1972 100HP V4 Evinrude on old 16' trihull ski boat

[wireready]

I have a 1972 100HP V4 Evinrude with a trihull 16' small glassmater skiboat from the early 70's era. The motor manual recommends any of a half dozen props, for example 13.25x17, 13x19 or 12.75 x 21 etc..

Will lowering the pitch shorten the time it takes the prop to reach maximum RPM from a dead stop?

Currently I have a 12.75 x 21.

I'm trying to achieve one specific improvement- I'm looking for it to reach maximum RPM under load from a dead start quicker than it currently does. - I'm not really concerned with top speed (it already goes a little faster than the people I'm pulling prefer). Not sure whether I should bump one pitch lower or jump several steps lower in pitch, and I don't want to buy more than one more prop to try this out.


When I start off pulling a ski-er or if I'm pulling a tube. at WOT the motor eventually hits around 5800 RPM with 2 people on board plus a person on skis. So I'm fine at the top end, and I'm satisfied with my top speed of around 30MPH depending on load. However when I throw the throttle fully forward pretty much all at once, RPM climbs very quickly to around 4000, and then takes a good 30 or so seconds to get up to full RPM if I'm pulling a someone on skis or a tube - tube is worse. I haven't timed it exactly but it really takes a while for the motor to reach maximum RPM between the 4000 and 5500 range. Does this mean my pitch is too high for this type of boating. Since we are using this ski boat as a ski boat, I'm looking to put the best prop on it. As it slowly speeds up between 4000 and 5500, the boat first pitches pretty high as it's coming up out of the water, then it slowly levels off a bit as the motor hits around 5000 and then as the boat pitch levels slightly the motor slowly gets up to 5500 and depending on water conditions we might get as high as 5800.

Is this normal, or would dropping the pitch with one of the recommended prop sizes for this motor make a difference at the upper range as I'm describing- for example the 13x19 or the 13.25 x 17, 13.5 x 15 etc..

I'm assuming that all of the recommended prop sizes in the manual are designed to let the motor get to around 5500 without exceeding it much- but perhaps the lower pitches help the motor get to the maximum RPM when under load faster? Or is it the opposite.. I don't want to change the pitch if it won't help me do what I"m wanting.

With larger adults, it's hard for beginner waterskiers to get up on plane because the boat doesn't accelerate smoothly as it's transitioning up on plane.. it gets up going quick and then between 4000 and 5500 RPM the person on ski's perceives (complains) as if I'm slowing down, but I'm not- it's just leveling off on the acceleration as the boat's attitude is shifting while waiting for the motor's RPM to fully move in the 4000 to 5500 range.

My motor is working great, no sputters, or struggles.. recently serviced.. etc.

The boat is an old 70's era fiberglass trihull skiboat, 16' long and probably less than 6 feet wide and very low draft.. it's load max capacity with people and all gear is rated at 1600 pounds or 6 people.

I never plan to have more than 2 or 3 people on board plus a ski-er.


For all I know the 21 pitch is biting too much at lower RPM- and this is what really puts the boat at a really steep angle if I try to accelerate to full all at once.

Anyway, my main concern is whether I need to completely go to the opposite pitch extreme recommended for my motor like a 13.25 x 17 or 13.5 x 15 or if I should only try to bump down from the 21 pitch I have now to a 19 and see what effect that has.

 

[CaneCutter79]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

Dropping the pitch will cause the boat to reach max. RPM faster and pull people out of the hole on skis faster. If you are reaching 5,800rpm with two people in the boat and pulling a skier, that's a lot of RPM for only 30mph.

Sounds like you have a factory prop and try a good ski prop in a 19 pitch. I have a 1979 85hp Evinrude on a trihull fish/ski and I get 5,000rpm at 40mph with a factory prop. With a really good performance prop, I'm hoping to get 5,300rpm, more bow lift, and est. top end around 45 by going up in pitch to a 21p performance prop.

 

[hwsiii]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

Wire, you don't say what your GPS speed and maximum RPM are at WOT with only 1 or 2 people on the boat. My information shows that your motor should turn 5,500 RPM at maximum WOT and that you have a 2:1 gear ratio, please verify this.
And if you are TRULY turning a 21" prop at 5,800 RPM and a top speed of 30 MPH while pulling a skier you have a prop slip of 48%, with a 21" pith and at 5,800 RPM you should be doing 52 MPH with 10% prop slip. And that is unbelievable, there is a major problem here somewhere, and I suspect it is your tachometer, but it could be something else. If you were only turning 4,200 RPM at 30 MPH you would still have a prop slip of 28%. You need to turn the switch on the back of the tachometer 5 or 6 times back and forth to try and clean the electrical contacts and then bring a hand held tachometer with you next time to verify the RPM, as well as a GPS whether it is hand held or even a phone to verify speed.

