Trying to improve top end speed.

[gpatton]

Hello I have a 17ft Alum Garmen boat with a 60hp 1986 Merc. Pictured here

I've had this boat for close to a year and it's my first boat that wasn't a 9.9. She seems to be very fast out of the whole and in my bass tourneys it out performs the fiberglass boats until they just over power me. This is of course expected but my top end seems to be a little slow compared to other boats that are similar. I have a friend of a friend who has a 17ft alum and a new 50hp. My boat beats it fairly bad out of the whole but it has a higher top end speed by a fair amount. My boat can get about 31mph on wot at around 5500~ RPMs. The thing is we don't run over 4500 RPM WOP much because were talking about only 2-3mph diffrence. I can also and a third person and have them site between us or even on the back deck seat and we can still run at about those speeds. In fact it even seems to run on plane better. I just went out and checked my prop and I have a Quicksilver 48 918 18 A5 6p prop. Any type of advice or guides that could help me improve my top end would be much appreciated. I'm new to all this and I'm trying start to lean about all the concepts involved in boat performance.

I appreciate your time and advice.

 

[steelespike]

Re: Trying to improve top end speed.

Are you sure that last digit is 6P. if I'm not mistaken that is usually the pitch.
Could that be 16p?
It appears the motor is trimmed in all the way.Do you trim out for best speed?
Motor height is also important.The antivent plate(just above the prop) should be visable at speed.Even with or slightly above the bottom of the boat.
If the motor has a foil or whale tail get rid of it.
Check that the throttle opens all the way at the carb(s).
Is the motor in excellent tune?

 

[gpatton]

Re: Trying to improve top end speed.

Are you sure that last digit is 6P. if I'm not mistaken that is usually the pitch.
Could that be 16p?
It appears the motor is trimmed in all the way.Do you trim out for best speed?
Motor height is also important.The antivent plate(just above the prop) should be visable at speed.Even with or slightly above the bottom of the boat.
If the motor has a foil or whale tail get rid of it.
Check that the throttle opens all the way at the carb(s).
Is the motor in excellent tune?

Here is a pic of the proposed numbers

I do trim it for best results and that 31 is about all I can get. However the motor is need of some tunning. It runs rough at idle and does often want to die on me when it just idling in one place. I plan on taking it in to see if I can improve that. I honestly can't remember if I can see the plate. I'm going out Friday hopefully so I'll be able to look at that.

 

[kenmyfam]

Re: Trying to improve top end speed.

I'm with Steelespike. Set up needs to be optimised before considering a prop change.

 

[muskydope]

Re: Trying to improve top end speed.

I've been playing the same game myself earlier this last fall. Set up is key...
Step1. With the boat on the trailer take a straight edge and hold it flush to the underside of the hull, leave about 12" hanging out the back, trim the motor to even trim. Now you can see if the hull is straight in back, you can also see if the motor is too high or low on the transom ( engine height). The cavitation plate should be even with the bottom of the boat, if not, raise or lower the motor so it is, next check if your trimmed up running position is truly that, the cavitation plate should be on the same angle as the straight edge.
Step 2. On the water test ( in step 1 I would make a mark on the engine mount bracket and another on the motor at the hinge point where it tilts when the motor showed even trim with your staight edge). Back to the water... make your run and trim for best speed, now stop without touching the trim and check your marks.... Are you trimmed up or down from even? If up your weight balance is probably close, if down, your bow heavy and plowing (speed loss). Try shifting weight around, move gear and troll motor battery/s around and see what works best. Always watch not just speed but RPM's also, note both as you make changes/move weight.
Remember when checking the hull/motor in the driveway with your straight edge ( a straight board or yardstick will do) that if your hull is not straight(flat) it will affect performance usually for the worse.

 

[captkevin]

Re: Trying to improve top end speed.

Do you have a tach on your boat?

If so what rpm's are you achieving?

 

[Slip Away]

Re: Trying to improve top end speed.

Do you have a tach on your boat?

If so what rpm's are you achieving?

ANSWER:
He said 5500 RPM @ WOT

His prop is:
Mercury Marine Vengeance Boat Propeller:

* Manufacturer: Mercury Marine
* Prop Family: Vengeance
* Part Number: #48-91818A5

* Rotation: Standard Rotation
* Diameter: 10?
* Pitch: 16?

 

[smallboatlover]

Re: Trying to improve top end speed.

keep the cavitaion plate on or the wail tale it give you better performace and better plannin at low speed and faster pick up

 

[captkevin]

Re: Trying to improve top end speed.

What is the suggested rpm operating range for your motor?

 

[Slip Away]

Re: Trying to improve top end speed.

Answer: 4500 to 5500 RPM.