Prop change and fuel economy - Quintrex 540 Bowrider

[thylacene]

Having asked some questions here about props and then done some research I changed the prop on our 115HP 4 stroke merc from a 16" pitch Avenger to an 18"pitch. The boat is much more pleasant to use now, and if truth be known may even be better with a 20"Prop, but scored a 2nd hand 18" near new for $250.

The cruising speed is better, seeing at least 8 Kmh difference towards the top end, but haven't had it on smooth water to do a direct comparison. The new prop seems to have more "bite" when it is choppy and allows for significantly more trim without cavitation. It also turns a bit harder, as the passengers found out as three seals surfaced immediately in front of us at 50Kmh

We clocked up 90 km (using mapping GPS) over three trips on the weekend running around outside of Bermagui starting each time with a full tank (95 litres) and four adults on board with gear and used 42 litres of fuel in total, most of this at around 40kmh (4000 RPM) with the odd burst of youthful exuberance at 50-55kmh.

Don't know how this compares to anything else out there as this is our first boat, but it seems reasonable.

Have now been out 9 times, and clocked up just over 24hrs of engine time. Feeling quite comfortable with the boat generally, and it has come back dry each time (no water from bungs). The boat is relatively dry to ride in with only a small amount of spray when it is choppy.

I couldn't find this type of info when we were shopping so I figured this is as good a forum as any.

Even caught some fish this trip!!

[Noelm]

main thing is, what is your max RPM with a "normal" load"? speeds and so on mean jack, if the RPM's are not right, you can be slowly killing the motor.

[FNQCairns]

What Noelm says, I smell a real problem here on the surface 40 at 4000 just doesn't sit right.

cheers fnq

[thylacene]

Prior to the prop change the engine revved happily to 5400 RPM (with 2 persons and no gear and with 4 persons and gear) according to the tacho. Post prop change the engine revs to 5400 RPM happily, the boat just goes a bit faster, albeit slightly slower out of the hole.

The boat was purchased by the original owner for towing skiers, the 16"prop gets out of the hole quickly, making it more suitable for towing/getting skiers up quickly. The 115 is the max rating for the hull, and the additional pitch on the prop is not causing the engine to "lug"or load up.

I have a reasonable understanding of the principles of gearing, having experimented way too much with motor cars (diff & gearbox ratios) and motorcycles (chains and sprockets), and the changes made do not appear to have any adverse effects apart from allowing the engine to run slightly lower revs at the same speeds or a little more speed for the same revs.

I will get to the river to do some proper comparitive tests, taking both props and doing the swap at the pontoon. I will post results and GPS logs when I do.

