30 mph top speed?

[wyoboat]

I have a 20' 88 larson lazer with 4.3L in it runs great low hrs. I'm having trouble going faster than 30 mph in it. it has a 17 pitch 3 blade prop. seems like it should have a little more on the top than this?

 

[craze1cars]

Re: 30 mph top speed?

Yep. Should have a bit more top end than that.

What RPMS are you turning when going 30 mph? What is your outdrive ratio?

 

[wyoboat]

Re: 30 mph top speed?

Yep. Should have a bit more top end than that.

What RPMS are you turning when going 30 mph? What is your outdrive ratio?

about 4500, not sure what the outdrive ratio is, How do i find that out? this is my first boat i've ever owned. its a alpha 1 if that helps. I've ran it up to 5000 and only got to 32, Tried trimming it up and down and didn't get much response as far as speed.

 

[Maclin]

Re: 30 mph top speed?

If the prop pitch and mph and rpm is correct then that calcs to a 2.21 drive ratio. That is way too high. That engine should have a 1.7 ratio or so. I wonder if it had been swapped out earlier in life for one from a 3.0 or something.

I think the boat probably came with a 1.7 and 19 pitch prop. If the engine could hit 4600 rpm with a 1.7 ratio drive and 19 pitch prop the speed would be 43mph or so.

With those rpms you could go to a 19 prop easily to get to 33mph at 4600rpm (5000rpm is too high and over-revving it). A 21 would get you to 36. But your real problem, if these calcs about the drive ratio are correct, is that drive ratio of over 2.0

 

[wyoboat]

Re: 30 mph top speed?

I don't think it's ever been swapped I bought it from a guy here in town and he bought it from the original owner couple years ago. I just ran it up to 5000 to see what it would do. Although it did seem to sound good a that rpm ;} the prop says 19 2p 17 quicksilver on it. what does this mean?

 

[QC]

Re: 30 mph top speed?

How are you getting your speed? Should be a 1.84 ratio . . . I'm getting 27% slip. One of your numbers is wrong. Speed, RPM, pitch or gear ratio (my number I guess . . . )

 

[craze1cars]

Re: 30 mph top speed?

about 4500, not sure what the outdrive ratio is, How do i find that out? this is my first boat i've ever owned. its a alpha 1 if that helps. I've ran it up to 5000 and only got to 32, Tried trimming it up and down and didn't get much response as far as speed.

Ok....so your absolute top speed/RPM is actually 32/5000? And it's not 30/4500? Your motor's target is supposed to be 4,400 to 4,800 I believe. So this makes an enormous difference. If you're spinning 5,000 max you are out of range and need to add pitch. If you are at 4,500 you are at the low end of the range and I might actually suggest you take away some pitch.

So if 5000 is correct, you are underpropped a bit. A 19 pitch would probably be in order to reduced your RPMS and it would probably increase your speed a little bit. What is the condition of your current prop? Any nicks or dings?

This 19 recommendation assumes your tach is accurate. Old boat tachs usually are NOT accurate, and it would be smart to verify it with another known good tach. Same with boat speedos (old and new)....GPS speed numbers are far more useful, which is why QC is asking his question above mine...

How to determine drive ratio: 1. Look it over carefully for numbers...often times it is stamped on the housing of the outdrive. or 2. With motor OFF, turn the motor over by hand exactly ONE full revolution, while a helper counts exactly how many times the prop turns around in that one turn.

Lastly...your name is wyoboat and your profile doesn't provide us a location....could this name have anything to do with Wyoming? What elevation are you getting these speed/RPM numbers at? If you're at high elevation, 30 might actually be fully expected...high elevation sucks power away in droves due to less oxygen, and it causes ALL naturally aspirated engines to slow down due to HP loss substantailly....

The greater the accuracy and quantity of data you provide us, the more likely we will be able to help you and not misguide you...

 

[wyoboat]

Re: 30 mph top speed?

