Boat speed UPDATE

[boaterntexas]

UPDATE okay got the tach work and the speedometer. The tach read about 4500 rpm at 35 mph. Not sure if the speedometer is accurate. ON shore I can get the motor to tach out @ 6000 rpm. But off shore the best I can get is 35 to 37 mph and 5000 rpm everything is trimmed out riht and sounds great, on and off shore. what should I do?<br /><br />________________________________________<br /><br /><<First Post>> 3-22-03 >><<My boat is a 1976 17.5' haydrasport fiberglass bass boat, which I belive whiegh in the nieghboor hood of 1000 lbs. , the boat has a 155 hp merc with thunderbolt, the motor is 1976 also I belive and its a inline 6 cly. The boat does not seem to be reaching it potential speed, not sure how fast it moves but I would say around 25 or 30 mph, any idea on how fast this boat should be able to go. Oh yeah the prop is in okay shape few small niks, and a tad out of pitch. any suggestions to increase the speed<br /><br />Thanks,<br />Thomas

 

[FlyBoyMark]

Re: Boat speed UPDATE

A 155 Merc? The inlines were only rated to 150 HP at the power head maybe put out 140 at the prop....your boat should do between 50 and 60 MPH at least unless your weight figures are way off...how about model and serial numbers on the engine? Maybe someone put a cowel switch on you..Rpm sounds slow...get us serial number and model number....OR is it a I/O stearn drive? A handful of early bass boats had stearn drives.

 

[Valv]

Re: Boat speed UPDATE

You shouldn't look at rpm to speed. Go step by step reaching max RPM for your motor, then you will see which speed you can reach.<br />Mid 30s.mph is slow for a bass boat with such a big motor, I have a '91 Sprint with Yamaha 150 at I reach 72mph with a speed prop.<br />Also try to get a GPS unit, even a handheld, they are much more accurate than regular boat tach.<br />If you have some obstructions in water line to speedometer it will read incorrectly.<br />Maybe we are just talking for nothing since your boat already goes as fast as it can and you don't even know it.

 

[rickdb1boat]

Re: Boat speed UPDATE

boaterntexas <br />With the 115 Merc and your boat, you should be running 5000-5500 RPM's and hitting around 45-50 MPH. Sounds like you may be a little underpropped. You should be running around a 13" X 15" pitch Prop. Check out the pitch on the prop and let us know. Might want to pick up a GPS for $100.00 to verify speed.

 

[FlyBoyMark]

Re: Boat speed UPDATE

rick is rite...even its a 115 it should be doing close to 50 MPH or better...

 

[ajp]

Re: Boat speed UPDATE

I have a 1969 115hp johnson on a 18ft deck boat. Full speed shows 28mph on my speedometer. It is plenty fast for me to get to where I am going. I would guess that in the last 34 years of the boats life, the speedometer hose has got some accumulation of crud and slowed down the speedometer. If your speedo is 27 years years old, you are probably going faster than you think.<br /><br />Are you in a race to the best fishing spot? Enjoy the ride a little.

 

[Kenny Bush]

Re: Boat speed UPDATE

Tex, I have a 17' Del Magic (1976) with a 150 merc and it does 47 mph at 4900 RPM's with a 21 pitch prop. Something looks to be wrong with the motor. Has the boat sat up a while? You could have a gummed carb. Have you run a compression check to make sure the motor is balanced at around 120 lbs in each cylinder. Run a compression check on each cylinder and post results. We need to make sure that powerhead is good before we check ignition, fuel, and other notables. Goodluck......BushCat

 

[one more cast]

Re: Boat speed UPDATE

You probably already checked,but just in case,your not taking on water are you? That would slow you down.<br />-----------------------------------------------<br />OMC

 

[boaterntexas]

Re: Boat speed UPDATE

A update. The motor is actually a 115 hp Merc serial # is 4570020 not sure of what model though, where would that be found? What method should I use for trying to get the rpms up while in the water Tilt/and trim wise.<br /><br /> Thanks,<br /> Thomas

 

[roscoe]

Re: Boat speed UPDATE

Start with motor all the way down and slowly raise it. There should be a sweet spot where the boat lifts slightly in the stern and may lower slightly in the bow, and the rpm's increase slightly.<br /><br />If there is nothing mechanically wrong with the motor, I would try a prop with less pitch. This will allow the motor to reach its optimum rpm range. Even though the pitch will be less, the higher rpm's will propell you faster. I would drop the pitch down 2 or 3 inches and hope for 600 more rpm's.<br /><br />BTW, my boat with me in it (1200lbs +) will hit 39mph, powered by a 70hp Force.

 

[slow_boat]

Re: Boat speed UPDATE

Motor probably won't last too long running it at 6000RPM especially with no load.

 

[boaterntexas]

Re: Boat speed UPDATE

Thanks for all your help guys,, I have a follow up question, my motors tilt and trim move real slow should this be a concern, and it take a while for the motor to get going when trimmed all the way down, I have to play with the trim and find the right spot to take off, with a load it's even more tricky, once I get here going she will tilt and trim fine, it's just the "Out of the hole I'm having trouble with." also is there a way to test the accuract of the speedometer with out spending money on a gps?<br /><br /> Thanks, Thome

 

[tmcalavy]

Re: Boat speed UPDATE

Have someone with a good GPS or speedo run along side you to compare the speedo reading at different speeds. Don't run too close.