Effect of Motor hight and prop selection

[FishHunter]

Today is the 1st chance I have had to do some wot testing.
boat is a 18ft Reef Ranger and motor is a 1996 150hp Evinrude
Prop is a Stainless steel Viper 14 1/2 x 17
160litres of fuel fishing tackle and 2 people
4000rpm = 40km/h
4500rpm = 50km/h
5600rpm and 70km/h
Motor is on lowest hole and cav plate is well under water, I will be lifting the motor 2 holes which I suspect is going to increase the speed and the rpm it will rev to at wot.

I want to drop the cruising rpm a bit without overloading the motor, holeshot isnt a big issue for me.

I am leaning towards a 14 1/2 x 19 which should drop my rpm by around 200rpm.

Are my guesses and feelings in the ball park?
PS at 70km/h I could still see bottom on the sounder, i know it wasnt bullshit because I saw Dangerous Reef as I passed over it.
Was only 15m of water though ;D

[STUIE63]

what is WOT for that motor

[FishHunter]

what is WOT for that motor

I did some googling and it seems to be 4500 to 5500

[Fed]

The rule of thumb is about 200 revs per inch but it's a very flexible number and depends a lot on prop design & cupping.
Lifting the motor will help you make a decision, too bad you can't borrow a 19" prop to test it with.
I'd be very interested to know how the boat handles steering wise after you raise the motor up as I'm thinking about doing the same to lighten my steering and make my boat a bit more flighty at speed.
Take notes for me. LOL

[STUIE63]

give brp a ring and make sure what the WOT is 02 9794 6600
Stuie

[FNQCairns]

Something that gets missed a lot is your 70km/h at 5600 means you are around the top of the rev range when at the very same time the hull is giving it's maximum lift so not much hull in the water = very little drag.
If one extrapolates down to fuel efficient cruising speeds and even snotty water speeds, in comparison to your top speed there will be lots of increased hull in water = drag.

In essence the propping on your boat because it can travel at that speed will not be by a linear line throughout it's rev range.

if it where me I would raise the engine and test but unless I was seeing 6k or above rpm from that 17 I would hesitate to pitch up without also decreasing diameter down at the very least and as a last resort.

Those engines last forever if they do all of the rev range easily, a 19 to me anyway might be pushing the boundary's to far toward lugging it when at the lower rpm/speeds.

[NAGG]

The rule of thumb is about 200 revs per inch but it's a very flexible number and depends a lot on prop design & cupping.
Lifting the motor will help you make a decision, too bad you can't borrow a 19" prop to test it with.
I'd be very interested to know how the boat handles steering wise after you raise the motor up as I'm thinking about doing the same to lighten my steering and make my boat a bit more flighty at speed.
Take notes for me. LOL


Be careful with the rule of thumb ...:-? 1" drop in pitch just gave me 600 rpm (WOT from 5300 to 5900rpm)

Chris

[Moonlighter]

I'd lift the motor first then re-test it to see what happens before playing around with props - the old story: change one "variable" at a time and then test.

Once youve done that you will have a new base-line set of data and can then think about changing the next variable (ie the prop).

I've put this link up a few times before - it is still the best info on props and engine height that I've seen - read it and learn!

http://www.veradoclub.com/smf/index.php?topic=137.0 (http://www.veradoclub.com/smf/index.php?topic=137.0)


Cheers

ML

[FishHunter]

I'd lift the motor first then re-test it to see what happens before playing around with props - the old story: change one "variable" at a time and then test.

Once youve done that you will have a new base-line set of data and can then think about changing the next variable (ie the prop).

I've put this link up a few times before - it is still the best info on props and engine height that I've seen - read it and learn!

http://www.veradoclub.com/smf/index.php?topic=137.0 (http://www.veradoclub.com/smf/index.php?topic=137.0)


Cheers

ML
thats exactly what I am doing and I have read that page a few times

[FishHunter]

Raised the motor 2 holes, havent had a chance to check wot and look over the back yet. The noticeable difference Is that the
steering is much lighter and trimming the motor makes a much bigger difference.

[STUIE63]

Well you will be getting closer then
Stuie

[Fed]

Thanks for the continued feedback FishHunter.

[FishHunter]

No problem Fed.
I might be in for a surprise, Something felt a bit off on the throttle adjustment and I just had a look and found it was only getting 3/4 throttle, so i guess its going to rev a little bit higher.
Now to wait for some flattish water.