Gulfstream bracket setback?

[mhff34]

I recently purchased a 2008 gulfstream. Performance is terrible. I'm pretty sure the motor is too low and I am out of holes to raise it.

It's a Yamaha 300hp on a bracket with a 3' setback. Going to hire a aluminum welder to shorten and raise the lip of the bracket.

Would appreciate intel on anyone with a similar setup. What's the setback? How high off the keel are your cavitation plates?


Thanks in advance!

 

[VeroWing]

"Rule of thumb" approach to outboard mounting is to locate cavitation plate 1" up for every 1' back from transom, meaning on your single engine 3' setback should have your cavitation plate 3" higher than keel. I also have a Gulfstream, but with twin 150s, and they are also 3' back and cavitation plate is 3" higher than hull bottom area directly in front of each outboard. Are you achieving wide open throttle RPMs of somewhere between 5000-6000? If not, you may need a different prop, or could be other issues.

 

[DennisG01]

Just seconding the rule of thumb - approximately 1" up for every 1' aft of the transom - measuring to the anti-ventilation plate.

Good question about the Max RPM, too. Ideally, you want to be at the top of that range with a light load.

Did you have the same issues during the sea trial or is this new?

It's not an "extra long" motor, by chance, is it?

 

[mhff34]

"Rule of thumb" approach to outboard mounting is to locate cavitation plate 1" up for every 1' back from transom, meaning on your single engine 3' setback should have your cavitation plate 3" higher than keel. I also have a Gulfstream, but with twin 150s, and they are also 3' back and cavitation plate is 3" higher than hull bottom area directly in front of each outboard. Are you achieving wide open throttle RPMs of somewhere between 5000-6000? If not, you may need a different prop, or could be other issues.

I think I'm within the rule of thumb. Probably 3'' to the cavitation plate. Wot is around 5200 doing 38mph. Cruise is terrible around 4500 rpm doing 25mph getting 1.5 mpg. It feels and looks underwater. It's hard to get up on plane and the motor ends up halfway swamped getting on plane or coming off.

I'm willing to try a different prop, currently running 15.5x 15 or those numbers in reverse.

 

[mhff34]

Just seconding the rule of thumb - approximately 1" up for every 1' aft of the transom - measuring to the anti-ventilation plate.

Good question about the Max RPM, . Ideally, you want to be at the top of that range with a light load.

Did you have the same issues during the sea trial or is this new?

It's not an "extra long" motor, by chance, is it?

Yes I had the issue during sea trial. Walked away from deal after leaving deposit. Came back a month later and walked away again. Third time just bought it and figured I would deal with it after owning it.

What's an extra long motor? How do I measure? Assuming it is 25" but wouldn't know

Thanks for reply

 

[mhff34]

Measured mid section 25''. This boat left the factory with a 350hp. The second owner blew it up and gave the boat to a broker. The broker put a new 300 on it, that's when I came into it.

It's a great boat, I absolutely love it just don't like the performance. It sits nice in the water, scuppers are totally out of water fully loaded.

 

[Legend]

The prop will make a big difference. I had a similar situation on twin 250 I bought last year. The right props were backordered for a few months and I was using props from my 225's. I was only getting around 5200. New props I am now just shy of 6000 and performance exceptional. I went from 15 1/4 x19 to 15 1/2 X 17.

 

[Ky Grady]

You're leaving aroung 800 rpm on the table somewhere. Do you have clean fuel filters, fresh plugs, are fuel injectors clean, fresh fuel, how many hours, any updates need to be performed on the motor? All of these play into a good running boat. See if you can borrow another prop and try that. Might want to reach out to Ken at PropGods and run your numbers by him. He's very knowledgeable and helpful when it comes to propping. He reccomended a 4 blade prop for my setup and it is spot on for what I wanted out of my boat.

 

[DennisG01]

30" would be extra long.

You should have good performance with that boat. I have a 250 on my boat - a bit lighter - but my boat absolutely screams out of the hole. The 5200RPM with only a 15" prop is concerning. I would think a 17" or 19" would be the norm. And there is NO WAY dropping down to a 13" is the solution.

Take a helper with you and go for a run on some smooth water. Get going about 30MPH and observe the anti-ventilation plate. It should be skimming over the top of the water. This is the best way to see if your engine is at the right height.

If the engine is in the right place, which it sounds like it's at least close enough... then as mentioned, we gotta go down fuel or mechanical issues. Speaking of fuel... how old is it? What does it look like/smell like? Dump the filter into a clear, glass jar.

Do you use your tabs?

