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Old 08 May 2021, 16:12   #1
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115hp EFi only 36 knots?

Hello everyone

Got a 5.3m Tornado with a 2001 Mercury (yammy) 115hp 4 stroke efi.
Top speed at 5000 revs (wot 5-6k) is only 36 knots with only me & 1x 22L tank on board. Prop is 19".

I wouldn't of thought a 115hp would struggle with a 19" prop, & im sure I should get 40 knots plus.

I've had the injectors cleaned by injectortune as 2 were really bad (only 2 cylinders running). They came back looking brand new.

Prop is decent condition, Hull is clean with few chips, from memory the cav plate is inline to bottom of Hull.

On muffs, Its hesitant from tick over to light throttle & sometimes stalls, in water its fine (due to back pressure?)
Had a plug in & everything is good.
New fuel lines & tanks, high flow filter.

What speeds are people getting with a similar set up?
Could still be my injectors? Or 17" prop the way to go?

Thanks in advance
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Old 08 May 2021, 17:02   #2
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Hello everyone

Got a 5.3m Tornado with a 2001 Mercury (yammy) 115hp 4 stroke efi.
Top speed at 5000 revs (wot 5-6k) is only 36 knots with only me & 1x 22L tank on board. Prop is 19".

I wouldn't of thought a 115hp would struggle with a 19" prop, & im sure I should get 40 knots plus.

I've had the injectors cleaned by injectortune as 2 were really bad (only 2 cylinders running). They came back looking brand new.

Prop is decent condition, Hull is clean with few chips, from memory the cav plate is inline to bottom of Hull.

On muffs, Its hesitant from tick over to light throttle & sometimes stalls, in water its fine (due to back pressure?)
Had a plug in & everything is good.
New fuel lines & tanks, high flow filter.

What speeds are people getting with a similar set up?
Could still be my injectors? Or 17" prop the way to go?

Thanks in advance
This may be helpful ,i have r/c 5.3 with 50 hour honda 100 ,but these tests show very similar to my findings ! not quite what i was expecting either !
https://www.ribcraft.co.uk/documents...erformance.pdf
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Old 08 May 2021, 17:15   #3
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Hi thanks
Do you get into the higher rpms? As mine doesn't have the guts to get past 5k. 19" prop too?
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Old 08 May 2021, 17:41   #4
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Hi thanks
Do you get into the higher rpms? As mine doesn't have the guts to get past 5k. 19" prop too?
I am still trying to understand too, most of investigation done last season,i have a 19 pitch prop but only 4 blade which i tried last year and had full 6000 wot but imho it seemed like a poor ride ,so i bought a 3 blade 21 prop ,left me at 5300 ish but a super ride into the chop, i started with a 17 prop which was lively to say the least ,but soon reached wot
So to be honest i hav,nt really done this carefully enough to be in a position to help that much ,but i do plan to log all so i can create a good comparison ,my feelings at the moment is leading me to think genuine honda 3 blade 19 will give me 37 knots and wot ,i have had a very informative chat with cc at ribcraft with regard to ride quality which tbh is my main goal
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Old 08 May 2021, 18:17   #5
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Yeah you want to be closer to 6000 unloaded, so your around 5500 when loaded.

There's definitely something not right with mine then not pushing past 5000 with a 19".

Maybe get your injectors cleaned, cost me £56 I think it was. That could get you those extra few knots.

I know engine height causing drag can be an issue.
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Old 08 May 2021, 18:46   #6
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I'd raise the engine at least one hole but think you are likely over-propped. Set realistic expectations, 17" seems to be middle of the road. In my (limited) experience I'd expect 19p or 21p to be paired with 140hp.

You have low slip but too heavy boat/not enough power to reach the upper end of WOT. With a 17" prop you will actually go faster and have better holeshot which is far more useable than a couple of knots on the top end.

Ribcraft recommend a 19p for 90hp in their brochure which is at odds with the stated performance of 34.8 knots at 6000 rpm. Must be a mistake, clearly incorrect as a 19p would give over 40 knots at that rpm.
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Old 08 May 2021, 18:58   #7
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Appreciate your time in doing that, thanks.

So think my Tornado (quite broad & high bow) is much heavier than than "Orwell Boy's" ribcraft? If he's getting 5300 on a 21" prop by 100hp?
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Old 09 May 2021, 01:28   #8
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my humber 5.5 with mariner 115 and a 19p four blade alloy is hitting 6k at 37 knots .i have tried a 21p but it didn't improve top end and i couldn't get over 5.5k revs plus it had lost any mid range punch . mine is the newer 2.1 liter and has the command thrust gearbox though so different final drive
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Old 09 May 2021, 03:10   #9
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my humber 5.5 with mariner 115 and a 19p four blade alloy is hitting 6k at 37 knots .i have tried a 21p but it didn't improve top end and i couldn't get over 5.5k revs plus it had lost any mid range punch . mine is the newer 2.1 liter and has the command thrust gearbox though so different final drive
Interesting. The 2.1 litre 115hp Command Thrust engine has the same 2.07 gearbox ratio as the 1.7 litre. Feed this into the prop calculator and you have a shocking prop slip of 18%. Therefore you require more blade area to bring this figure down.

