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Old 06 December 2018, 02:21   #1
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The right speed ?

Hi Everyone,
I have a 8.6m Cobra rib with a Yamaha 225hp 2 stroke engine with 440 hrs of use, WOT on a flat sea with 2 people on board and half a tank of fuel (150lts) and no head or tail wind, the max I get is 38 MPH at 47,000 revs.

Reading here I should be getting nearer 5500 revs the prop I am using sayes 19-M on it, so my question is :

Are these realistic speeds and revs?
If not is it the prop or the engine tuning at fault ?

Any advise much appreciated
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Old 06 December 2018, 03:35   #2
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That's a very big, broad boat for a 225. I think that speed is realistic for what you have. I have a Humber 8.0m with a 225 2-stroke an that will only do about 38knts (although my throttle only goes 90% at WOT (don't ask)).

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Old 06 December 2018, 04:36   #3
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Is this a new installation or a new to you boat ? If not has it ever reved harder? Or has it always been down on revs if its never reved harder then its possibly just overpropped and dropping to a 17p may cure your issue & possibly gain a knot or two but should help with holeshot in anycase.
Running overpropped risks possible engine damage due it being overloaded
If it used to rev out further but doesnt now then it may indicate several other potential issues
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Old 06 December 2018, 06:33   #4
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Quote:
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That's a very big, broad boat for a 225. I think that speed is realistic for what you have. I have a Humber 8.0m with a 225 2-stroke an that will only do about 38knts (although my throttle only goes 90% at WOT (don't ask)).

Regards

Steve
+1

That's a fairly big boat for not a lot of engine, my old 8m ran a 300hp and I'd imaging that would be a min for a 8.6, I'd also drop the prop a pitch or two to get the rev's up - and as pointed out if you've recently noticed a drop in revs / performance then there's an underlying problem, good luck
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Old 06 December 2018, 11:49   #5
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There could be many reasons for the craft not reaching its full potential but you are definitely down on RPM at full throttle. I assume you have trimmed the craft out to attain the optimum speed and that the engine is mounted at the right height.

What is the overall weight of the craft and has it a clean hull? A 19" pitch propeller sounds about right but that really depends on the overall weight of the craft and whether the engine is mounted high enough on the transom.

Generally speaking and providing you have a stainless steel propeller, the anti-cavitation plate (situated just above the propeller) should be at least 1" above the bottom of the transom for the optimum efficiency.

If the cavitation plate is parallel or just below the transom, try raising the engine so it is at least one inch above and this should give you between another 300 and 400RPM and increase your top speed significantly. Ideally at full throttle and trimmed out, the engine should be revving at 5,500RPM.
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Old 07 December 2018, 05:49   #6
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Something is wrong.
Nobody in their right mind would rig a boat and engine to only reach 4700rpm.

Is the bottom clean?
Is the hull full of water?
2 easy things to check.

If the above are OK then i'd be taking a look at the engine.
Is it running on all 6 cylinders? Running on 5 it will still sound and feel ok.
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Old 07 December 2018, 09:41   #7
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If your engine is a Yamaha HPDI, it's recommended max rev band is wide - 4500 to 5500rpm. So you'd be within the recommended engine loading...just.

I'd guess a 19" pitch is a bit high for a big boat with only 225hp and at the revs you're getting 38mph isn't unreasonable though a little low. Reducing the prop pitch to 17" will give you approximately 200-300 higher rpm providing you stay with the same make and type of prop. If you change prop make and type, you are stepping into the unknown and effectively starting over.

Using a prop of less pitch is a plus/minus situation - you gain RPM but you loose out on boat speed because the pitch is less. The ideal is to reach maximum power within the recommended engine loading (4500 to 5500). To do this with any certainty you need a graph of your engine power. Given that Yamaha give a wide range of 4500 to 5500 for max prop loading, there is a clue there that within that rev range the power curve is pretty flat so propping down in pitch is unlikely to see much of a gain in speed but it will be a little easier on the engine and generally boat response and handling will improve.

