I don’t think my boat is hitting the speed it should

[Chopp]

Hi, I bought a 1993 Sea Ray 170 Bowrider over this winter and I did some things such as bellows, tune up including cap and rotor, plugs, wires and a new 140 T-Stat. This has the 3.0 Mercruiser. The previous owner bought a new aluminum 3 blade prop, it’s a 17P. So taking it out, it is hitting about 35mph fluctuating between 4900-5300 rpm. Depends on the day I guess. One day it’s 5300 the next it’s 4900. So I decided on a 14.5 x 19P aluminum 3 blade. Same mercury blackmax prop (I believe that’s what it is) and the RPMs are now 4500-4600 like it should be... but still goes the same speed. The compression is 140-150psi. The carb seems to be in healthy condition too. What are your guys’ thoughts?

 

[Chopp]

And to add, I do trim it up. The speed is GPS, and the speed is 35ish if by myself, and with another 2 adults. Same speed. And to add, it had a wake board tower, but I doubt that has anything to do with it.

 

[Chopp]

Has a wakeboard tower***

 

[jimmbo]

Most often the Top Speed is the same when there is a Pitch change, the engine is turning a Higher Pitch prop but at a lower RPM. Often addition weight int he boat has no effect on speed if the added weight improves the Weight Distribution.

Verify that the Carb is opening fully with the Throttle lever at WOT., is the Timing set correctly, and is it advancing properly?
Does the boat have any unneeded Ballast?, As in Saturated Flotation Foam
Almost every Owner of a 17 ft boat with a 3 liter that I have encountered, want it to go faster. If the Boat and engine are rigged correctly, with the engine in good Mechanical shape and Tuned correctly, and Best Prop chosen, there isn't much more to be found.

 

[Chopp]

I understand that possibly that is it’s top speed, which I’m fine with but I only have a concern since others with 18 or 18.5ft boats hit much higher top speeds with the same motor. As far as floatation foam, I doubt it’s wet. The floor is solid, everything appears dry. And towing it, it feels to be the weight it should... unless there’s another way to check? I haven’t checked the timing, and I will have a marina check and adjust that as I don’t have the tools. I will check the carb tomorrow. I will note, the cap and rotor and the entire tuneup I did yesterday. I have the 17P on today as my 19 is being fixed... already. The cap and rotor were in horrible condition, I’m surprised it ran. So maybe, I will see a difference once I get it back.

 

[TunaFish389]

Get your max RPM in the 4400 to 4800 range .. The 17p is not the right prop for your boat, letting rpms too high. The 19p is about right. 35mph is a good speed for year and length of boat. That's about all you are going to get and is completely normal. No sighs of wet foam..

You here about higher speeds from 3.0 and you could if you shed weight but not much more. Your max rpm range is much more important than your max speed.

Just fyi I have the same length of boat and older 3.0. I run a 19p three blade at 4400 and at 33 max. I run a 17p four blade for pulling and it runs 4600 and slightly slower than the three blade.

 

[Chopp]

Okay, thank you!

 

[Scott Danforth]

have you weighed your 27 year old boat to see if you are carrying around 800# of water in the foam?

 

[Chopp]

I have not... correct me if my findings are incorrect, the boat weighs in at 1900 dry, and the trailer is around 500. Do those numbers look to be where they should be? I will measure that one day here...

 

[Scott Danforth]

correct, 1900 dry weight. each gallon of fuel is 6.82#. add any gear

trailer should be a single axle, if its a shoreland'r, it may be a bit heavier than 500, however not much. the dual axle shoreland'rs under the 19 footers weighed 1200#

 

[Chopp]

It’s not a shorelander. I don’t remember the brand but it’s the oem trailer, which is single axle. Don’t have much gear and the fuel tank I don’t think is even 10 gallons. Probably around 100-140 pounds there including gear.

 

[QBhoy]

Hi. For me this would all depend on if you are meaning gps speed or not.
I also think your tacho Could be a little off too.
All being well and the likely assumed gear ratio..I’d say a 19” would be good middle of the road prop for that set up and you should be seeing around the 40 mph mark gps, when well trimmed and not lots of weight onboard.

I would also say that if you are just going by the dash speedo...don’t believe a word it says. At the top end of things on my boat the speedo will either get a reading or sometimes even drop off a little with high trim...despite the GPS speed increasing.

You’ll also probably know that even things like a Bimini being up can lose you 5 mph...or a full tank of fuel. I know mine does. If I have even more than 2 people on it and Especially someone in the bow seats...all losing speed.

 

[QBhoy]

Just saw you said it’s GPS. All else I’ve said still applies though.

 

[Chopp]

I do have a bimini up almost all the time, I will take all things considered. Thanks

 

[QBhoy]

I do have a bimini up almost all the time, I will take all things considered. Thanks

Ah. Worth considering. I know that my boat will not go more than early 60’s with the hood up. With it off, it’s late 60’s. That’s the difference.

 

[JimS123]

Just for comparison...

We had a 1984 Stingray 19' bowrider with a 3.0 liter 140 HP. Always ran with the convertible top up (not a bimini). The boat weight was about 2400 pounds. I had a 21" cupped SS Michigan Wheel, that was re-pitched to about 20". Top rpm was 4700-4800 depending on trim and she would easy hit 46 mph GPS in a very light chop.

 

[QBhoy]

Just for comparison...

We had a 1984 Stingray 19' bowrider with a 3.0 liter 140 HP. Always ran with the convertible top up (not a bimini). The boat weight was about 2400 pounds. I had a 21" cupped SS Michigan Wheel, that was re-pitched to about 20". Top rpm was 4700-4800 depending on trim and she would easy hit 46 mph GPS in a very light chop.

Stingray always punch above their weight though. Fast fast hulls compared to your average 18ftr

 

[Chopp]

Just for comparison...

We had a 1984 Stingray 19' bowrider with a 3.0 liter 140 HP. Always ran with the convertible top up (not a bimini). The boat weight was about 2400 pounds. I had a 21" cupped SS Michigan Wheel, that was re-pitched to about 20". Top rpm was 4700-4800 depending on trim and she would easy hit 46 mph GPS in a very light chop.

This is why it raised the question, but mines only the 115hp version, but it’s also 500 pounds lighter. But QBhoy may be right, maybe my hull isn’t very hydrodynamic

 

[JimS123]

This is why it raised the question, but mines only the 115hp version, but it’s also 500 pounds lighter. But QBhoy may be right, maybe my hull isn’t very hydrodynamic

I believe the later 3 liters were 130 HP. Also, I don't think a SeaRay is a bad hull. Ours 19 footer weighs about 3000 pounds and only has a 150, but she'll do 50 mph with the engine trimmed right.

 

[QBhoy]

This is why it raised the question, but mines only the 115hp version, but it’s also 500 pounds lighter. But QBhoy may be right, maybe my hull isn’t very hydrodynamic

I’m almost certain that there isn’t likely such a thing as a 115 hp 3.0 like yours. As far as I know the 3.0 GM merc has only ever been badged as 135hp and before that, 140hp.
you should see 40 mph GPS with the right prop and no big parachute holding you back like a Bimini.