Finding the right pitched prop

socket985 · 08-02-2008, 07:55 AM

Hi,

I have a old 19 ft Sabrecraft boat that I repowered with a 1989 70 HP Johnson outboard, It has a 14 x 19 pitch prop.

I would guess the boat and engine weigh around 1200 pounds.
(the trailer doesn't have electric brakes and anything combined over 2,000 lbs must have trailer brakes)

In Fresh water with 3 adults and 1 child it will go 28 to 29 mph WOT

In salt water it goes 25 to 27 mph WOT regardless of how many people I put in the boat.

If it's just me and my son, or 7 people totaling 4 adults, 1 teen, 1 pre teen and 1 child, it still planed the boat and traveled the same 25 to 27 mph in salt.

Is my boat pitch prop wrong? I would like to gain 5 more mph if possible but still have plenty of engine torque.

Maybe this is all I can expect for a 70 hp two stroke. It just seems it could do better. It increases about 4-5 mph between 3/4 throttle and WOT

The V is similar to the older 18 ft bayliners,glassply etc. it's relatively flat bottomed and not a deep V at all.

Any advice is greatly appreciated.

Thanks

SouthCoastStu · 08-02-2008, 08:21 AM

You'll need to know the RPM's at WOT before knowing if a higher pitch will do you any good. I'm not sure, but I think you should be around 5800-6000 at WOT. May be able to get a loaner prop to see how you like it before purchasing one

Danno · 08-02-2008, 08:22 AM

Adjusting your prop is about rpm's, not speed. Determine what the recommended top rpm range is for your motor. You should be able to reach that with a light load. Every change in prop pitch by 2" will change your rpm by about 400. As you increase your load, the availale rpm will drop but not drastically.

Tinman · 08-02-2008, 12:44 PM

Seems to me you're getting pretty good performance from that 70 hp, especially with as many people as you are carrying.

ET · 08-02-2008, 04:40 PM

Quote:

Originally Posted by socket985

Hi,

I would guess the boat and engine weigh around 1200 pounds.

That's a good one! .....19' fiberglass boat with motor and gear weighs 1200 lbs...

How much you want to bet that your weight is closer to 2400 than 1200? Stop on a highway scale and check it out sometime. Trailer is probably about 500.

You're actually doing pretty good with a 70hp. As others are saying....speed doesn't tell you anything...what are your engine rpms at WOT?

socket985 · 08-02-2008, 05:02 PM

I will get it weighed and let you know.

Road Rage · 08-02-2008, 05:20 PM

I'll bet the difference in performance you've noticed has more to do with altitude than prop. If your old two-stroke is tuned for higher altitudes, it won't perform as well at sea-level. Was there much elevation to the fresh water you were on?

Over-all prop diameter makes a difference too... not just pitch.

socket985 · 08-05-2008, 08:39 PM

I was fishing Drano Lake off of the Columbia river
I haven't weighed the boat but my dad thinks it weighs between 1500-1700 lbs with outboard.

Starfish · 08-11-2008, 09:39 PM

As others have said, adjusting your pitch without knowing your WOT RPMs is just guesswork. But as long as we're guessing, I'd say a 17 or 19 pitch is probably going to be about right for your rig, more likely the 17. And in addition to adjusting your prop, does your engine have power trim/tilt? If not, have you tried adjusting the manual trim setting (usually a pin or two in holes on the engine mount)? I've seen boats that were set up completely wrong that had great improvements in performance with just a few tweaks.

Finally, I wouldn't expect rocket speed with a 70hp on a 19' boat. You may not be doing too badly.

DJD · 08-15-2008, 01:39 PM

Sounds like you're there already! Probably not going to get much better. Just my .02

Phil Layer · 08-15-2008, 03:10 PM

Quote:

Originally Posted by socket985

(the trailer doesn't have electric brakes and anything combined over 2,000 lbs must have trailer brakes)

Can you tell me where you heard this? Is this in Washington or Oregon? Thanks.

Orca · 08-15-2008, 03:36 PM

Oregon - no law stated for trailer brakes
Washington - Over 3000 lbs

But, it only makes sense to have brakes on your oregon trailer at 3000 lbs (or less) as well.

http://www.towingworld.com/articles/TowingLaws.htm
http://www.tiedown.com/atowlaws.html
http://www.2rvguys.com/towlaws04.html
...

ET · 08-18-2008, 09:14 AM

Quote:

Originally Posted by socket985

I was fishing Drano Lake off of the Columbia river
I haven't weighed the boat but my dad thinks it weighs between 1500-1700 lbs with outboard.

Great....you think 1200, your dad thinks 1500-1700 and I think more like 2200, add the trailer and you are towing at least 2700 lbs. Could be more unless you have taken everything out of it. How much gas are you carrying?

Stopping at a scale off the interstate is free... Just look for one that is closed and take the exit. Pull onto the scale and look around for your weight. They leave the scale on so people can do this.

BTW....it sounds like you are doing well with 70hp. Chasing 5 mph is usually a waste of time and money. Especially after you find out your real weight....

What are your rpms at WOT?