Boat speed, Propellor pitch and diameter

[JohnO]

This started from the thread Disappointing Speed and I wanted to capture it all in one place.

First hull form discussion

With 10 people on board (~1500 lbs) plus your gear, 20 mph would be wishful thinking. Go out by yourself with the least gear possible and the GPS and see what you get. Mine is half again bigger than yours, with twice the motor, and with a load of passengers I don't think I can break 15 mph either, but I can make 26 mph alone. I'd bet with the extra people you can't get up on plane, so you're driving in "displacement mode" which has a mechanical limit defined by physics. Tubes and strakes only provide lift to some limit in weight, and with that many people you've probably exceeded that.

For Displacement Hulls:
Max Speed = Length-at-Water-Line (LWL) x .5 x 1.34
For yours: 24 x .5 x 1.34 = 16 mph

This suggests that a little more motor would get you to 16 mph but no higher without planing.

http://www.2hulls.com/archive/Gen%20Art ... aning.html Discusses the differences between planing and displacement catamarans. They mention an additional factor called "Hull Fineness" which affects the calculations.

The equation above is for an average monohull displacment hull. A pontoon is closer to a catamaran (or trimaran) so the speed will be a bit more than what the equation gives. Catamarans tend to have very "fine" hulls separated by a space. They outperform single hull displacements ships.

I've not been able to find exactly what the change would be but it does exist. (read a lot about catamaran design and everyone mentions it but no one defines it)

This website:Speed Calculator gives an equation with some hull correction factors. Since the standard equation MrGadget provided does not describe my hull form (My real speed with partial planing exceeds the limits of that equation) the truth must lie somewhere in between. I've not been able to find an easy equation to describe a tri-toon hull form top speed yet.

On to props:

My theoretical speed limit based on my prop is 42 (See the prop page at the speed calculator website) . My real world speed is about 37.5 (GPS) So my slip is about 11%.


The question I have not been able to find an answer to is the role that diameter plays in all this.

I have a 15.25x15 aluminum prop which turns at 5500 WOT. Exactly on spec. Providing ~37.5 mph.

Increase in pitch to 17 should drop WOT RPM to 5100. BUT what if diameter decreases (and by how much should it decrease)? There should be a point at which decreased diameter regains the RPMs lost with greater pitch. A 17 pitch yields a theoretical top speed of 47.6 which should get me to 42.3 if slip remains somewhat constant.

I'd like to get over 40MPH (Yes I know that none of us could really "feel" 5mph difference but we all tend to be a little warped about certain things on our boats).

Does anyone know the pro's and con's of trading diameter for pitch? Or at least someplace else to look for this info?

(I hope this whole thing hasn't been answered before but if it has I couldn't find it.)

[GXPWeasel]

I have wondered the same thing about diameter. I am not real sure of my diameter of my prop, but I do know that it is a 17 pitch. This sounds big ( in tooning terms ) but I can EASILY get to 6,000 rpm on my 2 stroke 90 Merc, and only going about 20 mph. I am going to trade my extra brand new 17, for a 21, and hope I can pull another 5mph out of it. These speeds are by my speedo, as I haven't had a GPS on the boat yet. I am thinking that most people who are running smaller pitch than me, must have a bigger diameter prop to get the speeds they are getting. My toon is underskinned, and sits relatively high in the water, compared to many I've seen on our lakes.

[mjlyn]

I wonder about the diameter also. The Mercury Bigfoot motors use a larger diameter. I currently have a 10 3/4 x 11prop on my 60 4 stroke. I only pull 5400 rpm at wot and the max range is 5500-6000 for that motor. I called Mercury today and they recommended a 14x10 with extra cupping.

The mercury rep says that this increase rpm at wot but i wonder if the the increased diameter will negate the effect of the decreased pitch.

Seems like somewhere online there should be a test results of varing pitch/diameter and its effect on speed.

I'd love to do that test myself if someone would loan me all the props

[specialk]

I have wondered the same thing about diameter. I am not real sure of my diameter of my prop, but I do know that it is a 17 pitch. This sounds big ( in tooning terms ) but I can EASILY get to 6,000 rpm on my 2 stroke 90 Merc, and only going about 20 mph. I am going to trade my extra brand new 17, for a 21, and hope I can pull another 5mph out of it. These speeds are by my speedo, as I haven't had a GPS on the boat yet. I am thinking that most people who are running smaller pitch than me, must have a bigger diameter prop to get the speeds they are getting. My toon is underskinned, and sits relatively high in the water, compared to many I've seen on our lakes.

