Prop slip Q for FNQ

[Wahoo]

Scott ( FNQ)
my other laptop shat other week and i lost all the info i had on the prop slip and cals

if a prop has to much slip say 20%-25% , would this change the dia of the prop in the water? if this makes sense
i know i had very little slip, which is not very good, is it?


Daz

[FNQCairns]

Hi Daz, dunno what you mean by change the diameter but if you had 25% prop slip and a 17p prop it would propel the boat 25% less than 17inches in each propeller revolution.

Less is good you are probably seeing 7% slip at full noise, around 10 at cruise and lots like 20% or so just before the boat gets onto the plane out of the hole.

Did that help?

cheers fnq

[Wahoo]

Hi Daz, dunno what you mean by change the diameter but if you had 25% prop slip and a 17p prop it would propel the boat 25% less than 17inches in each propeller revolution.

Less is good you are probably seeing 7% slip at full noise, around 10 at cruise and lots like 20% or so just before the boat gets onto the plane out of the hole.

Did that help?

cheers fnq

Scott i found this and C/Paste

Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency (see Efficiency, below). Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack (see Angle of Attack, above). For example, in Figure 4-28, a 10" propeller actually advances only 8-1/2" in one revolution. Eight and one-half inches is 85% of 10", leaving a slip of 15%. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust.

[FNQCairns]

Daz I am just a little daft at times, less slip is always better slip can be countered by diameter more diameter can mean less slip and the converse applies.....closer to what you are chasing?

cheers fnq

[Wahoo]

Thanks Scott
im just trying to squeez a little more out of it
im hopeing to get 47knts fully loaded with this prop, i dont know weather to send it away to Merc of have a chat to NB props

Daz

[FNQCairns]

No worries Daz without knowing where you are ATM you could have some left.

If all you wanted was balls and all top end, that's easy but slip will compound at lower speeds and the boat will be less efficient overall, how much you are prepared to accept as less efficient (than is typical), lower speeds will be the decider in how much extra top end you end up with.

If you have no RPM left at WOT fully loaded your only choice is to go up in pitch and down in diameter to balance the equation, if it takes too large a diameter decrease to balance back RPM then slip will be too large for a fishing rig at the lower speeds and it will be a dog (you might not think so, it's subjective as what a person likes they like regardless of cold numbers).

If you have an off the shelf prop, leg height correct and it all works to what is considered acceptable by the manufacturers recommended max WOT you have nowhere outstanding to go off the shelf again.

Some handy work is needed to go further/faster (extra pitch AND less diameter) and your props are expensive!

Good luck

PS dont for one minute think that just because Merc make a linup of props to suit your Engine that they and their respective recipe is the best for your engine. I cannot find a prop on the market that works better than the one I made to suit my engine, drawback is for every new prop I need to get the scribe, grinder, file and sandpaper out or else run an ineficient and slow rig with an off the shelf prop.

cheers fnq

[Wahoo]

on the motor ATM is the Enertia 14.65x17p WOT is 6250 full loaded, i would really like to get 6400
i also have the Enertia 14.375x19p WOT is 5750 fully loaded getting 48.6Knts
as you can see there is a huge difference in RPM:-/
for some reason, i cant understand is the smaller pitch the bigger diameter:-/

Daz

[FNQCairns]

Ok get the 17 tipped, which is just a little off the diameter this will increase WOT revs but do nothing to increase slip in such a large diameter prop, you will go faster with this.

Alternately you could get the cup decreased or taken out totally but at the speeds you are traveling it adds to rake so may be helping you just a little and who wants to risk a backwards step with such expensive props. The tipping of the prop is not a risk IMO with such a large diameter.

Yes they do not make the props to suit your boat/engine they make them to fit nothing and sadly this is what they usually fit well.

You could probably run the 19 quite happily if you took all of the cup out then progresively decreased the diameter till you found your recommended WOT RPM again and this WILL work, you will FLY, how much it impacts on your overall efficiency will be what doing the experiment will discover and only doing the experiment will discover it.

cheers fnq

[Wahoo]

thank you very much Scott


PS: would you recommend NB props for this or just send it away?

Daz

[FNQCairns]

I will PM you Daz.

cheers fnq