mercury 25HP strange occurence... prop?

[anonOmattie]

No Title

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

hey there! we posted at the same time so i missed your reply! the first video in the first post of me was not my boat!

the second video i posted was the video where my OB was sitting at original height and after a 1inch elevation (which resulted in a 4kmh increase). the OB elevated is the position which im using now, so you could estimate the height of the OB with that video, as i sadly don't have a photo which shows the height (i should have made one when it was out of the water :/)!
also, the OB and the prop are both in very good conditions! OB starts always on 3rd pull and runs very smooth. havent had any problems with it. prop is also in good condition. very smooth, sharp edges and just minor dents on the edge of every blade.

the water was splashing before the height adjustment, but now it gets immediately shut off by the upper water deflector. when on WOT and making sharp turns it does not ventilate or loses grip (as it is a 4 blade?)

the hull has been cleaned when i was in the water. i saw a minor increasement, but nothing to drastic. the hull is smooth and not filled with algae as you can see on the back of the transom (mainly surface water is contaminated. below there's not much going on)

1.5K revs missing.... i think where all the performance is lost!!
better prop should result in better acceleration and more speed

 

[Sea Rider]

A L shaft OB will match nicely a 20 inch transom height, When water flow was skimming right under small upper plate as suggested, what was your trim position, assume is a manual trim OB, right ? what's the current prop as of right now, a 3-4 blade prop, which size ?

Happy Boating

 

[roscoe]

Late to the discussion here, but I have looked at the videos and it sure looks like engine is still way too low in the water.

Doesn't matter where the deflection plate is.

The large plate just above the prop ( anti-ventilation plate ) NEEDS to be even with the bottom of the hull.

See the lower red line in the diagram below.

 

[anonOmattie]

A L shaft OB will match nicely a 20 inch transom height, When water flow was skimming right under small upper plate as suggested, what was your trim position, assume is a manual trim OB, right ? what's the current prop as of right now, a 3-4 blade prop, which size ?

Happy Boating

its the 3rd trimhole from the top, so right in the middle. i think that it is still slightly undertrimmed, but the next trim position is slightly to high (that was my beginning position). so just to get on plane i thought this one was the right one . prop is a solas 4x10x12!

roscoe sea rider is telling something different! i also read that a lower prop position is better for heavier boats. as mine is not light i think this is about right? also, i cant raise it more... the clamps of the OB will ot grab the transom anymore, and ill risk losing my OB when driving?

 

[Sea Rider]

Gee don't understand why boaters like giving wrong advise, "NO" OB brand Owner's Manual that have read, and have read them all states AV plate must be even with rear keel/boat's bottom. This is a fallacy invented somewhere, the height will vary depending on the HP, more on portable to medium sizes, less on larger OB's. Technically speaking doesn't say anything, when hull is out of the water it's other story. Was reserving this pic for a future lower leg essay, but what the heck...



Happy Boating

 

[anonOmattie]

So engine is properly mounted, at the right hight. OB is in good condition. Bottom is mostly clean.
Current prop is an 4x10x12. This shop suggested a 3x10x11.... the prop i could buy then is a 3x103/8x11. Would that be about right?

 

[Sea Rider]

Does water flow at speed skims right under upper plate while OB sits at 90? while deck weight was evenly distributed to say 100% shure that OB is sitting at the right height. If not, it's not sitting at the sweet OB/Transom height. A 25 HP isn't a wow OB, worse if badly height seated, badly trimmed. Need to take all the horses out of the barn fast. Confirm by visual inspection if OB sits or not correctly.

None of the last 2 mentioned props would be about right on a theoretical dry evaluation, needs to be water tested including spending some money water testing other pitch props. Best combo performance is dialed usually on second prop selection and even on third one. Going from 4 blade props to 3 balde ones screws the equation.

BTW, this post is getting extremely long with no positive avail so far and losing summer boating fun.

Happy Boating

 

[anonOmattie]

So engine is properly mounted, at the right hight. OB is in good condition. Bottom is mostly clean.

as i stated mutiple times my OB is sitting at the right height, trimmed as best as it would go. 25HP is what i have to deal with, so i would like to get the maximum performance out of it! i will buy a new 103/8x11 and find out the results. if they do not match i can still swap them for another model.

i will post my findings here! thanks for all the help!

ps. summer in the Netherlands is just starting

 

[anonOmattie]

damn.... so much confusion. today i went for a last trip before ordering the prop. upped the trim by 1, so its not 90deg with transom but 90deg with water.

result: 500rpm more (5000 total) but almost no speed increase.

