mercury 25HP strange occurence... prop?

[anonOmattie]

Tomorrow ill experiment a bit with the 14.5p prop and height, but today i noticed something else...

We were with 3people. For this motor, thats too heavie to get speed, but just enough to get out of the water. When i apply throttle slowly, the boat also slowly increases in speed. If i stop right before wot the boat holds its speed. If i apply the last bit of throttle the motor revs a lot more, but the boat decreases in speed!!

I cant figure out where this comes from. Prop slipping on the hub is impossible!

 

[anonOmattie]

so yesterday i received my tach, and today I installed it in my boat.

it idles at around 1800rpm, and maxes out with 5000rpm (or 900rpm and 2500wot... my tach does funny stuff) with just me inside the boat.... i dont think thats a great rpm number :/
definitely need a lower pitch prop if i say so myself. the current one is a 4x10x12 but i think 3x10x11 or 12 is just about right!

 

[Sea Rider]

Which brand model is the current tach ? Is it spark firing adjustable ? If so, at which firing number was adjusted to ? If it's fixed will be useless, will read whatever rpm wants to read solo or with 4 up. Posted idle speed is way high, should be under 800-900 revs.

Happy Boating

 

[anonOmattie]

this is the one i bought: https://www.banggood.com/Wholesale-...auge-Motocycle-ATV-p-46889.html?rmmds=myorder

yeah, i know right! when i adjusted the tach to read 900rpm at idle, top was only 2500 at WOT...otherswise, if i doubled it which i later did, idle is at 1800 and wot at 5000.

either way, both do not read the optimal rpm for this motor, which should be around 800 higher. when i calculated my slip % at 5000rpm, 14 pitch (12x4) and 20mph max speed it resulted in about 32% slip, which is way to high right?

 

[Sea Rider]

Those cheapo meters are available in 3 different brands, none of them mentions being spark imput adjustable. Anyway which spark imputs adjustments are avilable on that model ?

First reading, 900 rpm at idle, and only 2500 at WOT seems right but at wot with a probable spun hub, check prop condition if doing its homework.

Happy Boating

 

[anonOmattie]

the available ones, which are mentioned in the manual are: 2stroke 1 cilinder, 1 stroke 1 cilinder and 1 stroke 2 cilinders. i chose the frist one, as i saw this one idling at 900 rpm. it is almost impossible the hub is spun... it was spun, and i replaced the hollow spring-like inserts with solid inserts.

 

[Sea Rider]

That tach is useless, that's the problem with $ 12-14 cheapo tachs. Although you can adjust number of strokes & cylinders can't adjust spark imput which is essential for correct rpm readings throughout the whole throttle range. Go for a Hardline hour/tach model [FONT=&quot]HR-8061-2 to read well, are just $ 22-24 at Amazon...Money well spent and boating fun guarateed...

Don't go for a new prop with that inaccurate tach, could be dangerous for the OB..

Happy Boating[/FONT]

 

[anonOmattie]

I think there is nothing wrong with the tach, but i do think it is interfering with something else. I was boating today and kept an eye on the tach. I mounted it at the steering wheel and added about 1.5 m of wire (soldered).

When i was throttling up, i noticed the tach was jumping from 1500 to 1800 to 2500 and back. At low rpms it read fine, but after half throttle it stops. I think i mounted the wire from the tach on cilinder 1 to close the other spark ignition, so ill try to get some more distance between them to up the accuracy. Otherwise ill buy a more accurate one!

 

[Sea Rider]

Same issue happened to me moons back, bought a Kholer induction tach and when installed in a OB read double than needed to be, expensive & useless...

Happy Boating

 

[anonOmattie]

so, i think i solved the RPM problem!

i rerouted the wires and moved the other spark-wire away as far as possible. the rpm readout droppen from 900 to 450 so i changed the settings again to match the real rpms. in gear its around 1100rpm idle, but on WOT its where the problem starts!

on my own, with a full tank and battery, i only manage to pull 4500(!!!) RPM WOT! i think i found the problem here, with all the help of you guys! (and especially to you Sea Rider )

now, the next big question is what prop to buy! the current is a 4x10x12, and would like to get a 3 blade next round. is somewhere around 3x10x11 a good first run?

 

[flyingscott]

Take another pic of the motor underwater and get the cavitation plate this time. It looks like you have a longshaft motor on a short shaft boat or measure the transom and tell us what the measurements are. A 13" pitch prop was standard for that motor and they run that on a lot of 16' fiberglass boats around me.

