Wrong prop or faulty outboard rpm gauge?
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Category: General Forums
Forum Name: The Boat Shed
Forum Description: Discuss all things boating.
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=126060
Printed Date: 09 Jun 2026 at 7:28pm
Topic: Wrong prop or faulty outboard rpm gauge?
Posted By: Dunwurkn
Subject: Wrong prop or faulty outboard rpm gauge?
Date Posted: 31 Oct 2017 at 12:00am
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Hi all, Recently upgraded the boat and have had the new one out a few times full loaded with my standard fishing crew and gear. Boat runs great however when I open it up to full throttle the rpm gauge needle goes off the clock past 6000rpm limit. The outboard is a 200hp 2004 mercury carby which has a max recommended revs of 5600rpm and is running a 19p vengeance prop. The boat is a ramco Renegade 620. The guy I bought it from had owned it from new bought from a reputable ramco dealer and I find it hard to believe the prop could be set up so wrong. A few Questions then. 1. Is it possible the rpm gauge is faulty and reading to high? How would I test this? 2. What am I loosing with the current prop set up....speed, fuel efficiency ect? 3. If I don't push the motor above The recommended 5600rpm is there any potential risks with leaving the current prop as is? Boat runs great but want to understand a bit more before forking out $1000 for a new propellar and also need to get the right one first time around if it makes sense to Change it. I know a Increase in pitch gives a drop of 200rpm however hard to Know What he current starting point is when the needle goes past the max. Cheers |
Replies:
Posted By: Don18025
Date Posted: 31 Oct 2017 at 6:50am
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Dunwurkn, if you are in Auckland you could get the Rev counter checked out by Robinsons. They did a similar service of my VDO rev counter a few years when it began stopping and it has run okay since. http://www.robinsoninstruments.co.nz/ri_services.html" rel="nofollow - http://www.robinsoninstruments.co.nz/ri_services.html |
Posted By: MacSkipper
Date Posted: 31 Oct 2017 at 8:48am
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My mercury 2006 50hp tacho has an adjustment on it visible from back of tacho - I had to get a another tacho and run WOT and set to same - is a bit tricky an optical tachometer on flywheel is easier. It may mean tacho is knackered anyway? as mine failed completely a short while later? I now have a Kus tacho (sold on trade me) which is mercury style and has a combined hour meter. ------------- Good fishing trip nothing breaks, great trip catch fish. |
Posted By: Tagit
Date Posted: 31 Oct 2017 at 9:46am
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Next time you are out do some runs on a flat sea (no tide etc) and measure your speed (gps) vs rpms at say 4000, 4400, 4800, 5200, 5600. Then run some numbers through a prop slip calculator. If the slip numbers look silly then chances are that your rev counter might be off. A 19P might be a little at the smaller end for that engine on that hull so there is a chance that your rpms really are going too high. You won't do any damage though if you just keep the rpms below the recommended WOT number. You might be burning a bit more fuel than needed however if the prop is significantly under pitched. Most marine mechanics will have an infrared tach gun that they can shoot your flywheel with and get an accurate rpm count to compare with the engine tachometer. 15 minute job. You can also buy a similar tool from TradeMe etc for ~$20 and do it yourself.
