Boat performance

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
Hello,

I would like to have your opinions about my boat's performance.

Attached are some pictures of my boat, which is around 12 ft long and around 4ft to 5ft wide, made in fiberglass. In terms of weight, I'll say it's around 200-250 lbs. I have it running with a 1975 25 hp Johnson 16-inch shaft (around 110 compression on both cylinders).

In terms of trim, the cavitation plate is pretty much flat to the bottom of the boat, so the trim is ideal ( based on what I've read).

The problem I'm having is that I cannot get more than 7 mph, even at full throttle. What happens is that as soon as I accelerate, the bow of the boat lifts up, which makes sense since the outboard weight and I are in the stern, but it does not compensate after the acceleration. I cannot make it plain.

I understand that I won't be flying with that 25, but I would think that I would at least double that speed (I'm saying this because I've used this outboard with three people around my weight, a 17 ft boat and a on-boat live well full of water and we were planing).

My assumption is that the boat is too short and there is not enough weight on the bow to balance the weight (I've tried sitting on the bow and my dad on the stern, but it does not improve at all, I'm around 220 lb, and my dad is about 130lb)

I've tried two different props (10° and 9° pitch), but I'm still not getting more than 7-9 mph. I've also tried with all the trim options on the outboard but does not get better.

I've been looking at hydrofoils, do you think they would solve or improve the performance? Or what would you recommend?

I was also thinking on extending the stern of the boat 3 to 4 ft ( make it a 16 ft boat )so it will be more balanced but this would be more expensive.
 

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Mc Tool

Senior Chief Petty Officer
Joined
Aug 7, 2024
Messages
676
In the 5th pic ( view on water from side ) that outboard looks to me to be trimmed up to far .
 

cyclops222

Commander
Joined
Mar 21, 2024
Messages
2,849
Need to do a test with ONLY 1 cylinder running at a time. In forward gear at WOT. Should sound the same. Many ways to SAFELY do it.
I strip the insulation off of some wire and wrap one end around the top of the plug. And the other bare end around some metal ground part. This test will catch a bad spark plug also.
 

Mc Tool

Senior Chief Petty Officer
Joined
Aug 7, 2024
Messages
676
I say motor is running on 1 cylinder.
Yeah , or lacking power for some other reason , I cant help thinking that with 25hp it should leap out of the water , but if you have 25hp pushing the back end down maybe not so much :)
 

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
Need to do a test with ONLY 1 cylinder running at a time. In forward gear at WOT. Should sound the same. Many ways to SAFELY do it.
I strip the insulation off of some wire and wrap one end around the top of the plug. And the other bare end around some metal ground part. This test will catch a bad spark plug also.
So the idea here is to not let the coil charge get to the sparkplug right?

I am 100% sure that there is spark on both cylinders, a pretty strong one, i also replaced both sparkplugs with the service i did last month.
 

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
By sound, coul you guys tell if this sound like is runing with 1 cylinder ?

Motor shoul is runing with like half, no more than 3/4 throttle
 

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CaptnKingfisher

Petty Officer 1st Class
Joined
May 14, 2017
Messages
269
I had a 25 HP on a 13' fiberglass boat, I think that boat weighed around 600 lbs, then with me, a passenger, the outboard and fuel I'm sure I was over 1000 lbs and I was traveling 29 mph.
1000003633.jpg

It doesn't sound like one cylinder to me but Im not mechanic. but I agree with what's been posted a cylinder drop is an easy way to decisively tell for sure. Does it sound like the throttle is really opened up all the way at wide open throttle? the plate in the carb wide open? I would suggest a spun prop hub, but you've tried 2 props (are they both used props or did you try with a brand new one?)
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,947
Does spark jump a gap of at least 8 mm on both leads , yes or no?----Check fuel pump diaphragm.-----Check reed valves.
 

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
I had a 25 HP on a 13' fiberglass boat, I think that boat weighed around 600 lbs, then with me, a passenger, the outboard and fuel I'm sure I was over 1000 lbs and I was traveling 29 mph.
View attachment 411087

It doesn't sound like one cylinder to me but Im not mechanic. but I agree with what's been posted a cylinder drop is an easy way to decisively tell for sure. Does it sound like the throttle is really opened up all the way at wide open throttle? the plate in the carb wide open? I would suggest a spun prop hub, but you've tried 2 props (are they both used props or did you try with a brand new one?)
Both were used props.

Yes, throttle linkeage is as it should be, butterflies open as they should. At WOT i can hear rpms going up and the bow goes up as well, but still does not plain.
 

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
Does spark jump a gap of at least 8 mm on both leads , yes or no?----Check fuel pump diaphragm.-----Check reed valves.
Sparkplug gap is set to 0.030 inches (0.76 mm) spark is really strong. Fuel pump was changed a month ago.

I did everything i should acording to the manual, thats why i think is not the outboard itself.

But im not a mechanic so im here for suggestions.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,947
Read the question 2 or 3 times.----Does spark jump a gap of 8 mm , yes or no ?----The question does NOT ask what your sparkplug gap is !----Check the reed valves !
 

diegomzt

Cadet
Joined
Jun 28, 2025
Messages
17
Another video, this is the other boat i mentioned but just me and my brother
 

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