How to guess prop with unknown WOT RPM?

loss4wrds

Cadet
Joined
Jul 10, 2013
Messages
15
[FONT=&quot]I have a 98 Sprint 296 Pro (19.5ft bass boat) with 91 Johnson 200
The prop on the boat is stock Johnson 3 blade stainless 19P (13.25" I think)
When I took it out for WOT the motor revs past 6k @ 53 mph, I ran it to 6200 and it was still climbing steadily when I let out of it for fear of damaging the motor. At the time I had myself (220lbs) wife (100lbs) my 9 and 10 year old boys (75lbs each) 120lb great dane puppy - half a tank of gas (24-26gal)
I have found where guys with the same hull guys are running 25-27P props and getting 70+mph out of theirs in the 5800 rpm range.

I plan to use the boat for recreational fishing in the mornings and throwing the ski's out or pulling kids on tubes when it starts to get hot. I am not worried about top speed more being practical for my needs. Was thinking a 4 blade prop in the 21-23p range but am just guessing.

I am currently trying to find a shop locally that will let me test before I buy props but haven't had any luck so far. Is there any online resource that will let you rent props or has a great return policy?[/FONT]
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,296
going to assume your tach is accurate. you may want to verify that

if your going to use the prop both for zipping along to your next fishing hole and pulling a skiier. use the skiing as your determining factors in the prop. can your boat pull your 220# out of the water now on one ski?

if you have plenty of power to spare, then go up in pitch a bit.

for fishing, you can always back off the throttle or run a rev limiter
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Going along with what Scott said: If you are happy with your hole shot with your heaviest load.....you on a slalom, leave it be and back off the throttle when running light....no skier and such. You start messing around from where you are and the $$$$$ start flying and results may be less than stellar, especially when you start intermixing different brands, types, and pitches of props.

Your comment about try before you buy is the way to go if you must change but you have to return the prop in the condition in which you received it and that means get in clean water, run your test, and take it off. Also you will pay more for that prop but that would be cheaper than throwing props at the problem till you got it right.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
What's the max wot rpm range for that engine ? If 6K and running at 6200 as currently loaded, + 200 wot revs won't blow the engine. If like runnning wot, just throttle bit less, when skiing lightly loaded, watch tach with both eyes not to over rev....

If still wanting running at max 6 K, re pitch current prop 1/2 up at a prop shop, should put wot rpm arounf 6 K.

Happy Boating
 

loss4wrds

Cadet
Joined
Jul 10, 2013
Messages
15
Forgot to menton the boat also has a manual jackplate. I am going to measure the depth it is currently set for and add that to the info.
The prop that is on it now will snatch me out of the water with plenty to spare. (When boat was lightly loaded, was the only time I have tried it so far, I suck at getting up on slalom and usually just kick off once up)
The existing prop is fairly rough with the leading edges of all three blades blunted and dinged with at least 1 half dime size peice missing from the tip of one blade. To get it reworked locally was priced at $120-200, I priced over the phone without them actually seeing the prop. I have always perfered to have a spare prop in any boat I have had just in case. I spun a hub once way back the in swamps in Louisiana and never want to have that experience again. So was thinking of getting a new / new to me prop and throwing the old one in the back as a spare.
As for the tach I threw an old automotive tach I had on it to compair and they were within about 80rpm of eachother and the stock one doesn't hop or bounce around so I think I am good there. Was thinking 4 blade as the boat has slipped considerably in corners with the current configuration but that may be due to the jackplate settings from what I have read so going to spend some time measuring that and checking for proper height as I have never had a boat with jackplate before and am just learning about it's set up.
I have only had the boat for a couple months and have been working the bugs out as apparently one of the previous owners was a complete idiot or had never owned/worked on a boat before as there were mutiple easy fixes that were just done completely wrong or just left broken / neglected.
Thanks for all the help and info guys, you are telling me the same things I was thinking already, so it gives me more confidence I am moving in the right direction.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Recommended operating range for motor is 5400-5800


For peace of mind, plan revving at 5800 wot revs as usually loaded ? If so, in theory will need to go 2 + pitches to achieve that, but going per the Rule of Thumb that one + pitch lessens -200 wot revs could not be so, usually dials a lot more. Personally would try next + one pitch as off current damaged prop. Will need to find that out going for a wot test run as usually loaded.

Can you post a pic of the lower leg shot sideways to have a look ?

Happy Boating
 
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