prop or fuel problem need help

[sstoc]

hello I have a 1978 mercury 115 hp inline 6 cylinder blue band motor took it out for the first time yesterday. its on a 16 foot bass boat. The numbers on the prop are 48 72506A-4 21. When I opened up the throttle it would only go up to 2000-2500 rpms and was going to slow to get up on plane. me and my fishing partner are around 250 lbs each and i figure the boat, gas, passergers, and equipment are around 2000 lbs. after a minute i told him to sit up front and then in about ten seconds it came up on plane i adjusted the trim and we ran 40 miles to the fishing hole everything ran great, the speedometer said 40 mph and I kept the rpms around 5300 - 5500. I still have some throttle left but if I opened it up it went to 6000 rpms and I thought that was bad for the engine. also at 6000 rpms the speedometer went to 45-50 mph. I guess my question is do I need a new propeller or trim tabs to be able to get up on plane without having to get in the front of the boat each time to get going? And when I open it up from the sitting still position why does it only go to 2000 rpms and very slow. If i could get a better hole shot it would take care of this whole problem. If you think a new prop would fix this what size and pitch would you suggest. i have a fishing tournament next week and plan on running alot of miles and alot of starting and stopping. thanks

 

[QC]

Re: prop or fuel problem need help

Lower pitch will help, lower weight would help, trim tabs would help. Short of throwing your buddy over the side I like option three Kind of one of those problems with what is a relatively fast boat and engine combo. You need the high pitch to get there, but then you sacrifice hole shot. I like the tabs because it allows you to keep the good top speed and higher pitch. I am guessing she just runs out of power at the point of climbing over the bow wave, but there could be some other problem limiting power down low. Your results tell me that power is good at higher revs though. Can you verify speed GPS? I think I'd want that to draw any real conclusions. 50 in a 16 footer should've felt really fast . . .

 

[Silvertip]

Re: prop or fuel problem need help

If you've ever tried to take off from a traffic light with your car in high gear you would have noticed it was pretty much impossible or at best, tough on the clutch. Well -- the prop is the transmission for your boat and it only has one gear -- high gear. That's why prop selection is such a crap shoot. You need to get out of the hole but going down in pitch sacrifices top end. Going up in pitch sacrifices hole shot. Happiness is somewhere in between and it becomes a compromise.

 

[Frank Acampora]

Re: prop or fuel problem need help

Your boat is very tail heavy. when you give it the throttle from a stop, the bow rises and it plows water. The engine is so loaded down that it can not make its horsepower and can not get the boat "over the hump" onto plane.

Investigate a vented prop! The vent holes bleed exhaust into the prop blades and increase slip at low speeds. This lets the engine rev up, develop its horsepower, and get out of the hole. The vent holes have no effect at speed or on top speed. Lots of bass boats use them.

You can try venting your existing prop by drilling holes in the hub between the blades. These holes should be about an inch in from the spot where the prop enters the lower unit case. Start small! About 1/4 inch and work up. Since too large hole diameters will adversly affect performance and since you can not un-drill them, go up in small steps.

It is easy to do and if you have any doubts, look on line for performance prop photos and see how they are drilled.

Also investigate smart tabs. They are automatic in operation and relatively inexpensive.

 

[QC]

Re: prop or fuel problem need help

Investigate a vented prop!

I agree!