will prop pitch increase help mid range economy?

[truebrit]

I have a 20ft Bayliner with OMC 5 ltr , the prop is 19" pitch and I presume it is pretty well matched for top whack in that she will reach around 40-42 kts. most of my forays involve a mixture of idling about at 6 kts or so and cruising at around 3000 rpm which with the 19" prop equates to around 23kts. I realise going to 21" pitch will drop top speed , which is very rarely approached ( fuel consumption of around 18 galls per hour of unleaded holds me back a lot !!!) but would it help fuel economy at my "normal" speed ranges ?? Any comments appreciated.

<hr width=100% size=1>Mike....... V8s are sweet music

 

[boatmike]

No. Probably make it worse.

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[commissioner]

Moving up to a 21" pitch will actually increase your top speed but knock about 400-600rpm off your max revs at the moment, in theory providing your engine has the power to pull the bigger pitch you should be able to cruise at the same speed with less rpm, therefore using less fuel, unless the engine doesn`t have enough power in which case you will have to use more throttle to get the same speed, hence using more fuel. In any case expect a drop in acceleration. I personally would take prop to a someone like propellor revolutions in poole and get them to re-pitch to a 20" first and see how that goes, if it goes in your favour then you can always have it adjusted to a 21" later.

Hope this helps.
Jaime

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[danielbroad]

If your RPM at top speed is already at the low end of whats specified for your engine you could damage something by over propping it - but if its at the top end you'll definately get better cruising economy by increasing your pitch.

I get vastly better fuel consumption with my Mercruiser 5.0 at 3250 rpm than at 3500 rpm so i've recently changed my prop from 15 to 17inch so i can cruise at the lower rpm - i've got around a 33% increase in mpg doing this.

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[adarcy]

Yes but your reasoning is wrong Jamie.

A higher pitch is like having a high overdrive ratio on the car and driving using it all the time. I agree he'll lose the top end speed which he may not care about but as long as he can get up on the plane easily enough he should see reduced consumption.

At cruising speed with a higher pitch he still needs to put the same hp to the prop but the engine is working a lower revs - lower friction losses- and a WIDER throttle. This is more efficient than revving faster with a more closed throttle because there are losses in the actual throttling effect and it is better to have more filling of the cylinder - closer to the maximum torque condition.

If he can't borrow a 21" I'd have thought your idea of going gradually is eminently sensible

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[commissioner]

I think you have confused yourself, what I have said is correct and true.
How do you work out that with a bigger pitch he will lose top end speed, are you saying that if you were flat out in a car in 5th and had passed your maximum torque band, you could grab another gear and you would bring yourself back towards your max torque figure and increase speed!! Same in boat. Hence as I said, same speed would require less throttle hence better economy.

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[adarcy]

Re: I am sorry but

you are mistaken or, more likely, I have not explained myself clearly.

You said <<Moving up to a 21" pitch will actually increase your top speed but knock about 400-600rpm off your max revs>>
I said <<he'll lose the top end speed >> well of course either could be true and it would depend on what his max revs are on his present prop. If he is revving beyond peak power now a greater pitch will make him faster but if he is under-revving the new prop may make his engine fall down the power curve so much that he cannot attain his present top speed. IMHO I think it is likely as most small fast light boats are already slightly overpropped to give the best top speed and consumption figureswhen brand new but then boats gain with spares, cruising clobber etc.

Using cars as a comparison, german cars used to be slightly undergeared so that their top speed was a bit over the max power of the engine but on meeting a hill or wind would slow the engine and one would then get max power to maintain as much speed as possible. When the 70's fuel crisis bit, they brought out much higher geared cars (remember the VW Formel E's?) then, as now with most 6 speed cars, one could not reach top speed in top gear as the engine was not revving fast enough to give max power, it was a true overdrive.

I think I may have confused the thread by talking about max speed and max torque, my apologies. However, I am afraid your reasoning is still wrong. The maximum torque point of an engine is when it is most efficient ie filling the cylinders and burning it. This may not be the point of the lowest specific fuel consumption but is the maximum BMEP (Brake Mean Effective Pressure). Torque is the thrust, power is torque x speed, torque drops off beyond the max torque speed leading to a power hump, revving faster still the power drops off. One needs power to drive boat/car and the speed tops out when the engine cannot produce more power than the drag which increases with speed. The gap between the power needed to drive at a speed and the maximum power that the engine is capable of giving at those revs is the spare available for acceleration.

