How do I understand prop slip and use it?

[Thad]

OK, I did the prop slip calculator.
Conquest 2100 VBR
I have a Volvo Penta 5.0 with an SX drive.
The dealer told me the hp is 260 but I think it is 220. It is carbed not EFI.
The drive is a 1.60-1 ratio.
I can't find dry weight anywhere, but she is 21' w/ 8'4" beam. Fibreglass.
50 gal. fuel cell.

25p Volvo aluminum prop
21p "" "" "" ""
19p "" "" "" ""
4800 WOT. I can get higher with all of them.
The 25p maxes out at 5100rpm.
Right now I am going off of what I remember from last year.
The 25p at WOT would get 49mph with normal load. I did hit 54mph with a half tank of fuel and two occupants.

The caclulator is putting me between 24 and 31% slip.
What does that mean? Am I wasting power? Do I need different props?
I will have new "actual" speeds with gps to post on Saturday.

 

[hwsiii]

Re: How do I understand prop slip and use it?

Thad, that is very high prop slip for a boat that size, what are the prop slips for the other two props. It sounds like you need a lot more blade surface area and the right blade geometry from your numbers.
Lets start with the 21" pitch prop for now. The 25" pitch is a reworked prop from the 23" and they could have compromised it by using the wrong pitch block when reworking it. You also said you have a Michigan prop as well but I didn't notice you mention what pitch it was. Lets use these two props (the 21" Volvo and the Michigan) this weekend for an analysis of what is happening with your vessel. I would also like to know the model, diameter and pitch for the Michigan prop.
I would like you to write down the RPM and speed for both props starting at 3,000 RPM in 250 RPM increments all the way to WOT. You need to make two runs with each prop in 180 degree opposite directions, then add the two numbers together and divide by two. This will account for any differences in river or tide movement and skewing of the numbers. Then we can talk about the boat itself.
When you give me these number I will run a complete analysis of motor and torque, required prop power needed, prop slip and some charts to see what is happening.

H

 

[Thad]

Re: How do I understand prop slip and use it?

Thank you.

You've been paying attention
The Michigan prop is a three piece unit. The hub, ?spline?, and prop.
It is a 19p that has had cup added to it after the fact. (I noticed that it was a lot flatter than my other props.) It has never been on my boat. I bought it on sale at a dealer going out of business for a spare.
The numbers on the prop are: 992004-14.5X19 VORTEX

I took notes and will use the 21 Volvo and the 19 Michigan tomarrow.
I will get GPS speeds and calculate, then report back the numbers.

I assume you read an earlier post of mine to know I had a Michigan prop, I should add, I figured out why it would not fit on my drive...turns out, you gotta really press those two pieces together.

Thank you very much...but still, what is slip and what does it mean?

 

[hwsiii]

Re: How do I understand prop slip and use it?

Thad, prop slip is the difference between how fast a boat should go with a particular pitch prop and how fast it REALLY goes. Think of it like a screw that has a 10" pitch lets say, for everytime that screw is turned one revolution it should move forward 10". But if it only moves forward 9" that means it has a 1" slip in 10" and that works out to 10% prop slip.
So when your prop has a slip of 24% that means that in one revolution of that prop, instead of moving forward 25" as it should with no prop slip it only moves forward about 18.75". I look at it as the efficency of that particular prop with your boat and motor combination. The lower the efficency of the prop the slower the boat will go at the same RPM. The actual efficency of a prop is not figured that way, but I look at it that way for any particular boat, motor and prop combo.
Many things affect prop slip in a boat, for example: Engine height, prop diameter, pitch, rake, cup, regressive pitch, progressive pitch, blade surface area, and number of blades are some of the variables in how much prop slip you experience in a particular boat, motor and prop combination, that is why it is sometimes called a Black Art. But the manufacturers enhance this reputation by not giving the public all of the specifications for their props, if we had this information on every prop then the black art would diminish significantly, and they would sell less props. Since this information is not readily available it makes it much harder to pick the right one. Blade surface area can really be a big variable when you are trying to fine tune a boat.

I hope this helps you understand prop slip better.

H

 

[Thad]

Re: How do I understand prop slip and use it?

That could not have been described any better. I understand what it is now.
There is a lot more to a prop than I ever thought. I wish I'd have known this three props ago. I might have the right one by now.

Thank you!!!!

 

[TilliamWe]

Re: How do I understand prop slip and use it?

"25p Volvo aluminum prop
21p "" "" "" ""
19p "" "" "" ""
4800 WOT. I can get higher with all of them.
The 25p maxes out at 5100rpm."