WOW! I had no idea the 2 strokes would turn that many more rpm's than the four strokes, that's very impressive. I bought an adjustable prop last weekend and tried it on my 90 four stroke (2007 merc) @18 pitch 4400 rpm 23mph. then I adjusted it to 16 pitch rpm went to 4900 and speed went to 24.5mph. this was a 13" dia. 4 blade prop. my boat came with a 13.5 dia x 15 pitch which woul do 5500 rpm at 27-28 mph new. this was a larger dia prop but was a 3 blade. I got some digs in the prop which caused a reduction in speed,and ultimately caused the prop to spin the hub due to excessive heat buil up from cavitation and also due to heavy load (tubing). Just ordered the 14 X 13 pontoon prop from mercury. according to the tech there, this should put my rpm's in the 58-5900 rpm range to produce max hp and torque, plus it is double cupped for more traction and thrust. but as I witnessed this weekend, going to a larger pitch can slow you down, especially on a toon. have to keep your motor where it has max performance.... I am sure a corvette could pull monster truck tires,but it would take all day to get to top speed. put those same tires on a 1500 hp monster truck, and they'll spin em'.

[beermunk]

I propped "up" from a 15x15 to a 14x17 on our tri-toon with a mercruiser 3.0. I must say I love the difference. It runs WOT about 700rpm lower than with the old 15x15 prop but the speed is a bit faster. I like that I can keep the RPM's down below max WOT.

[mjlyn]

[quote="GXPWeaselJust ordered the 14 X 13 pontoon prop from mercury. according to the tech there, this should put my rpm's in the 58-5900 rpm range to produce max hp and torque, plus it is double cupped for more traction and thrust..

Special K....I am thinking of ordering the double cupped prop recommended by the mercury tech. I have a 4 stk 60.

Please update us on the performance once you get the new prop installed. I imterested to see your results.

[specialk]

[quote="GXPWeaselJust ordered the 14 X 13 pontoon prop from mercury. according to the tech there, this should put my rpm's in the 58-5900 rpm range to produce max hp and torque, plus it is double cupped for more traction and thrust..

Special K....I am thinking of ordering the double cupped prop recommended by the mercury tech. I have a 4 stk 60.

Please update us on the performance once you get the new prop installed. I imterested to see your results.

I sure will.... maybe every one on this forum could pitch in a few dollars and we could have one of the mods buy several props and pick different boats to try them all on.. you know 20,22,24 footers with 50-115 hp motors and also different size toons.then we could get a better idea of what works best with each set-up... anyway, my prop should be here friday, and as long as it isn't raining saturday, I will try it out,and post the results..... again,as a reference point I have a 2007 bentley 240 cruise(24') with 90hp fourstroke and 26" toons not a tri toon, just a 2 tooner. It ran 54-5500 rpm new @ 27-28mph with a 13.5dia x 15 pitch prop. I'll let you know the results as soon as I come back from the lake.

[dockholiday]

I propped "up" from a 15x15 to a 14x17 on our tri-toon with a mercruiser 3.0. I must say I love the difference. It runs WOT about 700rpm lower than with the old 15x15 prop but the speed is a bit faster. I like that I can keep the RPM's down below max WOT.

I got the same 700rpm difference with the 17 to the 15 but my speed went up with the 15. Not much but something like one and half to two mph.
doc

[specialk]

I propped "up" from a 15x15 to a 14x17 on our tri-toon with a mercruiser 3.0. I must say I love the difference. It runs WOT about 700rpm lower than with the old 15x15 prop but the speed is a bit faster. I like that I can keep the RPM's down below max WOT.

I got the same 700rpm difference with the 17 to the 15 but my speed went up with the 15. Not much but something like one and half to two mph.
doc

yeah, I got 500 rpm and 2 mph sunday going from 13x18 to 13x16 on a 4 blade adjustable composite prop! funny how that works huh? less pitch more speed that's back asswards, but it's how it works sometimes