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Posted By: Steps
Date Posted: 31 Oct 2017 at 9:50am
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1. Is it possible the rpm gauge is faulty and reading to high? How would I test this? Yes but very unlikely...a $35 tach off trademe.. just wind a wire around a spark plug lead, and that ill give up the rpms out on the water. Taking rpms up on land without a loading prop.. is not a good idea.. thu once should be ok ...remove prop clamp an induction automotive tach on a ht lead and compare... be a where that with modern spark plug systems may fire on more than just the compression cycle. And /Or post up data..rpms/ speed reading , aprox 3000 rpms thru to WoT and speed and gear box ratio. See if numbers match up ... 2. What am I loosing with the current prop set up....speed, fuel efficiency ect? If the prop is incorrect, be it pitch or grip/ diameter the data will show that... The prop is about making efficient use of the power at the prop .. " efficient = best economy and use use of power, speed for the intended application of the bat. 3. If I don't push the motor above The recommended 5600rpm is there any potential risks with leaving the current prop as is? Maybe. If the boat is well or over powered, no, depending how far out diam and pitch the prop maybe....if under or min, hell yeah... manufactures require the engine to operate @ WoT within a given rpm range... what they dont also tell you is when that is correct the the loading on the power head in the lower rpms is not full use of available hp..therefore not running as a fully loaded racing engine.. which is hard on longevity reliability want to understand a bit more before forking out $1000 for a new propellar and also need to get the right one first time around if it makes sense to Once u have good data rpms/ speed at best trim for each of the rpms runs, on flat water, and knowing the current prop diam/ pitch etc.. selecting the corect prop 1st time damn spot on or so close doesnt matter is not hard... Once you know what you are looking for, one can pick up good 500/ 1k props from $100 to 250 on trade me ... This is all assuming you need a diff prop. I know a (1 inch) Increase in pitch gives a drop of 200rpm however hard to Know What he current starting point is when the needle goes past the max. that is a VERY loose guide line even if changing between 2 props of the same make and model.. Then one considers you may require a diameter and or cupping change , not a pitch...and that will change WoT rpms also. OK so now , with very limited info/ data lets see wat you have The boat is a ramco Renegade 620. What do you recon the total gross weight, ppl everything is .. on the water? 200hp 2004 mercury (good engine) and at 1st glance/ impression , well powered , subject to knowing weight. IF that is the case then a 19 pitch is likely to be correct or close to right prop.. and likely require a grip/ diameter change. ie you maybe driving with the equivalent of a slipping clutch in a car... At a guess you should have at least a 14 1/2 inch to 15 1/4 diam prop. All subject to good data... Ok now lets see what we can crunch on only guestimate/ estimate data gearbox 1.87:1 (please confirm) rpm range 5000 to 5600 What is the current top speed? Guestimate of weight 1950 kg would in theory give you a top speed around 42 mph With a 19" pitch on your data so far assuming rpms reading is correct.. that means the prop is way too small... and a 17/18" pitch more appropriate Which would with correct prop, correct grip give a cruise around 30 mph@3900 rpms. So on that , without accruate data..with a 19" and too small a diam diameter it is very likely to be able to rev out a good 500/ 600 rpms above the 5600... ,just as you suspect, wrong prop Ideally would be looking for WoT around the 5000/ 5100 fully loaded, which if not fully loaded, less ppl gear will still keep in the top limit of 5600 Need good data 1/ setting rpms @ about 3200, trim to best trim.. just begin to porpoise, just before hear ventaltion on flat water...travel 100m take rpm and speed reading repeat timing each time 4000, 4500 5000 5500 6000 2/ Confirm gearbox ratio is 1.87:1 3/current prop spec on prop, usually the hub diam and pitch 4/Estimate as accurate as possible fully loaded weight on the water... when I open it up to full throttle the rpm gauge needle goes off the clock past 6000rpm limit. 5/When you do that do you reach top speed? and what is that speed? |
Posted By: Wefaknis
Date Posted: 31 Oct 2017 at 2:49pm
| my boat came from the dealer with the wrong prop Chris... its just a matter of working through the issue.... money will fix.. |
Posted By: MikeAqua
Date Posted: 01 Nov 2017 at 8:43am
