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

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?

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?

+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. 



 

and if you do need to get another prop, try these guys. top service. about 7 days from there to here.

If you just want to check if the tachometer is correct (logical first step) then you can do that anywhere you have sufficient water.

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.

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.

Update on this thread, the issue was a faulty gauge so a easy fix before stuffing around with props!

Cheers for the feedback

2 stroke fires every time the piston goes TDC so should be 1 to 1 spark to rpm if I have my thinking right.