Upping pitch on Yamaha 115 OB

[SimpleMan]

I'm looking at a 13 x21 prop as a replacement for my stock 1988 Yamaha stainless 13x19-K. We proped down to 13x19 when the boat was purchased in '89 to get better hole shot for skiing but we aren't doing much skiing now. So the reason for changing the prop is just to improve top end speed and efficiency (at the expense of hole shot). I have given some boat specifics at the bottom of this post.

Any first impression recomendations?

My questions ( I know some have been asked before but I'm looking for a prop for my boat and the answers always seem to "depend on the boat and the desired results": That being the case, here we go:

How much impact will a little bit of extra diameter have if the prop is not available in exactly the same size? what effect on hole shot? effect on top speed?

How does the MW Apollo prop compare to the MW Rapture and Quicksilver Lightspeed (which doesn't come in a 21) and the Laser II? Others that should be considered?

Do people normally have to go down an inch when switching to the Lightspeed?

I'm thinking that the vented Lightspeed prop might be nice as I'm going to be losing out in the hole shot with the increase pitch. Maybe a drilled hole in the Apollo would help?

Is the prop vent a gimmick or will it improve the hole shot for my 115 2-stroke?

Is the current Rapture XHS vented?

Are there disadvantages to the vent?

Any idea how these props compare to the 13x21 stock stainless Yamaha props like the 13 x 21- K that would have been on the boat originally.

Now to blow the whole thing off in another direction, the local prop shops are all pointing to aluminum. Is there any hard quantitative proof of the improvement that can be expected from stainless?

This prop stuff is way too mysterious for me. Thanks for helping me wade through.


Currently, on a flat cool day with the 13x19 on our '88 115hp outboard I can get 5500 RPM at WOT at 40 MPH (speedo) and a medium boat load. Recommended range 4500-5500 rpm. This is the top end of the recommended rpm range and just at the redline of the motor. The boat is a 19' Spectre, which is a Yamaha endorsed 4 seater basic runabout with a slight tri hull. The specs claim it weighs 1250 lb+ 200lb fuel +350 lb engine + 200 lb stuff + 400 lb people = 2400 lb.

I seem to remember the boat doing about 45 before we propped down.

 

[Dhadley]

Re: Upping pitch on Yamaha 115 OB

The numbers don't quite match with th 19 but that could be a combo of things. The speedometer is proabably a bit "happy". And the tach might not be reading out the whole story. If I remember correctly the Yamaha tach only reads out the first 2 numbers (55 at 5500) so you could be at 5590 and still only show 55(00).

Anyway you sure don't want to drop any more rpm. 5500 with an average load on todays fuel is about as much load as you want on it for a long life. If it were mine I'd play with the set up to see what could be gained with the 19. Then, if things work out, go to a 21 stainless. A 21 Stiletto probably wouldn't be a bad direction.

 

[SimpleMan]

Re: Upping pitch on Yamaha 115 OB

Yes, you could be right about the tach and speedo beign off. The tach is an analogue gauge so who knows on accuracy. What sort of "setup" things might a person play with? (There are hydrofoil fins already on the AV plate).

 

[steelespike]

Re: Upping pitch on Yamaha 115 OB

I think I would start with removing the hydrofoil.Try something like smart tabs.Be sure the motor is in tip top tune.Check the verticle height start with the antivent plate even with the bottom raise the motor a hole at a time till just before venting becomes a problem.Be sure it pumps water.
In the process you might try to check the tach. and borrow gps.

 

[SimpleMan]

Re: Upping pitch on Yamaha 115 OB

I'll try the height suggestion and removing the hydrofoils. I visited the local prop shop on Thursday and I let them sell me a Korean made aluminum 12-3/4 x 21 Yamaha knockoff prop. "Almost as good as SS" "You'll never know the difference". I figured these guys work with props all day every day, they should know a thing or two. Right?

Hmmf. No change from the exisiting 13 x 19. Nada. So what is the point? We'll see if he'll take it back on Monday.

 

[Dhadley]

Re: Upping pitch on Yamaha 115 OB

If you're willing to work with the set up, mainly height (X dimension) I still think you'll end up with something like the 21 Stiletto. Especially if your "experts" don't want to take the prop back. Might not hurt to look into the Iboats exchange program.

 

[SimpleMan]

Re: Upping pitch on Yamaha 115 OB

Sorry, X dimension? Is this just height from CL of prop to water line while running?

Also, what factors make you lean toward the Stiletto's?

Iboats seems to have a good program setup for exchanges. Unfortunately, I'm up here on the West Coast of Canada and I don't think it's worth the shipping cost. Nonetheless, thanks for the suggestion.

 

[steelespike]

Re: Upping pitch on Yamaha 115 OB

X demension is the verticle height of the motor.There is no specific measurement,just raise it to the highest point just before venting becomes a problem.Generally the antivent plate(just above the prop)about even with the bottom of the boat is considered a starting point.Allways be sure the motor pumps water when working with the X demension.