I'm over proped

Bait O' Eggs · 02-13-2005, 06:21 PM

With a shortage of test left hand props in stock to test before I bought one, I purchased a 21 pitch stainless prop and hoped it would close to right.

Now that I got a working tach from Honda

I got a chance to see how far it would wind up.

Wide open I could only get the motor wound up to 4900 rpm, and I am looking for 6000 rpm redline I was told.

It appears current prop will be backup and I need about a 19 or so.

What is the downfall of running this prop? am I working the motor to hard even though it is not anywhere near the top RPM?

I really wish I would have played around with the trim to see what that did to the max rpm, I just had the motor down and ran it.

With 1100 rpm to get to red line, I suspect I wouldnt have gotten there no matter how much trim I played with.

jet · 02-13-2005, 07:28 PM

a 19p should work best for the tin tuna hauler, maybe a 17P due to the big difference between your current max and the published max rpm. The 21p is like running a tall set of tires on your truck. Some motors do not have enough poop to push the taller pitch so you can not acheive proper max rpm. The 21P might work well as a cruiser prop, but not a real good load hauler.

It is always good to have a spare tire on the boat. By the way if there is no damage on the prop many retailers will let you exchange them.

Jet~~~

Kamalot · 02-13-2005, 07:56 PM

Another factor in prop selection is the diameter. I spoke with a couple of prop experts before I made my selection, and was told with a big 4-stroke motor choose a large diameter first and then pitch accordingly. I also run a Lefthand motor, LF225 Yamaha on a 23' NR Seahawk Hardtop. We determined a 15.5 X 17p prop would work well for my setup, and when trimmed up I get 5800rpm with great performance. My motor specs call for it to peak between 5000-6000rpm. The prop experts I spoke with said for best performance on light load and smooth water trimmed up I want to get as close to 6000rpm as possible.

Kamalot

corrirod · 02-13-2005, 09:00 PM

For each inch of pitch you can expect a 100-200 rpm change. So in your case if you are only getting 4900 rpm out of a 21 pitch prop you will likely need to reduce the pitch by at least 4 inches to get to your ideal performance. I'd try a 17 pitch and see where you end up. You should see an increase of 800 rpms roughly.

Kamalot · 02-14-2005, 06:53 AM

Corrirod is right on, if you stay with the same diameter prop you have now. If you went a half inch larger diameter you'd have to drop down to a 15 pitch. Just another hint if you think you may be going stainless I was told Merc Props quality wise are top notch.

Good Luck
Kamalot

trap50 · 02-14-2005, 07:56 AM

Check Seattle propeller. They have a web site.

Poi Dog · 02-15-2005, 02:50 PM

My boat is rated at 4400 rpm WOT (it's a diesel), and it was propped from the factory at 4000-4100 rpms (for whatever reason, Trophy's seem to under-prop their engines). This was fine if I was running with normal load, but if I had 300+ lbs of ice and the tuna to match, I found it was somewhat under-powered, especially if the seas were rough. I didn't want to buy a third prop, so I had my 19/14.5 re-pitched to a 17 pitch. It took them about a day to re-pitch, and they cleaned it up and repainted it while they were at it, all for $63 - considerably cheaper than buying a new one. However, this work was to an aluminum prop, and I'm not sure if they can re-pitch. There are several prop shops in the Seattle area that do this kind of work, but I am sure Portland would have some as well. All they do is mechanically bend the blades to a different angle and rebalance the prop using a computer. Initially, I was worried about a loss of structural integrety after bending the blades, but they insisted the effects are negligible.

I took the boat out this weekend to do some tuning on my radar, and what a difference!

I picked up an additional 400 rpms and the boat shot out of the hole much more rapidly. I only lost about 2 mph at WOT, but of course I was running without a load, and unfortunately, will be doing so for another 4-5 months

until it's TUNA TIME again.

Like Corroid said, you can figure about 200 rpms per degree of pitch. If you want information or have questions, pm me and I can put you in contact with the propeller shop that worked on mine.

-Poi Dog

SouthCoastStu · 02-15-2005, 04:42 PM

I don't know if this is right, but I was told (once your up on plane) to keep the throttle constant and trim up until you start to loose speed (rpm should go up as you raise the prop). Then back the trim down a hair. Anybody hear/know different?

icU812 · 02-15-2005, 09:37 PM

What is your top end speed with that prop? I am currently running a stock 14.5 in. 19p on my Yami 225 pushing a 22' NR Seahawk and clocks 55 mph on the GPS. It takes me 3 boat lengths to plane though... It raps out at 6K rpm.

Thanks in advance.

Brandon

icU812 · 02-15-2005, 09:53 PM

Quote:

I don't know if this is right, but I was told (once your up on plane) to keep the throttle constant and trim up until you start to loose speed (rpm should go up as you raise the prop). Then back the trim down a hair. Anybody hear/know different?

I have never been advised how to until now. But this too is how I do it.. With the throttle at max, just varying the trim tabs, I can run anywhere between 40 and 55 mph. I change this according to the water chop/ smooth ride. Rougher the chop, the deaper I push the bow into it, the slower I go...

Cheers...

Kamalot · 02-16-2005, 07:31 PM

Quote:

What is your top end speed with that prop? I am currently running a stock 14.5 in. 19p on my Yami 225 pushing a 22' NR Seahawk and clocks 55 mph on the GPS. It takes me 3 boat lengths to plane though... It raps out at 6K rpm.

Thanks in advance.

Brandon

Brandon, on my 23' NR HT I top out at 45mph/GPS. The intent of the larger diam is less slipage, but sounds like your setup is just right. I plane out very quickly with gradual throttle, I haven't really hammered the throttle yet from a stop. It also trolls nicely when there's just too much wind and chop for the kicker. I have a digital fuel meter, and have notice much better economy if I stay under 4500rpm though.

Kamalot

crabbait · 02-17-2005, 04:56 PM

Roy, I pick up 300-400rpm just by trimming. I say get her back out there with a reasonable load and trim the nose up until you get your best rpm. Find that by trimming up until she either cavs or porpoises, then trim down until the boat runs stable with no porp/cav.

ron m · 02-20-2005, 04:19 PM

BOE, what size is your Honda? I think our boats are the same size, 23 ft EdWings, though with my pilot house mine will be slightly lighter. I have a 200 hp Honda. The aluminum prop that came on mine was 3 X 14.8 x 19. I want to try 4 blades, same diameter, 17 pitch and stainless steel.

Thanks for posing the question bout props, I can tell by reading this thread I better get another one soon. I did a search on "choosing props" and got lots of resutls, several of them were helpful. There is also a book available on choosing props.

some of the tests that were done on props included getting data for rpm and top speed, speed at 2K, 3K, and 4K rpm, time to 20 mph, and slowest speed for the boat to plane. None of the tests included results re: fuel usage, I'd include that if I had a fuel flow meter. (which I need to get) Not sure I want to get quite so scientific and besides, probably can't get that many props to try.

Good luck and let us know what you find out works best for you.

ron m

Bait O' Eggs · 02-20-2005, 06:38 PM

Ron I have the 225, I am negotiating on a stainless 18 pitch 3 fin prop which should be about the right one, maybe an inch more than I need

but the price is right.

ron m · 02-20-2005, 10:18 PM

hope it works well, maybe they'll let you try it before you buy.