 

[Sea Rider]

The correct trim angle is when OB sit's at 90?, that's perpendicular to water level. Once there both upper and lower plates, along prop shaft will ride parallel to water surface. Should worry about wot rpm achieved as currently loaded, not speed increase. What were the max wot range rpm increase this time compared to before ?

Middle wot rpm will give slight more top end speed, but will lose best hole shot. OTOH, running top wot rpm will achieve best hole shot with slight top end speed decrease. Which modality do you prefer ?

Happy Boating ?

 

[anonOmattie]

the 3x10x11 prop had been received and tested. these were the results:

5300rpm, with much better hole shot, nice cruising speed and no noticeable loss of speed. OB also sounds better then before.

the prop grabs the water much better, whereas the other prop only grabbed the water when full throttle and slightly backing off from there. when from zero to full i get out the hole very Quick and no noticeable disadvantages.

after i add some load i will see if this prop really makes a difference or not, but as of yet it looks good!

 

[ondarvr]

Late to the discussion here, but I have looked at the videos and it sure looks like engine is still way too low in the water.

Doesn't matter where the deflection plate is.

The large plate just above the prop ( anti-ventilation plate ) NEEDS to be even with the bottom of the hull.

See the lower red line in the diagram below.

​This needs to be posted again and it's the correct line of thought, there are some other odd ideas on how to set up a motor for best performance out there.

 

[Sea Rider]

With adequate deck load distribution a less pitch prop will make hole shot day/night difference, lightly loaded, what was max wot rpm achieved ? You shouldn't go for 4 blade props when haven't tried lower pitch way to go 3 blade props...

Happy Boating

 

[anonOmattie]

​This needs to be posted again and it's the correct line of thought, there are some other odd ideas on how to set up a motor for best performance out there.

as i changed the prop, i will again experiment a bit with the OB height. as it has much better acceleration i can lift it up as far as possible and see what the RPMs do!

when with 4 people on board, the boat could still plane!! we hit 20kmh, which is a huge difference, compared to the 20kmh with 2 people on board before! the rpms went up to about 4750, so still on the low side, but this was a worst case scenario.

when slightly loaded, with 2 people on board, i get around 5100rpms!

 

[Sea Rider]

If water flow at speed skims right under upper plate, leave OB sitting there at that particular transom height. If raising too much prop will develop aeration issues, water bed is insufficient for prop to grip top, best prop thrust is lost.

With 2-4 boaters load variation can't possibly have a single prop that makes its homework right for both scenarios, unless counting with a variable pitch prop which are quite expensive...

What you can do is have 2 separate props, one for boating say at full wot revs with 2 up and another spare for boating with 4 up at say middle or max wot range, what do you prefer, top hole shot or better end speed ?

Happy Boating

 

[anonOmattie]

If water flow at speed skims right under upper plate, leave OB sitting there at that particular transom height. If raising too much prop will develop aeration issues, water bed is insufficient for prop to grip top, best prop thrust is lost.

With 2-4 boaters load variation can't possibly have a single prop that makes its homework right for both scenarios, unless counting with a variable pitch prop which are quite expensive...

What you can do is have 2 separate props, one for boating say at full wot revs with 2 up and another spare for boating with 4 up at say middle or max wot range, what do you prefer, top hole shot or better end speed ?

Happy Boating

holeshot is my go-to... as said this is a very underpowered boat and for fun i much prefer hole-shot over every mph.

as said with this prop the hole-shot im increased very much! as before, with 2 people we couldn't reach 20KMh, but with this prop i get around 28kmh!! that's almost an 8kmh increase with much better holeshot and nicer to drive the boat!

2 props will be to (time)expensive to have so 1 prop is the max. i quess with the pulling power this prop has it is sufficient enough to still get somewhere quick, have a heavy workload and maybe even do some fun water sports!

i think i will stop the search of a perfect prop, as i think this is keeping me away to much of the fun, and im quite happy with the results im getting!