slip factor

[Zaxxon II]

Hi guys,<br />After moving my motor up one hole and using the slip factor formula I came up with a .256 slip. This seems high but I don't know what to do about it. Data: 2000 Johnson 150 on 17ft Stratos with a 14 1/2 X 23 aluminum, 5600 rpm, 49 mph (gps). Since the move I can't trim much at all without some prop slip, and I now get some slip on WOT turns. I'll try the 15 X17 prop tomorrow and see what that does. Any other advice?

 

[pwiseman]

Re: slip factor

What did you get before moving up a notch and why did you move up a notch. Sounds like you are sitting too high now.

 

[Zaxxon II]

Re: slip factor

I was only getting 5200 rpm and 39 mph. If I keep the motor trimmed down a little it doesn't cavitate and still runs better than before. I also drilled my ports slightly bigger - 1/4"

 

[pwiseman]

Re: slip factor

My guess is you were trimming as much as you could before and not getting where you needed to be. Moving up a notch lets you get where you need to be with a lot less trim. <br /><br />400 RPM would not account for 10 MP, so moving up must have solved some of the slip issue.<br /><br />25% still seems high. Hopefully somebody with more experiance on this hull type can comment on this. A stainless prop will help some too. They are not cheap, but they are easy to install.

 

[Dhadley]

Re: slip factor

The increase in rpm indicates that the hull responded very well to less drag. The limited trim angle indicates that the current prop has very little cup left near the tips. <br /><br />The 17 should give a heck of a hole shot but should hit the limiter fairly easy. At 5600 youre in the right ballpark. Maybe just a better design prop is called for.

 

[Zaxxon II]

Re: slip factor

Dhadley,<br />I tried the 17 before moving and you're right H*** of a hole shot and it ran at about 5600 before moved but less speed. When I trim the top of the motor cowl is just slightly over paralell with the water. Is this enough? I don't really understand cup and which prop has how much. Can you recommend a good prop and would a stainless be better. The other factor is prop diameter. The 23 is only 14 1/2" and the 17 is 15". How do you think a 21 pitch by 15" would do? As usual thanks for all the expwert advice.

 

[Zaxxon II]

Re: slip factor

bump

 

[Dhadley]

Re: slip factor

I would suggest a good 21" ss prop with the largest diameter you can get. Something like a 21 or 22 Turbo Lightning with 14 3/4" diameter.

 

[Zaxxon II]

Re: slip factor

Thanks for everything. I know I'll have many more questions before I become a competent boat owner, but I've sure learned a lot so far.

 

[Zaxxon II]

Re: slip factor

Well I tried the 17 today and it turned 5800 rpm at 42mph (gps). That figures to only .04% slip. Does that sound right. I sould expect the smaller pitch at higher rpm and less speed to give me more slip. Any thoughts?

 

[Zaxxon II]

Re: slip factor

bump

 

[Dhadley]

Re: slip factor

I get about 18% slip. But something doesnt sound right. Going from a 23 to a 17 you only gained 200 rpm. Is that right? You should have gained a ton of rpm.<br /><br />I have basically the same motor you do, unless yours is injected, on a 20' Dusky (heavy) and spin a 19" Turbo 5800 or so. <br /><br />Is your motor carb'd?

 

[walleyehed]

Re: slip factor

Just for comparison sake, my 18ft alum deep-v with 82 rude turns the 15X17 well over 6k. My data shows 47 MPH at 5800. alum prop. I'm around 2400lbs. 7-8% slip is good for my rig...you should get a bit better when you get it dialed in.<br />I gotta follow Dhadley on getting your hands on a good SS, Turbo in particular if ya can.

 

[Zaxxon II]

Re: slip factor

I guess it's carbed. At least there are 6 units that feed fuel. They don't look like any carbruator I've ever seen on older outboards. I did miscalculate the slip now I got 16%. I sure don't know about the strange rpm either. Could the 15 in. prop vs the 14 1/4 account for some of it. When I got the 5600 with the 23 I was tilted up a bit. I just kept tilting until rpm's and speed topped out. Water was glass compared to a slight chop with the 17 and an extra 150 lbs aboard.<br />I will look for a prop. Any thoughts on a 4 blade? Thanks again. Bill

 

[Zaxxon II]

Re: slip factor

bump