prop choice

on my 21' boat i currently have a 14 1/2 X 19 degree pitch prop at full throttle it is turning approx 3800 rpms giving me about 28mph. if i switch to a 17 degree prop i know i will have better take off but will i lose much top end or maybe even gain a little more speed with the extra rpms i should gain. also with the the smaller pitch prop will pick up and extra fuel mileage?

Re: prop choice

Should be posting this in the "Prop Questions & Topics" area. And, it would help to know more about your rig.

However, I run a OMC Cobra 4.3L V6 powered 21 footer that tips the scales at 3800 lb. dry. My prop is a stock 15 x 17 aluminum 3-blade. I turn 4500 - 4600 RPM easy & top out consistantly at 37 kts on the GPS. And, I get a nice easy, fuel conserving cruise of 25 kts at around 3500 +-.
JSMoore

Re: prop choice

john cole said:

on my 21' boat i currently have a 14 1/2 X 19 degree pitch prop at full throttle it is turning approx 3800 rpms giving me about 28mph. if i switch to a 17 degree prop i know i will have better take off but will i lose much top end or maybe even gain a little more speed with the extra rpms i should gain. also with the the smaller pitch prop will pick up and extra fuel mileage?

Click to expand...

Yeah this a Prop Forum question for sure.

BUT, If your RPM and speed are correct (GPS and verified tach) you are seriously OVER-PROPPED. I would absolutely put a 17p on it (like stainless one below) and see what it does.

Not only will you get better hole-shot, but your top speed will improve too because the engine will be developing rated power at the right RPM... Lugging it at 3800 could eventually damage the engine.





Cheers,


Rick

Re: prop choice

if it used to go faster with the 19P, don't change props, fix the engine's lack of power

Re: prop choice

sorry about not posting in the proper forum. but my to is a 4.3l v6 pushing a 21 foot boat that also weights in around 3600 dry. this is the prop i got with the boat. i will definitly try out the different prop, i need a spare anyways.

Re: prop choice

If you read the stickys at the top of the page and the helps on the prop store page it will help you understand what info we need to help solve your problem.
We need the details of your whole setup.prop size,motor size,hp, brand, year.
with the barest of info it appears you have issues other than prop selection.

Re: prop choice

ok i'm in the right forum now anyways. on my prop issue i have a
1987 Mark Twain 21' stern drive boat
4.3l omc with cobra outdrive
it currently has a 14 1/2 X 19 three blade prop on it
i can get about 27 mph at 3800 rpms.

i just got this boat a couple of month ago and have done a bunch of work to the engine. such replacing a bad piston and valves. i couldn't get it on plane and found that my point where messed up. i replace them and then i could go on plane at least.
but from the sounds of it i need to switch to a marine grade point set up rather then the napa ones i just put in according to a boat guy i know locally because of a low voltage issue with them he claims.
but either way i need to get the right prop on this boat so i don't kill the engine like the previous owner did.

Re: prop choice

I am in central NY.
According to my figures at 3800 your speed should be about 37.
also your motor should be close to or at its max rpm 4600.
So it would appear either your tach is wrong or your speed is wrong and the vmotor has some issue.Speedos are often not very accurate ,a gps is best.
I never heard of such a thing.NAPA used to have standard points and better points.
But I haven't bought a set of points in 30 years.
But you do have some sort of issue.Check that the throttle opens all the way,that timing is right and advancing as it should.Verify your tach and/or your speed.

Re: prop choice

well i know the speed is right, using gps. and i am working thru some points issues on another forum. so i do have 2 different issues going on. main thing on this forum is trying to figure out which prop is the overall right one, if there is such a thing. i believe the tach is right too, just can't swear to it. and i haven't looked at the throttle cable adjustment yet, still not satisfied with other problems being solved yet.

Re: prop choice

If...big If...your tach is correct, changing from a 19" to a 17" pitch (not degrees) is not going to increase your rpm enough to get into the proper range. There will only be a 300-400 rpm gain...based on your numbers you need double that.....something is wrong with the engine or your tach is outta whack.

