17p al. to 21p st. and same rpms.

[snake91876]

i have an imperial deep vee bowrider with a 3.0l merc the prop on it was a beat up aluminum 17p and the boat turned around 5000 rpms at 32mph i put a stainless 21p and it still turned almost 4800 rpms and only gained 4mph. i also put smart tabs on to try and stabilize the ride and plane better. it almost planes instantly but im getting diving off to one side bad. could i have the wrong outdrive ratio could the tabs be to tight they are at notch 3.

 

[steelespike]

Re: 17p al. to 21p st. and same rpms.

I think you have another post regarding leaning.You don't say what size the boat is.My experience on the forums here is you very seldom see a leaning problem in an I/O.They occasionally lean a little when steering against the wind but they don't dive. I believe you may have a hull problem. probably a hollow spot Sometimes called a hook. just ahead of the transom or possibly slightly farther forward.Boat usually planes very easily but you can't keep the bow up.The tabs would probably make it worse.Does it trim out all the way?
Also if the motor is sound/well tuned and boat about 18ft max speed would be around 40 with a 21 stainless.When changing prop material usual pitch
change reaction is unpredictable when going from a dinged up prop to any prop adds even more unpredictability.If you can solve the leaning problem I think your speed and rpm may be even higher.You may want to varify the tach is accurate.By the way a 4mph increase isn't bad when you consider the boat seems to be running too flat.

 

[snake91876]

Re: 17p al. to 21p st. and same rpms.

what exactly do you mean a hollow spot or a hook

 

[snake91876]

Re: 17p al. to 21p st. and same rpms.

i took the foam out of this boat it was wet would that cause it to be unstable

 

[steelespike]

Re: 17p al. to 21p st. and same rpms.

The hook is when a portion of the bottom normally near thr transom that has been pushed up usually from improper storing or trailering.
lay a straight edge along the bottom front to back in the 4 to 5ft section
just in front of the transom. Should pretty much be straight.Any concave area
will cause the boat to lay down too much.and resist coming up when trimmed out.Removing wet foam just lightens the boat improving hole shot etc.
You should replace the foam.It is there to float the boat in the event it
fills with water or capsizes.If water was allowed to stand in the bildge
for an extended time it could rot the framing in the bottom.usually visable
in the areas we discussed.Sometimes its not evident unless in the water.

 

[snake91876]

Re: 17p al. to 21p st. and same rpms.

meant to tell you boat seems to almost level up as i trim higher and higher but eventually cavitates the prop , is there any way the trim cylinders are uneven i put new limit switches on them before i ever ran this boat

 

[Bondo]

Re: 17p al. to 21p st. and same rpms.

is there any way the trim cylinders are uneven i put new limit switches on them before i ever ran this boat

Nope,.... You've got Other Problems...

 

[steelespike]

Re: 17p al. to 21p st. and same rpms.

meant to tell you boat seems to almost level up as i trim higher and higher but eventually cavitates the prop , is there any way the trim cylinders are uneven i put new limit switches on them before i ever ran this boat

Do you have a trim indicator? do you feel it works right?There is a trim limit
I don't know that much about I/Os if the trim stops by itself at the limit or if you are suposed to stop.If you trim too far it will ventilate (cavitation is different, not something you can feel) Also trimming too far strains the universals.If you are trimming far enough to vent then the bow should be right up there.Can you put the boat on a trailer and check the bottom.
Just because a boat indicate it has a certain amount of trim doesn't mean that you can trim that far.It depends on the prop,load and water conditions.
If the tabs are too tight it might cause the diving/leaning.You might try releasing the trim tabs and see if it still has that quick hole shot.I/Os are notorious for slow hole shot when not assisted by tabs or prop selection.If it still leans without the tabs something is up.Would be good to find out if the bottom has a hook.Doesn't take much and if its weak it would flex as you apply power.

 

[snake91876]

Re: 17p al. to 21p st. and same rpms.

the boat has plenty of bowrise im sorry i kinda lost you earlier in the post it gets the bow fairly high out of the water on plane its just when you shift around the weight say to port side it starts diving off to the port and wont hardly come back up , it will come back up with some trimming. should i try the smart tabs in notch 1. are there times when tabs make a boat worse, im going to check for hook now , got it on a trailer.

 

[snake91876]

Re: 17p al. to 21p st. and same rpms.

boat has a spot about 36" in from the transom where the rollers hit on the trailer that may be flexing. when i lay a straight edge that is 36" long there is a spot thats 6 inches long that is about 3/16" to 1/4" raised would that cause problems and is it correctable from the bottom

 

[steelespike]

Re: 17p al. to 21p st. and same rpms.

boat has a spot about 36" in from the transom where the rollers hit on the trailer that may be flexing. when i lay a straight edge that is 36" long there is a spot thats 6 inches long that is about 3/16" to 1/4" raised would that cause problems and is it correctable from the bottom

That doesn't sound excesive as long as you are sure the bottom isn't flexing.
I would release the tabs all the way and make careful note how the boat handles.Then increase tabs carefully one step at a time.
Did it dive to one side before the tabs? How is the hole shot without
the tabs?Is the floor good and solid,no soft spots?
Does it lean only to one side?