Average slip ratio

cobra 3.0

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Jul 31, 2003
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Knowing that there are many different hull designs out there and looking specifically at bowriders. What would the normal or average slip ratio be of a 16-17 ft bowrider?

I ask this because I am starting to think that my foam is water logged and making the boat heavier and slower than it should be.
 

Silvertip

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Sep 22, 2003
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Re: Average slip ratio

Your question is sort of like asking "how high is up"! d:)Doesn't matter whether you have a row boat or a barge, slip is a function of hull, load, engine power, engine height, prop selection (design and pitch) and wide open throttle RPM and true speed. Since we know none of those factors -- we can't even guess (at least I can't). Calculating slip with a light load results in a different number when you increase or decrease the load. To determine if the boat is water logged, weigh it and compare with published weight minus engine wieight. Or you can pull up a section of floor and check the foam.
 

QC

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Mar 22, 2005
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Re: Average slip ratio

Although I tend to agree with Silvertip, I would repost this on the prop forum and see if you can get some ideas from those guys too. If you give a LOT of info about your specific boat, engine/drive package, prop, speed, WOT RPM, gear ratio, average load etc. etc. They might be able to give you some more clues.

I assume that you are dissatisfied with your top speed, but without all of that other stuff they won't be able to compare and give you some ball park numbers for you to work from . . .
 

Bondo

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Re: Average slip ratio

Although I really Doubt anybody has figured out the Average Prop Slip of all the boats out there,..........

I think you'll find that what seems to be the Magic # is 10%,...................

The Idea is to get % of Slip Down, as Low as Possible,.......
And,......
On Most boats,......
IF you Can get Prop Slip down To, or Below 10%,..........

You've done as Well as Can be Expected..........................
 

cobra 3.0

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Re: Average slip ratio

Thanks guys. Bondo's reply is what I was looking for. I know there are tons of factors affecting slip ratios. When I punch in my numbers, I get a slip ratio of 17% which I think is high for my boat with a light load.

Unfortunately, for my 1986 Peterborough 16 ft bowrider with 3.0 OMC (gearing of 1.71 to 1) there isn't any weight numbers available anywhere anymore...that I can find. Hence, my approach using the slip calculator below.

http://www.go-fast.com/Prop_Slip_Calculator.htm

I am trying to determine if my low top end of 36 mph with only one 200 lb person aboard (17" pitch 4600 rpm) is attributed to a tired engine or a water logged boat or a non-stepped hull design or a combination of the above. From what I can tell so far, I'm leaning more towards a water logged boat.
 

QC

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Mar 22, 2005
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22,783
Re: Average slip ratio

Has it always been that slow? What are you doing with trim at that speed? Can you make her porpoise at all? If not you need to move some weight from the bow to the stern. What's the props condition? SS? Aluminum?Also, I don't believe it is the lack of a stepped hull although that would be faster. The poor slip % also shows that it is probably not the engine if your calcs are right . . .
 

Fl_Richard

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Jan 21, 2005
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Re: Average slip ratio

4600 RPM sounds like it will be lugging.. Should be over 5400 No?
 

cobra 3.0

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Re: Average slip ratio

(4600 is top recommeded rpm on the 3.0 inboard.)

I've only had the boat for 3 seasons now as I am not the original owner. Yes it always has been this slow during my ownership. I have power trim and it is trimmed just before it porpoises to be able to reach that speed. And it can porpoise if I trim her too high.

It is a 14 x 17 aluminum prop that has great torque for skiing. I also have a 13(?) x 19 OMC stainless prop which brings down the rpm about 200 rpm for a WOT of 4400 or so for about the same top speed, but blows out more in turns and has slower acceleration. So I don't use it much because I mainly water ski, wake board, knee board and tube with it, so the 17" pitch is best.

I am thinking water logged because I recently fixed a small portion of flooring which was rotten.


...of course that's if it really has a 1.71 to 1 drive ratio. I see drive ratios of 1.86 to 1 on later models, but have no idea what I have on there. I know the drive had been worked on before I bought it. So I'm not sure what's really on there.
 

Texasmark

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Dec 20, 2005
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14,544
Re: Average slip ratio

Cobra, I think your mention of a non-stepped hull is a key point. I have run pretty much every type of hull out there and hands down a padded boat will run circles around everything else. Once you get her up and on the pad it's like hooking a 150 hp outboard (give or take) .....to a waterski. 8)

Mark
 
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