 

[mhff34]

Grady white just told me all there boats are set up with 30"" shafts. Mine is 25"

 

[mhff34]

Ab

You're leaving aroung 800 rpm on the table somewhere. Do you have clean fuel filters, fresh plugs, are fuel injectors clean, fresh fuel, how many hours, any updates need to be performed on the motor? All of these play into a good running boat. See if you can borrow another prop and try that. Might want to reach out to Ken at PropGods and run your numbers by him. He's very knowledgeable and helpful when it comes to propping. He reccomended a 4 blade prop for my setup and it is spot on for what I wanted out of my boat.

Less than 10 hours on motor. Just installed 2 months ago

 

[Tuna Man]

Was not aware that they used 30 inch shafts on the Gulfstream. In 2000 I'm almost certain they used 25 inch shafts. Your performance does appear to be lacking. Is the boat painted, if so is the bottom smooth? I'm very surprised your motor does not spin up any higher, also surprised by your top end and cruising performance. I was under the impression that the only older boat that utilized a thirty inch shaft was a single Islander, learn something new everyday.

Are you trimming the boat (engine) properly or are you leaving the engine trimmed all the way down. Reason I mention this is a close friend of the family put a deposit down on a used boat last spring. When he and my son went to test drive the boat a week later, the owner and his buddy were amazed at the performance of the boat. They had never seen the boat hit more than the low to mid- forties at full throttle, with my son at the helm the boat topped out at 51mph briefly before pulling back on the throttles. They were the original owners and were not aware how critical a properly trimmed boat is.

 

[mhff34]

Bottom definitely could use some attention. Painted and needs to be sanded. I trimmed up the motors probably could go a little more just not used to the boat yet. Thing is I know the motor is to low. Feel s like I'm pulling two skiers behind the boat to get it on plane

Just don't know why it's low the only thing that looks different is my setback being 3 feet. Other Gulfstream s don't seem setback as far

 

[DennisG01]

Bottom definitely could use some attention. Painted and needs to be sanded. I trimmed up the motors probably could go a little more just not used to the boat yet. Thing is I know the motor is to low. Feel s like I'm pulling two skiers behind the boat to get it on plane

Just don't know why it's low the only thing that looks different is my setback being 3 feet. Other Gulfstream s don't seem setback as far

I thought you mentioned that you think it's OK - roughly 3" higher than the transom bottom?

My motor is roughly 3' back, but I will say that looking at pics on the internet always seems to make them look like they shorter. Maybe yours is different, though.

Speaking of trimming... are you trimming down all the way when you take off? ALL they way? What happens if you use your tabs?

When trimming up, at WOT, you have to keep trimming up till speed no longer increases and/or you start to ventilate. Just before it ventilates is your Max RPM and Speed. Be sure to do this with a light load.

As long as the bottom isn't growing anything or has any slime on it, an "old paint job" shouldn't make the difference you are seeing. It could make a slight difference - but not what you have going on there.

Have you ever checked the compression on the engine?

What's the fuel quality like? Dump the filter out into a glass jar - smell and observe.

EDIT: Disregard some of my questions - I just realized that I already asked you some of them.

 

[mhff34]

30" would be extra long.

You should have good performance with that boat. I have a 250 on my boat - a bit lighter - but my boat absolutely screams out of the hole. The 5200RPM with only a 15" prop is concerning. I would think a 17" or 19" would be the norm. And there is NO WAY dropping down to a 13" is the solution.

Take a helper with you and go for a run on some smooth water. Get going about 30MPH and observe the anti-ventilation plate. It should be skimming over the top of the water. This is the best way to see if your engine is at the right height.

If the engine is in the right place, which it sounds like it's at least close enough... then as mentioned, we gotta go down fuel or mechanical issues. Speaking of fuel... how old is it? What does it look like/smell like? Dump the filter into a clear, glass jar.

Do you use your tabs?

My cavitation plate is buried. At least a couple inches. What kind of setback do you have on your bracket?

 

[DennisG01]

I'm not at the boat right now, but I'd say it's easily 2-1/2'. FYI, I took that silly fin off of the anti-vent plate.

Yeah, if your anti-ventilation plate is buried, that's definitely a problem and it brings us back to wondering if someone put the wrong length engine on there. It certainly seems like that's the case from the info here?

 

[Ky Grady]

Can you post up a pic from the side like Dennis's?

Give us a look at your setup.

 

[mhff34]

Here are some photos

 

[mhff34]

Pic

 

[mhff34]

Well I think I solved the riddle. Just spoke with Yamaha. I was told that the 350 and 300 have different bolt patterns. That would explain why the new motor is sitting too low. The boat left the factory with a 350 on it.

Don't know why Grady White didn't know this?? I asked this specific question to them. I also asked the closest dealer neither seemed to know this.

My plan than is to have a welder modify the bracket in order to get the motor higher. I figure why I'm at it I would like to shorten the bracket as well. No need for a 3' foot setback that was probably required for a 350. I would appreciate any measurements you guys with a gulfie could provide. Setback- Height from keel to cavitation plate

Thanks for the interest in help!
Mike