want to go from 2 blade to 3 blade prop

[dlp123]

hello im new to the site and im need some advice. i have a 1972 sleekcraft speed boat with a 115hp mercury same year as the boat my question is the motor has a 2 blade brass prop Michigan 13 1/8x19. the boat only weighs about 500 hundred pounds or less needless to say the motor weighs more than the boat does. my top speed is only around 30 mph even with 4 people in the boat it goes the same speed my question is what 3 blade would you recommend me running to get a little more speed out of the boat.

 

[steelespike]

Re: want to go from 2 blade to 3 blade prop

Welcome,You should read the stickys at the top of the page and the helps on the prop store page.
Generally speaking a 2 blade prop is the best bet for speed.A 3 blade usually is a better all around prop.At 30 mph I think your at least 10 mph short and very likely have more going on than prop selection.Could be your setup,could be a boat problem,could be a motor problem or a little of all that.Get us as much info as possible and you should probably start with the motor.Be sure it is tuned right,carbs opening all the way,good compression,timing set right and advancing as it should.You wouldn't be the first to not realize your 2 stroke wasn't running on all cylinders.

 

[dlp123]

Re: want to go from 2 blade to 3 blade prop

Thanks for the reply i just redone the whole boat the engine is hitting on all cylinders and i checked all the cables they are opening all the way my tach doesnt work atm moment need to get one but it sounds like im getting full rpm one reason i started this thread is all the 115 mercury motors ive seen have 3 blade props instead of 2 and when im going it feels like the prop is sliping in the water i have tried tilting the trim up or down but get the same results.

 

[steelespike]

Re: want to go from 2 blade to 3 blade prop

Props do wear out especially al props, perhaps a 39 year old Bromze prop is worn.
While it was state of the art in 72 it probably couldn't measure up to todays props.
Does the boat itself respond to up trim the bow should come up and it should gain speed.
If it seems to lay down flat and is un responsive it could be the hull or the prop.
We still need to know what size Sleekcraft?Does it have a relatively flat or slight v bottom or is it a true deep V all the way to the transom.
You do need your rpm to evaluate your results.An induction tach like a Tinytach or an optical tach that counts a reflective spot on the flywheel.Without your rpm your pretty much lost as far as the right replacement prop and still no way to know if the replacement is right.
Depending on the size and weight of the boat the 19 is in the ball park but I wouldn't make any changes without a tach.

 

[dlp123]

Re: want to go from 2 blade to 3 blade prop

sorry about that its 17 foot long i think its a true v hull when you first take off the front will rise out of the water then plane out. im going to order a tach asap so i will know what rpms im turning. what im confused about my pitch on the 2 blade prop is 19 does the pitch stay the same on a 3 blade prop? or is it diffrent condering more blades i can live with the speed just want to get a newer prop.

 

[steelespike]

Re: want to go from 2 blade to 3 blade prop

I'd wait for the tach before making any selection.
Depending on the rpm and speed you may drop an inch.

 

[Mi duckdown]

Re: want to go from 2 blade to 3 blade prop

Without a tach you will be chasing your tail.

 

[dlp123]

Re: want to go from 2 blade to 3 blade prop

got a tach today wot is 4800 rpm at 30 mph also talked to a boat shop and they said i will need to go down on pitch since going from a 2 blade to a 3 blade does that sound right.

 

[steelespike]

Re: want to go from 2 blade to 3 blade prop

Assuming the boat is planing right responding to up trim it appears you need to go to a 15" prop.
The more changes you make (pitch, material,number of blades, Brand,30+ years of technology)
the more variables you introduce.A 4" pitch change should add 500 to 700 rpm.

 

[dlp123]

Re: want to go from 2 blade to 3 blade prop

thanks for the reply im going to try a 17'' prop and see what that does i really dont wont to be at max rpm on this old motor so thats why i chose the 17 i ordered this one off iboats http://boatpropellers.iboats.com/Me...1-1317_Propeller/294/10001/?*******=772188776 so hopefuly it works out for me all in all i just wanted a prop with newer tecknology than what i have. thanks for the help everone

 

[steelespike]

Re: want to go from 2 blade to 3 blade prop

Forcing the "Old" motor below its designed rpm range is like putting you car in overdrive and pulling a nice big trailer up a long never ending hill.Your motor will be much happier being able to rev. If you want to be easy on it prop it for the max wot rpm and then run at the speed you feel is best for the motor.