Propping for skiing

[kswiatk]

Looking for options on how to improve acceleration in order to pull a slalom skier and barefooter.

Boat is a 16 ft Baja Sport with a closed bow, and 1985 merc inline 6 115 on the back, with a usual load of 4 people.

With a 19 pitch 3 blade aluminum, turns 5500 at 48 mph gps. - 1 person
With a 17 pitch 3 blade aluminum, turns 5800 at 44 mph gps - 4 people

The top speed is fine I am just very disappointed with acceleration to pull up a skier, takes too long to get to on plane. When full throttle is applied from a full stop, sits at about 2k- 2.5k rpm for a few seconds before planning out and allowing rpms up.

Is my best option to go with a lower pitch and just be more careful with not exceeding max rpm.
Or is it best to experiment with vent holes on the prop to allow the motor to spin up a bit more?

 

[Starcraft5834]

4 blade prop is your answer........................those rpms say outboard to me..........Michigan Match makes a 16p 4 blade that might get you in the same 5500 rpm range and will profoundly improve your hole shot....

 

[Scott Danforth]

Is my best option to go with a lower pitch and just be more careful with not exceeding max rpm.

Yes

 

[jimmbo]

My 15 1/2 crestliner(glass boat) with an 84 115, using a 19” prop(5400 rpm) used to pull slalom skiers up with 3-4 people in boat. Even pulled a person barefoot, out of the water without skis.
Try enlarging the vents a bit, if you go too big, a bit of epoxy to fill, then redrill smaller

 

[Chris1956]

I would think the 17P prop would work OK. Try not to load the boat with people as well as a skier.

I used a 21P prop on my 16 footer w/ an IL6, but it is likely flatter bottomed that your Baja.

I don't think you want a 4 blade prop, on that sport boat.

 

[ahicks]

I can tell you that unless things have changed drastically, 115hp is pretty marginal power for slalom skiing, and way too small for anything but a flyweight bare footer.

A 4 blade prop will help when pulling, but it's going to cost you some speed for sure. Need to decide on a priority, or plan on running different props for different types of play.

And I agree with Chris. Unload that boat the best you possibly can when skiing.

 

[kswiatk]

Thanks for the responses!
I think for now Ill try just keeping to keep the boat unloaded.. Beyond that I may try a 4 blade if a good deal comes along. I figured there is only so much that old motor can have.
I already plan on swapping out props as needed since I trailer the boat anyways.

 

[kswiatk]

Bit of an update
Tried pulling a single slalom skier up with just the driver in the boat and the engine lugs and will not pull the skier up.

I don't know whether this is normal for this motor or whether this is a sign of running issues.. I opened up the fuel screws half a turn and it did not make a difference. It still gets to full rpm.

 

[Chris1956]

Unless you are pulling a big guy who is fighting the boat, I would expect a 17P prop to get him up, on that boat.

You might check the spark plug color to see if all are firing. Compression is good?

 

[kswiatk]

Compression was all good as of 3 outings ago.This motor is rebuilt for this season. Just replaced both switchboxes, trigger, and one sticking needle and seat to clear up some running issues.
I will check plug color today.

 

[wahlejim]

Where is your trim set? I am assuming you have a closed bow and your passengers are sitting at the stern based on the Baja Sports that I have seen. Try setting your trim down more. If you have power trim, easy to do. If you have the pin system, not as easy, but not terribly difficult. Also, do you have access to the bow? Get some weight up there. A cooler full of stuff, sand bags, something.

I grew up skiing behind a tri-hull with a 115 Johnson hanging off of it. It had plenty of power to pull a slalom u out of the water. I am not a small guy either, over 6 feet and over 200 lbs. It did take some work to balance the weight in the boat and trim properly, but we got there and it was smooth skiing when we figured it out. We did keep a separate prop for skiing and cruising and kept our RPMs in check. It just takes some tinkering.

 

[wahlejim]

And I should add that the 115 would pull up a slalom skier with 9 people on board. I have a large family and that motor was a workhorse for 30 years before the hull cracked on us. 115hp is not your problem as previously mentioned. It is trim, prop, and weight distribution.

 

[kswiatk]

Thats a good thought. It is a Baja with a closed bow that is accessible. I keep some spare fuel, anchor or whatever up there.

I do have power trim and I keep it tucked all the way under. However now that I think about it I have the limiting pin set 2nd from the last. I will move it to the last pin if possible to allow it to tuck under more.

