prop upgrade question

[dorcutt]

Her goes. Boat is a 2007 19ft CHawk, center console wt is apr 1850 lbs dry, Engine is a 4 stroke 115 Yamaha, prop is a 13 1/4 x17 aluminum. WOT is about 5500 rpm, don't know what the speed is. My issue is it is slow to get up on plane. I dropped the engine to the second mounting hole because it was putting up on a bit of spray but have not had a chance to really test it since then. I would like to upgrade the prop to a stainless steel but don't know if I should change my pitch at the same time. In reading the Yamaha perfomance charts and I see some of the same wt boats with same engine with the same pitch and some with a little higher pitch. I am less concern about top end speed and more concern to get it up on plane and get decent fuel economy. Any guidance for me. Dave

 

[steelespike]

Re: prop upgrade question

If possible I think your performance could be better with the motor as high as possible without
creating venting/cavitation issues.
The Few boats I checked were close on weight and had 16" or 17" ss props..
They all ran 5950-6000.speeds 37-40. about 4.44 to plane 8.50 0-30 6.6 0-20.
By your figures I came up with 38.7 assuming a reasonable 6% slip.It would be better if you could get us a gps speed.
Assuming our calculations are correct it would seem a 16"al prop might net about 5700-5800 and about 37.8 mph.
with a slight improvement in hole shot. Not having your actual speed does make this a best guess.
Another way to go would be a 16" Solas Amita 16" 4 blade. This should noticeably improve hole shot,
stay on plane better at reduced throttle and handle better at speed and around the dock.
You will likely lose some top end.The Solas props have been predictable.

 

[dorcutt]

Re: prop upgrade question

Thanks for the feed back. Correction from my end I in fact raised the engine to the second hole not dropped it. According to every thing I have read this should make a difference. If I understand you correctly you are suggesting I go to smaller pitch 16 verses the 17 that I'm at now. I was expecting to some one to tell me I had to increase the pitch, pitch being the amount of distance the prop goes in one full revolution correct? Again, thanks for your input. Dave

 

[steelespike]

Re: prop upgrade question

Increasing pitch will lower rpm and slow hole shot,may make more difficult to push a load.
I feel your motor needs to reach closer to its max rated rpm. That should inprove hole shot
ability to push a load and might improve speed.
The added blade of a 4 blade usually will lower rpm slightly so a 1" lower
pitch is usually suggested to maintain your rpm.The 4 blade should do a good job
improving those points I made in the other post.It may cost a little top end.

 

[dorcutt]

Re: prop upgrade question

Thank you for taking the time to explain the logic. Its winter time here where I am so I will wait till I can put the boat in the water so that I can get a clear comparison. Again thanks. Dave

 

[steelespike]

Re: prop upgrade question

Won't be long now. remind us of you post and get us your info.

 

[Silvertip]

Re: prop upgrade question

Increasing pitch is no different than shifting to a higher gear in your car. Speed will increase IF and ONLY IF the engine has the power to pull that higher gear. Ever try to start out in hgih gear with a stick shift car???? A boat has only one gear so with more pitch you might increase speed a little but you hurt hole shot. There is no free lunch.

 

[dorcutt]

Re: prop upgrade question

Thanks again. Great analogy, lower pitch = torque and I need torque to get up on plane quicker.