Trouble to plane, Top speed.

[Jimmy1960]

I am running a 1992 Rinker Fiesta Vee this year. The boat is new to me.

Engine is 7.4 with Bravo drive. Prop pitch is 15.

Last weekend I was not able to get onto plane with 5 people about 1000 pounds on board. I moved two of the heavier bodies to the front of the cabin to get the boat on plane.

I doesn't seem this weight should affect the ablility to plane to this extent. Top speed is also about 25 mph.

Would appreciate comments from anyone with this engine and boat size combination to let me know if I should start looking into this.

Thanks for any comments.

 

[QC]

Re: Trouble to plane, Top speed.

I moved this to Boat Topics. Probably belongs in Mercruiser or Prop sections (you had it in Mercury OB), but let's do some fingering out first.

Any additional info on the boat at all? Length? Anything from previous owners? Trim tabs? Do you know how to use them? Trim? I am assuming a Bravo 2, and probably a 300ish 7.4 Carbed?

 

[jl_photo]

Re: Trouble to plane, Top speed.

One of the first things to check when having problems like this is to check the prop. Any damage, even damage not easily seen, can drastically affect performance. If the prop has any irregularity it will create turbulence instead of thrust.

The prop also has a pressed in rubber hub that slips when you hit something. They can wear out and slip all the time.

If the prop is the right one for the boat/motor combination you will have a cruising RPM of about 3500 and a maximum RPM of 4500. If you boat in an area where you may occasionally hit bottom a stainless prop is often better than an aluminum one.

 

[mr 88]

Re: Trouble to plane, Top speed.

No if you are in rocky waterway you want a aluminum,not stainless.If you hit with aluminum in most cases you take out the prop,stainless will not give so there goes your prop shaft at the very least along with the prop still needing work at the prop shop.

I cannot believe with a 15" prop that it would not jump out of the water and then rev to the moon and in that case 25 mph might be tops.How many RPM's were you turning that is the big question.Did you try and contact the previous owner to see if he put the wrong prop on when he turned it over? Engine running good? No misses,good compression? In any case start with the prop.

 

[spoilsofwar]

Re: Trouble to plane, Top speed.

As mentioned by others, we can't help you diagnose this problem properly without knowing your RPM @ WOT.

 

[QC]

Re: Trouble to plane, Top speed.

I cannot believe with a 15" prop that it would not jump out of the water and then rev to the moon and in that case 25 mph might be tops.

This is probably a Bravo 2, so I would reserve judgment on that until we get more info

 

[Jimmy1960]

Re: Trouble to plane, Top speed.

THanks for all the replies.
Drive is a Bravo ll.

I am going to have to check the RPM'S, at the moment Tach is not reliable. On the list to be replaced.

The prop is stainless. Engine does not seem to miss sounds good. I do not hear any misses.

 

[QC]

Re: Trouble to plane, Top speed.

Bravo 2 was 2:1 ratio and only 300 propshaft horsepower. We could still use some info on the size of the boat, trim tabs etc. But I used 25 MPH and 15% slip and the numbers look fairly right. RPM is still the next step unless you can answer more questions. Mo info = Mo betta

 

[Chris1956]

Re: Trouble to plane, Top speed.

What size is that boat? If 25-27 feet, even that big block will have difficulty planing that setup.

 

[QC]

Re: Trouble to plane, Top speed.

Yep ^^^^^ horsepower is horsepower. Older big blocks and a B2 are outperformed by modern small blocks and a B3. That's why there are so many single 300 Mags in pocket cruisers these days. The B3 solves the planing issue and geared properly, 300 bhp is the same whether it's made by a 6000 RPM OB or a 4200 RPM Big block.

 

[Jimmy1960]

Re: Trouble to plane, Top speed.

The boat is a rinker 260 which is 28 feet OAL

What size is that boat? If 25-27 feet, even that big block will have difficulty planing that setup.

 

[QC]

Re: Trouble to plane, Top speed.

Well, if this can be believed then you have power issues: http://www.rinkerboatowners.com/forum/index.php/topic,15759.0.html

I have a hard time believing that 45 MPH is in the cards. Just does not seem possible, but I can't argue, I don't have one of these. However . . . I found a bunch of these boats on line with 5.7Ls which were only 230 - 240 bhp at the propshaft. You have 300, so I believe the power as installed is adequate and is actually at the high end of what's out there. I have to believe this engine is sick, or you have a dirty bottom, or a bad prop, or you don't know how to use the trim and tabs together. I am not of the low power mind set anymore unless, again, the engine is not developing the power it should. I would start with all of the basics and you need an accurate tachometer. Fuel condition? Fuel filter? Tuneup, compression check. And of course the boat stuff too: bottom condition, prop condition, drive condition, etc, etc, etc. All of this, or some of this, or even none of this, could be your issue . . . Have to start at the beginning. Visual checks first.

 

[Chris1956]

Re: Trouble to plane, Top speed.

Wern't those 7.4 L motors rated at 330HP? They used to come with Alpha Drives, although the Alphas were marginal for all that power. I also have a hard time believing 45MPH, but would expect 35MPH. Maybe it's time for a compression test?

 

[QC]

Re: Trouble to plane, Top speed.

In 1991 and 1992 (most likely mfg dates) the 7.4L with a Bravo 2 was rated 300 at the propshaft. In 1990, when they rated at both the crank and the prop it was 330/300 respectively. Almost all Mercruiser I/O products show a similar 20 - 40 bhp discrepancy between crank and prop ratings. 1990 they listed both, so it is very revealing.

 

[Jimmy1960]

Re: Trouble to plane, Top speed.

Thanks everyone. There are several the Rinker 260's at the Marina close to the same year. I will talk with them and get some feedback.

I don't expect 40 MPH but I was surprised by the difficulty to plane with 1000 pounds of people aboard. This is about as big a boat as you can get before twins are required.


Will do some analysis and give an update.

 

[Jimmy1960]

Re: Trouble to plane, Top speed.

Ok I did discover that the trim tab will not extend. The motor operates for the retract but does not for the extend. So when I thought I was extending tabs I was not.

There is a three prong quick connect for the trim tab. Does anyone have experience with diagnosing the problem. I suspect the switch but would like to test at the pump prior to pulling out the switch.

Thanks.