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+1 for putting a second tach on it. I have a cheap multimeter from Super Cheap Auto that has RPM mode. As Steps says, plenty of induction tach's on TradeMe. As Tagit says an optical tach is another option. |
Posted By: Dunwurkn
Date Posted: 02 Nov 2017 at 11:45pm
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Thanks for all the feedback much appreciated. I have been offered a optical tachometer to borrow so will try that and see what the additional reading is sayin. I'm thinking I will go for a run down the river where it is calm and take cowling off to do the reading so the prop is under load. I don't have any of the suggested data unfortunately as have not had the boat long. If rpm data is correct I will gather the required data and post up the results. Cheers |
Posted By: wopass
Date Posted: 03 Nov 2017 at 12:01am
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and if you do need to get another prop, try these guys. top service. about 7 days from there to here. http://www.boatstoreusa.com/propellers/turning-point-propellers/solas-3553-143-17-prop-yam-ss4-v6-17/" rel="nofollow - http://www.boatstoreusa.com/propellers/turning-point-propellers/solas-3553-143-17-prop-yam-ss4-v6-17/  just bought a replacement S/S HR titan 4 for my westy after someone wrecked my last one...  owes me just under $600 on my door step
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Posted By: Steps
Date Posted: 03 Nov 2017 at 9:21am
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I will go for a run down the river Fresh water? that makes a difference to fine tuning props...less dense Most of your use of the boat on fresh water? Mention that when post up data. 4 blade props Im definitely a fan .. they are the way to go for a under or min powered boat.. not so much the grip but more combined with the increased rake.. small diameter 'thrust' cone out the back, breaks surface further back. Less rake breaks surface close and has a levelling effect to the bow due to more force lost at the top , levers the bow down.. visa versa more rake. And for larger boats 6m up off shore means more grip for less diameter, power power used for thrust less for turning the larger prop... also same effect on min under powered.. and the increased rake in more heavy swell / choppy seas . |
Posted By: MikeAqua
Date Posted: 03 Nov 2017 at 4:54pm
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If you just want to check if the tachometer is correct (logical first step) then you can do that anywhere you have sufficient water. |
Posted By: Steps
Date Posted: 04 Nov 2017 at 11:59am
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As I said above.. its not good to rev up an outboard without the prop loaded. See this at ramps.. not just dangerous but power head issues. Occassionally for short periods nps. Remove the prop.. most properly due to check any fishing line behind the prop ready to rip out the gear box seal... and give a good re grease anyway.. Muff, preferably drum, cover off engine, tach ready set up to go.. one person read tach other dash tach. fire up let warm up... increase rpms say around 4k call readings, repeat quick upto 5K then max manufacture for a sec Drop back rps in a smooth manner. All over in 30/ 45 sec let engine idle for a few minutes and shut down All over in 30/ 45 sec But take the bloody prop off!!!!!! Would lay huge odds you problem is the prop.. not the tach. A data run would show if is.. and would at the same time check tach.. that would be my number one choice as i would lay HUGE odds the issue is prop not tach. Tachs read wrong if the incorrect (johnson/ evinrude, think suzi, mercs) little selector switch on the back is selected wrong... Or simply dont read at all. |
Posted By: Tagit
Date Posted: 04 Nov 2017 at 1:23pm
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I wouldn't recommend running the engine at high rpm on the muffs.Whilst it won't overheat on a quick burst of revs you have potential to damage the impeller if the water flow is too restrictive for the rpms that you are pulling. A drum setup should be good. Always interested to here why people say not to rev your engine with the load off? I believed this myself for years until I had an engineer from an engine manufacture perform the installation certification on my new diesels years ago. Very first test he calmly sat there taking readings as my brand new (zero hours) engine spun in neutral on the rev limiter for several seconds. I asked him about this and he said (in broken english) something like - tell me what is different under load other than the higher combustion pressures when loaded? So what is the difference?
