Zodiac Pro 5.5 - Only 5100 rpm

[MrMusli]

Hi,

My Zodiac Pro 5.5 with Mercury 115 pro xs will only reach 5100 rpm, with a top speed of 37 knots (42,6 mph).



The prop says 12.7x19p RH, and according to the reseller, both engine and propeller were decided by Zodiac.

According to the specifications, Mercury 115 pro can go up to 6300 rpm and I expected the boat's top speed to be roughly 40 knots.

Any ideas? Is anyone else with the same setup?

Regards
Olov

[beamishken]

Originally Posted by MrMusli

Hi,



My Zodiac Pro 5.5 with Mercury 115 pro xs will only reach 5100 rpm, with a top speed of 37 knots (42,6 mph).







The prop says 12.7x19p RH, and according to the reseller, both engine and propeller were decided by Zodiac.



According to the specifications, Mercury 115 pro can go up to 6300 rpm and I expected the boat's top speed to be roughly 40 knots.



Any ideas? Is anyone else with the same setup?



Regards

Olov

Assuming the engine is running correctly it would suggest you need to reduce pitch. A 17pitch should increase rpm to 5500ish which would hopefully put the engine into a rpm band where it makes more hp & hopefully equates to more speed, bringing the wot rpm up further & into the correct range.
That is assuming engine height & trim are correct, the hull isnt full of water or loaded down with gear or people & engine is healthy & tach is telling the truth.
5100rpm isnt healty for the engine as its overloaded across the rev range

Sent from my SM-G950F using RIB Net mobile app

[MrMusli]

Originally Posted by beamishken

Assuming the engine is running correctly it would suggest you need to reduce pitch. A 17pitch should increase rpm to 5500ish which would hopefully put the engine into a rpm band where it makes more hp & hopefully equates to more speed, bringing the wot rpm up further & into the correct range.
That is assuming engine height & trim are correct, the hull isnt full of water or loaded down with gear or people & engine is healthy & tach is telling the truth.
5100rpm isnt healty for the engine as its overloaded across the rev range

Sent from my SM-G950F using RIB Net mobile app

Thanks beamishken. What if the engine isn't mounted high enough, would that give these symptoms? I haven't seen to boat out of the water yet so it is a possibility.

I can trim the engine a bit before the bow starts to "bounce". The hull is dry, gas tank at 25% and only the driver in the boat when I tested max speed.

[beamishken]

Originally Posted by MrMusli

Thanks beamishken. What if the engine isn't mounted high enough, would that give these symptoms? I haven't seen to boat out of the water yet so it is a possibility.



I can trim the engine a bit before the bow starts to "bounce". The hull is dry, gas tank at 25% and only the driver in the boat when I tested max speed.

Engine sat low will hamper performance but probably not as much as your experiencing check the av plate height in relation to the hull when the engine is trimmed level it should be level or higher than the hull

Sent from my SM-G950F using RIB Net mobile app

[Swordfish]

Originally Posted by MrMusli

Hi,

My Zodiac Pro 5.5 with Mercury 115 pro xs will only reach 5100 rpm, with a top speed of 37 knots (42,6 mph).

Olov

Hi Olov, is this the Command Thrust or standard version of the Mercury 115?

[MrMusli]

Originally Posted by Swordfish

Hi Olov, is this the Command Thrust or standard version of the Mercury 115?

Hi Swordfish,

It's the standard version.

[Swordfish]

Hmm, I thought that might be the issue but sounds like the right gear casing for the size of boat at 430kg. I'm considering buying the same engine but I'd be expecting to be able to use a prop with a pitch of 19" or even 21". Have you considered fuel or air starvation? Have you tried running from a different tank with a different fuel line? How fresh is the fuel you are using? I take it the air breather is open? I had an engine many, many years ago with a similar issue it was straight from the factory and it turned out the be a blockage in the fuel filter / line. Replaced this and it ran perfect. Good luck.

[MrMusli]

Originally Posted by Swordfish

Hmm, I thought that might be the issue but sounds like the right gear casing for the size of boat at 430kg. I'm considering buying the same engine but I'd be expecting to be able to use a prop with a pitch of 19" or even 21". Have you considered fuel or air starvation? Have you tried running from a different tank with a different fuel line? How fresh is the fuel you are using? I take it the air breather is open? I had an engine many, many years ago with a similar issue it was straight from the factory and it turned out the be a blockage in the fuel filter / line. Replaced this and it ran perfect. Good luck.

I did not try with a different tank / fuel line. It is fresh fuel directly from a boat gas station.

Fuel filter looks fine.

Air breather, I don't think there is one to close on this boat. I will look into it.

Now the boat is out of the water for a couple of months. Might test it in the water if I replace the prop during the fall.

Thanks for the ideas!

[Swordfish]

Suggest you make sure your fuel tank is properly ventilated and no kinks in the fuel lines, any fuel shut of valves should be wide open. I guess that dropping the pitch to less than 19' would be a last resort if you are not entirely sure the engine is running correctly. Good luck and let us know how it goes!

