Frustrated with 25hp Merc on 16 1/2 ft aluminum Sea Nymph Fishing Machine

[wolfee]

This boat has been in the family since the 80's and I do remember the 25hp merc getting this boat on plane but it just isn't happening anymore. I just put a hydro-foil on it after being told it should do the trick but still no luck. With the motor shaft perpendicular to the water, the cavitation plate sits ~1 inch above the bottom of the transom and I can't adjust/mount it any lower, there's no room. I've been looking at motor brackets and was wondering what you thought? Would a motor bracket allow me to get the motor down in line with the bottom of the transom? Would a motor bracket allow me to switch to a 20" shaft vs. 15"? (currently 15", always has been) OR do I just buy a bigger 40-50 hp outboard and give up? Problem is a new 40-50 hp costs more than buying a whole boat with a 50hp already on it. (used of course) I could go that route but I love the boat … any help/advice appreciated.

One more thing, how can you check full throttle rpm's on the lake w/o killing yourself?? The #1 plug wire is like 6 inches long and the clamp gets interference from touching the motor, hard to get a reading w/o falling off and I'm doing this solo.

 

[racerone]

Is the transom 20" high , yes or no ?-------What are the results of a compression test ?------Has the water pump impeller been replaced about 3 or 4 times since motor was new ?-----Have you confirmed it is running on both cylinders ?

 

[wolfee]

The transom is 15", I don't have a compression tester, rpm's are normal at idle, water pump impeller was replaced last year (steady stream of water). Both cylinders are firing, been taking it out fishing this summer + a Canada trip just can't get on plane.

 

[flyingscott]

Take a pic of the boat from the side. That was a pretty common set-up in the 80s. When was the carb last serviced? Have you tried pumping the ball and see if that changes anything?

 

[Tassie 1]

Isn't a 25 hp a little on the low side though?
( a similiar size boat here in Australia would have a 70hp or similiar size motor )

Just thinking out loud...

 

[wolfee]

flyingscott I'll get a pic, the carb was serviced last year, I changed to 0% ethanol last year as well after having it rebuilt 3 times. I haven't tried pumping the ball, will have to get it back on the water somehow. I read about that, it will tell me if I have a gas line issue/vacuum issue.
Tassie 1: The max hp is stamped 50 hp. My dad bought it in the 80's and just had a 9.9 on it, then it went to a 25hp when my brother took ownership, I've had it for about 10 years now and have redone everything including the transom but was stupid by using the old transom as a template = 15", when I could have just gone straight across and kept it at 20".

 

[wolfee]

[No message]

 

[flyingscott]

Your transom is fine. Do a compression and spark test.

 

[Faztbullet]

Does it not build RPM and go or does it sound like its spinning but not going anywhere?? Where the inlet screens????

 

[wolfee]

Spark test is fine + new plugs (out once with them). It does seem like it loses power at full throttle. When I was in Canada we took the carb apart and the screw that held the needle in had actually fallen out. We thought we fixed it but it kept losing power at full throttle. When I got home I put a new fuel pump/bladder in and it seems better but still not on plane. Then I put the hydro-foil on (in the pic) thinking that would do the trick, no luck. Then I checked the rpm's at idle and it was fine but I can't check the rpm's at full throttle on the lake by myself. I'd have to lean way back over the transom because that plug wire is so short. So here I am with my original post. If the motor looks like it's ok with the bottom of the transom...I'm gonna have to take it to a shop to have the compression checked...right? Timing and rpm's were good at idle, can't tell at full throttle. Thanks for the info/help guys.

"Where the inlet screens?" Good question! Didn't notice that, is that something that could cause my problem?

 

[roscoe]

Motor length and transom match pretty well, motor does not need to be lowered.

That said, if you had rebuilt the transom and made it 20", you would not be able to use this motor.
But... it would open up more possible motors for you to upgrade to, like a 40 or 50, as most of those are 20".

Compression can be checked with a gauge, often available to buy, borrow, or rent from a decent auto parts store.

Looking at the photos, I doubt the foil will be of any benefit.

No screens means that debris, paper, plastic, weeds, sticks, could go in, directly into the water pump, not good.

 

[Sea Rider]

You can't possibly know it the combo has a spot on motor/transom match by looking at pictures, you'll need to find it for yourself on a wot run and check if motor works as a charm on all water cond, that's from flat, calm to bit rough water cond and at tight close turns at speed. Without any posted wot rpm data won't know at which wot rpm the motor is actually working at.

That combo needs a visual inspection on what's going on at back transom while running at wot. If that motor still has the prop it was factory delived with which should be a medium pitched one needs to be changed for a less pitch one for motor to rev at least middle to max wot rpm range as usually loaded. Is it possible to pass that 25 to a 30 HP one along a top prop maximization ?

Happy Boating

 

[roscoe]

The fact that it worked and planed for many years, and now it doesn't, means something has changed in the motor.
That needs to be fixed.

 

[Sea Rider]

The fact that it worked and planed for many years, and now it doesn't, means something has changed in the motor.
That needs to be fixed.

Touche, need to have the motor running top before venturing to better its water performance. Check compression, prop hub with one step prior to become fully spun or the actual boaters have gained extra weight since the good old days and now the motor needs a less pitch prop to get the combo out of the hole faster than before. LOL!!

Happy Boating

 

[Faztbullet]

The missing inlet screens can cause turbulence in front of prop thus reducing prop bite.(aeration)

 

[roscoe]

The missing inlet screens can cause turbulence in front of prop thus reducing prop bite.(aeration)

Interesting, good point.
May be the whole problem.