I realize that one of the props has been tinkered with, but what you post here should not be possible. If a "25" pitch props can turn 5100 rpms, then there is no way that a 19p could even be remotely useable. It should turn 6500rpm!
But more realistically, a 21' boat with a 1.6 ratio drive and only 220hp should not be able to turn a 23 pitch prop to 5100 rpm. It's just not enough horsepower per weight of the boat. I really don't think your tachometer is giving you accurate readings.

Confirm your tachometer, please.

And yes, picking a prop is part math, part science, part experience, part feel, and part voodoo!

 

[hwsiii]

Re: How do I understand prop slip and use it?

We, that is why I recommended using the 19" and 21" for testing, I don;t trust the 25". My software should tell me if his tach is correct or not, so I am not worried about that at this time. I am like you though but I believe it is the 25" that has a problem, but it could be the tach.

H

 

[Thad]

Re: How do I understand prop slip and use it?

I put the Michigan prop on yesterday and it does not fit.
The hub and spline insert are too long. The nut is the only thing that goes. I put a larger washer on behind the nut, but then found the prop has a wobble play in the hub insert. So I have decided that it would not be safe to use it.

I put the 21p on, got on the water, started heading out to mark a 1 mile point with gps for test runs, and to keep my bad day from getting any better, the batteries died. It did read out 46.6 mph at 4900rpm's and I got a reading 33.7 at 3700rpm's.
I had apx. 1,000# of people and gear. 50 gal. of fuel.

If the rain gets out of here today, I am going to get out and run all of the numbers that you requested. And I will use the 19p Volvo prop instead of the Mich.

 

[TilliamWe]

Re: How do I understand prop slip and use it?

I put the 21p on, got on the water, started heading out to mark a 1 mile point with gps for test runs, and to keep my bad day from getting any better, the batteries died. It did read out 46.6 mph at 4900rpm's and I got a reading 33.7 at 3700rpm's.
I had apx. 1,000# of people and gear. 50 gal. of fuel.

Now I am totally with you hwsii, that 25 prop is not a 25pitch. The WOT RPMS with this 21p are good. Especially with the load you had aboard.

 

[hwsiii]

Re: How do I understand prop slip and use it?

That makes sense to me Thad, sorry about the battery.

H

 

[hwsiii]

Re: How do I understand prop slip and use it?

We, that is why I didn't trust the 25", something is wrong there but time will tell what.

H

 

[hwsiii]

Re: How do I understand prop slip and use it?

Thad, when you get a chance will you give me the year, make and model and/or model number of the boat. I want to fine tune my calcs with the weight of the boat and I think I can probably find it if i know that information. How big is the fuel tank as well.

H

 

[walleyehed]

Re: How do I understand prop slip and use it?

Not to butt-in here, but for the sake of saving time and confusion, you're not going to get accurate numbers or repeatable performance with the props listed.
This is a game where no real "rules" exist and although it's tuff to swallow, there is no formula that can tell us the right prop for every boat.....even 2 identical boats will run-out different.
The bottom line in set-up is "what do you want the boat to do".
Most will say "I want speed and good hole-shot with good mid-range acceleration with good economy." This is not possible but we have to settle for a happy medium...best of all, if you will. And, if your serious about getting the best performance be ready to run many designs, and don't expect any real "performance" from an aluminum or even a general "replacement" SS.
The info above is not bad info, but there is no calculation that will put you ON a particular prop band or style. The Calculations will be figured on a base by base case and until you hit the mark of where a particular prop has been run against any other given prop in testing can we make an educated "guess" (I won't say recommendation just yet) on the direction, ie style, type, brand.
I know you've been through this but I'm going to say it again...Verify the tach.
I know you have issues with a certain 25P but we don't know how much wear this prop has (HUGE factor), and we don't know what the manufacturer intended when stamping 25P on it.
They all measure pitch differently and that's why it's near impossible to compare unless you have 3 or 4 25's of the same DESIGN to run against it.
I've seen this many, many times.
BTW, the hub you have that has too long of splines is for a Merc....again, a very common issue.

p.s. the carbed 5.0 SX is "usually" 230HP, at sea-level.

 

[Thad]

Re: How do I understand prop slip and use it?

A lot of good information. And I really appreciate all of the time and effort you guys are putting into this to help me out.

The boat is a 2001 Conquest 2100VBR. Made by Conquest Marine MFG Inc. According to NADA, the net wt. is 3175#. It was the last year this company made fiberglass boats. I bought it new.
It has a 50 gal. stainless steel fuel cell.

The 25p prop is not an original 25. It has been mopdified by a shop. So yes, I agree that something has gone wrong with that prop. It used to perform better.
The 21p and the 19p have both been to the shop as well. They were checked for balance and had cup added to them and he said something about rake?