My advice with regard to looking for top speed is to remember that if you gain a couple of MPH it may seem good and worthwhile but that extra speed is about the speed my granny walks to the shops! Keep it in perspective!....
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Old 10 December 2018, 10:28   #8
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Hi Thanks for all the info, I have had the boat 1.5 years and its always been the same only just getting round to trying to find out why!

I am more interested if there is a problem with the engine than gaining a few more MPH and also thought if set up better it might improve the fuel efficiency.

The hull is clean and it is kept out of the water when not in use so no water in the hull, the engine is trimmed right when on WOT and I have checked the heights and the cavitation plate is 40mm above the bottom of the boat, this I have measured when the engine if fully trimmed down.

The main 3 bladed prop I use is just a standard steel one, I do have another 5 bladed stainless steel one a 19-M which I tried and had exactly the same results same speed and same WOT revs.

I did notice that it was less responsive maneuvering when mooring with this prop on so will probably change it back to the steel one.

How can I check easily that he engine is running on all 6 cylinders without specialist equipment ?

By using this prop and only achieving 4800 revs is this not better for the engine ? less wear and not screaming its *alls off going flat out ?
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Old 10 December 2018, 13:25   #9
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By using this prop and only achieving 4800 revs is this not better for the engine ? less wear and not screaming its *alls off going flat out ?
Doesn't work that way, it is effectively like driving your car about in 6th gear all the time but with an engine not making max hp. (Outboards make max power typically right at end of rev range or close to it)

You are putting more strain on engine by over propping and in all likely hood using more fuel into the bargain.
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Old 10 December 2018, 14:21   #10
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By using this prop and only achieving 4800 revs is this not better for the engine ? less wear and not screaming its *alls off going flat out ?[/QUOTE]

Nope its actually worse for the engine as its running overloaded you potentialy risk burning a piston running as you are
Engines dont blow up cos there being revved they have rev limiters to prevent that
I'd try and borrow a smaler pitch prop to try and see how it goes.
If it had another problem I.E. a cylinder off or a misfire it would have developed further or have shown other symptoms in 1.5 years of use . Problems that restrict power tend to get much worse rapidly if not sorted out
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Old 10 December 2018, 16:14   #11
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Surely if the manufacturer recommends a max range of 4500 - 5500 it's not being "overloaded" at 4800. Maybe not the optimum figure but still within range.
Two inch drop in pitch will possibly give an extra knot or two letting it rev to 5150-5200 but it will have a marginal, detrimental effect on fuel consumption at cruising speed.
Will almost certainly improve acceleration though.
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Old 10 December 2018, 16:57   #12
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Evinrude Modified their operating rpm range and added another value of RPMs called optimum. This is the holy grail of rpms. Anything else will get you spat at and cursed on an etec forum. It sits roughly in the top 150-200rpm of the WOT.

Unfortunately for me I started changing props before discovering I had an injector fault. In a way it was the only way to verify I couldn’t pull more than 5000rpm.

I know nothing about Yamaha but I am sure they have a dedicated forum and you may be able to acquire diagnostic software to connect to the motor.
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Old 10 December 2018, 17:31   #13
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Quote:
Originally Posted by Last Tango View Post
Surely if the manufacturer recommends a max range of 4500 - 5500 it's not being "overloaded" at 4800. Maybe not the optimum figure but still within range.
Two inch drop in pitch will possibly give an extra knot or two letting it rev to 5150-5200 but it will have a marginal, detrimental effect on fuel consumption at cruising speed.
Will almost certainly improve acceleration though.
Op hasn't stated exactly what model or year it is but the 225hpdi manual I looked at says 5000-6000 rpm & I'd wager the straightforward old school 2 stroke is meant to be nearer 5500rpm
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Old 11 December 2018, 13:06   #14
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Op hasn't stated exactly what model or year it is but the 225hpdi manual I looked at says 5000-6000 rpm & I'd wager the straightforward old school 2 stroke is meant to be nearer 5500rpm
Yeh, you're correct but I've given a bum steer earlier so Last Tango may be using my mis-information. The 150 is indeed 4500-5500 rev band as I stated but the intermediate sized engines are 5000-6000rpm rev range and the 300 in this series is very wide band from 4500 to 6000rpm so apologies for that. It's interesting that max power is in the centre of the power band in all cases indicating a fairly flat power curve.

