What speed can you get up to with your motor?

[Lone Ranger]

What speed can you get up to with your motor?
Well, I'd be interested to know what max speed your motor can do, and how many HPs yours has? I am mostly interested in those from 25 - 40hps, as mine is 30hps and max I can get to is only 23 knots.

[Rod]

Not fast enough unfortunitly:(

Want more speed, must have speed:( :(

I do not know what has been posted already or deleted but am happy to assist if I can without causing any angst among others or to yourself.

There is a small program I have here called 'prop calc', which allows you to determine if you are getting optimum speed from your boat and also the proper fuel consumption.

I can however do this manually for you as well - tis something they teach you at boat school.

So - would you like a copy of the program?, or would it be an educational excercise to work out your particular figures here manually so everyone else can go away and do theirs?

In order to know anything worthwhile about your particular figures - you need to gather some information to help us out!

Your engine manufacturer, will have a brochure or web site with some "specifications" for your engine.

These relate to a lot of the engineering of the outboard, but for our purposes we only need a few figures to get started.

The first is something called WOT (which stands for Wide Open Throttle) RPM's (Revolutions Per Minute) that your engine is designd to run at, when you twist the hand throttle full open.

Usually on a 2 stroke this is a figure somewhere around 6000 rpm's but we need to know that figure to work out the theoretical top speed and eventually actual max top speed achievable!

The next useless bit of info we need is something called the "gear box reduction ratio"...

The gear box is the bulbous bit on the bottom of the outboard leg in front of the propellor ahich has a shark fin upside down attached to the bottom of it (called a skeg!).

Inside the bulbous bit is a 'crown wheel and pinion type gear' for want of a better description - which reduces the engines WOT rpms down to a final drive RPM...and also changes the direction of engine drive from straight up and down to forward and back (Verticle to horizontal).

We need to know that particular gear set reduction ratio..

Usually it's somewhere around 2:1 (2 revolutions of the engine crankshaft / flywheel yeilds one revolution of the propellor shaft and hence propellor)

Some ratio's are 1.8:1 or thereabouts - the engine specifications brochure/website will have that detail on it!.

Next we need something called the diameter and pitch figures of your propellor. Usually these are stamped somewhere on the propellor and also mentioned on the specifications sheet accompanying the OB as long as no ones changed it from standard!.

The figures usually look something like 13 & 3/4 (inches) x 12 (inches).

This would indicate a propellor with a diameter of 13.75 inches and a rake angle on the blades equivalent to 12 inches of forward movement thru a semi solid substance (Gellatine?) for one complete revolution!.

Lastly we need an ACCURATE speed you are getting now at WOT (Wide Open Throttle) measured by hand held GPS. (You could however time an EXACT known distance and work it out if you don't have access to a hand held GPS!).

If you had access to a engine rpms gauge that would be very handy too - to see if you are in fact achieving the manufacturers specified WOT rpm's at the top speed you are getting now!

Some mobile mechanics may have a portable Tachometer (RPM's gauge) that they could lend you to ascertain this.??

From all this information - we can calculate several things!

1. If your engine is achieving it's specified WOT rpm's. If not it is Bad for a engine on a boat to lug all day...it's like trying to drive a car up a steep hill without changing to a lower gear - bad for the car engine bad on fuel consumption. This happens if you have a propellor with a pitch (blade rake angle) too steep for your shape length and weight hull.

2. If the engine is achieving it's specified WOT rpm's...then we need to see if the pitch (blade rake angle) is optimum for your particular engine / boat / weight combination. We do this by calculating the amount % of "slip".

That is a measure of how much water slips past your rotating propellor without imparting any forward thrust. You could equate it to spinning the wheels in a car on say a wet road or lose surface like gravel....the car will rev up pretty fast - but not actually go anywhere very fast for all the fuel it's burning and noise it is making.

A well set up combination of prop / OB / boat / weight, will achieve somewhere around a 9% slip factor. Any more than that and something is not right!

From your figures - we can calculate manually a theoretical top speed, from your boat OB & propellor thru a semi solid substance like gelatine with no slip...just with simple math.

Then we can subtract the actual speed you get from your GPS reading...

This difference - when divided by the theoretical top speed with no slip is your % slip factor!

If it is more than 9% we know you probably have either too steep (tall) a pitch prop (too steep blade rake angle) or alternately too small a diameter propellor and it isnt getting sufficient surface area to grip the water sufficient to propell your boat forward with optimal efficiency!

From all this info - we can use the computer program to speculate what correct propellor specification would achieve the optimal results for you!

I hope that makes sense!

