what happens when you over prop a boat? Under prop it?

Ideas on a 31', twin 454, 440 hp each, 380 avg hp, bravo ones at 1.5:1 ratio, boat weighs 6900 unloaded..........

thanks!

Over Propping leads to low RPM numbers or lugging the motor for your application.
Under Propping leads to high RPM or over reving the motor or putting you in the rev limiter.

I can run your numbers for you but I need to know the fuel tank capacity. or if you can give me the wet weight I ill run your numbers.

Julie

Engine damage can occur from over propping or under propping your boat. Even with rev limiters, damage can be done.

In addition, performance at some throttle settings can be hampered. On the other hand, we have over or under propped boats for certain race applications.

Under propping can increase acceleration drastically. Over propping can improve cruise speeds drastically. Propping correctly can give you the best of all worlds.

weight weight is about 8900lbs, fuel capacity is 150 gallons....

I am looking primarly for top end speed, I guess my priorities would be:

Acceleration at low RPM = 20% of concern
Handeling and manuverability = 35%
Top end speed = 45%

We use the boat about like most people do....

Thanks!!!!!

What is the year, make and model of the hull?

Julie

1993 Profile, 31' V hull. Thanks!

I have calculated 420 HP for each motor, at the shaft, for a total of 840HP. This will give you a projected speed of 69 MPH. Keep in mind that this is a HP to weight ratio only.

At what RPM is your peak HP produced? I want to run the convertions to make a propeller and pitch recommendation.

Julie

I have calculated a slightly higher speed possibility. At 8900 lbs. and with your boats hull running 820 hp at the props you could see 73-75 mph. To achieve this you would need to turn Lab Finished 28 Bravo 1's in the 5100 rpm range. I'm assuming this is the rpm range you are going to turn the motors.

amazing, simply amazing.... you guys are great and I sincerly apprecieate the info....

The first motor came ofd the dyno the other evening, the second is going on probably tomorrow.

The 440 hp is from the dyno print outs, I will post them when I get them, the motors should run the same. I was hopeing to get to hte 70's range, the boat has been terribly slow in the past, barely reaching 60, but I think I was over proped as well with 4 blade Hydromotives in a 24 pitch...