What to check when boat does not run fast

[banhtrang]

Hello,
I ve just purchased a boat with Yamaha 115 4 strokes, the engine ran great, started right up, but it wont go over 8 miles/hr on the lake. I ve tried all the trim and jack plate position but it did not help.
I stop to give it more throttle when water pressure above 20psi and RPM above 4000. what do I need to check for and why would it go so slow?

appreciate your input

 

[BonairII]

Re: What to check when boat does not run fast

If your getting above and beyond 4000 rpms and not going over 8mph......you probably have a spun prop. Which means that the rubber hub inside your prop is slipping and the prop isn't turning.

I don't understand what your doing when you mention "I stop to give it throttle when water pressure is over 20psi".

 

[N1NE1]

Re: What to check when boat does not run fast

from what your saying,I also think it could be a spun prop .With the boat OFF,put it in forward gear. Grab the prop ,turn it the way it would be if it were pushing the boat forward and see if it slips on the hub

 

[BonairII]

Re: What to check when boat does not run fast

Better yet...draw line thru the hub/prop. if the line is broken after running the boat...the hub is spinning. Unless the hub is completely FUBAR'd...doubtful that it will slip when trying to spin the prop by hand.

 

[banhtrang]

Re: What to check when boat does not run fast

I checked with the previous owner, he said I need to put the jack plate all the way down, then trim all the way in and slam hard on the throttle to get the boat on plane. This is my first tunnel boat with Jack plate I may not know how to get it on plane. I will try it again next week to see if it actually can get on plane. I never have any problem get on plane with my previous walk thru windshield boats. I also installed a trolling motor and two batteries on the front, it may be harder to get on plane??? I just have to try again next week. Also when I "slam" on the throttle what RPM is the maximum that I should stop before it harm the engine

thanks

 

[N1NE1]

Re: What to check when boat does not run fast

It should not be that hard to get a boat on plane unless its farrrrr under powered

 

[99yam40]

Re: What to check when boat does not run fast

high RPMs and no speed means prop is slipping or prop cavitation /sucking air where it cannot bite enough to produce forward motion.

Has this rig ever been run before with that motor and prop?

Is the boat hull full of water?

Is the nose pointing into the sky?

You said you tryed trim and the jack plate, but now are saying you have not had the jack plate down or the trim down when starting.
are you sure you know how to trim a rig when taking off from stand still?

 

[MikDee]

Re: What to check when boat does not run fast

What I want to know is, was the prop fully submerged? Or, jacked up 1/2 way out of the water. Tilt, & Trim should be all the way down, Plus you shouldn't hafta "slam" the throttle, just advance it in a sweeping motion. If the right prop is on the motor, it should not over rev. RPM should be about 5500-6000 usually for an outboard, but check your particular engine specs.

 

[T_Herrod]

Re: What to check when boat does not run fast

Go to this link and see if you can find your engine/boat combo. It should tell you what you want to know. But i would have to agree that it sounds like a spun prop or operator error. Do you have a spare prop you can try on it?

Performance Bulletins | Yamaha Outboards

 

[QC]

Re: What to check when boat does not run fast

The previous owner has given us all the clues, and 99yam got it above except it sounds like ventilation, not cavitation. Clues are "jack plate", "trim" and "tunnel". If it is spinning 4000 RPM and not going anywhere, yet the previous owner can tell him how to plane it, then I suspect it is a setup issue and ventilating as is. If it is setup for top speed and there is a few tricks to get it to bite from a stop then he may be stuck with it unless he changes a lot of things. If trim and jack plate adjustments solve it, then I don't believe there is a hub issue at all.

 

[banhtrang]

Re: What to check when boat does not run fast

The prop is fully submerged... I will push the RPM higher, I stop pushing the RPM at 5000 for fear of damage. With other boat when I got on plane it was around 4500 max so I will try again and see

 

[banhtrang]

Re: What to check when boat does not run fast

Thank you for the link, but BAYMASTER is not on the list it's happened to be the brand of my boat