70hp evinrude pitch size advice

[High_flyerrr]

I have a 16ft custom craft with a 70evinrude on the back. It has a stainless 19 pitch prop but pushes past 6000 rpm wot. Boat does 36 at 5400. Will moving up to a 21 pitch get me to 40 mph and also help keep the rpm down. She’s a light boat maybe 1200lbs loaded down

 

[Texasmark]

I think OMCs pretty much ran 4500 to 5500 with the HP established at 5000. Also assume OMC 70's were geared about 2.5-1. If I am wrong, just change the gear ratio on the calculator, re calculate max speed, keep the same prop slip and solve for speed.

Go-Fast.com > Knowledge base > Find your prop slip > Prop slip calculator

On the answer to your question, plugging 21 into the equation and figuring 200 RPM per 1" change, you are looking at 5000 which is where your HP is measured.....If I remember my OMC days correctly. Assuming you will be using the same prop mfgr, model and all that you should expect the following and I wouldn't be worried about over loading your engine:

Go-Fast.com > Knowledge base > Find your prop slip > Prop slip calculator

Nope you are wasting your money! Would need more ponies as the faster you go the more it costs...one way or the other.

 

[racerone]

A 49 or 56 cubic inch model ?----Model # is ?

 

[willium8567]

A lower pitched propeller will create more power due to more engine RPMs, but the boat will move slower. A higher pitched prop allows the boat to move faster by travelling a farther distance with each rotation. When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range

 

[High_flyerrr]

A lower pitched propeller will create more power due to more engine RPMs, but the boat will move slower. A higher pitched prop allows the boat to move faster by travelling a farther distance with each rotation. When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range

What I’ve learned over the last few times when I took it out is that if the boat is loaded down with three or four people, ice, chess and equipment, it will top out at 5600 RPM but if it’s just two people or by myself, it will go over 6000.

 

[racerone]

Did a test myself,----Some years ago , same day , same boat , 3 different props.----19" , 21 " and 23" pitch.-----Boat speed was the same at 54 MPH.----Rpm of engine changed.-----Fact----For most folks , power torgue , RPM and boat speed are a mystery.

 

[JimS123]

You can change the pitch and get any rpm you want. But if you want to get to 40 you need more HP.

Is your boat an "original" Custom Craft tri-hull?

 

[jimmbo]

I would verify that the Tach is set correctly, and that it is Accurate. 6000 rpm, with a 19 in, on a 16ft, the numbers appear a bit off

 

[jimmbo]

Did a test myself,----Some years ago , same day , same boat , 3 different props.----19" , 21 " and 23" pitch.-----Boat speed was the same at 54 MPH.----Rpm of engine changed.-----Fact----For most folks , power torgue , RPM and boat speed are a mystery.

I did the same test, in the early 70s , when I was a Teenager, and found very similar, results. Acceleration did change though.

 

[racerone]

Agreed----Huge difference in acceleration.-----I normally ran the boat with 21" stainless.----A 1970 115 Evinrude on a 16' Kennedy.

 

[High_flyerrr]

You can change the pitch and get any rpm you want. But if you want to get to 40 you need more HP.

Is your boat an "original" Custom Craft tri-hull?

Yes a 1981

 

[JimS123]

Yes a 1981

The "original" Custom Craft was located in Buffalo, NY. They made kit boats and are quite an iconic brand. Out of business a long time ago, and the current company is called "Clark Craft".

The "originals" went like a bat out of heck and needed low HP. Based on your pic yours is of a different manufacture. Yours is a true "Tri hull", so you need to expect a higher HP requirement.

 

[High_flyerrr]

Gotcha. Hard to find any info on my boat

 

[High_flyerrr]

The "original" Custom Craft was located in Buffalo, NY. They made kit boats and are quite an iconic brand. Out of business a long time ago, and the current company is called "Clark Craft".

The "originals" went like a bat out of heck and needed low HP. Based on your pic yours is of a different manufacture. Yours is a true "Tri hull", so you need to expect a higher HP

Quick question. Whenever I gun it for a hole shot with trim down I get a lot of steering torque and boat wants to go left until I raise trim up some, is there any way to counter this?

 

[JimS123]

Quick question. Whenever I gun it for a hole shot with trim down I get a lot of steering torque and boat wants to go left until I raise trim up some, is there any way to counter this?

Your Trim tab located on the bottom of your Anti-Ventilation plate needs to be adjusted. With the right setting you can eliminate the torque completely.
Move the trailing edge of the tab in the direction of the torque. It's a trial an error procedure, so it may take several adjustments until you're happy with it.

A quick google should give you lots of info.

 

[racerone]

If boat goes LEFT, you turn trim tab to the LEFT

 

[JimS123]

If boat goes LEFT, you turn trim tab to the LEFT

That's what I said, but yours was a better explanation.

 

[jimmbo]

That's odd. Boats usually want to pull to Starboard when engine is trimmed in/down, then even out as engine is trimmed out/up, then pulling to Port when trimmed further out/up. That of course us assuming the Prop is not a Counter Rotating

 

[JimS123]

That's odd. Boats usually want to pull to Starboard when engine is trimmed in/down, then even out as engine is trimmed out/up, then pulling to Port when trimmed further out/up. That of course us assuming the Prop is not a Counter Rotating

Maybe someone already tried to adjust it but moved it too far.

 

[Texasmark]

Quick question. Whenever I gun it for a hole shot with trim down I get a lot of steering torque and boat wants to go left until I raise trim up some, is there any way to counter this?

Expect the trim tab to have different effects as its contact with the water changes as the angle (trim) changes. If you don't want prop torque feedback, install a "no feedback" steering system...solves that problem but is a little harder to steer...but no biggie for me, I'll take the NFB hands down and have it on my boat. They are readily available.