Next post I will do a theoretical example for you to copy and save - then you can diagnose your own particlar vessel and wont have to rely on anyone elses best guesses!

Cheers!

[Lone Ranger]

Jesus Shann! Do you always have to write in such long paragraphs. I didn't finish reading it! I couldn't went cross eyed! I will have to print it out and read it tonight so it puts me to sleep! :D :D Read a bit, sounds interesting but my motor is still under warranty. I am just curious what max speed others can do on theirs that's all. Just curious, not interested in going too fast but will let you tell me all about it one day. Looks like I will need 2 weeks down there! Man I will have to come see you in person with my ear muffs? :D Many thanks! You got a tape recorder? :D Seriously now, I am confident there is lots you can teach me indeed!

EDITED: Gees, ta JP. Interesting to note that it's doing well for its HP then. Maybe one day I will get the transom weight increased like you did and go bigger! :D Thanks!

[madsurfe]

Hi Carol

23 knots is pretty good for your setup. You cannot expect much more than that without spending considerable time and effort.
Your speed is indicative of your hull type and hp and I doubt it could push a taller pitch prop. You may only get 1 knot more by going to a taller prop and that move will cost you somewhere between $220 and $440. Don't use the Muddleduck speed as your benchmark because I only just managed to pull that off after a lot of hard work.

[Stu_000]

I get 23 knots out the punt and I reckon that's enough. Probably by tweaking this and that I may get 1-2 more but what are we talking here? Probably 1-2 mins saved over a moderate journey.

As they say "the minute you save could be your last" ;)

[madsurfe]

Hi Trouty

very true about the theory on getting more speed but not necessary in this case. It is a Dinghy/Punt crossbreed that has all the mod cons. The little 30hp is using all it's grunt to drive the boat and if Carol is getting 23 knots then she has it trimmed spot on. Any attempt to get more out of it is futile if you weigh up the $ against the benefit.

OK so you post that you are getting 23 knots!

Lets assume for this example the following specifications:-

6000 RPM's = Max WOT RPM's specified by engine manufacturer
2:1 Gearbox reduction ratio!
8inch diameter x 10 inch pitch prop fitted standard to the engine when you bought the boat.

Some Maths conversions figures so we all work with the same data!

12 inches to 1 foot
1760 Yrds to a Statute (Land) Mile
5280 Feet to a statute (Land) Mile
1.18 Statute Miles to a Nautical Mile (At the equator).
6230.4 Feet to a Nautical Mile
60 seconds to a minute
60 Minutes to an hour

So...

1). We assume - you are acheiving max engine manufacturer Specified WOT RPM's at your measured 23 knots top speed!
which is 6000 RPM's

2). At 6000 RPM's, your propellor spins at 3000 RPM's (Because gearbox reduction ratio of 2:1 reduces it by half)

3). If the propellor would travel forward 10 inches thru a semi solid substance like gelatine in one revolution with no slip then in One minute it would travel?

3000 RPM's x 10 inches = 30,000 Inches in one minute!

4). Therefore in one hour the same prop under the same condtions would travel 60 Minutes x 30,000 Inches = 1,800,000 inches!

5) How many nautical miles is that in one hour? 1,800,000 inches / 12 inches = 150,000 Feet in one hour.

6) How many nautical miles (at the equator) is 150,000 feet?
150,000/6230.4 = 24.07 Knots!

So we know that If all my above guesses about your OB specifications were correct that theoretically your boat should travel at 24.07 knots if there was no slip on the water of the propellor (because we assumed it was a semi solid substance like gelatine).

If we also know that you claim your boat only does 23 knots then:-

7). 24.07 knots subtract 23 knots is 1.07 knots!

Now to calculate % slip factor?

8). 1.07 knots / 24.07 knots = 4.45% slip factor - which would be very very good and indcate you MAY have almost the prefect setup for propellor your boat or be slightly underpropped!

It could however indicate you are underpropped and could also possibly go a little faster!

For each additional 2 inches of prop pitch you add to your propellor you will lose approximately 150 rpm's off your WOT RPM's, if you are already at optimal pitch, but will gain 5 knots!

There is a balance point at which you cease to gain extra top end speed with adding more pitch to the propellor - but instead get more % slip and lower WOT RPM's.

The trick...is to get a match of prop pitch and diameter with 9% slip or less for the very best WOT top speed!

The computer program allows you to plug in your known figures & % ages etc and then allows you to pretty accurately predict what results you would get from altering pitch of the propellor or increasing the HP of your OB etc etc.

It is what some dealers use when they first set up a new boat for a customer (if they are clever - some are not so!

