Looking to Increase Top Speed

[fisherman67]

I have a 1997 18' Kenner with a 1996 115hp Johnson with a 13 7/8" diameter 17 pitch aluminum propeller. With a full tank of gas (30 gals) 2 average size people and fishing gear it cruises at 30mph and 4800rpm. If I were to install a 19 pitch stainless steel prop, what kind of mph increase should I expect? I do notice that if I throttle too quickly from a dead stop the engine revs high as if it were cavitating also does it when I am cruising and make a turn semi sharply.

 

[Bifflefan]

Re: Looking to Increase Top Speed

Just the basic's, to go from 17p to 19p will gain you about 2 mph or drop your rpm about 200.
There are a ton of factors that go with that. First, make sure you have a prop in top shape, then make sure the prop is deep enough, but not to deep in the water. As for a SS prop vs alum, I would not waste your money. You will not see any difference with 115 HP.
You can go with a bigger motor, but you are talking 50 HP just to make a difference that you can see. Maybe 5 to 10 mph.
The best choice is;
If you want a faster boat, buy a faster boat.

 

[V153]

Re: Looking to Increase Top Speed

What are your rpms @ wot?

 

[fisherman67]

Re: Looking to Increase Top Speed

About 5200-5300rpm and 33-35mph.

 

[steelespike]

Re: Looking to Increase Top Speed

Is the Kenner a deep V? I believe the dry weight is 1100 lbs. Your present slip is about 18%.
Assuming your tach is accurate and speed is by gps. Your performance numbers seem normal but for the slip.
And the slip may account for your occasional ventilation problems.
Its not unusual to trim down for turns and hole shot.
High normal slip is about 12%,depending on the hull style and total weight.
I feel that a 17" is an ideal prop for your setup.
With props nothing is carved in stone but:: If you go to a 19" prop your max rpm could be about 4900 your hole shot may be slower
and it may struggle more with added weight.
A 19" prop ,assuming 12% slip, could produce about 38.8 mph.The Solas Amita prop has a excellent record for good predictable performance and should improve your slip numbers.
Now if you got a new Solas 17" prop you should maintain good performance with more normal rpm and slip.
At 12% slip 5300 rpm would produce about 37.5 mph.
Before any changes you may want to verify your tach against a shop tach or other source
and get a gps speed.

 

[jestor68]

Re: Looking to Increase Top Speed

My choice for a better prop for your boat would be the BRP Viper at 17" pitch( 13-7/8 X 17 ).

While the top speed increase might be moderate, that prop would reduce(or eliminate) your ventilation issues and provide better all around performance.

While the degree of improvement varied, I have not seen a case where a SS prop did not out perform a aluminum prop.

 

[steelespike]

Re: Looking to Increase Top Speed

Do you think the Solas Amita or Rubex would be a reasonable choice if they can't afford a ss prop at this point?

 

[Slip Away]

Re: Looking to Increase Top Speed

I Think Mi Wheel Vortex or Michigan Match would be a great choice if they can't afford SS.

 

[fisherman67]

Re: Looking to Increase Top Speed

I also forgot to mention that the motor has a dolphin/whale tail fin on the cavitation plate, don't really know if these help or hurt performance.

 

[jestor68]

Re: Looking to Increase Top Speed

I also forgot to mention that the motor has a dolphin/whale tail fin on the cavitation plate, don't really know if these help or hurt performance.

The only way to compare performance is to run with and without the fin. Is it a real Doel Fin?

I guess I missed the part where you couldn't afford a SS prop. In that case, the next best thing would be a Solas aluminum prop of the same pitch(17). Since you're only getting 52K-53K WOT rpm now, increasing pitch 2 inches will likely pull you down below 5K, which would hurt power out put.

Is your motor a 60 degree or 90 degree design. The 60 degree loopers like to run faster rpm.

 

[V153]

Re: Looking to Increase Top Speed

Please post a pic of your set-up.

 

[steelespike]

Re: Looking to Increase Top Speed

The only way to compare performance is to run with and without the fin. Is it a real Doel Fin? View attachment 220465

I guess I missed the part where you couldn't afford a SS prop. In that case, the next best thing would be a Solas aluminum prop of the same pitch(17). Since you're only getting 52K-53K WOT rpm now, increasing pitch 2 inches will likely pull you down below 5K, which would hurt power out put.

Is your motor a 60 degree or 90 degree design. The 60 degree loopers like to run faster rpm.

There wasn't any discussion about cost.
I was only concerned that if he chose not to do ss that a Solas prop would be satisfactory.

 

[dazk14]

Re: Looking to Increase Top Speed

There wasn't any discussion about cost.
I was only concerned that if he chose not to do ss that a Solas prop would be satisfactory.

+1. Amita is a great value and excellent performance, if that's a priority.

That is a common size, you may find a used SS deal on ebay.