Changing diameter of prop?

[TwoFish]

I am having trouble with prop slippage at slow & mid range speeds. Higher speeds are not as bad. The motor is a 1998 75 hp Force. They only run a 10 ? inch diameter prop.

I may go to a 4 bladed prop but a local prop repairer has suggested adding metal to the 3 blade aluminum prop to make it a larger diameter. Is this a safe thing to do?

 

[Bondo]

Re: Changing diameter of prop?

but a local prop repairer has suggested adding metal to the 3 blade aluminum prop to make it a larger diameter. Is this a safe thing to do?

Ayuh,... As long as you maintain 1/4" clearance,... Go for it...

 

[Frank Acampora]

Re: Changing diameter of prop?

That's a Mercury lower unit and uses the same props as Merc 40 and 50. Before you go modifying your prop with unknown results, why don't you see if Michigan (and its other brands) sold right here on iboats, or Merc offer a prop with different characteristics or a larger diameter.

 

[steelespike]

Re: Changing diameter of prop?

According to the prop chart there seems to be an overlap of the smaller and larger
gearcases if its a 13 spline then its the smaller gearcase.The larger is 15 spline.
I think a 4 blade could be a good move it should solve your venting and improve hole shot,stay on plane with less throttle and diddling,handle better on plane and at the dock.

 

[180shabah]

Re: Changing diameter of prop?

Before we get too deep here, why don't you describe your "slip" problem and the condition of your current prop.

 

[TwoFish]

Re: Changing diameter of prop?

Thanks for the replies.


The boat is 16 feet long and weighs about 1700 pounds loaded. The gearbox reduction is 1.64 : 1. Recommended rev range for the motor is 4700 to 5200. The motor has the 13 spline gear box. The motor is mounted on a pod about 1 foot behind the boat.

The boat has difficulties getting to & staying on the plane (Planing speed is about 14mph) at under 3500 revs. I was putting this down to the prop slipping because of the small diameter. I have tried moving as much weight forward as possible but still am having problems.

With the (3 blade Aluminum) props I have these are the results

RPM, Pitch, Mph, Slippage

4000,.. 12,.. 20,.. 27%
5600,.. 12,...33,...13%
3400,..14,..11,..59%
3500,..14, 13,..55%
4000,..14,..25,..23%
5000,..14,..31,..22%
5550,..14,..36,..18.5%


Are the slippage figures high?

Revs are too high so I will need to take this into account as well.

Having the prop altered will be relatively inexpensive but as suggested my be a bit of a gamble as far as results.

 

[steelespike]

Re: Changing diameter of prop?

I would say a planing speed of 14 is good.
Slip figures are only as accurate as the input.The only figures you know are accurate is the prop size.If the speed is by gps then that is likely accurate.13% slip is ok
18 is a little high but 36 is reasonable.I would start by verifying the tach is accurate.and varify speed with a gps.
Also consider the setup.Is the antivent plate(just above the prop) visable on plane? Usually even with or slightly higher than the bottom.

 

[TwoFish]

Re: Changing diameter of prop?

Thanks steelspike

Speed was by gps but I will need to check the tach for accuracy.

The antivent plate height (3-4inches above the bottom of the hull) could be a part of the problem at lower speeds as the pod on this boat sets the leg of the motor about 1 ? feet behind the transom. The antivent plate & a good portion of the leg is under water until reaches about 25 mph. At WOT it just touches the surface of the water with my weight out over the back of the boat. I thought of setting up a camera to get a better idea of what is happening with normal weight.

Thanks for all the previous replies. I now have a few things to check out & some ideas to go on with.

 

[TwoFish]

Re: Changing diameter of prop?

An update on prop diameter change.

Had the prop diameter made about ? of an inch larger & reshaped. Metal was added all the way around the outside edge of each blade. Seems to have done the trick.
The motor can hold the boat on the plane at 2800 revs instead of 3600. WOT is now 5200 instead of 5450, with the same top speed for 36 mph. The prop has a lot more grip in the mid range. It has added about 4 mph to mid range speeds. I now don?t know the true pitch of the prop so can not work out slippage figures but I?m happy with the end result so far. Need to run it a bit longer to see it goes with different sea conditions. Possibly will need to reduce the blade size if I feel the motor is laboring but it is looking OK at the moment.

Thanks everyone for your input.

 

[RicMic]

Re: Changing diameter of prop?

You might want to check the Mercury prop selector website, they have props with extra big blades on there now.

 

[sethjon]

Re: Changing diameter of prop?

I am having trouble with prop slippage at slow & mid range speeds. Higher speeds are not as bad. The motor is a 1998 75 hp Force. They only run a 10 ? inch diameter prop.

I may go to a 4 bladed prop but a local prop repairer has suggested adding metal to the 3 blade aluminum prop to make it a larger diameter. Is this a safe thing to do?

Sounds iffy at best. Why not just get a larger diameter prop? that way you will have peace of mind and in fact have a stronger prop too.

 

[TwoFish]

Re: Changing diameter of prop?

A new prop would be my first choice but could not find a 13 spline prop with a larger diamiter. Plenty of 15 spline props but this would require replacing the gears and the bottom end.
Does anyone knows where I can purchase an 11.5 inch diameter, 14 pitch, 13 spline merc prop?

 

[RicMic]

Re: Changing diameter of prop?

The diameter is not the important part the surface area of the blades is. You can add surface area, by adding more blades or making the blades bigger, longer or wider. So I wouldn't get to hung up on the diameter, thats just the easiest way for the prop shop to add surface area. I'd go to the Mercury prop selector web site and plug in your numbers.

 

[Robbabob]

Re: Changing diameter of prop?

An update on prop diameter change.

Had the prop diameter made about ? of an inch larger & reshaped. Metal was added all the way around the outside edge of each blade. Seems to have done the trick.
The motor can hold the boat on the plane at 2800 revs instead of 3600. WOT is now 5200 instead of 5450, with the same top speed for 36 mph. The prop has a lot more grip in the mid range. It has added about 4 mph to mid range speeds. I now don?t know the true pitch of the prop so can not work out slippage figures but I?m happy with the end result so far. Need to run it a bit longer to see it goes with different sea conditions. Possibly will need to reduce the blade size if I feel the motor is laboring but it is looking OK at the moment.

Thanks everyone for your input.

Looks like you solved it! Hole shot improved, planing easier, WOT rpm where you want it and you're top speed reamins constant with improvement in the mid-range.

Happy boating... with the time we have left this year
!