Everything you add to your boat in the way of weight, once you have set the rpop up for optimum performance will take away from the performance..so it is best to get your boat setup for hw you want it to be for the rest of it's life before you go to all the trouble of exchanging props to get a few more top end knots!
(Otherwise you just have to do it all again!).

Ther tricky balancing act is related to a lot of variables like planing surface area of tyhe hull, dgerees of deadrise angle of the hull, weight of the vessel plus payload, density of the water (your boat will be faster by a couple knots in salt water), slight ripple on the surface or aerated water from another boats wash in front will all help to increase speed!

And of course - lastly the boat will go faster downhill toward the south pole than it will toward the north pole which is uphill! :p

(Just kidding on the very last sentence!!) :rolleyes: :D ;)

Cheers!

I was already typing the second reply wen you all made yours....I did not realise it was just a rhetorical question, I actually thought someone wanted to know the method and how it all works for future reference maybe?

I figured maybe the young ones here might actually learn something!

I usually go to such trouble in case I'm not around any more and some kid in the future asks a questuion and can't get the full response - from anyone, maybe the search engine will turn up a old reply from me - with the method to work it all out.

Lots of people read posts but don't participate and take away a LOt of education...

I try and write with them in mind too!

I'll try hard to go back to the readers digest version in future!:rolleyes:

Just coz they are little boats don't mean they won't respond to some fine tuning..

Ok a props expensive but if you save that much in fuel in 12 months...and get more top end - then it wasn't all that dear was it?:confused:

If the boat is overpropped and lugging and you save blowing up a OB powerhead at $2K+ to replace then a correct prop was probably cheap at a few hundred $ no?

I guess it's all in how you look at it!

Thre was a closing down clearing sale last sat at Yunderupo marine - lots of dofferent props all going out for pennies!;)

Also breamers could probably "swap props" to test out alternative pitches etc, at probably 'no cost' if they just wanted to try before they buy!

Me - I just love messing with boats and discussing them!

As an afterthought - if Carols boat is struggling with 30 horses it may not be prop or HP as much as lack of planing area so maybe a foil (dole fin etc etc) would add enough extra planing area for the 30 ponies to get up a decent canter or support a better pitched prop?

All food for thought, but if costs an issue - then you only get what you pay for!

I also replied to your PM Carol but your mailbox was full and it got lost in the internet ether! Thats happened a couple times now!

Cheers!

[Ozzy1]

I run a 40 Yam 2 st on my 4.1 Poly and get 26 knots 2 up with all gear

Cheers
Ozzy

[Simmsy]

trouty hats off to ya mate. you gotta be one of the most clued up blokes going around.
Thanks for the info you posted here, I am going through similar issues with my hornet and 25hp honda and will use your info to suss out what to do next.

Cheers.

[Lone Ranger]

Hey trouty sorry re PM. Hey I think you are a rather helpful man indeed, always wanting to help, yak a lot, type a whole essay ;) but always genuinely wanting to help! You have great knowledge. Hope your kids appreciate it. I always print everything that you advise on. Will clear PM. Go Trouty! Cheers LR

Some OB's are made in "Pairs" by OB Manufacturers..

Bye that I mean, they may for example make a30 hp & a 40 Hp model whichh are exactly the same OB's.

The lower HP model may be rev limited (say the 30 Hp will rev to 55oo rpms while the 40hp will rev to 6000 rpm's

Others just have a throttle travel stop..while yet others have smaller jets in the carby and a carby spacer plate with air restriction and so on..

So- you may be in a position where a couple hours work by a mechanic can remove the rev limiter or carby restrictor plate - put in new jets and get 10 extra ponies for very little cost when the need arises.

Then with those 10 extra HP you can twist a steeper pitch prop at WOT for a lot of extra top end speed...

Just another way to skin the same cat!

Cheers!

[Brad Y]

Trouty, i have the same model outboard as carol (lone ranger) and i get about 22 knots when i have two people and a half full well ( measured by my new navman) For some reason when i do a bit of a left turn and the transducer is skimming the surface it gets to 35 or so knots....

The outboard is a 30hp mercury lite weight (the japanese model) I think from memory it revs to 5500rpm and i think the prop is a 9.9x 11? dont quote me as its late and ive just go home from the pub....

Im considering getting the 40hp lite weight model just for sh!ts and giggles and extreme fun... I crave fishing time and while 22 knots is good for a dinghy, i cant wait til the day she reaches 30 ligitimate knots...

Hi Brad,

Mate, I'm not sure how you are measuring that speed...

It sounds to me like you have a paddlewheel speedo attachment alongside your normal sounder transducer, (which is quite common!)

