why only 19p?
 

hi
my boat has got 2 330hp 7.4 bravo1 drives with 19p mirage props. it goes upto 49mph @ 4400rpm i am going to change my engines to 425hp 496HO's and the advice i got was to try to keep the same props since there is a 600rpm increase between the engines.

my question is the following, how come most of the people on this forum use props with not less than 23p and most with bravo 1 props which are 4 blade and larger dia props. are my engines that weak that they can't turn higher pitched or larger and 4 bladed props? can someone please tell me what could be wrong with my setup and what i should do?

34 foot italian make offshore boat. non stepped hull. deep v. heavier than american made boats.

thanks
jack

Re: why only 19p?
 

It is your HP to weight ratio that you should look at, as this will determine the max speeds you will attain. Now as far as pitch is concerned with heavier boats it may take a smaller pitch to attain the appropriate RPM at WOT. However you must also consider the set-up of yor boat ie: Drive type, gear ratio and drive height. Give us some specifics and perfromance numbers and we can run thru some of the calculations with you.

Julie

Re: why only 19p?
 

A hundred hp increase will bring you up about 2 inches of pitch (3-4 for 200hp). Thats a 600-800 rpm gain. Even if you gain 800 rpm, your still just tapping the rev limiter for your engine. This makes the props you already own a great way to get a new base line. The advice you got was sound.

Don't be concerned with the size of the number you have stamped on the propellers. Get the set that carries the boat the best in the upper end of the rpm range the manufacturer suggests for your engine.

Re: why only 19p?
 

thanks for your reply and input, here are some more details about my boat;

drive type: bravo 1 (1995)
gear ratio: 1.5 to 1
not sure about drive heights
max speed trim up(drives) 49mph @ 4400 rpm
max speed trim down(drives) 45-46 mph @ 4200 rpm
cruising @ 3800 rpm 37-39 mph depending on load

these figures are based on my old engines 2 x 7.4L/310HP bravo drives with mirage 19p props.

my new engines will be 496HO/ 425HP on the same drives.

can you advise what performance gains i will get with 4 bladed props and which model would be more appropriate the bravo 1 or the revolution 4. taking into consideration that cruising speed is more important than flat out max speed.

hope this is enough info

thanks
jack

Re: why only 19p?
 

Jack,

What is the weight of your boat without fuel?

I need this to run thru some of the calculations, please.

Julie

Re: why only 19p?
 

not less than 11000pounds

jack

Re: why only 19p?
 

julie,

it weighs 11500 pounds approx

jack

Re: why only 19p?
 

I suspect you will run around 60 MPH at 5000 RPM with 22P props. This is were I would start with your testing.

Julie

Re: why only 19p?
 

22p of which props? bravo1 or mirage+ or revolution 4. according to my boats configuration and info i have provided, which would be best suited?????

Re: why only 19p?
 

What year is the hull? I am concerned as to whether your boat will perform well with 4B props.

Julie