Since you mention you had engine trouble and you're messing with the ignition (Napa points are just fine....just put a set in my Century), you need to rule out a problem with the engine. Points, condenser, rotor, cap, plugs need to be properly installed....timing and dwell must be set properly.....rule out any issues with the engine first. Have you checked compression?

It is entirely possible that this is an ignition issue. I gained 300 rpm at wide open throttle just by replacing my plugs, points, rotor, cap and condenser, setting the proper point gap, checking timing and dwell.....and prior to performing this routine maintenance the engine seemed to be running just fine. So if you get the ignition issues resolved, and you are still 200-400 rpm short on your wide open throttle rpm...then a reduction in prop pitch would be in order.

Re: prop choice

john cole said:

well i know the speed is right, using gps. and i am working thru some points issues on another forum. so i do have 2 different issues going on. main thing on this forum is trying to figure out which prop is the overall right one, if there is such a thing. i believe the tach is right too, just can't swear to it. and i haven't looked at the throttle cable adjustment yet, still not satisfied with other problems being solved yet.

Click to expand...

The 19" is in the ball park. It should produce about 4600 rpm and over 40 mph.
Your final prop size would depend on your results with the 19.
Its not out of the question that with the right prop size and fine tune up speeds in the upper 40s are possible.

Re: prop choice

steelespike said:

The 19" is in the ball park. It should produce about 4600 rpm and over 40 mph.
Your final prop size would depend on your results with the 19.
Its not out of the question that with the right prop size and fine tune up speeds in the upper 40s are possible.

Click to expand...

Where Abouts in CNY if you don't mind

Re: prop choice

Is it possible that this boat has water in the hull somewhere adding a bunch of extra weight? ie: water logged foam under the floor?

Re: prop choice

no i don't think so, seems like everything is dry where i can get to. thinking i may have some voltage issues on the ignition side of things.

if anyone knows, i would like to know what prop came on the boat from the factory just for reference. and i do need to get second prop as a spare so maybe i will get the 17 pitch prop.

Re: prop choice

Did you check compression in all of the cyls. after replacing the piston and valves? It really seems like something is robbing HP here.....I wouldn't expect the 19" pitch to lug the motor. Does anyone know what the gear ratio is for this outdrive. With the ratio we can calculate what the theoretical speed should be at a engine rpm of 3800....?

Using the numbers provided 19" pitch, 27 mph, and 3800 engine rpm....the theoretical prop rpm would be 1500.6...that calculates to a 2.5:1 ratio. Obviously I'm neglecting prop slip, but the 2.5:1 seems out of line......I'm typing out loud here...tossing out things to discuss. Even if the theoretical speed would be 32 mph (5mph more) without slip the ratio would still be over 2:1.....Is this possible? Does the 2.5:1 make sense?

Re: prop choice

1980Coronado said:

Did you check compression in all of the cyls. after replacing the piston and valves? It really seems like something is robbing HP here.....I wouldn't expect the 19" pitch to lug the motor. Does anyone know what the gear ratio is for this outdrive. With the ratio we can calculate what the theoretical speed should be at a engine rpm of 3800....?

Using the numbers provided 19" pitch, 27 mph, and 3800 engine rpm....the theoretical prop rpm would be 1500.6...that calculates to a 2.5:1 ratio. Obviously I'm neglecting prop slip, but the 2.5:1 seems out of line......I'm typing out loud here...tossing out things to discuss. Even if the theoretical speed would be 32 mph (5mph more) without slip the ratio would still be over 2:1.....Is this possible? Does the 2.5:1 make sense?

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My info indicates 1.68 on both 4.3s in 87. 3800 with 1.68 19", arbitrary 9% slip=37 mph.
I think the tach is wrong Compute for rpm at 27 mph I get 2770.Again thats with a guess of 9% slip. This all disregards the size/weight of the boat A heavy boat might create higher slip but would be off the scale to make only 27 at 3800.
I am assuming the boat is fully on plane at 27.

