Alum Verses SS

[Maximerc]

Ok I am looking for more bow lift. I have a 21 pitch Stainless prop on the boat now .. It is 3 blade. If I put on a 4 blade Alum propeller will I gain any lift ?? not changing pitch boat runs 5500 RPM at 55 MPH.

I just would like opinions on the 3 blade SS as apposed to 4 blade alum .. I keep hoping to get a fair deal on a SS 4 blade but Wow some are VERY pricy .

Its been a long time since I posted here !

 

[hwsiii]

Re: Alum Verses SS

Maxi, bow lift can be created in two or more ways
1. In general a high raked prop creates more bow lift and if the prop has parabolic rake it can create even more bow lift.
2. Bow lift can also be enhanced by adding cup to the blade tips, not the trailing edge.

It all has to do with blade geometry, tell us what make and model of prop you are running now and we can help more.

H

 

[Maximerc]

Re: Alum Verses SS

It is a 3 Blade , SS precison propeller the prop says 22 pitch but when I inquired at the factory they say that it really should have been classified as a 21 I also think that it was around 1984 vintage. The prop seems to have cupping but its pretty minimal and it is on the trailing edge , I think. The prop has a couple of scuffs and a burr where it must have hit a rock flat on the edge, its noticable but not bad.

Hope this helps. I have read about the rake and all that , just not sure what this prop carries

 

[hwsiii]

Re: Alum Verses SS

Maxi we need a lot more information, but with having cup on the trailing edge that will help raise the stern, not the bow.
We will try to help you as much as we can, but there is certain information that is needed in order to help you. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information, and most other people who are trying to help you will need the same information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the exact prop you need for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM.

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
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 and model of motor
7. HP and gear ratio of motor
8. Manufacturer?s recommended WOT range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Does it have a hydrafoil or trim tabs
11. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from prop, how much gas and how many people in boat for test data

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

 

[Maximerc]

Re: Alum Verses SS

Ok .. I was trying for a basic Idea but I am glad to get detailed ..

184 Baja Sunsport 1984 vintage It has a V bottom fairly flat not sure what angle looks like it is a 4 in rise in 12 inches . Boat was waterlogged ,, compleatly rebuilt 4 years ago by me. YES the bottom is FLAT.. Epoxy glass used and Marine Plywood. Boat porpoused before rebuild and still does above 45 MPH. Just as it always has. I installed smart tabs.. no diffrence, though it helped boat handle better. 17 gallon gas tank is located 2 inches forward of where it was when it was built. All Foam was removed and new foam poured.

Boat does not have a base weight on it. I had it weighed before the rebuild on hte trailer and it was 2600 pounds .. I am sure that it is less than that now with all the water logged foam gone but not sure what the trailer weighs .

Fuel tank full or empty no diffrence. Anchor is in the bow now though I have run with it everywhere with no change.

Boat is normally used for pleasure , some ski ... like speed but would rather have the darn thing quit hopping. ..


150 V6 Black max power trim oil injection removed Carbon Fiber Reed Valves
110 PSI per cylinder WOT recommended is 5500 so I think I am right on the re though if the boat was running flat and smooth It should run a bit faster. this is with three people in the boat one in the bow (open bow) and two in the (drivers seats).

Cavitation plate is maybe 1/4" above the bottom of the V . ( I lowered the engine last fall nearly a inch) . When it is level with the boat . I ran a Bob's speed shop cavitation plate with no gain so I removed it.

Again its a 22 Precision Propeller Turbo that the factory claims that if they made it today they would call it a 21. 14.25 in diameter

Stock gears on the 150 not sure about the ratio .. cannot find that info.

I think you have all the info on the boat that I have now.. I had it down to a bare fiberglass skin .. I know it thru and thru..

 

[Maximerc]

Re: Alum Verses SS

BTW yes I will share with you improvments to help your data base

 

[hwsiii]

Re: Alum Verses SS

I need to know what year the motor is to try and find the gear ratio. That is why I ask the questions, it makes it harder.

H

 

[Maximerc]

Re: Alum Verses SS

1984 mercury black max V6 2.0 2 to 1 gear ratio on the lower unit.

 

[hwsiii]

Re: Alum Verses SS

Maxi. I am getting ready to run some numbers for you since you got the gear ratio for me. LOL But i don't see the length of the boat anywhere in our dialogue and I definitely need that as well.

H

 

[hwsiii]

Re: Alum Verses SS

Maxi, even though I didn't have the boat length I assumed an 18' from the model number you gave us and I also assumed a base boat weight of 1,600 pounds since you didn't know what that was and I used the 2:1 gear ratio you specified and then I changed the weight for the base boat to 1,200 pounds and I came up with a 19" pitch prop and you are already running a theoretical 21" prop, so something is wrong, my assumption is the gear ratio and that is because if I run the numbers for a theoretical 21" pitch prop I come up with a real world negative prop slip of -1% and that is impossible. If I run it with a 22" prop it has a prop slip of 4% and that isn't right either. Here are the numbers.

Real World Prop Slip

If you want to run some more speed tests on the water with a GPS that is fine and we can run the numbers back through and see what we get, just let me know.

On another note lets talk about the bow lift that you are looking for to keep the boat from porpoising. If you try to use a 4 blade aluminum prop to raise the bow it won't work, because all 4 blade aluminum props that I know of do not have a high rake and that is what is needed to raise the bow of your boat. They are designed as low rake stern lifting props in aluminum 4 blades.
You can buy a higher rake SS prop in either 3 blade or 4 blade configuration and if you got one with cup on the blade tips that would increase the bow lift even more. The cup that is on the prop you have now is on the trailing edge and that just makes the bow lift even less because cup on the trailing edge of a prop induces stern lift and with a 14 to 1 power to weight ratio you don't need any stern lift.
I hope this explains the differences so you have a better idea what the problem is with going to a 4 blade aluminum prop.

H

 

[Maximerc]

Re: Alum Verses SS

boat length is 18'4" yes ....

Gear Ratio is 2 to 1 two turns of the crank to exactly one turn of the prop checked this numer physically .

I have not run a GPS on the boat but I will.

you did answer my original question ... that a alum blade will not lift like a SS blade.

What is a real world slippage number ??

 

[hwsiii]

Re: Alum Verses SS

Think of a propeller as if it was a screw, a 21" pitch prop theoretically moves forward 21" for every 1 complete turn, just like the lower unit. When I show the prop with a negative prop slip that means it is turning more than 21" in one complete revolution and that is impossible.

H

 

[Maximerc]

Re: Alum Verses SS

I understand the numbers ,, just looking for what pecentage amount a "standard" or "normal" slip is.

 

[hwsiii]

Re: Alum Verses SS

On your boat you are looking for between about 7% to 11%.

H