props and speed

[newish]

i have a volvo penta sx outdrive with a 15x17 prop on it and it tops out at 32 mph i bought a 15x19 to increase the speed but one i got to looking while i had the prop off the diameter on the used one is about an inch less than supposed to be how much would that affect the speed of the boat?

 

[steelespike]

Re: props and speed

What horsepower.?What is the rpm at 32? So your saying its a 14 but numbers indicate a 15? Results would depend on the boat
and if a 14 X 17 is a normal size for the motor/drive combination.
A 14 X 17 would tend to rev higher than a 15 17. Is the 14 a 3 blade?

 

[newish]

Re: props and speed

Its a 17ft its 125hp. It revs 4400 which is normal. The boat just had a lot of work done to it last summer and I had it out twice after I got it fixed. I done wanna have that prop on there n constantly labor the engine. I'd like to be able to cruise at 35 instead of being wide open all the time.

 

[newish]

Re: props and speed

Oh and its also a 3 blade aluminum prop and yes the numbers say 15x17 but its wore out to 14

 

[steelespike]

Re: props and speed

If the prop has worn down an inch it is also very likely more primitive than todays props.
So your probably allready getting more rpm than a new 17 will allow. But the new prop may help the hull respond
better as well. so rpm may balance out.
If your motor only makes 4400 with a worn 17 a 19 will knock the heck out of your rpm.
It will lug the motor.Your motor needs to rev freely within its rpm range.
Boats aren't like cars.Marine engines are working like they are going up hill pulling a trailer all
the time. If you put too high a pitch on its like putting it in overdrive and forcing it to pull that trailer.
Lugging the motor will cause premature repair problems.If you want to ease the rpms you'll have to throttle back a little.
I would go to another 17" prop Perhaps a Solas,they have a good reputation.
There are props that will make it easier to stay on plane at reduced throttle.
The new prop may also help maintain plane at reduced throttle.

 

[newish]

Re: props and speed

i had the boat tuned to that prop well the carb anyway its going back in a month to get a few things done to it depth finder gel coat in a couple spots little stuff like that. i guess the only thing i can do is try it out on the water and then see if i need to go back to the 15x17 instead of the 15x19. also im adding the dolphin tail to help plain out.. so ill keep ya posted

 

[steelespike]

Re: props and speed

Do you know your gear ratio?
While the Dolfin foil could help staying on plane and hole shot They can rob some top end and and in the extreme can cause weird
handling. I think your better off with something like smart tabs. They will help hole shot, help stay on plane better,keep the bow from bounding around,limit no wake speed wander and may even add top end.
Any tabs need to be added carefully , applied just enough to accomplish the desired result.
A 19" prop could lower the rpm as much as 400.If it revs too much with a new 17 you can still use it by watching the rpm
It will be peppier than the 19 and will stay on plane better than the 19.

 

[newish]

Re: props and speed

hmm im kinda new to boats and ive not heard anything about these tabs? ive heard nothing but good about the foil. hints why i was gonna have it installed. what do you mean by the no wake speed wonder? i do use the boat to ski and wake board with but i also want to cruise and not have to push the boat the whole time im cruising around.

 

[steelespike]

Re: props and speed

No wake speed wander: some I/Os have a tendency to wander at no wake speed I believe something to do with deep V hulls..It can be a pain until you learn how to handle it trim tabs are said to help resist wander.
We had a 15 ft outboard and a 18ft I/O with a foil and they definitely helped. But I have seen reports of plowing and strange handling at speed,quick hole shot but a lack of top end.The ideal fit will definitely help hole shot and ability to stay on plane
but will lose some speed. You would likely need a speedometer to tell the difference.
Tabs carefully selected and adjusted will do all the good things mentioned and are said to increase speed sometimes.
As with the foil ;poorly selected and adjusted tabs will do all the bad things.
Smartabs are very effective and have excellent customer support.
I suggest a new 17" prop perhaps a Solas and don't buy a foil or tabs until you do some test runs.

 

[Silvertip]

Re: props and speed

Its a 17ft its 125hp. It revs 4400 which is normal. The boat just had a lot of work done to it last summer and I had it out twice after I got it fixed. I done wanna have that prop on there n constantly labor the engine. I'd like to be able to cruise at 35 instead of being wide open all the time.

If 4400 RPM is normal, then increasing pitch is going to lower the rpm which will lug the engine and possibly COST you speed rather than increase it, and at the same time increase fuel consumption. For certain it will also drastically affect hole shot. You need to understand the "physics" involved with prop selection as it affects engine rpm and speed. There is only one best prop. That prop allows the engine to operate at or very near the upper end of the engine manufacturers recommended RPM band. Speed will be whatever that prop design provides. Any deviation from that causes lugging due to too much pitch or over reving due to too little pitch.

 

[newish]

Re: props and speed

i already bought the 19 and have it on the boat. i dont understand the "physics" so to say but i do have a buddy with a 19 ft ski boat with an alpha 1 with a 15x19 and his boat is a 125hp and it handles the prop really well. by the stick weight mine out weighs his by 300 lbs i also dont know about the gear ratio either? id like to figure out my gear ratio if you guys could help instuct on how todo that?i know that when the boat went down i was reading alot about the motor and what rpms i should be running on top end and never had a problem reaching them. like i said before i dont need the speed but i want a little extra speed to be able to cruise and not have it topped out..

 

[newish]

Re: props and speed

he has a 17 ft. not 19 sorry about that

 

[steelespike]

Re: props and speed

Can you return the 19 if you test run it and its too much pitch?
Is Your friends boat the same brand and model?
Unfortunately the Volvo nomenclature can be confusing.
Some of their little motors run around 4800-5000 rpm.
Look for gear ratio on your outdrive on a a tag or numbers stamped there.
Usually near the top.
Nothing about props is carved in stone but it's just about a sure thing that the 19 on your boat will lower the rpm about 400,
slow hole shot and be more difficult to keep on plane with reduced throttle.
Your somewhat low rpm with a undersized worn 17" prop puts your motor under suspicion.
If you can't return the prop if you test it you could sell it or keep it for a spare.

 

[newish]

Re: props and speed

i got the prop from this website its a solas so idk if i could or not?

 

[newish]

Re: props and speed

oh and i was also wondering how these smart tabs will affect backing up? are they spring assist or fixed? ive never been around them!

 

[steelespike]

Re: props and speed

I call these passive tabs
I think they have pneumatic pistons That can be adjusted to various positions.
Do a search on the net I'm sure there must be lots info and probably a phone number.
I don't know how they back up I haven't seen any one commenting on reverse.So I'm guessing its not terrible.

 

[steelespike]

Re: props and speed

i got the prop from this website its a solas so idk if i could or not?

I don't know about exchanges/returns. Give them a call.