Until then, here is the log from a known section of our trip at 4000 RPM

431 1/08/2009 5:21:23 PM 12 m 181 m 0:00:15 43 km/h 203° true S36 22.143 E150 05.749
432 1/08/2009 5:21:38 PM 4 m 355 m 0:00:30 43 km/h 206° true S36 22.233 E150 05.702
433 1/08/2009 5:22:08 PM 8 m 247 m 0:00:20 45 km/h 204° true S36 22.405 E150 05.597
434 1/08/2009 5:22:28 PM -1 m 301 m 0:00:25 43 km/h 207° true S36 22.526 E150 05.529
435 1/08/2009 5:22:53 PM -1 m 179 m 0:00:15 43 km/h 202° true S36 22.671 E150 05.438
436 1/08/2009 5:23:08 PM -4 m 184 m 0:00:15 44 km/h 202° true S36 22.760 E150 05.393
437 1/08/2009 5:23:23 PM 6 m 358 m 0:00:30 43 km/h 201° true S36 22.853 E150 05.347
438 1/08/2009 5:23:53 PM -1 m 239 m 0:00:20 43 km/h 201° true S36 23.033 E150 05.261
439 1/08/2009 5:24:13 PM -8 m 118 m 0:00:10 42 km/h 200° true S36 23.153 E150 05.204
440 1/08/2009 5:24:23 PM -2 m 179 m 0:00:15 43 km/h 205° true S36 23.213 E150 05.178
441 1/08/2009 5:24:38 PM 17 m 247 m 0:00:20 44 km/h 205° true S36 23.300 E150 05.127
442 1/08/2009 5:24:58 PM 15 m 128 m 0:00:10 46 km/h 200° true S36 23.420 E150 05.056
443 1/08/2009 5:25:08 PM 15 m 218 m 0:00:20 39 km/h 202° true S36 23.485 E150 05.027
444 1/08/2009 5:25:28 PM 10 m 263 m 0:00:25 38 km/h 205° true S36 23.594 E150 04.972
445 1/08/2009 5:25:53 PM -3 m 191 m 0:00:20 34 km/h 201° true S36 23.723 E150 04.898
446 1/08/2009 5:26:13 PM 3 m 516 m 0:00:45 41 km/h 196° true S36 23.819 E150 04.852
447 1/08/2009 5:26:58 PM 25 m 247 m 0:00:20 44 km/h 198° true S36 24.085 E150 04.754
448 1/08/2009 5:27:18 PM 12 m 301 m 0:00:25 43 km/h 192° true S36 24.212 E150 04.703
449 1/08/2009 5:27:43 PM 7 m 425 m 0:00:35 44 km/h 193° true S36 24.371 E150 04.662
450 1/08/2009 5:28:18 PM 11 m 120 m 0:00:10 43 km/h 194° true S36 24.594 E150 04.597
451 1/08/2009 5:28:28 PM -8 m 591 m 0:00:50 43 km/h 198° true S36 24.657 E150 04.578
452 1/08/2009 5:29:18 PM 5 m 174 m 0:00:15 42 km/h 198° true S36 24.960 E150 04.457

[FNQCairns]

Sell all those props and buy/beg borrow a Vengeance 13 1/8 x 16p and start again from there. The change in your boat will startle you, not wishing to offend but ATM it is dog on the water and cannot do anything right.

After you have ran the V16 and with fingers crossed you may be able to move up to say a 13.25 x 17p equivalent aftermarket prop for that little bit of added efficiency.

The deal with props is that the pitch only matters when considered in concert with the diameter, the engine only has so much twisting power if the diameter is large the pitch need to be small to compensate and visa versa but with a condition or two, still you are way overproped ATM and it was when the ski bloke owned it...real slow onto the plane.

Consider also looking at your engine height but ATM that's of secondary importance being so badly overproped, still if you do both it will all come together better.

cheers fnq

[Jasinex]

We have a 90 up opti on the back of our 5.3 runabout. Using the 16p vengance and the engine at the correct height we get about 70 km/hr @ 5400 and bout 55 km/hr cruise @ 4000 . Every boat will differ but this seems to be great overall performance , excelent fuel economy , and great hole shot and we can maintain planing at a reasonable pace

[thylacene]

Sell all those props and buy/beg borrow a Vengeance 13 1/8 x 16p and start again from there. The change in your boat will startle you, not wishing to offend but ATM it is dog on the water and cannot do anything right.

After you have ran the V16 and with fingers crossed you may be able to move up to say a 13.25 x 17p equivalent aftermarket prop for that little bit of added efficiency.

The deal with props is that the pitch only matters when considered in concert with the diameter, the engine only has so much twisting power if the diameter is large the pitch need to be small to compensate and visa versa but with a condition or two, still you are way overproped ATM and it was when the ski bloke owned it...real slow onto the plane.

Consider also looking at your engine height but ATM that's of secondary importance being so badly overproped, still if you do both it will all come together better.

cheers fnq

Thanks for that.

I will get the engine height checked by the local Merc dealer in Qbn next time I take it over.

The prop I started with is a Mercury Vengeance Part no 48-16986 16P and it was replaced with part no. 48-16988 18P. Both props are the same diameter measuring 6 9/16"from centre to tip, so I assume this is what you are calling 13 1/8. A piccie of the original is here http://www.flyfishcuttyhunk.com/mercprop1.gif

I will measure the time taken to get on the plane next time we are out, but do not consider it slow. Have to admit to almost losing the wench over the back when I took off a bit enthusiastically ;^)

So to start a whole new set of views, how long should a boat take to get on the plane?