[paintman]

If all else is correct then try entering your details into one of the prop finder apps & see what's recommended.
http://www.mercurymarine.com/en/us/p...or/#!/step-one

I've had a very brief play with it guesstimating some of the info & for 'all round good performance' it does suggest 17 pitch.
If you enter all details you'll get a better result.

[Swordfish]

What a fantastic link Paintman! Very helpful. If you run it on Performance / RPM then that assumes that the engine is running correctly. If you run it on weight you may get a different result. At 430Kg boat weight + "hull is dry, gas tank at 25% and only the driver in the boat when I tested max speed" lets say 650 Kg in total then the calculator (set at overall good performance) still suggests a pitch of 19.8 and indicates a speed of 49.5 mph or 43 knots. Load up another 200Kg and it drops to 17.8" and 44.5 mph or 38 knots, which is about what Olov is currently topping out at. I'm still not 100% convinced its the prop at fault but ultimately it depends on how much weight Olov will typically have in the boat.

[paintman]

They make a reasonable guide.

Load makes a major difference. As you've found.
You need to keep WOT RPM within the engine's specified band.
So testing must be done with the normal loading.

FWIW my own boat - 17' Dory - came with a 19 pitch ally prop. Very quick onto the plane & with 4 people on board within WOT RPM.
Normally I'm either solo or occasionally have one adult & a 21 is much better - currently on a s/s.
Did try a 25 but way too low on WOT RPM & decided not to bother looking for a 23 although solo that might have worked.

Ebay was the source for the props (S/H) & the unsuitables sold for what I'd paid for them.
BUT you can finish up spending more time changing props looking for the perfect compromise than enjoying the boat!

[beamishken]

I've always found the mercury prop selector to be a bit "optimistic". If your underreving with a 17p your going to be even worse with a 19+ which kinda makes the prop calc look wrong. You need to reduce pitch to gain rpm, bring the engine rpm up so its making more hp which should bring the speed up further & increase the rpm into the correct rated range. Regardless of performance you need to prop the boat lightly loaded for top end of rated rpm range. If the boat has been in the water a while it could also be a dirty hull holding it back, even light slime can reduce speed considerably.
Once its out the water you can see what the hull is like & check engine height etc then go from there

Sent from my SM-G950F using RIB Net mobile app

[MrMusli]

Thank you all for your excellent insights.

I have been able to check the fuel filter and the fuel shut off valve, without finding a problem. When I get a chance I will investigate the boat further. The hull is certainly 100% clean and the only load when tested is fuel (roughly 50%) and me (roughly 100kg).

As a reference I found this from Yamaha:

https://yamahaoutboards.com/en-us/ho...10-21-2019-inf

They reached 5900 RPM FUT with a three-blade 19" pitch prop.

The gear ratio on the Yamaha is 2.15:1 and on my Mercury it is 2.07:1.

Given that I have a 4-blade prop with the same 19" pitch, and a lower gear ratio, it supports that the prop on my boat should have a lower pitch.

Do you guys agree with my conclusion based on the Yamaha paper?

[Swordfish]

Hi Olov, I hadn't realized it was a 4 blade prop. The speed you are getting sounds about right. 4 blades will create more drag than 3 and it's stainless steel so those 4 blades are heavy to swing which will impact the revs. Seems an odd choice of prop for this boat but perhaps they expected it to be heavily loaded and would need more thrust so went for a four blade prop without making the necessary adjustment to the pitch? This guy reckons he's running a 23" on a similar sized/weight boat and getting 50 knots.

https://www.youtube.com/watch?v=eUFNrCMQDvo

If you read the comments and replies he's added a jackplate to get it up on the plane so it's likely not performing well in other performance aspects and certainly wouldn't perform well with a heavy load.

I am frequently wrong so please don't ever take my advice but if it was my boat I would try a 19" three blade stainless steel Enertia.

I plan to go with the same engine and try to get away with 21" but my hull will hopefully be lighter than the Zodiac 5.5 and the BRIG in the video, I won't have a jackplate and I sometimes have more than a couple of people in the boat.

[office888]

Trim it up until the prop blows out at speed, then lower it a bit. Note the indicator. What percent is it trimmed?

I’m running a similar setup.

Avon D560
Mercury 115hp CT
Spitfire 4 blade 14x23 ALU

Max speed ~48 mph, my trim is at about 80%.

[Swordfish]

I’m running a similar setup.

Avon D560
Mercury 115hp CT
Spitfire 4 blade 14x23 ALU

Max speed ~48 mph, my trim is at about 80%.[/QUOTE]



That’s pretty darn good. What weight is the hull and what RPM are you getting WOT with a 23”?

[Swordfish]

Hi Olov, I put the order in for the same engine so will let you know how it goes. I ordered a standard gear casing and a stainless steel 20" pitch prop with three blades. If it was the CT version I believe I may have been able to run a 21" or 22".

Will let you know what RPM I get and whether I need to switch to the 19" or the 21". I'm currently running a 19" aluminium prop with 90hp and a similar gear ratio so I'll be disappointed if I have to drop a pitch..... However, the guys at the Mercury stand at the Southampton boat show suggested 17" would be a good choice for my set up, so I'm not at all sure I've made the right choice!!