another prop question

[fishingman220]

I have a 96 proline that weighs 3450 dry, running a 5.7 w 210hp and a 1.81 drive alpha. i guess you would call it a deep vee, right now i am running a 13 3/4 x 19 mercury vengence and with 3 or 4 people on the boat(normal load) i can spin up to about 5100rpms, now my wot should be 42-4600. now my boat is preatty slow out of the hole, doesnt slip i guess just a heavy boat, i have started to get it up faster with using my hydrylic trim tabs, but from what i have heard i need a 15in miragex21 pitch, any thoughts?

 

[steelespike]

Re: another prop question

I'm not an expert but:: I think your tach is reading high.You need to verify it is accurate.
Your hole shot now is probably about average,You would need to use your tabs normally.If you go to a 21 your hole shot will be worse.
I think the 19" is close to what you need.Diameter usually simply follows design and application.If you look up 19" or 21" 3 blde Vengence for your system diameter will follow.You don't usually consider diameter except in certain specific problems and to be sure it will fit.

 

[fishingman220]

Re: another prop question

well a few people i talked to said i needed a 15in diam. because of the weight of the boat and smaller prop is trying to dig a hole in the water when i go to take off insted of pushing the boat. i dont know if that is right figured i would see what all you say

 

[Silvertip]

Re: another prop question

Going to a 15P prop would increase RPM at the high end so you would be extremely over the max rpm for the engine. The 15P prop is the one that would likely dig a hole and ventilate. You probably have the best prop for the boat and current load. You can throttle back a bit if you are worried about top end RPM being high. That's what it and the tach are for. Verify that your tach is accurate before spending money on props.

 

[hwsiii]

Re: another prop question

You don't say how fast you are going at 5,100 RPM, that is very important information.


H

 

[fishingman220]

Re: another prop question

no, a 15in diam 21in pitch, at 5100 i am running about 40-41 by the speedomiter, and fishfinder, i dont have gps

 

[hwsiii]

Re: another prop question

Fishing, Spike is right, your tach is not giving you a correct reading in my opinion, it is reading high if the information I have is correct. I show your motor should turn between 4,200 and 4,600 RPM at WOT and your gear ratio should be 1.47:1, and I also show the Mercury Vengeance prop for your motor should be a 14" x 19" pitch, please verify this. Try turning the switch on the back of the tach back and forth 6 to 8 times to clean the contacts to see if you can get it to read right. And take a mechanics hand held tach with you next time you go out to verify your RPM and bring a hand held GPS or even some phones have a GPS built in to them. The reason I say your tach is wrong is because I ran some numbers on your boat and I show at 40 MPH and 5,100RPM your prop slip is 36%, and that is extremely high.

Prop Slip

H

 

[fishingman220]

Re: another prop question

my gear ratio is a 1.81

 

[hwsiii]

Re: another prop question

I corrected for your gear ratio and it dropped your prop slip to 21%, which still is not good. If you will bring your prop to a reputable prop shop and ask them to cup the prop a theoretical 2" on the trailing edge your RPM will drop about 500 RPM and put you right at the correct RPM, and hopefully give you more stern lift to help with your speed. Just make sure they have the correct pitch block for your Vengeance prop, it might help your hole shot but I am not sure about that.

Prop Slip

If you want to go another route, I would recommend you buy a Solas Amita 4 blade ALUMINUM prop in a 21" pitch and that will increase your hole shot as well as reduce your prop slip and you will have better acceleration through out your whole RPM range, and you can keep your Vengeance as a spare.


H

 

[fishingman220]

Re: another prop question

cool, thanks for the info