More blade area can come from a different design prop, an extra blade or by fitting a three blade with a larger diameter. To drive a prop with more surface area you'll need to reduce the pitch but you won't lose any top end. You'll also gain on fuel economy by reducing the slip and the boat should feel more responsive.
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Old 09 May 2021, 03:17   #10
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command thrust final drive is 2.38 to 1. and i am using mercury spitfire 4 blade prop in 14x19p. having put this in the calculator i have a 7% slip so pretty much as good as ill get
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Old 09 May 2021, 03:27   #11
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command thrust final drive is 2.38 to 1. and i am using mercury spitfire 4 blade prop in 14x19p
Ok that changes things. I did use two sources for the figure of 2.07:1 but got it wrong.
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Old 09 May 2021, 03:52   #12
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my humber 5.5 with mariner 115 and a 19p four blade alloy is hitting 6k at 37 knots .i have tried a 21p but it didn't improve top end and i couldn't get over 5.5k revs plus it had lost any mid range punch . mine is the newer 2.1 liter and has the command thrust gearbox though so different final drive

So I'm assuming a 4 blade gives you better acceleration & grip. But not less revs & top speed compared to 3 blade?

Is your humber a destroyer? A deep V like my Tornado
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Old 09 May 2021, 03:59   #13
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Limecc

What's your thoughts on Orwell boy's Ribcraft getting 5300 on a 21p with 100hp
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Old 09 May 2021, 04:22   #14
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So I'm assuming a 4 blade gives you better acceleration & grip. But not less revs & top speed compared to 3 blade?

Is your humber a destroyer? A deep V like my Tornado
yes mate 5.5 destroyer carrying 2 robust people and probably in the region of 150 litres of fuel
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Old 09 May 2021, 04:25   #15
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Ok that changes things. I did use two sources for the figure of 2.07:1 but got it wrong.
no worries 6.1 % im happy with that looks like im spot on
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Old 09 May 2021, 04:37   #16
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So I'm assuming a 4 blade gives you better acceleration & grip. But not less revs & top speed compared to 3 blade?

Is your humber a destroyer? A deep V like my Tornado
So really then, my 115hp on 19p is under performing
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Old 09 May 2021, 04:55   #17
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So really then, my 115hp on 19p is under performing
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Limecc
What's your thoughts on Orwell boy's Ribcraft getting 5300 on a 21p with 100hp
No. It looks normal. You don't have the same 2.38:1 final drive as Beerbelly so you can't pull a 19p and if OrwellBoy pulled 6K with 19p/2.07:1 it's because his rig is much lighter and easier to push through the water. Maybe he had a following wind or tide.

Yours and Beerbelly's boats are similar and you both have comparable top speed and slip. This is what to expect with 115hp. Yours is doing well because it's a much older engine.

A good idea is to plot a chart of prop slip at 1000rpm intervals, having less slip at lower rpm is beneficial as it makes the boat more responsive and more fuel efficient. A four blade or well designed three blade will help reduce mid range slip. The boat spends more time at these rpm's than it does flat out so a slight sacrifice of top speed is worth it imho.
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Old 09 May 2021, 05:03   #18
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No. You don't have the same 2.38:1 final drive a Beerbelly so you can't pull a 19p and if OrwellBoy pulled 6K with 19p/2.07:1 it's because his rig is much lighter and easier to push through the water. Maybe he had a following wind or tide.

A good idea is to plot a chart of prop slip at 1000rpm intervals, having less slip at lower rpm is beneficial as it makes the boat more responsive and more fuel efficient. A four blade or well designed three blade will help reduce mid range slip. The boat spends more time at these rpm's than it does flat out so a slight sacrifice of top speed is worth it imho.
Ok so dropping to a 17" definitely worth considering then. Would a 4 blade be 17" too if same diameter?

Mercury vs Solas, much in it?
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Old 09 May 2021, 05:13   #19
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Ok so dropping to a 17" definitely worth considering then. Would a 4 blade be 17" too if same diameter?

Mercury vs Solas, much in it?
I have no idea.
I know that Solas know a thing or two but so do Mercury so either would be a good choice.

Personally went with a four blade Solas and because I purchased it from Steel Developments they give one free 'adjustment' where they convert it one pitch up or down for just the cost of the postage.

Comparing four blade to three blade Solas shows they reduce the diameter slightly to maintain the same blade area/power requirement.

It's said that having less flex of a stainless prop is worth a pitch or two over an alloy prop.
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Old 09 May 2021, 05:25   #20
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I have no idea.
I know that Solas know a thing or two but so do Mercury so either would be a good choice.

Personally went with a four blade Solas and because I purchased it from Steel Developments they give one free 'adjustment' where the convert it one pitch up or down for just the cost of the postage.

Comparing four blade to three blade Solas shows they reduce the diameter slightly to compensate for the extra area.

It's said that having less flex of a stainless prop is worth a pitch or two over an alloy prop.
Thanks for that
I see steel developments do a repair service too, handy to know
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