twin 03 250 evinrudes

[jserb]

I need your help on what kind of props to use for my 30? scarab center console. I have been watching on ebay and asking those people selling the same boat I own what type of prop they are using, and I gotta tell you I am so confused I just don?t know where to start.
My Evinrude dealer said he wanted to start me off with a 17 pitch prop, however this seems a little shallow to me? I mean he is a pro at these motors but the props that came with the motor when I bought them off a guy with a 26ft boat were 19 pitch.
This boat is a 1985 scarab sport with twin 03 250 Evinrudes on it. The max RPM at WOT is 5200, it is a 30? boat. I have never ran this machine before so I don?t know what it will do, im just looking for something that will give me a happy meadium between high speed and good cruising. The boat with motors and full tank will weigh around 5500 to 6200,

Some of the motors and props used on other scarab sport owners I have spoken to:


1. 35? scarab super sport with 02? 225 evindures runs with 19 pitch ( but says he?d like to put 21?s on them?)
2. 30? scarab sport with 89 OMC Evinrudes runs on 19's and it run's 4600-5000
3. 30 scarab sport with 08 yammah 200hp, runs with 21 pitch.


So im confused more pitch = more thrust, why wouldn?t everyone want the most possible thrust off the props? But its wired cuz racing boats have little pitch yet they go so fast, SO CONFUSED what should I use???

 

[Dhadley]

Re: twin 03 250 evinrudes

My buddy is going to test the 19 4 blade Turbo's I had on Ebay last week. He's got a 30 Scarab with twin 250 Yamahas but they're 4 strokes. I can tell you the 17's were too small. I'll keep you posted.

 

[junior1113]

Re: twin 03 250 evinrudes

you need more rpms 18 rakers would be ideal prob bring you up to 5500

 

[jserb]

Re: twin 03 250 evinrudes

Well i was talkin to a guy in the bar last night and he was trying to tell me how to do the simple math on how to convert the mathematical speed of my boat with a certain pitch. he said that the pitch is how many inches a prop "theoretically" moves then you take that and time is by the gear case ratio? anyone know what an 03 250 evinrude ratio is?>? Then you take that and times it by RPM, then divide by 60sec, and 5280ft??? Does that make sense? Im just trying to do the theoretical math as to what pitch i should start with one my boat. From what this guy in the bar said was that 17, 18, 19p, no way im gonna push 60 with those that i need to start at a 21 and go up from there, im just so confused cuz you'd think my evinrude dealer would have at least been somewhere in the right ball park if this other guy is right. Can you help me with how to do this math and what is the advantage of shallow pitch from deep, what about a clever? What good would that do me?

 

[steelespike]

Re: twin 03 250 evinrudes

Are you sure you have your weight right every CC in the 30ft range were
at least 5,000 to 6,000.dry w/o motors. they were tested 2 men 500 and 200 to 300 gals of fuel.basic weight motors 1,100lbs. running 19 and 21 stainless props 5,400 to 5,700 rpm. new Etecs. 1.85 ratio.
It would appear that a 17" would be very light for your application.
I checked this at the BRP Etec site. 8 or 10 boats similar to yours tested.

 

[jserb]

Re: twin 03 250 evinrudes

Steel- Yeah it might be closer to the 6800 area but its not much more than that!

 

[triumphrick]

Re: twin 03 250 evinrudes

You will usually find the cleaver props on smaller lighter boats with pitches up in the 20-24 range. Don't think one of them is going to work out for you. I too think 17 - 19 will work. Dhadley is the prop guru on this site. Hopefully he will pick up. I haven.t mentioned these guys in a while, but they have a prop wizard site you can click on to get really close..here's a link.
Go halfway down the page and find prop wizard..

http://www.dansdiscountprops.com/

 

[Dhadley]

Re: twin 03 250 evinrudes

We ran the 30' with the 19 4 blade OS Turbos and they bite a little hard. The 19 Turbo 1's were real close but we're going to look at the Vectors or maybe the new style Lightning. Right now it looks like a set of 20 Lightnings may be the way to go.

Stay tuned.

 

[jserb]

Re: twin 03 250 evinrudes

DH you think there will be much difference between yoru buddies 4 strokes and my 2 stroke, if anything i think whatever proformance your buddy gets i think my 2 stroks would only improve, do you concure?

 

[Dhadley]

Re: twin 03 250 evinrudes

Yes, your performance will (should) be better. I'd almost think both boats will swing the same prop, the difference will be in the gear ratios.

 

[jserb]

Re: twin 03 250 evinrudes

DH, yeah i looked on a site and it looks like they dont make the Vector in a 19 pitch, it starts at a 21, so i assume if the 19 was close your thinking that the 21 will dial you right in. What was the turbo 1 doing? was it a little sluggish for your motor? what was your top speed with those?



P.S. man i am getting excited; you have become the only guy who is helping me with performance on this project. I have mounted my bracket specifically to your directions, with the ratio of 1 to 5, and now i am going to be trying that props you suggest! Man i hope your a pro at these outboards and rigging! ;-)

 

[Dhadley]

Re: twin 03 250 evinrudes

I'm going to talk to my buddy at Turbo tonight about these props. We feel that the standard T-1s are OK but would like to use something with just a touch more blade area.

Ha! He just called and he concurrs that the Vectors should be fine. We're going to try them and will keep you posted.

