need suggestions on next prop size

[monk-monk]

I have a 1975 Glastron V-178 (17.6 feet) bow rider with a 1977 mercury 1500 6 cylinder. I am currently running a mercury black max prop of 13" dia. and 19 pitch. This prop is just a little too big for the boat and normal load I carry...plus I can only get to 4400 RPM's and motor specs say 5800 RPM's...so im losing 1400 RPM with current prop..current prop is also a cupped prop..if you had to decide, what would be the next size prop to try...Thanks for any help!

 

[Georgesalmon]

15p

 

[Maclin]

What MPH do you get at the 4400rpm? Just wondering if the tach is not accurate. For example if you are getting to 40mph or faster then I bet the tach is not set correctly. If you are only getting to 30 then your engine is struggling to hit top rpm.

I have a 90hp on a 16 foot Glastron SSV with a 17p, and it will get to almost 40 at top rpm. With light load it is under-propped, so by way of comparison a 19p on your 17 footer with 150hp should be ok if the engine is in good condition and tuned and running well..

 

[monk-monk]

Thanks!...Georgesalmon, that's what I wanted to go to was a 15p...but I can't find a black max prop of 15p with a dia of 13"..at that pitch (15) the dia range is all over the place, but not in 13" dia...it's always way smaller, or way larger dia in 15p category. Maclin, I had the same thought about the tach, on the flip-side of your reply, sad to say, but at this time I have no accurate way to measure my speed. I will say this: im going to investigate the accuracy of the tach...because of my experience with over propped boats, I m very sure it is over propped because the previous time I had it out, it got on plane a little sluggish, the most recent time I had an extra 100 lbs and it struggled to get on plane...I know that extra weight made the difference in how it performed. Thanks for all the help!

 

[steelespike]

If you look through the prop charts you may note that as pitch goes down usually diameter goes up.
​ If the prop is intended for your motor it will be right. Some cells have a GPS app.
GPS is the only way to get accurate speed even then you need to be aware of currents of any consequence.
You have a 150 hp on a boat that is about 1,000 lbs dry weight.. I don't think a 19" prop is excessive. perhaps not ideal
for water sports but I don't think it should struggle when you add 100 lbs.
Your 77 has the Thunderbolt ignition which uses electronic ignition with a distributor.
I believe timing is critical on these motors both for accuracy and correct setting.
There are also I believe 3 carbs any one or more of which could need work.
It's possible you could be on 5 cylinders and not realize it accept for the poor performance.
​ If you think about it a 3.0 135 hp I/O about 2000+ lbs runs about a 19 or 21" pitch at speeds close to 40.
A 19 being adequate for water sports.
​ If your going to be doing your own maintenance get your self a service manual for the carbs and ignition.