They aren't a bad way to measure speed at fast speeds but quite unreliable at slow speeds and also susceptible to fouling with weed or any air bubbles fed to them by thru hull skin fittings in the underside of the hull forward of the transducer etc...

Another common speed measuring device in boats is a water pressure sensor type called a pitot speedo- which has a very small hole in the leading edge of the gear case with a rubber/plastic tube leading all the way up the leg - then thru the boat to the dash mounted speedo.

Again they are also not bad at high speed as long as the tube isn't split, damaged, kinked, or clogged with dry salt or the tiny pitot hole in the front of the gearcase, jammed up with mud or shell grit etc

However at slow speed they again aren't all that "accurate' for such prop calculations!

The best way of course is to measure speed by GPS!

From the sound of widely varying speed readings in a tight port turn it sounds to me like you have a paddlewheel speedo getting fed aerated water from the sounder transducer? (OB leg induced turbulence?) in a tight turn affecting your speed reading!

Thats my best guess anyway!

With the 3 common ways of measuring speed, all have their limitations - but GPS is usually regarded as the most accurate of the methods.

You see GPS measures speed over "ground" (the river / ocean floor) - just like your car does!

The others measure speed either "against" or "with" or "across" current - which can totally make readings outta whack!

Imagine drifting down stream at say displacement speed at 8 knots due solely to current and also making way of 1 knot downstream with your engine to maintain steerage! (so you don't hit any bridge pylons!)

Technically (by GPS) you would be making 9 knots over ground (8 knots current plus 1 knot propulsion to maintain steerage!) Theoretically Marine and Harbours (OK DPI Water Safety) could book you for exceeding 8 knots! :rolleyes: :mad:

So - measuring 1 knot downstream on your paddle wheel or pitot speedo's when the currents adding another 8 knots you have no conntrol over suddenly makes you a criminal! :rolleyes:

This is why GPS is preferred as a measurement method for speed calcs and the same applys for working out prop figures, when tweaking for performance!

These days you can get a hand held Garmin GPS for not much over $100...so for getting accurate speeds (not to mention saving waypoints of favourite fish producing structure in unfamiliar rivers, estauries, bays that you might want to come back to again), is probably a worthwhile investment IMHO.

30 knots is not unreasonable as aspeed to expect from a 30 Hp motor.

I seem to recall only 27 knots from my 16 ft River runner with 50 HP 4 stroke Merc!

That was because it had the low gear ratio bigfoot gearbox (designed for pushing heavey slow houseboats, where torque & longevity of the gears under heavey constant load was more important than top speed!) as well as a low pitch prop for the same reason!

That same 50hp OB with a better ratio gear case / lower unit and higher pitch prop could probably have pushed her to near on 40 knots I reckon!

So - just like cars at the drags can only go as quick as they can get the power to the ground, boats have to get the best setup for traction on the water and get all those horses into pushing the boat forward and not up over planing wedges etc etc.

Planing surface area can be a help or hindrance!

Not enough planing surface area, combined with insufficient HP and you can only push the boat at it's nominal displacement hullspeed...extra rpm's and HP don't achieve much if any extra speed, they just burn more fuel andcreate more heat in the engine for very little extra speed.

Some of the older fly bridge cruisers were actually designed this way - to only "half" get on plane i.e. to "sit on their planiong wedge" for a comfortable ride. It was in days when engines were much heavier and hulls as well and power output was uch lower than today - it was the "best they could do" at the time.

With a small boat - adding more planing area (such as adding a foil) - will sometimes give amazing results - the hull may achieve plane (climb over it's own planing wedge) at much lower speeds and ride higher in the water and thus have reduced drag from reduced wetted surface area and hence actually go a lot faster!

You would think that more planing area normally means more wetted surface area therfore more drag & therefore less top speed BUT if lack of planing surface area is stopping a hull from planing and all those ponys are struggling to get over the hump - the prop has insufficient grip on the water - then you reach a point where the hump + prop slip just mean you aint going to go any faster!

However - once you change something..i.e a prop with bigger diameter for more surface area hence grip might just do the trick, or a foil might add enough lift toget over the planing wedge and suddenly - the engine will "wind out" to max RPM's rather than bogging down and your suddenly up and away at MUCH higher speed!

It just takes tweaking.

Sometimes having a little more 'cup' added to an existing pitch prop is all ypou need!

Understanding the problem is half the battle to getting the speed you want and it's NOT always a case of just more horses!

Hope that helps....

You guys should experiment with each others props (if they are different) to see what happens!

Can't hurt can it?

