh,.... Welcome Aboard,..... Merc hasn't used the 5.8l motor since the '70s,....

Just to clarify, ya got a 5.7l, Right,..??
Efi,..??
Carbed,..??

Is it a Dock boat, or Trailer boat,..??
 

1986_Horizon

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Thank you for the responses. Yea...5.7 sounds better. Sorry about that It is an EFI engine. The boat was a docked boat until I purchased it 4 months ago in which I have it on a trailer now..
 

thumpar

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What gear ratio and drive is it? 23 pitch seems kind of high unless the drive ratio was change at some point.
 

Fly'n Family

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Not familiar with your specific boat, but a Merc 5.7L should be running around 4800 rpms at WOT. Sounds to me like an improper pitched prop for your set-up. You're leaving a lot of rpm in the water doing nothing.
 

Walt T

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A 23 pitch at 32 mph at 3500 rpm assume 15% slip is a 2.02 ratio. Tach or mph is way off. I suppose he could have the 1.81 or a 1.94 but I seriously doubt it unless he's way up at altitude. No matter how you plug in his numbers either the tach or the MPH or both are wrong. I run a 23 pitch duo prop with a 1.95 ratio at altitude. 23 isn't an unreasonable pitch if geared correctly.
 
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DaveG55

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I think it's unlikely he has a 2.02 drive ratio. The 5.7 usually came with a 1.47 or 1.5 ratio drive. Usually, the smsller the engine the bigger the ratio drive. For exampl the 3.0 is often coupled to a 1.94 drive
 

haulnazz15

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I think it's unlikely he has a 2.02 drive ratio. The 5.7 usually came with a 1.47 or 1.5 ratio drive. Usually, the smsller the engine the bigger the ratio drive. For exampl the 3.0 is often coupled to a 1.94 drive

It's unlikely, but shouldn't be ruled out. A 17-year old boat could have had any number of different drives installed on it if the original went kaput. I would confirm the GPS speed, and prop pitch, then get a spare tach or timing light with RPM readout to verify rpms.
 

1986_Horizon

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Thank you all for your responses. I verified several items that could weigh in on my particular dilemma. The prop is a 19 degree (not 23) and the speed I take directly off my Garman GPS/Chartplotter.The RPM's I am taking directly from my Tach gauge. I have no clue what my drive ratio is. Is there a quick way I can locate that? Thanks in advance for your time.
 

haulnazz15

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Thank you all for your responses. I verified several items that could weigh in on my particular dilemma. The prop is a 19 degree (not 23) and the speed I take directly off my Garman GPS/Chartplotter.The RPM's I am taking directly from my Tach gauge. I have no clue what my drive ratio is. Is there a quick way I can locate that? Thanks in advance for your time.

Well, with 12% prop slip, 32mph, and a 19p prop/1.9 outdrive ratio, you should be running around 3800rpm. If you have a 1.5 ratio outdrive it would be around 3,000 rpms, and a 1.65 ratio is 3,350 rpms. So your tach may be off no matter what drive you have. I'd still get a timing light with RPM readout and verify the tach before you went further.
 

DaveG55

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Drive ratio is stamped / written on the outdrive on one of the sides near the top, if I recall, mine is next to the drive serial number.

I agree, you need to verify accuracy of the tach before going much further. Much like a computer, garbage in = garbage out.
 

HT32BSX115

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Drive ratio is stamped / written on the outdrive on one of the sides near the top, if I recall, mine is next to the drive serial number.

I agree, you need to verify accuracy of the tach before going much further. Much like a computer, garbage in = garbage out.

Yes.

The drive ratio must be determined AND a known tach must be used or you're wasting your time. It's even possible that someone had that drive rebuilt (and changed the ratio) in the last 17 years and didn't bother to change what is stamped on the side!

Since the boat is on a trailer, you can positively verify the ratio by pulling the spark plugs, placing it in gear (FWD or REV) and have someone turn the crankshaft and count turns (10 or so) .............. Count prop turns, then use your calculator to divide prop-turns into crank turns.

Since you probably have a 3-blade prop you can easily estimate a third of a prop turn................. 0.33 turn= 1 blade..........with a 4-blade, 0.25 turn= 1 blade etc etc.........

When you determine your RPM and drive ratio accurately, you can then investigate engine problems, water-logged hull, dragging the anchor:eek:, forgot to remove the trailer:facepalm: etc!!!
 
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