 

[hwsiii]

Re: twin 03 250 evinrudes

JS, these are the numbers for your boat, motors and props. These are formulas I have put together to pick the correct prop for any planing hull boats, a couple of them are even proprietary and do not match up with the universal formulas, which you will found out if you try some other variations of these formulas. These are calculated for aluminum props, because when you get into stainless steel there are so many variables involved, with things like progressive pitch, rake, cupping, blade area and many other variables. Each one of the aforementioned variables changes the THEORETICAL Pitch of a prop, which is why you normally can't just change models of props and expect the same performance and handling characteristics from each one. They all have the potential to change handling characteristics of how the boat rides and feels at higher speeds especially so. Adding rake increases the bow lift on a boat and usually increases speed, because it decreases the wetted surface area of the boat, and hence less drag. But some boats cant stand much bow lift without porpoising, on these style boats usually you want a zero rake prop. Progressive pitch props have a lower pitch on the leading edge of the prop and as it progresses to the trailing edge it steadily increases in pitch, such that there can easily be a difference of 4" or more in theoretical pitch on a prop. The advantages of this style prop is that since it has a lower pitch on the leading edge it will give you a better hole shot and acceleration. Whereas the higher pitch on the trailing edge will help to give you a higher top end speed. A fixed pitch prop cannot do this. Cupping on the trailing edge of a prop will increase the theoretical pitch of a prop for high speed and also helps decrease ventilation and cavitation, as well as it adds stern lift if you have too much weight in the stern and your boat is stern heavy. But, if you cup the blade Tips it actually increases the Rake of the prop, which helps with ventilation and cavitation as well as increases bow rise. Hopefully this gives you a better understanding of prop geometry and how the different elements of a prop controls different aspects of how it will react with different hull forms, height of motors and different speeds. Every stainless prop has some of these blade geometries designed into them, the question is which ones and how much and I have not found any manufacturers that will really tell you that information. I give you all the numbers you need to know below for an aluminum prop. If you just transpose these pitch calibrations from aluminum to the particular stainless prop you want to use you will find that you will get even better performance numbers than I say, because the stainless props are designed with thinner blade sections because the stainless has superior metallurgical characteristics than the aluminum does. I recommend probably not going above a 21" Pitch in most props, but that is not always true.

This is the correct numbers for your prop

This is the Pitch at a theoretical 10% Slip

This is the Real Slip according to the speed

Usually a good prop shop can tell you how much difference there is between the aluminum prop and any particular model of stainless prop you are interested in, by that I mean the differences in Theoretical pitch. That will tell what pitch in each particular model of prop you would want. Don't forget you want to have the anti ventilation plate riding just at the top of the water for the least drag and highest performance. I doubt that you will get the actual RPM shown but you should get the speed, because of the differences in theoretical pitches, blade area and other variables. I used Stelle's projection of the weight at 6,800 pounds. I found your boat to weigh 3,000 pounds and the gear ratio for your motors at 1.85 to 1, if these numbers are wron then my calculations will be wrong. I also ran the numbers if the boat actually weighs 6,300 pounds and if this is so then you can expect 61 MPH running with a 22" prop. I am sorry this has been such a long dissertation, but I was trying to give you a better understanding of the differences in blade elements and geometry, and how they affect performance and handling characteristics. This should help you pick the right prop for your boat. If you decide on a 4 blade prop then you should drop the pitch by 1" in most cases.


H

 

[jserb]

Re: twin 03 250 evinrudes

HWSII, thank you very much for that explination! I printed it out and will take it with me when i go to the prop shop.




Now maybe you can help DH here a little bit....


DH! K so i am getting nervous i mounted the motors today and we have the engines as low as they can possibly go and i HOPE that i have not set the bracket to high. I have attached some photos, if you could take a look at them and tell me what you think.... i did like you said and mounted the AV plate 5'' above the bottom of the hull (relative to their position on the transom) and when i put them on today i put a straight edge on the hull and measured and my AV plate is more like 6'' from the bottom of the hull.....(thats when the outboard and AV plate are trimmed level). So i was getting nervous cuz im worried about that water intake on the Lower unit, and i found that when i set the straight edge on the bottom of the hull and ran it back to the outboard, that the bottom of the hull was falling right in the middle of the water intake! PLEASE TELL ME THAT THIS DOSENT MEAN THE BRACKET IS TO HIGH?!?!?!?!?!? if you could take time to explain why we even raise the bracket at all it would help settle my worries!!!! its a 25'' set back, thus i went up 5'' (but like i said accual numbers when outboard was mounted was 6'' above hull)

 

[Dhadley]

Re: twin 03 250 evinrudes

The motors look OK but those tabs might create an issue. What are you going to use for alignment (toe in/out)?

 

[jserb]

Re: twin 03 250 evinrudes

Toe in , i think i JUST made it with the tabs, cuz they are an 1 1/2 away from the lower unit when you put a straight edge on it. Can you explain to me again the reason why we raise the motors are we go back 25 inches? What makes you "feel" like were in good shape with there current location? I just need something to put my mind at ease....

 

[Dhadley]

Re: twin 03 250 evinrudes

As the waqter comes off the hull it comes off at an angle. Also the balance of the boat changes compared to the motors being mounted on the transom. However, the tabs being that close to the motors changes things. They will push the water down and act like the transom is much further back. You really don't want anything closer than 12" to the propshaft centerline.

 

[jserb]

Re: twin 03 250 evinrudes

DH- how we doin on your buddies boat, did he test that 20 pitch prop???

 

[Dhadley]

Re: twin 03 250 evinrudes

The 19 Vectors came in but he was at a drag race and won't be back until tomorrow. He'll probably test them on Friday.

 

[Dhadley]

Re: twin 03 250 evinrudes

He ran the boat today with the Vectors. 1/2 load of fuel and otherwise an average load. Motors turned 6000 and ran 51.5 mph. Nice!