I have 2 props and neither are ideal... Why?!?

[GoldLS1]

Here goes... I have a 16.5 foot alum flat bottom, 87 85hp force, few hundred pounds of gear. It had a 13x15 SS prop on it when I bought it. It ran wot at 4500 rpm. Seemed odd to me, so I found a 13x17 SS and now the rpms are real high no matter trim and the motor doesnt sound like it turning near the displayed rpm. Rpms are 6k with plenty more throttle to go. A blip of the throttle showed 8k+ I was running around 35mph at 5500 with the 17 prop trim down all the way. The 15 prop has a round back side of the blade, the 17 is straight.

Any ideas?

 

[jspansel]

Re: I have 2 props and neither are ideal... Why?!?

Could the 17 be slipping?

 

[Bondo]

Re: I have 2 props and neither are ideal... Why?!?

Here goes... I have a 16.5 foot alum flat bottom, 87 85hp force, few hundred pounds of gear. It had a 13x15 SS prop on it when I bought it. It ran wot at 4500 rpm. Seemed odd to me, so I found a 13x17 SS and now the rpms are real high no matter trim and the motor doesnt sound like it turning near the displayed rpm. Rpms are 6k with plenty more throttle to go. A blip of the throttle showed 8k+ I was running around 35mph at 5500 with the 17 prop trim down all the way. The 15 prop has a round back side of the blade, the 17 is straight.

Any ideas?

Ayuh,.... Something is Very Wrong,...

Goin' to the 17" pitch, Shoulda lowered the rpms...

 

[GoldLS1]

Re: I have 2 props and neither are ideal... Why?!?

Im new to boats... Had seadoos for years. I'm not sure of what the differ prop issues feel like?

I can feel the 17 prop do something when the motor is trimmed up. Feels normal when trimmed down minus the high rpms (didn't sound like it was running that high tho).

 

[GoldLS1]

Re: I have 2 props and neither are ideal... Why?!?

Here are the two props:

View attachment 122602

They look quite different. But I have no idea if the blade shapes matter much. The left one (not on the boat) is dinged around the edges. The one on the boat (right one in pic) is in pretty good shape with only 1 slight ding mid blade.

Not sure if this matters, but the bottom fin on the motor is broken off... again I have no idea of what the piece is actually called or if it serves a purpose besides protecting the prop blades from hitting the bottom first?

Thanks for any and all help!!

 

[steelespike]

Re: I have 2 props and neither are ideal... Why?!?

I would say your tach is way off (high).it may be set wrong.Check all connections,operate the switc on the back to clean the contacts,
Prop calculator says 5500 rpm,2.00 gear ratio,17" prop at 12% slip should be about 39 mph.
So your 35 isn't unreasonable. Is your speed by gps?
What is the weight of your bare boat?Is it a bare bones boat?No floor or built ins?total weight?Depending on the weight a 17" prop is reasonable.

 

[Lowe170]

Re: I have 2 props and neither are ideal... Why?!?

If this helps for comparison, my 16.5 ft aluminum mod v hull bass boat (2000 Lowe 170) with a Johnson 70hp runs much higher rpms at wot (like 6K+, so I don't run it wide it open) with a 13.25x15 prop vs the 14x17 that I had on it before it met a submerged tree it didn't like. Both are aluminum props.

 

[GoldLS1]

Re: I have 2 props and neither are ideal... Why?!?

Tach is good. It is new, on right pole, and wired in this summer. Idle reads good in the water and on muffs (800-900 and 1100-1200).

I went through the calcs too... I'd expect the 5500RPM with the 17 would be close to max throttle and it isn't even close. There is still 2500RPM left?

Weight: has floor, (2) big batteries, and (2) 12 gal tanks usually w/ <12 gals combined.

Boat pic:
View attachment 122611

Lowe I figured the 15 I had would run 6k plus also. I'm shocked it doesn't. I was really shocked when the 17 did!! The motor will take full throttle w/ the 15 and not overrev. With the 17 it doesn't sound over reved, but the gauge says it is.

I'm completely lost!

 

[steelespike]

Re: I have 2 props and neither are ideal... Why?!?

Taking the calculator another way 35 at 5500 indicates 20% slip,very high. What is the speed with the 15 at what rpm? Where is the antivent plate (just above the prop) in relation to the bottom of the boat Should be even with or slightly above the bottom.
Are the props cupped?Is your speed by gps? A speedo could be off 10% either way.
What is the speed when you take it to 6,000 with the 17" prop?

 

[GoldLS1]

Re: I have 2 props and neither are ideal... Why?!?

I don't know the exact mph with the 15...

Antivent is jump up from the bottom of the boat.

I don't think they are cupped.

I dont know the speed at 6k.

 

[steelespike]

Re: I have 2 props and neither are ideal... Why?!?

What does "Jump up from the bottom of the boat" mean?
Can we get the wot speed and rpm with the 15"?
and at 6000 with the 17?

 

[GoldLS1]

Re: I have 2 props and neither are ideal... Why?!?

Haha stupid auto correct!! It should read just up from the bottom of the boat.

I put the 15 back on today and can hopefully get a wot mph and rpm tomorrow pending the wind. I won't be able to get the 17 numbers until Thurs or Fri.