 

[anonOmattie]

Take another pic of the motor underwater and get the cavitation plate this time. It looks like you have a longshaft motor on a short shaft boat or measure the transom and tell us what the measurements are. A 13" pitch prop was standard for that motor and they run that on a lot of 16' fiberglass boats around me.

Thanks for the reply! My transom is 52cm, or around 20 inches, so i would say thats about right for a longshaft.

As you can see in the plane video i posted with the upper water deflector in shot, it comes out of the water and the water skimms along the bottom of it as suggested by Sea Rider.

As for the prop, i think 13p would still be to much for the motor. Its not a light open boats, with an estimated weight of around 500kilos with 1 person in it. Also, the bottom is not butter smooth, as im always struggling to get the boat out of the water and deal with this problem.

 

[Sea Rider]

Ok if tach is reading correct at least idle rpm, would you say that when tach was jumping numbers on display, was doing same with audible to the ear wot rpm, or wot rpm's were stable and even ?

Personally would be scared going for a huge prop maximization with a tach that doesn't count with adjustable spark timing as could blow OB if max wot read out is way less than should be...

What's the min.max wot rpm range factory recommended, is it 5000-5500, 5500-6000 rpm ? If firstly OB is losing 1K revs, if secondly, is losing 1.5 K rpm. Both way low on the extreme of the lugging side.

Which year is the OB, is it a Merc brand or a rebadged Tohatsu as this one. Asking because you can upgrade that 25 to a 30 if it were a rebadged OB for penauts costs and way to go.




Checking the video, it were a L shaft OB sitting on a S transom, there would be water splashes back or even over transom which vid doesn't show whatsoever. Middle wake is nice and even, at least water flow at speed is passing under upper water def plate, how under, can't be determined, only with a visual inspection.

Report stated factory wot range, can look forward to better its water performance by engine upgrade, if applicable along a prop maximization.

Happy Boating

 

[anonOmattie]

after a cold start it was jumping a bit, but it slowly stabilized after a few minutes of running. when slowly increasing throttle I could not see any inaccurate or jumping readouts. when on full throttle the readout was between a margin of around 50rpm in a duration of 20 second on plane and WOT.

the motor is an mercury 25XD long shaft, presumably from 1986. sadly it is not a rebadged tohatsu!
as i found on multiple sources, the factory wot range is between 5000 and 6000rpms, so im around 1K rpm to low with this prop.

 

[Fed]

Be aware guys the first video is NOT mattie's boat, just so you know.
Cut the grass.

 

[ondarvr]

Be aware guys the first video is NOT mattie's boat, just so you know.
Cut the grass.

​I didn't catch that the first time around, it explains a few things.

 

[Fed]

Yeah the videos caught me out as well.
I did a lot of pausing in the underwater video, growth looks to be 3"-4" long & clumpy, even the water coming off the bottom of the hull is massively disturbed at speed.
There's a huge streak of white water slightly to the Port of the prop & a corresponding extra large clump of growth on the hull. LOL.
I've had 40mph boats that couldn't get on the plane with less growth than that.

 

[Sea Rider]

if not Mattie's video, start checking what goes on at back transom at speed, if with middle water splashes out, over transom, prop aerations issues at close turns, any vice that combo might have and were not aware of.

It's quite normal for tach to have those mentioned read outs on cold starts till engine warms up and stabilizes. Check if OB and prop are in a heathy state cond. Check if boat has a waterlodgged hull issue along an impeccable clean bottom condition. All those issues contributes dramatically for engine to achieve less wot rpm than it should if without those.

Anyway, OB is 1000-1500 wot revs way off from middle/max wot range factory stated. Need a urgent prop maximization.

Happy Boating

 

[anonOmattie]

Be aware guys the first video is NOT mattie's boat, just so you know.
Cut the grass.

indeed! the first video is not my boat, but someone elses. it shows what my boat does when i was with 2 people. changing the OB to 1 inch higher resulted in a 4km/h increase with 1 and 2 people on board. it comes out of the water and planes, but nothing more.

Fed the water where the boat is moored (??) is very contaminated with alges. when i bought the boat there were around 25Kilos of mussels clamped to the bottom of the boat. mostly is gone, but some residu from those mussels are still left on the bottom (small hairs). the algae have been cleaned, but i think a new layer of anti fouling would not be so bad of an idea! im trying to clean most of it away by hand when swimming, as im not in a very good position to get the boat out of the water.

as for the propeller, are there any suggestions about the pitch?

 

[Sea Rider]

Need to check if everything previously mentioned is 100% correct before going for a further prop maximization. Is there a available pic of that OB sitting out of water to have a look, post one if so. Will help dialing best OB/transom height.

Happy Boating