Cheers

T

[FNQCairns]

Ok then so you didn't go from a 16p Michigan Match Avenger to the 18p Avenger, they where both mercury vengeance propellers with the small diameter and the 5400rpm you mention is not your full achievable rpm ATM on the vengeance 18p???

just trying to understand because the pitch alone is half the battle and diameter is the other half.

What is your wide open throttle ATM? still sounds like the 16 is closer to the prop you need but still without a WOT it's all academic....could your rpm gauge be incorrect?

The vengeance 16 should as you say and all things being equal should fairly make it jump onto the plane.

cheers fnq

[thylacene]

Tacho could be out, but have no way of measuring that. Will see if the dealer can check it at service. Tacho showed 5400 RPM @ WOT with either prop, my assumption was that sufficient torque was available to spin either prop as max revs did not seem to decrease when 18P fitted.

Revs read the same for both props at WOT, so I assume that unless it is stopping at a set point on the guage, that the extra couple of kmh is a result of the pitch change. The boat "feels" better with the 18P prop, but I will have to wait until I can do some controlled tests on really flat water.

The engine does not seem to be labouring at all, engine pitch (the sound it makes according to my hearing at least) sounds better, hard to explain clearly as it is something I have picked up with bike engines over the years, sort of like a sweet spot where everything "feels"good, crisp responsive and not sounding like it is free revving. I will post an update once I get time to get to Nelligen for a play day.

There is a plate on the motor that states maximum revs is 6900, tacho only shows to 6000, maximum reading has been 5400 and the specs here http://www.boatsplus.com.au/mercury-outboards/efi-fourstroke/115hp-efi-4-stroke-mercury/prod_399.html state 5800 - 6400 RPM.

Anyone else out there got a 115 EFI 4 Stroke that can offer some comparison?

Cheers

T

[FNQCairns]

Yeah interesting, it might be worth discounting a problem with the tacho first, totally lost in the wilderness without trusting in one of these (also the GPS) and trying to get the prop right.

6900 is one hell of a high top rpm limit, will be interested also to hear from anyone with the same engine on an equivalent hull.

cheers fnq

[Moonlighter]

Thanks for that.

I will get the engine height checked by the local Merc dealer in Qbn next time I take it over.

The prop I started with is a Mercury Vengeance Part no 48-16986 16P and it was replaced with part no. 48-16988 18P. Both props are the same diameter measuring 6 9/16"from centre to tip, so I assume this is what you are calling 13 1/8. A piccie of the original is here http://www.flyfishcuttyhunk.com/mercprop1.gif

I will measure the time taken to get on the plane next time we are out, but do not consider it slow. Have to admit to almost losing the wench over the back when I took off a bit enthusiastically ;^)

So to start a whole new set of views, how long should a boat take to get on the plane?

Cheers

T

Hi T

With respect to your dealer, unless they do an on-water test they will not be able to tell whether yout engine height is correct or not.

I was researching this issue before I re-propped my boat and was referred to a very informative article on the Verado members website posted by one of their technical experts - basically it says that before you start playing with props you FIRST need to make sure the motor is at the correct height - that means that the cavitation plate must not be underwater when you are at cruising speed with the motor in its normal trim position.

So you have to be underway to check this factor.

Engine height has a huge impact on your boat's performance - acceleration, speed and fuel economy all impacted. In fact, you could already have the correct prop but if the engine is too low, it could be the one thing that is limiting your performance.

See this link, it really is worth a read and a look at the pics:
http://www.veradoclub.com/smf/index.php?topic=137.0 (http://www.veradoclub.com/smf/index.php?topic=137.0)

It includes excellent pictures taken when the boat is at speed to show you how a cav plate looks when its too deep and when its at the right height.

It also tells you in very precise and easy to understand terms how to determine if you have a prop fitted that suits your boat/motor.

Cheers

ML

[thylacene]

Thanks, that is a very informative article. I will follow the instructions and put up my results.