Prop Slip

As it slowly speeds up between 4000 and 5500, the boat first pitches pretty high as it's coming up out of the water, then it slowly levels off a bit as the motor hits around 5000 and then as the boat pitch levels slightly the motor slowly gets up to 5500 and depending on water conditions we might get as high as 5800.

It sounds like you are experiencing bow lift from the wrong blade geometry prop from the bow rise you are experiencing.

I would also like a couple of pictures of the stern from the side of the boat taken from down low with the anti ventialtion plate parallel to the keel using a straight edge on the keel reaching to the anti ventilation plate to see if your motor is too far down on the transom, here is how to do it. And one picture down low from about 2' behind the motor as well. In my opinion you are probably at least 1 1/2" too low for a trihull.

I'm assuming that all of the recommended prop sizes in the manual are designed to let the motor get to around 5500 without exceeding it much- but perhaps the lower pitches help the motor get to the maximum RPM when under load faster? Or is it the opposite.. I don't want to change the pitch if it won't help me do what I"m wanting.

No, this is not true, running a 13" pitch prop on your engine could possibly even create a failure of the power head from the excess RPM you would turn. But you do need to change props from what you are running right now for sure.

If you will fill out this form I will help you as much as I can.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gal. Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H

 

[wireready]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

It's a 16 foot 1968 Glassmaster trihull. It has a 1972 model year 100ESL72R Johnson 100HP V4 2 stroke on it.

I don't know the gear ratio on this motor.

I have typically 6 to 12 gallons of fuel (two 6 gallon plastic tanks)

aprox 27-30MPH top speed was GPS verified with two people on board. I'm confident my tach is providing accurate readings.

I have a spare 13x19. I'll put the spare on the boat next weekend. I don't think there is prop slippage at WOT but it's possible.

My boat is moored in the water- I don't plan to pull it out anytime soon. When I do, I can recheck the motor mounting you requested and provide more data. I do have the motor currently trimmed in all the way to the transom. Perhaps I should bring it out a notch or two and try that?

Pictures of the hull are attached. These are the only two pictures I have with it out of the water.

I'll answer your other questions when I have time. Thanks for the very detailed suggestions

 

[wireready]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

Here are some answers to your questions. See below:

1. Year, make and model of boat
=1968 GlassMaster SkiBoat (it just says glassmaster - no other model info)

2. Length, width and base weight of boat, look for boat decal on back of boat
=16 feet long. Aprox 6 feet wide. It says 1600 pounds but I think that's the the max carrying load of the boat (i.e motor, plus 6 adults, plus gear) It doesn't show the weight of the boat.

2a.What is the maximum recommended HP for your boat

=100 HP

3. Number of people and gallons of gas normally on boat
= 2 or 3 150 pound young adults, and one portly 250 pound adult.
6 to 12 gallons max.

4. What do you use the boat for
=water skiing and tubing.

5. Is it a Deep Vee and if so how many degrees of deadrise
=can you define what deadrise is? It's a pretty flat boat. Very shallow draft, and I'm assuming the front shape is considered a tri hull.

6. Year, make. manufacturer and model of motor

=1972 Johnson 100 ESL72R 2 stroke

7. HP and gear ratio of motor IMPORTANT
=100 HP
=13:26 Proppeller gear ratio (from service manual see note below)

Note: I have a 1972 Evinrude 100HP Starflite serivce manual, I assume the Evinrude and Johnson 1972 100HP V4 are identical. All the parts, diagrams and all pictures etc from the manual match my motor. I used to have a 1972 Evinrude and this Johnson appears identical to it.

8. Manufacturer?s recommended Wide Open Throttle (WOT) range
=4500-5500 rated 100HP at 5000

9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene

=Won't be able to do this measurement for a while as boat is in the water.

10. Is it a bass boat or does it have a pad bottom

=Not sure what pad bottom means- but it's pretty flat. It's slightly V'd but more flat than V'd. Other than the trihull front.

11. Does it have a hydrafoil, dolefin or trim tabs

=No- none of the above.

12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop

=Currently 12.75 x 21 pitch Aluminum 3 blade prop. I will try swapping pitch with my spare which is a stainless steel 3 blade 13x19 which came with the boat. I never used it because it had a couple of small dings on two of the blades.

13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS.

=Last test was 26-29MPH with 3 people (total weight of people was 550 pounds).

Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating.

=I've never done that - I may have this wrong- the prop is currently pitched all the way in towards the transom. My previous boat was a pontoon boat and I figured that was standard. I realize now that my trim may be off- I was afraid that raising the trim up would pitch the front of the boat too high- and since it comes up pretty high when it first gets on plane, I figured that was correct.

By ventilating- do you mean cavitating? Please describe.

If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gal. Gas ________

at 5500 I was going 28MPH or so GPS verified with 550 pounds worth of people (3 of us) and aprox 8 gallons of gas, plus my battery a small trolling motor, life jackets, and maybe 50 pounds worth of other crap (skis etc).