Likely yours and Carols are the same propsif you have the same engines - often the manufacturers send out the smaller HP outboards with just a standard prop...one prop suits all boats theory which is seldom correct!

But - there also often arent a large selection of varying pitch propsavailable for smalloutboards like their might be for larger ones... so sometimes your between a rock and a hard place!

A good prop man tho - will get it right with some help from prop tables, and prop calc programs as well as some good ol fashioned trial and error!

If so many of you breamers all have these lightweight mercs 30 Hp jobbies...(Tohatsu?) then maybe someone should do a bit of experimenting to get the right details before everyone has a go..and pass on his results so others can take a shortcut to the solution!

Hope this helps!

I did suggest I think once before - that maybe breammasters should load the prop calc program to their site as a service to boaters so anyone can log in and with the right numbers self diagnose their boats performance..and select the correct pitch props to get the most out of their boats..

Maybe there are software copyright issues or something which is why it isn't done - I can't recall right now!

All I know is I have a copy of the program and it works for me!

I did once have a link to a website prop calculator program...but seem to have lost it now in one of my hard drive crashes over the years - no doubt a google would find one or two such examples for people to use!

Cheers!

[BIG PETE]

trouty
is there anything you dont know about im not being a smart ass its just you have an answer or a solution for anything i find it amazing good work mate

Undoubtedly there are things i don't know about!

I have spent some 5 years or so as a hobby hanging around some US boating web boards, and learned a heck of a lot! And i've been fortunate that a lot of very helpful and decent people helped me learn stuff along the way - i try to return the favor..but it does seem to tget me into strife with people who get cranky about me writing too much or knowing to much or something!

Heck sometimes I get it wrong and even give bad advice!

All the same - I don't ask anything in return, I get enjoyment from helping people - I'm a people person and I get lonely on my own!

Still - it gets me into plenty of hot water from time to time, only 2 days ago old mad max garth threatened (for the 2nd time in 10 years) to introduce me to his baseball bat! :D :cool: :rolleyes:

Geeze - I hope when I'm his age, I'm as much of a reprobate as that ol codger - giving all you young-uns what for!;) :p

Cheers!

[BIG PETE]

trouty
good on ya mate givem sh+t some advice is better than no advice

[Simmsy]

I just googled prop calc and came up with this site:

http://homepage.ntlworld.com/john.weale/navsoft.htm

The program seems pretty good and it is a free download.
The only problem it is in the imperial system so you might need a conversion chart with you to translate all the figures.

But it does give you your prop pitch and size in 2,3 and 4 blade props.

[TopEnder]

Try this link for a top conversion chart(s) (http://www.spunge.org/~aag/units.htm)

Regards Dave:D

p.s. GO TROUTY!!!!!!!!!!!!!:p :p :p

[Rod]

I think i have resolved my need for more speed:D Photos to come tomorow...

:D :D Warning warning Danger Will Robinson..

Do not encourage me!:D

Besides boats and fishing, my "other hobby' is theoretical physiscs so unless you want me posting my proofs that Einstein was wrong and E Does NOT = Mc^2 then you should refrain from any encouragement!:p ;) :D

After that you'd likely get a rendition of much vaunted Russian Physicist Leonid Lebedevs Dark Sucker theorem! :cool: (This is the one where light globes are not light globes but are in fact dark suckers!). You see Leonid contends that Light globes do not in fact emit light at all - but do in fact "suck dark" and it is only the absence of dark which we percieve as light!

So - the reason light globes blow out at unpredictable times is because they are full of dark and can't hold anymore - which is why they cease to function! When we then dump them in landfills - they get broken by the bulldozers and the sudden release of dark energy that was sucked and stored over such a long period, in such a short burst, is the true root cause of global warming! :eek:

So - there you have it!

Cheers!

[Brad Y]

Trouty your a legend mate

I know the value of a good foil. The grandads runabout would barely plane with 4 guys going deep sea fishing and with an se400 foil it is amazing. Not only in planing but we reckon it has made it a hell of alot faster.

I have an se200 foil on its way soon. Im not too fussed prop wise. When the one i have gets buggered i will get a descent prop that will push the boat faster. You were right about the transducer, it has the little wheel on it and varies between 22 and 24 knots at WOT again low speeds dont seem that accurate but i can calibrate it on the sounder with a gps which is good.

At the moment i am not complaining about speed. 22 knots is perfectly fine with two guys and water in the well. As long as i get to where i want to fish and do it safely i think thats the main thing. If i want to go hyper speeds i will buy a stratos or similar type of boat. But then i might as well write off fishing out in the ocean....