The main reason for the different aluminum props is that I have been trying to find what I am needing for a good playing prop that can push a lot of weight while pulling very large inflatables. My daughter has huge tubes and she and her friendes like to be pulled on two and even three of them at the same time.
The second reason is for a good mid-range to top-end prop when I do not have a lot of people and toys on the boat. My wife likes to cruise and I like to go fast once in a while. So hole shot is not an issue.
I thought if I could get close with alum. it would save alot of time and money when I buy the SS.

I would put a picture of my boat on here but my files are to big and I do not know how to size them dowqn to fit.

Thank you all again. I hope to get out after the parade today. Get some accurate readings on what I have to work with.
Thad from Dixon, IL.

 

[TilliamWe]

Re: How do I understand prop slip and use it?

Thad, if you go to photobucket.com, create a new account, then upload pictures, it will give you some automatic resizing choices, and one of them says "for bulletins boards" or something like that. It's very easy, even I did it.

Just from your last post, you will need two different props to accomplish your two very different goals. The one that does the best pulling your gaggle of tubes, will not be the best for cruising and top speed.

 

[Thad]

Re: How do I understand prop slip and use it?

I plan on buying two SS props. That is why I have been working with the three alum.'s I have now. I change them quite often. That reminds me, I will have a new thread in stupid human boating tomarrow. I still can't believe what happened.
I will try the photobucket, but as for now, I figured out how to do an attachment.

 

[Thad]

Re: How do I understand prop slip and use it?

Hey fellas',
It took me a lot longer than I thought to get out and get some numbers. I trashed my 21p prop so all of the speeds are on the 19p ONLY.

I ran down river for seven miles. Thought that would give enough time at each RPM to allow for variance and get a reliable reading. I then ran upstream for the same seven.
Down stream I had a head wind of apx. 15mph. Water was choppy with 8 to 14 inch chop.

hwsii,
I do not know how this plays in, but I consitantly had an avg of 2mph higher going down stream into the head wind than I did the opposite. Maybe my trim was'nt quite the same. But, at 5000rpm, the speeds were exactly the same.?.?

Enough of that. Here are my numbers and stats.

The boat has a dry weight of 3,175#. 50 gal fuel cell, full. Me at the helm @ 225#. My wife in the port bow @ 140#. My daughter and dog in the aft seat @ a combined weight of 80#.
I would estimate the total weight at 4,220#.

RPM SPEED(avg)
3000 25.5
3250 29.1
3500 32.3
3750 35.5
4000 38.0
4250 40.7
4500 43.8
4750 45.3
5000 48.8

I should add that I can run the rpm's WOT at 5200rpm but did not run that for this test. I only know this because I was not paying attention one time, and wooooops


I would like to get a 4-blade aluminum. I think for all around performance that might be the ticket.?. I do not want to buy a SS yet because I primarily boat on the river with changing depth and "obstructions". The SS will come later for the lake house.

I think I covered just about everything, I posted a pick of the port sode of the boat so maybe you could get an idea of the hull design. If you need more, let me know.

THANK YOU GUYS

 

[hwsiii]

Re: How do I understand prop slip and use it?

Thad, you got some good numbers to work with this time and your prop slip is 13% at your top speeds and that is about normal for aluminum 3 blade props. If you change to a 19" pitch 4 blade aluminum prop your prop slip should come down as well as the RPM should reduce to between 4,700 and 4,900 RPM and you should gain a little speed and a good bit of hole shot and acceleration. If you went to a 21" pitch you would reduce you RPM to about 4,400, Here are your prop slip numbers.

Prop Slip

MY recommendation for a better prop than you have is the Volvo Penta 4 Blade, but they also have a Solas 4 blade a little cheaper but if I remember correctly it needs an insert, bit that would be useful for resale value.

Price: $219.95
Add SX Drive Aluminum Propeller 3587522 To Your Cart

Propeller Specifications:
SKU:#3587522
Manufacturer: Volvo Penta
Brand: SX Drive
Material: Aluminum
Diameter: 14 - 1/4?
Pitch: 19?
Blades: 4
Rotation: Standard (Clockwise)
Usually Ships Within: 1 Business Day


H

 

[Thad]

Re: How do I understand prop slip and use it?

Thank you very much.

The Solas Rubex looks to have a larger blade surface. It also is $100.00 less and that includes the nub and free shipping. If this is a bad idea, let me know.

I am going to get the 19p because I will be using this for when I have a lot of weight in the boat and still want to play.

What would you recommend for a "speed" prop? Both aluminum and SS.
Thanx again

Now I have some props to get rid of I guess. Or just put them on my "wall of shame"

 

[hwsiii]

Re: How do I understand prop slip and use it?

Thad, lets wait until you get the new prop and get some more test results from the new prop before we look at a high speed prop. The new results will give us more data to work with.


H