14. Are you at sea level or a higher elevation, give us the elevation in feet

=Kansas - I'm guessing around 900 feet above sea level

15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, - YES
checked compression - YES all 4 cylinders between 117-125 PSI

My guess is this is a well used motor. My previous motor was an Evinrude 1972 100HP (basically identical motor) and the compression was 130 to 135 on all 4 cylinders

, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM

YES- motor seems to be running strong - note I am attaining full RPM. My problem isn't reaching full WOT- while everyone seems concerned about my top speed (I'm interested in that too) my main concern is the time it takes to go from 4000 RPM to 5500 as the boat pitches up onto plane.. it seems the transition takes longer than it should. then again I have nothing to compare.
If I am pulling a skier out of the water the engine goes from 1000 to 4000 RPM within a couple seconds- - but then the RPM grows very gradully from there only about 100RPM every couple seconds- and it seems related to the angle of the boat (and what my skier is doing) worse if I pull a tube. Once the boat seems to relax on the plane - the RPM gets from 5000 to 5500 faster than it did from 4000 to 5000. After running a minute at 5500 depending on the conditions it might go to 5700.

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop

= This is a "new" old boat for me. No prior experience. I just put the 12.75x21 3 blade alum prop on. I've only be running the boat a few weekends. I don't know if the prop is wrong - maybe the motor is trimmed wrong for the boat.. or maybe the prop is indeed slipping. I have a 13x19 prop that came with the boat. I pulled that prop because it had a couple small dings on the blades. I'm assuming it's stainless because it was rusty and very heavy compared to the aluminum I've been using.


17. Does the prop show any damage that you can see

=no damage- the 12.75x21 prop that's on the boat was advertised new- but I got it on ebay and I believe it was re-manufacturered. however the blades look perfectly- not even a scratch.

18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
= see original post- I didn't think my top speed was that bad (until the other posts told me I should be going faster) My problem was the transition it makes when it comes up on plane, and the long amount of time it takes to get from 4000RPM to 5500RPM (or even from 4000 to 4500 for that matter). It's difficult for my ski-ers- because while the boat pops quite nicely up to about 4000 and pulls the ski-er up- it takes a long time for the boat to level off (while it's slowly going from 4000 to 5000) and the ski-er really has to struggle and stay up until we get up more speed. The heaviest ski-er I've tried so far is only around 150 pounds. I don't think I could this with a big adult. And if I try to pull a big tube- it really takes a long time for the motor to get from 4000 to 5000 RPM.

I'm beginning to wonder if this is a trim issue so I might verify the position of my motor relative to the boat when it's in the water and retest and let you know. I will also try the other prop in case this 12.75 prop is junk.

19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

I will do this in a week and get back to you.

 

[wireready]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

The service manual says that a 13.25 x 17 pitch prop was supplied with the motor back in 1972. However it lists the following alternate 3 blade props

Aluminum
==========
14x11 pitch
14x13 pitch
13.75x15
13x19
12.75x21
12.75x23

Stainless Steel
==========
13x19
12.75x21
12.75x23
13 3/8 x 17


I realize the alternate props are a guide and depend on boat, load - usage etc.

Since my motor seems to take a long time going from 4000 RPM to 5000RPM as I'm transitioning up on plane, short of double checking trim of motor/height on transom - my best bet is to try a smaller pitch than the 12.75x21- so other than the 13x19 SS prop I have - I'm thinking of trying the 13.25x17 since that is what OMC was supplying with the motor way back when according to the serivce manual.

And I'll be very careful to watch my maximum RPM at WOT - I realize I don't want to run this motor above 5500 since the stated operating range is 4500-5500 in the service manual.

 

[wireready]

Re: 1972 100HP V4 Evinrude on old 16' trihull ski boat

I replaced the 12.75 x 21 prop with a new aluminum Solas 3 blade 13.25 x 17 prop and things have improved a lot. The boat gets on plane immediately with a consistent rise in the RPM all the way up to WOT. I can now pull skiers immediately out of the water. I also now do think my tachometer was wrong all along and I was previously reporting a much higher RPM than I was actually spinning. So when I said I was going 26MPH at 5500RPM I think I was going closer to 4000. I won't know for sure until I replace the tach, but I believe my 1972 Johnson V4 requires a 6 pulse tachometer, and the 70's era tach I have has a jumper system with C, 2, 3 and 4 and while the manual says jumpering all 4 works with V4 2 strokes, I suspect that doesn't equate to the 6 pulses I believe my electric shift 1972 puts out. In any case- I clocked a speed with my GPS at 33MPH with 2 people onboard and with less wind and chop I think we'd go faster. In any case- my main problem has been solved by going to a smaller pitch prop - like night and day.