Prop for wakeboarding w/outboard

[tjatech]

Ok - quick question for ya'll. Probably a common one but here goes:

1) Performance issue you are trying to correct.

Current prop is sized right for the boat. However, I have started to do a lot of wake boarding, and would like to give up some top end speed for a little better hole shot.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.

Don't know manufacturer, aluminum, 3 blade (I'm guessing the original from 1974)

3) Current prop diameter and pitch (required).

13 and 3/4, 15" pitch.

4) Wide open throttle RPM and speed with an average load (very helpful)

WOT 5500 RPM, about 35 MPH

5) Engine/drive make, model, year, and HP

1974 Johnson 115 outboard.

6) Boat make model, year, length and weight

1974 Reinell T900 (Tri-Hull), 18' 6", guessing the weight is around 2000 lbs dry.


How much pitch change would be recommended to give up ~5 mph top end speed for a little better hole shot?

 

[MikDee]

I have read that a Mercury Spitfire 4bld in a matching 15" pitch to your prop, would give you more power, & hole shot, then a 3 blade, plus the most speed of any 4 blade. Check it out!.

 

[SkiDad]

I'm surprised you have much trouble with a 15 pitch on that engine but you could try a 13 pitch 4 blade possibly.

 

[tjatech]

Thanks for the input - I'll look into a 4 blade and report back.

To your point, I may just be expecting a little too much here - I don't really have trouble pulling people up, just takes longer to plane than the "nice" boats I've wakeboarded behind. May just be a function of the style of boat though.

 

[SkiDad]

To your point, I may just be expecting a little too much here - I don't really have trouble pulling people up, just takes longer to plane than the "nice" boats I've wakeboarded behind. May just be a function of the style of boat though.

i feel you on this one - i got spoiled behind a Ski Nautique Air and you are up almost instantly.

 

[Texasmark]

In 1974 OMC was powerhead/break hp rated. You can take 10-15% off that 115 for prop hp (like engines are rated today)plus a few more for age wear and tear.

I had an 18' Caravelle tri-hull which was a heavy boat with a fairly large center hull; might call it a mild trihedral hull. Had some dead rise at the stern. I had a 1972 125 Johnson on it and did a lot of skiing. I had a 17P 3 blade OMC SST prop. I had no tach but you could tell the engine was happy. I could pull 2 kids on slaloms with wife and 2 more in the boat or they could pull me up reasonably fast on slalom with all 5 of them in the boat. With me alone in the boat 39 was tops. Had no power trim which could have made things a lot nicer....didn't offer it back then. It had a lot of ballast in the stern with a stern mounted 29 gallon tank and jump seats adjacent....where an I/O engine would normally sit. This didn't help the hole shot at all, but still it was not that bad. (Posted as reference for comments)

The prop you have is, as you said, right for your rig. Adding a 4th blade with the same pitch will lug somewhat over what you have as 4s aren't as efficient as 3s although they give you better hole thrust and better cruising control on boat attitude.

The BAM prop slip calculator from go-fast.com says that if you dropped pitch to 13 and still maintained the 5500 rpm you would drop 5 mph in WOT no slip speed and would probably be a wise choice in 4 blades. You definitely will have a much better hole shot and your engine will love you while wake boarding and all since having the hull in the water and plowing through it just below planing speed is a high load situation and 2 strokes don't like to lug. Just have to live with it at WOT (but you said that was ok) and keep the throttle cut back to maintain your 5500 max rpms.

Oh and on the comparison to the Ski Nautique, as stated by mitsysman.....................................

Mark