mercury 25HP strange occurence... prop?

anonOmattie

Seaman Apprentice
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May 26, 2017
Messages
42
hello. ive got a new boat, a 4.5m speedboat, with a 25hp mercury XD longshaft on the back. im struggling to get good performace on this boat so far. on my own i get almost 28kmh, but with 2 people inside it barely reaches 20kmh. the prop it is running is a 4blade 10x12.

today i was driving on a small canal, and the following happened. i was full throttle, going 20kmh, when i was taking a slight turn. suddenly my motor dropt in rpm and picked up some speed! it was not making high waves anymore and was planning quit nice, even better than when im driving on my own. also, when slightly turning the boat wobbles a bit, only to one side (i quess the rotation of the prop?).

when with 2 people, this is almost the same situation: https://www.youtube.com/watch?v=E-IuE-Klacs&index=7&list=WL
high rpms, but not a lot of speed... when after a turn it dropped in rpms and picked up speed.

could someone help me with this situation? i thought it was an oversized prop, but now im thinking more of the outboard height...
 

jimmbo

Supreme Mariner
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May 24, 2004
Messages
12,944
Welcome to iboats
Can you post a picture of the unit, on the trailer with motor down. Taken level at the height of the boat bottom, side and rear shot. No wide angle focal lengths either

What rpms are you getting at WOT, and what is your enigines WOT operating range?
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
Which year is that 25 HP ?

Are there transon back or over transom water splashes at speed when OB sits at 90? and boat runs parallel to water level ? Check pulling head out transom. Without wot rpm won't know if current prop is what your combo needs, assume don't have a tach, right ? Prop hub could be shot, make a hub slip test and go from there...

Happy Boating
 

anonOmattie

Seaman Apprentice
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May 26, 2017
Messages
42
thanks for the responses!!

jimmbo i am currently not able to take the boat out of the water. i will try to make some reasonable pictures of the motor and boat. the operating range is 5000-6000 rpm. it hits (from hearing) the top of that rpm every time, but yesterday, as i said, it suddenly dropped rpm and increased in speed. (more like 5000rpm)

Sea Rider i think it is an 1986, or somewhere in that direction. i dont have a tach indeed, but those or not to expensive around here ^^
hub slip is excluded... had that before this and fixed it.

ill try to experiment a bit with the height of the outboard today, whilst filming with an actioncam... thanks for the help!!
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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AnonOmattie,

"Hub slip is excluded".. had that before this and fixed it.. So current prop has a new hub, or it's a new prop ? Buy a Hardline or Tiny Tach meter to test prop, with ear taching revs will be way off of what you think they are.

Happy Boating
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
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42
it has a new hub, and will not slip anymore. i noticed when moving the boat up a little it was going faster, but im still not sure that is the problem
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Well it does seem like the motor is too deep in the water, but we would need a picture from the side, level with boat bottom to be sure. If you can't pull the boat out of the water, do you have a waterproof camera? Or can you take a tape measure and measure the transom height? And the length of the engine from the bottom of the cowl to the anti-ventilation plate?
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
Messages
42
Well it does seem like the motor is too deep in the water, but we would need a picture from the side, level with boat bottom to be sure. If you can't pull the boat out of the water, do you have a waterproof camera? Or can you take a tape measure and measure the transom height? And the length of the engine from the bottom of the cowl to the anti-ventilation plate?

yes! i have a waterproof cam and will try to make a small video of the engine under water!
the length of the transom (the back of the boat where te outboard mounts right?) is around 50cm, so i don't think a shortshafted outboard will fit.
 

Sea Rider

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Does anyone knows if props that have removable hubs have a specific torque adjustment ? If so, prop was adjusted to light and is probably slipping, remove split pin, adjust nut CW to next groove, re install split pin..test.

Happy Boating
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
Messages
42
The hub my prop uses has 3 holes in it, where a "spring" has to be placed. In case of a propstrike the springs collaps and your gearbox is saved. I have replaced the springs with solids so the prop cant slip anymore. I will replace those again when i find some that fit...

I have a small video of the boat and motor underwater, and will upload it. Today i did another speedrun and i have to say the 2cm of height adjustment made a huge difference. It grips the water better, and my top went from 28 to 32. Thats a little more then a 10% increase!! Its nice to have a little playroom with the throttle without falling back out of plane.

But... i still think its to far down. I only have 1cm to spare otherwise the clamps will not secure anymore.
 

Sea Rider

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Rising OB has eliminated extra lower leg drag and increased wot speed. Ideal is for water flow at speed to skin right under small upper water deflector plate when on plane. Flow is not passing parallel to water level, seems OB is a little under trimmed.

With deck weight evenly distributed, trim OB to sit at 90? if with manual trim, make 2 wooden shims, one 1.0 cm and the other 1.5 cm, test which one dials water flow to skim right under upper plate, right under the front protuding edge if OB has one. Something like this : Don't let flow pass over upper plate, not even 1 mm.

http://www.youtube.com/watch?v=oBeZjaA76VM&feature=youtu.be

Once you got it right, buy an induction tach and go for a prop maximization, will love what a height optimization along prop maximization will do for your combo. Day & night difference.. Water test must be made on flat calm, no wind water cond..

Post which shim height dialed the ideal sweet transom height..

