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Author Topic:   Prop Slip/Efficiency
John Day posted 09-24-2001 06:45 PM ET (US)   Profile for John Day   Send Email to John Day  
I followed your prop formula to calculate the potential max speed for my Wellcraft Martinique 3300 with the following data.
Prop size : 18x19 (pitch : 19)
Gear reduction : 2:1
Engine RPM : 4600 (at WOT)
The calculated potential max speed is 41.38 MPH. However the actual measured speed is only around 32 MPH. This means the prop efficiency is 77%. Is there a way to improve the prop slip/efficiency?



John Day posted 09-24-2001 07:41 PM ET (US)     Profile for John Day  Send Email to John Day     
I forgot to mention that the "normal operation range" is 4200~4600.
Hank posted 09-25-2001 12:08 AM ET (US)     Profile for Hank  Send Email to Hank     
How big an engine do you have ? How heavy is the boat ? I'm not familiar with your boat but I presume the 18" diameter is appropriate for it. Prop slip/efficiency depends on lots of factors. Prop pitch and RPM are not the only characteristics that enter into the performance. An earlier post by Clark Roberts presented a formula relating boat weight per HP and a boat type "constant " with maximum speed.
(I believe its Crouch's Formula. Unfortunately, I don't have my references with me. Perhaps some one else on this forum can help.)

Obvious factors like the condition of the boat bottom, a bent prop, etc. could contribute to your slip.

jimh posted 09-25-2001 09:29 AM ET (US)     Profile for jimh  Send Email to jimh     
One factor that determines the slip or efficiency of the prop is the angle of attack of the blades. This angle is variable, depending on the speed at which the prop is moving through the water.

At low speeds and with a high pitch prop, the angle of attack is very high and slip is high. This allows the engine to turn the prop at lower RPM even though it does not have enough torque (to turn the prop if it had NO slip).

As the flow of water across the blades increases with increasing speed, the effective angle of attack reduces, until it reachs a more efficient value, and at that point the slip is reduced to a low value, say under five percent.

Drawing from this information, I would conclude that the prop you are using is not reaching an effective angle of attack, and this is because it has not reached the speed it needs to gain this more efficient operation.

Now WHY that is, I cannot tell you!


Dr T posted 09-25-2001 11:41 AM ET (US)     Profile for Dr T  Send Email to Dr T     

Have you optimized the trim angle.

On my old (non-PT&T) 13 with a 35 HP Johnson, it makes a world of difference.

For example, with a 10.5 x 11 prop:

Middle hole (third from the top), 5300-5400 rpm and 23 MPH

Hole next to the top, 5600 rpm and 28 mph.

John Day posted 09-25-2001 06:11 PM ET (US)     Profile for John Day  Send Email to John Day     
Thanks for all your reply. Here is more info about my boat. It is a year 2002 brand new Martinique with twin Merc 8.1 Horizon in-board engines w/ total 740 HPs. I also found out that the props are 18x19 #6 cup Nibral. I made a mistake when uploaded my post. The WOT RPM is 4900 (not 4600) which is higher than the max operation RPM 4600. Hence, when I drove my boat, most of time, it is not at WOT. I kept the RPM around 4600 so I don't damage the engine. Then using GPS, 32 MPH was measured. I personally think that my boat is under propped. I'm thinking moving to 18x21 (2" larger pitch) but not sure whether it will improve the slip/effciency. Someone told me that it could be so-called "blowout" and changing to a bigger prop will fix it. Does it make sense?



acseatsri posted 09-25-2001 06:30 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
WHOA!! You shouldn't run any inboard engine at more than 3500 rpm or so for any length of time. You'll be rebuilding them in no time! It's also not efficient at that rpm. You definitely need another 2" of pitch or more diameter, though. You may want to read some of the articles at this website as well. Good luck.

Hank posted 09-25-2001 11:01 PM ET (US)     Profile for Hank  Send Email to Hank     
John I wouldn't fool with the prop without a little more investigation.
I was intrigued by your case and went to Wellcraft's website to look up the Martinique 3300. What a classy boat!

The specifications for your boat show some performance data with twin VOLVO 5.7 GXI engines, (not your Mercs). Max HP is 760 HP
Time to plane 4.5 sec.
Top speed 39.3 MPH @ 4980 RPM
Cruise speed 27.6 MPH @3500 RPM

Unfortunately they don't indicate the prop size for comparison.
I don't understand your statement about max operating RPM being less than WOT. Is this a
peculiarity of the Merc engines ?
At any rate it looks like your performance at
4600 RPM isn't that bad. Also, perhaps you should be following acseatsri's advice and cruise at 3500 RPM rather than 4600 RPM. What is your speed at 3500 RPM. Is it much different from the 27.5 MPH shown with the Volvo?

