1976 170 Prop size

[76_170_Clemente]

Hello!

I have a 1976 170 and run a 2007 yamaha f50 on this hull. I recently had the ECU re-flashed and removed the restricter plate in the air intake the motor does 65HP now.

My question, since the 1976 170 hull is quite a bit heavier than more modern boats - currently running a 13" x 13pitch 3 blade prop but its sluggish out of the hole and seems like it could do better at WOT. I am considering going up to a 13" diameter 15 pitch 4 blade prop? In theory I am hoping it does better out of the hole and lifts the stern some in midrange as well as increases speed at WOT all while keeping RPMs in the sweet spot 5500 to 6000 rpm....

Any and all input would be helpful!

Thanks,

[wingtime]

Increasing pitch will only make the hole shot even more sluggish.  It will also lower your WOT RPMS considerably.  What are your WOT RPMS now?

[76_170_Clemente]

Well heres the thing I haven't run it since making the conversion to the ECU/air intake.

with 50HP I was running 13x13 3 blade prop and was about 5200-5300 RPM at WOT and ~22mph

With the decent amount of extra HP I wanted to go up in pitch and switch to 4 blade which I've read will help with stern lift (getting the boat on plane) and hole shot. I am also thinking not changing the diameter (leaving a 13") will allow for slight increase in RMPs to somewhere between 5500-6000.

I haven't purchased the prop yet so trying to get input on which to try first with the extra HP, thanks!

[RickK]

My  , a 50hp on a 170 is really not enough to get it up and over the prow wave very quickly.  I think I would save your money from the prop and see if you can find a used 70 or 90.

[boatnamesue]

I have the same boat as you, '76 170.  Mine is powered by a Yam 115.  When I read your initial post, and that this hull is powered by a 50hp engine, I had to re-read it a few times to make sure I was reading correctly. 

50hp, whether that be a 2 stroke or 4 stroke, restricter plate or no restricter plate, mounted on this boat that weighs 1100 lbs dry, is WAY underpowered.  Add the weight of a full tank of fuel, engine weight, gear weight, etc, and you're near 2,000 lbs!  A 50hp powering a 1 ton boat?!?  And to be even more frank with ya, it wouldn't matter what pitch, how many blades, SS or AL, you put on it...you will continue to experience a sluggish boat. 

This hull naturally sits ass heavy, it's the way it was built.  So in order to raise the stern you need POWER.  Prop pitch and number of blades only tweek the boats performance.  They don't provide miracles. 

Save your money on a new prop.  Buy a bigger engine, 90hp at the very least hp.  If I re-powered my boat I'd put a 130hp on it.  And the only reason I wouldn't put a 150hp is because a 150 Yam is 100lbs heavier than the 115 and 130, which are the same weight.