Why should I care about prop slip?

You know the drill..

Moderators: Redneck_Randy, badmoonrising, lakerunner

Post Reply
Message
Author
User avatar
Marc K
Posts: 742
Joined: Fri Jun 01, 2018 4:35 am
Location: Eustace, TX

Why should I care about prop slip?

#1 Post by Marc K » Mon Aug 03, 2020 9:19 am

Yes, I do understand what prop slip is and how it works.

I need to buy a new prop because mine is looking pretty rough. My boat does what I think that it should, so I never considered a prop change until now. In doing A LOT of reading, I have come to the conclusion that calculating prop slip is next to worthless for my boat. My prop slip calculates to about 30% WOT at 27 MPH.

My heavy boat does not get any major bow lift when trimmed running WOT. The best I can do is to reduce the wake from the nosecones by about 50% – unless I have 4+ people riding in the front seats, where everyone loves to ride.

My boat is 20 years old, 25’ with two 23” logs, no strakes. Full couches with heavy wooden frames.
My motor is a 25 year old Yamaha 115 HP 4-stroke, rated 5300 to 6000 WOT. Gear ratio is 2.15
Prop is a stock Yamaha 13.5” x 15P aluminum.

All by myself, I can run 27 MPH WOT at 5800 - 5900 RPM when trimmed up to the edge of ventilating.
With 4-8 people aboard we run 23-25 MPH @ 5600 to 5900 RPM depending upon weight distribution.
With 4 people aboard and towing two adults in a couch style tube – 23 to 25 MPH @ 5600 to 5800 RPM

The only way to see increased efficiency (lower slip %) is to lower the RPM and/or raise the speed. I don’t think that this boat is going go any faster without lots more HP to lift the front of the tubes out of the water. The motor is currently operating nicely in the specified range. “If it ain’t broke, don’t fix it?”

What am I missing?

Marc
Big and ugly but we love our 1999/2000 Crest II DL with a Yamaha F115

MattGent
Posts: 62
Joined: Wed Apr 12, 2006 10:27 am
Location: South FLA

Re: Why should I care about prop slip?

#2 Post by MattGent » Mon Aug 03, 2020 10:50 am

I don’t think you’re missing anything.

Prop slip and efficiency are not exactly the same thing. Pontoons, especially ones without planing aids, will run at high slip. Your performance sounds just fine.

You may be able to bring the slip number down with more blade area, but then need to adjust pitch to get back to your peak RPM. I think you would get a little better performance with a stainless prop, just due to the blade stiffness.

Your performance seems pretty well in line with the Yamaha performance bulletins for 115 pontoons.
https://yamahaoutboards.com/en-us/home/ ... -bulletins

User avatar
Marc K
Posts: 742
Joined: Fri Jun 01, 2018 4:35 am
Location: Eustace, TX

Re: Why should I care about prop slip?

#3 Post by Marc K » Mon Aug 03, 2020 12:25 pm

Matt - thanks for that link! That is a whole bunch of info in one place.

I'll likely stick to exactly the prop size and pitch that I have but look at SS per your suggestion.
Big and ugly but we love our 1999/2000 Crest II DL with a Yamaha F115

User avatar
steve1313
Posts: 679
Joined: Tue Aug 22, 2017 6:33 am

Re: Why should I care about prop slip?

#4 Post by steve1313 » Mon Aug 03, 2020 12:41 pm

Mark-
The simple answer to your question is you shouldn't care. Why? You answered that in your post............ "My boat does what I think that it should"

If you're happy with how it's performing, than don't worry about it. Sometimes we all tend to worry about things instead of just enjoying them! Now stop thinking about it and go get on the water! :)
Steve
"Serendipity" - 2017 Sweetwater 2286 WB Tritoon w/ 150HP Yamaha
Slipped at Lake Anna, Virginia
Primary Residence: Richmond, Virgina
Lake House: Lake Anna, Virginia

User avatar
wwind3
Posts: 1515
Joined: Tue Jan 13, 2009 10:03 am
Location: Lake Livingston Tx

Re: Why should I care about prop slip?

#5 Post by wwind3 » Mon Aug 03, 2020 12:53 pm

Sounds like a pretty good setup... I do have a question for the class tho' :biggrin2

My aluminum prop is 1-2 mph faster than my SS with same pitch-- 16".. Pretty sure diameters are same too.

Can't 'splain that.... anyone? Anyone? Bueller? Bueller??

Mine ran 26 yesterday with just me. 27-28 last couple times I had the alum on when SS was getting rehubbed. Both 3-blade Merc props on 90 Merc Optimax 90.
Boat is 20 ft Lowe with 4 fishing seats and 25" tubes.
2009 LOWE SS204 25in logs
90 Merc Optimax
Motorguide W55 Wireless
Underskinning
28 GPS

MattGent
Posts: 62
Joined: Wed Apr 12, 2006 10:27 am
Location: South FLA

Re: Why should I care about prop slip?

#6 Post by MattGent » Mon Aug 03, 2020 1:56 pm

It’s not always 1 for 1 going aluminum to stainless. Blades can be thinner, often design isn’t exactly the same, and the more stiff blade...may need to drop one pitch.

User avatar
wwind3
Posts: 1515
Joined: Tue Jan 13, 2009 10:03 am
Location: Lake Livingston Tx

Re: Why should I care about prop slip?

#7 Post by wwind3 » Tue Aug 04, 2020 3:32 am

MattGent wrote:
Mon Aug 03, 2020 1:56 pm
It’s not always 1 for 1 going aluminum to stainless. Blades can be thinner, often design isn’t exactly the same, and the more stiff blade...may need to drop one pitch.
Cool--thanx. I did increase my pitch on my SS from 16 to 17p as the 16 puts me right at the alarm overrev rpms of about 5800+ Lost almost 400 rpm and some speed as I recollect.. Havent reduced from 16 to 15 as I am at max rpms with the 16 and OK with speed, no blowout etc.. Trims out well w/o ventilating etc.

True story.. when rig was new I was fishing Sam Rayburn in Texas out of Powell Park Marina. Coming back in I was in a headwind and boat was running 29--which I attributed to getting lift from the wind and the underskinning. Then I hit one area where the speed hit 30. Only time that has happened. Only lasted a few seconds. Then dropped to 29. Early spring and cold water helped too I'm sure. Water more dense.

Went back over the area later and discovered I had run over a spot where the water depth changed abruptly-got shallower.. I am sure now it was a ground effect ---boat compressing water down and pushing bow of boat up--pretty cool.

Happened once before in late 60's--going down Intercoastal Canal east of New Orleans duck hunting. Old 50's model runabout boat with probably a 50 at most motor. 4 guys and gear. If we were close to the bank boat would get on plane and make pretty good time. Drop off into deeper water and it would fall off plane & speed would drop off significantly...
2009 LOWE SS204 25in logs
90 Merc Optimax
Motorguide W55 Wireless
Underskinning
28 GPS

Post Reply