Just another prop question

[Vinny L]

Hi all, first post on here and this is also my first powerboat being a sailor all my life so I have a lot to learn about outboard performance.

I would like to know if my prop is matched well to my application.
Here are the specs.
1) Performance issue: pontoon seems slow and can’t get above 20mph according to GPS.
2&3) Current prop: Evinrude 13.75 x 13, aluminum 3 blade, model 765181.
4) Wide open throttle 5000 RPM and speed 20mph
5) Johnson 88HP SPL, 1990 model, gear ratio 0.5, WOT rangw 4500 to 5500 RPM
6) 1988 Suntracker 24’ DL Party Barge pontoon. Not sure, about 3 adults and a toddler, cooler and picnic lunch.

From my calculations I have 30% slip at 20mph. I’m sometimes able to get to 22mph, but with the load above it seems like I hit something like a displacement hull speed limit. This is because with an increase in throttle I don’t always get an increase in speed.

Normally I have at least 4 adults and two toddlers and 2 coolers onboard. I’d like higher speed if possible to get around larger lakes quicker, but I would also like to do some mild tubing with an adult or 2 at the back.

Is my prop selection good?
What prop can I get do to get better performance?
Or, am I at the limit of what an 88HP can do in my setup?

Thanks in advance!

 

[flyingscott]

Your set up is running pretty well the way it is. Pontoons live up to the old adage, If you put a big enough motor on a brick you can make it faster.

 

[SeaCrow]

30% slip is excessively high. I've been told 15% is at the max and anything less is good. You may want to talk to someone that really knows props.

 

[Vinny L]

Hi all, so this weekend I ran into some shallow water and damaged the prop a bit. I can feel it’s not properly balanced, and the leading edges are bent and roughened up.

With that in mind she still runs well and still only gets to 21mph - I dont see 24mph anymore after adding a trolling motor with 3 size 27 batteries.

I’m now looking for a new prop, and I don’t quite know what to do since my RPM is in the required range. I would like more speed when loaded, or at least reduce slip. I’m still seeing the effect that I max out on speed after reaching about 80% WOT. The motor sound changes but not RPM and speed.

Should I try lower pitch together with a 4 blade or lower pitch with more surface area like a Hydrus prop?

Also, can anyone please point me to a reputable online or Alabama store that can offer good advice? I’m still only interested in aluminum props while I’m learning and making mistakes, so I doubt I qualify for an exchange program.

Thanks

 

[Texasmark]

I wouldn't be surprised you are having the slip you have with your setup-load. Planing hulls and trim positions can have a tremendous effect on speed and slip. Taking a hull that will run in single digits slip and 50 mph, with the trim alone you can drop the speed down to 40 and pull your rpms down and slip up with it just by putting more hull in the water and increasing drag. I agree, toons are displacement devices vs planing hulls, even with 3 tubes.

Since you were running 5k and increased your load you are down in the lower half of the power band. Need to bring it up. 4 blades push big loads better. Drop 1" for the 4th blade, and 1" for the low rpms putting you at 11" pitch.......

http://boatpropellers.iboats.com/Ev...11.00&chart=18&engine_id=87&cart_id=484099240

 

[Vinny L]

Thanks Texasmark,
So before I spend money on a prop I thought to play around with the setup I have to get the best baseline I can. I moved the motor up to the highest position since it was at the lowest height on the transom. I read somewhere that I could have too much drag if the motor is too deep. I didn’t get any cavitation problems, and the spray plate is now about an inch above the bottom of the motor pod.

With only me on board I got up to 23 almost 24 mph and my RPM was down to 4700 ish. Before the height change I got 21mph under the same conditions. That means my slip has dropped to 20% - great.

So now I’m in a bit of a tough spot, go to a high surface area 14x11 inch prop designed for a toon or a 4 blade 14x11”. The problem is assuming better slip performance I’ll end up at the same max speed. If it means I can pull faster with a tube it would be worth it.

On the other hand I’m about to replace the plywood framed seats with Deckmate seats which should make the boat lighter. Maybe I should wait until then and just get the same prop but in 14x13”, maybe I could get more speed then... will the weight make that much of a difference on a heavy toon with narrow pontoons?

I did hammer the prop back into shape and filed it smoother so the shakes are gone and I can delay the new prop for a bit still.

Vinny

 

[Texasmark]

On pulling anything, with your prop closer to the surface ventilation can become a problem....sucking surface air...follows the axiom as to the path of lease resistance, into the prop wash.

Getting a prop with the trailing edge of the blade "cupped" helps to hold better in those conditions. Accepted theory on cup effect on prop performance is that at the higher RPM ranges is amounts to one inch increase in pitch. So, if you do decide to do something line a Turning Point Hustler aluminum you will normally get the cupping and ports. Ports ventilate the prop at low boat speeds and tend to act like a prop with a couple of inches of less pitch....course if the Ports are problematic, you can just seal them off with epoxy or JB weld.......total confusion I know......gotta run it to know for sure.