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Posted By: Steps
Date Posted: 04 Nov 2017 at 2:12pm
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Yep totally agre with Tagit on the muffs as allude to in my post above We have around 85lb/sq" mains pressure.. and heaps of volume.. muffs go on water sprays out around and water will come out telltail...just The leg/ impeller not dry.. Even so I use a drum... Because 1/ Our local service workshop says so.. for the same reasons 2/ The Official manufactures installation, maintenance and service manual says best to.. As to high revs , flushing motors, the manufacturer... manual says dont do it in large letters... Why? Well breaking in a new engine, several things must be pre set right.. besides bleeding oil... timing must not be not be so it will be over advanced when bee cam and bores in.. and must not be lean.. be on rich side The engine is not under load. low compression, over advanced and or lean.. and because not under load tends to lean out and has low vaccuum/ imap readings.. This lean over advance or even one of them can very quickly create very high combustion temps, resulting in a piston skirt touching a cylinder wall then as happens in 99% of cases the machinist get the blame.. When breaking in the engines and rpms increased around 2500 rpms held there and listen very carefully till hear the ring bed , and the cam beds in. Especially touchy these days as oils .. most oils.. no longer have heavy metals added. Not apply the above.. and engine thats been dialed in and tuned to those specs...free rev, low compression, no vacuum fuel system leans out and as no load the spark advance increases.. combustion temps rise very quick and that heat transfers into the piston walls.. the heat expansion between the different alloys.. walls , piston , rings is greater at these temps if rpms maintained and a good likely hood something grabbing/ twist on the gudgen etc. This is attributable to many failures on dynos usually because of basic training at the most by those who have not read the manual for the last couple decades.. Again engine machining/ assembly gets the blame.. Then us say.. what about high performance race engines... well its like this for example.. the crank, piston to wall ring tolerances will be something like1 1/2 -2 times Then throw in the alloys used in pistons rings etc are different. But still not a good idea to take rpms well up and hold for any more than a few seconds. |
Posted By: pjc
Date Posted: 04 Nov 2017 at 3:33pm
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in the old days of cars pre 1980s use to drive around for 500 miles there abouts at about 50 mph with the odd burst to 60 mph,why to break engine in and parts to seat correctly,now turn the key and give it death. what is one to do with a launch?idle every where for a few hours with a burst now and again? brought a new motor in the 80s 2 stroke,run for 25hrs at 50;1 at 1/2 throttle then go to 100;1,notice same motor today says run 100;1 no break in period. cannot see what harm will be done above as he is hooking up a new rpm counter to see which is accurate.
mate has a new engine fitted in his stock car,60k,ran on dyna tune,just went out and gave it death 6 races on no problem.
------------- Sex at 58.Lucky I live at 56 |
Posted By: Steps
Date Posted: 04 Nov 2017 at 8:25pm
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Something many dont know..American cars from the mid late 60s roll off the production line get hooked up to a Propane and are 1st fire up on it It is a vapour fuel and an octane around 110/115.. because of that issues like advance compression , lean rich are not an issue as has a narrower air fuel ratio mix... British aussie kiwi assembled where fired on liquid fuels. This has a huge effect of beding in rings bearings etc and having to 'run an engine ' period. Like I said race engine have huge tolerances hence no run in time required except 1st fire up and the 1st 20/ 20 sec bedding rings and in particular cam lobes.. if the cam doesnt bed you wil wipe a lobe in the 1st 10,000 miles. As to oil ratios .. it to do with huge stride in oil technology as to old mineral oil , semi synthetic and fully synthetic. So logically they will have different lube/ burn qualities and therefore mixture ratios And Then add to that.. I would not be surprised the 1st fire up on propane is far more widely common now with engine manufactures ... I have spent since pre 80s on designing from cam profiles thru chamber design , customn ign curves.. in the 'old days machine customising dizzies/ vacuum advance units etc. building LPG engines.. mostly Chev small blocks. I ran the 1st LPG muscle car down meremere at an inter club meet. |
Posted By: Dunwurkn
Date Posted: 04 Feb 2018 at 3:33pm
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Update on this thread, the issue was a faulty gauge so a easy fix before stuffing around with props! Cheers for the feedback |
Posted By: MikeAqua
Date Posted: 07 Feb 2018 at 9:41am
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Rookie question as I lost the instructions that came with my multimeter and can't remember ... When using an induction clamp on a single spark lead on a two stroke outboard (in-line 3-cylinder) ... What is the relationship between spark frequency and rpm? |
Posted By: Tagit
Date Posted: 07 Feb 2018 at 9:58am
| 2 stroke fires every time the piston goes TDC so should be 1 to 1 spark to rpm if I have my thinking right. |
Posted By: Foxtrot Oscar
Date Posted: 07 Feb 2018 at 11:42am
Correct sir. ------------- ><(((*> J ><(((*> |
Tagit wrote:Posted By: MikeAqua
Date Posted: 07 Feb 2018 at 1:08pm
| Thanks. |