No hole shot

[GonzagaCRO]

Hello,

I have 1974 blue band Merc 500 50HP Thunderbolt on a 2008 16.4ft fiberglass open fisherman;
http://www.autonautica.hr/proizvodi.aspx?cat=72&ID=215 .
Total weight without persons and with 15gallons of fuel is around 1300-1400lbs.
With only me WOT is around 5900rpm and it runs great, hole shot is very good.
But problems start when more people are in the boat, with 2 people (me+1) it still OK but with three or more it can`t plane.
I noticed that I have a 250rpm drop per person so with 5 person on board WOT is at 4800-4900rpm.
I have a OEM Merc prop 12p on it and was think to go to 4 blade 10p, is this OK? Would this help?
I don`t care about top end speed I just want to have hole shot, any kind:redface: .

Best regards,

 

[GonzagaCRO]

Re: No hole shot

I was thinking maybe
Solas Amita 4 11.4x10p 4 blade, PN 1313-114-10 ?
or
Solas Amita 3 12x10p 3 blade, PN 1311-120-10?

Best regards,

 

[j_martin]

Re: No hole shot

Yer marginal on power to start with. It's well worth while to change props for different loads.

I change props on an 18 footer with a hot V6 150 on it. (lightly modded XR4)

The main benefit is keeping the torque down to within specs on the engine. It's really hard on em to lug like that.

hope it helps
John

 

[hwsiii]

Re: No hole shot

Gonza, I must have he wrong information on your motor as I show your RPM range should be 4,800 to 5,300 RPM and your gear ratio to be 2:1, please verify this. I also need to know your speed at 5,900 RPM and then I fan help you.


H

 

[GonzagaCRO]

Re: No hole shot

TNX hwsiii,

RPM range is 4800-5500rpm, I am still searching to verify gear ratio.
I don`t have speedometer installed so I don`t know exactly but I think it`s around 20knots. My friend was driving next to me in his boat and at WOT (5900rpm) speed was around 18-20knots, I know this doesn`t help much.
Usually I am not alone in the boat (2 persons) so the WOT is around 5500-5600rpm, depends on the sea that day.

Best regards,

Igor

 

[GonzagaCRO]

Re: No hole shot

I found this;

Motor Specs
Manufacturer: Mercury
Model: "Merc 500; Model 500"
Year: 1973
Horsepower: 50.0
Cylinders:
Type: 2 stroke
Gear Ratio: 1.7:1
Min WOT RPM: 4800
Max WOT RPM: 5500
Spline Count: 13

On this page; http://www.piranha.com/

 

[steelespike]

Re: No hole shot

H will get you setup in the mean time your asking a lot of a 50 hp.
Be sure it is well tuned,that the throttle opens all the way. The antivent(cavitation) plate just above the prop starting point is even with the bottom of the boat. raise the motor to a point just before venting becomes a problem.
Trim in as you add people try to balance the load perhaps move preople forward to help it plane then back a little for better balance.Eliminate as much weight as possible plastic instead of metal fuel tank move the tank forward move the battery forward.

 

[GonzagaCRO]

Re: No hole shot

Cavitation plate is 1cm above the lowest point of the boat, I don`t have any problem with ventilation.
I have a plastic fuel tank and the battery is in the middle of the boat
Trim helps but not enough and the motor is working OK.
I know that I am expecting a lot from a fifty but if I can`t get a hole shot then just to get it in optimal operation range when under heavy load. (5 persons on board).

2Hwsiii
WOT 4800-5500rpm
Gear ratio: 2:1

Best regards

 

[steelespike]

Re: No hole shot

A 4 blade should help. H should be along with a suggestion he is usually right on.
You will most likely have to watch the rpm with a light load.

 

[hwsiii]

Re: No hole shot

Gonza, using your numbers of a 2:1 gear ratio and your boat running 23 MPH at 5,900 RPM I show you have a prop slip of 31% and as you load the boat with more people I am positive your prop slip is getting even worse, as well as your RPM are declining as well. The prop you have on there right now does not have sufficient blade geometry or blade surface area to carry the boat as it should with that modified vee hull.

Prop Slip

You need a 13" pitch Solas Amita aluminum 4 blade prop to lower your prop slip and get a better hole shot and top speed. Your RPM should be reduced to about 5,300 RPM and you should gain about 3 MPH and be able to carry heavier loads with less problems. I would have picked a 12" pitch but they don't make that prop in 12" pitch.


H

 

[GonzagaCRO]

Re: No hole shot

Hwsiii thanks a lot!!

I`ve looked on Solas web page and there is a 12p prop, this one is OK?


10.8 x 12p 4 blade

Best regards

 

[hwsiii]

Re: No hole shot

If it fits your motor it will be fine.


H