Cheers

T

[Jasinex]

we never take ours to wot . We limit ourselves to 5500 rpm as wot takes us to 5900 @ 75 km/hr unloaded . Loaded it's approx 70 onvthe digital gauges

[Malcolm W]

But less than 5800 is going to do long term damage to the motors. It will cause the motor to add gas to the oil, raising the oil level and diluting the protection the oil gives the motor. Eventually the crank will fail and it will not be warranty.

Thats the one I found interesting in MLs post as I would imagine a 2 stroke with VRO would be the same, adding fuel but if the revs dont come up no extra oil would be added. My 2003 115 Cv premix Yamaha may have one good point to premix- at least it will run at 50-1 no matter how much load its under. ( smokes at idle though). I did the prop calculator and came up with the figures of .08 to .142 prop slip depending on load? Not sure if I have done it right or if thats good or bad? 4200 rpm, 2-1 gear ratio, 17 pitch and 29-31 mph.

[Noelm]

Thylacene, I think you are missing the point, we don't care and it is completely irrevelant how fast you are going at 4,000 RPM, your motor needs to be able to achieve max manufacturers RPM, whether you run it there or not does not matter, it needs to be able to get there reasonably easy, thats the important info needed.

[thylacene]

Thylacene, I think you are missing the point, we don't care and it is completely irrevelant how fast you are going at 4,000 RPM, your motor needs to be able to achieve max manufacturers RPM, whether you run it there or not does not matter, it needs to be able to get there reasonably easy, thats the important info needed.

Noelm,

Thanks, I have worked that out from the thread to date. I will follow up on the engine height and RPM@WOT as soon as I can get to the river. I figure that if I can get it to run @5800RPM minimum, and see the cav plate when it is trimmed up. This has been really informative I now have a much better understanding.

Cheers

T

[Noelm]

that's the way, if you can get to those RPM, and the Motor height is good, then the only increase can come from very expensive and time consuming prop brand and design selection, and unless you have a very willing dealer who has a range of props to try then just settle on the what you get with equipment you have now.

[thylacene]

Yep, primary objective is to get optimum performance and economy with minimal/no expenditure.

Setting up a race bike can give you that little bit extra, and the experimentation will increase my understanding. Thanks all for the advice.

[thylacene]

OK, had a chance to do some more testing, although not that much, a seven hour return trip to the water meant that fishing took priority.

With the 18P Vengeance the engine pulls 5800 revs @ WOT according to the tacho. From all said, it appears this is OK.

Looked at the engine height while underway, the plate is clear of the water.

Testing was done with four adults and gear on board (this is normal for us). Guess I should be happy.

Top speed @59kmh by gps.

Thanks all for advice.

Cheers

Thy

[Moonlighter]

Good on you, Thy. That's the only way to do this and to know exactly what the results mean.

The only thing I didn't see mentioned earlier in this thread was the recommended rpm range for your engine.

So I had a quick look at the merc website and found the specs for the 115 4 stroke - I assume (always a dangerous thing to do!) that is the motor that you have. It says there that the max rpm range for that engine is 5800 - 6400. See: http://www.mercurymarine.com.au/mpi/mmproddb.nsf/CLUP2/25A989F67BC13E49CA25709700145A28?OpenDocument

So at 5800rpm you seem to be right on the lower rpm range recommended - maybe go back and have a look at that Verado website article again and satisfy yourself that you're happy with 5800rpm at WOT, rather than say 6000rpm.

A one inch change in prop pitch might make a diff and put you right in the "zone". You can get many props repitched professionally (and tidied up and recupped to look like new again) from about $120.

Cheers

ML

[thylacene]

I will run it in smooth water before anymore changes and check. Will try it with the max no of people/gear we expect to carry, and then do the prop swap and test again. If it doesn't fall below 5800 @ WOT with the 18P I will leave it as is, if it does, I will go back to the 16P and keep an eye out for a 17P as the 16P does not "feel" right. We are heading down for a week at the end of September so there will be time to fish and test also.

Thanks all for the input.

Cheers

Thy