 

[jimmbo]

If it is an 1985, you probably won't be able to install the pin in the hole closest to the Transom, just leave it out. An 84 and newer had the the 2nd style trim system with one tilt cylinder and 2 rams for trim, The 1st generation of Power Trim used 2 rams outside the swivel/transom bracket, and had the pump inside the boat. I've seen those run without the Trim Pin as well. It was last used on the inline 6 in 1983

 

[kswiatk]

If it is an 1985, you probably won't be able to install the pin in the hole closest to the Transom, just leave it out. An 84 and newer had the the 2nd style trim system with one tilt cylinder and 2 rams for trim, The 1st generation of Power Trim used 2 rams outside the swivel/transom bracket, and had the pump inside the boat. I've seen those run without the Trim Pin as well. It was last used on the inline 6 in 1983

Mine is an 83 gearcase and mid/transom with a 85 powerhead, so it has an onboard pump. Ill have to give it a try tomorrow. Would be nice to be something so simple.

 

[kswiatk]

Changing pins made no difference, still lugs.
Squeezing primer bulb no difference.

I did notice that around 1500 rpm under mod the stator only produced 160 volts, which is below the spec of 180.
i will probably change the stator to my spare just to see.
if no change I will drill vent holes.

anything else I should try?

 

[jimmbo]

Is the timing set to spec? Drilling the prop is an option, but it must be done with care with regard to location and size.

 

[Texasmark]

Looking for options on how to improve acceleration in order to pull a slalom skier and barefooter.

Boat is a 16 ft Baja Sport with a closed bow, and 1985 merc inline 6 115 on the back, with a usual load of 4 people.

With a 19 pitch 3 blade aluminum, turns 5500 at 48 mph gps. - 1 person
With a 17 pitch 3 blade aluminum, turns 5800 at 44 mph gps - 4 people

The top speed is fine I am just very disappointed with acceleration to pull up a skier, takes too long to get to on plane. When full throttle is applied from a full stop, sits at about 2k- 2.5k rpm for a few seconds before planning out and allowing rpms up.

Is my best option to go with a lower pitch and just be more careful with not exceeding max rpm.
Or is it best to experiment with vent holes on the prop to allow the motor to spin up a bit more?

I was running that engine on a 17' Ranger (brand) Pad hull and running 48-9 (never could get it to 50) and 5500-6000 rpm with me. I was running a Laser SS propeller (Laser I essentially since the Laser II has since been produced). On several occasions my lightweight son and I would go water skiiing.

With me running around 230 at the time and slalom ski, he didn't have a problem getting me up nor running me 35 for some fun. What made is work for top end and hole shot was the ports. They allowed exhaust gasses to pass over the blades, reducing the density of the water allowing the engine to rev very fast even though it was under a heavy load and get me up fast. Once at speed, the ports closed off, the prop locked into the thrust you would get from an unported prop and away we went. Essentially I had 2 props in one.

Otherwise you have to get a 4 blade of less pitch which would give you your hole shot but your top end would really suck and you would be disappointed having to keep the throttle (and speed) cut back to control your rpms.

4 people and a slalom is a load on that engine and barefoot, requiring 35 mph for it to work right, forget it!

 

[Texasmark]

Is the timing set to spec? Drilling the prop is an option, but it must be done with care with regard to location and size.

I added 5/16" ports to my 13 ¼ x 24 Ballistic SS on the boat in my Avatar. SS on a round barrel, even on a drill press where you can control the angle is a challenge. Lots of pressure, slow speed and lots of cutting oil. Expect to consume a couple of bits.....aluminum, no biggie. A fixture (home grown) to hold the prop is a must.

Start small if you choose to go that route, like the ¼" in jimmbo's cartoon (not a funny book, the name illustrators give to illustrations). Increase till you get where you want to be....if that's possible. If you get too large and rpms shoot up too high in the hole shot or it takes too long to lock up at speed, if it ever does, Tap the holes with a coarse thread tap (helps to hold the epoxy where you put it), fill with JB weld, and after it has had a week or so to cure, start over or leave them out and get another prop.

 

[Chris1956]

I doubt drilling some vent holes in the prop is going to solve your problem. That motor (if healthy) with a 17P prop should pull up a slalom skier, with the driver and observer in the boat. It will take several seconds to get him up, but the issue should not be in doubt.

I suspect the boat is waterlogged, or more likely, the motor is running on 5 cylinders. They run pretty good on 5. Dis you ever check the spark plug color? If any spark plugs look real clean,it is a sign of not firing due to water infiltration or something else.

Just so you know, barefoot skiing requires a lot of speed. My buddy would wear a pair of sneakers, and get up on a slalom ski. I would pull him up to mid-late 40s and he would drop the ski. I did that with my Merc 1500 IL6, on a 16 footer, with a standard 21P prop. I once pulled a skier at full throttle (about 55MPH), but that is a different tale.