I only ran it up to 5000 to see if would make a difference, 4500 is most i run it. But ya i guess my very best is 32 at 5000. Yes i am from wyoming and the elevation i've been boating at is probly 3800. the prop i have now looks old but doesn't have any dings or nicks in it. I will check the ratio tonight, gonna take it out tomorrow so maybe i can do some more testing with it. The boat is fairly heavy i think 2500 is what i read, It does get on plain fairly fast, Guess i want to be like all the cool guys and go fast anyway thanks for all the in put. I've been reading alot on this forum site and have all ready learned a great deal.

 

[Maclin]

Re: 30 mph top speed?

Here is a link to a simple prop calculator, it needs 4 parameters and can give you the fifth....:

http://www.marksmarineinc.com/prop_calc.aspx


I use .12 - .15 slip as a typical number. Over 15% is pretty high and the prop would need work, so the numbers you give the calculator need to be accurate and the prop needs to be in good shape or the numbers will not make "sense".

 

[45Auto]

Re: 30 mph top speed?

How to determine drive ratio: 2. With motor OFF, turn the motor over by hand exactly ONE full revolution, while a helper counts exactly how many times the prop turns around in that one turn.

The prop is going to turn much less than one turn using this method. With one revolution of the motor, a 1.5 drive will turn the prop .67 of a revolution, a 1.67 drive will turn the prop .60 of a revolution, a 1.84 drive will turn the prop .54 of a revolution, and a 2.0 drive will turn the prop .50 of a revolution.

 

[QC]

Re: 30 mph top speed?

Right ^^^^^^ so you need to count how many times the engine turns for one prop revolution. I am guessing 1.84 . . .

3800 ft. is worth about 10% horsepower loss.

 

[Maclin]

Re: 30 mph top speed?

Spinning the engine 4 or 5 full rotations while counting the prop full rotations will give more precision for the calculation. With ratio numbers below 2 it is harder to tell with just one rotation.

 

[QC]

Re: 30 mph top speed?

Agree ^^^^

 

[craze1cars]

Re: 30 mph top speed?

I agree as well....didn't explain it very fully.

 

[wyoboat]

Re: 30 mph top speed?

thanks guys for the info. I'll get to it today.

 

[wyoboat]

Re: 30 mph top speed?

took it out today and still only saw 30 at 4500. Looks like my drive ratio is 1.97. so maybe 30 is my limit. anyone have any prop suggestions to get more speed and still pull a skier out?

 

[craze1cars]

Re: 30 mph top speed?

If you're propped at 4,500, that's really not too bad. Your current prop calculates to 19% slip. Not great, but not all that surprising for that particular speed on a 20 foot hull. So honestly I'd take a close look at a number of other things before messing with a prop change...your 3,800 elevation shouldn't slow it down THAT much...it's not that high up in the sky, and Mercruiser usually specs the same carb jetting for zero to 5,000 rpm.

What are your compression readings from each cylinder? Base ignition timing? Ignition timing advance? How to these compare with specs for the motor? Have you comfirmed your tach is 100% accurate? Most old boat tachs are not. Are your throttle butterfies opening fully at the WOT lever position? Choke releasing fully after warm-up? Flame arrestor clean? Fuel filter clean? Carb properly jetted for modern fuels (not 1988 fuels) and for your elevation? Carb jets all clean? Carb float at proper spec? Color of spark plugs/piston wash? Are your spark plug wires all within proper resistance specs and routed without crossover? How recently were distributor cap/rotor replaced?

I think you're missing some of the potential HP from your engine, and I don't think your prop is the primary reason for your lack of speed. Correctly propping a motor with a possible problem is a waste of money...It's more economical to correct any potential engine problems first and make sure the motor is running absolute peak, and THEN shop for a prop to match it...

In the meantime you should systematically attack every system of your engine and check everything out, with your goal being to increase your speed and RPM with your current prop. If this is happening, it's because you are restoring some of the lost HP to your engine.

Back up in post #5 you said the boat could spin 5,000. Now you're saying 4,500. I'm very confused by your differing posts...and your conflicting data can easily result in me and others giving propeller advice that will either destroy your performance, or possibly destroy your engine and outdrive. So the answer to this question is CRITICAL: Is your current prop able to spin your motor up to 5,000 rpm? Or is 4,500 rpm absolutely all it has in it no matter how hard you try to make the boat/motor go faster? Keep in mind it is positively critical that your answer be based on data from a known good tach....not necessarily your current 23 year old boat tach...

