75hp Force Struggling under load

[thew4]

I have a '96 75hp force on a bayliner 1600LS. Since owning the boat I seem to have issues with it running on plane with a decent load. I have bought 3 different props from a marine dealer 13, 15, 16 pitch. With 2 people on board and the 16 prop I can run about 26mph gps. If I have 3-5 people on board I need to switch to the 13 prop and I'll be lucky if I hit 15mph. Most the time with 3 or more people it struggles to get up on plane.

After trying different pitches I am stumped. It seems like my OB doesn't have the ummph that I thought it would. I'm not looking to break speed records but being able to run on plane with 3-5 people should be a given. Now all she does is sink the back end down and "plow" water, putting off huge wake.

This is very frustrating. I'm thinking maybe my OB is not at proper height?? Ice tried diff pitch props supposedly recommended by manufacturer accusing to marine shop. Is trying an increase in diameter worth it?

Any help/suggestions/tips are greatly appreciated as this is frustrating me. I just have to believe this OB can do better!!

 

[fucawi]

Re: 75hp Force Struggling under load

Well with that range of pitches you should not have this problem ..first what RPM at WOT do you get with each prop when its light loaded ? Without a tacho you are wasting your time ....Is the boat water logged at the rear ..do you have any hatches you can check through ? Is the outboard tucked under fully when you take off ?? I have had great success with whale Tails to lift the back quick and flat onto the plane without the nose up attitude ...tuck it under first 16 pitch .

 

[TwoFish]

Re: 75hp Force Struggling under load

Hi thew4.


I have similar size boat an Allison Fisherman 5 http://www.boatpoint.com.au/reviews/older-than-1999/runabout/allison-fisherman-5-9000 and a 1998 75 force.


Most of these engines came out with a Mercury 50 hp gearbox, 13 spline, ratio 1.64 to 1. This makes them extremely highly geared. They run small diameter low pitch props to compensate. There were some that came out with Mercury ?Big Foot? gearboxes. 15 spline, 1.23 to 1 and larger props 12? to 13?. Count the splines on the prop shaft to see which gearbox you have. You get a lot of slippage at lower speed until you go fast enough for the prop to grip with the 13 spline gearboxes.


Most other 75hp engines run around 2.3 to 1 gearboxes with larger pitch (17? or 19?) and larger diameter (12? or 13?).



Having such a small diameter prop means they lack grip at takeoff.



I have been through a similar process regarding props. I run a 12 inch pitch 10.38 in diameter at the moment. I also have a 14 inch pitch. With the 12 inch prop the motor will reach 5800 rpm with a light load 5500 with a heavy load (4 people). The 14inch prop will get 5600 with a light load and 5200 with a heavy load. Neither prop gives much grip at low speed but the 12 inch is the better of the two. With the 14 inch prop and 4 people on board the boat would bog down and not be able to get up on the plane until everyone move forward in the boat. It would plane OK after but would drop off the plane very easily if the speed was reduced.



I had the 14inch prop made slightly larger in diameter (11.25inches and slight cupped). It now gives great grip at lower speeds and does not slip if it grabs any air. It tends to clear the air away very quickly. IT is now very sensitive to weight in the boat. I can get 5000 rpm with just one person aboard but this drops very quickly to about 4750 rpms (a bit low for my liking) with 3 people and fishing gear. I?m running the 12 inch prop now as I noticed the gearbox oil has been going dark very quickly. I need to look at the oil after running the 12 inch prop to see if problem is the 14 inch prop or something else. The 14 inch prop had slippage rates of 19% at top speed before it was enlarged and 11% after.



I don?t have the solution yet but the larger diameter prop makes the boat much nicer to drive. I?ll wait to see what happens with the oil before I look at having the 14 inch prop reworked.



The other thing to look for is a waterlogged boat. Check to see if your boat has any water trapped in it anywhere. The extra weight will make a big difference.

 

[thew4]

Re: 75hp Force Struggling under load

Thanks for the Responses..

@fucawi - I forgot to mention that I also run it with a Whale Tail hdryofoil. While that does seem to help a bit in "lifting" the boat on plane, when I have 3 or more people on board it struggles big time. 5 or more people and it wont even plane out. The only WOT I can say for certain is with the 16 pitch, that's 5200. The lower pitches Redline so I can't see what they run at WOT.

There is no water trapped in the boat, and nothing is water logged.

I guess I am curious to know if this is some thing that I will always have to deal with on this boat. I honestly didn't know it was going to be so sensitive to weight before I purchased it. Again, speed is not my concern, having the nose of my boat 4ft in the air with the throttle pegged is!

@TwoFish - It is a 13 spline shaft...so does that mean I should quit complaining and learn to deal with it?? I like you idea of expanding the prop diameter a bit. Would running a prop that's too big for the outboard cause any damage to it??

What about position of the outboard? Any other advice is always appreciated.

 

[TwoFish]

Re: 75hp Force Struggling under load

Running a larger prop could possibly cause damage. I?m still looking at what effect it is having on my boat.

The height of the motor on the transom is important. Too low causes drag too high and the prop will suck air in and will slip. When the boat is at planing speed and trim correctly the cavitation plate should be slightly out of the water.

 

[fucawi]

Re: 75hp Force Struggling under load

I have had a lot of success with 4 blade props though for the same diameter you need to drop 2 inches in pitch .....the 16 pitch seems good WOT figures so a 14 pitch 4 blade if you can get one