Ben Paquin
Seaman Apprentice
- Joined
- Sep 2, 2007
- Messages
- 45
Bought a used boat recently
1982 16" fiberglass sportcraft open bow with 1990 Yamaha V4 115 2-stroke.
The boat is a light (1400lbs loaded with 2 people in it) 16" v-hull/hybrid boat. It is rated for a max of 110hp.
The propeller is a yamaha 13" X 21 pitch stainless. Max speed is 39mph alone (GPS tested). 37mph with 3 people. This boat defies logic and I'm trying to find the reason why it's so slow.
I researched other similar boats and most of them are well over 40mph. Most of the them in the 45mph-47mph range and are able to hit 5300rpm+ with this size of propeller.
What I've checked:
-120psi compression on all cylinders.
-new spark plugs well gapped
-tried mounting the engine 1" higher to make the anti-cavitation plate line up with the bottom of the boat. This result in the same speed +-.5 mph with a lot slower acceleration due to prop venting. Also the boat can't turn hard now for the same reason. With the engine in the original position when the trim is down the boat handles very well.
How the engine runs:
never stalls seems to run really well, no hiccups or up and down rpm at WOT. Excellent acceleration out of the hole, shoot out and gets on plane very quickly. Hit 30 mph in around 5-6 seconds.
But it just seems to not want to go over 4600rpm. Unless I overtrim the engine and lose a bit of mph. Trimming from bottom adds about 2.5mph.
Next thing I need to check is Ignition timing and I ordered a 19 pitch aluminum propeller by turning point prop.
Another note for the performance aspect is the boat has been repaired a few times and the bottom is kinda rough. A few patches not well sanded down and such. I don't know how bad this can affect top speed.
Any other ideas or comments ?
1982 16" fiberglass sportcraft open bow with 1990 Yamaha V4 115 2-stroke.
The boat is a light (1400lbs loaded with 2 people in it) 16" v-hull/hybrid boat. It is rated for a max of 110hp.
The propeller is a yamaha 13" X 21 pitch stainless. Max speed is 39mph alone (GPS tested). 37mph with 3 people. This boat defies logic and I'm trying to find the reason why it's so slow.
I researched other similar boats and most of them are well over 40mph. Most of the them in the 45mph-47mph range and are able to hit 5300rpm+ with this size of propeller.
What I've checked:
-120psi compression on all cylinders.
-new spark plugs well gapped
-tried mounting the engine 1" higher to make the anti-cavitation plate line up with the bottom of the boat. This result in the same speed +-.5 mph with a lot slower acceleration due to prop venting. Also the boat can't turn hard now for the same reason. With the engine in the original position when the trim is down the boat handles very well.
How the engine runs:
never stalls seems to run really well, no hiccups or up and down rpm at WOT. Excellent acceleration out of the hole, shoot out and gets on plane very quickly. Hit 30 mph in around 5-6 seconds.
But it just seems to not want to go over 4600rpm. Unless I overtrim the engine and lose a bit of mph. Trimming from bottom adds about 2.5mph.
Next thing I need to check is Ignition timing and I ordered a 19 pitch aluminum propeller by turning point prop.
Another note for the performance aspect is the boat has been repaired a few times and the bottom is kinda rough. A few patches not well sanded down and such. I don't know how bad this can affect top speed.
Any other ideas or comments ?