How much rpm's do I lose with one extra person?

[spacerust]

I have a 15' shallow sport sled with front and just added a rear tower. I also just got a new prop to replace the original one that had a spun hub. It is powered by a 88 Nissan 90HP with 132psi compression on all cylinders. Today I had three people with a combined weight of 550 lbs. The prop is 13x17p and at WOT I was running 4500-4600 consistently. How many rpm's would a person weighing approx 200 lbs make. I am asking because I normally will fish two but today the third had me to the 4500-4600. I can't really say what rpm's I had before because I just recently got this and think the prop had been slipping a little since i first got it but last time out is when i really realized it was a spun hub. I am going order a new prop and keep this as a spare, but I'm figuring I need to gain almost 800 rpm's to be between the 5000-5500 mark. Then what if I switch to a 4 blade?

 

[jestor68]

Re: How much rpm's do I lose with one extra person?

My preferred way of selecting a prop is to test with a light load(one person), and prop it to turn at or near the max rpm that it's rated for.

That way, when you add a person(or two), it will pull it down some, but usually stay within the recommended rpm range.

How much effect the additional person will have varies from boat to boat.

The rule of thumb is 150-200 rpm per inch of pitch change. But I wouldn't decide on a prop until I run it alone to see what it does; then adjust your pitch to get it as close to 5500 rpm as possible with the light load.

That the motor is rated 90 HP @ 5000 rpm. If it can't turn up to 5000 rpm, it's not developing full power.

 

[spacerust]

Re: How much rpm's do I lose with one extra person?

okay, i can try that but will it hurt to run it at 4500-4600 at WOT until I figure out the right prop. I have a fishing trip tomorrow but can ask if I can ride it alone to check the rpm's. I just don't want to mess up the motor. Today we rode for about thirty minutes, stopped, and ran back to dock. The prop worked great, got on plane fairly easy, no one got wet, but I want to order another to compensate and have this one as a spare since you can't return to iboats once used.

 

[jestor68]

Re: How much rpm's do I lose with one extra person?

Your full throttle operating range is 4500 -5500 rpm. It won't hurt anything to run at 4500 rpm, as that is still within the operating range(barely).

The correct prop pitch should be 15"; that will gain 300-400 rpm and put you near 5000 with your buds along with you.