Does "slippage" increase with twins?

[Fl_Richard]

Not having any problems but when I run a slippage calculator my boat comes up at about 14%. Does "slippage" increase with twins?

I know 10% ish is about normal but no one specifies single or twin.

Thanks!

 

[steelespike]

Re: Does "slippage" increase with twins?

I would say no.but what boat,motors,props are you running.

 

[Fl_Richard]

Re: Does "slippage" increase with twins?

1995 Wellcraft Coastal 238 with Johnson 150 twins - currently set up with 14.5x19 SS OMC props - 5800 RPM @ 42-43 knots

 

[steelespike]

Re: Does "slippage" increase with twins?

Your performance seems good to me at close to 50 mph. a 25 footer at 4,000lbs dry.

 

[Fl_Richard]

Re: Does "slippage" increase with twins?

Yea I thought she runs pretty good too. That was what made me wonder about the slippage. The calculators all show more than "normal" and I was wondering if the twin setup was to blame.

 

[Texasmark]

Re: Does "slippage" increase with twins?

Being a mechanism guy, I'd ask what is the mechanism that would cause what you said? Personally I don't see one. Each engine has it's load to push. Performance numbers are good so you aren't concerned that you are overloading anything. The only negative is double weight and drag. Course 2 wheels pushing ought to give you better thrust which one would think would balance things out somewhat.

I think the design of the wheel (when operated within it's design envelope) has more to do with slippage than tandems.

MY 2c

Mark

 

[hwsiii]

Re: Does "slippage" increase with twins?

If your motors have the Right rotation motor on the starboard side and the Left rotation motor ion the port side, then you should not experience any extra prop slip from twins than having a single motor.


H

 

[Texasmark]

Re: Does "slippage" increase with twins?

If your motors have the Right rotation motor on the starboard side and the Left rotation motor ion the port side, then you should not experience any extra prop slip from twins than having a single motor.


H

Yeah but you are dragging 2 lower units rather than one and they do make a difference. Counter rotating props just stabilize steering and can reduce any yaw caused by propeller torque.

 

[hwsiii]

Re: Does "slippage" increase with twins?

Course 2 wheels pushing ought to give you better thrust which one would think would balance things out somewhat.

I think the design of the wheel (when operated within it's design envelope) has more to do with slippage than tandems.

You have exactly the same answer, so why are you questioning my knowledge of prop slip from twin engines. LOL

Theoretically, a single engine with the same horsepower as the twin engines would go faster because of less drag, BUT that has NOTHING to do with the amount of Prop Slip encountered from the twin engines.
Prop Slip always goes back to the props you are using combined with motor height, hull design and weight, if you have enough horsepower.

You might try reading this for a better understanding of changing props for better performance.

http://forums.iboats.com/showthread.php?t=365653



H

 

[Texasmark]

Re: Does "slippage" increase with twins?

You are correct HW. I was typing the answer while thinking about possible other things involved and got side tracked. Course, nitpicking: If the weight/drag amounted to more load on the prop (which they surely do) then there would be cause for additional slippage but it probably isn't even measurable.

Mark

 

[hwsiii]

Re: Does "slippage" increase with twins?

Mark, in my opinion, what you are not taking into account is that by having two props instead of just one there is a lot more blade surface area with the two props instead of just one prop on a 300 HP motor. If the prop on the 300 HP motor was exactly the same manufacturer and model of the two props that are on the 150's then the two props combined would have more blade surface area than the single 300 HP prop and thus they would have LOWER loading than the the single prop, even though they increase the overall weight and produce more drag because of two lower units in the water, kind of like adding a 4th blade.
The difference in weight between the two 150's and the single 300 is right at 185 pounds, and when you consider this boat weighs about 6,000 pounds all up weight that is miniscule or about 3% of the total weight. The extra drag from the other motor does not change the amount of prop slip for each independent motor. In my opinion, with the right props, Richard should have a prop slip Under 10% very easily.


H

 

[Texasmark]

Re: Does "slippage" increase with twins?

Amen Brother! There is truth to your speech. Grin

Mark

 

[hwsiii]

Re: Does "slippage" increase with twins?

LOL Mark


H

 

[hwsiii]

Re: Does "slippage" increase with twins?

Mark, having read many of your posts in many different forums on Iboats, you have muchhh more knowledge than I do in many aspects of boats and motors. My only knowledge is limited to boat hulls and propellers, I applaud your help you give so freely to people that have very limited knowledge about boats and help keep them on the right path to fix their problems.


H

 

[Texasmark]

Re: Does "slippage" increase with twins?

Thank you sir. I applaud your knowledge also and am impressed by your ability to access accurate data to support your theories.

So for you folks on the sidelines, looks like hw is a good source of info; tells it like it is.

Mark