How much PROP SLIP do I have ?

[Baba]

Wondering if maybe someone can explain how slip is relative... if my prop is ok? Do I have too much slip? 5%-25%?? What's the good - bad - and the ugly ?

Click the attachment below for prop comparison results. It will open in your downloads as a PDF.

6100 WOT at 31.4 mph
I get her up on a plane at ~3700-3900 rpm @ ~14-16 mph, but loses plane coming down in speed from up above going down at 18-20 mph.
Very (ass) heavy boat, lots of stuff, (not counting the fishing gear; downriggers or crab puller, pots, gear, and the 1st mate (wife).

1985 SC 17'
Yamaha F90TLR 2008 4 stroke, Permatrim,... is my gear ratio 2.15:1 ?
Stainless 4 blade ; SOLAS H4 Titan  13.5 x 13.  I dont see nayone with this size on a 17SC, (... it tested about the same as a SOLAS 4 blade aluminum 13x15 tested prior).


Got alot of stuff, test numbers with 1 person,  Yamaha T8 Kicker, semi loaded (ya think?)... trimtabs, hyd steering, Auto pilot, big radar, heavy captain chairs and seaspension pedestals, and alum wheelhouse; (3/4 full 23 gal gas tank, aft starboard 2 gel batteries, aft port 3 gal gas tank, ladder, etc).

Im no expert, (but I have tried a lot of props). I already raised the motor up a hole.

I tried looking up slip calculators, but i get lost. 

The pitch is the theoretical distance the prop should move through the water in 1 revolution. A 24" pitch prop would move 2 feet. The theoretical speed is figured by multiplying the pitch x engine rpm and dividing by (gear reduction x 12) = feet per minute. Divide by 5280 = Miles per minute, multiply x 60 = miles per hour. This can be shortened to (Pitch x RPM) / (Reduction x 1056) = MPH.

Slip is figured by comparing this theoretical speed to the actual measured speed. By the way, for these calculations, add 1" to the pitch if your prop is cupped. (Just about all hi-performance props are)

Turning Pt 08 11 23 after raising motor.pdf

[Fisherdv]

I would say your performance numbers sound good based on the weight of the load and being able to hit 6100rpm@31mph. If you have both a fin on the motor, and trim tabs I would only run one or the other. Probably lose the fin.

[JDB]

I would NEVER have the phrase "(ass)heavy" and "(wife)" ever, ever ever ever in the same sentence... 

However, the rpm you are able to turn is good, the characteristics of the prop *may* not be a great fit for the boat(I'm not all that family with the h4 titan).  The aluminum Amita 4 ... Good stern lift- really good all around prop, lots of people here like the prop on lots of different boat/motor combinations.

[Tom C]

OK, there are a LOT of things to discuss here. I'll take it bite by bite. Simple and obvious things first.

31.4 MPH at 6100 RPM with 13" pitch prop on a motor with a 2.15:1 gear ratio is about 10 percent calculated propellers slip.

This is typical of an aluminum prop, but might be a little high for a good stainless prop. I do not have personal experience with the intermediate size Solas Titan HR4, so it might be perfectly normal, or not.

Overall the speed is not bad, but I suspect there is room for improvement, and I'm not talking about going faster at WOT, but overall performance during acceleration and cruise speed, both in terms of subjective handling, but also fuel economy.

[Tom C]

The pitch is the theoretical distance the prop should move through the water in 1 revolution. A 24" pitch prop would move 2 feet. The theoretical speed is figured by multiplying the pitch x engine rpm and dividing by (gear reduction x 12) = feet per minute. Divide by 5280 = Miles per minute, multiply x 60 = miles per hour. This can be shortened to (Pitch x RPM) / (Reduction x 1056) = MPH.

Slip is figured by comparing this theoretical speed to the actual measured speed. By the way, for these calculations, add 1" to the pitch if your prop is cupped. (Just about all hi-performance props are)

I believe you copied that from the BAM Go-Fast site [ https://www.go-fast.com/Knowledge-base/Find-your-prop-slip ]

The advice is rather dated, and one should NOT add one inch of pitch when making the calculation with a cupped propeller.

No need to do boring arithmetic, just use a prop calculator. Here are my two favorites:

https://continuouswave.com/calculators/propCalc.php

https://www.mercuryracing.com/propellers-overview/prop-slip-calculator.html

[Tom C]

Always remember that calculated propeller slip is just a tool we use to analyze our propeller's performance. It is NOT a goal by itself. If a given boat/motor/propeller perform better with higher slip, so be it.

Every model of outboard propeller made today, and there are hundreds, will have its own typical range of calculated propellor slip. Some will yield modest slip numbers, some much higher, other will be near zero, or even negative. You need to know the models, and have experience with them in order to gauge this.

[Tom C]

As a corollary to the above, propellers are not generic pieces of hardware that you can just describe by diameter, pitch, and number of blades. Each model will have its own characteristics.

[Fisherdv]

Looks like the best performance you had (according to your chart), was with the aluminum 13x15 prop, and no motor fin.

[Tom C]

Yes, 33.4 MPH at 6100 RPM with the 15" pitch Solas Amita 4 (17% slip) Not bad.

But is the better performance because of the prop, or the lack of the Permatrim?

[Fisherdv]

I would try your current prop, but without the fin. The fin and trim tabs together are going to end up fighting each other and adds to much drag.

[DARice]

A pilot house adds a lot of wind resistance. Small effect at low speeds, but meaningful at higher speeds. My max speed has dropped 1.5-2mph. For me, some reduction in WOT rpm, but maybe an increase in prop slip, too? I don't yet have detailed performance numbers on my current setup to make a fair comparison.

Dave

[KEM KAI]

JDB, loved the advice about"ass/wife" pretty funny but good advice.  Unless your wife is like mine and just laughs, lucky me. I have a sticker that says"My wife gives me sound advice-99% sound 1% advice".

[Old No7]

I think that ensuring your motor attains the "max RPM per spec" at WOT is much more important than any paperwork calculation about prop slip.  My Arima only turned 5,200 RPM with the old 3-blade prop that was on it when I got it, and the performance of the boat are SO much better with the 4-blade prop that I changed to -- that also got my max WOT RPMs up to 6,000 which matches the Honda spec.

The outboard was lugging when run at 5,200 at WOT before (luckily my other brother didn't run it much), and while she hasn't told me so, I honestly believe "she runs better" now at 6,000 RPM.

Old No7