Re: prop choice

steelespike said:

My info indicates 1.68 on both 4.3s in 87. 3800 with 1.68 19", arbitrary 9% slip=37 mph.
I think the tach is wrong Compute for rpm at 27 mph I get 2770.Again thats with a guess of 9% slip. This all disregards the size/weight of the boat A heavy boat might create higher slip but would be off the scale to make only 27 at 3800.
I am assuming the boat is fully on plane at 27.

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My math agrees....hmmm.....at 27 mph slip is 34% with that ratio. Even with a heavy boat it shouldn't be that high. Could be both a bad tach and engine problems.....the tach would have to be off by over 1000 rpm, which means the engine isn't coming up to rpm. I don't believe this is being caused by the 19" prop being too big. Need to rule out bad compression....ignition problems....throttle not opening all the way...etc. and verify tach. before changing props.

Re: prop choice

well dont know anything about prop slip, but i do know that the compression is good and even across all cylinders. i just got a meter to were i can confirm the tach. and i do have several more things to check. and the boat is on plane at 27mph. will reply more in another post my connection keeps dropping on me. thanks for the help

Re: prop choice

The numbers that you provided point to engine problems. This is the link to the sticky to troubleshoot low WOT RPM. Go through it one item at a time and make sure that each item is checked. There is something that just isn't right about the engine that is causing the problem. If the compression is good like you said then you probably either have a fuel delivery issue or an ignition issue. I would check the ignition first and pay close attention to the timing. Timing that is advanced too far will still allow the engine to run (and fairly well at lower RPM's) but the engine will just loose all its power in the upper RPM range. Get the ignition right, then the fuel mixture and I bet you will have solved your problem. DO NOT change your prop yet..... this IS an engine issue and probably very easily fixable. Good luck.

http://forums.iboats.com/showthread.php?t=295070

Re: prop choice

john cole said:

well dont know anything about prop slip, but i do know that the compression is good and even across all cylinders. i just got a meter to were i can confirm the tach. and i do have several more things to check. and the boat is on plane at 27mph. will reply more in another post my connection keeps dropping on me. thanks for the help

Click to expand...

Slip is the difference between the actual and theoretical travel of the boat based on the pitch, and rpm. A 19" pitch should theoretically move the boat forward 19" in 1 revolution. When you use the motor rpm and the outdrive ratio to calculate the rpm of the propeller you can then calculate how fast the boat should be going at that rpm. When you take the actual speed of the boat at the given rpm and compare it to the theoretical you can calculate slip. In your case you are getting 27 mph @ an engine rpm of 3800. Using the drive ratio of 1.68:1, the prop is spinning at 2262 rpm. Doing the math 19"/rev x 2262 rev/min x 60 min/hr x 1 mile/5280 ft x 1ft/12 in = 40.7 mph.

Comparing your actual 27 mph to 40.7 mph......27/40.7 = .663......66.3% of theoretical......100-66.3 = 33.7% Slip. I would think this number should be in the 10%-15% range...maybe a little higher with the heavy boat...but definitely not over 30%.

Most likely your slip numbers are not actually that high....that's why we believe the rpm you see on the tach is erroneous......and since we do not believe the 19" pitch prop should cause the engine to lug there has to be another problem causing the lack of rpm. Typically a rule of thumb would be 150-200 rpm gain by a decrease in 1" of prop pitch. Dropping from a 19 to a 17 then should increase rpm by about 300-400 rpm given the same type and diameter of prop. I'm just trying to give you the thought process we are going thru to help troubleshoot the problem.

Since you've checked compression, start looking for other issues that would rob horse power....ignition, fuel system, etc....

Since we have more than likely ruled out a prop issue you may want to start a new thread over in the I/O forum seeking more engine specific help.....not all of those engine gurus check the prop thread.