prop question

Currently I have a 13 3/4″x21p aluminum prop on a ’05 lund fisherman 1800 with an ’04 yammie f150
loaded with typical gear for an afternoon of fishing and full livewell (no fish!!)
tm batteries and starting battery
2-guys and half tank of gas (20 gallons)

Speed is 50 mph gps tach is 5600 with the boat slightly porpoising trimmed as far as it will go without blowing out if i go down to 5400 it will ride like a dream. I can get the recommended 5800 rpm only if i go straight and it’s smooth

Question: can i get the same size prop in stainless and keep same stats or will the rpm go down? Will it hold without porpoising at 5800? Or is the 19 pitch better?

First let me say i know nothing about aluminum boats, so take what i say with a grain of salt. If 1800 means you have a 18ft boat then i would think your speed is way down. As for the difference between a 19 an 21pt prop my first question would be how is your hole shot. Do you come out of the water good. If you feel your hole shot is alittle slow then a 19 would get you on plane faster but you would lose top speed and your rpms would go up. If with the 21 you come out of the water good then i would go up to a 23p. I had a 19.6 ft glass boat with a 150 rude on it that i ran a 23p. Another thing , do you have a jake plate on your boat? If so you can raise that up alittle and try it. The porpoising is an easy fix. Get yourself a StingRay Hydrofoil Stablizer. Best one on the market and easy to install. I hope this has helped but like i said i know nothing about aluminum boats but i dont really see how there would be alot of difference in them. Good luck

If you keep the same pitch when you switch to stainless, you will lose RPM. The rule of thumb is pitch down when switching to stainless. I use a 21 aluminum and a 19 stainless on my boat. The stainless runs 200 rpm higher but has the same top speed and far better handling and prop lift. A hydrofoil should be your last resort – try a stainless prop to cure the porpoising. Your current prop is loosing bite and allowing the bow to drop and cycle up and down. My last three boats have been walk thru windshield aluminum and they perform best with a good bow lifting SS prop.
DB3

thanks for the replies, just picked up a demo 19p prop mercury mirage (didnt have the tempest) and will try that out this weekend so if you see a boat flying up and down the st. croix doing circles and acting weird all together its me

Here is a site for you to check out http://www.ptprop.com http://www.dahpropellors.com

thanks klh ill give john a call tom. i talked to his son jeff today at dah and got a lot of info out of him alone.

tried the rental prop and man is it slow! full gas 3 people and only 46 mph! i could trim the boat all the way up though and hole shot was good. but I still had a cruiser pass me. Sweet sound and roster tail!! might have to try the tempest next.

The results are in!!!!!!!!!

Loaded with one person and 60lb. springer spaniel
full livewell (no fish)
Average gear.
Tested on the st. croix above Prescott

Boat trimmed all the way up as far as she’ll go:
5800 rpm’s
47 m.p.h.
no porpoising
good holeshot
2.8 trolling speed (upriver) @800 rpm
Shooting at least a 30 ft. rooster tail out the back!

I talked to john at DAH this a.m. and he suggested trying the 21p tempest (i tested the mercury mirage 19p)

I am sold on stainless and dont wanna go back to aluminum!

Here is something for yah to check out,

http://www.go-fast.com/prop_slip_calculator.htm

http://www.boatramp.com/prop_applet/propanalyzerappletg.html