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Prop Slip Test
Handymans342
Member Posts: 10,375 ✭✭✭✭✭
Has anyone done a Prop slip test on their boat?
Here were the results of mine which shows I need prop work.
WOT RPM: 3250 both engines. 25 MPH was the speed. I was not on the boat so I dont know what the wind or current conditions was. I do know that I have hit 28 MPH on the GPS. The mechanic wants my engines turning 3700 RPM. I have 19 diameter by 23 pitch. Anything below 3000 RPM and I start losing plane.
Here were the results of mine which shows I need prop work.
WOT RPM: 3250 both engines. 25 MPH was the speed. I was not on the boat so I dont know what the wind or current conditions was. I do know that I have hit 28 MPH on the GPS. The mechanic wants my engines turning 3700 RPM. I have 19 diameter by 23 pitch. Anything below 3000 RPM and I start losing plane.
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PC BYC, Holland, MI
WOW I never herd of a Prop Slip Test, learn more every day
I also just found out that my engines are not running at full RPM when not under load. 300 RPM low. So my props may just be fine. Stay tuned
Prop RPM = engine RPM / gear ratio
= 3250 / 2.0 = 1625 RPM
Next, you need the "theoretical speed" the boat would go if the propeller didn't slip at all. For a 23-pitch prop, the boat should move forward 23 inches for each prop revolution, so:
Theoretical speed = Prop RPM * pitch / 1056
= 1625 * 23 / 1056 = 35.4 mph
However, the boat's actual speed is always slower than the theoretical speed - in this case, 25 mph. So the prop slip is just the percent difference:
Prop slip = (Theoretical speed - actual speed) / theoretical speed * 100
= (35.4 - 25)/35.4*100 = 29.4%
You can do this for any combination of boat speed and engine RPM.
@LaRea, great guideline for prop slippage.
@Steve, remember too that your engine's ability to generate HP and its WOT are dependent on load, humidity and temperature. At 20 hours we took my boat up to WOT for about 10 seconds. The RPMs were 4950 with the GPS showing 52.9 MPH.....but that was with low fuel, water, waste and practically no gear. As well the temps were in the low 70s with very low humidity (less than 40%). We repeated at 40 hours at WOT for about 5 seconds and got 4750 RPMs and 48 MPH but that was with a more normal load and temperatures in the low 80s with humidity in the 70% range.
There's slippage and then there are water conditions, temperature/humidity conditions and boat load all of which will affect your WOT. As well, I've experienced "wild card" boating situations that defy theory. My 190 Rinker with a 383 bravo 1 and 23" SS prop demonstrated virtually no slippage at all because the hull was so light with a low windage profile in comparison to the amount of horsepower. BTW a "loose" rule of thumb is that it takes 12 HP to increase a boat's speed 1 MPH above its normal MPH.
Anyway, imo, the really relevant thing - in your situation - is that your boat's WOT should fall within its designated RPM band most of the time you will be using it - and - with the load you will usually be putting in it. Of course, it will vary quite a bit in SW Florida with the considerable temp/Hum. swings from season to season and that's why the rpm bands are flexible. H
However, if your tech is correct and your engines are off by 450 rpm for how you intend to boat then, imo, something is way off. Could be your props - and they might not be slipping, they might just be too "tall" as in too high a pitch.
if it IS in fact 2:1, you're looking at 20% slip.... that's a lot of wasted engine....
on a gasser, what is the RPM and speed where the boat planes? What is the average top speed for the boats with like gasser engines? the hull isn't any different for your diesel, is it?
you may have to play with your trim a little differently to get it to pop up on step quicker- getting your timing just right from full flap and throttle and not hitting the flaps or trim until you're nearing the power curve- then all at once setting her loose.
I ain't the one to better explain this, but I'll spill what I do know...
there is a relatively flat area on your hull as there is all planing hulls, and it's called the 'pad'. It may not be flat like a bay boat, and it may retain a good bit of the Hull's V, but other than that it is a relatively gentle entry angle of attack, and offers little other obstruction...
once the boat is pushed up and on the pad, it takes only a fractional amount of power to keep it there. where energy is expended to lift, and plow through the water, once on the pad the direction of energy is used to maintain angle of attack and push it just hard enough to stay on the pad. The angle of entry your 'pad' makes is based largely on your trim and/or tabs... dragging the tabs will force the leading edge of the pad to make entry, which may be bad as that angle may be steeper than what is an inch or two behind it. The amount of force on your outdrive is ridiculous, as it is the hinge point for the front of the boat (or the entire boat, actually), and just a few degrees alteration of the attack angle will make a big big difference.
my question is if your boat has the 'speed' to get on the pad... it no doubt has the power, but 'speed' is what is required to push it up there. if it doesn't have the speed, my question is if your hull is built differently than others powered by gas- with the pad area slightly forward- and I doubt it is...(edited to add: diesels are powerful in terms of TQ, and TQ is what you need, but they're NOT known for fast spooling up as witnessed on a RPM gauge, and THAT may be what you're missing that is keeping you off the pad)
my next question is:
what device do you use to trim and tab the attitude of the boat? a gauge, or, your instincts?
the thing about finding this sweet spot is that it can't easily be trimmed/tabbed once you're already out of the water and planing.. it's better served jumping all the way up on the pad and then moving the trim/tabs until water it cutting right behind where the hull transitions into the pad- which means you get one shot (hole shot to plane) to push the boat up on the pad, and THEN transition...
most your heavy boats don't have the power to ease on the pad... few do... if you don't believe me, head out underway, and slowly move the throttle to WOT, not by popping it but by a slow steady transition to WOT taking maybe 30 seconds to transition from idle to WOT... I bet a dollar you'll find yourself plowing off plane with a heavily loaded engine... the boat and engines demand that heavy powerful 'pop' to move that hull up as well as forward...
if you google hull design and theory, I wager there is enough information to keep you reading for days on this subject.