New Prop - Now No Speed

[Frank Chyz]

1988 Mercury 100HP OB. Replaced the original prop (48-77348-A40-21P) with Solas MD3916 1411 130 21. Now it won't do more than about 10 miles per hour WOT with only me in the boat. Before the change it would go up to 25 or 30 MPH. I have the trim set down. What might I be missing here?

 

[Sea Rider]

Trim set down means fully trimmed down ? If so, boat is plowing bow down and loosing speed along more hull drag. Trim slight up, go full throttle, once on plane adjust trim for combo to plane parallel to water level.

Report max wot rpm with you alone when combo planes better to check if prop performs right.

Happy Boating

 

[alldodge]

Have to agree something is wrong. Does appear you went from a 12 3/4 diameter prop to a 13 diameter prop, but a 1/4 greater should not have made you loose the much. I'm wondering if the prop is misidentified. Lay your original prop next to the new and the angle should be right about the same. Other then that need to take it to a prop shop and see what they tell you

 

[Frank Chyz]

Sea Rider, I tried trimming up a fair amount but did not notice any change. AllDodge, I took numbers directly off both props. New prop was ordered by and installed by a very well known tech in our area. I have not spoken with him about it yet but plan to on Monday.

 

[jimmbo]

Anything else besides the prop changed, or adjusted? is the engine revving up?

 

[Frank Chyz]

Jimbo, impeller was also changed. Every once in a while the motor made a sound that lasted for a second. Perhaps it was trying to kick it up a notch?

 

[89retta]

I'm thinking your motor is not running on all cylinders. With a 100hp mercury you should be around 40-45 mph . You stated with the previous prop you were getting 25-30 mph. Which is telling me something is wrong engine wise. Did he forget to put the thrust washer on ? That could be the reason your speed dropped.

 

[Texasmark]

I guarantee you that adjusting trim from parallel to the hull to full down WILL make a significant change in the boats speed at WOT if that's all there is to it.

 

[Frank Chyz]

Texasmark, are you saying that the motor should be trimmed almost parallel to the hull and not anywhere near full down?

 

[roscoe]

motor should be 90* to the water surface to launch out of the hole.

This may be full down, or not, depends on the angle of the transom.

 

[Texasmark]

Texasmark, are you saying that the motor should be trimmed almost parallel to the hull and not anywhere near full down?

I used to punch out of my hole shot with my engine partially pushed out to about 100 degrees with 90 being parallel to the hull. I'd blow a lot of water and come out porpoising and venting, but every time the bow came up and the prop bit, I'd shoot forward. Took about 3 oscillations and I was gone. I never tucked in on a hole shot on that, my last boat an 18' tin lizzy with a 90 Merc, Ballistic SS 24 XL.

 

[dussbake]

Im no prfessional but given what you have described I would think a cylinder is misfiring. Maybe bad coil or plug. But if you are gaining RPMs and just not accelerating with a new prop and hub your dog gear may not be engaging or slipping under load.

Two things to consider here... may be a simple shifter cable adjustment to fully engage the prop or you may have rounded of cogs in your foot that isn't allowing the dog gear to "grab".

The latter would be bad but repairable. Also consider the fact that I could be way off but if new prop and hub is close to the same it shouldn't make that drastic of a difference. Secondly, I'm a newbie at this forum thing but hopefully something I said can point you in the right direction.

 

[Stumpalump]

Prop hub may have spun. Try your old prop.

 

[Sea Rider]

If prop is found ok, is OB sitting at the sweet transom height ? A tip top OB sitting on transom with a maximized prop will give a fantastic hole shot and impeccable close turns at speed. Independantly of the transom angles, you cal always trim an OB to sit at 90? or very close by adjusting manual trim thrust rod at different trim positions or with remote trim.

If using any prop without a thrust washer, in a very short time front prop will be literally eat prop shaft housing to the bone due to prop forward movement when pushing very bad music....

Happy Boating

 

[ajgraz]

Prop hub may have spun. Try your old prop.

I'm thinking the same thing.

 

[Texasmark]

Mark the prop from outer casting down thru the brass/bronze hub. Punch it out a few times and pull the prop off and look at it. If your hub is slipping the mark you made will no longer be lined up. Takes just a few minutes to know for sure.

 

[Texasmark]

On the spun prop, didn't read anything about rpms. If you have a spun prop you are taking the load off the engine and at a given throttle setting the rpms will be higher than usual and the throttle response will feel mushy, like you change the throttle position and nothing really happens to boat speed but the rpms respond with an increase type thing.

 

[Frank Chyz]

Spoke with my tech today. Asked me to put the old prop back on and see what happens. He suspects a stopped up carb. Tells me if the old prop makes no difference he will know what to look for when I bring it back to his shop.

 

[Frank Chyz]

Looks like 89retta is our winner here when he said "I'm thinking your motor is not running on all cylinders". Tech just called and said it looks like the needle is stuck on the bottom cylinder so no fuel is getting to it. I'll post again when I get back on the water in a few days.

 

[89retta]

Good to hear you found the problem. Now you should get some power and speed.