Grady 208 top speed with new mercury 4 stroke 225 hp

[Joesialiano]

I just installed a new mercury 225 4 stroke on my 2003 Grady 208. The boat top speEd is 38 mph at wot running 5200 rpm’s. I am seeing people’s post with the same boat traveling at 48 mph. I replaced my the 150 4 stroke because I was only getting 30 mph. Is it the boat? I’ve checked the hull and bilge. Everything looks good to me....any suggestions?

 

[ScottyCee]

That’s not enough RPMs. Shaft length? Propeller? Is it mounted in the correct set of holes? How far up are you able to trim it? Where is your anti-cavitation plate with regard to the water surface at speed?

we can definitely help you but you will need to provide far more information. These questions I have asked are the beginnings of troubleshooting.

 

[VeroWing]

There are many things that affect your boat's performance and top-end speed. barnicles/growth on hull bottom, excess water water in bilge areas, improper engine height mounting, improper trimming of outboard, are just a few. I would advise you to first find out the manufacturer's recommendation for redline rpms on your outboard and then compare with what you are acquiring. Let's say, for example only, that Mercury sets redline at 5600 rpms and you can only get 5200 while trimming your outboard to optimal running location. If all other items are correct, then you may want to change prop to one that will propel boat and reach the 5600 rpm redline. A general "rule of thumb" prop guide is that changing prop size 1" will change redline rpms by 200. You always must keep in mind though taking off and getting up on plane too.

 

[Joesialiano]

It is a 225 xl. I’m not sure about the other information

 

[Parthery]

What hole is the motor mounted in? Are you trimming it out or running it all the way down?

Are you measuring speed with a GPS?

 

[seasick]

Not a lot of info here. Is the motor the V6 or the V8? What are the prop specs, pitch and diameter?
The new motor may be heavier than the previous one. Other factors could be trim as mentioned, you should experiment with small changes to motor trim and watch your speed for changes. You should measure the speed with a GPS plotter.
If you have trim tabs, they should be fully retracted for the test.
The amount of gas in the tank will affect top end and trim.
How does the bottom look? Is it painted and is it clean?
Do you have a hard top?
How many people on board during the test?
You WOT revs seem a bit low and that can be excess drag or just the wrong prop.
I am somewhat skeptical of the claims that others have a top speed of 48 MPH. Assuming folks adhered to the max HP specs, that year hull is rated for 230 Hp max. An older 2 stroke HDPI will probably make more speed since it will usually be lighter than a 4 stroke. That brings me to my last comment; A heavier motor will make the hull sit lower in the water and as also mentioned may make the gear case sit too low under water causing excessive drag. Check where the cav plate rides at at speed

 

[coldpizza]

I think you are over-proppped (although I didn't see info on prop like diameter or pitch). Seems like the operating range would be up to 5800-6400 rpm for a Verado, so if you are only seeing 5200 rpm and hull is pretty clean, I would link a inch or two in pitch based on same diameter (for example, 15" diameter) would be better to engine and probably run faster. That's my $0.02.

 

[seasick]

Coldpizza,
I see you had a 208 and now have a Gulfstream. I am going to post a new thread with the topic 'Thinking Gulfstream' and would appreciate your input.
Thanks

 

[SkunkBoat]

225 should push a 20 footer into the mid to high 40s
Did you install it or a dealer?
not enough info.

Prop size?
motor height?
max rpm on plane , motor trimmed out

Trim tabs up all the way?(UP, so they don't do anything)

GPS speed or are you looking at a speedometer?

 

[DennisG01]

Joe, this might be a silly question, but do you know what "trimming" an engine means? If so, can you explain your trimming procedure - use as much detail as you can think of.

What were the RPM's (max) with the 150HP?

 

[coldpizza]

I'll look for the thread. I like both the 208 and 232.

 

[Lt.Mike]

GPS speed or are you looking at a speedometer?

You make a good point.
Boat speedos are notoriously inaccurate.
Even GPS averages speed so if your doing 40 then briefly run it up to max speed say 48 then let off back to 40 the GPS may only display and record the max speed averaged out to 44 or 45. There is a lag to it, you have to maintain a given speed for it to display true.
That might only need to be a 100 yards, maybe more depending on the unit and satellite signal.
On prop size, 20’ers with 200hp ob’s like the Yamaha F200 in line 4, 4 stroke run best with a 19 pitch prop.
My 225 runs a 20 pitch prop and though its a 2 stroke engine I’d expect a 4 stroke 225 to be the same.

 

[wrxhoon]

Mike , looks like you have a Suzuki 200, they have 2.5 gears , he has a Mercury 225 ( I assume V6 4 stroke) , they have 1.85 gears. You have to use a much bigger pitch if you have a 20" pitch he probably needs a 17". His engine maybe mounted too low, his speedo maybe wrong, bottom growth , could be anything. If all is good he should be getting mid 40's trimmed out on calm water even loaded.
As he only gets 5200 RPM his prop pitch maybe too much, he could probably go down 2" if the motor is fitted at the right height.