His OMC 5.0 is, I am sure, a petrol and one must be clear different conditions apply in a diesel but, as I said, a wider open throttle at a lower engine speed is more economical. You seem to be under the impression that the only thing that matters is the position of the throttle yet I am sure that you do not drive at 40mph on the flat or even better up a slight hill revving away in 3rd gear when you have a 4th or 5th (if it will pull it). You would certainly have a wider open throttle position in the higher gears. Compared with 3rd, 5th gear means the cylinders are being filled less often, filling each time with more fuel and air and operating closer to their ideal efficiency. In 3rd they fill much more often and are "sucking" against a strangulating throttle with inevitable frictional pumping losses added to the friction loss of the faster engine revs. It may seem counterintuitive but it is more efficient to slog along with a wide open throttle than revving faster with less throttle. On the wife's VW Passat years ago it had a 'change-up' light when the system worked out the engine could pull a higher gear. This is/was much more sophisticated and accurate than the misleading economy guages which only measured manifold pressure, these encouraged people to trickle along gaining speed slowly in the lower gears. Scientific papers at the time showed clearly it was more efficient even to use full throttle in the lower gears until past the max torque point to get to cruising speed and get into top as soon as possible but the most efficient was to use about 2/3rd throttle until it would pull a higher gear and to change up into it as soon as one could.

To come back to his boat, higher pitch = lower engine revs for the same boat speed. He needs the same hp output to the water to get the same speed as before. The slower the engine runs the lower the frictional losses. The slower the engine runs the wider the throttle has to be open to fill each cylinder and the less the throttle butterfly/slide is disturbing the airflow in. Each cylinder is banging less often but is operating closer to its maximum efficiency and overall his consumption at that speed will be better. As he goes faster and faster, the power required increases in both prop pitch situations supplied by opening the throttles. Now, if the greater pitch does not allow his engine to reach peak power revs he will then have a lower maximum speed than the finer pitch which will allow max power output - OK ?

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[commissioner]

Re: I am sorry but

I`ll send you a book, then after you have read it you can come back and agree with me. ok?

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[adarcy]

Re: I am sorry but

Fine

Please tell me which book. It would be easier to read something different than dig out the old articles + references on which I base my understanding

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[duncan]

Re: now now

I can't see a fag paper between you two except possibly with regard to the starting point - ie the relationship between his existing engine/gearbox/prop to hull/drive drag.
If (big if) the existing set up is 'perfect' then increasing or decreasing the pitch shouldn't improve things across the range but may, as you both state, result in an improvement in economy at particular hull/engine speeds - effectively you are selecting a speed then choosing a 'new gear' specifically for that speed.
Can I suggest a further option for consideration - I am assuming that you are using a standard 3 blade prop, designed for an across the board average performance - have you looked at a prop specifically designed for your cruising economy?

<hr width=100% size=1>madesco madidum ..../forums/images/icons/smile.gif

 

[adarcy]

Re: Ever the diplomat Duncan

I certainly hope we are not having a row but a gentlemanly agreement on the outcome of economy but difference on the reasons why.
The fag paper is the throttle setting.
Agree 100% we can go no further on the top speed issue until we know the WOT revs and the peak power band. Comissioners first posting said "will increase...but lower revs" the bit about "providing your engine" I still take to be at his cruising revs. I was sort of agreeing with danielbroad about what his revs were initially but did not make it clear. Later response clearer with me saying repitching may increase or decrease top speed.
The fag paper is whether the agreed increase in fuel economy of the higher pitch is caused by having a wider open throttle at lower revs (me) or you use less fuel because you have a lesser open throttle because the engine is revving less (Comissioner).

Nitpicking maybe, because we agree on the result but I always like to know why and how rather than just know it works.

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[mercman]

any light on the matter a decrease in or increase in pitch equates to a differential of approx 200rpm at WOT. i.e. say max rpm with 21" = 4400rpm going to 19"=4200rpm which you will find will decrease top speed . Planing times will be increased unless the prop is too small and just cause more froth than anything else. I believe if you went to the smaller prop you will use less fuel when opening the throttle because she will spin up quicker where as the 21" will labour a little longer.

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[duncan]

I think you will find the 19 will run up around 200 more rather than less than the 21 at WOT.................

<hr width=100% size=1>madesco madidum ..../forums/images/icons/smile.gif

 

[mercman]

apologies your right got confused myself and wrote it the wrong way round

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[jon_bailey]

I assume that you are using a three bladed prop. Why not go to a four? Slightly lower WOT speed but faster pull onto the plane and more efficient at cruising speed hence better mpg. I did this and it certainly improved things although difficult to quantify improvement in fuel consumption. I would estimate 15% at same cruising speed.

If you are using ally prop then you may also want to consider stainless. Thinner material allows more efficient prop design that will also improve economy and top end speed.

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<A target="_blank" HREF=http://home.btconnect.com/bailey>BOAT NO LONGER FOR SALE</A>

 

[truebrit]

Re: I am sorry but

Many thanks for your informative reply to my post about prop pitch change, much appreciated.

<hr width=100% size=1>Mike....... V8s are sweet music