RPM & Speed Relationship

[Sea Rider]

Sorry for the dumb question : Is there a direct relationship between rpm and max speed ? Scenario a given boat/engine combo running 25 knots at max wot 6,000 rpm with 13 pitch factory delivered prop. Iif changing prop to a 14 pitch, definitely will lower max wot rpm to say 5800. Will the max speed achieved with a 14 pitch be slight lower than with a 13 pitch accordingly?

For the given example both props are same diam, engine trim & height are spot on. Hole shot will not be taken into consideration.

Happy Boating

 

[Texasmark]

Not all that simple. Basically you have a screw turning in something other than a solid so there is some slippage, usually the worse at low operating speeds when there is a lot of boat in the water and a lot of drag (resistance to movement). Once you get up and running, the drag decreases, amount depends on a lot of things but hull shape has a great big impact...softer the ride worse the drag as more boat is usually in the water at WOT.

Takes so much propeller thrust to push a certain drag to a certain velocity. Thrust vs rpm is the pitch and to a lesser extend diameter of the prop, like a transmission gear in your car.

On changing the pitch and getting the rpms up, it's a matter of what else changes in the process and that is the caveat that only testing will reveal.

So, generally you get more thrust with more diameter and less pitch for a given rpm. Since the distance the prop theoretically moves forward for a given rpm one would expect the boat speed to diminish. However the other gotchas come into play. Some folks have a variant of this but I always marched to the "normal load" at WOT looking for the rpms to be at or very near the top end of the recommended range for setting the prop pitch. For a given size range, diameter moves around as pitch changes but that's for the prop designers to figure out. Especially with 2 strokers since the torque isn't there you have to get your HP from revs (HP = (torque x rpm)/5252) and HP is what spins the prop shaft and makes the prop thrust available unlike something like an OTR diesel truck that has syrup buckets for pistons and cylinders so long you can't see the bottom sort of thing giving you the torque at a lower rpm.

Hole shot is always better with less pitch due to the increased torque. Sometimes porting of the prop (holes under the blades) is used on high pitched props made for high speed running to get the desired hole shot. Big bass boats usually run a high pitched, high performance, ported prop. If you have a higher pitched prop and really like your cruising and WOT performance, porting can make the difference in the hole. Like having 2 props in one.

HTH,
Mark

 

[eavega]

There is definitely a relationship between RPM and Max Speed. For a given pitch, the faster you can turn that prop, the faster you will go. The key measurement, though, is engine RPM. A properly trimmed and rigged boat should achieve its max WOT RPM, so that is your first step. Then you will find out if you have any wiggle room to move your pitch up or down.
Given your scenario, you need to find out what your max WOT RPM for your motor. If you are in the range with both scenarios (i.e. MAX WOT is 5500-6000 RPM), and note most folks like to run at the top of their WOT Range, you can play around with the two pitches. So, if you are turning 6000 RPM, and the 13" Pitch means the prop turns 13" for a full revolution, you are moving 6000 Rev/Min x 13 In/Rev = 78000 In/Min x 60 Min/hr = 4680000 in/hr x .0000158 mi/in = 73.944 mi/hr. Now that is completely disregarding something called slip, which accounts for things like friction, water resistance, etc, etc. so the number is basically meaningless other than for the sake of this discussion of how a change in pitch affects speed.
Going through the same exercise with a 14" Pitch Prop at 5800 RPM: 5800 Rev/Min x 14 in/rev = 81200 in/min x 60 min/hr = 4872000 in/hr x .0000158 in/mi = 76.9776 Mi/hr.

The higher pitch at the lower RPM will yield a net gain of 2 MPH (approximately), or a 4% increase in speed. Given that slip should be constant for your boat (again, in theory), the 4% should be valid, so if your boat is running 25 knots at WOT 6K RPM with a 13" P prop, you should be able to increase speed to 26 Knots with the change in prop at the lower RPM.

Again, I may be totally blowing smoke here, and if I am off with my calculations, I'm sure someone else will pipe in. I know there are good calculators online that will let you play with numbers once you have actual measurements on your rig of current prop, WOT RPMs, and speed.

Rgds

Eric

 

[steelespike]

Keeping it simple 25 knts converts to 28.769 we'll call it 29. 13" at 6000,2.42 ratio,calculates to .049 slip. Unlikely so low but we'll use it simply for calculation.
5800, 2.42, 14" using .049 slip calculates to 30.2. Slip is unlikely to remain the same with a pitch change.Probably would increase with the drop in rpm.
Rpm change for 1" pitch might be as little as 50 rpm and possibly more than our 200 rpm figure.
If you look up a 50 hp Etec motor there are 13 prop sizes with 14" total change.If you look up a Etec 25 there are 7 sizes and 6" in total change.
If you add a 140 lb person in a 25 hp boat you will lose more rpm than adding 140 to a 50 hp.
Thus I think you can see a 1" pitch change with a 25 would lose our gain more rpm than a 50.

 

[Sea Rider]

Muchos thanks to Mark and Eric. Forgot to add to the given scenario, that the max factory stated wot rpm for that portable engine is 6K fully achieved with "just driver" on ideal flat no wind water cond, Combo runs full stated wot with a 13 pitch prop, so if going 1 pitch up will lose some revs but will slightly increase top speed compared to. What about hole shot, if perfect with 13 pitch, what with 14 pitch, better or worst compared to a 13 ? Seems you can't have 3 in 1 top rpm, speed and hole shot, right ?

Eric, will all the data that you currently post under this section, would you say those are plain theory numbers from where to go or account for ? you're not taking hull shape into consideration, right ? so, will this data still be spot on while testing combo in plain no wind water cond regardless of hull shape ? What other issue would you need to add to the equation for those numbers not to work spot on ?

Happy Boating

 

[Texasmark]

Seems you can't have 3 in 1 top rpm, speed and hole shot, right ?
Happy Boating

Well, actually you can and it's called a padded hull, aka "bass boat" with a high rake (for bow lift) high pitch (for speed) and ported (for hole shot) prop. Getting the boat up on the pad makes the hull perform like a water ski with comparable displacement connecting the surface of the water reducing the associated drag. Oh and a high enough hp engine to get you where all that will happen. Go to a boat mfgrs. site, like Ranger brand bass boats for one, and look at the boats they advertise on the water on the pad. There you can pick up on the low water contact area.....aka low drag, huge hp making it happen, and all you need next is to see what's bolted onto the prop shaft. Mercury is what I used to run with a Merc. Quicksilver Laser Prop.....before the advent of the Laser II and all the other high perf. props available today which are available on this site.

Good boating,
Mark

 

[eavega]

Eric, will all the data that you currently post under this section, would you say those are plain theory numbers from where to go or account for ? you're not taking hull shape into consideration, right ? so, will this data still be spot on while testing combo in plain no wind water cond regardless of hull shape ? What other issue would you need to add to the equation for those numbers not to work spot on ?

Happy Boating

Yeah, what I threw up there was pure theoretical based on pitch and RPM. Doesn't take anything else into account (hull shape, wind, wave action, water density, etc). All I did was demonstrate the simple relationship based on the fact that pitch represents the linear distance (in inches) a prop will move horizontally through one revolution. It would be like taking a wheel's diameter and translating that into a linear distance as the wheel moves through one full revolution.

-E