48SPL Stops Making MPH Before It Stops Making RPM

[tinskin]

Long story short I bought a boat with a 1987 48spl, upon opening the throttle it would bog, then slowly build rpm and plane off in probably about 8 or ten seconds. Brought it home pulled and cleaned the carbs, then noticed as I was putting it back together it had 2 very different bore sized carbs. I bought a matched set from a 1995 48spl, strapped em on, problem mostly solved. It idles smooth, and clean, accelerates well, planes quickly, and gained a little top speed, but it seems to reach top speed before it reaches top rpm. The last 3 or 4 hundred rpm don't make anymore mph. Pushing the key in at speed slows the motor, and my prop does have a little rash on the leading edges. Is there a way short of buying a new prop to isolate this condition? I did find the formula to determine prop sufficiency, and if I am doing it right my slip percentage is high. My prop is a Michigan Vortex 11.?x17p which seems to be steepest prop I can find for my gear case. The hull is a 600lb aluminum pro bass style, and I travel light. There is no trim, and I run it in the third hole which is right on the verge of porpoising. My top speed is 33mph, theoretical top speed is a little over 41 according to formula I found.

I had the gear ratio wrong, using 2.42 makes my TTS 37mph, makes slip 11% which I reckon is pretty good.

 

[DaleH]

11% slip is pretty good ...

But FORGET about speed ... you TOTALLY missed that proper RPMs are what you want. Find out what the maximum RPMs are for that motor and I would prop it so you are no less than 200 off the maximum, with you and 1/3 to 1/2 fuel and all the gear typically carried.

Best performance, carrying capability, and longgevity for the motor will be when you are wearing the correct prop and are turning towards the higher end of that maximum recommended RPM range.

 

[gatorglenn]

As Dale said is correct. Your motor is to high ( for this prop) you are venting. This is the problem with the slip so high. Is why your speed is not increased with the rpms, and porpoising. Two choices are to lower motor one hole with current prop. Or reprop to a 4 blade if you’re going to run that high. But get the max rpm rating first.


Sent from my iPad using Tapatalk

 

[DaleH]

Actually you need to set the motor height correct for blade installed on the motor, then assess if the RPMs are too high or too low.

I believe that is a 5,500 RPM motor.

 

[tinskin]

I don't know why you are yelling at me, but I do understand rpm's do determine pitch, but there is no higher pitch prop that I can find. One would think though that as long as the motor is gaining rpm the speed should also increase. Now if the carbs I have now are from a more free flowing leg (the jet size is much bigger on these) maybe I have exhaust restriction, maybe I have cavitation, is there a way to tell? The sensation is like floating the valves on an old v8 car when you wait too long to shift, the tach keeps going, the speedo not so much. I also wonder do reeds float? On the other boats with trim if I go too high and vent the prop those last rpms jump up real quick, this one has a linear climb. I have been using 5600 rpm as max, my other 2 cylinder lists a range 5000 to 6000 with max hp @5500. I don't know where I got 5600, there is a non rev limited power pack available though. What height should the cavitation plate be above / below the hull bottom? Now I am about an inch above.

 

[DaleH]

No one is ‘yelling’ at you, don’t be soooo sensitive ... but you still have avoided any answer about RPMs you are getting.

Vent plate, not a cavitation plate, should be at the surface or out of the water when at the correct height.

 

[tinskin]

Anti-cavitation I should have said. There is no tach, but I have had enough boats with them to know what sounds right. This one is going a little past right. Normally I would just get the next wheel size up and see if I could turn it, and I think this one would if it was out there. The prop I have does hold the bow up nice and high, and holds plane well at low speed. All would be well if I didn't have that gps. If all alse fails I guess I could have it re-pitched.

 

[Pappy]

Get a tach on it. I have run one Hell of a lot more boats than you will ever run and I cannot pinpoint RPM by ear. I believe that power pack has a 6100rpm limiter in it and you can hear that when it kicks in.
There are no less restrictive midsections ("Legs" as you called it). It is not a reed issue....how can reeds allow an engine to continue climbing in RPM without a boat speed increase?
What you probably have is a propeller with torn up leading edges cavitating in the water and forcing air past the rest of the blades making the blades slip. Send the prop off or buy another prop. No easy way out of that.
You are being given good information. Use it wisely.

