Disappointing Power & Speed

T

Tim Sullivan

New Member
Jun 29, 2020
7
Boat Info
2006 Sundeck 200 with 4.3 MPI
Engines
Mercruiser 4.3L MPI Alpha I
i just bought a 2006 Sundeck 200 with a 4.3 MPI engine. I put a new prop (14.25 and 17 degrees) on it. It has very little power out of the hole and takes a while to plane. It tops out at 4,100 RPMs and only 28 mph. The engine only has 153 hours on it. It is disappointing. Is there anything I can do?
 
Not to be rude, but buy a boat with a bigger engine. Seriously.

I had a 20' with a 4 cylinder thinking it wouldn't burn much gas. WRONG!! I usually ran almost WOT and it burned a ton of gas. Worst boat I've ever owned. Hole shot, top speed all sucked, regardless of prop I installed.

Another factor is how you load it. Too many people or stuff on board affects performance. That same boat required people to move forward to get on plane with more that 3-4 people on board.

After that experience, I want the biggest (most h.p./torque) engine combination available. You want to be able to cruise with your normal compliment of people fuel and beer/cooler at a reasonable RPM usually around 3000- 3500 for most gas engines.

I quickly sold and moved up to a 24 ft with a 5.7 small block. Great boat.
 
dtfeld said:
Not to be rude, but buy a boat with a bigger engine. Seriously.

I had a 20' with a 4 cylinder thinking it wouldn't burn much gas. WRONG!! I usually ran almost WOT and it burned a ton of gas. Worst boat I've ever owned. Hole shot, top speed all sucked, regardless of prop I installed.

Another factor is how you load it. Too many people or stuff on board affects performance. That same boat required people to move forward to get on plane with more that 3-4 people on board.

After that experience, I want the biggest (most h.p./torque) engine combination available. You want to be able to cruise with your normal compliment of people fuel and beer/cooler at a reasonable RPM usually around 3000- 3500 for most gas engines.

I quickly sold and moved up to a 24 ft with a 5.7 small block. Great boat.
Click to expand...
I
scoflaw said:
Do a complete tune up
Change your fuel filter
Check your compression while you have the plugs out
Check your fuel pressure at WOT
Click to expand...
i did change the fuel filter and have put two tanks of new gas in it. Maybe I will have get a tune up.
 
Better take it to a dealer/marina with the electronic equipment to diagnose the engine. Ask them to take it out for a test cruise. Your first problem is finding a reliable mechanic who knows what he is doing and wants to keep your business in the long term. He is likely to be busy now. A family owned marina would be good. You are going to have dealer only maintenance in the future. I analyzed my 2003 5.0 MPI and realized the computer was not automotive and required a marine Rinda $500+ code reader.
 
Thanks. I am in CT and Marinas are three weeks out.

I do have vessel view. Is there a reading in there that might might give me clue?
 
Before you go down the mechanical road - which could certainly explain it - anything from a plug wire not secured to, well, just about anything...

Is the boat bottom and running gear perfectly clean?

Are you trimming the drive properly?

Did you check speed with GPS or just the speedo?

When you changed the fuel filter, did you dump the gas into a glass jar to check quality? If the gas was old, it may have gummed/fouled the injectors.
 
That boat should be running mid 40MPH range.

Definitely something amiss.

Boats often present with a shine outside but mechanical maintenance has been neglected.

it’s almost always worth the time and money to have a mechanical survey before purchasing.
 
That engine should turn around 4700-4800 RPM's and run at least 40 MPH with the correct prop. As others have said above, there are other issues preventing top performance.
 
Brought it to a mechanic and had it checked out. Getting better power and speed. Wasn’t trimmed properly.

4400 rpm
37 mph
 
4400 rpm & 37 mph sounds about right based on variable loads to include amount of fuel. I'm still figuring mine out. I was getting 4600-4800 WOT and 40-45 mph with a 14 x 23 aluminum prop with just 2 adults and about a 1/4 tank of gas when I first had it in the water. I ended up having to replace the prop after I introduced it to a sandbar in Great Bay. I put on a 14 1/4 x 21 stainless steel prop and now get 4200-4400 and 38-41 mph with 2 adults and about 3/4 tank of gas. I think I'm good with that and attribute some of the loss to barnacles, bottom paint chipping and whatnot from having it slipped in brack water for almost 3 months. Good luck and have fun out there.
 
Tim Sullivan said:
Brought it to a mechanic and had it checked out. Getting better power and speed. Wasn’t trimmed properly.

4400 rpm
37 mph
Click to expand...
So for now, the only thing done is better use of the trim? OK, that's better, but you're still missing some RPM's and speed - assuming a light load in the boat. Are you now trimming it as far as you can go? There were some other questions asked above that are still questions... ;)
 
paulswagelock said:
That is plenty of motor for that boat. Get the engine checked- not running to its fullest potential.
Click to expand...
We have a different scale for plenty of power. A 20 foot boat with a 5.0 FI motor is underpowered to me:D
 
Tim Sullivan said:
Brought it to a mechanic and had it checked out. Getting better power and speed. Wasn’t trimmed properly.

4400 rpm
37 mph
Click to expand...
What do you mean by "not trimmed properly"? How was it trimmed before and what is the way thats giving you better performance now? I am curious because I dont know the best way either. This is our first year on the boat and if anyone could share some advice that would be great!
 
Point the boat straight, take it up to cruising rpm or above, start trimming up and watch speed and rpm. As speed increases, so will rpm. When additional trim causes rpm increase but no speed increase, that is too much trim, bring it back down a touch.
You will eventually learn the right sound and feel and not need the trim gauge at all.
 
paulswagelock said:
Point the boat straight, take it up to cruising rpm or above, start trimming up and watch speed and rpm. As speed increases, so will rpm. When additional trim causes rpm increase but no speed increase, that is too much trim, bring it back down a touch.
You will eventually learn the right sound and feel and not need the trim gauge at all.
Click to expand...
AS above but I will add that you start trimmed full "in" to get on plane and then trim out (drive up) until you just start to hear or feel the prop start to blow out or ventilate (you will hear the engine rev up because the prop is not biting the water), or the boat starts to porpoise (bow bouncing up and down rhythmically but not due to waves) then trim back in in down a bit. Your boat may not porpoise at all. Some do and some don't. That is normally the sweet spot for cruising on plane.

If you have to slow down to right around the planing speed you will want to trim in again to hold the boat on plane and bow down as much as possible. If you don't you will be running bow up in the air and will not have control or good visibility. Also trim the bow down if you are hitting heavy waves from the front. It will smooth out the ride a bit.
 
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