need more top speed!

[yfz450guy]

i have an 88 bayliner 18' bass boat and an force 125hp. have an 13x19 and a 13x17. when i run the 13x19 i can get to 35 to 40 mph at 4500 rpm. with the 13x17 i get to the same mph but my rpm's are at least 6000 rpm's. any ideas as to how i can get more top speed?

 

[roscoe]

Re: need more top speed!

Your numbers don't compute.

You should only get 400 more rpm by switching to the 13 x 17p prop.

Only way to measure speed accurately on the water, is with gps.

Given the numbers, going up in pitch will lug the engine, and drop the mph.

So you have all the speed you are going to get, unless you want to try a few expensive high performance props, to gain 2 mph.

 

[pnwboat]

Re: need more top speed!

More horsepower! Might want to check your engine compression to get an idea of what kind of shape the rings are in. Possible that your engine is not putting out maximum HP.

 

[yfz450guy]

Re: need more top speed!

compression is in the mid 135's.

 

[jerryjerry05]

Re: need more top speed!

If you are running 35-40 thats about the best that boat and motor is gonna do.The boat probably came with a 17c.If you use the 19 it will cost more in fuel. Stick with the 17 Just don't over do.Your hub could be spinning on the 17 at high rpm.Jerry

 

[steelespike]

Re: need more top speed!

If your really only gettiing 35 that seems slow.If your getting 40 by gps your close.
Try to get a speed by GPS.Does the boat seem to lay dow flat like it can't get the bow up?Are you running with a foil?
Be sure timing is right and throttle opens all the way.
Clean bottom, prop in good shape.
Optimize the setup If you can raise the motor vertically you could gain rpm and speed.Under ideal conditions 2 to 3 mph.Just be sure it pumps water.

 

[Frank Acampora]

Re: need more top speed!

Are your props the stock black aluminum, semi-cleaver, open hub pieces of crap? A michigan stainless 19 will get you 5-7 MPH more at 2-400 RPM more.

 

[yfz450guy]

Re: need more top speed!

what would a 4 blade prop do? i found two that are 12.7x 17 and 12.7x19.
perfomance opinions?

 

[Frank Acampora]

Re: need more top speed!

If I remember correctly, a four blade prop gives more stern lift than a three blade. The diameter is slightly smaller to help maintain the same blade area as a three blade. I don't know if this will affect top speed. Best to ask this question on the prop forum. Those guys know it like the back of their hands.

I didn't know four blade props were available for Force engines. Which ones are they? Are you sure those props are for the small gearcase--4 inch diameter? Or are you intending to modify Merc props? They are the same shaft diameter and splines as Force.

By the way: Last year I rebuilt a Force 125 on an 18 foot bayliner bass boat. With the stock 17 pitch aluminum prop it was turning 6000 and 40 MPH--Sound familiar? When I changed to an uncupped Chrysler 19 pitch 3 blade stainless, RPM went down to within recommended range and speed went up to 47, with myself and the owner in the boat. Prop selection can be critical.

Stock Chrysler props were uncupped and made by Michigan. They had a different blade shape than the branded Michigan; I always thought they performed better. If you do get a 19 pitch, uncupped would probably be better unless you are having cavitation/ventilation problems. The cupping reduces RPM by 2-400 and provides NO INCREASE IN SPEED. It merely helps the prop get a better "bite" in the water to reduce ventilation. Because of this, you CAN USUALLY run a cupped prop with a little more trim (higher) and this will SOMETIMES yeild a bit more speed. ----If the engine has the guts to turn it. A cupped prop may actually cost some speed over the same uncupped prop. This is an area where you need to gauge your engine's performance and make an educated guess as to prop selection.

 

[yfz450guy]

Re: need more top speed!

ok, i will post this in the prop section and see what they say.
thanks guy's.

 

[CharlieB]

Re: need more top speed!

The 19 is slightly too much prop, the 17 is just a hair too small.

Stay with the 17, you can always throttle back a bit to keep RPM down, but over loading that engine will shorten its life.

Switching to a stainless will make a difference, but you will still need to try both pitches before you can be sure which is correct.

Also, loading of the boat will make a difference, where the 17 seems too small, if you add a couple of people to the boat then the 17 may be just right.