Please recognize that "4,500 at 30 mph" is a completely meaningless spec, if your boat is actually capable of spinning up to 5,000+ and going a few MPH faster by just giving it some more throttle...we need to know your absolute MAX rpm and the absolute MAX speed that the boat can do. If your absolute max is 4,500 we will likely recommend going down in pitch. If your absolute max is 5,000+ we will most certainly recommend going UP in pitch. So you can see why it is so important to answer this question accurately...if your data is inaccurately stated or misinterpreted, you stand a strong likelihood of getting advice that is OPPOSITE of what you need to do.

 

[wyoboat]

Re: 30 mph top speed?

I see, Ok the boat will run up to 5000 "on the old tach" but only gains 1-2 mph when i do it. and i guess i don't like holding it there. the motor has about 550 hrs on it. I changed what looked to be the original plugs and and cleaned the air filter, It is running way rich looking at the plugs and the filter was dirty, after doing this it did run great and was better on fuel yesterday, I plan on jetting the carb this week and making sure everything is functioning properly. probably change wires too. what would be the easiest way to confirm the tach is right? I don't want to take one out of my old hot rod.

 

[Maclin]

Re: 30 mph top speed?

Wyoboat,

You can use an older style dwell meter that has a tach on it that uses an inductive signal from one of the spark plug wires.

The rest of this post may not apply, but the figures that come up are still askew somewhat and I still need to know more.

Did you do the rotation counting and determine the actual drive ratio? I think Volvo offered a 1.61, 1.89 then it jumped to a 2.15 from what I can gather about single prop drives of that vintage.

When I use your 4500rpm and 30mph and 17p and 1.89 ratio I get that 19-20% prop slip craze1cars got. Any slippage over 15% is considered high and needs to be addressed.

When I use 2.15 as the ratio the slip show only 10% which is closer to normal 10-12%. So it is very important to get the numbers right. If your prop slip is truly 20% then your problem is in that area. Since you say the boat accelerates good and you can actually gain 1-2mph when revving higher than the recommended 4500 range then I think prop slip is not the root problem.

Certainly keeping an engine in tune is always worthwhile but it will not change the mechanical item's characteristics and 30mph is it for the combo you have (ratio + prop @ 4500).

Th effects of an incorrect ratio can be minimized by prop pitch but the "top" mph for the drive combo you should have will be hard to get to with just prop pitch.

For example:
4500rpm with 2.15 ratio and 17p and 11% slip calcs to 30mph.
4500rpm with 1.89 ratio and 17p and 11% slip calcs to 34mph.

Because your engine can rev well past the 4500 range then it seemingly is not overpropped, and it could probably drive a 21 pitch easily and that would calc to 37mph at 4500rpm.

With a "more correct" 1.89 ratio the same rpm with 21 pitch will put the boat at 44mph, assuming the engine can drive to that rpm under those new load characteristics.

When the ratio installed was intended for an engine of lower output a stronger engine can over-run the prop and that is why it is hard to "make up" for the ratio with just prop pitch changes. I think a 17p is too low for your boat no matter what the actual ratio is right now, I base this on the ability of it to rev well past the 4500 range under load.

I would try a 19 first to see if it can still get to 4500 rpm, if so and speed should increase to 33-34. If that works, and the engine feels like it has more then try a 21 to see if you can still get to 4500 and the speed should be 37 or so. There are prop dealers that allow some swapping to get to the correct pitch as you experiment.

Be aware that if your prop is truly slipping that much then a new prop will put more load on it, so jumping from 17p to 21p may be too much.

 

[Maclin]

Re: 30 mph top speed?

Wyoboat,

Somehow I got it in my head that you have a Volvo drive when I put in the available ratios. Merc has more ratios I believe, sorry for any confusion. The "physics" still applies. Use that prop calculator for comparisons.