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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)).

Regards

Steve
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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:
Originally Posted by Dry Run View Post
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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Quote:
Originally Posted by FrankfromCrete View Post
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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