Help with prop!

[Slummy]

I have a 08 bass tracker 175 pro with a yamaha 70 2 stroke bored 20 over. It has a 13 1/4 x 17p ss prop on it now. Full loaded boat I'm getting 5200rpm with 34mph gps. I've been told to move to a 13 1/2 x 15p would get my boat performing better. My dealer does not have props for me to try so I would like to get it right on the first order. Any help would be greatly appreciated! And suggestions on solas prop would be helpful to....thanks!

 

[SkiDad]

I think your current prop is about right. Unless you want more take off power I think the 15 will slow you down.

 

[Slummy]

What way should i go if I wanted a little more top end? Would a 4 blade do more harm then good?

 

[fhhuber]

It may be counter-intuitive, but sometimes reducing pitch SOMETIMES gives more speed.

Its thrust vs drag. More blade area (13 1/2 > 13 1/4) to provide more "bite" essentially pushing a larger column of water back.

Prop RPM * pitch / 1050 = mph the prop is TRYING to drive the boat. Above a certain % slippage, all you are doing is trying to turn the water into whipped cream.

Not sure what your gear ratio is... Close to 2:1 is typical. At 5200 engine rpm I'll assume your prop is doing close to 2600 rpm.
2600 * 17 / 1050= 42.09....
You are showing 34/42 = 80% of what the prop is trying to do or 20% slippage. (if I guessed the right gear ratio... but its NEVER exactly 2:1, so I didn't.)

http://www.mercuryracing.com/prop-slip-calculator/ (They aren't dropping a few digits on the conversion as I did above)

Reduce the slippage and give the prop a bigger bite and you MIGHT go faster. Trading 2 inches pitch for the larger blades of 1/4 inch more diameter could be doing just that.

PLUS the engine could see lower load and turn higher rpm...

You'll never get rid of slippage.
The only way to find minimum slip and max speed for your boat is... try the different props on the lake.

4 blades may or may not help... More blades is a "trick" for packing more blade area into the same diameter. Generally good for hole shot but not so good for top speed.

 

[Slummy]

Thanks for explaining the slippage part...makes more sense to me now. I don't have a boat shop around me with test props so I wanted to get it right the first time. Shelf full of props can get exspensive!

 

[Maclin]

Do you know the RPM range recommended for that engine? The 5,200 might be a little low, just depends on the lower number from the MFG recommended range.

If is is on the low side of the range, or even under, then getting the engine to be able to rev to the top end of the range with typical loading will get you better hole shot with about the same top speed in my opinion. If it is lower than the the "low" top range then you may even see a slight increase in the top speed...

 

[Slummy]

The dealer that rebuilt the motor told me to run it at 6000 rpm max.

 

[fhhuber]

You have pointed out something that SCREAMS change the prop.

Recommended peak 6000 rpm and you are maxing at 5200. You are overloading the engine and not hitting its peak power.

 

[Slummy]

I think I'll try going to a 15p and see what that does for me. I was figuring 5800rpm with the 15p

 

[SkiDad]

you need to take into account the gear ratio - i think you have a 2.33. That is 94% efficient and you can't do much better than that. I might be wrong but it my experience my boat turns a 17p AL at 5300 and 38 mph (10% slip) but turns a 15p AL 5800 and 36.5 (11% slip) - i have a different gear ratio though. i run a stainless that gets 7% slip

I might be wrong on your results, so it will be interesting to see what happens.

 

[Slummy]

How do I figure out my gear ratio?

 

[fhhuber]

You look it up.

Or you rotate the engine (spark plugs out and ignition off, in gear) 10, 20, 50 or 100 (more is more accurate) turns and count turns of the prop.
Helps to put a tape marker on the flywheel and one on the prop.

Engine turns / prop turns = ratio

 

[Slummy]

Yes your right I looked it up I have 2.33 so I guess I'm not going to get anything else out of it. Maybe change the lower unit to get a taller gear.

 

[SkiDad]

does your boat take off fast ? If so, you might just need to raise the motor up a hole or 2 to get your rpm up ? that might get you a few mph.

are you after more speed - you really didn't say. You could try a 17p Hustler AL prop - it might get your 2 mph as well - it did when I tried on my boat but i didn't keep it b/c it wasn't my favorite prop for skiing.

seems like you have another few mph hidden somewhere unless your engine just isn't running strong.

 

[Slummy]

It takes about 5-6 sec to get out of the hole. Not real happy with that. It also has a jack plate on it but I have no clue when it comes to that. I wanted to hit a topend of 40mph with decent holeshot. I thought for sure it wouldn't be a prob with the bored out 70 but I wasnt think out the low gearing.

 

[SkiDad]

is your stainless prop vented ? if not I think the 17p turning point might help you - just a guess though - or maybe try to get some help from a local fisherman that is willing to help with your setup. Have you checked your compression per cylinder ?

you could also check with Ken at propgods.com - he could help with a different style prop