New boat need help with prop choice

[haikt]

Hi,
I just purchased a Nitro 185 sport (force120)fish and ski and have some questions.
Out of the hole it is very quick but when it gets on plane and I start trimming up I almost have no room to trim. can go less then 1/4 on the trim gaugee before the prop seem to come out of the water and rpm go up and speed down. With 2 people on the back seats I can trim higher and gain 10 more mph. The current prop is a Mercury laser 2 (has 20 p 45 marking)
What can I do to fix this issue if it is an issue at all. The only other boat I have used is a Javelin with a 120 jhonson and never had this issue.
Please give me some pointers if possible.
thank you

 

[haikt]

Re: New boat need help with prop choice

Also the engine is rated for WOT 4750 - 5200 RPM. I am getting 5000 rpm at 45 mph but the rpms are jumping around 5000 +/-100. with people on the back seats it will push 50-52.

 

[hwsiii]

Re: New boat need help with prop choice

Haikt, you dont say what the RPM are when you are doing 52 MPH. But at 45 MPH with a Normal 20" prop and 5,100 RPM and a 2.07 gear ratio you have a slip of 4%, I would have to say the prop is a theoretical 21" pitch as that slip is normally too low. Check to see if your gear ratio is 2.07 please, that is what I show.
But to reach 52 MPH you have to be turning at least 5,400 RPM with a theoretical pitch of 21" and 0% Prop Slip.
We need some GPS numbers with RPM to make any recommendation.

H

 

[haikt]

Re: New boat need help with prop choice

Hi,
I went out today to test the boat alone.
Best result I could achive was 45-47 mph at 4900rpm
I found that if I trim to fast and high the boat slows down to 40 and needs to be trimed down all the way to (catch again) if I trim up slow I can go 1/4 trim before getting the prop out of water ( I assume)
I am going to get GPS reading nex time out.
I think the prop is 20" only because it has 20P45 marked on it.
Thank you

 

[hwsiii]

Re: New boat need help with prop choice

You are welcome. and as soon as we have the numbers we can help you.

H

 

[haikt]

Re: New boat need help with prop choice

Hi,
I went out with GPS yesterday and got 40mph highest while my speedo was reading 45. At WOT I could not get any higher then 5000 RPM. I have installed a hydrofoil and it seems to give me wider trim range but for some reason made hole shot longer. Seems like the prop is bogging the engine down.

 

[hwsiii]

Re: New boat need help with prop choice

Haikt, I can't seem to see where the hydrofoil helped you. Your speed did not increase any and now it takes longer to get on plane, being able to trim the motor more only helps if it raises the boat and increases speed. You are showing a prop slip of 13% at 40 MPH with that prop, and it should be closer to about 8%, if you changed to a 19" pitch in the laser II it would brig your RPM up to about 5,200 RPM and probably gain 1 MPH in speed and help with slip, but that doesn't seem justified for the cost involved.
I would take the hydrofoil off and install and reinstall vent plugs on the prop until I got the quickest hole shot I could get and let it go at that, your motor only will turn 5,250 max RPM and you are within 5 % of that now. There may be better props for your boat and motor combination out there, but it would require a lot of effort and expense on your part to find them, I would settle for your current setup.

H

 

[cleve]

Re: New boat need help with prop choice

sounds like your engine is mounted to high on the transom.

 

[steelespike]

Re: New boat need help with prop choice

Get rid of the foil for now.Is your prop in excellent condition?
Where is the antivent plate(cavitation plate) in relation to the bottom of the boat? I hate to give up the height but it may actually run faster
one hole lower if you can use the trim more effectively.

 

[hwsiii]

Re: New boat need help with prop choice

Great idea Steele, I dint think that far ahead,

H

 

[haikt]

Re: New boat need help with prop choice

Hi,
Thank you all for the info.
I measured the cav plate is about 1.5 inches from the bottom of the V the lowest point. engine is mounted on the last hole and is on the transom, I dont think I can get it any lower then this without a Jake plate. My speedo seems to be off also, about 4~5 mph. I have a Johnson 120 on my Javelin 363 (17') and a 13.5 X 21 and I can get up to 5800 RPM if trimed up and run at 50 MPH. I know these are not comparable but this is my only other exeriance with a boat.
I was thinking to try a prop at 13.5 x 21 on it with less prop blade area?
thank you again.
PS,
the prop looks brand new on the boat.