Happy Boating
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Hi
I would imagine that two things are clear;

1 you need to get it out the water and have a look at the prop, set up height and hub condition

2 a 25hp on that boat is probably a little underpowered. In the first video, it would appear the prop pitch is too small, but this may be a compromise in order to get the boat up on the plane in the first place.
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
Messages
42
Rising OB has eliminated extra lower leg drag and increased wot speed. Ideal is for water flow at speed to skin right under small upper water deflector plate when on plane. Flow is not passing parallel to water level, seems OB is a little under trimmed.

With deck weight evenly distributed, trim OB to sit at 90? if with manual trim, make 2 wooden shims, one 1.0 cm and the other 1.5 cm, test which one dials water flow to skim right under upper plate, right under the front protuding edge if OB has one. Something like this : Don't let flow pass over upper plate, not even 1 mm.

just to make this clear.. the small upper plate is the one above the cavitation plate right? and the cavitation is the one above the prop? im sorry if i sound noob but im originally from the Netherlands so some words are new for me :p

undertrimmed, so the leg of the OB (outboard?) has to get a bit further back. 90deg. with the boat or with the water?
the wooden shims are for the height adjustment? from the video i would say the height is about right, and from what i read online it should even go up a bit more... lowering will only increase drag right? ill try to find a tachometer!
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
Messages
42
Hi
I would imagine that two things are clear;

1 you need to get it out the water and have a look at the prop, set up height and hub condition

2 a 25hp on that boat is probably a little underpowered. In the first video, it would appear the prop pitch is too small, but this may be a compromise in order to get the boat up on the plane in the first place.

thanks! i sadly don't own a trailer, so i have to improvise a bit to get all the information... the prop is in good condition. small dents but no real damage. the hub is also in good condition, and does not slip.

i know its a bit underpowered, but as im just 17yo i cant really go bigger... im already going faster then legally allowed here! :p
the prop is a solas 4x10x12, but i do have an 3x10(9 3/4)x14.5 prop laying around. i have had that on this motor and gained around 1.5kmh, but all other aspects went worse... maybe with this new height i should try it again, although i think this prop is a bit too heavy!
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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12,345
Some tech clarifications about your last post :

Small upper plate is just a water deflector plate, it withou it water flow at speed will simply climd upwards lower leg edge and hit middle OB leg and create heavy water backwards or over transom splashes. Larger plate avove prop is the AV plate (Anti Ventilation Plate).

Ideal is for combo tu run parallel to water level when on plane, if OB sits at 90? will achieve hull and AV plate runnning parallel to water level too with minimum water drag on AV plate. If it's an issue, just float boat alone near shore and manually adjust trim pin for OB ro sit at 90?.

The boater on video had severe water back splashes over transom, OB sat to low. Ideal is for water flow to skim right under upper water deflector plate, all my transoms were modified to dial "sweet transom height" running very underpowered OB's.

As long water flow at speed doesn't pass above deflector plate will be just fine. Test with 1 cm shim and OB fully trimmed to 90? and report your findings. When on plane pull head out transom or film a video to check if lower leg has lowered a bit. If with water splashes out transom will need a 1.5 cm wooden shim.

Besides a transom height optimization a prop maximization is the way to go, but with a tach. Being in Holland can order them through Amazon, E Bay

If it's a 2 cylinder OB :
[FONT=&quot]Hardline Hour/Tach model HR-8061-2

If it's a 3 + cylinder OB :
Hardline Hour/Tach model :
HR-8062-2
[/FONT]



Happy Boating
 

Fed

Commander
Joined
Apr 1, 2010
Messages
2,457
Mattie I think you're wasting your time until you harvest the growth from the hull.
 

anonOmattie

Seaman Apprentice
Joined
May 26, 2017
Messages
42
Ideal is for combo tu run parallel to water level when on plane, if OB sits at 90? will achieve hull and AV plate runnning parallel to water level too with minimum water drag on AV plate. If it's an issue, just float boat alone near shore and manually adjust trim pin for OB ro sit at 90?.

The boater on video had severe water back splashes over transom, OB sat to low. Ideal is for water flow to skim right under upper water deflector plate, all my transoms were modified to dial "sweet transom height" running very underpowered OB's.

As long water flow at speed doesn't pass above deflector plate will be just fine. Test with 1 cm shim and OB fully trimmed to 90? and report your findings. When on plane pull head out transom or film a video to check if lower leg has lowered a bit. If with water splashes out transom will need a 1.5 cm wooden shim.

Besides a transom height optimization a prop maximization is the way to go, but with a tach. Being in Holland can order them through Amazon, E Bay

thanks for the clarification!! ill try to put the trim one further away and film the consequences! maybe it will angle the motor to far up, but we will see.
as you can see in the first video i posted, i firstly drove the boat with the normal height, and after raising it about 1.5cm. i gained around 4kmh, but im still able to raise the OB just another .5cm! after that the clamps of the motor will not grap the transom anymore.

for the tach, ill order a digital induction one, and mount it at my steering wheel. this will take about 2weeks, so after changing the height (and maybe cleaning the hull from all the growth, all though it was just cleaned 1.5 months ago... :/) i cant do much until then...

for now, i want to thank all of you who are trying to help me out!! im glad you guys are willing to help me, because here in the Netherlands its almost impossible to find these kinds of forums! :p
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
OK, firstly clean hull, install induction tach, trim to 90?, go for a wot spin and report your findings. BTW shim enough for water flow to skim right under upper plate, dont pass over it or will have water splashes rear of transom. The upper plate is the frontier between excellent lower leg water performance and so-so one.

Happy Boating
 
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