I would consult with Wellcraft before I changed the prop.

acseatsri posted 09-25-2001 11:16 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
"Then using GPS, 32 MPH was measured."

Was this knots or MPH? Most GPS read in knots. If it is, multiply your reading by 1.15 to get MPH. My guess is you're probably running closer to 37 mph!

John Day posted 09-26-2001 12:50 PM ET (US)     Profile for John Day  Send Email to John Day     
According to the Merc 8.1 inboard Horizon spec, the full throttle RPM range should be 4200~4600. My boat at full throttle (WOT) is around 4900 RPM. I personally think this boat is under propped. Yet even that, I'm only getting 32 MPH measured by GPS and it is only 77% of the calculated potential max speed which is 41.38 RPM. THis is why I have posted the qurestion regarding improving the prop slip/efficiency. I've talked to Wellcraft, so far they are not able to release any performance data for this boat w/ Merc 8.1 engines. The data they published on the web look impressive w/ 5.7 engines. My boat has 8.1 engines and it is supposed to perform better than 5.7. But this is not the case. The top speed w/ the current props I can ever get is 32 MPH only!!

I will double check my GPS device and make sure if it reads in knots. If this is the case, I'll feel better.

Thanks to both of you.


acseatsri posted 09-26-2001 01:04 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
John, another thing to keep in mind is that most boats will never live up to the manufacturers claims on maximum speed. I'm sure they attained this speed with minimal fuel load, no equipment, a 50 mph tailwind, and probably a horse jockey at the throttles. When you do your speed check, run your course in opposite directions and take the average. This will take into account wind, current, etc.
John Day posted 09-26-2001 02:17 PM ET (US)     Profile for John Day  Send Email to John Day     
Agreed. I'm now focused on getting WOT RPM back to the manufacture's spec (4200~4600). Hopefully changing the prop from 18x19 to 19x20 will do it. Yet, I still have concerns about the prop efficiency. Many articles show that the normal prop slip should be less than 15%. As I mentioned, my boat is only getting 77%. I suspect that it is due to the fact that my boat is under propped. Again, hopefully changing to a bigger prop will improve the prop efficiency as well. Correct me if I'm way wrong.



acseatsri posted 09-26-2001 02:27 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
If your boat is doing 37 and not 32, it seems slippage is not a problem. I'd check before I did anything. Also keep in mind that as the motors age and you carry more stuff on the boat, that max speed and rpm will come down.
Hank posted 09-26-2001 10:44 PM ET (US)     Profile for Hank  Send Email to Hank     
After reading your posts more closely you're probably correct in trying first to achieve a balance with your props that matches the WOT rating of your engines. Your boat does sound underpropped. As you say, you're about 300 RPM too high at WOT.

Incredible that a manufacturer can sell a boat like yours without having some idea as to the appropriate props to install in order to achieve proper WOT engine RPM ! Have you checked with Mercury for some guidance as to how much pitch increase and/or diameter increase ?

Hope you can overcome this annoyance and enjoy your boat.


John Day posted 09-27-2001 12:29 PM ET (US)     Profile for John Day  Send Email to John Day     
Thanks. I'll definitely check whether it reads in MPH or knots. I wish it is knots, then I feel better about the slip.

I did talk to Wellcraft. The also agreed that 18x21 (ie. 2" larger in pitch) is more appropriate. However, they don't have this kind of size. Instead, they said they have 19x20 (1 " larger in diameter and in pitch)which pretty much has the same effect. I hope this prop will bring the WOT RPM back to manufacture's spec range and increase little top end speed.



bigz posted 09-27-2001 12:58 PM ET (US)     Profile for bigz    
John, this ain't my cup of tea --- based on the rule of thumb quick top end mph guide used by Clark Roberts (might add it hit my top mph pretty much on the head with the 27WA) "the formula for max performance ( sq root of hp/gross weight X 180) Using a constant of 200 instead of 180 (180 is general constant for deep V hull and 200 is for fast tri-hulls and semi V's) only way to exceed this top mph is to lessen weight, increase hp or modify hull..." = your top end is in the area of 27 to 30 mph.

Seems "lite" to me but that's what this comes out to using approximately 20,000 lb gross weight (estimated fuel water miscellaneous etc ... maybe I am a little heavy on this area) anyway try it your self. I might have goofed.

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