 

[tinskin]

I do need a portable tach, any recommendations on one I could easily move from boat to boat? Maybe a tiny tach?

I am concerned that I may have moved the power band of this motor by switching carbs and not the exhaust housing as well. OMC introduced this motor as a 100:1 premix, and were jetted for thinner premix, so when I found out I needed carbs I got 50:1 carbs from a later year. Now I see that OMC changed the exhaust housing when they changed carbs. I remember from the motorcross days and later even the street bikes if you change pipes you have to re-jet the carb/s. And on the 2 strokes changing pipes moved the power band. So I guess carbs and exhaust housings are a matched set. Cause even in same hp, same year same everything else, short shaft motors take different jets than long shafts. Is it possible I could have moved the rpm range down a little, maybe from a little more back pressure?

 

[thedude]

I think you are over analyzing this. 1st step is to confirm the rpm (tiny tach is a great idea). I dont think the carbs will be an issue on this motor. I also know a lot of people use much older carbs and change the mix to 50:1 without changing anything else.

 

[thedude]

Also does this prop have a shear pin? Would be good to eliminate possibility of early stage of spun prop.

 

[tinskin]

Tach ordered. Not a shear pin prop. The Michigan features a user switchable hub, which means I can replace just the metal part of the prop and save $25. Just wish they made a 19p.

 

[Pappy]

I do need a portable tach, any recommendations on one I could easily move from boat to boat? Maybe a tiny tach?

I am concerned that I may have moved the power band of this motor by switching carbs and not the exhaust housing as well. OMC introduced this motor as a 100:1 premix, and were jetted for thinner premix, so when I found out I needed carbs I got 50:1 carbs from a later year. Now I see that OMC changed the exhaust housing when they changed carbs. I remember from the motorcross days and later even the street bikes if you change pipes you have to re-jet the carb/s. And on the 2 strokes changing pipes moved the power band. So I guess carbs and exhaust housings are a matched set. Cause even in same hp, same year same everything else, short shaft motors take different jets than long shafts. Is it possible I could have moved the rpm range down a little, maybe from a little more back pressure?

No, OMC did not jet for a thinner mix.
The 100:1 engines were basically forced upon engineering as a marketing ploy to gain market share. The 100:1 mixture was rolled back to a 50:1 mixture do to corrosion issues that started surfacing during off-season storage. Dealer bulletins were sent out to pull the 100:1 stickers off engines and notify customers to change to a 50:1 mixture.
As I said in an earlier post there were no changes to the exhaust housing to allow for a better exhaust flow. Changes were made to go to a different trim system and lower pan system.
You would do well to quit putting your guesses down in writing as fact should someone take your word for it and make changes to their engine based on a guess rather than fact.

 

[earl60446]

I doubt you will get 41 mph. I have the same motor, 48 spl johnson and 560 lb boat, so very similar to yours and my top speed is 31 mph.
JFYI Tim

 

[tinskin]

If I ever touched 35 I would be tickled. I do have a real nice 70 that I thought about swapping before I replaced the carbs , I thought maybe it would hit mid 40's then. Overall I am happy with the way the boat runs now, I haven't tried but I think it would pull up a skier as is, and planes almost instantly. Being able to achieve top speed at lower rpm might be a good thing at the gas pump too. How long it lives?, time will tell. I did pick up a flapper wheel today, I will clean the edges up and see if it makes a difference.

 

[Stumpalump]

Drop your motor mounting height to keep the trim height and loose the cavitation rpm. My 48 SPL runs out like yours does then I put a 19p OMC stainless on it. All the mods and numbers are in this thread.
https://www.tinboats.net/forum/viewtopic.php?f=3&t=43165&hilit=Bayrunner&start=0

 

[tinskin]

Thanks for sharing that, good to know there is a steeper prop out there. My motor has brackets and OMC accessories trim switch, and wiring, I was told this used an electric actuator, I have not been able to find any further info on it though.

 

[BillPlayfoot]

The prop on my 1989 48SPL is a 12 X 17 and according to my GPS does 30.3 mph on my 18 foot AceCraft. I'm happy with that.