Here's a clip from the manual...
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Old 11 December 2018, 13:36   #15
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With regard to checking the engine for non-running cylinders; there is two primary causes, one is no spark and the other is no fuel. However, the first and easy check to make is to see whether there is a cylinder not firing. You need to start the engine and put it into gear and remove a plug cap one at a time and note the engine's response. If the boat is in the water it's not so easy to tell because these engines idle very quietly but if you sense no change at all then you need to investigate further. If the boat is out of the water and running on a hose it's easy the tell if a cylinder drops out but then take great care because you will be driving the prop - mind your legs and feet! Perhaps remove the prop first.

If the engine is not in gear it only fires four of its cylinders.

If you are missing firing cylinders come back and tell us what you've found.

Edit: OP, you could do with telling us exactly which version of the Yamaha engine you have because there are model variations with differing max rev bands. Since yours is 225hp I've presumed the VZ model.
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Old 11 December 2018, 15:12   #16
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I agree with JW. Missing cylinder is possible. The evinrude exec software allowed me to remove fuel or spark from each cylinder in turn until to spot the difference. Oddly I couldn’t sense I was running on five at idle, in gear or even whilst running At 60mph. Only at river speed around 1500rpm was it noticbly missing. The drop test in the software saved my bacon.
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Old 11 December 2018, 18:38   #17
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Im not buying the "got a cylinder off" theory, I dont believe anyone could run a boat for 18 month (assume regularly) and not notice its on 5 cyls and it hasnt exhibited any other symptoms or done serious damage to itself
Knowing what model the engine is would definitely help
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Old 11 December 2018, 19:00   #18
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Im not buying the "got a cylinder off" theory, I dont believe anyone could run a boat for 18 month (assume regularly) and not notice its on 5 cyls and it hasnt exhibited any other symptoms or done serious damage to itself
Knowing what model the engine is would definitely help
Yeh, you might think so but the way these engines work means that the cylinders are properly lubricated at all times unless there's an oil feed problem, if an injector stops fueling then the engine simply feels a bit flat and down a little on max revs. You might expect it to sound as a typical misfire but the engines sound surprisingly normal. Dunno why. On idle the tick-over is typically 2stroke grumpy and burbly, it's very difficult to tell. If a fault is on one of the two cylinders which are switched off at idle there won't be a clue there either.

I'm not suggesting that this is the problem with the OP's engine but because of their characteristics it is worth a bit of time checking it out.
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Old 11 December 2018, 19:12   #19
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Yeh, you might think so but the way these engines work means that the cylinders are properly lubricated at all times unless there's an oil feed problem, if an injector stops fueling then the engine simply feels a bit flat and down a little on max revs. You might expect it to sound as a typical misfire but the engines sound surprisingly normal. Dunno why. On idle the tick-over is typically 2stroke grumpy and burbly, it's very difficult to tell. If a fault is on one of the two cylinders which are switched off at idle there won't be a clue there either.

I'm not suggesting that this is the problem with the OP's engine but because of their characteristics it is worth a bit of time checking it out.
I get that it might not be immediately obvious but id expect if it was a lack of fuel and ran for 18 months the piston would have burned out by now and if lack of spark it would be obvious raw fuel was being poured out of the exhaust
If it was a recent problem then I'd agree but after 18 months of use id not put my money on it being a dead cylinder.
My money would be on either a tacho telling porkies or the boat being overpropped
A high idle test or a shop tach would answer the tacho theory or trying to borrow a prop should answer the prop question.
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Old 11 December 2018, 19:51   #20
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Yes, I know how you feel but my recent experience with having two HPDI on my Delta and both having running faults without any engine damage has shown me a different conclusion. It's possible for an HPDI to run indefinitely with an injector not functioning, even the spark plug gave no clue! Bear in mind too that they're designed to run without cylinders firing.

On idle, with only 3 cylinders firing on every other beat the engine sounds real smooth! Don't ask me how I know.. When idling on four as is intended it sounds really scaggy!
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